DOI: 10.002/anie.201105813

Strong Bonds: International
Collaboration in Chemistry
Special Insert in Angewandte Chemie
from the Deutsche Forschungsgemeinschaft

SPECIAL INSERT

Foreword
Dear reader,2011 has been announced as the International Year of Chemistry to celebrate
the achievements of chemistry and its contributions to the well-being of humankind. The versatile and creative world of chemistry is presented in worldwide events. In Germany, various activities have been organised by national
chemical organisations, funding institutions and companies in which both science and the public are invited to participate and encouraged to discover the
fascinating world of chemistry.
With this special edition, “The German Research Foundation (Deutsche Forschungsgemeinschaft, DFG)” provides you an insight in the work of selectedinternational chemical research projects and its funding programmes. You canimagine the diversity and interdisciplinarity of chemical research just by looking
at the table of contents – I wish you a fascinating read!

Prof. Dr. Matthias Kleiner
President of the Deutsche Forschungsgemeinschaft

Table of Contents
Editorial
F. Schth

A4 – A5

ArticlesS. GrandelInternational Research Supported by theGerman Research Foundation (DFG) –Examples of Activities in the Field ofChemistryA6 – A10

A. Cattani-Scholz, K.-C. Liao, A. Bora, A. Pathak,
M. Krautloher, B. Nickel, J. Schwartz, M. Tornow,
G. Abstreiter

A New Molecular Architecture for MolecularElectronics

A11 – A16

J. M. Englert, A. Hirsch, X. Feng, K. Mllen

Chemical Methods for the Generation ofGraphenes and Graphene Nanoribbons

A17 – A24

X. Wang, D. Zhang, Y. Wang, P. Sevinc, H. P. Lu,
A. J. Meixner

Interfacial Electron Transfer Energetics
Studied by High Spatial Resolution TipEnhanced Raman Spectroscopic Imaging

A25 – A29

A2

A. Llobet and F. Meyer

Water Oxidation in the Context of the Energy
Challenge: Tailored Transition-Metal Catalysts
for Oxygen-Oxygen Bond Formation

A30 – A33

F. Liebner, G. Pour, J. M. de la Rosa Arranz,
A. Hilscher, T. Rosenau, H. Knicker

Ammonoxidised Lignins as Slow NitrogenReleasing Soil Amendments and CO2-Binding
Matrix

A34 – A39

P. J. di Dio, M. Brssel, K. Muñiz, R. S. Ray,S. Zahn, B. Kirchner

Pd-N to Pd-O Rearrangement for a Carbamate
Synthesis from Carbon Dioxide and Methane:
A Density Functional and Ab Initio Molecular
Dynamics Metadynamics Study

A40 – A45

K. Gruber, B. A. Hermann, P. H. Seeberger

Sensing Carbohydrate-Protein Interactions at
Picomolar Concentrations Using Cantilever
Arrays

A46 – A51

H. Faber, S. Jahn, J. Knnemeyer, H. Simon,
D. Melles, M. Vogel, U. Karst

Electrochemistry/Liquid Chromatography/
Mass Spectrometry as a Tool in Metabolism
Studies

A52 – A58

S. Wilhelm, T. Hirsch, E. Scheucher, T. Mayr,
O. S. Wolfbeis

Magnetic Nanosensor Particles with
Luminescence Upconversion Capability

A59 – A62

F. Lequeux, D. Long, P. Sotta, K. Saalwchter

Constrained Dynamics in Interphases ofModel Filled Elastomers: Role of Interface
Chemistry on Crosslinking, Local StressDistribution and Mechanics

A63 – A70

A3

Articles
                      !"#$% &'       (   ) &   &
&& *      +  +                  && ,                      &   &  + &   +          ' $  &      ++     &       ' &     + &      +  ',  ++     +'       ++   + ' -  ' &     '  $  &  -'     +&     ,  ++  .!/  0&    +     + &        +     +               &        + '      +  '$  &&         +   $1   '     +         
      , %  ' &++   &    
0&    &   '   '  '     & )  &             &       +                ' .!/$ 2     '       &   '  ' 
3+                  0&       &'   +    '     +           $
4 ' 5   ++&     +& 
2 67     + & '      8     &     + &    &   $       9   + +   +     + +      8     &    '
 +     +           & '      &   '$ 4+ &         0&          -'    +      $ '       +       &'     +    &   +  +   '      ' &  &   $                ' '      '  )  +      $ %   &  0&        &    
& , :    '   ; 2   +    &    &      + & 
''        ( '*; 2  
 &   + 3   &    '    + & &     '     '



&  ; :   0&      &   '  &   '     ( &  *  &           
0&     +     + $                 0&  &  & & $ 2   
 +     +  6
 & & $ :  & &   3  
 +   8  ; : -            &        ; '       &  +  &+ +         $ :  & ''    ++ &   $ +     0&     &   ++   &  +                    +$ %   &           &'            '  + + &     && ,     +   '          -   &  - -   &           +      &$     &    3
 + &'$ /4<      &   &              ' '
 <   '   '   &   - &     ++   &  +    +           3
 $ :       &    '      +   & '  ) &   &     0& 
&  )            + '  &   +   + '            $             '  
-  & &      !& +     $ /4<  +   !& +  &''       +   &   +    !& +       !=6 !%  $ % +     &      !& +   &' 6      +    ' +       & $ :  +   & 6       /4<   '  &++    (#+  *    !=6       &     3        &'$   &   +  & '        -  0&  &   /4<6&    +  $    
 8       -    
0&  &        +  &    +  $ %  &     &'     /4< +     '     $ % +   & ' '  +   /4<$  +   +   +     ++          
&      &        "  <    & $   &   &  +  
&     +          &    ,     '     0&  &    '    '    ' &     ++    +     &  +   $'        + '      

 
 '   $ # '    0&    '     #.!/   ' + $  '    
&     &       &  &+ &        +&  +   - '  &     6 +& '      $ 6"  8  '&          &    '         '6++ $      +  
+ +   6         '6++   &   '  +   &' &   $
'   '       ++     & 
   
' +  6   $    &  8   & & ' &   &'    +  + +    ' +  6+  $ 4       +6 ++   
$ $ 3    &      6&+  '  $ $       + 
$ %    
 ' &+    2    ? & 1@     
++     ' '    +     $     '  '         +   +     & & '   '   &  0&       '  &       &  & '   +     & 
 '        $ %  0&   +     &   & , :'   $  '          +  %#&       +6     6  =+   + '   0& $ %      ' &    &'    0&      '
  &+ ' & 
+      8 A%#$"&     &  '  +    + '$ "&        &    '  &   .    1   & &    
 &       3       
 & '      +     -         '      '$   (      #  1  *  +    &     + $   '    ++    #     +     .   $ .'   + &    +& + ++ &   38   &    && & & $  #   &        8      + '  0& $ %  -  ' &+    & '     ' 6 '  '$ / /   '&        &          #     + & $ %    & '   -   +      +    +'    # 3   $   &    "'     ? ' &+ ' '         & '  +       $ % "'     +            6           6+          +      +  '  ' '  $%    +&   &        4  $     '   &'      '  & '   3    $ % &  '+      &     &    &'&     3   '  '& $              ' ++          &   &   '  +    &   :    - $ "&  +    & &        '   ' ''$          8     +     +  $ %     + 
& 

+   3  &    +     +  &   &   +8         &    &    &$ %     & . 0& &3   $       '  +  0&     &  +   $    & ' ' 3 +       ' +            &    /4<$
: +         ' +    &   &         $                       !
"#   $% !%&!

 ' &++  & &+   &   
&  0&    '    ' 
& $ % 
&++     +& '   3   '   &'    + &      $ :      + &     -    3  
&  +              &  &  #6+ $ !=6    B  &  ' #
%   ( & )  *         * + ,   - *./.01    ! !  >Articles
4GUGCTEJ (WPFKPI   !"  #$ %  &'%#( ) *+$ , -.  % , /$"Sibylle Grandel%hemical sciences play an important role in addressing global challenges such asenergy supply and storage, health, or environment. Research in these complex
areas requires more and more international collaborative work. In fact
collaborative research has been common practice for many years in the field ofchemistry. This situation is also reflected by the increasing number of German
funding programs, which promote international and joint research collaborations,in order to achieve a more effective use of both national and international
resources in the field of chemistry.
 +PVTQFWEVKQP
Topics like energy, health and environment – key areas ofchemical research – have always been characterized bycomprehensive and cross^border questions and challenges. The
need for international collaborative approaches has increased
even more due to the growing social, economic and ecologicalchallenges in the course of globalization. Scientists, represen^tatives of national research councils and funding agencies thusaim ^ both in bottom^up and top^down approaches ^ to intensify
the already on^going international and interdisciplinary dialogue,
in order to achieve a more efficient use of national resources andknowledge. In addition, such international collaborations are
regarded as fruitful scientific and personal experiences andstimulating precursors for the careers of both young and moreadvanced researchers.
Germany provides a wide variety of funding opportunities fornational and foreign researchers: The main “players” among thefunding institutions are the Federal Ministry of Research and
Education (BMBF), the German Research Foundation (DFG), theGerman Academic Exchange Service (DAAD), and theAlexander von Humboldt Foundation. In addition non^university
research institutions like the Max Planck Society, the Hermannvon Helmholtz Association, the Fraunhofer Society, and theLeibniz Association provide funding and research positions inscience and industry. This short list reflects only a choice of thevarious funding opportunities in Germany, providing an entire
overview of all institutions and programmes including their
specific target groups, funding requirements, etc. would be far
beyond the scope of this article. The focus of this article is on thenational and international funding opportunities provided by theGerman Research Foundation. For information about the otherA6

institutions, the interested reader is referred to the individual
homepages, or to the Research in Germany portal(www.research^in^germany.de).
The German Research Foundation (Deutsche Forschungs^
gemeinschaft – DFG) is the self^governing organisation for
science and research in Germany, and the main national fundingorganisation in the field of basic research. The DFG promotes
research in all fields of science and the humanities. Some
numbers and statistics about the amount and distribution of DFG
research funding, particularly in the field of chemistry, are given
in Sections 2 and 3. The DFG is internationally active, bypromoting cross^border cooperation between researchers, foster^ing international collaborations, the mobility of researchers, andthe internationalization of German universities. It actively
encourages international research collaborations and maintains
partnerships and relations with a large number of partnerorganizations all over the world – which is also valid for the fieldof chemistry. Support for international cooperation and exchangeby the DFG is provided for individuals by specific grant
programs, or by coordinated programs which are described in
Section 2. Joint proposals, with two or more partners fromdifferent countries, follow in general the principle of mutual
responsibility. Researchers working in Germany apply to the
DFG while their partners abroad apply for funding at theirrespective partner organizations (see Section 3).
The aim of this article is to introduce selected examples of thefunding portfolio of the DFG, which support international
scientific collaboration and projects. The general program
introduction is illustrated by examples of funded research in thefield of chemistry. With this insight, national and international
scientists are informed and encouraged to use these options toestablish or intensify their international networking or to apply
for funding for research stays and visits in Germany or abroad.

 &() 4GUGCTEJ (WPFKPI 1RVKQPUThe DFG offers a large funding portefolio from Individual
Grants Programs to Coordinated Programs for scientists from thegraduate up to the senior level. A brief overview of DFG’s
research funding in the field of chemistry is given in Scheme 1.
Related to the total DFG funding budget, chemical researchaccounts for 7%, all natural sciences have a proportion of about
26%. [1] Funding is not only offered to German scientists, also
foreign researchers may be supported, if they fulfill the specificrequirements of the respective programs. (For details about
purpose, extent or requirements for all the programs described
below,
theinterested
readerisreferredto
http://www.dfg.de/en/researchfunding/programmes/index.html.)(KIWTG : Number of funded research fellowships (first grants) in
the field of chemistry between 2005 and 2011 (status July).
Different colors indicate the various host countries. [3]The two most prominent and prestigious individual grants
and fellowships are named by two eminent researchers of the lastcentury – Emmy Noether and Werner Heisenberg. The *$$"
2 34$ provides funding for outstanding researcheswith 2^4 years of postdoctoral and international experience toestablish an independent junior research group for up to five
years. In the natural sciences a total of 248 Emmy Noether
proposals were funded between 1999 and 2010, whichcorresponds to an average funding rate of 38%. [3] 28% (onaverage) of the candidates came from the field of chemistry(Figure 2). The 5!4 %1 provides funding for 5
years for outstanding researchers who qualified for aprofessorship (by Habilitation, Emmy Noether, or juniorprofessorship, etc.). Since 1978 a total of 173 Heisenberg
candidates have been supported in chemistry, thereof 85% comefrom the chemical subdisciplines of “Molecular Chemistry”(51%), and “Physical and Theoretical Chemistry (34%). The
other subdisciplines were each 5^6% (“Biological Chemistry”,“Polymer Research”), and 2% (“Chemical Solid State Research”,“Analytical Research”), respectively. [3]5EJGOG : Selected numbers and statistics on DFGs researchfunding in the field of chemistry.

 +PFKXKFWCN )TCPVU 2TQITCOU
Within the Individual Grants Programs the 
#0 (“Einzelantrag”) are the central funding form supportedby the DFG. [2] Funding is provided from the doctorate researchlevel upwards for a research project on a specific, defined topic
for a limited time period. The project is carried out at a German
research institution and enables international cooperation. The
 %1 ("Forschungsstipendien") support young
scientists at their postdoc level, and provide a basic fellowship fora research stay abroad for up to 2 years. Between 2005 and 2011a total of 243 research fellowships (first proposals) were fundedby the DFG in the field of chemistry (Figure 1). The USA was thehost country in almost 50% of cases, followed by the United
Kingdom (20%), Canada (7%), Switzerland (5%), and France
(4%).


funding also for foreign scientists under specific requirements(KIWTG . Number of funded Emmy Noether first grants in the fieldof chemistry compared to all natural science fields between 1999
and 2011 (state June 2011).[3] %QQTFKPCVGF 2TQITCOUThe Coordinated Programs include four main funding
options: /!.  / 6 // (“Sonder^
forschungsbereiche ^ SFB”) represent long^term (max. 12 years)
university research centers, where a cross^disciplinary research
program is performed. The traditional CRC is applied at one

A7

Articles
university, the CRC/Transregio is jointly appliedby generally up to 3 universities, where alsointernational partners can contribute (see Table 1).
The  74 # 6 7# (“Gra^duiertenkollegs ^ GRK”) are established byuniversities for doctoral research with a structured
program and may last up to 9 years. A specificform of the RTGs, the International RTGs provide
joint doctoral training programs between
universities abroad and from Germany. 3"34$ 6 330 (“Schwerpunktprogramme ^SPP”) enable a collaborative networked researchover several locations in an emerging field of
science and are established for a period of six years.
Once the program is established, the DFG
announces a call for proposals. The 
8 6 8 (“Forschergruppen ^ FOR”) comprisea team of researchers working together on a
research project which, in terms of thematic focus,
duration, and finances, extends beyond the funding
options available under the Individual Grants
Program or Priority Program. They generally last
six years and often contribute to establishing new
research directions. Funding opportunities for
Research Units are subject to the same principles
as Research Grants. Numbers of currently fundedchemical projects in the respective coordinatedprograms are summarized in Table 1.
(KIWTG . Graduate Schools and Clusters of Excellence of the German Excellence
6CDNG . Number of currently running Collaborative
Initiative currently running (map kindly provided by C. BrBck, DFG)
Research Centers (CRC), Research Training Groups(RTG), Priority Programs (PP), and Research Units (RU) in thefield of chemistry.[3] In parentheses,
 'ZEGNNGPEG +PKVKCVKXG 'Z+P
the number and location of international partners is given.[a]Chemical Subfield

CRC

RTG

PP

RU

Molecular
Chemistry3

7 (2 JP,
1 CH,
1 NL)

2
(1 CH)

Chemical Solid
State ResearchPhysical andTheoretical
Chemistry
Analytical Chemistry
Biological Chemistry
Polymer Research

3

1 (NL)

6 (USA,
NL, F, I,
BE, SK,
RU, TH)
3 (1 CH)

4
(1
CN)
1
4
3

2
(1 USA)

1

6(1 UK)

1 (USA)
2
1 (ES)

1
3
3

Total

18

1
2 (1 SE)3(1 ROK,
1 F)
16

14

16

1

[a]
BE: Belgium, CA: Canada, CH: Switzerland, CN: China, ES:
Spain, F: France, GB: United Kingdom, I: Italy, IE: Ireland, JP:Japan, NL: Netherlands, ROK: South Korea, RU: RussianFederation, SE: Sweden, SK: Slovakia, TH: Thailand, USA:
United States.

A8

In addition to the programs mentioned in the previous
Sections, in 2005 Germany started the Excellence Initiative. Thisnationwide competition among higher education institutions wasestablished, to strengthen their research capacity and make themmore attractive to highly qualified scientists from all over theworld. 1,9 billion  were invested between 2006 and 2011,
additional 2,7 billion will be provided for a second period from2012 to 2017.[4] The initiative offers three funding lines: 1)#   promote the education of young scientists. 2)/ , *+, which are internationally visible researchhubs, and offer research positions for PhD studies, postdocs,
junior professors, and professors. 3)  4
promote top^level university research to enhance the institutions’international competitiveness. Currently 15 Clusters of
Excellence and 14 Graduate Schools deal with chemical research
topics (Figure 3). Corresponding funding for the first and secondline in the field of chemistry amounts to 59.4 Mio . [1] 'ZCORNGU QH $K/WNVKNCVGTCN +PVGTPCVKQPCN(WPFKPI 2TQITCOU 'ZCORNGU QH 'WTQRGCP 2TQITCOUThe network *-6/$" was founded in the year 2004
with the aim to substantially develop and implement programs fortransnational collaboration in the field of chemistry in Europe
thereby significantly contributing to the creation of a harmonizedEuropean Research Area (see www.erachemistry.net). ERA^
Chemistry is a network of national research funding organisations
throughout Europe and was supported by the European
Commission between 2004 and 2008. All activities of ERA^
Chemistry are centered on the initiation of joint European
research programs and the facilitation of international
collaboration in curiosity^driven research in chemistry. Apartfrom the organization of research conferences, symposia, andworkshops for scientists and/or administrators ERA^Chemistry
has launched two Thematic Calls and four thematically open calls
within the Open Initiative (see Table 2). Nine national funding
agencies have already participated in the Open Initiative which islaunched once per year, allowing scientists from these nations tocollaborate with each other in bilateral or trilateral collaborative
arrangements. The program follows a two^stage procedure,
involving pre^proposals and full proposals. All proposals are
subject of a joint peer^review and joint decision process of thefunding organizations involved.
It is the purpose of ERA^Chemistry to establish the Open
Initiative on a long^term perspective, to improve its procedures
and to attract more Partners in Europe to participate. Moreover,
ERA^Chemistry has the aim to intensify its collaboration withother European organizations in chemistry in order to signi^
ficantly strengthen European chemistry, to provide efficient andeasily available programs for transnational collaboration and topromote young scientists.
6CDNG . Number of proposals and funding rates in the ERAC
Chemistry program calls. [3]Initiative[a]TC 2005
TC 2007
OI 2008
OI 2009
OI 2010Involved
funding
partners

10
12
7
7
6

Numberof preC
proposals

Number ofinvited full
proposals[c]Numberof fundedprojects(success
rate)

82
36
97
50
71

35
22
41
26
26

9 (26E)
13 (59E)
10 (24E)
8 (31E)
7 (27E)[b][a] TC 2005: Thematic call 2005: “Hierarchically organizedchemical structures. from molecules to hybrid materials”; TC
2007: “Chemical activation of CO2 and CH4” OI: Open Initiative
Apart from ERA^Chemistry, the DFG offers , !6
9 , 4 4$ with corresponding European
partner organizations, such as the French Agence Nationale de laRecherche (ANR), the Dutch Technologiestichting STW, or thetrilateral cross^border D^A^C^H funding program offered by the
research councils of Germany, Austria, and Switzerland.
Proposals in these programs can be submitted anytime within theregular program for national proposals. Provision of a complete
statistical overview bi^/trilateral programs funded to date isbeyond the scope of this article, therefore only some examples
are presented: Under the D^A^C^H umbrella 11 bi^/trilateralprojects have jointly been funded since 2009. With French
partners, almost 30 joint projects have been supported since 2009.
In the same period the DFG financed about 10 grants to initiate
and intensify the bilateral cooperation between research groups ofthese two countries.[3] 'ZCORNGU QH (WPFKPI 2TQITCOU ,QKPVN[ 1TICPK\GFYKVJ 2CTVPGT +PUVKVWVKQPU HTQO 1XGTUGCUOne of DFG’s major funding partners in the “International
Collaboration in Chemistry” (ICC) program is the 8:: 2
 %  &2%(. Since 2005 joint calls forcollaborative research proposals are announced once a year toestablish new bilateral projects between investigators in Germany
and in the United States. Applicants are asked to submit a joint
proposal. The complete peer^review process, including selectionof reviewers and decision^making is jointly carried out by NSF
and DFG. Since 2005, out of 177 proposals 44 have been
granted.[3] Due to the great success of the program, applicants
will also have the opportunity in 2012 to submit joint proposals.
In 2007 an international Committee on Chemistry Research
Funding (CCRF) was established under the umbrella of the ;6 8 , 3  - /$"; &83-/(.
The consortium of research funders aims at a better support topromote chemical basic research collaborations. As aconsequence a first multilateral Call for Proposals in Polymer
Chemistry was launched in late 2009. The goal of this pilot call
was to establish an efficient transnational funding program inchemistry, with a minimum of bureaucracy for the applicants, andto establish best practices for future calls of this type. This call
was supported by the Polymer Division of IUPAC and 7 national
funding organizations. A joint call secretariat at the IUPAC wasestablished, to enable a more efficient administration of the 28
multilateral proposals from a total of 88 applicants. Based on
written peer^reviews an international panel of 12 eminentscientists gave the final funding recommendations. As a result
seven proposals were announced for funding. In four of these a
total of six German research groups are involved. The membersof the CCRF intend to further continue and to intensify theiractivities. Therefore, further international calls in the chemical
sciences can be expected in the coming years.
The DFG has a successful research collaboration with /
which is underlined and supported by the Sino^German Center
for Research Promotion (SGC), jointly founded in 2000 by the
National Natural Science Foundation of China (NSFC) and the
DFG (http://www.sinogermanscience.org.cn/). One example forprojects currently running with China is the CRC/Transregio 61,

A9Articles
where the University of Münster, the Chinese Academy of
Sciences, the National Center for Nano Science  Technology,
and the Tsinghua University in Beijing are involved (Table 1).
Since 2007 almost 15 grants have been funded to initiateinternational cooperation between China and Germany in thefield of chemistry. With <, the DFG partners with the Japan
Society for the Promotion of Science (JSPS) in the field ofchemistry. Joint chemical projects since 2007 include about 10
individual projects, and an International RTG (see Table 1).

 5WOOCT[ CPF 2GTURGEVKXGUThe aim of this article was to give a brief overview onfunding programmes supported by the DFG with a special focus
on options for international activities. As an example, numbers
and statistics on projects funded in the field of chemistry were
illustrated, a discipline where collaborative research is common
practice. The presented data, make no claim to be complete,
nevertheless, they allow some interpretation, such as delineationof trends.
The increase in structured PhD education programs,
established by (International) Research Training Groups and the
Graduate Schools strengthened the internationalization ofGermany’s scientific chemical education. This is underlined by astudy on behalf of the German Chemical Society, which revealed
that during the last ten years the proportion of international
students obtaining a German PhD degree has increased from 5%
in 1998 to 25^30% in 2007^2009.[5] In contrast, the total numberof finished PhDs decreased from about 2200 in 1998 to about
1500 in 2009. In this context, a corresponding number of German
graduates who obtained their PhD abroad would be interesting,
unfortunately such data was not available.
Of the total number of first grant research fellowships fundedin 2010, 13% were in the field of chemistry. This proportion ismuch higher compared to other interdisciplinary andinternationally orientated natural sciences, such as geosciences(2%) or physics (6%)[3] (Figure 1). The proportion of chemical
Emmy Noether grants and Heisenberg fellowships on the total
number of funded first grants in 2010 was in the range of 12%
and 15%, respectively. These funding rates are similar to those of
the geoscientific and physical disciplines.[3] Collaborative
activities between German and international chemical researchgroups are reflected by the great variety of coordinated research
projects (see Table 2). The main collaboration partners fromoverseas are the United States, Japan, or China. In Europe groups
from Great Britain, France, Switzerland, or the Netherlands
mainly cooperate with Germany in chemical research (see also
Section 3).

A10

An intensification of the international collaboration describedis not exclusive for the chemical sciences, but a major futureobjective of the DFG. As stated by its president Matthias Kleiner
the international orientation of DFG activities is “to strengthen
existing international collaborations between researchers,
institutions and funding organizations as well as systematically
identify and tap into new potentials for cooperation”.[6] The long^term goal is to “pave the way to joint research areas”, which is,according to Kleiner, “the highest integration level of scientificcooperation”. Therefore collaboration with other internationallyworking funding agencies, such as the Humboldt Foundation, or
the DAAD will be expanded. “International cooperations shall be
particularly intensified with scientifically dynamic countries andregions. Of great interest for science in Germany are, therefore,
the countries of the European Union, especially Austria,
Switzerland, France, Great Britain, the Netherlands, and Poland,as are the USA and Canada, Israel and Japan, as well as Brazil,
China, India, and Russia”.[6]Sibylle Grandel works at the DFG in the
BMBFHfunded project NInternational
Research MarketingR and is associated to
the group NChemistry and Process
EngineeringR.

EHMail: sibylle.grandel@dfg.de

Acknowledgements:
I thank all my colleagues from the Chemistry and Process
Engineering group for their fruitful contributions and proof reading,in particular Markus Behnke, Bernd Giernoth, [ohanna KowolH
Santen and Kathrin Winkler.

[1]

DFG, Förder^Ranking 2009, Institutionen – Regionen – Netzwerke,
WILEY^VCH, Weinheim, ISBN 978^3^527^32746^1, 2009, p. 16.

[2]

DFG, Jahresbericht 2010, Aufgaben und Ergebnisse, 2011, p. 164(www.dfg.de).

[3]

DFG, Internal Statistical Data and Management Reports, status
July 2011.

[4]

http://www.dfg.de/foerderung/programme/exzellenzinitiative/
allgemeineinformationen/index.html[5]GDCh, Chemiestudiengänge in Deutschland – Statistische Daten
2009, 2010, p.6.

[6]

DFG press releases, No. 34 (13.7.2011). Working together for
Research –Transparent – Internationa. Annual Press Conference inBerlin, 2011.

A New Molecular Architecture for Molecular Electronics
Anna Cattani-Scholz, Kung-Ching Liao, Achyut Bora, Anschuma Pathak, M.
Krautloher, B. Nickel, Jeffrey Schwartz, Marc Tornow, Gerhard Abstreiter

1. Introduction
Our goal is to integrate fundamental surface science andinterface synthesis chemistry with novel device architecture thatis underpinned by an analysis of device physics to build nextgeneration molecular scale transistors with application to
microelectronic devices. The promise of molecular electronics
has been based on organic thin films in devices devised andstudied so far. Organic thin film transistors (OTFTs) mostcommonly use pentacene as the semiconductor in these devices;pentacene-based OTFTs have carrier mobilities in the range of 1
cm2/Vs and on-off ratios between 106 and 108 [1] which may rival
the performance of amorphous silicon transistors. There are,however, major drawbacks of pentacene-based OTFTs, including
processing issues and grain boundary limitations[2] poor subthreshold performance and large positive threshold voltages. In
order to truly realize the promise of molecular electronics, where
active devices are of the scale of molecules, new device
architectures must be devised where molecular control of thestructure is key. We propose to create new moleculararchitectures based on organized growth of surface-attachedorganic semiconductors on nanoscale devices through directed
regular stacking of bifunctional oligoarenes and organometallic
complex linkers. Our new architecture eliminates the need forsuperdeposited pentacene or other organic semiconductors bysynthesizing ordered, truly molecular devices based on new
concepts of device physics that employ our unique ways to build
ordered arrays of aromatics.
Our efforts are based on the application oforganophosphonate chemistry as an alternative to silanization forcovalent surface modification, specifically of silicon
semiconductor based devices.[3,4] In particular, we have been able
to prepare anthracene derivatives and analogs and to convert
them to phosphonic acids by standard routes (Figure 1). We havedemonstrated the growth of high quality monolayers of organics
that are capped on each end by a phosphonate group,
accomplished by the T-BAG method (tethering by aggregationand growth) using solvent mixtures.[5] Moreover we have learned
how to generate three-dimensional, organized multilayers using
techniques of coordination chemistry (Figure 2).
In this contribution we report on our preliminary
investigation about the thin film properties of these novel organo-phosphonate SAMs on Si substrates using several analysis
techniques, including AFM and X-ray reflectivity. In view ofelectrical passivation and transport properties we finally discuss
first impedance spectroscopy characterization in electrolyte
solution and electrical characterization in a vertical setup using a
Hg top electrode.

Figure 1. Chemical structure of 11-hydroxyundecylphosphonicacid (1) and of the anthracene derivatives 2,6-diphosphonoanthracene (2), 9,10-diphenyl-2,6-diphosphonoanthracene (3),10,10'-diphosphono-9,9'-bianthracene (4), and 9,10-dinaphthyl-2,6diphosphono-anthracene (5).

2. Preparation of Organophosphonate ThreeDimensional, Organized Multilayers on
Silicon Oxide
This procedure involved the synthesis of two difunctional
phosphonic acids and the formation of a monolayer SAMP ofeach on a silicon oxide/silicon wafer. This was accomplished by asimple solvent draw-down process called the T-BAG method.[5]After the SAMP was formed it was exposed briefly to vapor oftitanium tetra(tert-butoxide), which gives a titanium di(tertbutoxide)-phosphonate complex on the distal surface of the
SAMP. Because of the residual reactivity of the remaining tertbutoxide groups in the coordination sphere of the Ti, thiscomplex can serve as a linker to join together two equivalents ofa phosphonic acid, and because the coordination about the Ti
gives rise to a net linear arrangement of these two equivalents ofphosphonate, order in the monolayer SAMP is translated to the

A11Articles
second layer, giving a structurally ordered, three-dimensional
duplex (Figure 2). Multi-monolayer structures were preparedusing 2,6-diphosphonoanthracene (2) and 9,10-diphenyl-2,6diphosphonoanthracene (3).

7.69-7.61 (m, 8H), 7.48 (d, 3J (H,H) = 7.60 Hz, 4H) ppm; 13C
NMR (CD3OD, 125 MHz, 298K): δ = 140.4, 139.1, 132.9, 132.8,
132.3, 132.1, 131.5, 131.1, 131.0, 130.0, 130.0, 129.3, 128.5,
128.4, 126.4, 126.3 ppm; 31P NMR (CD3OD, 202 MHz, 298K):
δ = 15.42 ppm; HRMS (ESI-TOF) for C26H21O6P2: Calc’d491.0808 (M+H)+; found m/z 491.0806. Diphosphonic acids 2, 4,
and 5 were prepared similarly.

2.1. Preparation of the SAMP Duplexes of 3 on Silicon
Oxide [8]Figure 2. Synthesis of the duplex of 9,10-diphenyl-2,6-diphosphonoanthracene (3).

