Anal. Chem. 1997, 69, 4060-4064
Modeling Potentiometric Sensitivity of Conducting Polymers Agata Michalska,† Ari Ivaska,‡ and Andrzej Lewenstam*,‡,§
Department of Chemistry, University of Warsaw, PL-02093 Warsaw, Poland, Laboratory of Analytical Chemistry, A° bo Akademi University, FIN-20500 Turku-A° bo, Finland, and Faculty of Material Science and Ceramics, University of Mining and Metallurgy, PL-30059 Cracow, Poland
The influence of different polymerization conditions and electrochemical processes provoked by soaking on the potentiometric sensitivity of conducting polymer films is discussed. Poly(pyrrole) doped with hexacyanoferrate(II) is selected as a model polymer. It is shown that, depending on the conditions applied during film deposition and soaking, either anionic or cationic potentiometric responses can be observed and related to the composition of the film. The potentiometric sensitivity of the conducting polymer films is analyzed and interpreted by means of generalized theoretical schema. In recent years, conducting polymers (CPs) have been intensively investigated, being also of interest as a sensor material in the field of potentiometry.1,2 Electropolymerization of heteroaromatic compounds, in particular, poly(pyrrole) (PPy), offers new possibilities in the design of sensors. Relatively stable PPy films can conveniently be polymerized in aqueous solution on various substrate materials.3 This certainly ranks PPy among the most attractive sensor materials, especially for the construction of allsolid-state ion-selective electrodes.4-6 The potentiometric responses of PPy to anions, such as chloride,7-10 bromide,7 nitrate,7,10,11 fluoroborate,10 and perchlorate,12 have been reported. It has been shown that the potentiometric sensitivity of PPy is determined by the polymer layer, and not by the substrate,13 which is of fundamental importance when discussing the potentiometric response of a CP. In most cases discussed in the literature, the open-circuit sensitivity, characterized by the potentiometric slope, was found to be smaller than the theoretical Nernstian value, which indicates some contribution * Address correspondence to this author at A° bo Akademi University. † University of Warsaw. ‡ A° bo Akedemi University. § University of Mining and Metallurgy. (1) Ivaska, A. Electroanalysis 1991, 3, 247-254. (2) Josowicz, M. Analyst 1995, 120, 1090-1123. (3) Evanns, G. P. The Electrochemistry of Conducting Polymers. In Advances in Electrochemical Science and Engineering; Gerisher H., Tobias, