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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 2275 Received 25th September 2013, Accepted 29th November 2013
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Engineering hydrophilic conducting composites with enhanced ion mobility† Eleni Stavrinidou,a Orawan Winther-Jensen,b Bijan S. Shekibi,b Vanessa Armel,b a a Jonathan Rivnay,a Esma Ismailova,a Se ´bastien Sanaur, George G. Malliaras and b Bjorn Winther-Jensen*
DOI: 10.1039/c3cp54061h www.rsc.org/pccp
Ion mobility has a direct influence on the performance of conducting polymers in a number of applications as it dictates the operational speed of the devices. We report here the enhanced ion mobility of poly(3,4-ethylene dioxythiophene) after incorporation of gelatin. The gelatin-rich domains were seen to provide an ion pathway through the composites.
Conducting polymers (CP) can support both electronic and ionic transport (especially during the doping–de-doping process) making them attractive for a variety of applications. The mechanism of operation for electrochromic displays,1 pseudo-capacitors,2 light emitting electrochemical cells3 and organic bioelectronic devices4,5 relies on the mixed charge transport properties of CPs. While tuning their electronic properties has been addressed in numerous ways – for example by tuning the doping, the macromolecular chemistry, or the solid state/structure film morphology – relatively little has been done to refine the ionic properties of these materials. In the current work, we aimed to control the ion-mobility of PEDOT-based composite materials by establishing ionconducting pathways through biocompatible, hydrophilic ‘‘non-PEDOT’’ domains. The idea of incorporating a second material into the CP matrix that can enhance the overall ionmobility is one that has already seen success in the commercial PEDOT(poly(styrenesulphonate) (PSS)) material by Bayer.6 Within the film, a significant percentage of the PSS forms large domains that are segregated from the PEDOT-rich domains.7,8 Unfortunately, there are many limitations in tuning the properties of PEDOT(PSS) as additional chemistry may easily destabilize the PEDOT(PSS) suspension,9,10 and systematically changing the ratio of PSS can only be done on the synthesis level. a
Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia. E-mail: [email protected]; Fax: +61 3 9905 4940; Tel: +61 3 9905 5343 b Department of Bioelectronics, Ecole Nationale Supe´rieure des Mines, CMP-EMSE, MOC, 13541 Gardanne, France † Electronic supplementary information (ESI) available: Experimental details and additional results for DSC and CV. See DOI: 10.1039/c3cp54061h
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It is worth noting that, in the PEDOT(PSS) system, the ionconducting phase (immobilized PSS) is also the counter-ion for the doped PEDOT thereby allowing only for cation movement during the doping–dedoping process. On the other hand, in the current study, the ion-conducting phase in the PEDOT composites will not play a role in the charge-compensation of PEDOT which makes anion movement the main factor in the doping– dedoping process. It has been shown that the electronic conductivity of PEDOT can be significantly enhanced by the incorporation of polyethylene glycol (PEG).11,12 Likewise, the incorporation of extracellular matrix (ECM) molecules to ‘bridge’ the conducting platform for biological applications13–15 has been explored. The incorporation of hydrophilic polymers such as ECM molecules (i.e. proteins, peptides or biological derived molecules like gelatin) into CP films required a change away from the use of traditional solvents like butanol that is often used for the precursor solution. Acetic acid was found to be a suitable candidate for the choice of solvent as it is a weak acid that will avoid the denaturation of biomolecules while also preferably coordinating to Fe(III) – thereby preventing gelation between Fe(III) and gelatin. The ‘moving front experiment’ is a method based on electrochromism of a CP that is used for studying the ion transport occurring within a CP film. The doping state of the film changes upon movement of ions across the CP film/electrolyte interface, with the color of the film varying accordingly. Monitoring the change in the optical transmission intensity can be used to follow the kinetics of injection and transport of ions within the polymer film. This technique was introduced by Aoki et al.16 and was applied later by Johansson et al.17 and Wang and Smela.18 However, due to the complex geometry of the devices in each case, straightforward estimation of the ion mobility was not possible. Recently, Stavrinidou et al.6 proposed a one-dimensional configuration of the moving front experiment that allows straightforward measurement of ion mobilities in CPs. The mobility of the tosylate anion in PEDOT(TOS):gelatin composites was measured using a planar junction between the composite film and the sodium para-toluenesulfonate
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(NaTOS) electrolyte. When a positive voltage is applied to the Pt electrode in the electrolyte (with respect to the PEDOT film), tosylate anions are extracted from the film into the electrolyte while holes are extracted from the film through an Au connector in order to preserve the electro-neutrality of the film.6 Dedoping of the film during this process results in an electrochromic color change, and a subsequent ‘dedoping front’, which can be monitored optically. From the change in the transmitted optical intensity with respect to the initial state, we define the drift length of ions (l) at a given time (t). For the one dimensional geometry of the cell, ions and electrons move in the same direction, allowing the determination of ion mobility using the relationship l2 = 2mVt,
(1)
where m is the mobility of ions in the film and V is the applied voltage.