2,6-Bis(diethylphosphono)-9,10-diphenylanthracene (6). 2,6Dibromo-9,10-diphenylanthracene (7) (2 mmol) was dissolved in
flash distilled THF (70mL) under argon at -78 °C. tertButyllithium (Aldrich, 1.7 M, 2.5 mL, 4.25 mmol, 2.1 equiv.)
was slowly injected through a rubber stopper using a glass
syringe, and the resulting orange solution was allowed to stir for
30 minutes at -78 °C and then 1 hour at -20 °C. The solution wascooled back to -78 °C, and diethyl chlorophosphonate (Aldrich,
97%, 0.64 mL, 4.4 mmol, 2.2 equiv.) was added. The reaction
mixture was allowed to stir first for 3 hr at -78 °C and then for 12
hr at room temperature. The resulting yellow mixture wasconcentrated under reduced pressure, and the recovered solid was
purified by chromatography on a silica column (20% ethyl
acetate/hexane eluent) affording a yellow product (Yield 94%).
1
H NMR (CDCl3, 500 MHz, 298K): δ = 8.29 (d, 3J (H,P) = 17.00
Hz, 2H), 7.80 (d, 3J (H,H) = 5.15 Hz, 2H), 7.65-7.55 (m, 8H),
7.45 (d, 3J (H,H) = 6.35 Hz, 4H), 4.10-4.04 (m, 8H), 1.24 (dd, 3J(H,H) = 6.65 Hz, 3J (H,H) = 6.55 Hz, 12H) ppm; 13C NMR(CDCl3, 125 MHz, 298K): δ = 139.3, 137.4, 133.5, 133.4, 131.1,
131.0, 130.0, 129.9, 128.8, 128.2, 128.1,128.0, 126.6, 125.3,
125.2, 125.1, 62.3, 62.3, 16.3, 16.3 ppm; 31P NMR (CDCl3, 202
MHz, 298K): δ = 18.48 ppm; HRMS (ESI-TOF) for
C34H37O6P2: Calc’d 603.2060 (M+H)+; found m/z 603.2064.
9,10-Diphenyl-2,6-diphosphonoanthracene (3). Bromotrimethylsilane (Aldrich, 97%, 1.2 ml, 6 mmol, 6 equiv.) was added
to a solution of 2,6-bis(diethylphosphono)-9,10-diphenylanthracene (1 mmol) suspended in 25 mL of anhydrous methylene
chloride under argon. The reaction mixture was stirred overnight,
then anhydrous methanol (1.2 mL) was added, and the mixture
was stirred for an additional 6 hours. The solvent was removed
under reduced pressure, and the crude product was dissolved inmethanol (10 mL). The solution was filtered through a filter aid
packed on a fritted filter disk. The filtrate was concentrated under
reduced pressure, and the recovered solid was recrystallized fromethanol, affording the bright yellow product (Yield 98%). 1H
NMR (CD3OD, 500 MHz, 298K): δ = 8.31 (d, 3J (H,P) = 16.90
Hz, 2H), 7.76 (dd, 3J (H,H) = 9.00 Hz, 3J (H,H) = 3.65 Hz, 2H),

A12

SAMPs of 3 were grown from 5 M solutions in THF. Direct
quartz crystal microgravimetry showed the molecular loading tobe 0.24 nmol/cm2. The SAMP-coated substrate (S3) was thenplaced in a deposition chamber that was equipped with two
stopcocks for exposure either to vacuum or to vapor of titanium
tetra(tert-butoxide) (Strem). The chamber was evacuated to 10-3
torr for 15 min. Samples were then exposed to vapor of Ti(OtBu)4
at 10-3 torr for 12 minutes with active evacuation. This procedure
was repeated in duplicate. Samples were then evacuated at 10-3
torr for 15 min. to allow for complete reaction to adduct S6,
which was also measured by QCM to have an areal density 0.24
nmol/cm2. Adduct S6 was then dipped into a 10 M solution of 3
in methanol for 2 hr to give duplex S7, which was then washedwith methanol under sonication and measured by QCM for areal
density (0.22 nmol/cm2). Similar procedures were used to makeduplex structures of 2.

3. Characterization
Organophosphonate monolayers have been extensively
characterized by a variety of techniques, such as contact angle
measurements, X-ray reflectivity, ellipsometry and X-ray
photoelectron spectroscopy (XPS).[4,9] However, no extensive
studies on the electrochemical properties of the organophosphonate monolayers, in terms of resistive and capacitive
behaviors in electrolytes, have been carried out so far. In thissense we have investigated the electrochemical properties ofmonolayers of 11-hydroxyundecylphosphonic acid (1) and of theanthracene derivatives 2, 3, and 4 on silicon/silicon oxide
electrodes by using impedance spectroscopy. The resistive andcapacitive behaviors have been quantitatively measured inaqueous electrolytes, near neutral pH. Complementary surface
analysis by AFM and XRR is presented as supporting
characterization, demonstrating the quality of the investigatedmolecular layer systems.

3.1. Thin Film Properties
Fig. 3 shows representative topography images of themolecular layer surfaces of self-assembled monolayers (SAMs)
of 11-hydroxyundecylphosphonic acid (1) and 9,10-diphenyl-2,6diphosphonoanthracene (3) acid precursors on p-doped (boron,
1018 cm-3) silicon surfaces coated with a thin (~1 nm) silicon

oxide layer. Contact angle, AFM nanolithography and ellipsometry evidenced the homogeneity of the formed monolayers, andtheir thickness was determined to be 1.13+/-0.09 nm and 0.82+/0.07 nm, respectively. The surfaces were overall homogeneousand free of pin holes. Locally, spots of larger thickness could beobserved that may be assigned to residual multilayers. The
surface roughness of the samples was measured for a scan area 5
μm x 5 μm and compared to that of the bare substrate. The root
mean square (rms) values of roughness were found to be 0.21 nm,
and 0.23 nm respectively, which was very close to 0.19 nm forthe bare substrate. Complementary x-ray reflectivity
measurements have been performed to determine the thickness
and the scattering length density of the SAM layers of 2, 3, 4, and
5 (Table 1).

estimated the minimum volume of a rectangular box which
includes the molecule (Table 1). The experimental scattering
length densities SLDexp match closely with the density obtained
from the box model SLDbox. For the thickness, we obtain 0.70 nm,
0.80 nm, 0.81 nm, and 0.91 nm, respectively; this is about half ofthe full length of the molecules. This excludes an uprightorientation of the molecules. Improvement of the coverage, i.e.
initial grafting density, might induce a more upright orientation of
the molecules. Interestingly, x-ray studies on 2,6diphosphonoanthracene (2) bilayers[11] show that the addition of
the second layer results in a rather vertical orientation of bothlayers presumably for steric reasons.

3.2. Electrochemical Impedance Spectroscopy

Figure 3. AFM topography of self-assembled monolayers of (a) 11hydroxyundecylphosphonic acid (1) and (b) 9,10-diphenyl-2,6diphosphonoanthracene (3). Image size is 5 μm ×5 μm.

Monolayerof 2Monolayerof 3

Monolayerof 4Monolayerof 5

molecular
volume
3
[Å ]

255

400

409

481

box
volume
3
[Å ]

481

1294

853

1861

SLDbox-6 -2[10 Å ]

9.6

5.5

8.8

4.7

SLDexp-6 -2[10 Å ]

8.0

4.2

7.8

5.1

thickness
[Å]

7.0

8.0

8.1

9.1

Table 1. Calculated molecular volumes and experimental
scattering length densities for monolayers of 2,6-diphosphonoanthracene (2), 9,10-diphenyl-2,6-diphosphonoanthracene (3),10,10'-diphosphono-9,9'-bianthracene (4), and 9,10-dinaphthyl-2,6diphosphono-anthracene (5), as obtained from X-ray reflectivity
measurements.

To judge the packing efficiency, we have calculated themolecular volumes of the molecules using first the VABC
method[10] and second a van der Waals box model, i.e., we have

Fig. 4 displays a representative impedance measurement of a
Si/SiO2 substrate with and without 11-hydroxyundecylphosphonic acid (1) monolayer at Vbias = 0V. The recorded Bode
spectra are analyzed according to the basic equivalent circuit
models shown in Fig. 5. The data for the unfunctionalized sample
are consistent with a simple series of one resistor and one
capacitor (α). Herein, the lead resistances of the bulk semiconductor and the electrolyte, as well as the capacitances of thesemiconductor space charge region, the oxide layer and theHelmholtz layer are merged. While this circuit clearlyoversimplifies the realistic situation we find that the obtained
numerical values, in particular in comparison to the SAMfunctionalized case, can describe the system well and do capture
the main changes after coating. The oxide thickness derived fromthe oxide capacitance can be estimated to d=1.68 nm, in very
good agreement to the thickness determined by ellipsometry dopt=(1.62 +/- 0.06) nm.

Figure 4. Bode plot of the impedance (absolute value |Z| andphase) as function of frequency, for a Si/SiO2 sample with (red)and without (black) a 11-hydroxyundecyl phosphonate monolayer
coating. All shown symbols/lines are experimental data.

A13

Articles
After functionalization of the silicon/silicon oxide electrodes
with 11-hydroxyundecylphosphonic acid (1) both impedance
curves shift to higher frequencies as expected for an additional
capacitance CSAM, in series, lowering the total capacitance andthereby increasing the cut-off frequency. To model this behavior
we introduced an additional RC element to the equivalent circuitin Fig. 5 (β) and obtained CSAM=2.64 μF/cm2.

Figure 5. Equivalent circuits used to model the impedance data,
Rlead: lead resistance of bulk semiconductor and electrolyte.
CSC/SiO2/DL: total series capacitance of semiconductor space charge
layer (SC), oxide layer (SiO2) and Helmholtz double layer (DL).
RSAM and CSAM are the resistance and capacitance of the selfassembled monolayer, respectively.

Herein, we kept CSC/SiO2/DL fixed during the fitting procedure.
The result is in good agreement with previously determinedcapacitance values for alkylphosphonates on titanium oxide andalumina.[12] The thickness of the monolayer calculated with adielectric constant ε = 2.7 is obtained as dSAM = 0.9 nm, asomewhat lower value than the one measured by ellipsometry and
AFM scratching (1.1 nm).
Significantly higher capacitances around 7 to 10 μF/cm2 were
found for anthracene biphosphonic acid based films at 0V bias,
with parallel resistances of the same order of magnitude as for thealiphatic SAM (data not shown). Further measurements are
currently under investigation, in order to better evaluate thecontribution of structural defects (pinholes), conformational
rearrangements (stacking, tilting), and of electron delocalization
effects (polarizability, dipole formation) to the overall capacitive
properties of this new class of aromatic hybrid systems, inparticular addressing the controlled growth of novel bilayer
stacks by using metal ion coordination centers.[11]4. Electrical Transport Characterization
The electrical transport through SAM molecules deposited on
Si/SiO2 surfaces was investigated by measuring the currentvoltage (I-V) characteristics using a two terminal configuration,
where a degenerately doped p-Si substrate was used as one
electrode and a hanging Hg drop was used as another electrode(Fig. 6-a). The usage of a Hg drop as one electrode facilitatesformation of a quick and reliable contact on the SAM withreproducible contact area and negligible risk of metal penetration

A14

through the SAM due to the high surface tension of Hg.[13] We
have studied aliphatic as well as aromatic SAMs having identicalend groups.

Figure 6. Schematic of (a) electrical characterization using ahanging Hg drop as top electrode and (b) respective tunnel barriermodel under an applied bias V (not to scale). Note that the Si
chemical potential is located within the valence band due todegenerate doping.

Figure 7 shows current density vs. voltage curves for 9,10diphenyl-2,6-diphosphonoanthracene (3) SAMs in comparison toa model aliphatic compound 1,4-diphosphonobutane (purchasedfrom Acros Organics). Each experimental data shown here wereobtained by averaging over around 30 J-V plots measured at
different positions of a given sample. Since the total thickness of
the SAM and the SiO2 was about 2nm, tunnelling of charge
carriers could be a prevailing transport mechanism. Hence, we
modelled our system within the framework of non-resonant
tunnelling through a trapezoidal barrier consisting of the SAM
and the SiO2 (Fig. 6-b), fitting the measured J-V data to
Simmons’ equation[14]

J

4d
4d
 V
 V
e 
V  h 2 .me   2
V  h 2 .me   2 




e

e



2hd 2 
22

where d= effective barrier width, Ф = barrier height (Ф > V )and α is a correction factor to account for a non-ideal barrier
shape and effective electron mass, taking Ф and α as fitting
parameters. The measured J-V data for both SAMs can be
excellently described within this model. We observed differencesof barrier heights depending upon the bias polarity, which is
expected for dissimilar electrode materials. The barrier heights
for the 1,4-diphosphonobutane acid SAM were found to be muchhigher (3.63 eV and 3.18 eV at positive and negative biases to Si,
respectively) than those for the 9,10-diphenyl-2,6-diphosphonoanthracene (3) SAM (0.95 eV and 0.69 eV). This observation can

Figure 7. Absolute value of the current density as a function of substrate bias for 1,4-diphosphonobutane (top curve) and 9,10-diphenyl20-32,6-diphosphonoanthracene (3) (bottom curve) SAMs on highly p-doped Si (10 cm ). Open symbols are measured data and lines aremodel fits, with several of the fitting parameters displayed.

be explained by the fact that the height of the barrier would
depend on the molecular orbital level energies of the SAM
molecules. As supported by DFT calculation the aliphatic SAM
precursors have a much larger energy gap between their highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO), compared to the aromatic one
comprising a conjugated electron system. Details of this analysis
will be published separately.
Summary
Understanding the electronic transport through layered systems
comprising organic functional layers in direct contact tosemiconductor surfaces is of major importance for futureapplications in nanoelectronics, photovoltaics and sensors. We
report on the successful deposition of self-assembled monolayers(SAMs) of alkyl and aryl phosphonates onto silicon/silicon oxide
substrates. These SAMs further served as a basis for the
preparation of novel three-dimensional, organized bilayers ofanthracene diphophonates, using techniques of coordination
chemistry under controlled conditions by deposition oforganometallic linkers onto the monolayers. We havecharacterized the mono- and bilayer systems on planar surfacesby AFM and XRR measurements. Furthermore theelectrochemical properties of the organophosphonate monolayers,
in terms of resistive and capacitive behaviors, were quantitatively
analyzed by impedance spectroscopy. Finally we discuss
preliminary electrical characterization data of aliphatic as well
as aromatic organophosphonate systems in vertical transport
using a metal top electrode.

Zusammenfassung
Für zukünftige Anwendungen in der Nanoelektronik, Photovoltaik
und Sensorik ist ein grundlegendes Verständnis deselektronischen Transports durch Strukturen aus organischen
Funktionsschichten in direkter Verbindung mit
Halbleiteroberflächen von herausragender Bedeutung. Wir
berichten über die erfolgreiche Deposition selbst-assemblierter
Monolagen („self-assembled monolayers“, SAMs) aus Alkyl- undArylphosphonaten auf Si/SiO2-Substraten. Diese SAMs dienen
uns u.a. als Grundlage für die Präparation neuartiger, dreidimensionaler, geordneter Doppelschichten aus AntracenDiphosphonaten. Hierfür kommen Techniken derKoordinationschemie unter kontrollierten Bedingungen zum
Einsatz, bei welchen vor der zweiten SAM-Beschichtungorganometallische Linker auf die Monolagen deponiert werden.
Wir haben die Mono- und Doppellagensysteme auf planaren
Oberflächen mit Hilfe von AFM und XRR-Messungencharakterisiert. Weiterhin konnten wir quantitative Analysen vonelektrochemischen Impedanzspektroskopie-Messungendurchführen und so die Organophosphonat-Monolagen in Bezug
auf ihre resistiven und kapazitiven Eigenschaften
charakterisieren. Schließlich diskutieren wir erste elektrische
Charakterisierungen von aliphatischen und aromatischen
Organophosphonatsystemen in einem vertikalen
Transportexperiment, bei welchem die Monolagen mit eineroberen Metallelektrode kontaktiert wurden.

A15

Articles
Addresses:
1)*

Jeffrey Schwartz is Professor at the
Department of Chemistry at Princeton
University.

1)

Department of Chemistry, Princeton University, Princeton, NJ
08544, USA
2)

Institut für Halbleitertechnik, Technische Universität
Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig,
Germany

Email: jschwart@princeton.edu

3)

Zentrum für Nanotechnologie und Nanomatrialien, Walter
Schottky Institut, Technische Universität München, Am
Coulombwall 4a, 85748 Garching, Germany
4)

Department of Physics & CeNS, Ludwig-Maximilian Universität
München, München, Germany

2)

Marc Tornow
is Professor at theInstitut für Halbleitertechnik at theTechnische Universität Braunschweig.
Email: m.tornow@tu-bs.de3)*

Anna Cattani-Scholz is research
assistant at the Zentrum fürNanotechnologie und Nanomaterialien at
the Walter Schottky Institute of theTechnical University München.
Email: cattani@wsi.tum.de

Project description & Acknowledgement
In order to truly realize the promise of molecular electronics,
where active devices are of the scale of molecules, new device
architectures must be devised where molecular control of thestructure is key. The work presented here builds on our ability tosynthesize and characterize self-assembled monolayers andmultilayers of active molecules by incorporating them into new
device architectures. Our efforts are based on the application oforganophosphonate chemistry as an alternative to silanization forcovalent surface modification, specifically of silicon semiconductors based devices.
The authors gratefully acknowledge funding by the DFG (grants
AB 35/8-1, TO 266/2-1), by the NSF (CHE-0924104), by the
Braunschweig International School of Metrology IGSM, and by the
Nanosystems Initiative Munich.

* corresponding authors
1)

Working departments of coauthors: K.-C. Liao , A. Bora
2)
4)
3)
Pathak , M. Krautloher and B. Nickel , G. Abstreiter

[1]
[2]
[3]
[4]

[5]
[6]
[7]

A16

2)

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D. J. Gundlach, Y. Y. Lin, T. N. Jackson, S. F. Nelson, D. G.
Schlom, IEEE Electron Device Lett. 1997, 18, 87-89.
M. Shtein, J. Mapel, J. B. Benziger, S. R. Forrest, Appl. Phys. Lett.
2002, 81, 268-270.
A. Cattani-Scholz, D. Pedone, F. Blobner, J. Schwartz, M. Tornow,
L. Andruzzi, Biomacromolecules 2009, 10, 489–496.
A. Cattani-Scholz, D. Pedone, M. Dubey, S. Neppl, B. Nickel, P.
Feulner, J. Schwartz, G. Abstreiter, M. Tornow, ACS Nano 2008,
2, 1653-1660.
E. L. Hanson, J. Schwartz, B. Nickel, N. Koch, M. F. Danisman, J.
Am.Chem. Soc. 2003, 125, 16074-16080.
K.-C. Liao, A. G. Ismail, L. Kreplak, J. Schwartz, I. G. Hill, Adv.
Mater. 2010, 22, 3081-3085.
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73-74.

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2010, 26, 14747-14754.
Y. H. Zhao, M. H. Abraham, A. M. Zissimos, J. Org. Chem. 2003,
68, 7368-7373.
A. Cattani-Scholz, K-C. Liao, A. Bora, C. Hundschell, A. Pathak,B. Nickel, J. Schwartz, G. Abstreiter, M. Tornow,“Electrochemical Characterization of Self-Assembled
Organophosphonate Monolayers and their Three-Dimensional
Organized Bilayers on Silicon/Silicon Oxide Electrodes”, 2011, in
preparation.
H. Klauk, U. Zschieschang, J. Pflaum, M. Halik, Nature 2007, 445,745-748.
H. Haick, D. Cahen, Acc. Chem. Res. 2008, 41, 359-366.
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Graphene

Chemical Methods for the Generation of Graphenes andGraphene Nanoribbons
Jan M. Englert, Andreas Hirsch, Xinliang Feng, Klaus Müllen

Synthetic carbon allotrope chemistry is currently among the most rapidly growing
topics in materials chemistry. The youngest and at the same time probably the mostpromising representative of new carbon allotropes is graphene. In this article weoutline our recent contributions to chemical graphene formation andfunctionalization.
1. Introduction
In recent years graphene being the youngest representative of
the growing family of synthetic carbon allotropes has attracted
overwhelming attention due to outstanding and unprecedented
physical and materials properties, such as novel magnetotransport and high charge carrier mobility in the ballistic regime
at temperatures up to 300 K. Structurally, these single sheets ofgraphite can be considered as a mother of all expanded aromatic
carbon modifications and they have been considered for a very
long time to be an exclusively theoretical material. The firstpreparation of single graphene layers succeeded in 2004 via astraightforward mechanical exfoliation of graphite using a scotch
tape. This method, however, is not suitable for mass production,
which on the other hand is required when targeting large scale
practical applications of graphene. In principle, there are two
major strategies to prepare graphene or graphene derived
nanoforms, namely, the top-down exfoliation of graphite and denovo bottom up approaches using suitable small precursor
molecules that can be condensed to extended two-dimensional
conjugated π-systems. We report here on our recent
investigations in both directions, a) the wet-chemicalfunctionalization and exfoliation of graphite leading to free aswell as covalently and non-covalently functionalized grapheneand b) the cyclodehydrogentation of oligophenyl precursors as ahighly versatile bottom-up strategy for the formation of
atomically well defined graphene nanoribbons.

2. Exfoliation of Graphite
In order to generate individualized graphene sheets out ofgraphite the most obvious obstacle is to overcome the strong π/πinteraction adhering them in graphite. On the other hand,graphenecan undergo also pronounced interactions with
substrates, which initially enabled generation of single layer
graphene (SLG) from highly ordered pyrolytic graphite (HOPG)
by sticky tape exfoliation in 2004.[1] With suitable postprocessing the extrinsic corrugation of the substrate can befurther utilized to improve the single layer yield of mechanicalexfoliation on very large areas.[2]Figure 1. Wet chemical top down approaches for the generation ofgraphene sheets out of graphite. Non-covalently functionalizedgraphene mediated by surfactants such as amphiphilic
perylenediimides or SDBS (left) and covalently functionalizedgraphene generated, for examples, by reactions with nitrenes ordiazonium salts (right).

The cohesive energy gained by stacking graphene sheets onto
each other was experimentally determined to be as high as 61
meV/C-atom.[3] Similar challenges had to and still have to be met
for debundling and “unmeshing” carbon nanotubes, which share
the hexagonal sp2-carbon network with graphene but present a
convex outer surface. This significantly reduces the areal
interaction and facilitates exfoliation.. A promising approach to
the target of exfoliation was to search for solvents balancing thecohesive energy term, which could enable the generation of metastable dispersions of graphene, CNTs and other carbon based
nanostructures by application of ultrasonication.[4] In the case of
CNTs these considerations led to the identification of a series oflactam- or lactone based solvents providing appreciable
concentrations of dissolved carbon nanotubes.[5] Consequently, in

A17Articles
2008, this concept has been applied microcrystalline graphite,
where most effective solubilization after sonication andcentrifugation was found for those liquids that match a surface
tension of 40-50 mJ m-2. Among the most prominent
representatives of these solvents are N-methylpyrrolidone (NMP),
N-cyclohexylpyrorlidone (CHP), benzylbenzoate (BBZ), γbutyrolacton (GBL). These effectively reduce the energetic
penalty connected with dissolution of the graphite by balancing
the graphene/graphene interactions.[6]Encouraged by these findings we and other groups went
forward aiming for advanced noncovalent stabilization
techniques.[7] Next to the direct ultrasound aided solvent
exfoliation of graphite an alternative is taking advantage of thehydrophobic effect using suitable surfactants. While doing so theentropy penalty for the solvent, in particular water is kept low dueto the formation of surfactant cages around the material to bedispersed. Obviously, in order to fully exploit the hydrophobic
effect the use of very polar media, at best water, is mandatory in
the first place. Graphite does not constructively interact with thewater phase unless it is subjected to ultrasonication in thepresence of dissolved detergents. While the use of commerciallyavailable detergents like SDBS, SDS or bile salts enable e.g.
ultracentrifugation based purification protocols[8] and large scaleliquid phase production of graphene, specialized π-detergents
permit solution bound spectroscopies. Our previous
investigations on carbon nanotube dispersions suggested the useof electron deficient amphiphilic or bolaamphiphilic
perylenediimide (PDI) surfactants (Scheme 1),[9-11] which canundergo pronounced π-π stacking interactions with carbon
nanotubes in water.[12]
CO2 R

RO 2C

CO 2R

O
RO2 C

N
H

RO2 C
RO2 C

CO2 R
RO 2 C

O

O

N

N

O

O

O

NH

CO 2R

CO2 R

RO2 C
1: R = H
2: R = t Bu

CO2 R

CO2 R

O
CO 2R

CO2 R

CO 2R

CO2 R

Scheme 1. Bolaamphephilic perylenediimide (PDI) equipped withnd
two 2 generation Newkome dedrimers. The watersoluble moiety
1 is obtained after quantitative acidic deprotection of 2.

The features of these amphiphiles, namely dendrimer
mediated solubility in water, characteristic absorption andemission properties in the visible light region together with highfluorescence quantum yields paved the way to an in depth
understanding of carbon nanotube dispersion, individualizationand doping.[13] We expected that our experience with carbon
nanotube dispersion studies will provide insight into solution
phase graphite exfoliation. Indeed, we could show that
dendronized amphiphilic perylenes such as 1 (Scheme 1) are able
to exfoliate graphite in water.[9] The characteristic spectroscopic
properties of PDIs such as 1 allowed for the characterization andquantification of the perylene/graphene interaction. In particularpronounced π-π-stacking interactions were observed as

A18

2.1. Surfactant Aided Exfoliation

O
HN

HN
CO 2R

N
H

H
N

O

NH
O

Although graphite is chemically rather inert it has been
known for a long time that it can undergo a series of reactions
providing graphite intercalation compounds (GICs). GICs may beconsidered salts of the amphoteric carbon host classified as donoror acceptor compounds depending on the electronic nature of theguest which is to be intercalated. Intercalation usually occurs in a
stoichiometric fashion forming ordered layers of the guest locatedparallel to the graphene sheets, which in turn increases the
carbon/carbon interlayer distance and thus, depending on thestage of the GICs, virtually reduces or completely removes the
π/π interaction in the crystal. On the other hand, ionic or dipolarinteractions are introduced at the same time stemming, forexample, from electron transfer from the guest to the carbon hostor vice versa. Acceptor GICs of strong oxidizing acids likesulfuric acid or ferric chloride are known to form after chargetransfer from the sheets to the guest.[14] The opposite situation,
namely an electron transfer onto the graphene sheet, was foundwhen for the first time potassium was intercalated into graphite
crystals to form a first stage donor-GIC.[15] Evidence fordissolved negatively charged graphene generated from thepotassium GIC KC8 was found after the treatment with polar
aprotic solvents under inert atmosphere.[16] According to thisapproach we came up with the concept of a balanced combinationof intercalation combined with coulomb driven exfoliation andrepulsion. This opens the door for wet chemical functionalizationof the electronically activated graphene sheets via recombination
with reactive addends. In the following two sections these newdevelopments in non-covalent and covalent graphenefunctionalization will be outlined.

O
H
N

O

demonstrated by efficient emission quenching - even upon
resonant laser excitation of the PDI.[10]The surfactant based exfoliation in biocompatible nontoxic
and green solvents like water is one of the most promising routes
to large scale production of cost efficient graphene dispersions.
However, it remains challenging to carry out a reliable bulk
analysis of the degree of exfoliation and of the determination of
the number of layers in the resulting dispersed particles. As aconsequence, a model system that reflects the dispersion
properties of graphene without sacrificing spectroscopic insight
was required. Expanded polyaromatic molecules like hexa-peribenzocoronenes (HBC) can offer monodispersity with precisely
defined absorption and emission features while resembling manyof the aggregation phenomena of graphite. Thus it can be
anticipated that detergents and dispersion protocols which yield ahigh amount of HBC monomer in aqueous solution also would
result in appreciable yields of graphene monolayers. First
investigations based on commercially available surfactants suchas sodium dodecylbenzenesulfonate (SDBS), sodium cholate(SC) and sodium deoxycholate (SDC) (Scheme 2) were carriedout in our laboratories and revealed significant differences inmaterial uptake and individualization efficiencies.

SO3Naand alcohol groups including defects (holes) in the sheets.[18]These randomly formed functionalities are the reason for the easy
sodium dodecylbenzenesulfonate (SDBS)
O
O
dissolution of graphite oxide into single layer graphene oxide in
OH
OH
ONa
ONa water or solvents of comparable polarity.[19] These functional
H
H
groups greatly disturb the aromatic hexagonal carbon network ofH
H
H
H
the graphene basal planes. Attempts to regenerate the initial order
HOOH
HO
Hexa-per i-benzocoronene (HBC)H
H
by various reduction agents like e.g. hydrazine treatment[20] or
sodium cholate (SC)sodium deoxycholate (SDC)
physically via thermal annealing[21] so far failed. Anyway, thecarboxylic acid groups attached to the carbon framework
Scheme 2. Structural representation of hexa-peri-benzocoronene
resemble very useful and versatile anchors for further(HBC), sodium dodecylbenzenesulfonate (SDBS), sodium cholate(SC) and sodium deoxycholate (SDC)
functionalization by means of organic chemistry. Anotherpossibility for graphite functionalization is the direct treatment
with very “hot” reagents such as hydrogen plasma, fluorine gas or
While the highest overall material uptake could be observed
radicals. Evidence was provided that a large amount of sp3in amphiphilic SDBS dispersions, the use of SDC on the otherdefects was introduced this way, which is indicative of successful
hand led by far to the highest degrees of individualization.
addend binding. In the case of the treatment with atomic
Individualized HBC molecules could easily be identified by theirhydrogen even the perfunctionalization, namely, the formation ofcharacteristic emission pattern and quantified by fluorescencegraphane was discussed.[22] Recently also azomethin ylides[23] orlifetime analysis. In contrast, when graphite was used forbenzynes[24] provided covalently functionalized graphene via
surfactant assisted exfoliation into graphene, solution based bulk
cycloaddition
reactionsandfinallyedge
selective
analytical access is not easily provided with such surfactants due
functionalization have be achieved by Friedl-Crafts like acylation
to the lack of descriptive optical properties either of graphene orroutes.[25]the detergent itself. With the use of chromophors such as
perylenes as part of the surfactant structure these shortcomings
can be overcome. By doing so also enhanced interactions withgraphene layers can be achieved. Besides the ultrasound based
exfoliation of graphite down to high quality single graphene
layers in water, also electronic communication between theperylene-anchor and graphene could be investigated. This ismanifested in quantitative fluorescence quenching in the solid
phase that can be explained in terms of electron or energy transferof the photoexcited Sn>0 state into the graphene conduction band
(photoinduced n-doping of graphene) or by fast filling of theelectron vacancy in the photoexcited PDI S0 level by a graphene
valence band electrons (photoinduced p-doping of graphene).[10]Ground state p-doping of carbon nanotubes like CNTs by PDIs
has been carefully investigated[13] and justifies the assumption oflatter process being the reason for the efficient quenching of the
PDI emission after deposition. The pronounced interaction ofgraphene and the perylene moiety in turn even allowed theacquisition of a resonantly enhanced Raman spectrum of the
PDI/graphene complex which, without the strong interaction,
would just result in superimposition of the weak Raman signal by
the strong emission pattern of the PDI.

2.2. Covalent Bulk Functionalization of Graphene
The procedures to generate solutions or dispersions ofgraphene in water or special solvents discussed so far rest on noncovalent interactions. The oldest known approach to generatesoluble graphite or graphene however is based on oxidative
covalent chemistry (Fig. 2).[17] As early as in 1860 oxidation ofgraphite powder carried out by Brodie allowed to produce
oxidized material which resembles the properties of presently
known graphene oxid (e.g. swelling in water). The watersolubility is mediated by the presence of a myriad of oxygen
containing functional groups like carboxyl-, carbonyl-, epoxide

Figure 2. Covalently functionalized graphene by oxidation ofgraphite leading to graphene oxide (right) or via the recombinationof potassium GICs with acitved addends such as diazonium salts(left).

A19

Articles
One very versatile approach for covalent carbon allotrope
functionalization is aryl diazonium based derivatization. So far
diazonium cations have been used to successfully functionalize
e.g. carbon nanotubes, graphite surfaces or even glassy carbon
materials.[26] The reactants can be activated through thermal
shock, host material electron transfer or electrochemically. We
have recently reported on the first wet chemical graphenefunctionalization by the reaction of aryl diazonium compoundswith in situ activated, exfoliated and reduced graphene.[27]Reduced graphene sheets were obtained from spontaneously
dissolved GICs which have been synthesized in solutions ofliquid sodium/potassium alloys in 1,2-dimethoxyethane (DME)
as inert and electride/alkalide stabilizing solvent. Working in
DME in contrast to the more commonly used liquid ammonia(classical Birch conditions) offers the opportunity of working atroom temperature and also allows using most diazonium
compounds in solutions because DME does not decompose thelabile diazonium moiety. After addition of the diazonium salt tothe dispersion of reduced and exfoliated graphene sheets
functionalization takes place accompanied by fast nitrogen
evolution. The functionalized material can be recovered by meansof spin-casting, filtration or dip-coating for spectroscopy andmicroscopic characterization. High-resolution transmission
electron microscopy (HRTEM) allowed for the observation of
amorphous domains among crystalline areas in the flake’s surface.
After introduction of heteroatom markers into the diazonium salt
addends additional characterization by elemental and chemical
analysis through EDX and XPS was accomplished. The aryl
moiety was found to exhibit typical emission patterns and could
be quantified by thermogravimetry. Annealing of the material at
1000°C under inert gas allowed for the complete regeneration of
the hexagonal carbon network which is in stark contrast tographene oxide materials as up to now no possibility exists toheal hole defects which are generated during the cleavage of theaddends. This complete reversibility is combined with thepossibility of varying both the nature of the addend and thedegree of functionalization to a large extend. Hence, these newgraphene modification protocols open the door to a broad variety
of functional and processable graphenes with tailored properties.

