Post-Overoxidation Self-Recovery of Polypyrrole Doped with a

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Post-Overoxidation Self-Recovery of Polypyrrole Doped with a Metallacarborane Anion E. Crespo,† S. Gentil, C. Vin˜ as, and F. Teixidor* Institut de Cie` ncia de Materials de Barcelona (ICMAB/CSIC), Campus de la UAB, 08193 Bellaterra, Spain ReceiVed: July 16, 2007; In Final Form: August 29, 2007

By using the metallacarborane [Co(C2B9H11)2]-, [1]-, as a doping anion, PPy materials can be made 300500 mV more resistant to overoxidation. These materials are written as PPy/[A] to indicate a noncovalency between the anion and the PPy and have a clear overoxidation resistance limit, ORL, so that once this is surpassed the material becomes permanently nonelectroactive or dead. On the contrary if [Co(C2B9H11)2]- is chemically bonded to pyrrole through a spacer and then is copolymerized with pyrrole the resulting material, PPy-[1sp], has an electroactive behavior similar to PPy/[A] so that once an apparent ORL is reached the PPy-[1sp] becomes electroinactive but, very importantly, in few hours recovers the electroactivity. The situation is so attractive that even in the case that the overoxidation threshold was surpassed the system would react to restore itself. In this paper we demonstrate that [1]- not only enhances the overoxidation resistance but that, if grafted to the PPy strand, the applied voltage that can sustain the new material can go far beyond the overoxidation limit of the compositionally similar material with, however, no anion grafting. These properties are due to grafting of the metallacarborane to the PPy and have not been observed before. This behavior is not a consequence of the anion grafting to the PPy but depends on the nature of the anion.

Introduction Conducting electroactive polymers (CEPs) and their composites have been extensively studied as electroactive materials in recent years.1 Potential applications of these polymers include use in polymeric batteries, membranes, light-emitting diodes,2 sensing devices,3 actuators,4 circuits,5 and biosensors.6 Despite these possibilities one important challenge in CEPs research, and in particular polypyrrole (PPy), is in getting a true stable material. All known simple conducting polymers are chemically and mechanically weak. They need to be oxidized and doped by a counteranion to achieve significant conductivity. By controlling the dopant type and level their conductivity can be reversibly modulated over 15 orders of magnitude. The reason for the CEPs’ unusual electronic properties is the existence of a π-electron backbone that is responsible for their controllable electrical conductivity, low-energy optical transitions, low ionization potential, and high electron affinity. The extended π-conjugated system of the conducting polymer has single and double bonds alternating along the polymer chains. This alternating π-conjugated system that is the basis for the CEPs’ electronic properties is also the reason for their weakness. Overoxidation is the phenomenon usually attributed to the degradative process of PPy and is known to reduce the conjugation through partial oxidation of the double bonds and cross-linking of the polymer chains7 thereby producing a decrease in conductivity, and electroactivity.1,8 There seems to be two processes8 involved in the overoxidation, one of electrochemical nature and a second process that is caused by chemical attack by nucleophiles in the solid at the radical cationic centers. Thus, a high pH facilitates overoxidation.9 It has also been observed that an electroactivity degradation of 50% induces a 15% increase in the weight of the material, pointing to the presence of rigid and oxidized islands entrapped * Corresponding author. E-mail: [email protected]. † E. Crespo is enrolled in the UAB Ph.D. program.