Experimental Preparation of PEDOT(TOS):gelatin composites The PEDOT(TOS):gelatin composites were prepared as previously reported.15 Briefly, Fe(III)TOS, pyridine and various gelatin contents were mixed in a water–acetic acid solution. The solutions were spin-coated onto various substrates followed by VPP at 70 1C and finally washed in ethanol (see ESI† for more details). Ion mobility measurement The method used for direct measurement of ion mobilities in CPs was developed by Stavrinidou et al. and has been published elsewhere.6 The planar junction was fabricated using standard microfabrication techniques (see more details in the ESI†). The PEDOT(TOS) and PEDOT(TOS):gelatin films were coated on a glass substrate with a thin layer of plasma polymerized maleic anhydride coating19 to serve as an adhesion layer between the substrate and the PEDOT in order to avoid delamination of the polymer film during the experiment. A 100 nm gold electrode was evaporated on one end of the film using a shadow mask. A Pt wire electrode was immersed in aqueous solution of 0.01 M NaTOS in the polydimethylsiloxane well for application of bias. A 2.5 V bias was used to drive the
electrochromic change in the PEDOT films (please see ESI† for further details).
Results and discussion We first attempted to measure the tosylate anion mobility in neat PEDOT(TOS) and PEDOT(TOS):PEG samples, but this proved impossible since the dedoping front was pinned at the electrolyte/electrode interface (Fig. 1a). We attribute this to the structural collapse of the CP film after initial dopant removal, as reported by Otero et al.,20 which constricts ion transport pathways. When composite films were made by the addition of gelatin, a moving dedoping front was observed (Fig. 1a). For the PEDOT(TOS) : gelatin 1 : 0.5 composite, the front was parallel to the electrolyte/film interface. However, the dedoping front was found to exhibit zig-zag-like features which imply non-uniform dedoping (at the scale of the optical microscope, Fig. 1a), most likely arising from non-uniform gelatin distribution. The mean displacement of the front, which corresponds to the drift length of ions, is calculated from the change of the transmitted light intensity (DT). DT profiles are calculated along the x-direction (from the electrolyte/film interface towards the Au electrode) after averaging the y-axis values. The averaging ensures that the extracted value for ion mobility is that of an average lateral ion transport. By fitting the experimental data (Fig. 1b) to eqn (1) the mobility of the tosylate ions in the film can be calculated (Fig. 2a). For the PEDOT(TOS) : gelatin 1 : 0.5 (33% gelatin) composite, the ion mobility was found to be 1.2 10 5 cm2 V 1 s 1. When we increased the relative amount of gelatin, the front edge was uniform (Fig. 1a) and the ion mobility was found to be 2.8 10 5 cm2 V 1 s 1, 4.0 10 5 cm2 V 1 s 1 and 7.8 10 5 cm2 V 1 s 1 for PEDOT(TOS) : gelatin films with ratios of 1 : 0.8 (44% gelatin), 1 : 1 (50% gelatin) and 1 : 2 (67% gelatin) respectively (Fig. 2a). The reproducibility of multiple replicates was more than 15%. The mobility of the tosylate anion in PEDOT(TOS) : gelatin 1 : 2 is thus about one order of magnitude lower than reported values for the mobility of common metal ions in water21 and only 5 times lower than the mobility of organic cations in PEDOT:PSS.6 It is worth noting that the electronic charge carrier mobility in PEDOT has been reported to be in the order of 10 cm2 V 1 s 1 (ref. 22) – this
Fig. 1 Ion mobility measurement of PEDOT(TOS) and PEDOT(TOS):gelatin composites. (a) Micrographs of the dedoped area (initiating from the electrolyte/film interface) at t = 100 s, and (b) the square of the drift length of ions (l2) as a function of time (t).
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Fig. 2 Ionic and electronic properties of PEDOT(TOS) and PEDOT(TOS):gelatin composites. (a) Ion mobility of tosylate anions and (b) electronic conductivity as a function of the amount of gelatin in the composites.