3. Directed Synthesis of Graphene
Nanoribbons
Graphene nanoribbons (GNRs), which are narrow andstraight stripes of graphene, are expected to exhibit quantum
confinement and edge effects which make them attractive for the
fabrication of nanoelectronic devices. In general, all GNRs withwidth smaller than 10 nm show the semiconducting property,
while zigzag-type GNRs are always metallic.[28, 29] Therefore, theband gap of GNRs can be effectively tailored through the controlof their width and edge structure. Due to the localized states atthe edges, zigzag GNRs can allow spin-polarized electrontransport, which makes them interesting for application in
spintronics.[30]A number of top-down methods have been devised tofabricate GNRs. They were initially produced by physical

A20

methods such as high resolution electron beam lithography.[31]Later, chemical means like liquid exfoliation of graphene with theassistance of functional polymers,[32] controlled plasma etching ofgraphene[33] and unzipping of carbon nanotubes[34, 35] in solution
and on surface have been proposed to prepare GNRs with widths
down to 5 -10 nm which is needed to bring about semiconducting
characteristics with high on/off transistor operations. Due to theabundant availability of CNTs, the longitudinal unzipping ofmultiwalled CNTs may offer a feasible production of GNRs on alarge scale. However, all these approaches fail to control the sizeand edge structure and thus only provide poorly defined graphenematerials.
In order to achieve GNRs with defect-free edges and widths
below 5 nm, a bottom-up synthetic strategy starting from small
aromatic building blocks has been established in the past few
years.[36-38] In this approach, the final GNRs are exclusively
determined by the type and shape of precursor monomers
employed. Actually, this strategy has been widely adopted tobuild up extended PAHs of different size, symmetry andperiphery together with various functional substituents.[39,40]
Structurally well-defined PAHs with carbon numbers up to 222
have been synthesized and fully characterized in the past
decade.[41] These graphene-type molecules show tailorable
optoelectronic properties which are dependent upon the size andedge structure. They also display interesting supramolecularbehaviour at different length scales in solution, in bulk and on
surfaces which leads to promising applications in molecularelectronics and organic electronics.[42] Typically, two-step
syntheses are involved to make GNRs. In the first step, themonomers need to be coupled covalently for yielding preorganized polyphenylene precursors that have a (2D) planar
projection without any overlapping of benzene rings. In thesecond step, intramolecular cyclcodehydrogenation associatedwith planarization has to be carried out to furnish the final GNRs.
This synthetic strategy has been firstly developed for solution
synthesis for which the solubility of precursors and compatibility
with the reaction conditions need to be considered. Very recently,
we and co-operators hypothesized that a catalytic surface may
also provide a platform for the GNR and graphene synthesis withatomic precision.[43] In this way, the processing of single GNRs
can be facilitated by performing the reaction on the surface.
Therefore, in the following two sections, we will demonstrate thesynthesis of GNRs in solution and on surfaces, respectively.

3.1. Solution synthesis of GNRs
As we discussed above, the solution synthesis of GNRs will
firstly rely on the build up of oligo/poly-phenylene precursors.
Afterwards,
theScholl-type
intramolecularoxidative
cyclodehydrogation in the presence of Lewis acids and oxidants
is typically employed to produce GNRs. The use of the weaker
Lewis acid FeCl3 renders a sufficient oxidation potential for C-C
bond formation during cyclodehydrogenation.[40] Other oxidant
systems such as AlCl3/CuCl2,[44] PhI(OOCCF3)2/BF3.Et2O[45] andDDQ/H+ [46] can also be used for this purpose. In an early case,
the Diels-Alder reaction was demonstrated for the synthesis ofsoluble branched polyphenylenes with high molecular weight.[36]However, the presence of three structural isomeric units withinprecursor polymers caused by the irregular Diels-Alder
cycloaddition can not allow the generation of final GNRs with thesingly defined conformation. Nevertheless, the FeCl3 mediatedcyclodehydrogenation afforded a black, insoluble powder withgraphite-like appearance. Raman, infrared spectroscopy and highresolution transmission electron microscopy (HR-TEM) further
disclose that the final materials possess graphite-type features.
Later, one-dimensional linear GNRs (6) were obtained byoxidative cyclodehydrogenation of hexaphenylbenzene-type
polymers (5) derived from a sterically hindered Suzuki-Miyaura
coupling between 1,4-diido-2,3,5,6-tetraarylbenzene (3) and thebis-boronic ester of hexaphenylbenzene (4) (Scheme 3).[37] Gelpermeation chromatography (GPC) analysis indicates a numberaverage molecular weight of Mn=1.39*104 g/mol with a low
polydispersity value of PD=1.2, which is further confirmed by
MALDI-TOF mass spectroscopy. After FeCl3 based cyclodehydrogenation, a black solid was produced which dissolves
well in common organic solvents. This fact can be attributed tothe introduction of a large number of branched alkyl chainslocated at the adjacent position of the aromatic periphery, which
are known to reduce the aggregation in solution. High
cyclodehydrogenation efficiency can be confirmed based on themass analysis and UV-vis spectroscopy. The TEM study on asolid film of GNRs reveals a well-ordered stacking of thegraphene layers with a π-stacking distance of 0.34 nm, suggestingthat the GNRs prone to stack together as similar to the behaviourof graphene. After drop-casting the solution of 6 on the HOPG
surface, single objects of GNRs with lengths from 8 to 12 nm canbe clearly visualized by means of scanning tunnelling microscopy(STM). Therefore, this result clearly demonstrates that thebottom-up organic synthesis can serve as a powerful protocol for
fabricating GNRs with well-defined structure.

Scheme 3. Synthesis of linear (6) and kinked (8) graphene
nanoribbons.

It should be however noted that it remains a great challenge
to achieve long GNRs by using the above methods which
generally suffer from the difficulty of obtaining high molecularweight polyphenylene precursors. To overcome this obstacle in
the polymerization step, very recently, we improved the concept
by introducing nonrigid kinked polyphenylene backbones. This
strategy led to a significantly increased solubility ofpolyphenylene systems (Scheme 3).[38] The precursors 7 can beprepared by the microwave assisted Suzuki polycondensation ofortho-dibromobenzenes and benzene-1,4-dibornonic esters. For
polymer 7 with solubilising dodecyl chains, GPC analysis with
polystyrene (PS) standards indicates an average molecular weightof Mn=9900 g/mol with a polydispersity index as low as 1.40.
MALDI-TOF mass suggests that molecular weight up to 20000
g/mol can be detected. Thereby, full dehydrogenation ofpolyphenylenes with a length of more than 40 nm is possible
upon the treatment of FeCl3 mediated Scholl reaction. The
perfectly defined nanoribbons 8 are still soluble in common
organic solvents, which make further structural characterizations
and processing of GNRs from solution possible.

3.1. Surface synthesis of GNRs
Albeit great progress has been achieved in the wet chemical
synthesis of GNRs by means of the bottom-up strategy, several
obstacles may obstruct its general applicability. One particular
problem is the re-aggregation of graphitic structures during thecyclodehydrogenation step which always leads to the difficulty ofregenerating single graphene species. Therefore, further
processing of aggregated GNRs is necessary in order to uncover
their intrinsic electronic and optic properties. Like the solution
exfoliation of graphite and HBC with the assistance of surfactants
which has been discussed in Section 2, the exfoliation of GNRs
by means of covalent and non-covalent methods are being
pursued in our groups. It is also the motif of our joint project thatfully exfoliated single GNR objects can be readily accessed insolution environment or on surfaces. And eventually we expect togain further structural information of GNRs and push forward toelectron transport studies of single GNR.
An alternative approach to the synthesis of GNRs by the“stitching up” of oligophenylene precursors associated withintramolecular cyclodehydrogenation and planarization is
performed directly on a metallic surface that can provide a
catalytic role on stabilizing intermediates and lowing the energy
barrier of these reactions.[43] For this approach, the precursor
monomers should be firstly deposited on the surface withmonolayer coverage via sublimation. Afterwards, these carefully
selected molecular building blocks should react with each othervia two thermal activation steps. During the first thermal
treatment, the dehalogenated intermediates of monomers containenough thermal energy to diffuse along the surface and form
polymer chains through radical addition. In the second heatingprocess which is normally carried out below 500 oC, thethermally mediated intramolecular cyclodehydrogenation willtake place and generate final GNRs on the surface. For instance,
the intermolecular coupling of 10,10’-dibromo-9,9’-bianthryl (9)A21Articles
on a gold surface can be realized through a thermal annealing at
200 oC (Scheme 4).[43] The surface-stabilized biradical species
form single covalent C-C bonds to furnish linear polymer
precursors. The second step of thermal treatment to 400 oC
induces intramolecular cyclodehydrogenation of polymer
precursors, which can be monitored under the STM. Apparently,
as demonstrated in Scheme 4b and 4c, the topology of resulting
GNRs is exclusively governed by the precursor monomer utilized.
STM simulations are perfectly consistent with experimental
images, confirming the reaction products are atomically precise
N=7 GNRs with fully hydrogen-terminated armchair edges(Scheme 4c). High monolayer density coverage of GNRs on gold
surface can be achieved, and no obvious structural defects can beobserved in GNRs, which can not be accessed by the present topdown approaches. A simple “chip-to-chip press” method suggeststhat the GNRs can be readily transferred from gold films onto a
SiO2 substrate.

a)Br

200 °C

400 °C

Br

9

b)

c)

Scheme 4. Surface synthesis of straight graphene nanoribbons(N=7). a) reaction scheme from bianthryl monomer (9) to GNRs. b)ooverview of STM image after cyclodehydrogenation at 400 C. c)
High resolution STM image of the GNRs. (Copyright Nature 2010
publishing group)This renders a possibility to build up electronic devices onsuch GNRs. Raman spectra of these GNRs both on gold and SiO2
surface reveal the specific vibrational modes that arise from theirdefined edge and low dimensionality. In addition, this surfacemediated synthesis approach allows the fabrication of graphenewith much more complex architectures. For instance, a chevrontype GNRs with pure armchair edge structure and three-fold
GNR junction based on two different monomers can be also
efficiently produced.[43] Moreover, extended nanographene[47] andporous graphene[48] nanostructures have been demonstrated bythis synthetic approach starting from defined oligophenylene
precursors. There is no doubt that the surface-mediated bottomup approach will provide a route to GNRs with engineered
chemical and physical properties. It is also our aim within this

A22

project to realize the synthesis of novel graphene architectures
including the theoretically predicted graphene quantum dots,boron/nitrogen-doped graphenes, and GNRs with zigzag edge
peripheries.
Summary
Chemical entries into the constantly growing general field ofgraphene can be categorized in two major classes. On the one
hand there is the category of top-down approaches starting fromeasily available graphite sources following the exfoliation routeand on the other hand there is the bottom up technique offering
atomic precision of produced materials by means of organic
synthesis. The top-down approach - capable to produce large
amounts of graphenes - is further divided into two strategies
resting on different binding interactions. Non-covalentfunctionlization by surfactants and suitable solvents provides
defect free few and single layer graphene in processabledispersions which may be used for all kinds of coating techniquesfrom spray- to simple dip-coating. The exfoliation efficiency of
surfactants can be tailored allowing them to fulfill more thantheir sole dispersion purpose by implementation of electron
donating/accepting units. If covalent functionalization is utilized
while walking down the “top-down track” one usually does not
obtain defect free material as defects in the form of sp3-carbon
centers are introduced into the graphene lattice. However GICs
offer the possibility to minimize possible σ-lattice damage during
reductive activation of the parent graphite stack. The obtained
reductively exfoliated negatively charged graphene sheets can
successively be functionlized by mild organic oxidants leading tocovalent bond formation between the sheets and addend to be
introduced.

When control about local atomic structure is concerned e.g. inorder to define exact optical and electronic properties for narrowgraphene nanoribbons, the bottom-up synthetic strategy has to bechosen. Both solution and surface synthesis have been revealed
for the successful generation of polyphenylene precursors andfollowing graphene nanoribbons upon intramolecular
cyclodehydrogenation process. For both cases, the topology andedge periphery of GNRs are strongly governed by the precursor
monomers utilized. Suzuki-type polymerization enables one tosynthesize polyphenylene precursors in solution. The introductionof a kinked-backbone and attachment of suitable alkyl chains at
specific positions can improve the solubility of final GNRs in
organic solvents which are essential for further solution
processing and structural characterizations. Regarding thesurface-mediated synthesis of GNRs, the precursor monomers
with functional halogen groups must be firstly deposited on thesurface with monolayer coverage, and then a two step thermal
activation process must be performed. While the low temperature
treatment in the first step generates the polyphenylene chains via
radical addition, a higher temperature is required to fully
planarize the precursor by eliminating the hydrogen and affords
atomically precise GNRs on the surface. Naturally, extending thewidth of GNRs by selecting expanded precursor monomers
should pave the way to tailoring and engineering thearchitectures and properties of GNRs. This will finally bridge thebottleneck of current top-down synthesis of GNRs by means oflithography and chemical etching of graphene.

Zusammenfassung
Die chemischen Zugänge zum ständig wachsenden
Forschungsgebiet Graphen können in zwei große Unterkategorien zusammengefasst werden. Einerseits sind die „topdown“-Vorgehensweisen zu nennen, welche, ausgehend vonleicht verfügbarem Graphit, der Exfoliationsmethodik folgen,
während andererseits die „bottom-up“-Ansätze die atomare
Präzision der präparativen organischen Chemie nutzen. Der„top-down“-Ansatz – geeignet zur kostengünstigen Herstellung
von großen Mengen an Graphen – kann weiter in zwei
unterschiedliche Strategien unterteilt werden, welche auf den
jeweils ausgenutzten Wechselwirkungstypen beruhen. Die
nichtkovalente Funktionalisierung durch oberflächenaktive Stoffeoder geeignete Lösungsmittel kann genutzt werden, um
defektfreies, in flüssiger Form prozessierbares Graphen in
großem Maßstab zu erzeugen, das in fast jeder denkbaren
Beschichtungsroutine zum Einsatz kommen kann. Durchchemische Veränderungen im Aufbau der verwendeten Tensidekann deren Effektivität gezielt angepasst werden, was wiederum
die Möglichkeit eröffnet, Funktionalität über die reine
Dispergierungswirkung hinaus einzuarbeiten – beispielsweise
durch Implementierung von elektronendonierenden oder aufnehmenden Untereinheiten. Sollte im Zuge eines „top-down“Ansatzes die kovalente Funktionalisierung zum Einsatz kommen,
wird in der Regel kein störstellenfreies Material generiert, dawährend der Funktionalisierung sp3-Kohlenstoffzentren in das
Graphengitter eingebaut werden. Jedoch erlauben GraphitInterkalationsverbindungen (GICs) die Minimierung möglicher
σ-Gerüstschäden während der reduktiven Aktivierung desAusgangsgraphits. Die erhaltenen reduktiv exfoliierten Graphenschichten können anschließend mit organischen Oxidationsmitteln versetzt werden, was zur Knüpfung neuer Bindungenzwischen den einzuführenden organischen Addenden und der zu
funktionalisierenden Schicht führt.

Wenn Kontrolle über die lokale atomare Struktur erreicht werdensoll, beispielsweise um exakte optische und elektronische
Eigenschaften von dünnsten Graphen-Nanostreifen (GNS)
einzustellen, so muss eine „bottom-up“-Strategie gewählt werden.
Sowohl lösungs- als auch oberflächenbasierte Synthesemethoden
wurden zur Herstellung von Polyphenylen-Vorläufermolekülen
eingesetzt, wobei nach intramolekularer Cyclodehydrierung
ebenfalls Graphen-Nanostreifen erhalten werden konnten. In
beiden Fällen hängt die Topologie sowie die Randstruktur der
GNS stark von den verwendeten Vorläufermolekülen ab. Suzukiartige Polymerisation erlaubt die Synthese der Vorläufer in
Lösung. Die Einführung eines geknickten Phenylgerüsts sowie die
Verknüpfung mit geeigneten Alkylketten an bestimmten
Positionen kann die Löslichkeit der letztendlich erhaltenen GNS
deutlich verbessern, was essentiell für die weitere Prozessierung
und Strukturaufklärung ist. Bei der oberflächenvermittelten

Synthese von GNS muss das halogenhaltige Vorläufermonomer
erst dünnschichtig, wenn möglich in einer Monolage,
abgeschieden werden und durchläuft dann zwei weitere
thermisch aktivierte Prozesse. Im ersten Schritt werden bei
niedriger Temperatur Polyphenylenketten über radikalische
Addition aneinander gebunden. Anschließend sind höhere
Temperaturen notwending, um eine vollständige Planarisierung
dieser Ketten zu erreichen. Die Eliminierung von Wasserstoff
führt schließlich zu den atomar vordefinierten GNS auf der
Oberfläche. Natürlich sollte durch Wahl größerer Vorläufermoleküle der Weg zu maßgeschneiderten Architekturen und
Eigenschaften der GNS geebnet werden können. So sollten
schließlich die Problematiken der momentan verwendeten „topdown“-Verfahren, welche auf lithographischen und chemischenÄtztechniken beruhen, überwunden werden können.
1*Andreas Hirsch is Professor for
Organic Chemistry at the University
Erlangen-Nürnberg. His researchinterests are the chemistry of synthetic
carbon allotropes such as fullerenes
carbon nanotubes and graphene as wellas supramolecular and nanochemistry.
Email: andreas.hirsch@chemie.unierlangen.de
2)

3)

3)

Jan M. Englert , Xinliang Feng , Prof. Klaus Müllen ** corresponding authors
Addresses:
1)

University Erlangen-Nürnberg, Henkestrasse 42,D-91054
Erlangen, Germany
2)

ZMP-Institute of Advanced Materials and Processes, Dr.-Mack
Str. 81, 90762 Fürth, Germany
3)

Max Planck Institute for Polymer Research, Ackermannweg 10,55128 Mainz, Germany
Acknowledgement
We thank the “Deutsche Forschungsgemeinschaft (DFG)"(Directed synthesis of graphene nanoribbons – 559148), The
Interdisciplinary for Molecular Materials (ICMM) and the Graduate
School Molecular Science (GSMS) for financial support.

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+PVGTHCEKCN 'NGEVTQP 6TCPUHGT, * 7, *4   !" 54  76* $ 
$44
3iao Wang, Dai Zhang, 7uanmin Wang, Papatya Sevinc, H. Peter Lu,
Alfred J. Meixner+nterfacial electron transfer (ET) in TiO2@based systems is important in artificial

solar energy harvesting systems, catalysis, and in advanced oxidative waste water
treatment. The fundamental importance of ET processes and impending
applications make the study of interfacial ET a promising research area. Photo@excitation of dye molecules adsorbed on the surface of wide band gap
semiconductors, such as TiO2, results in the injection of electrons from the dyemolecules to the conduction band of the semiconductor or energetically accessible
surface electronic states. Using Raman spectroscopy and ensemble@averaging
approaches, the chemical bonding and vibrational relaxation of the ET processes
have been extensively studied. However, due to the complexity of the interfacial ET
energetics and dynamics, significant questions remain on characterizing the sourceof the observed complexities. To address these important issues, we have appliedadvanced spectroscopic and imaging techniques such as confocal and tip@enhanced near@field Raman as well as photoluminescence spectroscopic andtopographic imaging. Here we explore single surface states on TiO2 as well as theinterfacial electronic coupling of alizarin to TiO2 single crystalline surfaces.
 +PVTQFWEVKQP
nterfacial electron transfer (ET) plays a central role in dye^sensitized solar cells, photocatalysis, environmental chemistry,
surface chemistry, and molecular electronics.[1] A model dye^sensitized semiconductor system is represented in Figure 1. The
forward electron transfer (FET) kinetics in various dye^TiO2
systems has typical half^times ranging from femtoseconds toseveral hundred picoseconds.[2] When the adsorbed molecule isstrongly bound close to the TiO2 surface, then rapid FET isexpected. In the so^called wide^band limit, Franck^Condon
factors do not matter for FET and the transfer rate is primarily
controlled by the electronic coupling.[3] Following FET, theinjected electron relaxes and is localized to sub^band states.
Backward electron transfer (BET) from the semiconductor to theoxidized dye molecules often follows. Compared to ultrafast FET
processes, BET processes take longer, with lifetimes ranging
from sub^nanoseconds to several milliseconds.[4] The wide
timescale variation is most likely associated to theinhomogeneous molecular surface bonding and vibrational
relaxation energy barriers for the back electron transfer; although,other contributions can be from the existence of trap states andnon^Brownian electron diffusion in semiconductors. Furthermore,

the dynamics of the BET processes are also often multiexponetialand stretched exponential.[4b, 5]'PGTI[('6eCB
Conduction
Band
$'6

/ / 5νJν

Jνµ/GVCNNKE 6KR 'PJCPEGF
4COCP URGEVTQUEQR[

e

6K1

// 5#(/ CPF 56/ UKPING OQNGEWNG+OCIKPI

Valence Band(KIWTG . Diagram of interfacial electron transfer processes for thedyeCsensitized TiO2 system.

Designing an efficient solar energy harvesting system entails
controlling the rate of the BET process in order to generate long^
lived charge^separated states. Characterization of the electron trap

A25

Articles
and localized states are important because these states play aregulating role in BET dynamics and, in turn, the fluorescencedark^state lifetimes of the oxidized dye molecules. An efficientdye^sensitized TiO2 solar cell system should have fast forward
electron transfer from the dye excited state to the semiconductor
conduction band, and a slow backward electron transfer so thatthe excess electron has a higher probability of conversion toelectricity.
We have used confocal and tip^enhanced high^resolution
near^field luminescence and Raman imaging spectroscopy toanalyze the interfacial charge transfer energetics down to thesingle site and single^molecule level, well beyond the spatial
resolution of the optical diffraction limit.[6] We intend to extend
our Raman spectral analysis to obtain the vibrational relaxationenergy, surface bonding, and molecular vibronic coupling
information in order to dissect the interfacial electron transfer
energetics. Near^field scanning optical microscopy makes use of
the strongly enhanced electric field around a sharp metal tip
under laser illumination. In a near^field Raman spectroscopic
measurement, the excitation field is locally significantly
enhanced by a combination of an electrostatic lightning^rod effectand a tip^plasmon excitation induced by the laser field. The
spatial resolution achievable with this technique is determinedprimarily by the apex radius and the shape of the metallic tip, andthe signal^to^noise ratio is defined by the strength of the field^
enhancement effect. We have made significant advances towards
using near^field Raman spectroscopic mapping to analyze
interfacial electron transfer on the TiO2 surfaces.[6]molecule near^field Raman spectroscopic imaging has been done
for dye^sensitized TiO2 systems. All the measurements were
performed with a home^built parabolic mirror^assisted near^field
optical microscope.[7] A typical configuration of the microscopeis illustrated in Figure 2. The sample is illuminated from above
by a parabolic mirror which generates a diffraction limited
excitation spot on the sample surface and in return efficiently
collects the scattered and reflected photons and directs them onto
a confocal detection system consisting of an avalanche
photodiode (APD) and a spectrograph equipped with a CCD^
detector for spectral analysis. For perfect polarization control in
the focal volume, the parabolic mirror is illuminated with anaperture filling radially polarized laser mode. For super^
resolution optical imaging and topographic recording the tip isbrought through the axial hole in the parabolic mirror andprecisely centred into the focus. The tip^sample distance is kept
to about 3 nm by a shear^force feed back loop.

 +OCIKPI QH UKPING 6K1UWTHCEG UVCVGU CPF
KPVGTHCEKCN GNGEVTQPKE EQWRNKPI#W VKR(KIWTG . Schematic illustration of a parabolicCmirrorCassisted
confocal ultramicroscope. A sharp Au tip is adjusted into the centerof diffraction limited focus as created by an incident radiallypolarized laser beam. Laser illumination is applied in a radiallypolarized mode to obtain a well defined field distribution in the focal
volume. The tip serves as an optical antenna providing intense
local electromagnetic (EM) field enhancement at the tip apex for
local excitation and emission to the optical farCfield.

We have applied a special near^field spectroscopic imaging
microscopy to characterize the dye^sensitized TiO2 single crystal
surfaces, and we were able to demonstrate 15^nm spatial
resolution on the correlated topographic and Raman
spectroscopic images.[6] This is the first time that a single^

A26(KIWTG . Optical response of the abundant surface states of the
NbCdoped rutile TiO2 (110) surface. (A1) Confocal imaging and(A2) Raman/luminescence spectra of TiO2 (110) surface. (B1)
Confocal imaging and (B2) Raman/luminescence spectra of TiO2(110) surface with alizarin (1 OM).

Panels A1 and B1 in Figure 3 show confocal images of theNb^doped rutile TiO2 (110) surface under the conditions of
without (A) and with alizarin (B), respectively. Most of the hot
spots in the confocal image (without the tip) in Figure 3A1 show
as donut^shape features, and only a small number of them show
as circular or ellipsoidal^shaped spots. Such different diffraction
limited fluorescence excitation patterns can only be observed
when single quantum systems are raster scanned through the field
distribution of a tightly focused radially polarized laser beam.

Since the excitation rate is proportional to the square of the
projection of the transition dipole moment D onto the electricfield E as

 2
Rexc ∝ D ⋅ EDistinct excitation patterns are observed if single quantum
systems, such as a single molecule, defects, or surface states areimaged, revealing their orientation and the dimensionality of their
optical transition. We have calculated the optical patterns ofquantum systems with one dipole moment, two perpendicular
dipole moments, or three perpendicular dipole moments excitedby radially polarized laser beams as shown in Figure 4. Only forquantum systems with a two dimensional transition dipole
moment as recently found for excitons in quantum dots[8] we canobserve circular patterns. Here the ring patterns suggest that thetransition dipole moment of the surface states can assume two
perpendicular orientations with the same probability and bothlying in the surface plane.[8] This is consistent with recent
quantum^chemical calculations which suggest that O^vacancy
formation in rutile Ti(110) surface results in two excess electrons
occupying 3d orbitals on Ti atoms neighboring the vacancy.[9](KIWTG . (A) Simultaneously obtained topographic and near field
optical images of a clean NiobiumCdoped rutile TiO2 (110) surface.
Single surface states localized at the subCdomain boundaries can
be clearly observed. (B) For the Alizarin adsorbed TiO2 surface,resonance Raman spectrum (using 532 nm laser excitation) with
the TiCO peak at 646 cmC1 provides the evidence of the strong
electronic coupling at the interface.

Surface states have been demonstrated to play important roles
in interfacial electron transfer process.[10] Their density andenergy distributions are the possible parameters that affect the
charge transfer pathways. We have used TiO2/alizarin, a typicalmodel system with strong electronic coupling,[11] to probe theseparameters. However, as shown in Figure 3B1 and 3B2, weobserved less hot spots and inhomogeneous imaging patterns(ellipsoidal shaped and donut^shape) and clear blue^shifts in thephotoluminescence background of the spectra comparing with the
TiO2 only crystal surface. These observations can be interpretedby the intensity difference of the electronic coupling between
TiO2 and alizarin molecules.[6](KIWTG . Calculated optical patterns of quantum systems with one
dipole moment, two perpendicular dipole moments and three
perpendicular dipole moments excited by radially polarized laser
beams with parabolic mirror. The intensity in each figure isnormalized individually.

The feasibility of using the near^field Raman imaging
analysis for TiO2 single crystal systems is demonstrated in Figure
5. Brightly luminescent spots arise from localized surface states
which are distributed mainly along the sub^domain boundaries asevidenced in the superimposed topographic (in blue) and near^field optical (in red) image (Figure 5A). The interfacial electronicmolecule^substrate coupling has been characterized by probing
the molecule^perturbed surface states distribution and theassociated specific Raman vibrational modes. Figure 5B shows
the high resolution (15 nm) surface states imaging (left) and theRaman spectrum of the Alizarin adsorbed TiO2 surface (right).
We observed a direct evidence of the formation of the Alizarin^
TiO2 charge transfer complex: besides the normal vibrationalmodes from Alizarin, we also observed a new Raman peak at 646
cm^1, which is a typical Ti^O stretching mode from a bridging Ti^
O^C bond [11^12] The strong C^O stretching at 1330 cm^1 reveals
that the Alizarin^TiO2 charge transfer complex forms primarily
through the hydroxyl groups of Alizarin and a Ti atom of TiO2
surface.

A27

Articles 5WTHCEG UVCVGU GNGEVTQPKE EQWRNKPI EJCTIG
FGNQECNK\CVKQP CV VJG KPVGTHCEG CPF VJGKT TQNGU
KP KPVGTHCEKCN '6 F[PCOKEU
In a solar energy conversion system such as a dye^sensitizedsolar cell, one of the key parameters that limit the photo electricalconversion efficiency is the charge transport inside thesemiconductor electrode. Due to its complexity, energeticallytrapping, thermally activated detrapping, and Non^Brownian
electron diffusion have been proposed to describe the chargetransport process in the semiconductor. To improve the chargetransport efficiency of TiO2 electrodes, single crystal TiO2
nanowires, polycrystalline TiO2 nanowires, and TiO2 nanotubes
have been applied in solar cells. However, it was demonstrated
that electron transport rate of these materials can be as slow as in
TiO2 nanoparticle because the surface traps almost dominate thecharge transport pathways[10] indicating the important role ofsurface states in charge transport.
Surfaces states on rutile TiO2 (110) surface have been
investigated and visualized by traditional approaches such as
scanning tunneling microscopy.[13] The physical nature of surface
states have been attributed to bridging oxygen vacancies orinterstitial Ti^atoms.[14] We suggest that the single surface states
revealed by our nanoscale imaging are originated from thebridging oxygen vacancies, which can be deduced from thedonut^shape optical patterns under radial polarized mode laser
excitation because only for quantum systems with a twodimensional transition dipole moment can give circular optical
patterns. On the basis of a quantum^chemical calculation,[9] O^
vacancy formation in rutile Ti(110) surface results in two excess

As the electron trap center, surface states on rutile TiO2 (110)
surface has been proved to be active chemical reaction centers.
Toward the potential applications as electron traps forphotogenerated electrons, dye molecule (such as alizarin)
perturbed surface states in TiO2 is a critical research topic. The
strength of the electronic coupling between dye and TiO2
significantly affects the charge transfer rate at the interface.
Driven by a strong electronic coupling, electrons can directly
delocalize from the highest occupied molecular orbital of the dyemolecule to the trap center of TiO2 in forming the interfacial
charge transfer complexes. This will make efficient electron
injection for a dye^sensitized TiO2 system. In addition, as amodel system, many experiments and calculations also focus on
the fundamental understanding of the surface chemistry andphysics of TiO2 surface by investigating the diffusion, adsorption,
dissociation, and reaction of molecular Oxygen, H2O, and related
species on the TiO2 single crystal surface.[15] These efforts
actually also base on a thorough understanding of the electronic
structure of the semiconductor surface.
Summary
Interfacial electron transfer energetics has been probed using
nanoscale confocal and tip@enhanced near@field
photoluminescence and Raman imaging and spectroscopy. Single
surface states on clean and Alizarin@doped TiO2(110)surfaces aswell as strong electronic coupling at the alizarin/TiO2 interfaces
have been demonstrated by exploring the imaging andspectroscopic characters. Our results help for the understandingof fundamental processes in solar energy conversion,
photocatalysis, and molecular electronics.

