Letterpubs.acs.org/JPCL

Probing Interfacial Water on Nanodiamonds in Colloidal Dispersion
Tristan Petit,*,† Hayato Yuzawa,‡ Masanari Nagasaka,‡ Ryoko Yamanoi,§ Eiji Osawa,§ Nobuhiro Kosugi,‡
and Emad F. Aziz†,‡,∥†

Institute of Methods for Materials Development, Helmholtz-Zentrum Berlin, Albert-Einstein-Strasse 15, 12489 Berlin, Germany
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan§
Nanocarbon Research Institute, Shinshu University, Ueda, Nagano 386-8567, Japan∥
Freie Universität Berlin, FB Physik, Arnimallee 14, 14195 Berlin, Germany‡

S Supporting Information*

ABSTRACT: The structure of interfacial water layers around nanoparticles dispersed
in an aqueous environment may have a signiﬁcant impact on their reactivity and on
their interaction with biological species. Using transmission soft X-ray absorption
spectroscopy in liquid, we demonstrate that the unoccupied electronic states of oxygen
atoms from water molecules in aqueous colloidal dispersions of nanodiamonds have adiﬀerent signature than bulk water. X-ray absorption spectroscopy can thus probe
interfacial water molecules in colloidal dispersions. The impacts of nanodiamond surface chemistry and concentration oninterfacial water electronic signature are discussed.

T

structure than bulk water.1 TEY-XAS can though hardly beapplied to NDs colloidal dispersions because of the small
penetration length of electrons, which makes the method
extremely surface sensitive. We previously characterized theelectronic structure of NDs in aqueous dispersion by
ﬂuorescence yield (FY) XAS and XES.25 Although this previous
study provided new insights on the inﬂuence of aqueous
dispersion on the surface chemistry of NDs, the waterorganization around NDs could not be probed because FYXAS measurements are sensitive to saturation eﬀects, which
complicates the interpretation of XA water spectra,18 and thesensitivity of XES to the hydrogen bond network was not
suﬃcient to reliably estimate the water structure around NDs.25
In this communication, aqueous dispersions of NDs with
diﬀerent sizes and surface chemistries are characterized by XAS
in pure transmission mode. Transmission measurements are
particularly relevant because they do not suﬀer from saturation
artifacts of FY measurements and are sensitive to the wholevolume exposed to X-rays, which is adapted to nanoparticle
dispersions. Experiments were performed at the UVSOR-III
synchrotron facility using a ﬂow cell described elsewhere.26
Brieﬂy, the liquid sample ﬂows between two 100 nm-thicksilicon nitride membranes and is exposed to the soft X-ray
beam as shown on Figure 1. The thickness of the probed liquidvolume can be controlled down to 20 nm by modifying thehelium pressure in the chamber to have an optimal absorbance.
The transmitted X-rays are then detected with a photodiode.
Detonation ND aqueous dispersions produced by Plasmachem (NDs-PC, 3−5 nm), International Technology Center(NDs-ITC, 5 nm), and NanoCarbon Research Institute (NDs-he organization of water molecules close to solid
surfaces1−3 or around proteins4,5 diﬀers signiﬁcantly
from pure water. Structured water may impact protein
recognition2,6 but is also believed to aﬀect electrochemical
and catalytic reactions at interfaces.7,8 Reorganization of solvent
molecules is likely to occur around colloidal nanoparticles9 andits understanding is of outermost importance to better estimate
their reactivity and interaction with biological moieties inaqueous environment. Nevertheless, the structure of solvent
molecules around nanomaterials in colloidal dispersion remains
largely unexplored.9−11
Nanodiamonds (NDs) are of particular interest for theinvestigation of interfacial water since the existence of anordered water shell ranging from 2 to 4 water layers wasrecently suggested.11,12 The interfacial water layer may play anessential role in the colloidal stability of small NDs,13 in theirphotoluminescence14 or in the enhancement of magnetic
resonance imaging signal from Gd-functionalized NDs,15 but its
structure is still unknown. Unusual water adsorption on dry
NDs have already been characterized by infrared spectroscopy16 and diﬀerential scanning calorimetry.12,17 Recently,interfacial water layers around NDs were probed directly inaqueous solution by interferometry11 and Raman spectroscopy14 but provided limited information about its molecular andelectronic structure.
X-ray Absorption Spectroscopy (XAS) and X-ray Emission
Spectroscopy (XES) at the oxygen K edge, probing respectively
the unoccupied and occupied electronic states through theexcitation of a core electron, were extensively applied tocharacterize the solvation shells of ionic species in aqueous
solutions because these methods are sensitive to the hydrogenbonds network of water.18−24 Recently, Total Electron Yield(TEY) XAS measurements of gold electrode−water interface
demonstrated that interfacial water have a diﬀerent electronic© 2015 American Chemical Society