supports the notion that the mobility values measured here indeed originate from the ion mobility. Higher concentrations of gelatin (greater than 1 : 2) led to an excess of gelatin that could not be properly and uniformly incorporated into the polymer matrix. As such, no further enhancement of ion mobility was observed. There appears to be a threshold value before ion movement is significantly enhanced (Fig. 2a). It is believed that this threshold concentration (B25% gelatin) may correlate with the point at which gelatin-only domains begin to form an interpenetrating network inside the PEDOT matrix. This value is different from the 15 v/v theoretical percolation threshold suggested for spherically shaped binary composites where there is no miscibility of the two components.23 The difference is due to the partial miscibility of the components in the PEDOT(TOS):gelatin system. This phenomenon of partial miscibility is well-known for traditional composites of two polymers prepared by simple mixing and also where one polymer is polymerizing in the presence of the other.24 The partial miscibility is not indicative of an inhomogeneous material. In previous work,15 we have reported the elemental homogeneity of the PEDOT(TOS):gelatin composites (in X, Y and Z direction) using the Secondary Ion Mass Spectroscopy (NanoSIMS) technique. The uniform lateral distribution of gelatin domains within the PEDOT(TOS):gelatin film was observed to be in the submicron range.15 At higher gelatin contents, the response of ion mobility to gelatin content becomes more linear. This indicates that in this region the additional gelatin contributes to an increase in the volume of the gelatin-rich domains. By adding an electrical insulator in a conducting matrix, one would expect the electronic conductivity to decrease with the increasing volume fraction of the insulator. However, it has been reported that incorporation of the non-conducting PEG in the PEDOT(TOS) matrix results in enhanced electronic properties.11,12 For PEDOT(TOS) : gelatin composites up to a 1 : 1 ratio, the electronic conductivity is relatively constant at around 300 S cm 1 (measured using a conventional four point probe method at 40–45% relative humidity – see ESI† for further details), which is in the same range as the typical conductivity of dried PEDOT(PSS) films used in ref. 6 (B300 S cm 1). This is 25%
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less than neat PEDOT(TOS), but still higher than expected from simple dilution with an insulating polymer (Fig. 2b). The dilution effect becomes more obvious with PEDOT(TOS) : gelatin 1 : 2. At higher gelatin content beyond 1 : 2 ratio, a rapid decrease in electronic conductivity of the composites was observed (Fig. 2b). However, the normalised conductivity of the PEDOT fraction in the composites (Fig. S1, ESI†) indicates that gelatin does not disrupt the electronic properties of the PEDOT(TOS). In fact the normalised conductivity of the PEDOT fraction increases up to a 1 : 2 ratio. In order to further investigate the interaction of PEDOT(TOS) with gelatin, differential scanning calorimetry (DSC) was performed. Samples of various PEDOT(TOS):gelatin ratios were scanned from 80 1C to 200 1C to study the glass transition temperature (Tg), the PEDOT solid–solid state transition temperature and the water loss from gelatin. Typical DSC traces for PEDOT(TOS):gelatin composites are shown in Fig. S2 (ESI†) and the Tg, solid–solid state transition/water loss peak positions and transition energies are summarised in Table 1 (see ESI† for experimental details). Changing the Tg of a polymer blend is a common way to determine the interaction between the constituent polymers. An interaction is typically observed as a change in Tg of one polymer component towards the Tg of the other. The almost unchanged Tg (ca. 4 1C, Table 1) for all gelatin-containing samples strongly indicates that a significant part of the gelatin in the composites is in its ‘‘original’’ configuration i.e. not interacting with PEDOT. The transition energy for water (liquid to vapour) is 2260 J g 1.25 For gelatin with a typical water content of 15%,26 a peak around 100 1C27 with a transition energy of B340 J g 1 would be expected. Table 1 Glass transition temperature (Tg), solid–solid transition/water loss peak temperature and transitional energy (J g 1) of PEDOT(TOS):gelatin composites of various ratios
Transition/water Transition/water Tg (1C) loss peak temp. (1C) loss energy ( J g 1)
Sample PEDOT : gelatin 1 : 0.5 PEDOT : gelatin 1 : 1 PEDOT : gelatin 1 : 2 Gelatin
4.3 3.7 3.5 4.4
101 82 102 109
162 267 304 351
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increase in the mass of the composite materials with high gelatin content. When the gelatin content is sufficiently high, this effect becomes more dominant than the mass loss associated with the de-doping of the CP. These results are a significant demonstration of the role that the hydrophilic domains play in the composites, whereby the limited interaction between PEDOT and gelatin allows the composite to swell as a function of electrochemical potential rather than as a result of the oxidation state of the PEDOT. The cyclic voltammograms (CV) obtained during the EQCM measurements (see ESI,† Fig. S3) show that all composites have an electrochemical signature similar to that of PEDOT.11,12 This indicates that the incorporation of gelatin for the given ratios does not significantly influence the electrochemical properties of PEDOT. Fig. 3 Frequency changes vs. applied potentials for neat PEDOT(TOS) and PEDOT(TOS):gelatin composites scanned in 0.05 M NaPTS pH 6.9 at 20 mV s 1.
