Communication
pubs.acs.org/JACS

Nanostructuration of PEDOT in Porous Coordination Polymers for
Tunable Porosity and Conductivity
Benjamin Le Ouay,†,‡ Mickael Boudot,§ Takashi Kitao,† Takeshi Yanagida,§ Susumu Kitagawa,*,†,#and Takashi Uemura*,†,‡†

Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan‡
CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan§
Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan#
Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, JapanS Supporting Information*

already been reported to prepare conductive polymer chains in
one-dimensional (1-D) nanochannels,21−23 2-D nanoslits,24 and3-D pores.25−27 However, PCP/polymer composites withreasonably high porosity as well as conductivity have yet to beprepared. To obtain conduction isotropy and mass transport in
the occupied pores, we used a 3-D PCP, [Cr3(BDC)3OF(H2O)2]n (MIL−101(Cr), BDC = benzenedicarboxylate),28
denoted as 1. This PCP is readily available at the gram scale,
highly porous, and widely studied along with its functionalvariants.29,30
Composite materials between 1 and PEDOT were prepared in
two steps (Figure 1). First, a controlled amount (chosen below
the maximal loading capacity) of 3,4-ethylenedioxythiophene(EDOT) dissolved in diethyl ether was introduced into the host
PCP by evaporation of the solvent. This approach allowed ahomogeneous dispersion of monomers throughout 1 as well as aprecise control of the loading rate. Owing to the very highporosity of 1, the maximum EDOT loading was estimated to be
160 ± 10% of the empty PCP mass, a value comparable to that of
ibuprofen (density: 1.03; 140 wt %).31
The second step consisted of the oxidative polymerization of
EDOT in the pores of 1 by iodine vapors.32 Unlike othercommon oxidizers (e.g., FeCl3 and persulfate), iodine can easily
be sublimed and allow reaction between a gaseous reagent andthe monomers adsorbed on the pore walls inside the PCP. Notethat after evacuation of the unreacted I2, PEDOT remained in ahighly doped (and thus conductive) state, with ionic iodine
species acting as the counterions.
Preservation of the PCP framework during the polymerization
was conﬁrmed by powder X-ray diﬀraction (PXRD), whichindicated the presence of characteristic peaks of 1 (Figure 2).
Diﬀerences in the relative intensities of the peaks after thepolymerization can be explained by the presence of disordered
guest polymer chains in the pores33 and is especially strong in ourcase as iodine species are strong X-ray scatterers.
Formation of PEDOT and elimination of unreacted EDOT
were assessed using ATR-FTIR (Supporting Information).
MALDI-TOF indicated the presence of oligomers having
molecular weight up to 1286 Da. Meanwhile, number-average

ABSTRACT: A series of conductive porous composites
were obtained by the polymerization of 3,4-ethylenedioxythiophene (EDOT) in the cavities of MIL−101(Cr). By
controlling the amount of EDOT loaded into the hostframework, it was possible to modulate the conductivity as
well as the porosity of the composite. This approach yields
materials with a reasonable electronic conductivity (1.1 ×
10−3 S·cm−1) while maintaining high porosity (SBET = 803m2/g). This serves as a promising strategy for obtaining
highly nanotextured conductive polymers with very highaccessibility for small gas molecules, which are beneﬁcial to
the fabrication of a chemiresistive sensor for the detectionof NO2.

Over the past two decades, porous coordination polymers(PCPs), also known as metal−organic frameworks(MOFs), have appeared as a promising and versatile class ofmaterials.1−3 Their success is largely due to their exceptional
characteristics, among which are very high porosity, a variety of
topologies, and tunability of their properties through selection of
the metal centers or ligands or inclusion of guests. However,applications of PCPs in the ﬁeld of electrochemistry remain
scarce, especially when excluding the ﬁeld of ion-conductive
networks.4,5 The main reason for this is the lack of electronic
conductivity of most frameworks, with only a few PCPs reported
as electron conductors.6−12 Because they have active metal
centers easily accessed by diﬀusive species and electrons, thesematerials possess excellent electrochemical properties, as
demonstrated by recent reports.13−16 Unfortunately, andwithout denying the impressive properties of compounds already(and to-be) described, this class of materials suﬀers from limited
tunability, as small changes in the nature of the metal or ligand
resulted in a signiﬁcant decrease in their conductivity.17−19
Furthermore, because high electronic conduction requires
multiple conduction pathways, the porosity of this category of
PCPs remains relatively low.
To beneﬁt from both the versatility of PCPs’ functionality andthe electronic conduction, we propose to synthesize conductive
polymers such as poly(3,4-ethylenedioxythiophene)(PEDOT)20 in the pores of a PCP. Similar approaches have© 2016 American Chemical Society

Received: June 7, 2016
Published: August 2, 2016
10088

DOI: 10.1021/jacs.6b05552
J. Am. Chem. Soc. 2016, 138, 10088−10091

Communication

Journal of the American Chemical Society

Figure 1. Schematic image for the preparation of 1⊃PEDOT.