Received: April 21, 2015
Accepted: July 8, 2015
Published: July 8, 2015
2909

DOI: 10.1021/acs.jpclett.5b00820
J. Phys. Chem. Lett. 2015, 6, 2909−2912

Letter

The Journal of Physical Chemistry Letters

Figure 1. Transmission cell for XAS measurements of aqueous
dispersions of NDs.

NRI, 3 nm) were compared to a high-pressure, hightemperature ND aqueous dispersion (NDs-SYP from Van
Moppes, 18 nm). The hydrodynamic diameter and the zeta
potential (ZP) of NDs in aqueous dispersions characterized inthis study are summarized in Supporting Information (SI)
Table S1. NDs-SYP and NDs-NRI have hydrodynamic
diameter close to their core sizes, therefore these aqueous
dispersions are considered monodisperse. On the other hand,
NDs-PC and NDs-ITC form small aggregates with sizes up to
32.5 nm, which are probably induced by water dilution
performed before size measurement. Furthermore, detonation
NDs aggregates have a fractal structure with pores in thenanometer range in which water can diﬀuse.17,27 Consequently,
we estimate that the extent of the ND−water interface is not
signiﬁcantly aﬀected by slight aggregation of NDs. Except for
NDs-NRI, the NDs exhibit negative ZP, which is related tocarboxylic groups on their surface.
XAS of the carbon K-edges of the diﬀerent NDs aqueous
dispersions were ﬁrst measured (Figure S2). The pre-edge
region below 289 eV is sensitive to the surface chemistry of
NDs. Π* transitions from sp2-hybridized carbon at 285.0 eV are
observed for NDs-NRI but cannot be clearly resolved on other
NDs. This feature demonstrates the presence of fullerene-likereconstructions (FLRs) on the surface of NDs-NRI, which may
be at the origin of their positive ZP due to oxygen hole
doping.28 On the contrary, the shoulder at 287 eV, which waspreviously attributed either to FLRs29 or to CO bonds,30 isobserved on all NDs except NDs-NRI. Considering that thisfeature is not observed on NDs-NRI, covered by FLRs as
conﬁrmed by the π* component at 285.0 eV, it can safely be
assigned to CO bonds.
The oxygen K-edges of the diﬀerent NDs dispersions werethen compared to pure water (Figure 2). Typical water XA
spectra are constituted of three regions, the pre-edge at
534.7 eV, the main-edge between 537 and 538 eV and the postedge around 540 eV. The spectra were normalized before thepre-edge and after the post-edge to take into account variationsin the sample thickness (see SI). The overall X-ray absorption ishigher for NDs dispersions but also diﬀers with the samples. At
the main- and post-edges, the absorption coeﬃcient increases
for decreasing NDs sizes as discussed later. On the other hand,
the intensity of the pre-edge peak at 534.7 eV is stronger for
NDs-PC and NDs-ITC, having larger sizes than NDs-NRI(Figure 2).
Changes of X-ray absorption of the oxygen K-edge can beinterpreted in terms of modiﬁcation of the hydrogen bondFigure 2. (a) XAS of oxygen K-edge from water and 1% aqueous
dispersions of NDs with diﬀerent origins in transmission cell. (b) Pre/
main-edge ratios for the diﬀerent NDs dispersions. The sign of the
NDs ZP in water is indicated above the respective bars.