The experimental data for gelatin (Table 1) fit well with this proportional relationship. The water loss peak from gelatin and the solid–solid state transition from PEDOT (at ca. 137 1C with ca. 117 J g 1 for pure PEDOT(TOS)28) merge into one peak for all composite samples. Considering the different content of gelatin in the composites, the transition energy of the composites varied proportionally (Table 1). This indicates that gelatin in the PEDOT(TOS):gelatin composites maintains the same ability to take up water and thus there is little interaction between PEDOT and gelatin. The influence of gelatin on the overall mass-transport into and out of the composite films was measured using an electrochemical quartz crystal microbalance (EQCM) (see ESI† for experimental details). Fig. 3 shows the resonance frequency of the film during a potential scan from +0.5 V to 1 V (vs. Ag/AgCl), where the resonance frequency is inversely proportional to the mass of the film. In neat PEDOT(TOS), the resonance frequency increases continuously as the material is de-doped (from oxidized to reduced, from +0.5 V to 1 V) confirming that the mass of the film decreases as a result of the tosylate anions being extracted from the film.29 However, upon increasing the gelatin content the EQCM trace gradually changes therefore for PEDOT(TOS) : gelatin 1 : 2 the trace apparently shows movement of mixed ions. For this ratio, the EQCM reduction trace at higher potentials (and thereby the oxidation state) of +0.5 V to 0.2 V is typical for that of anion movement, as seen at lower gelatin concentrations. For reduction at potentials below 0.2 V, a mass gain is observed. This can be explained by the potentialdependent water uptake of gelatin. Gelatin is – as a derivative of the protein, collagen – characterized by an isoelectric point. The isoelectric point covers both sides of neutral,30,31 and can vary significantly depending on the origin and production method of the gelatin used. The increase in pH (or in our case a more negative applied potential) beyond the isoelectric point is known to increase the swelling of gelatin (gels) significantly – in some cases by more than 1000%.30,31 This charge-induced swelling results in an
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Conclusion We have developed a new method using acetic acid as a solvent in order to prepare vapour-phase polymerized PEDOT(TOS) composite films with hydrophilic biomolecules that show enhanced ion mobility compared to neat PEDOT(TOS). Gelatin was used as a model biomolecule and when incorporated into PEDOT(TOS) composites the gelatin rich domains facilitated enhanced ion mobility and biocompatibility.15 DSC and EQCM results support a model wherein the gelatin domains create interconnected pathways that facilitate ion transport. Conductivity measurements and CV results showed that the electronic properties of the PEDOT(TOS) : gelatin composites (from 1 : 0 to 1 : 2 ratios) remain largely unaffected with an electronic conductivity maintained at B300 S cm 1. The PEDOT(TOS):gelatin composites have only 5 times lower ion-mobility of organic ions than PEDOT(PSS) with similar electronic conductivity.6 However the PEDOT(TOS) composites offer the possibility to incorporate designed/selected bio-compatible molecules/polymers for the targeted application and also have the potential to be used in applications where anion movement is required. We expect that the method developed and described in this work will contribute towards the rational design of materials that simultaneously optimize electronic and ionic transport.
Notes and references 1 R. J. Mortimer, A. L. Dyer and J. R. Reynolds, Displays, 2006, 27, 2–18. 2 B. Chen, T. Cui, Y. Liu and K. Varahramyan, Solid-State Electron., 2003, 47, 841–847. 3 Q. Pei, G. Yu, C. Zhang, Y. Yang and A. J. Heeger, Science, 1995, 269, 1086–1091. 4 M. Berggren and A. Richter-Dahlfors, Adv. Mater., 2007, 19, 3201–3213. 5 R. M. Owens and G. G. Malliaras, MRS Bull., 2010, 35, 449–456. 6 E. Stavrinidou, P. Leleux, H. Rajaona, D. Khodagholy, J. Rivnay, M. Lindau, S. Sanaur and G. G. Malliaras, Adv. Mater., 2013, 25, 4488–4493.
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7 H. Okuzaki, Active-Matrix Flatpanel Displays and Devices (AM-FPD), 2012 19th International Workshop on, 2012. 8 Organic Semiconductors in Sensor Applications, ed. D. Bernards, R. M. Owens and G. G. Malliaras, Springer, Berlin Heidelberg, 2008. ¨s, Adv. Mater., 1998, 9 S. Ghosh, J. Rasmusson and O. Ingana 10, 1097–1099. ¨s, Synth. Met., 1999, 101, 413–416. 10 S. Ghosh and O. Ingana 11 B. Winther-Jensen, K. Fraser, C. Ong, M. Forsyth and D. R. MacFarlane, Adv. Mater., 2010, 22, 1727–1730. 12 L. H. Jimison, A. Hama, X. Strakosas, V. Armel, D. Khodagholy, E. Ismailova, G. G. Malliaras, B. Winther-Jensen and R. M. Owens, J. Mater. Chem., 2012, 22, 19498–19505. 13 Y. Sulaiman and R. Kataky, Analyst, 2012, 137, 2386–2393. 14 Y. Xiao, C. M. Li, S. Wang, J. Shi and C. P. Ooi, J. Biomed. Mater. Res., 2010, 92, 766–772. 15 M. Bongo, O. Winther-Jensen, S. Himmelberger, X. Strakosas, M. Ramuz, A. Hama, E. Stavrinidou, G. G. Malliaras, A. Salleo, B. Winther-Jensen and R. Owens, J. Mater. Chem. B, 2013, 1, 3860–3867. 16 K. Aoki, T. Aramoto and Y. Hoshino, J. Electroanal. Chem., 1992, 340, 127–135. ¨s, J. Electrochem. 17 T. Johansson, N.-K. Persson and O. Ingana Soc., 2004, 151, E119–E124. 18 X. Wang and E. Smela, J. Phys. Chem. C, 2008, 113, 369–381.