by cross-linking points that prevent any ionic interchanges.10 This weight increase is understood by the introduction of carboxylates, hydroxyl, and carbonyl groups.11,12 When overoxidation has taken place the switching between oxidized and reduced forms of the PPy becomes irreversible;13 therefore, it is usually unwanted.14 One of the main properties of CEPs is their ability, in liquid electrolytes, to be reversibly switched between different oxidation states,15 typically upon application of 1 V or less. However, they suffer from a relatively slow speed, and poor efficiency, usually 1% or less16 in converting electrical to mechanical energy. When a reducing potential is applied to oxidized PPy doped with a large anion, electrons are transferred to the polymer backbone and cations, which are typically solvated, enter the material in response to the electric field. When ions and/or solvent enter the polymer it expands, and when they exit, it contracts.17-20 This mass insertion is responsible for the volume increase that is exploited in actuators and particularly in artificial muscles. Space between the polymer chains must be created in order for the ions to enter, so the ion current depends on chain movements,21 as well as on the degree of polymer solvation, the ion size, ion-polymer interactions, and the electrical field applied, among others.22,23 Slow switching speed between oxidized and reduced states can be overcome to some extent through the use of thin films and, importantly, higher voltages, but how can the latter be used while avoiding the overoxidation problem?16 Another area of growing interest for CEPs is tissue engineering because new technologies will require biomaterials that not only physically support tissue growth but that are also electrically conductive24 and, thus, able to stimulate specific cell functions or trigger cell responses.25,26 Conjugated polymers are effective for carrying current in biological environments27 and are therefore being considered for locally delivering electrical stimuli at the site of tissue damage to promote wound healing.28

10.1021/jp0755443 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/20/2007

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These applications and many others would benefit from a faster actuator response which can be improved through the application of elevated potentials. It has been proven that they do not harm the polymer when applied for short times,29-32 but this may not be the solution.34 With the objective to produce more overoxidation robust PPy materials we produced PPy doped with the highly stable [Co(C2B9H11)2]- anion,[1]-.33-36 The reason for this choice originated in the high stability of this anion that can withstand high severe radiation situations in processes leading to pacificate nuclear waste.37-41 In this paper we demonstrate that [1]- not only enhances the overoxidation resistance, but if it is grafted to the PPy strand, the new material can go far beyond the overoxidation limit of the compositionally similar material with, however, no anion grafting. It will also be shown that the anion-grafted material phenomenologically dies at potentials similar to the nongrafted one, but contrarily to the second, it considerably recovers its electroactivity within a few hours. These properties are due to the grafting of the metallacarborane to the PPy and have not been observed before. Experimental Section Materials. Pyrrole was purchased from Aldrich, distilled under reduced pressure before its use, and stored under nitrogen at 0 °C in the dark. 1,3-Propanesultone was obtained from Aldrich. Potassium was purchased from Fluka, and it was refluxed in anhydrous THF prior to use. Cs[3,3′-Co(1,2C2B9H11)2] (Cs[1]) was provided by Katchem Ltd., Prague. K[3,3′-Co(8-C4H4N-(CH2)2-O-(CH2)2-O-1,2-C2B9H10)(1′,2′C2B9H11)] (K[1sp]), K[3,3′-Co(8-C4H4N-(CH2)5-O-1,2C2B9H10)(1′,2′-C2B9H11)] (K[1sp′]), and K[C4H4N-(CH2)3SO3] (K[(CH2)3SO3]) were synthesized following reported procedures.40,47,48 Instruments. All electrochemical measurements were performed using a Voltalab10 PGZ100 potentiostat/galvanostat. Electrosynthesis of the Polymer Films. PPy/[1], PPy-[1sp], and PPy-[1sp′] samples were galvanostatically electropolymerized in acetonitrile over glassy carbon electrode (Ø ) 3 mm) at a constant current of 0.5 mA during 225 s, in a conventional undivided cell with a standard three-electrode system. PPy[(CH2)3SO3] samples were galvanostatically electropolymerized in water at the same conditions. Platinum wire and Ag/AgCl/ [N((CH2)3CH3)4]Cl 0.1 M (ACN) were used as a counter and references electrodes (EFc+/Fc vs Ag/AgCl/[N((CH2)3CH3)4]Cl 0.1 M (ACN) ) 850 mV). PPy/[1] samples were synthesized from acetonitrile solutions (1% water) of 0.1 M pyrrole and 3.5 × 10-2 M doping anion. PPy-[1sp] and PPy-[1sp′] materials have been synthesized from acetonitrile solutions (1% water) of 0.1 M pyrrole and K[1sp] or K[1sp′] in a molar ratio 3:1. Identical concentrations were used to obtain PPy-[(CH2)3SO3] samples. Linear Sweep Voltammetry Experiments. The linear sweep voltammetry (LSV) for every sample was done in aqueous 0.1 M NaCl at a scan rate of 0.5 mV/s, from 0 to 2000 mV versus Ag/AgCl/KClsat. Insult of PPy Samples by a Potential Stimuli (EOX). Each PPy sample was exposed to LSV in aqueous 0.1 M NaCl, from 0 mV to the targeted EOX versus Ag/AgCl/KClsat. After the anodic stimulus process, each sample was subject to cyclic voltammetry (CV) in a fresh NaCl solution. Activity Recovery of PPy Samples. To study the activity recovery of each material, a sample that had been oxidized to an EOX that was above the apparent overoxidation resistance