Zusammenfassung(KIWTG  Conceptual view of the OCvacancy on rutile TiO2 surface.
The structure is plotted by ball and stick model (Red: O; Blue: Ti).
Because there are two excess electrons occupying 3d orbital of
two neighboring Ti atoms, two transition dipole moments will occur
under laser excitation. The two arrows show the direction of thedipole moments.

electrons occupying 3d orbitals on Ti atoms neighboring thevacancy. With laser excitation, electrons at the valence band canbe excited to higher energy states, for example, trap states below
the conduction band. Considering the spatial configuration of theO^vacancy and the two adjacent Ti atoms, there should be two
possible transition dipole moments with an angle close to vertical.
A conceptual view is shown in Figure 6. Actually, the real angle
between the two transition dipole moments may be affected by
the local structural distortion.

A28

Elektronenübertragungsprozesse zwischen Farbstoffen und einer
Halbleiteroberfläche wurden mittels hochauflösender konfokaler
und spitzenverstärkter nahfeld@optischer Raman@ und Lumines@
zenz@Mikroskopie untersucht. Es konnten sowohl einzelne
lumineszierende Oberflächendefekte bei undotierten als auch bei
Alizarin@dotierten Oberflächen als auch eine starke elektronische
Kopplung zwischen Alizarin und der TiO2(110)@Oberfläche beob@
achtet werden. Unsere Resultate tragen zu einem besseren
Verständnis der Photovoltaik, der Photokatalyse und dermolekularen Elektronik bei.

&1)o %)6/ 1) , is PhD student andworking with Alfred Mei ner and Dai
Zhang.
Email: iao.wang@uni tuebingen.de

1]a) B. Oregan, M. Gratzel, Nature  , 353, 737 740; b) C.
Joachim, M. A. Ratner, Proc. Natl. Acad. Sci. USA , 102,
8801 8808; c) P. V. Kamat, J. Phys. Chem. C , 111, 2834
2860.

[2]

a) J. Asbury, E. Hao, Y. Wang, H. Ghosh, T. Lian, J. Phys. Chem.
B , 105, 4545 4557; b) Y. Gao, Y. Georgievskii, R. Marcus, J.
Chem. Phys. , 112, 3358; c) R. Huber, J. Moser, M. Grätzel, J.
Wachtveitl, Chem. Phys. , 285, 39 45; d) P. Kamat, J. Phys.
Chem. B , 106, 7729 7744; e) A. Barzykin, M. Tachiya, J.
Phys. Chem. B , 106, 4356 4363.

[3]

R. Ernstorfer, S. Felber, W. Storck, E. Galoppini, Q. Wei, F. Willig,
Res. Chem. Inter. , 31, 643 647.

[4]a) V. Biju, M. Micic, D. Hu, H. P. Lu, J. Am. Chem. Soc. , 126,
9374 9381; b) Y. Wang, X. Wang, S. K. Ghosh, H. P. Lu, J. Am.
Chem. Soc.  , 131, 1479 1487; c) Y. M. Wang, X. F. Wang, H.
P. Lu, J. Am. Chem. Soc.  , 131, 9020 9025.

[5]

L. J. Guo, Y. M. Wang, H. P. Lu, J. Am. Chem. Soc. , 132,
1999 2004.

[6]

P. C. Sevinc, X. Wang, Y. Wang, D. Zhang, A. J. Meixner, H. P.
Lu, Nano Lett. , 11, 1490 1494.

[7]a) D. Zhang, U. Heinemeyer, C. Stanciu, M. Sackrow, K. Braun, L.
E. Hennemann, X. Wang, R. Scholz, F. Schreiber, A. J. Meixner,
Phys. Rev. Lett., , 104, 56601; b) X. Wang, D. Zhang, K.
Braun, H. J. Egelhaaf, C. J. Brabec, A. J. Meixner, Adv. Funct.
Mater., , 20, 492 499.

[8]

A. I. Chizhik, A. M. Chizhik, D. Khoptyar, S. Ba r, A. J. Meixner,
Nano Lett. , 11, 1131 1135.

[9]B. J. Morgan, G. W. Watson, J. Phys. Chem. C , 114, 2321
2328.

: These authors contributed to the work
equally.

)1 (0)6/ 1) is junior research groupleader working with Alfred Mei ner.
Email: dai.zhang@uni tuebingen.de: These authors contributed to the work
equally.

'=)6516 %)6/ 2) is postdoctoral fellow
working with . Peter Lu.
Email: yuawang@bgsu.edu

2))8)<A)  !->16+ is a PhD student
working with . Peter Lu.
Email: psevinc@bgsu.edu -<-: = 2) , Ohio Eminant Scholar is
Full Professor of Chemistry at the
Bowling Green State University.
Email: hplu@bgsu.edu[10] a) E. Enache Pommer, Phys. Chem. Chem. Phys  , 11, 9648
9652; b) E. Enache Pommer, J. E. Boercker, E. S. Aydil, Appl.
Phys. Lett. , 91, 113116; c) K. Zhu, N. R. Neale, A. Miedaner,
A. J. Frank, Nano Lett. , 7, 69 74.
[11] D. Pan, D. Hu, H. Lu, J. Phys. Chem. B , 109, 16390 16395.

 corresponding author[12] L. C. T. Shoute, G. R. Loppnow, J. Chem. Phys. , 117, 842
850.

4.:-,  -1@6-: 1) is full Professor for
Physical Chemistry at the University of
Tübingen.

[13] A. C. Papageorgiou, N. S. Beglitis, C. L. Pang, G. Teobaldi, G.
Cabailh, Q. Chen, A. J. Fisher, W. A. Hofer, G. Thornton, Proc.
Natl. Acad. Sci. USA , 107, 2391.

Email: alfred.mei ner@uni tuebingen.de corresponding author
Addresses:
1)

University of Tübingen, Auf der Morgenstelle 18, 72076
Tübingen, Germany[14] a) C. M. Yim, C. L. Pang, G. Thornton, Phys. Rev Lett, , 104,
36806; b) S. Wendt, P. T. Sprunger, E. Lira, G. K. H. Madsen, Z.
Li, J. O. Hansen, J. Matthiesen, A. Blekinge Rasmussen, E.
Laegsgaard, B. Hammer, Science , 320, 1755.
[15] a) M. Rasmussen, L. Molina, B. Hammer, J. Chem. Phys. ,120, 988 997; b) S. Wendt, R. Schaub, J. Matthiesen, E.
Vestergaard, E. Wahlstrom, M. Rasmussen, P. Thostrup, L. Molina,
E. L gsgaard, I. Stensgaard, Surf. Sci. , 598, 226 245.

2)

Bowling Green State University, Center for Phytochemical
Sciences, Deoartment of Chemistry, Bowling Green, O 43403
USA
Acknowledgements:
Lu acknowledges the support from the Basic Energy Science ofDepartment of Energy (grant DE FG02 06ER15827) and theDivision of Chemistry of National Science Foundation (grant C E
08226 4) of the United States Mei ner acknowledges the support
from the DFG, project ME 1600 11 1, Germany

A29

Articles
9CVGT 1ZKFCVKQP

E F+    /+ ,  *4" /4G
7 76H /" , F+"46F+"4
I %$
Antoni Llobet and Franc Meyer%nderstanding water oxidation mechanisms. The power of two cooperating metal centers.
 +PVTQFWEVKQPThe energy demand for the year 2050 is expected to be in therange of 30^50 TW,[1] which is a huge amount of energy thatcannot be covered by today’s energy resources. Further, given theincrease in the rate of CO2 production and the dramaticconsequences this increase has for our planet,[2] any additionalenergy should preferably come from carbon^neutral renewablesources and mainly from solar energy,[3] Today’s energy
problem[4] needs to be solved urgently in order not only to be able
to maintain the lifestyle of our developed societies, but also toattain decent living standards in developing countries. The
foreseeable increase in worldwide energy utilization has to comenecessarily from environmentally friendly and sustainablesources, so that future generations can still enjoy living on planet
earth.
Hydrogen is a clean energy vector that has attracted
substantial attention from energy related industrial and car
making corporations. H2^based technology is already quite
developed and can be used to provide electricity to a house or car,and it can also produce hot water or run a domestic central
heating. Particularly interesting is the Personal Power Plant (P3)concept that was put forward in 1994 by Honda[5] and more
recently by Audi.[6] This concept is reminiscent of the computer
revolution that took place in the 80’s when the centralized main
frame computers were substituted at home with local andindependent Personal Computers (PC). An important advantageof H2 technology is that it can be used independently of the
precise situation of the earth rotational period.
While the storage and use of hydrogen has already beenachieved with a significant degree of success and sees rapid
further improvement,[7] the question of a sustainable hydrogensource still remains to be answered.[8] In fact, besides being
utilized as fuel itself, solar hydrogen is also considered the key toa large^scale production of other renewable fuels such as theconversion of lignocellulosic biomass or the reduction of CO2 tomethanol.[9] One particularly attractive solution for generation ofmolecular hydrogen consists of water splitting by sunlight, asshown in Figure 1. That is sunlight is used to promote athermodynamically uphill reaction where water is transformed
into molecular oxygen and hydrogen. The inverse reaction thenA30

forms water and renders back the harvested sunlight energy thatcan be transformed into heat and/or electricity with no net
molecular loss or byproduct formation. This scheme represents an
example of the concept of storing energy into chemical bonds.
Using water as a raw material certainly is the most sustainableoption for providing the electrons that are required for generating
any non^fossil fuel.

(KIWTG . Sunlight water splitting and the P3 concept

Even though sunlight has sufficient energy to promote thewater splitting reaction shown in Figure 1, the uncatalyzed
reaction is extremely slow and essentially does not take place.
Actually the interaction of sunlight with water occurs solely at a
vibrational level, and thus it only produces heat. Therefore
alternative schemes need to be developed in order to couple theelementary steps of light harvesting, charge separation, electrontransfer and finally splitting of water. From a chemical view point
the water splitting reaction can be written as two half reactions,
namely(i) water oxidation to O2
2 H2O

O2 I 4 HI I 4 eC

(1)(ii) proton reduction to H2
2 HI I 2 eC

H2

(2)

The water oxidation reaction (equation 1) is athermodynamically demanding reaction since Eo=1.23 V (vs.
NHE) at pH 0. It is also of enormous molecular complexity froma mechanistic perspective, since formally it involves the removalof four protons and four electrons from two water molecules,
together with the formation of an oxygen–oxygen bond. The
present research project is aimed at the development andmechanistic understanding of transition metal complexes capableof acting as efficient water oxidation catalysts (WOCs).
Nature has been using water and sunlight as a source ofenergy in its photosynthetic processes in green plants and algae
for a long time. The photosynthetic process can be summarizedaccording to equation 3:
6 CO2 I 6 H2Ohν

C6H12O6 I 6 O2

scaffold that serves as a backbone to position the two Ru metal
centers in close proximity. In in,in^[RuII(trpy)(H2O)]2(O^bpp)}3+(=) two ancillary trpy ligands (trpy = 2,2’:6,6’^terpyridin) occupy
additional coordination sites to leave two positions within thebimetallic pocket for water binding (Figure 3). This places thetwo O atoms in a favourable distance of 2.48 . Upon treatment
with four equivalents of the 1e^ oxidant CeIV the (II,II) species =is oxidized to the (IV,IV) oxidation state, which advances to anintermediate that finally evolves dioxygen. Labelling experimentswith H218O and kinetic studies unambiguously revealed that thissystem operates according to the intramolecular I2M
mechanism.[15](3)

Incidentally, nature uses a similar strategy as the water
splitting depicted in Figure 1 in the sense that electrons andprotons are extracted from water to form dioxygen. In photo^
synthesis, however, the reductive equivalents and protons arecoupled to NADPH formation and subsequent CO2 reduction ^ toin the end form carbohydrates, C6H12O6. Nature uses a
sophisticated polynuclear Ca^Mn4 complex as an oxygen
evolving catalyst (OEC), which is situated in photosystem II(PSII). Recently substantial efforts have been directed at
elucidating the OEC^PSII structure and the biological wateroxidation mechanism.[10,11] Low molecular weight OEC models
are contributing significantly to this endeavor, since they do not
only provide critical information regarding the potential
mechanism/s that operate at the photosynthetic OEC, but may
also lead to efficient artificial catalysts that are part of the energy
challenge solution.[9] In fact, water oxidation is currently
recognized as one of the bottlenecks for photoproduction ofhydrogen from water and sunlight.

M

M

O

O

M

M

O

O
H

O
H

M

M

O

O
O

O

M

M

(KIWTG . Potential reaction pathways for the formation of oxygenC
oxygen bonds assisted by dinuclear transition metal complexes.
Left: intramolecular interaction of two MO units, I2M; Center: water
nucleophilic attack, WNA; Right: bimolecular I2M.

N

N NH

N

N

N

Hbpp

N

trpy

(KIWTG . Left: bis(bidentate) Hbpp ligand. Center: ancillary trpy
ligand. Right: calculated structure of in,inCK[RuII(trpy)2(H2O)]2(Cbpp)L3I. Color code: Ru, pink; N, blue; O, red; C, gray; H, white.

 6JG ,QKPV 2TQLGEVVarious transition metal complexes have recently been
described that are capable of oxidizing water to dioxygen,[12] butvery few mechanistic studies have yet been performed.[13]However, proper understanding of the potential reaction
pathways as well as the deactivation channels is crucial in order
to come up with a sufficiently rugged and efficient catalyst thatcan be used technologically for a commercial water splitting cell.
The majority of the well^studied systems are currently based on
Ru complexes. Most Ru WOCs share a common feature, namely,
the intermediacy of ruthenium^oxo species in which the metal
center is formally in a high oxidation state. This high oxidation
state species for dinuclear complexes can then potentially follow
three fundamentally different pathways depending on how the O^
O bond is formed, as depicted in Figure 2.
One of the best studied and most active dinuclear Ru wateroxidation catalysts to date is the Ru^Hbpp system (Hbpp = 3,5^bis(2^pyridyl)pyrazole) developed in one of our laboratories(Llobet group).[14] It consists of a dinucleating anionic pyrazolate

At present, the optimized turn^over^number (TON) of catalyst= is more than 500. This calls for further modification of thedinucleating scaffold in order to obtain more rugged and evenmore efficient systems. Routes to a diverse array of multidentate
pyrazole^based ligands have been developed in one of ourlaboratories (Meyer group),[16] including the bis(terdentate) Hbbp
scaffold (Hbbp = 3,5^bis(2^bipyridyl)pyrazole) that can beviewed as an expanded version of Hbpp (Figure 4).[17] It thus
suggested itself to team up for the development of improved
pyrazole^bridged diruthenium water oxidation catalysts.
M
N
N

N NH

N
N

X = H: Hbbp

N

N

Y^py

(KIWTG . Left: bis(terdentate)MCHbbp family of dinucleating ligands.
Center: substituted monodentate pyridyl ligands. Right: calculated
structure of a representative diruthenium complex,
K[RuII(py)2(H2O)]2(Cbbp)L3I. Color code: Ru, pink; N, blue; O, red; C,
gray; H, white.

A31

Articles
The main aim of the joint project is to obtain a T
understanding of the different factors that influence the O^O bondformation in dinuclear water oxidation catalysts. To achieve thisgoal a family of highly preorganized binucleating pyrazolate
based ligands and their diruthenium complexes such as the one
shown in Figure 4 are synthesized and studied in great detail. The
different X^ and Y^substitutents on the bbp scaffold as well as on
the py ligands, respectively, provide a handle for tuning the redox
properties of the metal centers, which also allows to potentiallycontrol the mechanism through which the water oxidation
reaction proceeds. Further the X substituent can be used to attach
an anchoring group that would permit to heterogenize the catalyst
system. Fixation of the individual catalyst species on aheterogeneous support is expected to avoid potential bimolecularcatalyst deactivation reactions as well as the undesiredintermolecular I2M type of mechanism. Initial results for thehighly preorganized new diruthenium complexes jointly
developed in the two participating laboratories are promising.

#PVQPK .NQDGV 1) is professor ofchemistry at the Universitat Autznma deBarcelona and Group Leader at theCatalan Institute for Chemical Researchin Tarragona (Spain).
Email: allobet@iciq.es(TCPE /G[GT 2) is professor of inorganic
chemistry at the GeorgHAugustHUniversity
G{ttingen (Germany) since 2001.
Email: franc.meyer@chemie.uniH
goettingen.de

Addresses:

Summary
Water oxidation (WO) catalysis is a research field that
presently experiences a tremendous upsurge. The interest for thisfield is dual and stems from the fact that WO is the reaction thattakes place at the oxygen evolving complex in photosystem II and
is a key reaction for the design of new energy conversion schemes
and of future scenarios for sustainable energy supply. In thepresent collaborative project we merge the complementary
expertise of two groups to synthesize and study a new family of
bimetallic transition metal complexes capable of acting as WO
catalysts. The findings contribute to a more thoroughunderstanding of the different potential reaction mechanism
through which this important reaction can proceed. It thus
represents fundamental work that is needed in order to progress
toward the discovery of rugged and efficient water oxidation
catalysts.

1)

Institute of Chemical Research of Catalonia (ICIQ), Avinguda
Paisos Catalans 16, EH43077 Tarragona, Spain
2)

Institute of Inorganic Chemistry, GeorgHAugustHUniversity,
Tammannstrasse 4, DH37077 G{ttingen, Germany.
Acknowledgements:
Funding of this joint project by the German Research
Foundation (DFG) and the Spanish Ministeria de Ciencia eInnovacion (MICINN) in the framework of the ERA Chemistry
program is gratefully acknowledged.

[1]a) Basic Research Needs for the Hydrogen Economy; A reportfrom the Basic Energy Sciences Workshop on Hydrogen
Production, Storage and Use; Department of Energy;
WashingtonDC, ?@@J; b) Solar Energy Utilization Workshop?@@AKBasic Science Needs for Energy Utilization (US Dept ofEnergy, WashingtonDC).

[2]

K. Caldeira, A. K. Jain, M. I. Hoffert, Science ?@@J, 299, 2052.

[3]“Energy: the 50^year plan” by Nancy McGuire. Web of the ACS(www.acs.org.; First appeared on September 13th, 2004).

[4]

A. Witze, Nature ?@@B, 445, 14.

[5]

http://www.alt^energy.info/hydrogen^power/hondas^home^hydrogen^fueling^station/. January 24th, 2008.

[6]

http://www.naikontuning.com/noticiasnt/2011/05/13/audi^
balanced^mobility^el^e^gas^project^a^metano/. May 13th, 2011

[7]a) L. Schlapbach, A.Züttel, Nature ?@@=, 414, 353; b) H. Lee,
J.^W. Lee, D. Y. Kim, J. Park, Y.^T. Seo, H. Zeng, I. L. Moudra^
kovski, C. I. Ratcliffe, J. A. Ripmeester, Nature ?@@A, 434, 743.

[8]a) N. Armaroli, V. Balzani, Angew. Chem. ?@@B, 119, 52; Angew.
Chem. Int. Ed. ?@@B, 46, 52; b) S. His, in “Hydrogen: An Energy
Vector for the Future”, Panorama, 2004, IFP^Information, France;c) J. S. Connolly, Photochemical Conversion and Storage of Solar
Energy, Academic Press, New York, =>D=; d) Energy Resources
through Photochemistry and Catalysis (Ed.: M. Grätzel), Academic
Press, New York, =>DJ; e) T. N. Verziroglu, F. Barbir, Int. J.
Hydrogen Energy =>>?, 17, 391.

Zusammenfassung
Die Wasseroxidations(WO)@Katalyse ist ein Forschungsfeld,
das derzeit aus vielfältigen Gründen einen enormen Aufschwung
erlebt. Zum einen ist die WO jene chemische Reaktion, die am
Sauerstoff entwickelnden Komplex des Photosystems II
stattfindetS zum anderen handelt es sich um eine fundamentale
Reaktion im Zusammenhang mit neuen Szenarien zur
Energiekonversion und zur nachhaltigen Energieversorgung. Im
hier skizzierten Kooperationsprojekt bringen wir die
komplementären Fachkenntnisse zweier Forschungsgruppen
zusammen, um eine neue Familie von maTgeschneiderten
bimetallischen Übergangsmetallkomplexen zu synthetisieren und
zu untersuchen, die als WO@Katalysatoren wirken. Die daraus
gewonnenen Erkenntnisse tragen zu einem besseren Verständnis
der verschiedenen möglichen Reaktionsmechanismen bei, nachdenen diese wichtige chemische Reaktion verlaufen kann. Es
handelt sich somit um grundlegende Arbeiten, die von Wert sein
werden, um zielgerichtete Fortschritte bei der Entwicklung vonrobusten und effizienten Wasseroxidations@Katalysatoren zu
erreichen.

A32

[9]

H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan, P. Strasser,
ChemCatChem ?@=@, 2, 724.

[10] a) W. Lubitz, E. J. Reijerse, J. Messinger, Energy Environ. Sci.
?@@DK 1, 15; b) J. Barber, Inorg. Chem. ?@@D, 47, 1700; c) Y.
Umena, K. Kawakami, J.^R. Shen, N.Kamiya, Nature ?@==, 473,
55.
[11] a) J. P. McEvoy, G. W. Brudvig, Chem. Rev. ?@@L, 106, 4455^
4483; b) M. Haumann, P. Liebisch, C. Müller, M. Barra, M.
Grabolle, H. Dau, Science ?@@A, 310, 1019.
[12] a) L. Duan, A. Fischer, Y. Xu, L. Sun, J. Am. Chem. Soc. ?@@>,
131, 10397; b) J. D. Blakemore, N. D. Schley, D. Balcells, J. F.
Hull, G. W. Olack, C. D. Incarvito, O. Eisenstein, G. W. Brudvig,
R. H. Crabtree, J. Am. Chem. Soc. ?@=@, 132, 6017; c) W. C, Ellis,
N. D. McDaniel, S. Bernhard, T. J. Collins, J. Am. Chem. Soc.
?@=@, 132, 10990; d) D. K. Dogutan, R. McGuire, Jr., D. G. Nocera,
J. Am. Chem. Soc. ?@==, 133, 9178; e) Q. Yin, J. M. Tan, C.
Besson, Y. V. Geletii, D. G. Musaev, A. E. Kuznetsov, Z. Luo, K. I.
Hardcastle, C. L. Hill, Science ?@=@, 328, 342; f) F. Toma, A.
Sartorel, M. Iurlo, M. Carraro, P. Parisse, C. Maccato, S. Rapino, B.
R. Gonzalez, H. Amenitsch, T. D. Ros, L. Casalis, A. Goldoni, M.
Marcaccio, G. Scorrano, G. Scoles, F. Paolucci, M. Prato, M.
Bonchio, Nat. Chem. ?@=@, 2, 826.

[13] See for instance: a) J. J. Concepcion, M.^K. Tsai, J. T. Muckerman,
T. J. Meyer, J. Am. Chem. Soc. ?@=@, 132, 1545; b) D. J.
Wasylenko, C. Ganesamoorthy, M. A. Henderson, B. D. Koivisto,
H. D. Osthoff, C. P. Berlinguette, J. Am. Chem. Soc. ?@=@, 132,
16094; c) S. Romain, L. Vigara, A. Llobet, Acc. Chem. Res. ?@@>,
42, 1944.
[14] a) C. Sens, I. Romero, M. Rodrguez, A. Llobet, T. Parella, J.
Benet^Buchholz, J. Am. Chem. Soc. ?@@C, 126, 7798; b) J. Mola, E.
Mas^Marza, X. Sala, I. Romero, M. Rodrguez, C.Viñas, T.Parella,
A. Llobet, Angew. Chem. ?@@D, 120, 5914; Angew. Chem. Int. Ed.
?@@D, 47, 5830.
[15] a) S. Romain, F. Bozoglian, X. Sala, A. Llobet, J. Am. Chem. Soc.
?@@>, 131, 2768; b) F. Bozoglian, S. Romain, M. Z. Ertem, T. K.
Todorova, C. Sens, J. Mola, M. Rodrguez, I. Romero, J. Benet^
Buchholz, X. Fontrodona, C. J. Cramer, L. Gagliardi, A. Llobet, J.
Am. Chem. Soc. ?@@>, 131, 15176.
[16] J. Klingele, S. Dechert, F. Meyer, Coord. Chem. Rev. ?@@>, 253,
2698.
[17] a) J. I. van der Vlugt, S. Demeshko, S. Dechert, F. Meyer, Inorg.
Chem. ?@@D, 47, 1576; b) B. Schneider, S. Demeshko, S. Dechert,
F. Meyer, Angew. Chem. ?@=@K 122, 9461; Angew. Chem. Int. Ed.
?@=@, 49, 9274.

A33

Articles#OOQPQZKFCVKQP-$$+  M4  1 2464 -$ $  /F?6I 4 H+Falk Liebner, Georg Pour, JosV Maria de la Rosa Arranz, AndrV Hilscher,Thomas Rosenau, Heike Knicker

0itrogen (N) is a major nutrient element controlling the cycling of organic matterin the biosphere. Its availability in soils is closely related to biological productivity.
In order to reduce the negative environmental impact, associated with theapplication of mineral N@fertilizers, the use of ammonoxidised technical lignins issuggested. They can act as potential slow N@release fertilisers which concomitantly
may increase C sequestration of soils by its potential to bind CO2. The idea of ourstudy was to combine an improved chemical characterisation of ammonoxidised
ligneous matter as well as their CO2@binding potential, with laboratory pot
experiments, performed to enable an evaluation of their behaviour and stability
during the biochemical reworking occurring in active soils.
 +PVTQFWEVKQP
Among today’s crucial global socioeconomic and ecologicalissues, erosion by wind and water and the resulting soil
degradation are most prominent ones. In many parts of the world,
agricultural areas are insufficient to guarantee for adequate
nutrition, in addition to a permanently growing population.
A major factor in maintaining soil productivity represents thereplacement of N used for plant growth which is mostly achievedby the amendment of mineral fertilisers. However, the unintended
costs to the environment and human health due to surplus andinefficient application are substantial. Runoff from farms can leadto contaminated surface and groundwater and ammonia andnitrous oxides released from fertilised cropland are considered as
major source of air pollution and potent greenhouse gases. Thus,
since fertilisation is essential, there is certainly an urgent need forthe development of efficient and environmentally sustainablefertilisers that are able to slowly release their N in accordancewith the needs of the plants.
A further matter of intensive current attention and debate is
the global climate change. Taking large^area deforestation andextensive utilisation of fossil fuels as most probable reasons, bothaspects have also caused a dramatic atmospheric increase in CO2,
the former by a decrease in the fixation of CO2 in carbohydrates
and the latter by release of CO2 upon combustion of carbon^
containing matter.
Current environmental research strategies focus on alternative
energy sources, on effective landscape and soil rehabilitation toincrease primary production and thus CO2 fixation in renewable

A34

matter, and on the usage of CO2 as a synthon in chemical
processes. This chemical fixation is superior to a mere CO2
storage or sequestration. The project “COBAL ^ CO2 binding ofammonoxidised ligneous materials” (since 2008) within theframework of the ERA^chemistry call „Chemical activation ofcarbon dioxide and methane” and jointly funded by the German
DFG and the Austrian FWF addresses these aspects.
Ammonoxidation of technical lignins or ligneous materials
under mild conditions is proposed as an approach to mimic thenatural humification process.[1] Technical lignins are by^productsof the pulp and paper industry or of biorefinery processes which
are so far almost exclusively “energetically used”, i.e. burned.
Upon ammonoxidation, ligneous matter reacts with aqueous
ammonia and oxygen in suspension or solution under slightly
elevated pressure and temperature. The N^enriched products are
expected to be valuable humus substitutes, which are able toreturn N (amines, amides, carbamates) as well as C (lignin C andCO2 bound as carbamates) to the soil and into mineralization
cycles, stimulating microbial biomass production and hence soil
development. Ammonoxidised lignins are likely to allow for along^lasting fertilisation due to the presence of different types of
N bonds that mineralise in soil at different rates. However, up tonow, neither the mechanisms of N enrichment during
ammonoxidation nor the chemical nature of the organic N or its
behaviour in soils is well understood. Therefore, the investigationof N binding in different ammonoxidised lignins as well as theirstability in soils where biological activity enforces highcompetition for the limited N sources has been a central task of
the COBAL project. As the N moieties are crucial in interactingwith CO2, comprehension of their structure and their role in CO2
fixation, both under the conditions of laboratory chemistry andthe complex situation of soil environments goes hand in hand.

Further, a better understanding of the chemical constitution andproperties of artificial and natural humic substances is generated.

O

Amongst several theories on natural humification, thepolyphenol theory is based on the assumption that low molecular
phenolic and quinoid intermediates are formed by microbial
lignin degradation products.[2] This is in line with the concept ofour studies on ammonoxidation chemistry. 2,5^Dihydroxy^[1,4]benzoquinone (DHBQ) and methoxyhydroquinone (MHQ)
were chosen as model compounds as they were the most likelyintermediates to be formed if the humification process follows thepolyphenol theory, i.e. oxidative, microbial degradation of lignin,
and subsequent re^condensation of the reactive phenol andquinone intermediates under incorporation of N.
Analytical data, e.g. 15N CPMAS NMR and XPS spectra, of
the ammonoxidised model compounds and the ammonoxidised
technical lignins Indulin AT (Pine kraft lignin; MeadWestvaco,Charleston, SC, USA), Sucrolin (autohydrolysis bagasse lignin;
Illovo Sugar Ltd., Sezela, South^Africa) and Sarkanda grass soda
lignin (Granit SA, Lausanne, Switzerland) are very similar,
indicating the reaction sequence to proceed via a common key
intermediate. To exploit the 15N NMR technique, we usedisotopic enrichment through 15N^labelled aq. NH4OH in many of
our experimental approaches. The data suggest furthermore a far^reaching lignin degradation to MHQ^analogous low^molecular
phenols at least under ammonoxidation conditions (100°, 5% aq.
NH4OH, 2 bar O2).
DHBQ was shown to react with ammonia to 2,5^diamino^[1,4]benzoquinone (DABQ) already at slightly elevatedtemperature. Interestingly, DABQ and its N^methylated
counterpart 2,5^bis(methylamino)^[1,4]benzoquinone were also
isolated from the reaction mixtures of MHQ with aqueous
ammonia and methyl amine, respectively. As the presence of
DABQ in ammonoxidised technical lignins was also confirmed(2D^NMR) we accumulated sufficient evidence to propose(substituted) 2,5^diamino^[1,4]benzoquinones as key inter^mediates in the chemical alteration of ligneous materials caused
by ammonia/amines under aerobic conditions. 2,5^Diamino^[1,4]benzoquinone can be perceived as a dimeric vinylogous
amide as the two amino groups of the symmetric molecule areeach located in β^position of an α,β,^unsaturated carbonyl moiety.
This accounts for the peculiar reactivity of the compound which
behaves more as an amide rather than an amine, and also thespectral data point to an amide. Another striking feature is thestrong stabilisation of the molecule by H^bonds in acidic or
neutral media and by resonance in alkaline medium (Scheme 1).
Nevertheless, the intermediate DABQ disappears after longer
ammonoxidation times as shown by UV/Vis and IR spectroscopy,
and the structural elucidation of the reaction products is currently
going on.

OH

H

O

O

NH

NH

I OHC

-

2e

HN

HN
O

H
NH

NH

H2N

 #OOQPQZKFCVKQP QH NKIPKPU CPF GNWEKFCVKQPQH FKHHGTGPV PKVTQIGP DKPFKPI

O

OH
NH2

NH

HN
O

O

O

5EJGOG  Stabilisation of the dimeric vinyloguous amide 2,5C
diaminoC[1,4]benzoquinone by tautomerism and resonance.