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19 Z. Ademovic, J. Wei, B. Winther-Jensen, X. Hou and P. Kingshott, Plasma Processes Polym., 2005, 2, 53–63. 20 T. F. Otero, H. Grande and J. Rodrı´guez, J. Phys. Chem. B, 1997, 101, 8525–8533. 21 P. Atkins and J. de Paula, Physical Chemistry, Oxford University Press, Great Britain, 2006. 22 B. Winther-Jensen, M. Forsyth, K. West, J. W. Andreasen, G. Wallace and D. R. MacFarlane, Org. Electron., 2007, 8, 796–800. 23 H. Scher and R. Zallen, J. Chem. Phys., 1970, 53, 3759–3761. 24 G. Qipeng, P. Xinsheng and W. Zhiji, Polymer, 1991, 32, 53–57. 25 J. H. Lienhard, A Heat Transfer Textbook, Phlogiston Press, Cambridge, Massachusetts, USA, 3rd edn, 1981. 26 S. Fakirov, Handbook of Engineering Biopolymers: Homopolymers, Blends and Composites, Carl Hanser Verlag, Munich, 2007. ˜ a, K. de la Caba, A. Eceiza, R. Ruseckaite and 27 C. Pen I. Mondragon, Bioresour. Technol., 2010, 101, 6836–6842. 28 B. Winther-Jensen, M. Forsyth, K. West, J. W. Andreasen, P. Bayley, S. Pas and D. R. MacFarlane, Polymer, 2008, 49, 481–487. 29 D. Orata and D. A. Buttry, J. Am. Chem. Soc., 1987, 109, 3574–3581. 30 K. De Wael, S. De Belder, S. Van Vlierberghe, G. Van Steenberge, P. Dubruel and A. Adriaens, Talanta, 2010, 82, 1980–1985. 31 A. I. Raafat, J. Appl. Polym. Sci., 2010, 118, 2642–2649.
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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 2275 Received 25th September 2013, Accepted 29th November 2013
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Engineering hydrophilic conducting composites with enhanced ion mobility† Eleni Stavrinidou,a Orawan Winther-Jensen,b Bijan S. Shekibi,b Vanessa Armel,b a a Jonathan Rivnay,a Esma Ismailova,a Se ´bastien Sanaur, George G. Malliaras and b Bjorn Winther-Jensen*
DOI: 10.1039/c3cp54061h www.rsc.org/pccp
Ion mobility has a direct influence on the performance of conducting polymers in a number of applications as it dictates the operational speed of the devices. We report here the enhanced ion mobility of poly(3,4-ethylene dioxythiophene) after incorporation of gelatin. The gelatin-rich domains were seen to provide an ion pathway through the composites.
Conducting polymers (CP) can support both electronic and ionic transport (especially during the doping–de-doping process) making them attractive for a variety of applications. The mechanism of operation for electrochromic displays,1 pseudo-capacitors,2 light emitting electrochemical cells3 and organic bioelectronic devices4,5 relies on the mixed charge transport properties of CPs. While tuning their electronic properties has been addressed in numerous ways – for example by tuning the doping, the macromolecular chemistry, or the solid state/structure film morphology – relatively little has been done to refine the ionic properties of these materials. In the current work, we aimed to control the ion-mobility of PEDOT-based composite materials by establishing ionconducting pathways through biocompatible, hydrophilic ‘‘non-PEDOT’’ domains. The idea of incorporating a second material into the CP matrix that can enhance the overall ionmobility is one that has already seen success in the commercial PEDOT(poly(styrenesulphonate) (PSS)) material by Bayer.6 Within the film, a significant percentage of the PSS forms large domains that are segregated from the PEDOT-rich domains.7,8 Unfortunately, there are many limitations in tuning the properties of PEDOT(PSS) as additional chemistry may easily destabilize the PEDOT(PSS) suspension,9,10 and systematically changing the ratio of PSS can only be done on the synthesis level. a
Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia. E-mail: [email protected]; Fax: +61 3 9905 4940; Tel: +61 3 9905 5343 b Department of Bioelectronics, Ecole Nationale Supe´rieure des Mines, CMP-EMSE, MOC, 13541 Gardanne, France † Electronic supplementary information (ESI) available: Experimental details and additional results for DSC and CV. See DOI: 10.1039/c3cp54061h