network in interfacial water layers surrounding dispersed
NDs.18−22 Other explanations for XAS changes are unlikely
as discussed in Supporting Information. First, the increased
absorption for smaller NDs may be due to higher surface area,
increasing the amount of interfacial water molecules. Nevertheless, if the X-ray absorption increase in the main- and postedges correlates with the respective sizes of the NDs, changes
in the pre-edge cannot be simply explained in terms of higher
surface area. Indeed, XAS of NDs-NRI, having a higher surface
area but a positive ZP, present a smaller pre-edge than otherdetonation NDs (Figure 2). As a consequence, the surface
charge of NDs also has to be taken into account.
A parallel can be performed with a recent report by VelascoVelez et al. on gold electrodes exposed to electrical bias.1 The
surface potential of the gold electrode was found to strongly
inﬂuence the XA spectra of interfacial water. In this study, thepre-edge feature was clearly resolved at negative bias, while it
fully disappears for positive bias. This was interpreted as a resultof orientation of water molecules with their H atoms pointing
toward the negatively charged surface, increasing the number ofdangling hydrogen bonds at the interface. This is in line withthe modiﬁcations induced by oxidized NDs observed in the
pre-edge region in our case. Carboxylate groups on their surface
promote the breaking of hydrogen bonds between watermolecules in the ﬁrst solvation shell. There are thus less watermolecules in double hydrogen bond donor conﬁguration, andthe pre-edge feature in XAS increases.
On the other hand, positively charged NDs (NDs-NRI) havelower impact in the pre-edge region, probably because thepositive surface charges favor orientation of water’s oxygen
atom toward the surface, leaving both hydrogen atoms available
for hydrogen bond formation. Interfacial water molecules
donating two hydrogen bonds are promoted, but this molecularorientation does not contribute to the pre-edge region.1
Hydrogen bonds between neighboring water molecules in theﬁrst solvation shell of positively charged NDs are thus
promoted compared to negatively charged NDs.
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DOI: 10.1021/acs.jpclett.5b00820
J. Phys. Chem. Lett. 2015, 6, 2909−2912

Letter

The Journal of Physical Chemistry Lettersamount of hydrogen bonds.32 In the same time, they are smallenough to oﬀer a large interface and the surface polarization
orients water molecules in the ﬁrst solvation shell. Further
investigations are required to estimate whether the long-rangeorder of water molecules around NDs may be correlated to thehigh viscosity of detonation NDs dispersions at concentration
around 8−10 wt %.13,17
The electronic structure of interfacial water on NDs can becompared to diﬀerent forms of ice previously characterized by
X-ray Raman scattering.33,34 Tetrahedral ice and low density
amorphous ice present a predominant post-edge structure,
related to the less distorted hydrogen bond environment
dominated by tetrahedral hydrogen bonds.33 The electronic
signature of high-density amorphous ice,33,34 on the other hand,is constituted of an intense main-edge and reduced pre- andpost-edges, which was attributed to a collapse of the second
hydration shell, resulting in higher water molecule density.20
This electronic signature is close to the one observed around
NDs, suggesting that the structure of the ordered water layer
around NDs might resemble high density amorphous ice. A
similar water structure was observed inside living cells,35
therefore NDs could potentially be used as a crowding agent tomimic cellular environment.6
In summary, the hydrogen bond network of water molecules
in aqueous dispersions of NDs was probed by transmission
XAS. Orientation of water molecules in the ﬁrst solvation shell
are found to depend on the ZP of NDs. In particular, morehydrogen bonds are broken at the surface of negatively charged
NDs due to electrostatic interaction with carboxylate groups
while water molecules donating two hydrogen bonds are
dominant on positively charged NDs. A long-range ordering ofwater molecules around NDs is also evidenced. At high NDs
concentration, the electronic structure of water molecules is
extremely diﬀerent from bulk water due to the strong
contribution from interfacial water layers. The ordering ofwater molecules on their surface might be comparable to thearrangement of water molecules around proteins, which could
potentially explain the high aﬃnity of NDs for protein
adsorption36 or participate to their low cytoxicity.37