1⊃PEDOT decreased as the loading rate increased. However,
due to the high porosity of the host material, the value of SBET athigh loading still remained reasonably high. For example, SBET of
1⊃PEDOT(57) was calculated as 803 m2/g, which is higher than
any conductive PCP reported so far. 1⊃PEDOT presented a
type I isotherm, indicative of microporosity. The micropore size
distribution, determined using the HK (Horváth and Kawazoe)
method34 (Figure 3B), indicated the presence of two types ofpores in 1 with diameters of 1.8 and 2.3 nm, corresponding to thetwo types of cages. For low loading rates, the apparent diameterof the cages decreased slightly, indicating that the polymer
formed close to the pores walls, while the volume in the cages
remained accessible.
For high loading, the stacking of several PEDOT layers on thepore walls decreased the apparent pore diameter. However, thecomposites maintained a broad distribution of pores withdiameters between 0.5 and 2.2 nm. Raman spectroscopy andmolecular dynamics (MD) simulations indicated that PEDOT
presented a coiled structure with a low degree of order for the
chains (Supporting Information). This eﬀect was attributed tothe curvature of the cage walls that prevented ordering of the
PEDOT chains over long distances.
Conductivity was measured using impedance spectroscopy(two electrodes) on pressure-sintered pellets. Pristine 1
exhibited almost no conductivity (σ < 10−11 S·cm−1), as expectedfrom the absence of electron-conduction paths. In contrast, theimpedance diagrams of 1⊃PEDOT had the shape of a semicircle
when plotted in the Nyquist representation and impedance
reached a pure real value at low frequency (Supporting
Information). This was attributed to an electronic conduction
through the PEDOT network, with iodine species acting as adopant. The conductivities of 1⊃PEDOT, as well as SBET arereported in Figure 4. The conductivity of the composites
increased greatly with the PEDOT weight fraction. For thehighest loading rate (1⊃PEDOT(57)), the conductivity reacheda value of 1.1 × 10−3 S·cm−1. This value was higher than those of
the composites between 1 and other conducting polymers, suchas unsubstituted polythiophene and polypyrrole, prepared in the

Figure 2. XRD diagrams of pristine PEDOT, 1, and 1⊃PEDOT. The
number in the parentheses indicates the mass fraction of PEDOT in thesample.

molecular weight Mn was determined as 2960 Da by gel
permeation chromatography (GPC). Discrepancies between
both techniques are discussed in Supporting Information. The
polymerization yield was estimated using XRF, based on the S/
Cr ratio. The polymerization yield was between 82% and 86% for
all samples, indicating the high reactivity of EDOT in the porous
network. In this study, we designate composites as 1⊃PEDOT(X), where X is the mass fraction of PEDOT in the composite(determined from the S/Cr ratio by XRF). X could be varied in 1
from 10% to 57% depending on the amount of EDOT loaded.
The porosity of the composites were assessed by adsorption of
N2 at 77 K (Figure 3A). Pristine host PCP 1 possessed a speciﬁc
surface area SBET of ∼3100 m2/g.28,30 As expected, SBET of

Figure 3. (A) Adsorption isotherms (N2, 77 K) of 1 and 1⊃PEDOT.
(B) HK plot of 1 and 1⊃PEDOT composites. An oﬀset of 0.5 between
each curve was added for readability.