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It is worth noting that, in the PEDOT(PSS) system, the ionconducting phase (immobilized PSS) is also the counter-ion for the doped PEDOT thereby allowing only for cation movement during the doping–dedoping process. On the other hand, in the current study, the ion-conducting phase in the PEDOT composites will not play a role in the charge-compensation of PEDOT which makes anion movement the main factor in the doping– dedoping process. It has been shown that the electronic conductivity of PEDOT can be significantly enhanced by the incorporation of polyethylene glycol (PEG).11,12 Likewise, the incorporation of extracellular matrix (ECM) molecules to ‘bridge’ the conducting platform for biological applications13–15 has been explored. The incorporation of hydrophilic polymers such as ECM molecules (i.e. proteins, peptides or biological derived molecules like gelatin) into CP films required a change away from the use of traditional solvents like butanol that is often used for the precursor solution. Acetic acid was found to be a suitable candidate for the choice of solvent as it is a weak acid that will avoid the denaturation of biomolecules while also preferably coordinating to Fe(III) – thereby preventing gelation between Fe(III) and gelatin. The ‘moving front experiment’ is a method based on electrochromism of a CP that is used for studying the ion transport occurring within a CP film. The doping state of the film changes upon movement of ions across the CP film/electrolyte interface, with the color of the film varying accordingly. Monitoring the change in the optical transmission intensity can be used to follow the kinetics of injection and transport of ions within the polymer film. This technique was introduced by Aoki et al.16 and was applied later by Johansson et al.17 and Wang and Smela.18 However, due to the complex geometry of the devices in each case, straightforward estimation of the ion mobility was not possible. Recently, Stavrinidou et al.6 proposed a one-dimensional configuration of the moving front experiment that allows straightforward measurement of ion mobilities in CPs. The mobility of the tosylate anion in PEDOT(TOS):gelatin composites was measured using a planar junction between the composite film and the sodium para-toluenesulfonate
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(NaTOS) electrolyte. When a positive voltage is applied to the Pt electrode in the electrolyte (with respect to the PEDOT film), tosylate anions are extracted from the film into the electrolyte while holes are extracted from the film through an Au connector in order to preserve the electro-neutrality of the film.6 Dedoping of the film during this process results in an electrochromic color change, and a subsequent ‘dedoping front’, which can be monitored optically. From the change in the transmitted optical intensity with respect to the initial state, we define the drift length of ions (l) at a given time (t). For the one dimensional geometry of the cell, ions and electrons move in the same direction, allowing the determination of ion mobility using the relationship l2 = 2mVt,
(1)
where m is the mobility of ions in the film and V is the applied voltage.
Experimental Preparation of PEDOT(TOS):gelatin composites The PEDOT(TOS):gelatin composites were prepared as previously reported.15 Briefly, Fe(III)TOS, pyridine and various gelatin contents were mixed in a water–acetic acid solution. The solutions were spin-coated onto various substrates followed by VPP at 70 1C and finally washed in ethanol (see ESI† for more details). Ion mobility measurement The method used for direct measurement of ion mobilities in CPs was developed by Stavrinidou et al. and has been published elsewhere.6 The planar junction was fabricated using standard microfabrication techniques (see more details in the ESI†). The PEDOT(TOS) and PEDOT(TOS):gelatin films were coated on a glass substrate with a thin layer of plasma polymerized maleic anhydride coating19 to serve as an adhesion layer between the substrate and the PEDOT in order to avoid delamination of the polymer film during the experiment. A 100 nm gold electrode was evaporated on one end of the film using a shadow mask. A Pt wire electrode was immersed in aqueous solution of 0.01 M NaTOS in the polydimethylsiloxane well for application of bias. A 2.5 V bias was used to drive the
electrochromic change in the PEDOT films (please see ESI† for further details).