Figure 1. Doping anions studied in this work: derivatives of [3,3′Co(1,2-C2B9H11)2]- ([1]-) and [Py(CH2)3SO3]-. Hydrogen atoms on B and C are not represented. When the anion is grafted to PPy, the pyrrole unit is shown to indicate the polymerizable part. This is the case for [1sp]-, [1sp′]-, and [Py(CH2)3SO3]-.

limit was immersed in a fresh solution of 0.1 M NaCl without applying any potential stimuli. After different periods of time new CV experiments were done. Results and Discussion Overoxidation Resistance Limit: Differences between PPy Materials with [Co(C2B9H11)2]- Bonded and Not Bonded to the PPy Strands. By the Overoxidation resistance limit, ORL, we understand the potential EOX at which one oxidized PPy material no longer can be reduced. Beyond this potential the conducting material properties are definitely lost. Linear sweep voltammetry has shown to be a good technique to learn on the ORL of a PPy material doped with the generic anion A, PPy/[A]. The trace represents intensity versus the applied potential and typically displays a maximum beyond which the electroactivity of the material becomes null. Having surpassed the ORL the conversion between the oxidized and reduced forms of PPy becomes irreversible. From previous works PPy materials doped with [1]- or with derivatives of [1](Figure 1) have been found to be 300-500 mV more anodic resistant than PPy doped with conventional anions. More recently even a more overoxidation robust material was obtained by using the bulky borane derivatives of [B12H12]2-, [1-(C6H5CH2)2HNB12H11]- and [1-(C10H7CH2)2HNB12H11]-.42 The beneficial influence on the overoxidation resistance enhancement of BH-containing molecules in the PPy material has been further proven by the work of Fabre´ and coworkers43-45 which has shown an enhanced overoxidation stability in PPy materials containing neutral BH-containing molecules, in this occasion bound to the PPy strands. Therefore, it has been demonstrated that borane and metallaborane anions disconnected from the PPy strands on the one hand and neutral carboranes connected to the PPy strands on the other produce unprecedented overoxidation resistance to PPy materials. At this stage we wanted to see if the combination of metallaborane anions connected to the PPy strands would produce any

Post-Overoxidation Self-Recovery of Doped PPy

Figure 2. Comparison of LSVs for PPy/[1] (a) and PPy-[1sp] (b). The first one (a) shows a clear maximum located near 1250 mV, the ORL of PPy/[1], whereas (b) does not show this well-defined trace.