 %1 DKPFKPI QH COOQPQZKFKUGF NKIPKPU CPFNKIPKP OQFGN EQORQWPFU
Several secondary amines are able to bind CO2, mostly in theform of 2:1 complexes.[3] These compounds can be describedboth as addition products or ion pairs (dialkylammonium
dialkylcarbamates, “dialcarbs”). The simplest candidate of thiscompound class, the dimethylamine ^ carbon dioxide complex (cf.
Scheme 2), is a good example to reflect the complex structural^
dynamic behaviour in these addition products. Many
dialkylammonium dialkylcarbamates are room^temperature ionicliquids or room^temperature salts, but they may reversibly
degrade into their gaseous components upon heating, and re^form
upon cooling. They can thus be “distilled” in contrast to theconventional heterocycle^based ionic liquids.
Ammonoxidised lignins are supposed to bind CO2 incarbamate complexes in an action mode similar to low^molecularweight secondary amines, especially if ammonia is (partially)
replaced by amines. This is due to the content ofalkylaminobenzoquinones, alkylamines and alkylamides, and has
been confirmed for the ammonoxidised models 2,5^dihydroxy^[1,4]benzoquinone and methoxyhydroquinone. Both significantly
increase the CO2^binding ability of lightweight bacterial cellulose
aerogels (8 mg cm^3) when finely dispersed as nanoparticles
inside the aerogel pore network. As mentioned, the content of
dialkylamines and dialkylamides as CO2^binding moieties in N^
lignin is further increased using (di)methylamine instead ofammonia in the preparation of the N^lignins. Dialkylammonium(dialkyl)carbamates are stable compounds that can be stored
without decomposition.
H3C
H
4

H3C
H3C

N H

O

TS60RC

H3C

O

TQ60RC

H3C

I 2 C

N C

I

N

CH3
H

O

O

O

O
H

H3C

I

N

C N

CH3
CH3

H
CH3

5EJGOG  Formation of the dimethylamine carbon dioxide(ODimcarbP) complex.

 %1 DKPFKPI QH CEVKXCVGF J[FTQZ[DGP\Q
SWKPQPGU
Recently, it was demonstrated that not only lignoid phenols,
but also cellulose is converted into 2,5^dihydroxy^[1,4]benzo^
quinone structures upon aging, with those structures mainly
contributing to residual chromophores and yellowing effects.[4,5]These findings are intriguing with respect to the current teaching

A35

Articlesof natural humification, claiming those structures in humic
substances to originate mostly from lignin and secondary
metabolic products. Hydroxy^[1,4]benzoquinones have found no
significant synthetic usage so far. This is due to their properties aspotent chromophores and their tendency to undergo condensationand polymerisation reactions. This behaviour, which is preventive
for synthetic utilization, renders them predestined for natural andartificial humification scenarios – and for the studies in thepresent project. Importantly, 2,5^dihydroxy^[1,4]benzoquinone
structures exhibit a peculiar behaviour that has been neither
recognised nor used so far. In weakly acidic to alkaline medium,
2,5^dihydroxy^[1,4]benzoquinones form symmetrical dianions
highly stabilised by resonance (cf. Scheme 3), with the remainingnon^substituted ring positions being able to attack carbon dioxide
and carbonic acid derivatives, by analogy to biochemical
pathways. Such carboxylation of 2,5^hydroxy^[1,4]benzoquinone
in neutral medium is one of the very few examples of binding
CO2 under carbon^carbon bond formation. The resulting alicycles
undergo fragmentation and rearrangements according to several
pathways (Scheme 3).
O

CelluloseOOH

ageing
HO

O

O

O

O

O
O
O

O

O

I 2 CO2

O
OH

O

O

HO
O

O

O

O
OH

O

O

2e

O

O

O

O

O
O

O

O

O

O
O

I OHC

O

HO
O

Malic acidPyruvic acid
Carboxymalonic acidO

5EJGOG  2,5CDihydroxyC[1,4]benzoquinone and its proposed
carboxylation reaction under alkaline conditions.

The main products are C2^C4 monoacids, oxoacids anddiacids. The average CO2^binding capacity of DHBQ at neutral
pH is 1.86 equivalents. In contrast to the temporary binding of
CO2 by N functionalities such as in Dimcarb, the CO2 binding by
DHBQ^type compounds is permanent. In N^lignins, both binding
modes are active.

 #OOQPQZKFKUGF NKIPKPU CU UNQY 0TGNGCUGHGTVKNK\GT
For the study of the effect of ammonoxidised lignin on plant
growth, a series of pot experiments were conducted for two and ahalf months. In the first, material of a Terra rossa representing atypical agriculturally used soil of Mediterranean regions, wasmixed with 15N^enriched ammonoxidised lignin (Indulin andSarkanda) on which grass seeds (Lolium perenne) were planted
and grown in a greenhouse under controlled conditions based on
an experimental design described in more detail by de la Rosa
Arranz and Knicker.[6] The above^ground biomass production as
well as the partitioning of the 15N label among the plant, themicrobial biomass and the soil was monitored as a function of
incubation time. A high 15N enrichment of 2.5 to 2.8 atom% wasused to enable 15N NMR spectroscopy of the soils amended withN^lignins. Their addition to the organic^matter^poor andcalcareous soil increased its N content from 0.9 mg g^1soil to 1.5

A36(15N^Indulin) and 1.6 mg g^1 soil (15N^Sarkanda). In order toaccount for possible N losses due to leaching, the bottom of the
incubation pots were perforated to allow the exit of excess water
after watering. For comparison, a control soil without any
amendment and a soil mixed with K15NO3 were prepared.
Because we expected a fast removal of the latter due to leaching,
the starting 15N concentration was 10 mg 100 g^1 soil.
Compared to the controls, the addition of K15NO3 resulted ina significant increase of the total above^ground plant yields. Mostof the growth occurred during the first 18 days for the control andduring 28 days for the fertilised samples. After that, the growthrate declined significantly which is best explained with the using^up of the bioavailable N pool. This assumption is supported by
the fact that after one month incubation time less than 20% of theadded 15N (15Nadd) were recovered with the soil matrix, whereas athird was already incorporated into the above^ground plantbiomass and no major increase was achieved until the end of theexperiment. However, in spite of the low 15N amendment,
between 40% and 60% of 15Nadd were already lost after one
month either due to leaching but possibly also by volatilisation.
With the expectation that ammonoxidised lignins act as a slow
release fertiliser and prevent fast N leaching they were applied inamounts in which 15Nadd was between 4 and 6 times higher thanwith the mineral fertiliser. This, however, did not result in higher
above^ground biomass production. Not earlier than after 42 days
(15N^Indulin) and 58 days (15N^Sarkanda) the accumulated
harvest overran, but only slightly that of the unfertilised pots. In
spite of the lower mass yields, the N concentrations wereapproximately 70% higher, suggesting that the above^groundshoots of the 15N^lignin^added pots were considerably richer inproteins than those from the mineral fertilised pots.
After 58 days, less than 7% and 20% of 15Nadd of 15N^Indulin
und 15N^Sarkanda were incorporated into the plant shoots. In the
15
N^Indulin experiment, most of the unused 15Nadd remained in
the soil (80% of 15Nadd), indicating that here, N was efficiently
sequestered from fast release into the soil solution. The 15N^
Sarkanda lignin, on the other hand, showed a different picture.
Here, already after 28 days of incubation only one^third of the15
Nadd were retained in the soil, most of which was recovered withthe microbial biomass, as it was determined with the chloroform
fumigation extraction methods.[7] The respective 15Nadd recovery
with the microbial biomass for the 15N^Indulin series was about
8%. Although for both experiments, the amount of 15N retained in
the soil matrix did not alter much until the end of the incubation,
15
Nadd associated with the microbial biomass declined to less than2%.
In order to obtain some more insights into the fate of the N^
lignins during plant growth, the soil material of selected pots
were dried and after removing the carbonates with HCl they were
demineralised with hydrofluoric acid[8] to increase their
sensitivity for solid^state 13C and 15N CPMAS NMR spectroscopy.
The latter was further facilitated by the high 15N enrichments.
The solid^state 13C CPMAS NMR spectra of the 15N^lignins
showed the typical lignin pattern[9] and the respective solid^state
15
N CPMAS NMR spectra demonstrated signals in the chemical

shift region assignable to pyrrole^type N (^145 to ^245 ppm)
(Figure 1), as well as considerable signal intensity in the chemical
shift
regionsofaminobenzoquinones
(^274
ppm),
aminohydroquinones (^304 ppm), aromatic amines (^330 ppm)
and ammonium (^358 ppm).[10,11] However, in the 15N NMR
spectra of the HF^treated soil material from the pot after 30 daysof incubation those typical signals disappear in favour to adominating signal around ^260 ppm and a small intensity at ^348
ppm, demonstrating a surprisingly fast transformation of thearomatic amino groups into peptide^type N, although someintensity remained in the chemical shift region assignable topyrrole^type N. The signals at ^260 and ^348 ppm are also
observed in the 15N NMR spectrum of the soils amended solely
with K15NO3 and are typical for organic N in biomass.[12](KIWTG  SolidCstate 15N CPMAS NMR spectra of the used 15NC
lignins and the HFCtreated soils amended with 15NClignins andK15NO3 after one month of incubation. Asterisks indicate spinningside bands. The spectra were obtained at the Lehrstuhl fBr
Bodenkunde, TUCMBnchen.

The shift of the 15N signal intensity, thus, is most tentatively
caused by the fast and efficient recycling of amino groups
released from the lignin backbone for the build^up of new
microbial biomass. The high 15Nadd recovery in the microbial
biomass of the pots incubated for one month after amended withthe 15N^lignins supports this conclusion. With respect to soilorganic C, the solid^state 13C CPMAS NMR spectroscopy
revealed no major alteration due to incubation.
For the next experiment, material of a Luvisol[13] derived
from the experimental station Viehhausen of the TU^München
was mixed with Indulin that beforehand was ammonoxidised withmethylamine (P1) or dimethylamin (P2). These lignins,
synthesized to increase the CO2 binding potential, were comparedto 2,5^diamino^[1,4]benzoquinone (P3) and 2,5^dihydroxy^[1,4]benzoquinone ammonoxidised with NH3 (P5) and control
soils with and without addition of KNO3. Although the basic
experimental set^up remained comparable to the first incubation,
this time, leaching was not enabled and the N^addition via KNO3
was approximately the same as for the other pots. The N content
of the soil matrix was with 0.15% approximately twice as high as
for the Terra rossa, which most tentatively contributed to adoubling of the yields with respect to the control of the firstexperiment. However, here the plant production could not be

increased by KNO3 addition. Comparable to the first experiment,
the germination was retarded with P1 and P2 by 2 weeks and with
P5 by at least one month. Whereas, relative to the control, after 8
weeks, P1 and P2 addition finally increased the harvest by morethan a factor 2, P3 and P5 could not achieve this threshold.
The C/N ratio of the grass grown on the control pots
confirmed that consumption of bioavailable N results in the
production of low^protein plant biomass. Concomitantly, the highavailability of KNO3 resulted in grass shoots with a very low C/N(w/w) ratio of 5.6. The excess supply of mineral N, however,turned to a disadvantage which led to the death of grass plants
after 2 months. Low C/N values were also detected for P1 and P2(7) but increased to 20 until the end of the experiment, indicatingthat here, too, the availabilityof the N source declined after afirst boost. Plant shoots frompots with P3 and P5 had C/N^
values of around 10 and 6,
respectively. Whereas no N
loss was detected for thecontrol approximately 23 and
32% of the KNO3^N was lost,
most of which already during
the
first
two
weeks.
Comparably high losses of Nadd
were observed for the N^lignins
and the N^model compounds.
It seems that similar to the first experiment, a part of the“amino”^N which has been shown to behave as an amide (seeabove) was relatively quickly released from the aromatic
backbone and subjected to N^cycling in the soil. Whereas the N^
recoveries remained constant for P1 and P3, for P5, they
decreased to 50% after 4 months.
In order to test the assumption derived from the two pot
experiments that in a soil system, some of the N of theammonoxidised lignins, may be quickly mineralised, theconcentrations of inorganic N were determined as a function of
incubation time.[14] After two weeks, for the soils amended with
P1, P2 and P3, 10% of Nadd was recovered as NH4+. Since, here,
15
N^labelling was not applied, we were not able to obtain
unbiased quantitative data about N partitioning into microbial
biomass. After two months, the concentrations of extractable
NH4+ decreased to values comparable to those determined for thecontrol pots. Because neither an increase of the N loss nor a
noteworthy transformation into nitrate was revealed, it can beassumed that after the first flush, the released N was used forplant growth, although some part was certainly incorporated into
microbial remains that subsequently contributed to the soilorganic matter pool. The model lignin P3 showed no major
accumulation of NH4+ at any stage of the experiment and for P5,
18% of Nadd was extracted as NH4+ after two weeks. Considering
the low N^recovery of 68% for this sample, it seems that at least
50% of Nadd have passed through the mineral stage before any
plant growth occurred. Comparable to P1 and P2, theconcentration of NH4+ were decreasing constantly during theexperiment, either by the mechanisms discussed above or by

A37

Articles
further N losses since only 50% of Nadd were recovered in plantand soil after four months.

 %1 DKPFKPI QH COOQPQZKFKUGF +PFWNKP KP CUQKN TGURKTCVKQP GZRGTKOGPV
The impact of N^lignin addition on the CO2^sequestration
potential of soils was elucidated in a model experiment
determining the respective CO2^release of soil incubates with andwithout addition of N^Indulin in a Respicond Apparatus IV (A.
Nordgren Innovations AB, Sweden). This was based on the ideathat efficient CO2 fixation by the N^lignins decreases the amountof CO2 released during soil organic matter degradation.
Nevertheless, we were aware that this experiment could only be afirst approach, since lower CO2^release rates can also be causedby i) a slower degradation of the added lignin, ii) by a possible
inhibition of microbial activity or iii) by the opposite, namely an
increased microbial growth due to higher amounts of substrateand N. For our experiment, we used a sandy and acid soil (pH: 4;
Histic Humaquept from the Doñana National Park, Spain) to
avoid possible CO2 adsorption to the matrix. Materials of two
horizons with different C contents (4% C and 2% C) were usedwith and without addition of N^Indulin and Indulin. All samples
were inoculated with a bacterial mixture isolated from agardening soil. After two and a half months of microbial
degradation at 28°C, all vessels showed a comparable C loss of 5
to 6%, thus no major differences of the C^sequestration potential
were observed.
Those results imply two major conclusions. First, thecomparable degradation rates evidence that lignins are degradedat a rate comparable to natural soil organic matter. Second, CO2
produced during soil organic matter degradation was notefficiently sequestrated by the added N^lignin under the usedconditions. However, this may change under neutral or weakalkaline conditions under which CO2 fixation according to the
proposed mechanisms (schemes 1 and 2) is expected to be much
more pronounced.
Summary
15

N CPMAS NMR and 3PS spectra of 15N labelledammonoxidised technical lignins, methoxyhydroquinone and 2,5@
dihydroxy@W1,4Xbenzoquinone were very similar, indicative for areaction sequence proceeding via a common key intermediate.
Therefore, (substituted) 2,5@diamino@W1,4Xbenzoquinones areproposed to be key intermediates of the chemical alteration ofligneous materials caused by ammonia or amines in the presenceof oxygen. 2,5@Diamino@W1,4Xbenzoquinones are vinylogous
amides which are highly stabilised by tautomery and resonance.
Both ammonoxidised methoxyhydroquinone and 2,5@dihydroxy@
W1,4Xbenzoquinone were confirmed to bind CO2. 2,5@Dihydroxy@
W1,4Xbenzoquinone was furthermore shown to react in alkaline
medium with CO2 to C2@C4 monoacids, oxoacids and diacids.
Subjecting the ammonoxidised lignins to pot experiments resulted
in a surprisingly fast formation of amides. During the first weeks
at least a part of the Nadd passes through the microbial biomass

A38

stage. The harvested plants showed an N enrichment caused by
the amendment. Respiration experiments revealed comparable
CO2 production rates in soil incubated with and without N@
lignins, possibly because microbial activity competed with CO2
for the available N groups.

Zusammenfassung
15

N@CPMAS@NMR@ und 3PS@Spektren von 15N@markierten,
ammonoxidierten technischen Ligninen, Methoxyhydrochinonsowie 2,5@Dihydroxy@W1,4Xbenzochinon wiesen eineüberraschende Yhnlichkeit auf, woraus auf eine
Reaktionssequenz mit einem gemeinsamen Intermediat
geschlossen wurde. Als Schlüsselverbindung der chemischen
Modifizierung von ligninhaltigen Materialien durch Ammoniak
oder Amine in Gegenwart von Sauerstoff wurden (substituierte)
2,5@Diamino@W1,4Xbenzochinone vorgeschlagen. 2,5@Diamino@
W1,4X@benzochinone sind vinyloge Amide die durch Tautomerie
und Resonanzeffekte in besonderem MaTe stabilisiert werden.
Sowohl Methoxyhydrochinon als auch 2,5@Dihydroxy@W1,4Xbenzo@chinon weisen nach Ammonoxidation ein erhöhtes
Bindevermögen für CO2 auf. Weiterhin konnte gezeigt werden,
dass 2,5@Dihydroxy@W1,4Xbenzochinon in alkalischem Medium mit
CO2 zu C2@C4@Monocarbonsäuren, @oxosäuren und @disäuren
reagiert. Topfexperiment zeigten, dass die Zugabe derammonoxidierten Lignine zu einer überraschend schnellen
Bildung von Amiden führt. Schon während der ersten Wochen
wurde zumindest ein Teil des Nadd in mikrobielle Biomasse
eingebaut. Elementaranalysen bestätigten eine durch die
Düngung verursachte N@Anreicherung im Grasmaterial. In
Respirationsexperimenten wurden für Böden mit und ohne N@
Lignin@Zugabe vergleichbare CO2@Mengen freigesetzt. Einemögliche Erklärung hierfür könnte die Konkurrenz zwischen
Mikroorganismen und CO2 um die verfügbaren N@Gruppen
darstellen.

Acknowledgements

The financial support of the German DFG (projet. KN 463/9@1)
and Austrian FWF through the project I154@N19 is gratefully
acknowledged. The authors would like to thank E. Brendler, TU
Bergakademie Freiberg, Germany, and J. Ralph, University of
Wisconsin@Madison, USA, for recording some of the 15N CPMAS
and the 2D@NMR spectra. Furthermore, we thank H. Hesse and P.
Streubel, University of Leipzig, Germany, for the 3@ray
photoelectron analyses. The IRNAS@CSIC, Spain, is gratefully
acknowledged for providing access to their greenhouse, their
respirometer as well as their laboratory facilities. Comparably,
the Lehrstuhl für Zierpflanzenbau of the TU@München, Germany,is thanked for allowing us to use their Phytotrons.

(CNM .KGDPGT 1) is Assistant Professor andhead of the group NBiomaterial ChemistryR at
the University of Natural Resources and Life
Sciences Vienna (BOKU), Dept. of Chemistry,
Chair of Wood, Pulp and Fiber Chemistry.
Email: falk.liebner@boku.ac.at)GQTI 2QWT 1) is PhD student at the Universityof Natural Resources and Life Science Vienna(BOKU), Dept. of Chemistry, Chair of Wood,
Pulp and Fiber Chemistry.
Email: georg.pour@boku.ac.at[1] F. Liebner, K., Fischer, J. Katzur, L. Böcker in The Loess
Plateau in Central China. Ecological Restoration and Management(Ed.: UNESCO office Beijing), Tsinghua University Press, Springer,
BeijingK ?@@A, pp. 183^207.
[2] M. Kästner, M. Hofrichter in Biopolymers I (Eds.: A.
Steinbüchel, M. Hofrichter), Wiley^VCH, Weinheim, ?@@=, pp. 349^
378.
[3] W. Schroth, H.^D. Schädler, J. Andersch, Z.. Chem. =>D>, 29,
129^135.
[4] T. Rosenau, A., Potthast, W. Milacher, A. Hofinger, P. Kosma,
Polymer ?@@C, 45, 6437^6443.
[5] K. Kraintz, A. Potthast, U. Suess, T. Dietz, N. Nimmerfroh, T.
Rosenau, Holzforschung ?@@>, 63, 647^656.

,QUo /CTKC FG NC 4QUC #TTCP\ 2) is aubiliary
researcher at the Nuclear and Technologic
Institute of Portugal.
Email: jmrosa@itn.pt[6] J.M. de la Rosa Arranz, H. Knicker, Soil Biol. Biochem. ?@==K
accepted.
[7] D.S. Jenkinson, D.S. Powlson, Soil Biol. Biochem. =>BL, 8, 167^
177.
[8} C.N. Gonçalves, R.S.D. Dalmolin, D.P. Dick, H. Knicker, E.
Klamt, I. Kögel^Knabner, Geoderma ?@@J, 116, 373^392.

3)#PFTo *KNUEJGT is PhD student at theLehrstuhl für Bodenkunde of the Technische
Universität MünchenHWeihenstephan.
Email: hilscher@wzw.tum.de[9] H. Knicker, H.^D. Lüdemann, K. Haider, Europ. J. Soil Sci.
=>>BK 48, 431^441.
[10] K. A. Thorn, M.A. Mikita, Sci. Total Environ. =>>?, 113, 67^87.
[11] M. Witanowski, L. Stefaniak, G.A. Webb. Nitrogen NMR
Spectroscopy, Academic Press, London, =>>J.
[12] H. Knicker, J. Environ. Qual. ?@@@, 29, 715^723.
[13] H. Knicker, P. Müller, Water Air Soil Poll. ?@@L, 6, 235^260.

6JQOCU 4QUGPCW 1) is full professor and chairof Wood, Pulp and Fiber Chemistry at the
University of Natural Resources and Life
Sciences Vienna (BOKU), Dept. of Chemistry(since 2005).

[14] J. C. Forster in Microbiology and Biochemistry (Eds.: K. Alef, P.
Nannipieri), Academic Press, London, =>>A, pp. 49^121.

Email: thomas.rosenau@boku.ac.at*GKMG -PKEMGT 4) is profesora de investigacin(since 2008) at the Institute of Natural
Resources and Agrobiology of Sevilla (IRNASH
CSIC) and apl. Professor at the Lehrstuhl fürBodenkunde, TUHMünchen.
Email: knicker@imase.csic.es

Addresses:
1)

KonradHLorenzStr. 24, AH3430 Tulln an der Donau

2)

Estrada Nacional 10, PH 2686Hw53, Sacavém

3)

EmilHRamannHStrae 2, DH85354 Freising

4)

Avda. Reina Mercedes, 10, EH41012 Sevilla

A39

Articles%CTDCOCVG 5[PVJGUKU

3 62  3 6F 4$ ,  /!$ ",$ /! '+   HG - '"%  -!  H '"$H "$  "Philipp J. di Dio, Marc Brüssel, Kilian Muñiz, Rupashree Shyama Ray, Stefan Zahn,
Barbara Kirchner

9e investigated the key step of Pd@N to Pd@O rearrangement from a model

catalytic cycle for the activation of carbon dioxide and methane with staticquantum chemical calculations and metadynamics simulation. Our calculations
show that different bottlenecks appear in the catalytic cycle but that theinvestigated rearrangement of the Pd@N to Pd@O bounded complex has a barrier\G#/\F# of approximately 20 kJ mol@1 and is therefore accessible at ambient
reaction conditions.
 +PVTQFWEVKQP
Since the first isolation and characterization of a carbon
dioxide^transition metal complex by Aresta in 1975,[1] thefixation and activation of carbon dioxide as a non^toxic, abundant,
and inexpensive C1 building block in organic synthesis was
extensively studied by experimental as well as theoreticalmethods.[2] There are several transition and main^group metal
complexes which are capable to engage in CO2 fixation andsubsequent chemical transformation. Usually, CO2 can be
activated within two different binding modes at the active site ofa transition metal: The transition metal interacts with one C^O π^bond in a η2^binding mode or a transition metal C^bond is formed
in a η1^binding mode (Figure 1).[3]synthesis of carboxylic acids.[5] Overall, a series of synthetictransformations is now available that demonstrates the usefulness
of CO2 as a C1 building block. Particular interest in CO2 fixation
has recently been devoted to the condensation of this moleculewith epoxides under metal catalysis.[6] Organic carbonates arewidespread feedstocks for industrial synthesis. Therefore, an easy,
cheap and save production process is desirable. While traditionalroutes for synthesizing organic carbonates usually involve thehighly poisonous phosgene (COCl2), several atom^economic
reactions, where CO2 reacts with peroxides in the presence of ametal catalyst, are now available (Figure 2).

(KIWTG  A simple example of an epoxide reacting with CO2 in thepresence of a catalyst.

(KIWTG  BallCandCstick model illustrating the η2Cbinding mode (left)and η1Cbinding mode (right) of CO2 with a transition metal M.

Both cases result in a non^linear CO2 geometry and allow
reactions of CO2 with various substrates. In this way, coordinatedCO2 can be reacted to form methanol, formic acid, ureas, lactones,
carboxylic acids etc.[4] Alternatively, CO2 can be functionalized
directly with organometallic reagents, for example in the

A40

Although detailed description about the mechanisms and thereaction conditions for several catalytic CO2 condensation
reactions with epoxides has been accomplished,[6] most reactions
still need vigorous reaction conditions and a further improvementof the catalysts is hence required.
Besides the CO2 activation, the activation of C^H bonds isalso of special interest to chemists. The direct functionalization ofalkanes from small, simple, and inexpensive molecules likemethane CH4, ethane CH3^CH3, and toluene C6H5CH3 leads toimportant synthetic precursors for academic synthesis andindustrial processes. This has been widely applied[7] andinvestigated[8] for different C^H bond types (Car^H, Csp2^H, Csp3^
H, ...) and different metal complexes (Ni, Au, Pt, Pd, ...) forming
different bonds (C^C, C^N, ...).[9]quantum chemical calculations of the transition state which leads
from  to  (step '). Finally, we present metadynamics
simulations of this rearrangement in carbon dioxide solution toinvestigate the dynamics and solvent effects in this reactions step.

 %QORWVCVKQPCN &GVCKNU

5EJGOG  Proposed and investigated catalytic cycle for theactivation and fixation of CO2 and CH4 forming the methyl N,NCdiC
methylcarbamate  at the palladium catalyst .

We currently pursue an approach to combine both carbon
dioxide and methane activation within a single catalytic cycle,
which is shown in Scheme 1 for a palladium catalyst  bearing
the bis(dimethylphosphino)^propane ligand. In this proposed
catalytic cycle, the reaction starts with methane activation (step#) through an intermediate  to form a palladium methyl
complex  under concomitant loss of HCl. Exchange of theremaining anion at palladium for dimethylamine (step %)proceeds with loss of HCl and forms a palladium amide complex
. Since reductive elimination of trimethylamine from  is a slow
process, addition of carbon dioxide should be kinetically feasible.
This step &, the coordination of CO2 to the amide ligand, wasinvestigated previously with ab initio molecular dynamics
simulations, in particular by metadynamics and thermodynamic
integration.[10] The comparison between solvated and gas phasesimulation gave insight into solvation effects. The results
demonstrated that the CO2 addition to  takes place with anactivation barrier of approximately ©F# = 45 kJ mol^1. The final
catalytic cycle should then proceed through a series of steps
including the rearrangement from  to  (step '), followed by
reductive elimination through transition state . This step
liberates the carbamate product  and generates a palladium(0)catalyst state that upon reoxidation (step )) regenerates the initialcatalyst .
In the following, we investigate step ' of the catalytic cycle
in detail. This reaction comprises a rearrangement to generate an
N,N^dimethylcarbamoyl ligand. After the computational details,
we first give the results of the static quantum chemical
calculations for the whole catalytic cycle, i.e., the reactionenergies of every single step. Thereafter, we show and discuss the

For the static quantum chemical calculations the program
package Turbomole 5.10[11] was used to optimize all structures
reported in this paper. No symmetry or internal coordinate
constraints were applied during optimizations. We used theB3LYP functional[12,13] and all calculations were carried out withthe triple^ζ basis set def2^TZVPP[14] which includes a relativistic
electron core potential for Pd.[15] The energy convergency
criterion was 10^8 Hartree. For frequency calculations the program
SNF 4.0 was chosen.[16] The simulations were carried out
employing Born–Oppenheimer molecular dynamics simulations
with Gaussian and plane waves using the program CP2k.[17] The
temperature was set to 350 K controlled by Nos–Hoover chain
thermostats.[18] A time step of 0.5 fs was chosen and the DZVP^MOLOPT^SR^GTH basis set[19] with the GTH^BLYP^qn
pseudopotentials[20] (H: n = 1; C: n = 4; N: n = 5; O: n = 6; Pd: n= 18) were used. The plane wave representation was truncatedwith a cutoff of 280 Ryd and the gradient corrected functional
BLYP[12] with Grimme dispersion correction[21] was applied. The
simulations box was taken from a previous work,[10] it contains
one molecule of complex / with 59 additional CO2 and has abox length of 1800 pm (ρ = 0.85 g cm^3) with periodic boundary
conditions. The technique of metadynamics simulations was
explicitly described in a previous work.[10] The height of theGaussian hills was lowered during the simulations (3.9, 1.32 and0.26 kJ mol^1). The time interval between adding two Gaussian
hills was increased according to the lowering of the hills height(2.5·10^2, 5·10^2 and 6·10^2 ps). The hill width was fixed to 0.3
rad for the simulation. For all three collective variables the force
constant was set 840.2 kJ mol^1 rad^2 and the fictional mass to 200
amu. The figures were generated with Matplotlib, which is a 2D
plotting library for Python,[22] and VMD.[23] 4GUWNVU CPF &KUEWUUKQPIn Table 1 we show the energies from static quantum
chemical calculations for the step ' of the catalytic cycle(Scheme 1), the reaction barrier for step ' ('65), and theoverall reaction energy for the whole cycle (Σ).
6CDNG  Reaction energy of the step ', reaction barrier '65 forthe investigated step ', and reaction energy Σ of the wholecatalytic cycle (Scheme 1).

'
'65
Σ

∆E[a]

∆G[a,b]

C84.2
22.5
C72.5

C99.5
22.4
C48.7

[a] In kJ molC1. [b] At 298.15 K.

A41

Articles
We find that the overall reaction energy is approximately ^70
and ^50 kJ mol^1 for ©E and ©G, respectively. The catalytic cycleis therefore exergonic, i.e., in the direction of the formation of 
thermodynamically preferred. Unfortunately, the different
reaction energies for the different steps may vary significantly
and different bottlenecks may occur in the catalytic cycle. Future
investigations of this and similar methane activation cycles must
therefore concentrate on the critical steps.
A detailed description of metadynamics can be found in theliterature[24] or in a previous work[10] wherein we investigatedstep &. Thus, we will present here only a short introduction. In
the metadynamics approach, a set of collective coordinates (CC),which are functions of the atomic coordinates, is selected. EachCC has a corresponding collective variable (CV), which is
coupled to its CC. The main idea is to explore the free energy
surface (FES) only in the phase space of the CVs. The FES isreconstructed by gradually building a potential in the CVs space.
This potential is referred to a history dependent potential in theCVs space. If the potential is converged, it corresponds(approximately) to the FES. The convergence of the potential is avery critical point achieved by sampling multiple reaction events.
The CVs should fulfill the following two criteria: Firstly, theyshould provide a good description of the reaction, i.e., every
characteristic and significant change during the reaction must be
described by the CVs. Secondly, it should be possible to clearly
distinguish with the values of the CVs the different structures andreaction pathways, i.e., in our case  and  must be uniquely
described by their CV values. For different reaction pathways thesame argument holds.

the carbon dioxide becomes more difficult with the angles, i.e.,several decomposition pathways are not easily accessible for theselected angle CVs and this leads to a better and more efficient
exploration of the interesting parts of the FES. Therefore, thecorresponding distances were not selected for the CVs but theangles, because the angles describe the reaction in the same way
but with the advantage of time saving in the computationallyexpensive ab initio molecular dynamics simulations. The timesaving occurs because the regions in the FES corresponding to
the decomposition reactions (side reactions, not of our primaryinterest) need not to be sampled.

(KIWTG  left and center: Atom labeling of molecules  and  forthe definition of the collective variables displayed at a ballCandCstick
picture. right: Snapshot of the simulation box with one molecule /and 59 CO2 molecules.

In the present simulation three angles were selected as CVs,
namely ∠(Pd^C^O1), ∠(Pd^C^O2), and ∠(Pd^C^N), see Figure 3
for atom labeling and the simulation box with / and 59
molecules CO2. For the rearrangement ' the distances r(Pd^O1),r(Pd^O2), and r(Pd^N) do also describe the reaction in a suitable
way and might be a more intuitive choice for the reaction
coordinate in the rearrangement. Unfortunately, the calculations
with the distance CVs revealed additional reaction pathways
during the simulation, namely dissociation of the amide ligand
and carbon dioxide from the ligand. Like the distances, the angles
are able to fulfill the criteria mentioned above in the same way.
The different species are distinguishable by the values of the CVs
and they provide a sufficient description of the reaction. Despite
the usage of the distances, a dissociation of the amide ligand or

A42(KIWTG  Dynamic of the three CCs/CVs and distances. The
verticle lines highlight points which corresponds to conformers of ,
. Top: Dynamic of the three CCs (solid) and the corresponding
CVs (dashed); bottom: Dynamic of the three distances (solid) in themetadynamics simulation.