Results and discussion We first attempted to measure the tosylate anion mobility in neat PEDOT(TOS) and PEDOT(TOS):PEG samples, but this proved impossible since the dedoping front was pinned at the electrolyte/electrode interface (Fig. 1a). We attribute this to the structural collapse of the CP film after initial dopant removal, as reported by Otero et al.,20 which constricts ion transport pathways. When composite films were made by the addition of gelatin, a moving dedoping front was observed (Fig. 1a). For the PEDOT(TOS) : gelatin 1 : 0.5 composite, the front was parallel to the electrolyte/film interface. However, the dedoping front was found to exhibit zig-zag-like features which imply non-uniform dedoping (at the scale of the optical microscope, Fig. 1a), most likely arising from non-uniform gelatin distribution. The mean displacement of the front, which corresponds to the drift length of ions, is calculated from the change of the transmitted light intensity (DT). DT profiles are calculated along the x-direction (from the electrolyte/film interface towards the Au electrode) after averaging the y-axis values. The averaging ensures that the extracted value for ion mobility is that of an average lateral ion transport. By fitting the experimental data (Fig. 1b) to eqn (1) the mobility of the tosylate ions in the film can be calculated (Fig. 2a). For the PEDOT(TOS) : gelatin 1 : 0.5 (33% gelatin) composite, the ion mobility was found to be 1.2 10 5 cm2 V 1 s 1. When we increased the relative amount of gelatin, the front edge was uniform (Fig. 1a) and the ion mobility was found to be 2.8 10 5 cm2 V 1 s 1, 4.0 10 5 cm2 V 1 s 1 and 7.8 10 5 cm2 V 1 s 1 for PEDOT(TOS) : gelatin films with ratios of 1 : 0.8 (44% gelatin), 1 : 1 (50% gelatin) and 1 : 2 (67% gelatin) respectively (Fig. 2a). The reproducibility of multiple replicates was more than 15%. The mobility of the tosylate anion in PEDOT(TOS) : gelatin 1 : 2 is thus about one order of magnitude lower than reported values for the mobility of common metal ions in water21 and only 5 times lower than the mobility of organic cations in PEDOT:PSS.6 It is worth noting that the electronic charge carrier mobility in PEDOT has been reported to be in the order of 10 cm2 V 1 s 1 (ref. 22) – this
Fig. 1 Ion mobility measurement of PEDOT(TOS) and PEDOT(TOS):gelatin composites. (a) Micrographs of the dedoped area (initiating from the electrolyte/film interface) at t = 100 s, and (b) the square of the drift length of ions (l2) as a function of time (t).
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Fig. 2 Ionic and electronic properties of PEDOT(TOS) and PEDOT(TOS):gelatin composites. (a) Ion mobility of tosylate anions and (b) electronic conductivity as a function of the amount of gelatin in the composites.
supports the notion that the mobility values measured here indeed originate from the ion mobility. Higher concentrations of gelatin (greater than 1 : 2) led to an excess of gelatin that could not be properly and uniformly incorporated into the polymer matrix. As such, no further enhancement of ion mobility was observed. There appears to be a threshold value before ion movement is significantly enhanced (Fig. 2a). It is believed that this threshold concentration (B25% gelatin) may correlate with the point at which gelatin-only domains begin to form an interpenetrating network inside the PEDOT matrix. This value is different from the 15 v/v theoretical percolation threshold suggested for spherically shaped binary composites where there is no miscibility of the two components.23 The difference is due to the partial miscibility of the components in the PEDOT(TOS):gelatin system. This phenomenon of partial miscibility is well-known for traditional composites of two polymers prepared by simple mixing and also where one polymer is polymerizing in the presence of the other.24 The partial miscibility is not indicative of an inhomogeneous material. In previous work,15 we have reported the elemental homogeneity of the PEDOT(TOS):gelatin composites (in X, Y and Z direction) using the Secondary Ion Mass Spectroscopy (NanoSIMS) technique. The uniform lateral distribution of gelatin domains within the PEDOT(TOS):gelatin film was observed to be in the submicron range.15 At higher gelatin contents, the response of ion mobility to gelatin content becomes more linear. This indicates that in this region the additional gelatin contributes to an increase in the volume of the gelatin-rich domains. By adding an electrical insulator in a conducting matrix, one would expect the electronic conductivity to decrease with the increasing volume fraction of the insulator. However, it has been reported that incorporation of the non-conducting PEG in the PEDOT(TOS) matrix results in enhanced electronic properties.11,12 For PEDOT(TOS) : gelatin composites up to a 1 : 1 ratio, the electronic conductivity is relatively constant at around 300 S cm 1 (measured using a conventional four point probe method at 40–45% relative humidity – see ESI† for further details), which is in the same range as the typical conductivity of dried PEDOT(PSS) films used in ref. 6 (B300 S cm 1). This is 25%