unprecedented effect. To this aim we reinvestigated the polymerization of the self-doped PPy in which nearly one-fourth of the pyrrole units are connected to a [Co(C2B9H11)2]- moiety through an O-CH2-CH2-O-CH2-CH2- dietoxy bridge. This material, PPy-[1sp], in which sp stands for spacer, which results from the copolymerization of [1sp]- with pyrrole, had been studied earlier,36 and its peculiar LSV did not show any welldefined peak (Figure 2). This led us to interpret that the overoxidation resistance of the material had been enhanced. Because the LSV trace of PPy-[1sp] did not present any maximum the conclusion was that the material was not overoxidized. As we shall see the absence of a maximum in the LSV trace of PPy-[1sp] had another possible interpretation that leads to a phenomenon not unveiled until this work. Tests To Quantify and Correlate the Overoxidation Resistance Limit of the Different PPy-Doped Materials. As we have indicated above, the very different LSV responses for PPy-[1sp] and PPy/[1] precluded any adequate comparison of their capacity to resist high anodic potentials. Therefore, an alternative method to LSV but providing a similar value of ORL was needed. After considering different possibilities we concluded that the electroactivity of the PPy/[A] materials could also provide adequate information on the ORL. This could be obtained by studying the decay of the CV cathodic or anodic peak currents IP,C or IP,A or both versus an increasing potential stimulus (EOX). IP,C and IP,A stand for the intensity of the cathodic and anodic peaks in the CV electroactivity tests. The concept on which this is sustained is based on the fact that if the applied EOX does not degrade the PPy/[A] material the IP,C or IP,A of the cations’ exchange test will remain constant; when the applied EOX gets closer to the ORL a decrease in the absolute values of IP,C or IP,A should be observed, and finally the IP,C or IP,A values should be null after the EOX has exceeded the ORL. Therefore, a trace of IP,C (or IP,A) versus EOX should provide ORL, and the dIP,C/dEOX should be physically very similar to the trace of the LSV. The difference is that, whereas ORL can be obtained from the LSV with only one experiment, getting it from the electroactivity tests requires the preparation of as many samples as data points are wanted and the oxidation of the different samples to the applied EOX, including obviously the ORL. Once each sample has been oxidized to the applied EOX it is subjected to the CV electroactivity test done in an electrolyte solution identical to that utilized to get the ORL from the LSV. To minimize the influence of the sample’s oxidation history in the electroactive tests the samples were oxidized following the same protocol as to run one LSV. The comparable ORL information given by the LSV and the electroactivity tests was proven by

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Figure 3. LSV for PPy/[1] in 0.1 M NaCl. The maximum of the trace is considered the overoxidation resistance limit (ORL). In the same figure, arrows indicate the chosen potentials at which individual PPy/ [1] samples were treated before the CV electroactivity tests shown in Figure 4.

Figure 4. PPy/[1] ion-exchange voltammograms in 0.1 M NaCl after oxidation at E below ORL (a) 400, (b) 1050, (c) 1150, (d) 1250 mV and E above ORL (e) 1300 and (f) 1400 mV. For values more positive than 1250 mV no ion capture is found and material overoxidation has already occurred.

Figure 5. Sigmoidal curve for PPy/[1] obtained from the cathodic intensity maxima (|IP,C|) plotted vs the EOX stimulus, and the dIP,C/ dEOX plotted vs EOX. A well-defined maximum is obtained with this method enabling to calculate PPy/[1]’s ORL as 1220 mV. This value is near the 1250 mV obtained with an LSV.

studying PPy/[1] by both methods. The ORL obtained by LSV is 1250 mV referenced to the Ag/AgCl/KClsat electrode as shown in Figure 3. The electroactivity tests were done based on eq 1.

[PPyn+(1-)n] + nNa+ + ne- h [PPy(1-)n(Na+)n] (1) In Figure 4 the CV sweeps corresponding to six PPy/[1] films exposed to different oxidation potentials EOX are represented. The applied EOX correspond to potentials shown schematically in Figure 3 as arrows. The peak intensities decrease from a to f, or conversely with the increasing EOX stimulus. It is noticeable that the intensity is practically zero for e and f films that were exposed to EOX larger than the material’s ORL. The dIP,C/dEOX obtained from the plot of |IP,C| versus EOX (Figure 5) provides a maximum at 1220 mV very near to the LSV value of 1250 mV.

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Figure 6. (a) PPy-[1sp] ion-exchange voltammograms in 0.1 M NaCl after oxidation at E (trace a) 600, (trace b) 1080, (trace c) 1150, (trace d) 1200, and (trace e) 1300 mV. (b) Sigmoidal curve |IP,C| vs EOX and the dIP,C/dEOX plotted vs EOX for PPy-[1sp]. The curve shows a maximum at 1120 mV; this value is considered the apparent ORL of PPy-[1sp].