It is noteworthy, that there is no physical difference between
the ligands coordinated via O1 or O2 to the Pd center. The two
isomers are referred to  and  in the following discussion. The
close resemblance between the selected CVs and the three
distances can be observed in Figure 4, although no restrictions to
the distances were applied during the simulation. The verticallines in Figure 4 denote some conformers of  and  to illustratesome of the reaction/rearrangement events which occurred within
the metadynamics simulation. Multiple events for therearrangement between  and  were observed as seen from thenumerous occurrences of  and  . For step ' only two reaction
events were observed, one from  to  and one from  to  event.
From the angle development (Figure 4) we find that the two Pd^
C^O angles change inversely. While ∠(Pd^C^O1) shows adecrease from ≈ 1 ps to ≈ 3 ps, the ∠(Pd^C^O2) increases. This
inverse behavior is found over the whole simulation time and is aconsequence of the fact, that both angles are connected throughthe planar geometry of the carboxyl group formed by the additionof CO2. Furthermore, at approximately 10 ps we find that ∠(Pd^
C^O1) increases once more, while ∠(Pd^C^O2) drops by morethan 100 degree, and at approximately 16 ps this process reverses.
∠(Pd^C^O1) decreases dramatically while ∠(Pd^C^O2) increases
again. This change can be identified as a Pd^O1 to Pd^O2
bounded complex rearrangement. At approximately 10 ps thesudden angle flip is accompanied by a drastic change in the Pd^
O1 and Pd^O2 distances. The Pd^O1 increases at 10 ps by morethan 200 pm while the Pd^O2 distance decreases by the sameamount. The Pd^O1 and Pd^O2 distances provide the same
inverse relation as the corresponding angles. This shows that adoubly bounded carboxyl group to the Pd center is not stable.
Despite the short simulation time, this instability is supported by
the reconstructed FES and sudden change in the simulations show
a more frequent rearrangement than the Pd^N to Pd^O
rearrangement, the Pd^O1 to Pd^O2 with a lower barrier than thefirst rearrangement.

state (µ65) corresponds to the Pd^O1/Pd^O2 rearrangement
between  and µ. Complex  lies approximately on the red
isovalue surface (F = ^79 kJ mol^1) while the transition state to/µ lies on the gray surface with F = ^60 kJ mol^1. Therefore, theheight of the transition state for step ' amounts to ©F# = 19 kJ
mol^1. The transition state is characterized by the constriction in
the dark gray surface indicating a saddle point. Both parts of thesurface, i.e., the first part surrounding  and the second partsurrounding  and µ, meet at the saddle point, i.e., the transition
state. The activation barrier of approximately 19 kJ mol^1 fromthe metadynamics AIMD simulation is therefore in close
agreement with the static barrier of approximately 22 kJ mol^1.
Dynamic and solvent effects have therefore only little or no
influence on the transition state barrier. The similar behaviour isalso observed on geometries. In Table 2 the angles for both,dynamic and static, transition states are compared.

6CDNG  Comparison of the structures obtained from the staticquantum chemical (QM) calculations and the AIMD simulation
AIMD

Angle[a]PdCCCNPdCCCO1
PdCCCO2

QM

µ





'65





'65

135
110
35

152
39
96

41
123
106

54
145
75

150
36
90

45
110
100

64
135
66

[a] In degree.

(KIWTG  Reconstructed FES of the metadynamic run. The green
points denote the minima and the transition state according to the
FES. The white points stand for the same structures obtained with
QM calculations. The isosurfaces are drawn at the following
values: orange (C141 kJ molC1), blue (C129 kJ molC1), red (C79 kJ
molC1) and gray (C60 kJ molC1).

Additionally, the reagent () and the product ( and µ)angles are listed. For the transition state '65, the difference inangles is about 10 degree. These small differences can be
explained by dynamic effects, steric obstruction from the solvent,and the method for locating the dynamic transition state. In
comparison to the static quantum chemical calculations where thetransition state is perfectly defined and refined by the geometry
optimization procedure, the dynamic transition state isdetermined from the time development of the angles (Figure 4).
The largest slopes of the angle time development were used todefine approximately the transition state. Despite these
difficulties in characterizing a transition state using molecular
dynamics, the characteristics of the static and the dynamictransition state are very similar, energetically as well as
geometrically. From isovalue surfaces, the reaction energy forstep ' is evaluated to be approximately ^62 kJ mol^1.
Determination from static quantum chemical calculations isaround 20 kJ mol^1 higher. This could be due to the different
methodology used in both the cases.

In Figure 5 the reconstructed FES from the simulation ispresented, i.e., its isovalue surface at four different energies. The
coordinates of ,  and of two transition states are mapped on
the FES. The green points indicate the characteristic points
obtained from the AIMD simulation and the white points
correspond to the static quantum chemical calculations. The
transition state of step ' is labeled '65. The second transition

The second transition state for the Pd^O1/Pd^O2
rearrangement is obtained on the blue surface (F = ^128 kJ mol^1).
The products /µ lie approximately on the orange surface (F =^141 kJ mol^1). Therefore, for the second transition state a similar
activation barrier of ©F# = 13 kJ mol^1 is found, in comparison to©G# = 30 kJ mol^1 from static calculations. Both transition statebarriers are approximately 19 and 13 kJ mol^1 high and are
therefore easily accessible at ambient conditions.

A43

Articles
Summary/CTE $TUUGN1) joined the group of
Barbara Kirchner at the University ofLeipzig, Germany, for his PhD studies.

We simulated the Pd@N to Pd@O rearrangement of reaction ' withthe metadynamics approach (Scheme 1). For the simulation weused angles as collective variables instead of distances. The staticquantum chemical calculations and the metadynamics barriers
for this reaction are \G# = 22 and \F# = 19 kJ mol@1,
respectively. Therefore, finite temperature as well as solvent
effects from the surrounding CO2 have almost no influence on theenergy. However, the structural parameters of the transition
states, like distances and angles, change slightly due to the
motion in the simulation. Static and dynamic calculations showthat the energy barrier for the Pd@N to Pd@O rearrangement is\G#/\F# ≈ 20 kJ mol@1 and therefore accessible at ambient
reaction conditions.

Email: marc.bruessel@uniHleipzig.de

4WRCUJTGG 5J[COC 4C[2) is presently
continuing as a Lecturer in the
Department of Chemistry, School ofPhysical Sciences, Ravenshaw
University, Odisha.
Email: rsray@ravenshawuniversity.ac.in-KNKCP /WwK\3) joined the Institute ofChemical Research of Catalonia (ICIQ)
as Group Leader, where he is currently
ICREA Research Professor for
Chemistry.

Zusammenfassung
Wir simulierten die PdN@zu@PdO@Umlagerung von Reaktion 'mittels Metadynamik (Schema 1). Für die Simulationen nutzten
wir Winkel anstelle von Abständen als kollektive Variable. Die
statischer Quantenchemie und Metadynamikbarrieren für diese
Reaktion sind \G# = 22 bzw. \F# = 19 kJ mol@1. Dies zeigt, dass
Temperatur@sowie
LösungsmitteleffektederCO2@
Lösungsmittelmoleküle kaum Einfluss auf die Energiebarrierehaben. Abstände wie Winkel in den Übergangszuständen
verändern sich wenig aufgrund der Bewegung der Moleküle inder Simulation. Die statischen und dynamischen Rechnungenergaben Energiebarrieren für die PdN@zu@PdO@Umlagerung von\G#/\F# ≈ 20 kJ mol@1 und damit eine auch unter milden
Reaktionsbedingungen überwindbare Barriere.

Email: kmuniz@iciq.es

5VGHCP <CJP4) started recently his
postdoctoral research in the groups ofEkaterina Izgorodina and of Douglas
MacFarlane at Monash University.
Email: stefan.zahn@monash.edu

Addresses:
1)

WilhelmHOstwaldHInstitut für Physikalische und Theoretische
Chemie, Universität Leipzig, Linnéstr. 2, DH04103 Leipzig(Germany)

Acknowledgement

2)This work was supported by the DFG, in particular by the project KI
768/6@1, the ERA@Chemistry, and the ESF. Computer time from the RZ
Leipzig was gratefully acknowledged.

Department of Chemistry, School of Physical Sciences,
Ravenshaw University, Cuttack, 753003, Odisha (India)
3)

Institute of Chemical Research of Catalonia (ICIQ), Av. PañȉsosCatalans 16, EH43007 Tarragona (Spain)
4)

School of Chemistry, Monash University, Clayton, Vic 3800
(Australia)
2JKNKRR FK &KQ1) is currently studying
mathematics at the University ofLeipzig and works as a research
assistant in the group of Barbara
Kirchner.

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A45

Articles)N[ECP %CPVKNGXGT #TTC[ 5GPUQTU4 /!" )3   3$/ 84 /. -"
Kathrin Gruber, Bianca A. Hermann, Peter H. Seeberger%arbohydrates are important mediators of many biological processes that
underlie cellular communication and disease mechanisms. Therapeutic agentsinclude carbohydrate@based vaccines and the potent anti@viral protein
Cyanovirin@N (CV@N). CV@N acts by specifically binding the carbohydrate
structures decorating the cell surface of deadly viruses including humanimmunodeficiency virus (HI@V) or Ebola. In search for new carbohydrate – binding
proteins and the development of sensors that exploit carbohydrate – protein
interactions the label@free cantilever array technique can provides a fast, parallel
and low@cost approach.
 )N[ECPU 5VTWEVWTG CPF +PVGTCEVKQPDiverse glycans decorate the cell surface of many organisms,
typically in the form of glycoconjugates such as glycoproteins,proteoglycans and glycolipids.[1] Carbohydrate – protein
interactions regulate cellular interactions with the extracellularenvironment, mediate cell adhesion, signal transduction and help
to organize protein interactions.[2] Specific carbohydrate
structures are expressed on the surfaces of bacteria, viruses,
parasites and cancer cells.[3] The function of many glycans
remains unknown and cell surface carbohydrates possess
underexplored potential as both diagnostic and therapeutictargets.[4] These glycans show promise as a safe, fast and reliablemeans to screen for the presence of cancer cells and pathogens.[3]The glycome is structurally more diverse than both the
genome and proteome, as was revealed by recent advances insequencing technologies.[5] Carbohydrate structure and glycanfunction have to be analyzed in detail both biochemically andmicrobiologically. In recent years, technologies such as glycan
microarrays have been developed to rapidly measurecarbohydrate – protein interactions.[6] However, additional low
cost technologies with fast response times are necessary tocomplement these approaches and identify specific carbohydrate
binding partners to fuel progress in the emerging field offunctional glycomics. Glycan cantilever array sensors offer label^
free detection and short measurement times for the detection ofclinically relevant proteins and pathogens that interact withcarbohydrates.

 %CPVKNGXGT #TTC[ 5GPUQTU
The cantilever sensor technique is based on micrometersilicon levers that are employed in atomic force microscopy

A46(AFM). Cantilever sensors use the lever surface itself for(bio)chemical reactions.[7] The resulting nanometer sized
cantilever deflection is read out via beam deflection or
piezoresistive elements. In principle cantilever sensor setups canbe operated in vacuum, air or liquid. Two typical modes ofoperation are employed. In the dynamic mode, very smallchanges in mass due to material adsorption are detected by thecorresponding change in resonant frequency.[8] In the static mode
the adsorption of the sample induces surface stress, which thecantilever relieves by bending up^ or downwards. This deflection
is tracked in real^time as a function of the amount of material
adsorbed.[9] The technique is highly mass sensitive andconstitutes a powerful, label^free transducer of biomolecularinteractions.[9,10] The glycome and specific carbohydrate – ligand
interactions are still largely neglected since most studies havefocused on DNA and protein interactions.

 5GPUKVKXG CPF 5RGEKHKE 2TQVGKP 4GEQIPKVKQPYKVJ )N[ECP %CPVKNGXGT #TTC[ 5GPUQTU
We describe the development of a carbohydrate based
cantilever array sensor for the sensitive and specific detection ofcarbohydrate binding proteins.[11] To create the sensing layers,
trimannose (=), galactose (?) and nonamannose (J) equipped witha thiol linker (see Figure 1a), were self^assembled on gold coatedcantilever arrays by inserting them in glass capillaries filled withcarbohydrate solutions. Figure 1b schematically depicts
cantilever arrays functionalized with trimannose (red) andgalactose (blue). Protein detection on this glycan cantilever arraysensor was tested with Cyanovirin^N (CV^N), an 11 kDa protein
isolated from cyanobacteria. CV^N has potent anti^vial activity as
it has been demonstrated to bind and inhibit the humanimmunodeficiency virus (HIV).[12] Due to the clinical relevanceof the protein, CV^N was chosen as an ideal candidate to test thefunctionality of the glycan cantilever array sensor.

Figure 1a), to determine the capability of the sensor setup to
resolve these fine differences. Indeed, a distinctively larger
averaged as well as differential signal can be observed fornonamannose J when compared to the trimannose = cantilevers.
The larger nonamannose molecule probably binds to more than
one protein. In some cases it is possible that CV^N attaches via
both binding pockets to the mannose layer. These so^called
multivalent and multisite binding effects, are more likely fornonamannose than for trimannose, and accordingly are presumed
to cause the larger sensor signal. Thus, the glycan cantilever arraysensor is able to discriminate different sugar structures via
smaller and larger sensor signals.

(KIWTG  Sugar Structures and Sensor Coating: a) Trimannose (1),galactose (2) and nonamannose (3) were used to functionalize thesensor surface. b) Schematic of a cantilever sensor array
containing eight individual cantilevers coated with mannose
structures for protein recognition (red) and galactose as reference
channels (blue).

 &GVGEVKPI %[CPQXKTKP0
A typical experiment with a CV^N sample results in negative
deflections for all cantilevers. The averaged signals for thetrimannose and galactose cantilevers of an array as shown in
Figure 1b were calculated and are plotted in the upper panel in
Figure 2. The trimannose signal is considerably larger than thegalactose signal and is attributed to both specific protein – sugar
binding due to the high affinity of CV^N for mannose sugars aswell as to nonspecific protein attachment (total signal). The
smaller galactose signal is attributed to nonspecific binding
effects of CV^N to the carbohydrate structure, the thiol linker orthe cantilever surface (nonspecific signal). In both cases thesignal is generated by the protein attachment on the carbohydrate
covered cantilevers inducing a difference in surface stressbetween the top and bottom side, which is relieved by thecantilever bending.[13] The lower panel in Figure 2 depicts thedifferential signal that is derived by subtracting the nonspecificgalactose signal form the trimannose total signal. The differential
deflection represents the amount of specific CV^N binding to the
cantilever sensor (specific signal).
As different mannose structures vary in their respective
binding strength to certain proteins, an array was functionalizedwith trimannose and additionally with nonamannose layers (see(KIWTG  Detection of the AntiCViral Protein CyanovirinCN (0.1mg/mL; 9.1 WM) with the Glycan Cantilever Array Sensor: Upper
panel: Averaged deflections of the trimannose cantilevers (red) andgalactose cantilevers (blue) of an array as shown in Figure 1b. As
CVCN specifically binds to the mannose sugars, the trimannose
signal is significantly larger. The smaller galactose signal
represents only the nonspecific part of the binding. Lower panel:
Differential signals were calculated by subtracting the galactose
reference signal from the trimannose signal. Thus the resulting
curve represents the specific recognition of CVCN proteins bymannose sugars.

 5GPUKVKXKV[The utility of any sensor critically depends on the detection
limit. Consequently the sensitivity of the glycan array sensor wastested with very low CV^N concentrations. Even after a highsample concentration of 10^2 mg/mL (0.9 µM) was injected into
the measurements chamber, and immediately following
experiment with only 10^6 mg/mL (90.9 pM) resulted in asignificant differential deflection. Both measurement curves (darkA47

Articles
and light green, respectively) are depicted overlaid for better
comparability in figure 3. The inset emphasizes the picomolar
sensitivity that is attributed to the high affinity of the protein to
the trimannose^coated sensors by enlarging the corresponding
section of the graph. Since the relevant CV^N concentrations forbiological applications lie in the nanomolar regime,[14] thepicomolar detection limit demonstrated here more than qualifies
this sensor setup as a valuable tool. The picomolar sensitivity of
the glycan cantilever array sensor exceeds the capabilities ofother cantilever assays that detect protein – protein binding[15]and matches an immunosorbent competition assay that wasdeveloped specifically for CV^N detection in plasma.[14b]where c is the sample concentration and a is a proportionality
constant. This model contains the assumption that individual
protein – sugar complexes on the cantilever surface are
independent and unaffected by neighboring binding events (1:1
binding model). With this analysis for the CV^N – trimannose
interaction an average Kd value of (1.06 µ 0.69) µM wasdetermined. The value for trimannose = compares well to a Kd
value for a CV^N ^ di^mannoside binding published as 1.5 µM.[16]The sensor accurately measures the binding affinity of the CV^N
protein to mannose carbohydrates.
To assess the reproducibility of the individual detection
experiments, measurements on six independent cantilever arrays
were examined. Initial results showed that five out of six signal
sizes agreed within a standard deviation of 30% ^ an acceptable
range for new sensor methods. Over time more experience wasgained in sensor handling and sample preparation to improve theperformance to values below 20%.

(KIWTG  Sensitivity of the CVCN Detection: Two consecutive CVCN
measurements were overlaid for better comparability. First asample of high concentration with 10C2 mg/mL (0.9 WM) shows a
distinctive differential deflection (dark green curve). The
immediately following measurement with a very low concentrationof only 10C6 mg/mL (90.9 pM) still caused a significant differential
signal (light green curve). The inset depicts an enlargement of therelevant section that demonstrates the picomolar sensitivity of thesensor setup. Reprinted with permission from the American
Chemical Society.

 %QPEGPVTCVKQP &GRGPFGPEG CPF &KUUQEKCVKQP%QPUVCPVTo test the applicability of the glycan cantilever array sensor
over a wide range of analyte concentrations, a series of increasing
amount of CV^N sample was conducted. The resulting
differential deflection increases with the corresponding protein
concentration (Figure 4). Concentrations over a range of fiveorders of magnitude can be detected down to picomolar amounts(Figure 3).
For a quantitative evaluation, a Langmuir isotherm was fitted
to the deflections of the concentration series (inset Figure 4).
Thereby, the dissociation constant was determined as a measure
to describe the affinity of protein – carbohydrate binding. The
Langmuir isotherm states:
maximum differential deflection = a x c / (Kd + c),

A48(KIWTG  Concentration Dependence and Dissociation Constant:
A series of seven consecutive measurements with increasing CVCN
concentrations shows increasing sensor signals. Together with thepicomolar sensitivity of the sensor these results demonstrate a
detectable concentration range for CVCN over five orders ofmagnitude. When plotted against the respective sample concenC
tration, the maximum differential deflections of the concentration
series can be fitted against a Langmuir isotherm, see the inset.
The Langmuir analysis results in a value for the dissociation
constant that characterizes the protein – sugar binding and iscomparable to other literature reports. Reprinted with permission
from the American Chemical Society.

 5RGEKHKEKV[
Analyte specificity is a valuable measure indicating how wella sensor differentiates the intended analyte from other agents
possibly present in the sample that might lead to false positive
results. To demonstrate the specificity of the CV^N – mannose
recognition, the binding was challenged by a competitive
inhibition assay. After an initial reference experiment, freemannose (100 mM) was added to the running buffer. For thefollowing injection of a CV^N sample with free mannose present
in the buffer, only about one third of the original deflection size
was observed. As the free mannose competes with mannose
sugars attached to the cantilever surface for binding with CV^N,
fewer proteins bind to the sensor and the resulting deflection is
reduced. The reduced signal size in mannose buffer confirms the
specificity of the CV^N – mannose interaction.

 8GTUCVKNKV[ QH VJG 5GPUQT HQT 2TQVGKP 4GEQIPKVKQP
For general utility, the sensor needs to be able to detect morethan only one protein. Thus, a second carbohydrate binding
protein was analyzed. The generic 104 kDa lectin Concanavalin
A (ConA) consists of four identical monomers with one highaffinity mannose binding site each and is well recognized as
standard protein to probe carbohydrate interactions.[17]layers. In analogy to the CV^N detection, negative deflections for
all carbohydrate^coated cantilevers were recorded. The respective
averaged and differential deflections for a typical ConA
experiment (2 mg/mL; 19.2 µM) are plotted in Figure 5. Both theaveraged and differential responses of the tri^ and nonamannose
functionalized cantilevers are significantly stronger than thegalactose reference cantilevers. As was observed for CV^N
recognition, the averaged and differential signals of thenonamannose sensors were significantly larger than that of thetrimannose cantilevers.
As the proteins attach to the cantilever surface, interactions
on the cantilever surface induce surface stress that leads to theobserved cantilever bending. ConA is more likely to bind to themultivalent nonamannose so that the differences in signal size
can be attributed to these effects. Again, the signals of thegalactose reference are assigned to nonspecific binding togalactose, the thiol linker or the cantilever surface.
As for CV^N, the parameters validating the quality of thesensor setup were examined. The sensitivity, concentration
dependence and specificity of the glycan cantilever array sensor
for ConA binding were verified. For ConA detection,
concentrations down to the nanomolar level (1 µg/mL (9.6 nM))
were detected even in direct succession of a highly concentrated
ConA measurement (10 mg/mL (96.2 µM)). This detection limit
compares well to standard surface bound techniques such as
surface plasmon resonance (SPR), quartz crystal microbalance(QCM) and glycan microarrays.[18] The sensor signal increases
with increasing protein concentrations. Here the Langmuir
isotherm analysis resulted in a Kd value of 15.3 µM in good
agreement with published values.[19] Also, a competitive
inhibition assay verifyed the specificity of ConA – mannosebinding. Finally, an experimental setup aiming at a more realistic
scenario involving complex solutions was examined. For
measurements in the presence of background BSA (0.007
mg/mL; 0.1 µM) in the running buffer, ConA sample
concentrations of 2 mg/mL (19.2 µM) resulted in signal sizes thatcompare well to the signals observed without BSA. These results
independently demonstrate the selectivity of protein binding to
the carbohydrates active layers on the glycan cantilever arraysensor.

(KIWTG  Versatility of the Glycan Cantilever Array Sensor forProtein Detection: A sensor coated with nonamannose, trimannose
and galactose (see inset) was tested for recognition of the lectin
ConA. Upper panel: The averaged deflections of the nonamannose
and trimannose cantilevers are significantly larger and indicate thespecific recognition of the protein. The larger nonamannose signal
is attributed to increased multivalent and multisite binding. In
contrast, the smaller signal of the galactose reference cantilevers
represents the nonspecific part of the binding. Lower panel: The
differential signal represents the specific protein binding and again
is larger for the nonamannose compared to the trimannose sensor.
Reprinted with permission from the American Chemical Society.

For the ConA measurements, a cantilever array was
functionalized with trimannose, nonamannose and galactose

 'ZVGPFKPI VJG 5GPUQT VQ &GVGEV $CEVGTKC
Accurate detection of pathogens and other microorganisms is
crucial for clinical applications and environmental control.[20] As
traditional methods rely on time consuming culturing of bacterial
samples that require from several hours to days,[21] the short
detection times of the cantilever sensor technique provide asignificant advantage. Certain Escherichia coli (E.coli) strains
recognize mannose sugars via the mannose binding protein FimH
at their pilii. Such bacteria can potentially be recognized against agalactose reference by the glycan cantilever array sensor setup, as
is schematically illustrated in Figure 6. The capability of thecantilever sensor to detect different E.coli strains is currently
investigated and first measurements show promising results.

A49

Articles
 %QPENWUKQPU
We describe the development and use of a glycan cantilever
array sensor to sensitively and specifically detect clinically
relevant carbohydrate – protein interactions. This sensor detects
concentrations down to picomolar amounts of CV^N, anoligomannoside binding protein with potent anti^viral activity.
This detection limit matches the best reported results for CV^N
detection with other methods. The glycan cantilever array sensor
is reliable, readily prepared and reusable. This cantilever sensor
setup represents the first carbohydrate ^ protein detection setup
based on the label^free, parallel and fast cantilever techniquedevised for the growing field of glycomics that using theinteraction of the anti^viral protein Cyanovirin^N (CV^N) andoligomannosides as example. Further development of this assay
and expansion to bacterial detection may lead to quick, sensitive
methods to study and detect medically relevant carbohydrate –protein and pathogen interactions.

(KIWTG  Detection of Bacteria with Glycan Cantilever Array
Sensors: Microorganisms specifically recognizing mannose sugars(orange) could be detected against a galactose reference (blue).
First testing with E.coli strains that carry a mannose specificprotein shows promising results.

Summary
Glycans on cell membranes play crucial roles in many cell – cellcommunication processes and the transmission of dangerous
diseases like AIDS or malaria. The protein Cyanovirin@N (CV@N)is able to bind and block nonamannose structures found on thesurface of the human immunodeficiency virus (HIV) and prevents
cell infection. To accurately determine the prophylactic ortherapeutic potency of such proteins, specifically designed
sensors are required. The cantilever array technique provides
advantages like label free detection, short measurement times
and up to eight parallel reference channels. A novel cantilever
array sensor for the detection of carbohydrate – protein
interactions is introduced. Parameters that describe sensor
quality such as sensitivity, concentration dependence of signal
size and specificity of the recognition, are discussed. To this end,a cantilever array was coated with trimannose and nonamannose
sugars to create specific sensing channels. Additional galactose
layers were applied on cantilevers of the same array to act as
reference and determine the amount of nonspecific binding. With
this setup CV@N concentrations could be detected over five ordersof magnitude down to picomolar levels. The sensor differentiates
trimannose from nonamannose coatings via smaller and larger

A50

sensor signals. Finally, the specificity of CV@N – mannosebinding was verified by a competitive inhibition assay. Additional
tests with the generic protein Concanavalin A (ConA)
demonstrate the versatility of this glycan cantilever array sensor
for protein detection. Sensitivity in the nanomolar regime and thedissociation constant are in accordance with literature reports.
The specificity of ConA recognition was independently
demonstrated by competitive inhibition. As certain medically
relevant bacteria also specifically recognize mannose structures,the detection of Escherichia coli (E.coli) is currently investigated.
First results indicate that this sensor setup can be expanded
successfully from protein to bacteria recognition. The describedglycan cantilever array sensor poses a potent and versatile tool
to analyze and detect carbohydrate interactions that may advancedrug design and diagnostic applications.
Zusammenfassung
Kohlenhydrate auf der AuTenseite der Zellmembranen spielen
eine entscheidende Rolle bei der Zellkommunikation und derÜbertragung von gefährlichen Krankheiten wie AIDS oder
Malaria. Das Protein Cyanovirin@N (CV@N) ist in der Lage, die
Nonamannosen an der Oberfläche des Humanimmunodefizienz@
virus (HIV) zu erkennen, zu blockieren und dadurch Infektionen
zu verhindern. Um die prophylaktische und therapeutische
Wirksamkeit solcher Proteine zu untersuchen, sind speziell dafür
entwickelte Sensoren nötig. Die Cantilever@Array@Technik
zeichnet sich dabei durch Detektion ohne Markierung und kurze
Messzeiten mit bis zu acht parallelen, internen Referenzkanälen
aus. Dieser Artikel stellt einen neuartigen Cantilever@Array@
Sensor zur Detektion von Kohlenhydrat@Protein@Interaktionen
vor und beleuchtet die für die Qualität eines Sensors kritischen
Parameter wie Sensitivität, konzentrationsabhängige Signal@
gröTen und Erkennungspezifität. Zu diesem Zweck wurden
einzelne Cantilever eines Array mit Trimannose@ oder
Nonammanose@Zuckern beschichtet, um daraus die spezifischen
Sensorkanäle zu erzeugen. Zusätzliche Galactose@beschichtete
Cantilever desselben Arrays dienen als Referenz und zur
Bestimmung der GröTe von nichtspezifischen Bindungen. Mitdiesem Setup konnten CV@N@Konzentrationen über fünf
GröTenordnungen bis zu pikomolaren Mengen detektiert werden.
Die Sensitivität und die Dissoziationskonstanten der Bindung
stimmen gut mit Literaturberichten überein. Der Sensor
unterscheidet Trimannose@ von Nonamannosebeschichtungendurch unterschiedliche Sensorsignale. SchlieTlich konnte die
Spezifität der (CV@N)@Mannose@Bindung durch kompetitive
Hemmung verifiziert werden. Zusätzliche Tests mit dem Protein
Concanavalin A (ConA) zeigen die vielseitige Anwendbarkeit
dieses Glykan@Cantilever@Array@Sensors zur Proteindetektion.
Sowohl die für dieses Lektin gezeigte Sensitivität im nanomolaren
Bereich als auch die Dissoziationskonstante entsprechen Litera@
turwerten. Die Spezifität der ConA@Erkennung konnte
unabhängig durch kompetitive Hemmung und durch Messungen
im Hintergrund von Rinderserumalbuminen (BSA) gezeigt
werden. Da auch bestimmte, medizinisch relevante Bakterien@
stämme spezifisch Mannosestrukturen erkennen können, wird
gegenwärtig die Detektion von Escherichia coli (E. coli)
untersucht. Erste Resultate weisen darauf hin, dass dieser Aufbau

erfolgreich von Protein@ auf Bakterienerkennung übertragenwerden kann. Folglich stellt der hier beschriebene Glykan@
Cantilever@Array@Sensor ein leistungsfähiges und vielseitiges
Werkzeug dar, um Kohlehydratinteraktionen zu untersuchen und
die Entwicklung von Medikamenten und diagnostische
Anwendungen weiter voranzutreiben.
-CVJTKP )TWDGT 1) is a Ph.D. student at theLudwigHMabimiliansHUniversität München.
Address:[4]a) P. H. Seeberger, D. B. Werz, Nat. Rev. Drug. Discov. ?@@A, 4,
751^763; b) P. H. Seeberger, D. B. Werz, Nature ?@@B, 446, 1046^
1051.

[5]

a) C. R. Bertozzi, L. L. Kiessling, Science ?@@=, 291, 2357^2364;b) O. J. Plante, E. R. Palmacci, P. H. Seeberger, Science ?@@=, 291,
1523^1527.

[6]a) O. Blixt, S. Head, T. Mondala, C. Scanlan, M. E. Huflejt, R.
Alvarez, M. C. Bryan, F. Fazio, D. Calarese, J. Stevens, N. Razi, D.
J. Stevens, J. J. Skehel, I. van Die, D. R. Burton, I. A. Wilson, R.
Cummings, N. Bovin, C.^H. Wong, J. C. Paulson, Proc. Natl. Acad.
Sci. U. S. A. ?@@C, 101, 17033^17038; b) S. Fukui, T. Feizi, C.
Galustian, A. M. Lawson, W. Chai, Nat Biotech ?@@?, 20, 1011^
1017; c) D. M. Ratner, E. W. Adams, M. D. Disney, P. H.
Seeberger, ChemBioChem ?@@C, 5, 1375^1383; d) J. C. Paulson, O.
Blixt, B. E. Collins, Nat. Chem. Biol. ?@@L, 2, 238^248; e) T.
Horlacher, P. H. Seeberger, Chem. Soc. Rev. ?@@D, 37, 1414^1422.