The strong impact of NDs-NRI on the water structure is
conﬁrmed by probing the evolution of oxygen K-edge XA
spectra at higher NDs concentrations up to 8.6 wt %. As seen in
Figure 3, the main- and post-edges increase signiﬁcantly

Figure 3. (a) XAS of oxygen K-edge from water and aqueous
dispersion of NDs-NRI at diﬀerent concentrations ranging from 1 to
8.6 wt %. (b) Pre/Main-edge ratios for the diﬀerent NDs-NRI
concentrations.

compared to pure water, while the pre-edge is only slightly
perturbed. The X-ray absorption increases in similar amount at
main and post edges between 1 and 8.6 wt % NDs
concentrations. On the other hand, the pre/main-edge ratio
at 8.6 wt % NDs concentration represents only 60% of thesame ratio calculated for 1 wt % concentration (Figure 3b). The
quenching of the pre-edge in the interfacial water layer
surrounding positively charged NDs may result from theorientation of water molecules with oxygen sites pointing
toward the NDs surface over several water layers.1 Due to thisarrangement, hydrogen bonds between the hydrogen sites of awater layer with the oxygen site of the next water layer would
be stronger than in bulk water, resulting in a higher amount of
double donor/double acceptor water molecules, poorly
contributing to the pre-edge.
As a result, the increase in main- and post-edges at highconcentration of NDs-NRI appears extreme compared to otherreports by transmission XAS on aqueous ionic solutions20 andmethanol−water mixtures,22 although higher mass concentrations were used in these studies. The oxygen K-edge XAS
spectra include the contributions of both bulk water and theinterfacial water around NDs. By increasing the NDsconcentration, the ratio of the interfacial water is increased
compared to bulk water, which causes the increase of the mainand post-edges for high NDs concentrations.
Whereas ions mostly perturb water molecules in their ﬁrst
solvation shell,31 the disruption of the hydrogen bond network
induced by small NDs is more extended, apparently over morethan four hydration layers according to the strong changes
observed on XAS. Unlike ions, NDs are too large to be
engulfed in a hydration cage which would break a limited■

ASSOCIATED CONTENT

S Supporting Information*

Experimental details, normalization procedure, DLS measurements, transmission XAS of NDs at the carbon K edge, anddiscussion of XAS interpretation. The Supporting Information
is available free of charge on the ACS Publications website at
DOI: 10.1021/acs.jpclett.5b00820.

■

AUTHOR INFORMATION

Corresponding Author* E-mail: tristan.petit@helmholtz-berlin.de.
Author Contributions

The authors declare no competing ﬁnancial interests.
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
T.P. acknowledges the Alexander von Humboldt Foundation
for ﬁnancial support. H.Y. is a Research Fellow of the Japan
Society of Division for the Promotion of Science. E.O. and R. Y.
acknowledge partial ﬁnancial support from G8RI/JSPS. This
work is supported by JSPS Grants-in-Aid for Scientiﬁc Research2911

DOI: 10.1021/acs.jpclett.5b00820
J. Phys. Chem. Lett. 2015, 6, 2909−2912

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(No. 26248010). We acknowledge the kind support by staﬀ
members of the UVSOR-III synchrotron facility.

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DOI: 10.1021/acs.jpclett.5b00820
J. Phys. Chem. Lett. 2015, 6, 2909−2912