The two measures on PPy/[1], 1250 mV from LSV and 1220 mV from dIP,C/dEOX, are very similar, more if we take into consideration that they were obtained from samples of distinct origin and size. This indicates that any of these tests gives very similar information on the ORL and that structure alteration by overoxidation generates electroactivity losses. To substantiate further the electroactivity/LSV data both experiments were also performed with PPy/[DBS] (DBS ) dodecylbenzenesulfonate). This material shows an electroactivity behavior comparable to PPy/[1], and as for PPy/[1], the matching of LSV maxima with electroactivity deterioration (dIP,C/dEOX) was excellent (see the Supporting Information). Overoxidation Process in PPy-[1sp]. Having proven that the ORL information provided by the LSV and the electroactivity tests is comparable for well-behaved LSV materials we studied again the overoxidation process of PPy-[1sp]. In this occasion, however, the studies were done on the basis of the electroactivity tests. These were performed as for the studies on PPy/[1] but including more samples and therefore a larger number of EOX stimuli to overcome the absence of information on the breakdown potential commonly obtained from the LSV. For the well-behaved LSV materials the breakdown potential is the ORL; thus, for the non-well-behaved LSV materials the breakdown potential has been named apparent ORL. EOX stimuli at 200, 600, 800, 1050, 1150, 1165, 1200, 1250, and 1300 mV were applied. The CVs of the last four samples reveal that their IP,C’s equal zero indicating the death of the material (Figure 6a). Plotting the |IP,C| maxima versus the EOX stimulus gives a graph similar to that shown in Figure 5. From the dIP,C/dEOX the apparent breakdown potential was calculated to be 1120 mV (Figure 6b). This value states that both materials PPy/[1] and PPy-[1sp] have similar characteristics (1220 vs 1120 mV) as could be expected from materials that have the same main components. Probably, the 100 mV difference can be attributed to structural differences caused by the spacer. But if they are so similar what is the property that causes so different LSVs? As mentioned earlier, once the ORL has been surpassed the degraded PPy/[A] material becomes electroinactive and the oxidized T reduced transformation is irreversible. Serendipity led us to perform an electroactivity test on a PPy-[1sp] material that had previously been stimulated to an EOX exceeding the apparent ORL, therefore having given a flat electroactive test. Evidence says that the serendipity test was done shortly after the overoxidizing experiment had taken place, near 15 min later, and the response was neither that of a just prepared thus nonpotential stimulated material nor of one that had been overoxidized. Subsequent experiments were carried on to interpret this contradictory measurement, and their results clearly indicated that PPy-[1sp] materials that had been oxidized to

Figure 7. Gradual activity recovery of PPy-[1sp] shown by ionexchange tests in 0.1 M NaCl: (a) immediately after the insult of PPy[1sp] at 1300 mV, (b) after 30 min in the same electrolytic solution, (c) after 1 h, and (d) after 18 h.