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less than neat PEDOT(TOS), but still higher than expected from simple dilution with an insulating polymer (Fig. 2b). The dilution effect becomes more obvious with PEDOT(TOS) : gelatin 1 : 2. At higher gelatin content beyond 1 : 2 ratio, a rapid decrease in electronic conductivity of the composites was observed (Fig. 2b). However, the normalised conductivity of the PEDOT fraction in the composites (Fig. S1, ESI†) indicates that gelatin does not disrupt the electronic properties of the PEDOT(TOS). In fact the normalised conductivity of the PEDOT fraction increases up to a 1 : 2 ratio. In order to further investigate the interaction of PEDOT(TOS) with gelatin, differential scanning calorimetry (DSC) was performed. Samples of various PEDOT(TOS):gelatin ratios were scanned from 80 1C to 200 1C to study the glass transition temperature (Tg), the PEDOT solid–solid state transition temperature and the water loss from gelatin. Typical DSC traces for PEDOT(TOS):gelatin composites are shown in Fig. S2 (ESI†) and the Tg, solid–solid state transition/water loss peak positions and transition energies are summarised in Table 1 (see ESI† for experimental details). Changing the Tg of a polymer blend is a common way to determine the interaction between the constituent polymers. An interaction is typically observed as a change in Tg of one polymer component towards the Tg of the other. The almost unchanged Tg (ca. 4 1C, Table 1) for all gelatin-containing samples strongly indicates that a significant part of the gelatin in the composites is in its ‘‘original’’ configuration i.e. not interacting with PEDOT. The transition energy for water (liquid to vapour) is 2260 J g 1.25 For gelatin with a typical water content of 15%,26 a peak around 100 1C27 with a transition energy of B340 J g 1 would be expected. Table 1 Glass transition temperature (Tg), solid–solid transition/water loss peak temperature and transitional energy (J g 1) of PEDOT(TOS):gelatin composites of various ratios
Transition/water Transition/water Tg (1C) loss peak temp. (1C) loss energy ( J g 1)
Sample PEDOT : gelatin 1 : 0.5 PEDOT : gelatin 1 : 1 PEDOT : gelatin 1 : 2 Gelatin
4.3 3.7 3.5 4.4
101 82 102 109
162 267 304 351
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increase in the mass of the composite materials with high gelatin content. When the gelatin content is sufficiently high, this effect becomes more dominant than the mass loss associated with the de-doping of the CP. These results are a significant demonstration of the role that the hydrophilic domains play in the composites, whereby the limited interaction between PEDOT and gelatin allows the composite to swell as a function of electrochemical potential rather than as a result of the oxidation state of the PEDOT. The cyclic voltammograms (CV) obtained during the EQCM measurements (see ESI,† Fig. S3) show that all composites have an electrochemical signature similar to that of PEDOT.11,12 This indicates that the incorporation of gelatin for the given ratios does not significantly influence the electrochemical properties of PEDOT. Fig. 3 Frequency changes vs. applied potentials for neat PEDOT(TOS) and PEDOT(TOS):gelatin composites scanned in 0.05 M NaPTS pH 6.9 at 20 mV s 1.
The experimental data for gelatin (Table 1) fit well with this proportional relationship. The water loss peak from gelatin and the solid–solid state transition from PEDOT (at ca. 137 1C with ca. 117 J g 1 for pure PEDOT(TOS)28) merge into one peak for all composite samples. Considering the different content of gelatin in the composites, the transition energy of the composites varied proportionally (Table 1). This indicates that gelatin in the PEDOT(TOS):gelatin composites maintains the same ability to take up water and thus there is little interaction between PEDOT and gelatin. The influence of gelatin on the overall mass-transport into and out of the composite films was measured using an electrochemical quartz crystal microbalance (EQCM) (see ESI† for experimental details). Fig. 3 shows the resonance frequency of the film during a potential scan from +0.5 V to 1 V (vs. Ag/AgCl), where the resonance frequency is inversely proportional to the mass of the film. In neat PEDOT(TOS), the resonance frequency increases continuously as the material is de-doped (from oxidized to reduced, from +0.5 V to 1 V) confirming that the mass of the film decreases as a result of the tosylate anions being extracted from the film.29 However, upon increasing the gelatin content the EQCM trace gradually changes therefore for PEDOT(TOS) : gelatin 1 : 2 the trace apparently shows movement of mixed ions. For this ratio, the EQCM reduction trace at higher potentials (and thereby the oxidation state) of +0.5 V to 0.2 V is typical for that of anion movement, as seen at lower gelatin concentrations. For reduction at potentials below 0.2 V, a mass gain is observed. This can be explained by the potentialdependent water uptake of gelatin. Gelatin is – as a derivative of the protein, collagen – characterized by an isoelectric point. The isoelectric point covers both sides of neutral,30,31 and can vary significantly depending on the origin and production method of the gelatin used. The increase in pH (or in our case a more negative applied potential) beyond the isoelectric point is known to increase the swelling of gelatin (gels) significantly – in some cases by more than 1000%.30,31 This charge-induced swelling results in an
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Conclusion We have developed a new method using acetic acid as a solvent in order to prepare vapour-phase polymerized PEDOT(TOS) composite films with hydrophilic biomolecules that show enhanced ion mobility compared to neat PEDOT(TOS). Gelatin was used as a model biomolecule and when incorporated into PEDOT(TOS) composites the gelatin rich domains facilitated enhanced ion mobility and biocompatibility.15 DSC and EQCM results support a model wherein the gelatin domains create interconnected pathways that facilitate ion transport. Conductivity measurements and CV results showed that the electronic properties of the PEDOT(TOS) : gelatin composites (from 1 : 0 to 1 : 2 ratios) remain largely unaffected with an electronic conductivity maintained at B300 S cm 1. The PEDOT(TOS):gelatin composites have only 5 times lower ion-mobility of organic ions than PEDOT(PSS) with similar electronic conductivity.6 However the PEDOT(TOS) composites offer the possibility to incorporate designed/selected bio-compatible molecules/polymers for the targeted application and also have the potential to be used in applications where anion movement is required. We expect that the method developed and described in this work will contribute towards the rational design of materials that simultaneously optimize electronic and ionic transport.
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