[7]

J. K. Gimzewski, C. Gerber, E. Meyer, R. R. Schlittler, Chem.
Phys. Lett. =>>C, 217, 589^594.

[8]

K. M. Goeders, J. S. Colton, L. A. Bottomley, Chem. Rev. ?@@D,
108, 522^542.

[9]

H. P. Lang, M. Hegner, C. Gerber, Mater. Today ?@@A, 8, 30^36.

Email: kathrin.gruber@wmi.badw.de$KCPEC # *GTOCPP 1) is group leader at
the LudwigHMabimiliansHUniversität
München.
Email: b.hermann@cens.de[10] P. S. Waggoner, H. G. Craighead, Lab Chip ?@@B, 7, 1238^1255.
2)

2GVGT * 5GGDGTIGT is director at the Mab
Planck Institute of Colloids and Interfaces
and professor at the Freie Universität Berlin.
EHmail: peter.seeberger@mpikg.mpg.de

Adresses:1)

WaltherHMeissnerHInstitute, LudwigHMabimilians University
München, WaltherHMeissnerHStr. 8, 85748 Garching, Germany
2)

MPI Colloids and Interfaces, Department of Biomolecular
Systems, Arnimallee 22, 141w5 Berlin, Germany
Project description

Acknowledgment

This projects aims to probe hierarchical selfHassemblies that are
relevant for drug and vaccine design based on the ebample ofcarbohydrate  CyanovirinHN interactions. Combining different
areas of ebpertise in a transnational collaboration, new insights
and methods regarding carbohydrate chemistry and novel sensing
techniques were targeted.
We acknowledge the generous support provided by the ERAH
Chemistry program, the Nanosystems Initiative Munich (NIM), theCenter for Nano Science (CeNS), the elite network of Bavaria, theWaltherHMeissnerHInstitute of the Bavarian Academy of Scienceand gumanities, the Swiss National Fonds (SNF) and the MabH
Planck Society.
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[15] a) M. Yue, J. C. Stachowiak, H. Lin, R. Datar, R. Cote, A.
Majumdar, Nano Lett. ?@@D, 8, 520^524; b) N. Backmann, C.
Zahnd, F. Huber, A. Bietsch, A. Plückthun, H.^P. Lang, H.^J.
Güntherodt, M. Hegner, C. Gerber, Proc. Natl. Acad. Sci. U. S. A.
?@@A, 102, 14587^14592.
[16] P.^H. Liang, S.^K. Wang, C.^H. Wong, J. Am. Chem. Soc. ?@@B,
129, 11177^11184.
[17] J. L. Wang, B. A. Cunningham, G. M. Edelman, Proceedings of
the National Academy of Sciences =>B=, 68, 1130^1134.
[18] S. Park, M.^r. Lee, S.^J. Pyo, I. Shin, J. Am. Chem. Soc. ?@@C, 126,4812^4819.
[19] Y. Zhang, S. Luo, Y. Tang, L. Yu, K.^Y. Hou, J.^P. Cheng, X.
Zeng, P. G. Wang, Anal. Chem. ?@@L, 78, 2001^2008.
[20] a) K. R. Buchapudi, X. Huang, X. Yang, H.^F. Ji, T. Thundat,
Analyst ?@==, 136, 1539^1556; b) M. Zourob, S. Elwary, A. P. F.
Turner, Vol. 333II, ?@@D, p. 970.
[21] M. D. Disney, P. H. Seeberger, Chem. Biol. ?@@C, 11, 1701^1707.

A51

Articles#FXCPEGF #PCN[VKEU*$"9MN /$4"9H
$"   7  H!$  Helene Faber, Sandra Jahn, Jens Künnemeyer, Hannah Simon, Daniel Melles, Martin
Vogel, Uwe Karst'lectrochemistry/liquid chromatography/mass spectrometry is a powerfulcomplementary tool for the simulation of the oxidative metabolism of drugs andother xenobiotics.
&TWI &GXGNQROGPV CPF 1ZKFCVKXG /GVCDQNKUOAlthough the combination of the two groups of analyticaltechniques electrochemistry (EC) and mass spectrometry (MS)has not gathered massive scientific or commercial interest yet, it
has a surprisingly long history. However, while it was initiallyonly used to investigate the gases formed in an electrochemical
reactor using MS with a membrane inlet, the number and kind of
its applications has expanded significantly in recent years.
Currently, EC/MS is on the brink of becoming a widespread tool
for metabolism studies. The reasons for this as well as thepotential and the limitations of the respective approaches arepresented within this paper.
In pharmaceutical industries, the fate of an active substance in the
body has to be investigated at an early state during drug
development to gather information on its distribution, metabolism
and excretion. Metabolites have to be identified and quantified inhuman body fluids, as some metabolites may be the cause foradverse reactions of the drug.[1] The oxidative metabolism of
drugs, which mainly takes place in the liver due to oxidation
reactions catalyzed by enzymes of the cytochrome P450 group, isgenerally considered as a detoxification mechanism of theorganism. As the formed functionalized (phase I) metabolites
typically are more polar than the parent drug, their watersolubility is increased and they are easily excreted via the kidneys
and the urine. However, some metabolites, e.g., dehydrogenation
products as quinoid compounds, which may be formed fromcatechols, are less polar than the parent drugs and may react bycovalent linkage to functional groups in the body. Quinoids, forexample, are susceptible to attach thiol groups of small molecules
as the amino acid cysteine or the tripeptide glutathione.[2] These
reactions are used to detoxify reactive metabolites, because thehighly polar products are again prone to rapid excretion from thebody. Most frequently, glutathione adducts of reactive
metabolites are formed as phase II metabolites. If theconcentration of reactive metabolites exceeds the available
concentration of thiols in the liver, covalent attachment of thereactive metabolites to free thiol groups of liver proteins may take

A52

place.[3] This may cause a limitation in protein functionality andthus liver toxicity may be observed.

%QPXGPVKQPCN /GVJQFU
For this reason and related toxicological aspects, pharmaceuticalindustries as well as pesticide manufacturers or producers of foodadditives and consumer products have great interest to investigate
new chemical entities with respect to their metabolism in the
body. A large number of testing systems has been developed forthis purpose, including in vitro assays based on isolated enzymesof the cytochrome P450 group, perfused liver, liver slices or liver
cell microsomes.[4] All of these methods are laborious and time
consuming and/or depend on the availability of biologicalmaterials in reproducible quality. In all of these cases, the formed
products are extracted from the biological matrices and aresubsequently analyzed by liquid chromatography/mass
spectrometry (LC/MS). However, reactive metabolites formed inthese experiments frequently experience covalent binding tobiological macromolecules and are therefore not detected in
LC/MS[5] in most cases. Additionally, EU regulations prohibit theuse of animal testing to assess the toxicity of constituents ofparticular groups of products, e.g., cosmetics.[6] Therefore, there
is a strong demand for alternative methods, which are less prone
to differ in their biological variability and which are easy to use
and well reproducible. For these reasons, these methods should
ideally be based on purely instrumental approaches, although thecomparability of the results with the biological reality has to beinvestigated carefully.

'NGEVTQEJGOKECN 5GVWR
The combination of EC and MS, sometimes assisted by a LC
separation prior to MS analysis, has proven strong potential forthe simulation of the oxidative metabolism of xenobiotics inrecent years.[7^10] The set^up of a respective instrument ispresented in figure 1. A solution of the compound of interest,typically as a 1^100 µM solution in a suitable aqueous/organic
solvent, is transported through the electrochemical cell by asyringe pump. The potential of the cell is controlled by apotentiostat. The formed products are either directly infused into
the mass spectrometer to record mass voltammograms (seebelow) or are loaded into the sample loop of an injection valve. In
this case, the valve is switched subsequently to start thechromatographic run with mass spectrometric detection.

(KIWTG  Schematic setCup of the EC/LC/MS system for simulationof phase I metabolism. In case of EC/LC/MS (upper pathway), theeffluent of the electrochemical cell is collected in a sample loop
mounted on an injection valve. By switching the valve, thereactions products are flushed onto the LC column, separated andanalyzed by ESICMS. In case of EC/MS (lower pathway), theeffluent from the electrochemical cell is directly transferred to the
ESICMS.

Two different types of electrochemical cells are currently used
for EC/MS experiments. While both consist of a three electrodearrangement with working, counter and reference electrode, they
are distinguished by different working electrode materials, sizes
and geometries. Large volume porous flow^through electrodes are
suitable for comparably large flow rates of up to several hundredµL/min with high conversion rates ("coulometric" cells). The
large surface area also accounts for the limited influence ofelectrode fouling and limited requirement for maintenance.
Traditionally, they are available with glassy carbon (GC) as
surface material. Thin layer cells, on the other hand, are more
flexible with respect to the exchange of working electrodematerials, as not only GC, but also boron doped diamond (BDD)
as well as metals (Pt, Au, Ag, Cu) are available. Furthermore,
maintenance is easy, as the electrode surfaces can be accessed,
polished and exchanged rapidly. While earlier EC/MS
measurements were mainly performed on GC electrodes, BDD isgaining interest due to the higher potential range, which may
allow to purposely generate radicals in the cell. While the appliedcounter electrodes in these electrochemical cells consist of
stainless steel or Teflon doped with graphite, the referenceelectrodes typically are Pd/H2 systems, which are depending on
the pH of the solution of the xenobiotic compound. Therefore, toassure reproducibility of the method, EC/MS data should always
be accompanied with information about the composition and thepH of the solution investigated.
/CUU 5RGEVTQOGVTKE &GVGEVKQP KP '%/5
Requirements for mass spectrometers suitable for EC/MS include
excellent full scan sensitivity and the possibility for structure
elucidation based on fragmentation, as unknown compounds haveto be identified. For this reason, ion trap instruments typically are
more useful in EC/MS than quadrupole mass spectrometers.
Another important aspect is rapid switching between the positive
and the negative ion modes, as there may be some metabolites,
which are favourably or even exclusively detectable in thepositive ion mode, while others formed at the same time are
preferably analyzed in the negative ion mode. Time of flight
instruments or orbitrap mass analyzers add high resolution andhigh mass accuracy, adding the possibility to assign molecular
formulae only from mass spectrometric experiments. Hybrid
quadrupole^time of flight, ion trap^orbitrap or the newquadrupole^orbitrap instruments combine excellent full scansensitivity, recording of accurate masses and structure elucidation
by fragmentation and are therefore ideally suited for EC/MS.
Regarding ionization techniques, electrospray ionization^mass
spectrometry (ESI^MS) has been used most frequently due to thecomparably high polarity of most compounds of interest.
Typically, [M+H]+ or [M^H]^ are the most abundant ions in thepositive and the negative ion mode, respectively, while adductions with ammonium, sodium or potassium (positive ion mode)or formate, acetate or chloride (negative ion mode) are frequently
observed as well. However, atmospheric pressure chemical
ionization (APCI) and atmospheric pressure photoionization(APPI) are attractive alternatives in case of less polar analytes.
If liquid chromatography is integrated between EC and MS, thereversed^phase separation mode is mostly used. For more polar
drugs or metabolites, hydrophilic interaction liquid chromato^graphy (HILIC) on zwitterionic phases is one suitable alternative.
Carbon^based stationary phases are also showing promisingretention properties for highly polar drugs and their metabolites.
In general, the LC separation should be carried out with thefastest possible separation times to minimize the use of theexpensive MS detection system per analysis and to increasesample throughput.

1RVKOK\KPI VJG 'NGEVTQEJGOKECN 1ZKFCVKQPThe basic approach in EC/MS consists of recording a mass
voltammogram, a plot of the mass spectrum depending onvarying working potentials. Over a time period of between fiveand 30 minutes, the electrochemical potential is constantly
increased and the mass spectrum is recorded continuously. The
signal^to^noise ratio may be improved by longer recording times,
although at the expense of analysis time. A typical mass
voltammogram for paracetamol (acetaminophen, APAP) over asmall mass range from m/z 148 to m/z 155 is presented infigure 2. While the [M+H]+ signal of the parent drug isdecreasing, the reaction product N^acetyl^p^benzoquinoneimine(NAPQI) is formed with increasing potential at its respective m/z
ratio.
The mass voltammogram may vary depending on solvent
composition, pH value and electrode material. Therefore, it
should either be recorded at varying conditions or under carefully
selected conditions. The mass voltammogram allows to find the

A53

Articles
most suitable cell potential for further EC/LC/MS or adductformation experiments.
In literature, many pharmaceuticals and other xenobiotics havealready been investigated using this approach in comparison toestablished techniques based on liver cell microsomes, animal(KIWTG  Electrochemical oxidation of paracetamol (110C5M in
NH4formate 110C2M, pH 7.4/ACN 70/30 v/v). The mass
voltammogram was recorded as described above, using a thinC
layer cell equipped with a boron doped diamond working electrode.

experiments or studies with patients. Despite the different
mechanisms associated with the instrumental and the enzymatic
approaches, a surprisingly good agreement has been found inmany cases, thus proving the usefulness of the approach formetabolism screening purposes.

)NWVCVJKQPG #FFWEV (QTOCVKQP QH 4GCEVKXG /GVCDQNKVGUThe EC/MS approach can also be expanded to phase II(conjugation) experiments by adding a solution of a trapping
agent (cysteine, glutathione or a protein) after the electrochemicalcell. Reactive metabolites are converted into products, which canbe detected mass spectrometrically. The experiment using small
thiols as trapping agents can be performed in a simple approach,where a solution of the thiol is continuously added to the effluent
from the electrochemical cell and is then directly transferred tothe mass spectrometer. An extract of the respective mass
voltammogram of the addition of the reactive metabolite ofparacetamol (NAPQI) with glutathione (GSH) in the m/z range
from 440 to 500 is presented in figure 3. It is obvious that thesignal intensity of the [M+H]+ and the [M+Na]+ signals of the
NAPQI^GSH adduct as well as the respective 13C isotope patterns
are increasing above a potential of 1000mV vs. Pd/H2 reference.
A comparison with the data from figure 2, where the signal of
APAP decreases at the same potential due to formation of thereactive metabolite NAPQI shows that these data correlate well.

A54(KIWTG  Electrochemical oxidation of paracetamol (210C5M) andtrapping with glutathione (110C4M). The potential was ramped from
0C2500mV in 250s and glutathione was added to the effluent fromthe EC cell via a mixing TCpiece and a second syringe pump.
Conjugation products were monitored with an ESICToFCMS in thepositive ion mode.

(QTOCVKQP QH 2TQVGKP #FFWEVU HTQO 'NGEVTQIGPGTCVGF
4GCEVKXG /GVCDQNKVGUFor trapping experiments with proteins additional measures haveto be taken: Due to the comparably high salt content of many
proteins and the resulting signal suppression in ESI^MS, a LC
separation has to be inserted after protein adduct formation withthe goal to obtain some retention of the protein adducts, while
inorganic and small organic salts, which are prone to cause ion
suppression, will elute within the void volume of the system. Thisapproach drastically minimizes suppression effects and increases
the signal to noise ratio for the protein adducts. Protein adductformation of reactive metabolites can best be tested with the
18kDa milk protein β^lactoglobulin A. Although this protein is oflittle relevance for toxicological studies, it is an excellent model
protein due to a reasonable size and structural homogeneity. It
can therefore be detected very well in ESI^MS, contains only one
free and well accessible thiol group and has shown excellent
reactivity with reactive metabolites, particularly quinoid
compounds.[11] While the proteins hemoglobin (from rat or
human origin) or human serum albumin (HSA) are more relevantwith respect to toxicological studies, they are more difficult toanalyze due to different subunits as the α^ and β^chains ofhemoglobin or the significantly larger size (approximately
66kDa) and structural heterogeneity of HSA. Therefore, it isrecommended to always start studies on protein adduct formation
with β^lactoglobulin A and to subsequently use more relevant
proteins in case the initial studies prove a reactivity of theelectrochemically generated metabolites. Figure 4 shows the
adductformationofelectrogenerated
NAPQI
with
β^lactoglobulin A. While the upper data show the ESI^MS of the(KIWTG  Adduct formation of electrochemical generated NAPQI with βCLGA. A solution of APAP (110C4M) was oxidized at a constantpotential (1500mV) and the effluent of the EC cell was collected in a vial containing βCLGA (110C5M). The mixture was injected onto the
LC/MS system and analyzed by ToFCMS. Blue: unmodified βCLGA, red: βCLGA after reaction with NAPQI.

native protein with the raw spectrum with multiply charged ionson the left and the deconvoluted spectrum on the right, the lower
data show the respective situation after addition ofelectrogenerated NAPQI. The deconvoluted data clearly showthat there is a high conversion yield of the protein with one
molecule of the reactive metabolite. With the high resolution/highmass accuracy MS used in this work, the observed massdifference can unambiguously be assigned to one molecule of thereactive metabolite.
Protein adduct formation with HSA or hemoglobin will lead tomore complex mass spectra, but also to more relevant
information with respect to possible protein binding in the plasmaor the red blood cells. Future work should concentrate on theattachment to other physiologically relevant proteins, e.g., thosemost frequently found in liver cells, to use this system for theprediction of liver toxicity.

%QORCTKUQP VQ 'UVCDNKUJGF /GVJQFUComparison of the data obtained by EC/MS with establishedapproaches is very important: As liver cell microsome
experiments are established tools for the simulation of theoxidative metabolism of xenobiotics, they should always be
carried out in parallel with the EC/MS studies. Most literaturestudies confirm a significant degree of agreement between bothmethods despite the different oxidation mechanisms.[12] However,there are aspects, where EC/MS provides important additional
information as in case of reactive metabolites, which are typicallynot detected at all in liver cell microsome experiments due tocovalent protein binding, which prevents them from being
extracted from the biological matrix and thus from the subsequent
LC/MS analysis. The structures of the products formed uponoxidation of amodiaquine (AQ) in electrochemical or microsomal
incubation are shown in figure 5, while a comparison of theformed products is presented in table 1. An excellent correlation

A55

Articles(KIWTG  Overview of possible oxidation products and metabolites derived from electrochemical oxidation and microsomal incubations.
For detailed information regarding their occurrence see Table 1.

at the qualitative scale is typically observed for N^dealkylation
reactions (see figure 5 and table 1), while the reactive metabolites
formed in the liver cell microsomes can only be detectedindirectly via their glutathione adducts, when glutathione is addedin excess concentrations as trapping agent.

6CDNG  Comparison of Oxidation Products/Metabolites derived
from Electrochemical Oxidation and In vitro Incubations with Rat
Liver Microsomesm/zMetabolite

356
354
328
326
300
299

AQ
AQQI
DESAQ
DESAQQI
bisCDESAQ
AQCAldehyde
AQQIC
Aldehyde
AQCNCoxide
AQQICNC
oxide

297
372
370
[a]

Electrochemical

Microsomal

Oxidation

Incubation

An important issue in drug metabolism is the unambiguous
identification of the formed metabolites. Nuclear magnetic
resonance (NMR) studies provide the most valuable information
for this purpose but do, unfortunately, require significant amountsof the generated metabolites. Due to this reason, theelectrochemical synthesis of the metabolites using larger
synthesis cells with similar functions as described above foranalytical use has been introduced. The metabolites are collected
after flow^through synthesis and are subsequently purified by(semi)preparative LC using stationary phases of the same kind,
but with larger dimensions than for analytical use.[9]

[a]

[a]





only detectable after trapping with glutathione

A56

2TGRCTCVKXG 5ECNG 'NGEVTQIGPGTCVKQP QH /GVCDQNKVGU

3WCPVKHKECVKQP QH 4GCEVKXG /GVCDQNKVGU
Quantification of the electrogenerated metabolites can be
performed with external or internal calibration in case that therespective pure compounds (external calibration) or
isotope^labelled analogues (internal calibration) are available.
While external calibration is well possible for the solutions used
for electrochemical metabolite generation due to their low matrixload, quantification in body fluids will require internal standards,
which are very difficult to obtain. Therefore, an alternative
approach based on inductively coupled plasma^massspectrometry (ICP^MS) is needed. Base of this method is the factthat in the ICP, all compounds are atomized and then ionized anddetected, so that the signal intensity of a detectable ion is only
dependent on its concentration, but not on its chemical nature orionization properties as in case of ESI^MS. However, theapproach is best suited for the electropositive elements, which
can most easily be ionized. While the analysis of metals leads toexcellent limits of detection down to the subnanomolar
concentration range even after separation, elements as
phosphorous, sulfur, chlorine, bromine or iodine can still be
detected with reasonable limits of detection.[13^15] Quantification
can be carried out with any standard of known concentration anda retention time in LC, which differs from that of the metabolites.
The electrochemically generated metabolites are separated
therefore by LC and subsequently detected by ICP^MS, and thesignal intensity of any peak at an element trace is directly
proportional to the concentration of the respective analyte. It
should be noted, however, that the signal is only independent on
the nature of the analyte, when identical plasma conditions areapplied. Therefore, isocratic elution in LC is preferable overgradient elution, as the change in mobile phase composition,
particularly a varying aqueous/organic content, will lead tovarying plasma conditions. For this reason, complex separations
that have to be carried out under gradient elution conditions
should be accompanied by a post^column countergradientapproach to adjust the solvent composition to a constant value.

(WVWTG 6TGPFU
Due to the strong development of this field within the last few
years, it is expected that there will be important method andapplication development in the upcoming years. This should also
include technical solutions for an improved possibility forscreening of larger numbers of compounds in less time. It can be
expected that these methods will not only find more widespreaduse in pharmaceutical analysis, but also in the food/beverages andchemical industries.
One major challenge associated with the rapid and efficient use of
the EC/MS technology is the removal of the starting materials
and the oxidation products from the electrode surfaces. This isparticularly valid in those cases where substances areelectropolymerized at the surfaces, thus leading to products,
which are only sparingly soluble in the solution of the xenobiotic
compound. Different approaches have been selected to overcome
this problem: Rinsing with solvents, which are known to dissolve
the products well, is an obvious solution, although it is not always
efficient. The use of a strong oxidizer as 0.5M nitric acid is more
efficient, but the electrochemical cells and all parts of the
instrumentation, which are contacted with the acid, have to bechecked previously with respect to their stability under these
conditions. A promising future approach is the application of
microfluidic chip^based electrochemical cells, as the respective
electrode surfaces and geometrical dimensions can be prepared ina very flexible way and as these cells are suited for single use
provided they can be produced at low prices in mass production.
In this case, the cells can simply be discarded after single use, andmemory effects from the cells will therefore not be an issue
anymore. Odijk et al. have recently described the design and theapplications[16,17] of such a microfluidic chip in detail.
Summary
The on@line combination of the three analytical techniques
electrochemistry (EC), liquid chromatography (LC) and mass
spectrometry (MS) has been introduced in recent years as a newtool for the simulation of the oxidative metabolism of drugs andother xenobiotics. The purely instrumental approach is useful forthe prediction of many phase I and phase II metabolic reactions,
including the adduct formation with small biogenic thiols as
glutathione or even thiol@containing proteins. While the different
mechanisms taking place in electrochemical andenzyme@catalyzed metabolic reactions do not allow a complete
simulation of the biological situation by the instrumental
approach, many reactions observed in vivo are observed by
EC/LC/MS as well. Particular advantages are found for the
instrumental system in case of reactive metabolites, which can beobserved directly when using this approach. Other advantages
are the possibility to synthesize small quantities of themetabolites in an electrochemical cell with the goal to allow theirfurther characterization by NMR spectroscopy and to generatecalibration standards of the metabolites. For selected
heteroatom@containing metabolites, quantification may be carriedout without a substance@specific internal standard by inductively
coupled plasma@mass spectrometry (ICP@MS) after LC separation.

Zusammenfassung
Die on@line@Kombination der drei analytischen Techniken
Elektrochemie (EC), Flüssigchromatographie (LC) und
Massenspektrometrie (MS) wurde in den letzten Jahren als neues
Verfahren zur Simulation des oxidativen Metabolismus vonPharmazeutika und anderen 3enobiotika eingeführt. Die rein
instrumentelle Methode ermöglicht die Vorhersage vieler
Phase I@ und Phase II@Metaboliten. Hierunter fällt auch die
Bildung von Addukten aus elektrochemisch generierten
Metaboliten mit kleinen biogenen Thiolen und Proteinen mit
freien Thiolgruppen. Obwohl eine vollständige Simulation der im
Organismus entstehenden Metaboliten bereits aufgrund derverschiedenen Oxidationsmechanismen nicht möglich sein kann,werden experimentell überraschend viele Übereinstimmungenzwischen den mit der EC/LC/MS erhaltenen Daten und solchen
aus biologischen Systemen gefunden. Besondere Vorteile weist
das instrumentell@analytische Verfahren für reaktive Metabolite
auf, da diese direkt nachgewiesen werden können. Ein weiterer
Vorteil ist die Möglichkeit zur elektrochemischen Herstellung
kleiner Substanzmengen der Metabolite für die weitergehende
Charakterisierung mittels der NMR@Spektroskopie und als
Kalibrationsstandards. Für heteroatomhaltige Metaboliten kanneine Quantifizierung ohne substanzspezifische Standards mit der
induktiv gekoppelten Plasma@Massenspektrometrie (ICP@MS)
nach flüssigchromatographischer Trennung durchgeführt werden.

A57

Articles
Prof. Dr. Uwe Karst 1)
is professor at the Westfälische WilhelmsH
Universität Münster at the Institute ofInorganic and Analytical Chemistry
EHmail: uk@uniHmuenster.degelene Faber, Sandra [ahn, gannah Simon and Daniel Melles
are Ph.D. students in the research group of Prof. Karst at the
University of Münster (pictures in the name order).
Dr. [ens Künnemeyer (left) is
postdoctoral research
associate and Dr.Martin Vogel(right) is staff scientist in the
research group of Prof. Karst.

1)

Address: Corrensstr. 30, 4814w Münster

Project description

Acknowledgment

The project LCyElectrochemistryyMass Spectrometry in(Bio)Analytical Chemistry, Drug Metabolism and Proteomics was
funded by the DFG (Bonn, Germany) under registration number
KA 10w3y7H1. A parallel project with the principal investigators
Dr. Wouter OlthuisyProf. Dr. Albert van den Berg (University ofTwente, The Netherlands) and Dr. Andries BruinsyProf. Dr. Rainer
Bischoff (University of Groningen, The Netherlands) was fundedby the Dutch Technology Foundation STW. The present article
highlights one major aspect of the project. The authors of thisarticle would like to thank the DFG for financial support.

A58

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!  $          %$      &               ! !   "  ' '((       $ $     $$)'* !            %     $ % $        !   ! !  %  " 
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! ! )+*   ! ! ),* !  "  )-*    ).* $!   ! !)/*  $     !       $         $   $    !   !    $)0* # $  %    %   
   123                 ! %    !   45 !      3  $#$$ 6  ! 67- 123        %    &8   8)9*     123   %  ! !     $ % % ! !      $ $): '(* 123  8 !    8          !    $  $   )''#'-*    !                $ 123   ! 8    ! ! ; $    <7=$   77 >   

 

Magnetic nanoparticles (MNPs) made from magnetite(Fe3O4) have been studied for biomedical applications.
Magnetitebased MNPs are virtually nontoxic and biocompatible.
They can act as magnetic single domain crystals below a criticalsize $ (i.e., a diameter of 25 nm) and exhibit superparamagnetic
behavior at room temperature.[15] Superparamagnetic NPs can bemagnetized under an external magnetic field (), reaching amaximum moment (  ! "  ) when the field
is strong enough. Their net magnetic moment is randomized tozero without an external magnetic field and therefore no
remanent magnetic moment exists. We have synthesized
monodisperse (average size is 10 nm) magnetite MNPs cappedwith oleic acid according to literature.[16] A typical transmission
electron microscopy (TEM) image of such MNPs is shown in Fig.
1.

    TEM image of oleic acid capped e34 magnetic
nanoparticles. Scale bar: 50 nm.   Colloidal solution of 
NaY4 (0% Yb, % Er) upconversion luminescent nanoparticles
under daylight ( ) and under 80 nm laser excitation (  ).

A59

Articles 


Nearinfrared (NIR)tovisible upconv
version luminescent
nanoparticles (UCLNPs) display the uniqueuproperty of
converting NIR light (with wavelengths of ty
ypically 800 to 1,000nm) into visible luminescence.[17] It mainly occursowith rareearth
doped solids and relies on the sequential absorptionaof two ormore NIR photons by the dopants. Three mainmmechanisms are
discussed, viz. excited state absorptionn, energy transfer
upconversion, and photon avalanche processes.[18] Hexagonal
NaYF4 nanocrystals doped with lanthanide ions are particularly
efficient in terms of upconversion. The upconverted
luminescence can be varied in terms of coolor and intensity byproper doping. For example, UCLNPs doped
d with Yb3+/Er3+ emit
mainly green light (510 to 570 nm) and red liight (630 to 680 nm).
The green emission is dominant in fluorideebased lattices (e.g.
NaYF4), while oxidebased lattices (suchh as Y2O3) display
predominantly red emission. Dopants such as Tm3+ result in blue
upconversion luminescence (450 to 500 nm) that is accompanied
by a weak red luminescence in certain host materials. The
transitions and energy transfers between Yb3+ acting as theg as the activators is
sensitizer, and the ions Er3+ and Tm3+ acting
illustrated in Fig. 2.

solvent.[19] A TEM image of NaYF4 (20% Yb, 2% Er)
nanoparticles and the respective R
RD pattern are shown in Fig. 3.

intensity / a.u.

       θ angle    TEM image of oleic acidcappedaNaY4 (0% Yb,
% Er) nanoparticles. Scale bar: 100 nm. The average size of the
UCLNPs is 30 nm.   XRD patte
ern of hexagonal NaY4 (0%Yb, % Er).

         ! "
Based on the experience withw
the magnetic and theupconverting nanoparticles deescribed before, we havesynthesized a new type of nanoscaaled hybrid core/shell particles.
These offer magnetic response andd upconversion luminescence atthe same time. Superparamagnetiic magnetite nanocrystals withan average size of 10 nm and capped with oleic acid were used asseed crystals. A layer of hexagonaal NaYF4 doped with Yb3+ andEr3+ was deposited on their surfacee as shown in Fig. 4.

   Energy transfer and upconversion emission mechanisms
in a NaY4 nanocrystal doped with Yb3+, Er3+, and Tm3+ under 80nm excitation. The dasheddotted, dotted, curly, and full arrows
refer to photon excitation, energy transfer, mulltiphoton relaxation,and upconversion emission, respectively. Reproduced from ref.
[17], ig , with kind permission from Springerr Science+BusinessMedia.

   Schematic representationn of magnetic UCLNPs,
consisting of a magnetite core and a shell of hexagonal NaY4
doped with 0% of Yb3+ and % of Er
E 3+ ions.

Compared to organic fluorophores and semiconductor
nanocrystals, UCLNPs offer high photochem
mical stability, sharp
emission bandwidths, and large antiStokes shifts (of up to 500nm) that separate discrete emission peakss from the infrared
excitation. The absence of visible autofluoreescence in biological
specimens under NIR light excitation alsoo is notable. Thesebenefits make UCLNPs ideal tools for use aas luminescent labelsand probes. We are using lanthanidedoped hexagonal NaYF4nanoparticles that can be prepared,  in esseence  according to a
known method where oleic acid is used as a kind of surfactant

Such a core/shell architecture represents an elegant way to
combine the magnetic properties of the MNPs with theupconversion capability of the UCLNPs.
U
Figure 5 shows two
pictures that illustrate the prroperties of such core/shell
nanoparticles. The particles were collected at a distinct spot byapplying an external magnetic field, as can be seen in Fig. 5A.
The visible luminescence on excittation with a 980 nm (30 mWcw) laser is displayed in Fig. 55B. The emission spectra ofupconversion luminescent nano
oparticles with and withoutmagnetic core do not show any sign
nificant differences.

A60   3   < ! CE  # &     !   $ $ D    "   $!$
F  8  " G $ C  !  ! !   B$ D $  "     C    
4  $ =# %!  >!$ $
!    !  $  & & 8 $  A! $     &      !     $> !   HB ! $ " $"&"  <  >!!  $  #  3    :9(# #< $  D 8   =8   !
     B$ $ 3 "" ! $
&% $     # Pictures of upconverting nanoparticles with a magnetic
core (UCLMNPs) in hexane. () Under daylight; () Under 80 nm
laser excitation. Also shown is the permanent magnet in the back() and the spot of the UCLMNPs collected (). () Emission
spectrum of the UCLMNPs under photoexcitation at 80 nm using
a 30 mW laser.