EOX exceeding the apparent ORL were capable of self-recovery or in other words to not fulfill the irreversibility rule of the overoxidized T reduced process. Whereas PPy/[1] after overoxidation stays indefinitely electroinactive, PPy-[1sp] films taken to EOX stimulus exceeding their ORL gradually restore their electroactivity. The restoration process takes place plainly, with no applied voltage. To avoid any misinterpretation due to the different ORL between PPy/[1] and PPy-[1sp], the latter was insulted at EOX 1300 mV well beyond the ORL of PPy/ [1]. Obviously the material gave an immediate flat electroactivity test, but even under these circumstances the insulted PPy-[1sp] reaches 39% of its original capacity in 30 min and 77% after 18 h. The gradual recovery of the activity of PPy-[1sp] is shown in Figure 7. Trace a corresponds to the CV test immediately after the insult at 1300 mV, trace b after 30 min, trace c after 1 h, and trace d after 18 h. These CV traces have been obtained in the same electrolytic solution in which the self-repair has taken place. It is worth it to say that identical recovery is obtained in plain water. The tendency of the material to self-repair is so manifested that even some recovery, 36% of its original capacity, has been observed after 18 h of having insulted the material at 1700 mV. Compositional and Morphological Implications of the Overoxidation on PPy-[1sp]. As described, the self-recovery of PPy-[1sp] is not total. It reaches 77% after 18 h for a material that had been insulted to an EOX of 1300 mV and 36% for another sample of PPy-[1sp] insulted to 1700 mV. Therefore, the higher the insulting potential the lower is the recovery. It could be inferred from this data that the higher the applied insulting potential the higher the damage on the PPy strands. This may not be necessarily true, or at least is not the only reason. In Figure 8 there are represented three SEM images corresponding to plain PPy-[1sp] (Figure 8A), nonelectroactive overoxidized PPy-[1sp] labeled as OPPy-[1sp] (Figure 8B), and self-repaired PPy-[1sp] (Figure 8C). An inspection to

Post-Overoxidation Self-Recovery of Doped PPy

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Figure 8. SEM images corresponding to plain PPy-[1sp] (A), nonelectroactive overoxidized PPy-[1sp] labeled as OPPy-[1sp] (B), and selfrepaired PPy-[1sp] (C).

Figure 9. UV-vis spectra of the electrolytes used during the overoxidation of PPy/[1] (b) and PPy-[1sp] (c), along with the spectrum of the doping anion [1]- (a).

sample A shows the absence of fissures that are otherwise observed in both B and C samples. These fissures are most probably due to the Joule effect as a result of applying a high voltage and may degrade the electroactivity of the self-repaired samples. In support of this is that little damage has been produced to the PPy strands as evidenced by the UV-vis spectra of the electrolytes on which the overoxidation has taken place. In Figure 9 the spectrum of the electrolytes for the overoxidation of PPy/[1] and of PPy-[1sp] along with the spectrum of the doping anion [1]- are presented as traces b, c, and a, respectively. The peak at 282 nm, typical of [1]- is also found in the electrolyte in which PPy/[1] was overoxidized but is not observed in the electrolyte for the overoxidation of PPy-[1sp]. This means that [1]- is not extruded during the overoxidation of PPy-[1sp], whereas it is the case during the overoxidation of PPy/[1]. As we have proven earlier and others had suggested42 during the overoxidation process anions are extruded out of the polymer. Thus, the chemical stability of the PPy strands in what concerns the doping anion has been preserved. On the other hand the attenuated total reflection infrared (ATR-FTIR) spectroscopy of the overoxidized samples OPPy/[1], OPPy/ [PF6], and OPPy-[1sp] show the existence of a band near 1700 cm-1 presumably due to CdO that was not observed in the plain samples PPy[1], PPy/[PF6], and PPy-[1sp] (Figure 10). This information relates to chemical changes having occurred on the surface of the materials, not necessarily on the bulk; however, in what concerns the surface it appears that the changes there present are independent of the nature of the bond between

Figure 10. ATR-FTIR spectra of the PPy-[1sp] polymer before (a) and after having been insulted at a potential above ORL (1400 mV) (b). In the overoxidized polymer the frequency at 2526 cm-1 corresponds to the B-H stretching vibration, whereas the one at 1697 cm-1 is attributed to the CdO group of the overoxidized material.