Figure 4. Conductivity (red squares) and speciﬁc surface area (bluecircles) of 1⊃PEDOT.
10089

DOI: 10.1021/jacs.6b05552
J. Am. Chem. Soc. 2016, 138, 10088−10091

Communication

Journal of the American Chemical Society

Figure 5. (A) Time-dependent variations of conductivity of a chemiresistive sensor prepared using 1⊃PEDOT(45) (plain line, left axis) under diﬀerent
partial pressures of NO2 (dashed, right axis). (B) Zoom-in on the low-pressure domain. (C) Linear ﬁt of the conductivity changes as a function of thepartial pressure of NO2.

sensitivity, a low LOD, and a wide linearity range, theperformances of our sensing device were very promising
compared with other systems based on nanostructured
conducting polymers38−42 and were indeed close to the mosteﬃcient and optimized PEDOT-based sensors for NO2, which
rely on the modulation of a p−n junction instead of a direct
Fermi level modiﬁcation.47,48
In summary, we have produced a series of porous electronic
conductors based on the polymerization of EDOT in 1, which
possesses a 3-D porous structure. By varying the amount ofmonomer introduced in the pores, it was possible to modulate
the conductivity as well as the porosity of the composites. The
conductivity of 1⊃PEDOT reached 1.1 × 10−3 S·cm−1 with thehighest loading of the polymer, while the speciﬁc surface area
remained as high as 803 m2/g. Furthermore, we deliberately
chose an archetypal PCP framework, as a proof-of-concept.
Preparation of PCP/conductive polymer composites thus
appears as a promising and versatile approach to obtain highly
functional porous conductive materials. We have demonstrated
the potential of this approach by producing a chemiresistive
sensor that proved very eﬃcient for the detection of NO2.

similar manner, indicating the signiﬁcant role of the conductive
polymer’s nature (Supporting Information). We also prepared
PEDOT in 1-D channels of La(1,3,5-benzenetrisbenzoate),35
where the conductivity of the composite (30 wt % PEDOT) wasmuch lower (2.3 × 10−8 S·cm−1) than for corresponding
1⊃PEDOT, probably because of the low interchain connectivityof PEDOT isolated in the 1-D nanochannels (Supporting
Information). This indicates the essential role of the topology of
the PCP’s pores for the design of a conductive composite. Whilethe conductivity of 1⊃PEDOT was lower than the highest values
published for conductive PCPs,10 it is still appreciable and allows
application in electrochemistry.
PEDOT is a p-type semiconductor and its Fermi level can be
modiﬁed by the presence of oxidizing analytes, resulting inmeasurable changes of its conductivity. For this reason, PEDOT
has often been used for the chemiresistive detection of NO2.36−40
The detection of this analyte is of very great importance, as it is acommon atmospheric pollutant produced during combustion
reactions, with a toxic eﬀect on the respiratory system atconcentrations of 1 ppm and below.41 Thus, we initially prepareda NO2 sensor using conventional bulk PEDOT obtained fromsolution polymerization. However, this PEDOT sensor
presented a signiﬁcant drift, along with a very low sensitivity(0.8% for a partial pressure of NO2 P(NO2) = 1 Pa, equivalent to
10 ppm), probably because of low accessibility for gas analytes(SBET of pristine PEDOT = 2 m2/g). Because the design ofmicroporous and conductive materials is highly sought after toachieve eﬃcient gas sensors,13,42−44 preparation of PEDOT in 1
is a reasonable strategy to induce a nanostructuration of PEDOT,
which could develop a very high accessible surface area. As
conductive polymer chains tend to aggregate because of πstacking, their structuration at the subnanometer scale (where
chains are isolated or form bundles of a few entities) is so far only
attainable by molecular engineering of the side-chains.45,46 Here,we employed 1⊃PEDOT(45) (SBET = 1038 m2/g) for
chemiresistive sensing of NO2 gas. This composite presented
the best compromise between high conductivity to reduce theeﬀect of electrical noise and high accessibility for gas molecules.
As seen in Figure 5A,B, the conductivity of the sensor increased
sharply when put in contact with NO2. The response time toreach the equilibrium state was below 30 s for low partialpressures and remained below 150 s for P(NO2) up to 20 ppm.
The conductivity increase was found to be linear (R2 = 0.9974)
for P(NO2) up to 10 ppm (Figure 5C). The minimum P(NO2)measured was 200 ppb, with a sensor response of 0.9%.
Considering the linearity slope α and the electrical noise standard
deviation δ, the limit of detection (LOD) at 3δ was estimated at
60 ppb. Being operated at room temperature and having a high■

ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b05552.
Synthetic procedure and additional characterizations
(PDF)

■

AUTHOR INFORMATION

Corresponding Authors*uemura@sbchem.kyoto-u.ac.jp*kitagawa@icems.kyoto-u.ac.jp
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work was supported by the JST, CREST program, and a
Grant-in Aid for Science Research on Innovative Area “New
Polymeric Materials Based on Element-Blocks” (No. 2401) from
the Ministry of Education, Culture, Sports, Science andTechnology, Government of Japan.