$ % & '!                     ! "#  

"$
"$
 !
!
%
% 

"
"
&& ""We are currently investigating on how magnetic upconverting
luminescent nanoparticles can be modified at their surface, forexample by click chemistries,[20] with the final goal to immobilize
molecular probes and biorecognition elements thereon. We also
note that the luminescence of such particles strongly depends on
temperature in the physiological range which suggests their use in
the determination of temperature, for example in hyperthermal
treatment of cancer. We expect the resulting magnetic UCLNPs
to offer a potential that is comparable to that of magnetic
quantum dots, for example as imaging probes for microRNA,[21]and for stem cell labeling,[22] or of magnetic nanoparticles
covered with fluorescent conjugated polymers.[23] 
   $  !  $ 8   %!  "  '( %  $  $      !8        % !     !  6 #$$  ! 67-     !    % ! $ !       %       !   ! ! ?  @ $    ! !              8 !      !     $        
!   $ 8     !    !$ !  :9(  $ $  8  !        '  ( ) '      (   * +  ,  *# !    -* +   . -"      ! *       * &  / * 01232  ! * 4 -4 5     +   *        6   * 722 4 5*   8  '+  '  964 ' 8 :(&;;0<-&    . 
=    $  '   '!  '    '  $ '      4 5    +   " ( $   '      ' '  '   $     '   '!           $       $5      !  $ >' ' "[1]

J. Park, J. Joo, S.G. Kwon, Y. Jang, T. Hyeon, >!8 2   ,'':, 47144754 >!8 2   4 $ , -/, 46304660.

[2]

T. Tanaka, T. Matsunaga, >  2   , 0+, 35183522.



[3]

W. S. Seo, J. H. Lee, . Sun, Y. Suzuki, D. Mann, . Liu, M.
Terashima, P. C. Yang, M. V. McConnell, D. G. Nishimura, H. Dai,    , ., 971976.

 &  !  7,- $    <   #!AB % '(  8$   C   &   !" $   D   6 #$  67- " !
<  D   A!     >&% ! %  [4]

. Hong, . Gao, L. Jiang, I >  2    , '+:, 1478
1479.

A61

Articles[14] L. H. Fischer, G. S. Harms, O. S. Wolfbeis, >!8 2    ,'+,, 46404645 >!8 2   4 $  , 50, 20, 4546–4551.

[5]

M. Kettering, J. Winter, M. eisberger, S. BremerStreck, H.
Oehring, C. Bergemann, C. Alexiou, R. Hergt, K. J. Halbhuber, W.
A. Kaiser, I. Hilger,   ! , '9, 175101.

[6]

M. hang, S. Shi, J. Meng, . Wang, H. Fan, Y. hu, . Wang, Y.
Qian, I   2   2 , ''+, 28252830.

[15] K. M. Krishnan, A. B. Pakhomov, Y. Bao, P. Blomqvist, Y. Chun,
M. Gonzales, K. Griffin, . Ji, B. K. Roberts, I    ,-', 793815.

[7]

W. W. Yu, E. Chang, C. M. Sayes, R. Drezek, V. L. Colvin,  ! , '0, 44834487.

[16] W. W. Yu, J. C. Falkner, C. T. Yavuz, V. L. Colvin, 2  
2  ", 23062307.

[8]

H. S. Mader, P. Kele, S. M. Saleh, O. S. Wolfbeis, 2 2   G    , '-, 582596.

[17] D. E. Achatz, A. Reham, O. S. Wolfbeis,   2 2    ,,((, 2950.

[9]C. Liu, H. Wang, . Li, D. Chen, I  2   !, ':, 3546
3556.

[18] F. Auzel, 2   5% ", '(-, 139173.

[10] F. Wang. Y. Han, C. S. Lim, Y. Lu, J. Wang, J. u, H. Chen, C.
hang, M. Hong, . Liu,   , -/,, 10611065.
[11] D. E. Achatz, R. J. Meier, L. H. Fischer, O. S. Wolfbeis, >!8
2    , '+,, 274277 >!8 2   4 $  , .(, 260
263.
[12] R. Ali, T. Lang, S. M. Saleh, R. J. Meier, O. S. Wolfbeis, > 
2    , 9+, 28462851.
[13] H. H. Gorris, R. Ali, S. M. Saleh, O. S. Wolfbeis, >$%   ,+,, 16521655.

A62[19] . Li, Y. hang, S. Jiang, >$%  , +(, 47654769.
[20] H. S. Mader, M. Link, D. E. Achatz, K. Uhlmann, . Li, O. S.
Wolfbeis, 2  # I  , '/, 54165424.
[21] D. W. Hwang, I. C. Song, D. S. Lee, S. Kim,    , /, 81–88.
[22] L. Wang, K. G. Neoh, E. T. Kang, B. Shuter, S. C. Wang,
G      , ,', 35023511.
[23] P. Howes, M. Green, A. Bowers, D. Parker, G. Varma, M.
Kallumadil, M. Hughes, A. Warley, A. Brain, R. Botnar, I > 
2     , ',+, 9833–9839

                                                 !"# $   ! % %" &   '        (   %   )  *"+       &  ,       ,   %  &  ) ,  % %        % %     %   &        



Almost any rubber that is used for reallife applications is ahighly complex, heterogeneous material consisting of a cross
linked polymer matrix (an elastomer) and significant amounts ofinorganic, often nanometersized particles such as carbon blackor silica.[13] Without such fillers, rubber materials would notexhibit the favourable and fascinating combination of properties
such as elasticity, large deformability, high toughness, durability(abrasion resistance) and, in particular when tire applications arein the focus, traction on wet or icy roads and low viscous losses(low rolling resistence). In particular the optimization of the latter
while not compromising the other relevant properties, related tothe development of green tires for lower fuel consumption, is afield of active industrial development that is characterized by alack of rational design principles and an indepth theoreticalunderstanding.
Some aspects of the complex mechanical behaviour of filled
elastomers are highlighted in Fig. 1, where in the sketch inFig. 1a it is emphasized that the spatial distribution of thenanoparticles plays a key role. In most applicationrelevant cases,
the particles themselves form a network with a complex structural
hierarchy over many decades in length scale,[1,3] starting atprimary particles on the fewnm range, via aggregates andagglomerates, finally forming a macroscopically percolated
structure that can transmit mechanical load. To illustrate its effect,
Fig. 1b shows the storage modulus (100 Hz) from frequencydependent shear experiments of the industrially relevant tire
material SBR (styrenebutadiene rubber) as a function of tempe
rature.[4] The large drop in  over 3 decades marks the glasstransition of the matrix at around 15 C. While in the ensuing
rubber plateau region, unfilled SBR exhibits the expected positive
temperature dependence as expected from the entropic models of
rubber elasticity, filled SBR has a much higher modulus whichusually   
significantly on heating. This is emphasized byplotting the reinforcement factor +(.) = ’filled/’unfilled[5] atconstant temperature difference to the glass transition shown in
the inset, which is seen to be highest at temperatures around andabove the glassrubber transition. While below, the low
reinforcement can be explained on the basis of simply
considering the additive effect of a certain volume of highmoduls material, values exceeding + = 10 indicate the importantsynergistic effect of a filler network. Importantly, the decrease of+ at even higher temperatures indicates nontrivial interfacial
effects of the polymerfiller system that are in the focus of ourresearch initiative.
Based on such observations, some of us[5] have previously
developed a model based on the existence of a glassy layer ofimmobilized polymer material on the particle surface that
constitutes a sticker between the particles and simply softens at
higher temperature.[6,7] Other important features of filled rubbersthat must ultimately be captured by a quantitative model are thewellknown dramatic decrease of + in the range of higher
deformations, the socalled Payne effect that can be attributed toa mechanical breakdown of the filler network, and history andhysteresis effects under conditions of cyclic mechanical load, so
called Mullins effects, related to the evolution of structural com
plexity.[2,3]Preliminary NMR experiments on a welldispersed model
system have indeed provided direct evidence of an interphaseconsisting of immobilized polymer.[8] These studies provided thestarting point for the present work, where we seek tosystematically study such interphase fractions and their impact on
the macroscopic properties. Notably, in systems that are closer toactual applications, having much more inhomogeneous particle
dispersions, interphases have as yet not been observed directly.
Rather, the existence of glassy layers was for the case of carbon
black indirectly inferred from socalled boundrubber extraction
or dielectric experiments,[13,9] or from fits of mechanical models
to rheological data.[3] Direct studies by advanced NMR methods
have as yet neither revealed significant amounts of

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once means that structural inhomogeneity, i.e., the coexistence of
lowly and highly crosslinked regions, is directly reflected in thedata. For the given method, it is possible to extract the )  of the RDC, which directly reflects the distribution in
local crosslink densities. The data in Fig. 1c is typical in that,first, vulcanized elastomers were commonly found to be very
homogeneous, and second, that fillers of any type hardly affectthis homogeneity. The only significant filler effect turned out tobe the observation that the crosslink density in a filled system isalways somewhat lower that that of the unfilled counterpart,which is straightforwardly attributed to a partial deactivation of
the vulcanization system by the highsurface filler.
The conclusion is clear: if effects of glassy layers or locallymodified crosslink density should be responsible for themechanical properties (Fig. 1b), their volume fraction must bevery low, i.e. below the limit of NMR detectability on the percent
level. If can of course be imagined that in aggregated/agglomera
ted systems, the internal surface is much lower as compared toisolated nanofillers, such that even small amounts of interphasematerial close to the particles can affect large changes of the total
system. The important question to be resolved in our work is now
to establish a link between tunable model systems, which as
shown below do exhibit the mentioned features of glassy layers
and inhomogeneous rubber matrix, and the systems of application
relevance.

               !   "  (   (a) Inhomogeneous nanoparticle distribution in a filledrubber for, e.g., tire applications. Tire image by courtesy ofContinental AG. (b) Storage moduls ’ at a shear frequency of
100 Hz of unfilled vs. silicafilled SBR as a function of tempe
rature. The inset shows the reinforcement  = ’filled/’unfilled as afunction of temperature difference to the glass transition. Data
from ref. [4]. (c) Distributions of crosslink density, as measured interms of the RDC taken from NMR experiments, for pure NR and
NR filled with silica or carbon black.[10]glassy layer, nor did they give any indication of substantialchanges in the network matrix. Corresponding results from theHalle lab in collaboration with the ICTPCSIC in Madrid areshown in Fig. 1c.
In a broad study comprising a wide range of traditional andmodern nanofillers in commercially relevant rubber materials,
NR (natural rubber) and SBR, revealed  substantial effect ofany filler on the actual rubber matrix.[10] One of the NMR
methods used, namely 1H multiplequantum (MQ) NMR,[11] isable to provide information on the crosslink density νc of thematerials based on network chain mobility. Its result is the socalled residual dipolar coupling (RDC) constant associated withthe protons in each monomer. Generally, the RDC is directly
proportional to the crosslink density, or to the number ofelastically active network chains of weight /c in the sample,
which in turn determines the modulus. Thus, RDC ∝ ’ ∝νc ∝ 1//c. The feature that 1H NMR detects all monomers at

A64 #   '"  $   !% &" Industrial samples are simply made by strong mechanicalmixing of all the constituents: solid particles, grafting agent andsurfactant, polymer, crosslinker, and various catalysts. The spatial
arrangement of the nanoparticles is far from equilibrium anddepends crucially on the energy and time of mixing. Indeed,
during the mixing, it is known that the molecular weights of thechains decrease, probably in a different manner for the polymer
adsorbed at the particle surface and in the bulk. Thus themechanical properties are relatively sensitive to mixing
conditions. In addition, it is also known that the time elapsed
between mixing and crosslinking (curing by, e.g. vulcanisation)
has an influence on the mechanical properties because of slow
rearrangements of the nanoparticles. Lastly, the surface agents –grafters and surfactants – are also known to modify the arrange
ment of the nanoparticles in the samples. This complexity has
unfortunately hindered the understanding of the physics of thesesystems. Thus, it is crucial – in order to get quantitative andcomprehensive experimental results – both to synthesize modelnanocomposites with extremely wellcontrolled arrangement andto be able to characterize their nanoparticles arrangements. The
characterization is rather simple if we are able to use sphericalnearly polydisperse nanoparticles. The most versatile particles forthat are silica particles that are easy to prepare, and to modify

chemically. Moreover, they exhibit a natural contrast against thepolymer for neutron scattering experiments.[12]The trick we have been using in recent years was to prepare
solutions of colloidal silica particles in a solvent consisting of themonomers with crosslinker and eventually additional solvent –and to polymerize and crosslink the monomer controlling thecolloidal stability during the whole process. This was quite
simple in the case of polyacrylates – specifically poly(ethyl
acrylate), PEA – and has allowed us to get systems with nearly
crystalline arrangement of nanoparticles.[12] Thanks to thesesamples, we were thus the first to put into evidence the effect ofgradient of glass transition temperature in nanocomposites[6] aswell as the role of particles arrangement on mechanicalproperties.[13] The next challenge is to use polymers relevant in
the industrial word – poly(styrenecobutadiene), SBR, or natural
rubber, NR, for instance – which are more convenient for large
strain tests than polyacrylates. The latter are in fact quite fragile
mechanically. In the case of SBR and NR, however, it is rather
difficult to polymerize the monomer in the presence of particles.
The project will thus consist in mimicking the industrial mixing
process grafting and/or adsorbing polymer chains and dispersing
them afterwards in the polymer matrix &#"         )           * + ( In previous NMR work, we have developed quantitative
approaches to the determination of the phase composition in
dynamically heterogeneous materials, based on simple 1H low
field spectrometers.[14] Generally, the distinction is based upon
the orientation dependence of interproton dipolar couplings andtheir potential averaging due to fast rotational motions of themolecules. Thus, immobile regions associated with strong dipolar
couplings are characterized by quickly decaying time domain
signals (broad lines in the frequency domain spectra), whilemore mobile components are only subject to weak or almost
negligible RDC, leading to slower signal decay. Using a set ofspinecho based magnetization filters, signals belonging todifferently mobile components can be measured separately,
allowing for a determination of fitting parameters for simple free
induction decay (FID) data. With such an approach, we have
performed an indepth study of the phases that can be identified
in the PEAbased model systems described above. Parts of theseresults have been published recently,[15] and we here summarize
these and some more current findings.
Fig. 2a shows a typical FID from a highly filled PEA network,where three components can be identified: glassy (fully rigid),strongly immobilized (intermediate: only local, highly
anisotropic smallamplitude mobility) and mobile (network
chains and free chains. The total amount of interphase material(glassy + intermediate) is seen to reach up to 20% of the overall
polymer signal. Detailed temperaturedependent studies (Fig. 2b)
provided a direct proof the hypothesis on the softening ofpotential glassy bridges between fillers that, according to ourmodel,[5] may explain the temperaturedependent decrease of(   (a) Lowfield 1H NMR freeinduction decay data reflecting
quantitatively interphases of reduced mobility in PEAbased
nanocomposites. (b) Temperature dependence of the three distin
guishable fit components, demonstrating an apparently increased
glass transition temperature of the interphase material. (c) Relationof the amount of interphase material with the known inner surface(total surface area per volume) of different samples for two cases ofdifferent surface interaction. See ref. [15].

the reinforcement. A notable difference was found between thematerials that are just characterized by silica particle surface thatprovide physical adsorption sites for the polymer, and those thathave a high density of chemical surface grafting sites (Fig. 2c).
We stress that even in the former systems, significant amounts ofimmobilized polymer could be evidenced.
Currently, we are analyzing data from NMR spin diffusion
experiments,[14] which are able to provide estimates of the
thickness of the different domains and their spatial arrangement.
First results are consistent with estimates based on the data in
Fig. 2 that indicate a total interphase thickness of 5–6 nm at thelowest temperatures, as simply estimated from the determinedvolume fractions and the known internal surfaces. This corrects
our previous lower, less quantitative results.[8] Analyzing indepth
the spin diffusion across the three distinguishable phases, we

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have indications that the region of intermediate mobility does not
simply form a homogenous layer around the glassy shell, but isinhomogeneously distributed.

Finally, we used the 1H MQ NMR method to probe thehomogeneity of the mobile network phase in these systems. The
results are summarized in terms of RDC ( crosslink density)
distributions in Fig. 3, and they differ significantly in some
aspects from previous findings on commercially relevant systems,
refs. [4,10] and Fig. 1c. First, the PEA networks appear much
more inhomogeneous (note the log scale), which is attributed tothe more complex spin system (side chain with lower RDC) andinhomogeneities intrinsic to the crosslinking process in thissystem. So even though the systems nicely exhibit average NMRdetected crosslink densities that follow the trend expected forincreasing crosslinker concentrations (Fig. 3a), the intrinsic
width of the distributions poses limitations to a fully detailed
analysis. This point will be improved upon by way of the up
coming model composites based on NR, with their high intrinsic
homogeneity. However, the findings for the filled PEA systems
are also rather clear already: while the fillers with only physicalbonds to the polymer phase (Fig. 3b) lead to no detectablechanges of the rubber matrix, in analogy to our previous work
(Fig. 1c), the fillers with dense grafts (Fig. 3c) significantly affectthe matrix in that its crosslink density is generally increased andappears more inhomogeneous. The latter effect is seen in terms ofbroader peaks and shoulders on the highcrosslink side of the
distribution functions, with even a trend to bimodality for the
samples with the highest internal surface.
These results give a convincing demonstration of the changesof the polymer matrix as arising from welldispersed highsurface
nanofillers. Ongoing mechanical studies on these and the newmodel systems, and in particular on systems with controlled
nanoparticle aggregation states, in combination with more refinedcomputer simulations, will help in elucidating the relevance ofpolymer interphases with modified properties in general.
, &#" -    

A second, important aspect of the NMR part of the project isconcerned with MQ NMR on deformed (stretched or compressed)
samples, giving access to the orientation distribution andstretching states of the network chains. In this way, we seek toaddress the question whether in filled systems there is aninhomogeneous local strain distribution that may play a role indetermining the unusual mechanical properties (highreinforcement, Payne and Mullins effects). In unfilled elastomers
quite far above .g, the effect of uniaxial strain on the distributionof residual NMR interactions (related to local strain at the scaleof network chains) has been analyzed within the confines ofstandard rubber elasticity theories and the assumption of affine
deformation.[16] In this case, the degree of anisotropy induced
upon stretching is related directly to the average residualinteraction measured in the relaxed state. In filled elastomers, afirst evidence for a local strain distribution was obtained.[17] The
distribution of local strain (at a given macroscopic strain) was
shown to be related to the filler morphology and dispersion state.
MQ NMR will give quantitative access to the distribution of
local chain stretching (or equivalently local strain) at a given
macroscopic strain, as regards both the magnitude and theorientation (local 3D effects). To discriminate magnitude andorientation effects on the distribution of residual couplings, theorientation of the stretching axis with respect to B0 is varied. The
angular variation gives access to orientation effect. Then, by
combining experiments done at many different orientations, aneffective powder spectrum can be reconstructed. Preliminary
work has shown that the proposed analysis is indeed feasible.
This analysis of the residual NMR interactions (MQ signal) canthen be done as a function of the macroscopic strain imposed tothe sample. We will then correlate the obtained distributions ofresidual NMR interactions in stretched samples to the tuned
surface interactions and to the related heterogeneities of elastic
modulus and the local (gradient of) mobility within the matrix.

(  , RDC distributions reflecting local crosslink density
distributions as derived from 1H lowfield MQ NMR experiments,
based on data from ref. [15]. (a) PEA networks with varying
amount of crosslinker given in wt%. (b,c) PEA networks with
0.3% crosslinker and varying amounts of welldispersed silica
spheres that are modified so as to provide (b) just favourable
adsorptive interactions or (c) a high density of chemical bonds(grafts) between the particles and the elastomer.

A66

.      $$ 

As described above, elastomers filled with carbon black orsilica particles have a shear modulus much (up to a few 100
times) higher than that of the pure elastomer and exhibit a highdissipative efficiency. Another important feature is their nonlinear behaviour. When submitted to deformations with amplitudes γ of the order of a few percent or more, the elastic modulus
'(ω) decreases down to values much smaller than the value inthe linear regime: this is the socalled Payne effect. During
subsequent deformations, the elastic modulus in the linear regime
is smaller than that of the initial system, but recovers
progressively (at least partially) to the initial value: this is the socalled Mullins effect. Recently, some of us have proposed amesoscale model that explains these basic features.[18] The modelis based on the presence of a glassy layer around the fillers when
the interaction between the matrix and the fillers is sufficiently
strong,[7,8,13] see Fig. 2 for our experimental quantification of thiseffect. We have proposed that the mechanical properties of nano
filled elastomers are governed by the kinetics of rupture and re
birth of glassy bridges which link neighbouring nanoparticles andfrom large rigid clusters of finite lifetimes. These lifetimes
depend on various parameters such as temperature, nanoparticle
matrix interactions, and distance between neighbouring fillers.
Most importantly, these lifetimes depend on the deformation
history of the samples. We have shown that the unusual nonlinear and plastic behaviour of these systems can be predicted bythis death and rebirth process.
A major challenge of the physics of filled elastomers is thatthe relevant space and time scales are very large compared tomolecular scales. Thus, they are totally inaccessible to numericalsimulations at the molecular scale, by many orders of magnitude.
One must thus devise mesoscale models. Since numericalsimulations cannot cover more than 67 decades in time, it isnevertheless an impossible task to address all the issues
mentioned above in a single picture. Depending on the issues ofinterest, one must primarily focus on some particular aspect ofthe physical behaviour and use a coarsegrained description of thesystem. Two different scales are of particular interest. (i) A scaleof about ten nanometers, corresponding to confined polymers
layers, e.g. between two fillers, or two thin films deposited on asubstrate, and (ii) the scale of a few hundreds of nanometers,
corresponding to the size of filler aggregates. The latter bridges
the gap between the former and the macroscopic continuous scale.






ζ






0

(  . Schematics of a film at a temperature  > g. In (a), themechanical probe is at a distance z from the substrate larger than
the size ξ of the slow aggregates. The slow aggregates can movefreely around it, and the viscosity is smaller than the viscosity at g.
In (b), the mechanical probe is closer to the substrate. Moving
parallel to the film requires deforming slow aggregates: theviscosity is larger than that at the bulk g.

An essential feature of polymers dynamics close to the glasstransition is that it is strongly heterogeneous on a scale of a few

nanometers, typically 3 nm.[19] Some of us have proposed thatthese dynamical heterogeneities correspond to density fluctu
ations and that the macroscopic dynamics is controlled by theslowest percolating subunits.[20,21] This socalled "Percolation ofFree Volume Distribution" (PFVD) model [22] allows to explain:(a) the heterogeneous nature of the dynamics, (b) the violation of
the Stokes law observed for small probes, (c) essential features ofageing and rejuvenation, and (d) the shift of glass transition atinterfaces. Fig. 4 illustrates the implications on the local
mechanical properties of this films.
The predicted glass transition temperature at a distance z
from an interface is described by⎛ ⎛ β ⎞1 /ν ⎞. ( 0 ) ≈ . ⎜1 + ⎜ ⎟ ⎟ ,⎜ ⎝0⎠ ⎟⎝⎠

(1)

where Tg is the bulk glass transition temperature of the purerubber. The exponent ν ≅ 0.88 is the critical exponent for thecorrelation length in 3D percolation. The value of the length βdepends on the polymersubstrate interactions. For strong inter
actions, it is of the order 1 nm. In its current development themodel does not incorporate the description of the mechanicalbehaviour explicitly, for instance for calculating a storage and adissipative modulus. It also does not include the effect of
imposed deformation on the dynamical state (e.g. plastic
deformation of polymers). One aim of this project will be toextend the PFVD model in order to describe and calculate the(linear and nonlinear) mechanical properties of confinedpolymers. We will thus be able to describe the evolution of aconfined polymer layer under imposed deformations with a 23
nm scale resolution and on time scales spanning from 10 ns to
104 s typically. For this purpose, we will develop a 3D model,
which we will solve by numerical simulations. In this 3D model,
the basic units will be the subunits (3 nm) of dynamical
heterogeneities. Their dynamical evolution (aging/rejuvenating)will be coupled to the stress field which results from the imposed
deformations. We will thus describe the stress field at thenanometer scale (23 nm), as well as the strain field, and the
dynamical state of each of the subunits.
The PFVD model was used by Berriot et al.[5–8] for explaining
the microscopic origin of the reinforcement in filled elastomers,
as a consequence of the presence of a gradient of glass transition
temperature around the fillers, as described by Eq. (1) andillustrated in Fig. 5. This provided the link between thin films
dynamics (a few tens of nanometers) and the physical behaviourof filled elastomers. This equivalence between thin film dynamics
and filled elastomer properties was subsequently stressed byexperimental[23] and simulation[24] studies. In the presence of a
local stress σ, we assume that the glass transition temperature in
between two fillers is given by⎛ ⎛ β ⎞1/ν ⎞ σ. ( 0 , σ ) ≈ . ⎜1 + ⎜ ⎟ ⎟ −⎜ ⎝0⎠ ⎟ ⎝⎠

(2)

The first term in the right hand side of Eq. (2) represents theeffect of the fillermatrix interactions. The second term is thedecrease of .g due to the local stress, which is the plasticizing

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effect of an applied stress.  depends on the polymer. Thisparameter is known from macroscopic experiments. It relates theyield stress σy to the temperature . and the polymer glasstransition temperature .g by σy = (.g – .), and is typically of theorder 106 Pa/K.

 
)

100 nmWhen glassy layers overlap, the macroscopic shear modulus
' is related to the shear modulus of the glassy polymer 'gthrough geometrical effects. Indeed, a macroscopic deformation εis amplified locally in between the fillers by an amplification
factor λ, which is the ratio between the diameter of the fillers andthe distance between two neighboring fillers. In a plane normal tothe direction of elongation, the stress is supported by glassy
bridges which represent an area fraction Σ 1. Both parameters λand Σ depend on the considered systems. The macroscopic
modulus is thus given by(3)

Assuming that the fillers are spherical particles of 10 nm diameter with typical interparticle distance of a few nanometers, we
deduce that λ is of order a few units and Σ is of order a few 102,
depending on the ratio between the glassy layer thickness and thenearest neighbor distance. Since 'g 109 Pa, we obtain amacroscopic shear modulus of about 108 Pa, which corresponds
to very strong reinforcement. Consider a macroscopic
deformation ε of a few percent. The local stress σ is then of orderσ λε'g 108 Pa. With  106 Pa.K1, a local .g reduction of
100 K is obtained. Therefore, glassy bridges yield, which results
in a lowering of the shear modulus, for macroscopic deformationsof order a few percent.
The glassy bridges are not permanent, rather, they breakunder applied strain. Within a glassy bridge in between two
neighbouring particles, at equilibrium, we assume that thepolymer has locally the dominant relaxation time τα given by theWilliamLandelFerry (WLF) law of the corresponding polymer,
modified by the .g shift due to interfacial effects and the local
stress σ. We assume that the breaking times of glassy bridges are

A68⎛ τ ( 0, σ ) ⎞⎛ τ (. − . ( 0, σ ) ) ⎞1 (. − . ( 0, σ ) ) (4)⎟ = log⎜ 2 ⎟=− 1log⎜ α
⎜ τ
⎟
⎜
⎟
τ
1


2 + (. − . ( 0 , σ ) )⎝⎠⎝⎠Here, τg = 100 s (the relaxation time at .g) and . is thetemperature. C1 and C2 are the WLF parameters of the consideredpolymer.

(  / The macroscopic stress in a filled elastomer is supportedby the glassy polymer fraction which bridges two neighboring filler
aggregates. Aggregates of about 100 nm made of primary particlesof 10 nm are schematized. They are surrounded by a glassy layer
which is sketched. The fraction of glassy polymer in a section
normal to the applied stress is Σ ~ 1%. The macroscopic
deformation is amplified between the fillers by a factor typically l ~10, which results in a macroscopic modulus G´ of the order 107 to
108 Pa.

 ′ ≈  ′ λ Σ .

comparable to the local dominant relaxation times τα of theglassy bridges. The breaking time is thus given by

Eq. (4) gives the equilibrium value of the breaking time,
which is obtained when the distance z, the local stress σ and thetemperature . have been maintained fixed for a long time. In
general, the breaking time depends on the history of the glassy
bridge and is denoted by τα(). We assume that, at any time, a
glassy bridge has a probability for breaking per unit time, d/d,
given byd
(5)d = ατ α ( )where α is a number of order 1, but smaller than 1. When aglassy bridge breaks, the local stress σ is relaxed and drops to amuch smaller value, which is the rubbery contribution.
Immediately after breaking, we assume that τα relaxes to a value
τ min γ −1 , where γ denotes the local deformation rate. In
practice, typical deformation rates in our simulations are of order1γ ≈ 0.1 s . The local breaking time τα() undergoes a subsequentevolution, analogous to an ageing process. Thus the evolution of
the breaking time of a glassy bridge, τα(), is given byif∂τ α ( )
=1
∂

(6)

τ α ( ) ≤ τ 2  (. − . ( 0 , σ ) ) .

(7)

By definition, we set the time τα to be bounded by the timeτWLF(. – .g(z,σ)) given by Eq. (4). Eqs. (5), (6) and (7) describe
the evolution of the local breaking times and lead to very
complex behavior. When simulated in the presence of a few
thousand beads, the evolution of these relaxation times deter
mines the mechanical behavior of filled elastomers, enabling to
explain or predict very specific nonlinear behaviours of filled
elastomers.[7], 0   
The research project of the DINaFil consortium aims at an in
depth understanding of the physical properties of filled
elastomers. As highlighted in the artcile, it combines (i) thesynthesis of wellcontrolled model systems, employing concepts
from surface chemistry and colloid science, (ii) the application ofadvanced NMR techniques for the quantitative study ofimmobilized polymer phases and fillerinduced inhomogeneities
in the rubber matrix, and (iii) the molecular characterization of
local deformations in samples studied under strain. These studies
are supplemented by dielectric and mechanical spectroscopy. A
major goal consists in feeding the experimental insights into
multiscale computer simulations in order to test and improvepredictive theoretical models of filledelastomer behavior.

So far, we have been able to quantify exactly the temperature
dependent content of polymer involved in glassy layers
between the filler and the changes in the elastomer matrix inpreviously established model systems based upon poly(ethyl
acrylate), and the studies are currently extended to other polymers.
Our NMR techniques proved applicable to the study of strained
samples, from which we are currently deducing the distribution of
local strain. With the recent improvements in our theoreticalmodel that is based on the temperature and yielding behavior ofglassy bridges between the filler particles, we are confident thatthe project will contribute significantly to the development ofrationaldesign approaches of filled elastomer materials.

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 6     
,  !.   $    #  4      6  $         ,    * % 2= / 6- 
6   G6 H  B 6 2    /6/   5 46 2    6 ,   F$- 6+. I6 +J 8 6  -  !#[1]

J. E. Mark, B. Erman, F. R. Eirich, .    .  %+ )) , Elsevier, Amsterdam !""#.

[2]

M.J. Wang, + ))  1 - . - $%%&, >?, 520589.

A69

Articles[3]

M. Klüppel, *,- - . !""', ?@A, 186.

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A. Mujtaba, M. Keller, S. Ilisch, H.J Radusch, T. ThurnAlbrecht,
K. Saalw chter, M. Beiner,  & &-[5]

H. Montes, F. Lequeux, J. Berriot, /  81078118.

!""', B@,

[14] M. Mauri, Y. Thomann, H. Schneider, K. Saalw chter,  " - /- + -!""&, BA, 125141.
[15] A. Papon, K. Saalw chter, K. Sch ler, L. Guy, F. Lequeux, H.
Montes, /   !"$$, AA, 913922.
[16] P. Sotta, /  $%%& B? 38723879.

[6]

J. Berriot, H. Mont s, F. Lequeux, D. Long, P. Sotta, L. Monnerie,
9 & - . !""', @A, 50.

[17] S. Dupres, D. Long, P.A. Albouy, P. Sotta, /   , !""%,
42, 2634–2644.

[7]

J. Berriot, H. Mont s, F. Lequeux, D. Long, P. Sotta, /
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[18] S. Merabia, P. Sotta, D. R. Long, /  
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J. Berriot, F. Lequeux, H. Montes, L. Monnerie, D. Long, P. Sotta,
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[9]J. G. Meier, J. W. Mani, M. Klüppel,  - + ,- 5 !""(, >C,054202.

[10] J. L. Valentn, I. MoraBarrantes, J. CarreteroGonzlez, M. A.
LpezManchado, P. Sotta, D. R. Long, K. Saalw chter,/   !"$", AB, 334346.
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G. Meier, H. Frielinghaus,   !""', AA, 49094919.
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A70!""&, A?, 8252

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[24] V. Pryamitsyn, V. Ganesan, /  !""*, BG, 844856.

Fachkollegienwahl
2011

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