the anion and the PPy strands and that some chemical oxidation has taken part, at least, on the surface. So, despite the fact that some chemical oxidation has taken place in the PPy-[1sp] upon application of a high EOX, the material gradually recovers its electroactivity with time. This has to do with the nature of the metallacarborane and the fact that it is bonded to the PPy. By applying higher EOX’s the ratio of chemically affected PPy shall increase, and a more limited recovery takes place with time. Relationship between Anion Grafting and Self-Repair Capacity. That the grafting of [1]- to the PPy strands was responsible for the self-repair mechanism was proven by producing PPy-[1sp′]. As can be seen in Figure 1, the length of the spacer is identical to PPy-[1sp] but the chain contains one ether group less. The performance activity and self-repair capacity of PPy-[1sp′] parallels this for PPy-[1sp]. Once the self-repair capacity of a PPy material had been established and proven, we wanted to prove if the phenomenon had to do with the anion being grafted to the PPy strand. To this aim we performed the above studies on the well-described [Py(CH2)3SO3]-. This was copolymerized with Py to obtain a new material PPy-(CH2)3SO3,46 which was exposed to different potentials stimulus above its ORL. Irrespective of the time given to recover no self-recovery of the electroactive properties was observed. Conclusions It has been then proven that by using special but not uncommon anions PPy materials can be made more resistant

18386 J. Phys. Chem. C, Vol. 111, No. 49, 2007 to overoxidation problems. This shall permit the use of a wider range of anodic potentials facilitating the application of PPy materials and devices and opening new possibilities. The situation is so attractive that even in the case that the overoxidation threshold was surpassed the system would react to restore itself. We are conscious that much work needs to be done to understand the phenomena presented here, and perhaps less unorthodox anions may give the same results, but we also would like to point out that [Co(C2B9H11)2]-, [1]-, is not at all a strange anion to work with. We also would indicate that some controversy may arise by maintaining the ORL terminology for these self-repairing materials taking in consideration that they really do not die as they permit the overoxidized T reduced process. We have maintained it because its value has been taken from the EOX at which the material gives a flat electroactivity test that corresponds in common materials at the potential at which the oxidized T reduced transformation is irreversible. Acknowledgment. We dedicate this work to Prof. R. Sillanpa¨a¨ on the occasion of his 60th anniversary for his great contribution to the determination of the molecular structures of boron clusters. This work has been supported by CICYT (MAT2006-05339) and Generalitat de Catalunya 2005/SGR/ 00709. The research of E.C. was partially supported by a Postgrado I3P Grant from CSIC. S.G. thanks the French Ministry of Foreign Affairs for a “Lavoisier” postdoctoral Grant. F.T. designed and conceived the experiments, and E.C. and S.G. carried them out; F.T. and C.V. interpreted data and cowrote the paper. Supporting Information Available: Tests to quantify the overoxidation resistance limit of PPy/[DBS] and the self-repair capacity of PPy/[1], PPy-[1sp′], and PPy-(CH2)3SO3 materials. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Handbook of Conducting Polymers; Nalwa, S. H., Ed.; John Wiley & Sons: New York, 1997; Vols. 1-3. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (3) Leclerc, M. AdV. Mater. 1999, 11, 1491. (4) Smela, E.; Ingana¨s, O.; Lunstrom, I. Science 1995, 268, 1735. (5) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Vos, B. M. W.; Spiering, A. J. H.; Jansen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685. (6) Gerritsen, M.; Kros, A.; Lutterman, J. A.; Nolte, R. J. M.; Jansen, J. A. J. InVest. Surg. 1998, 11, 163. (7) Zykwinska, A.; Domagala, W.; Pilawa, B.; Lapkowski, A. Electrochim. Acta 2005, 50, 1625. (8) Otero, T. F.; Boyano, I. Electrochim. Acta 2006, 51, 6238. (9) Beck, F.; Barsch, U.; Michaelis, R. J. Electroanal. Chem. 1993, 351, 169. (10) Otero, T. F.; Marquez, M.; Suarez, I. J. J. Phys. Chem. B 2004, 108, 15429. (11) Ghita, M.; Arrigan, D. W. M. Electroanalysis 2004, 16, 979. (12) Rodriguez, I.; Scharifker, B. R.; Mostany, J. J. Electroanal. Chem. 2000, 491, 117. (13) Fernandez, I.; Trueba, M.; Nun˜ez, C. A.; Rieumont, J. Surf. Coat. Technol. 2005, 191, 134.

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