■

REFERENCES(1) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43,2334.
(2) Ferey, G. Chem. Soc. Rev. 2008, 37, 191.

10090

DOI: 10.1021/jacs.6b05552
J. Am. Chem. Soc. 2016, 138, 10088−10091

Communication

Journal of the American Chemical Society(3) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. Science
2013, 341, 974.
(4) Morozan, A.; Jaouen, F. Energy Environ. Sci. 2012, 5, 9269.
(5) Xia, W.; Mahmood, A.; Zou, R.; Xu, Q. Energy Environ. Sci. 2015, 8,
1837.
(6) Givaja, G.; Amo-Ochoa, P.; Gomez-Garcia, C. J.; Zamora, F. Chem.
Soc. Rev. 2012, 41, 115.
(7) Sun, L.; Campbell, M. G.; Dincă, M. Angew. Chem., Int. Ed. 2016,
55, 3566.
(8) Takaishi, S.; Hosoda, M.; Kajiwara, T.; Miyasaka, H.; Yamashita,
M.; Nakanishi, Y.; Kitagawa, Y.; Yamaguchi, K.; Kobayashi, A.; Kitagawa,
H. Inorg. Chem. 2009, 48, 9048.
(9) Talin, A. A.; Centrone, A.; Ford, A. C.; Foster, M. E.; Stavila, V.;
Haney, P.; Kinney, R. A.; Szalai, V.; El Gabaly, F.; Yoon, H. P.; Léonard,
F.; Allendorf, M. D. Science 2014, 343, 66.
(10) Sheberla, D.; Sun, L.; Blood-Forsythe, M. A.; Er, S.; Wade, C. R.;
Brozek, C. K.; Aspuru-Guzik, A.; Dincă, M. J. Am. Chem. Soc. 2014, 136,
8859.
(11) Darago, L. E.; Aubrey, M. L.; Yu, C. J.; Gonzalez, M. I.; Long, J. R.
J. Am. Chem. Soc. 2015, 137, 15703.
(12) Chen, D.; Xing, H.; Su, Z.; Wang, C. Chem. Commun. 2016, 52,2019.
(13) Campbell, M. G.; Sheberla, D.; Liu, S. F.; Swager, T. M.; Dincă, M.
Angew. Chem., Int. Ed. 2015, 54, 4349.
(14) Campbell, M. G.; Liu, S. F.; Swager, T. M.; Dincă, M. J. Am. Chem.
Soc. 2015, 137, 13780.
(15) Aubrey, M. L.; Long, J. R. J. Am. Chem. Soc. 2015, 137, 13594.
(16) Miner, E. M.; Fukushima, T.; Sheberla, D.; Sun, L.; Surendranath,Y.; Dincă, M. Nat. Commun. 2016, 7, 10942.
(17) Kobayashi, Y.; Jacobs, B.; Allendorf, M. D.; Long, J. R. Chem.
Mater. 2010, 22, 4120.
(18) Park, S. S.; Hontz, E. R.; Sun, L.; Hendon, C. H.; Walsh, A.; Van
Voorhis, T.; Dincă, M. J. Am. Chem. Soc. 2015, 137, 1774.
(19) Sun, L.; Hendon, C. H.; Minier, M. A.; Walsh, A.; Dincă, M. J. Am.
Chem. Soc. 2015, 137, 6164.
(20) Kirchmeyer, S.; Reuter, K. J. Mater. Chem. 2005, 15, 2077.
(21) Wang, Q.-X.; Zhang, C.-Y. Macromol. Rapid Commun. 2011, 32,1610.
(22) Uemura, T.; Uchida, N.; Asano, A.; Saeki, A.; Seki, S.; Tsujimoto,M.; Isoda, S.; Kitagawa, S. J. Am. Chem. Soc. 2012, 134, 8360.
(23) MacLean, M. W. A.; Kitao, T.; Suga, T.; Mizuno, M.; Seki, S.;
Uemura, T.; Kitagawa, S. Angew. Chem. 2016, 128, 718.
(24) Yanai, N.; Uemura, T.; Ohba, M.; Kadowaki, Y.; Maesato, M.;
Takenaka, M.; Nishitsuji, S.; Hasegawa, H.; Kitagawa, S. Angew. Chem.,
Int. Ed. 2008, 47, 9883.
(25) Uemura, T.; Kadowaki, Y.; Yanai, N.; Kitagawa, S. Chem. Mater.
2009, 21, 4096.
(26) Lu, C.; Ben, T.; Xu, S.; Qiu, S. Angew. Chem., Int. Ed. 2014, 53,6454.
(27) Aliev, S. B.; Samsonenko, D. G.; Maksimovskiy, E. A.;
Fedorovskaya, E. O.; Sapchenko, S. A.; Fedin, V. P. New J. Chem.
2016, 40, 5306.
(28) Férey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.;
Surblé, S.; Margiolaki, I. Science 2005, 309, 2040.
(29) Hong, D.-Y.; Hwang, Y. K.; Serre, C.; Férey, G.; Chang, J.-S. Adv.
Funct. Mater. 2009, 19, 1537.
(30) Zhao, T.; Jeremias, F.; Boldog, I.; Nguyen, B.; Henninger, S. K.;
Janiak, C. Dalton Trans. 2015, 44, 16791.
(31) Horcajada, P.; Serre, C.; Vallet-Regí, M.; Sebban, M.; Taulelle, F.;Férey, G. Angew. Chem. 2006, 118, 6120.
(32) Bhattacharyya, D.; Howden, R. M.; Borrelli, D. C.; Gleason, K. K.
J. Polym. Sci., Part B: Polym. Phys. 2012, 50, 1329.
(33) Hafizovic, J.; Bjørgen, M.; Olsbye, U.; Dietzel, P. D. C.; Bordiga,
S.; Prestipino, C.; Lamberti, C.; Lillerud, K. P. J. Am. Chem. Soc. 2007,
129, 3612.
(34) Horvath, G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16 (6), 470.
(35) Devic, T.; Serre, C.; Audebrand, N.; Marrot, J.; Férey, G. J. Am.
Chem. Soc. 2005, 127, 12788.
(36) Bai, H.; Shi, G. Sensors 2007, 7, 267.

(37) Pinto, N. J.; Rivera, D.; Melendez, A.; Ramos, I.; Lim, J. H.;
Johnson, A. T. C. Sens. Actuators, B 2011, 156, 849.
(38) Yang, Y.; Li, S.; Yang, W.; Yuan, W.; Xu, J.; Jiang, Y. ACS Appl.
Mater. Interfaces 2014, 6, 13807.
(39) Shaik, M.; Rao, V. K.; Sinha, A. K.; Murthy, K. S. R. C.; Jain, R. J.
Environ. Chem. Eng. 2015, 3, 1947.
(40) Dunst, K.; Jurków, D.; Jasiński, P. Sens. Actuators, B 2016, 229,155.
(41) Hesterberg, T. W.; Bunn, W. B.; McClellan, R. O.; Hamade, A. K.;
Long, C. M.; Valberg, P. A. Crit. Rev. Toxicol. 2009, 39, 743.
(42) Travlou, N. A.; Singh, K.; Rodriguez-Castellon, E.; Bandosz, T. J.
J. Mater. Chem. A 2015, 3, 11417.
(43) Wisser, F. M.; Grothe, J.; Kaskel, S. Sens. Actuators, B 2016, 223,
166.
(44) Nguyen, N. D.; Yoon, H. Polymers 2016, 8, 118.
(45) Rose, A.; Zhu, Z.; Madigan, C. F.; Swager, T. M.; Bulovic, V.
Nature 2005, 434, 876.
(46) Pan, C.; Zhao, C.; Takeuchi, M.; Sugiyasu, K. Chem. - Asian J.
2015, 10, 1820.
(47) Zampetti, E.; Pantalei, S.; Muzyczuk, A.; Bearzotti, A.; De Cesare,F.; Spinella, C.; Macagnano, A. Sens. Actuators, B 2013, 176, 390.
(48) Hangarter, C. M.; Chartuprayoon, N.; Hernández, S. C.; Choa, Y.;
Myung, N. V. Nano Today 2013, 8, 39.

10091

DOI: 10.1021/jacs.6b05552
J. Am. Chem. Soc. 2016, 138, 10088−10091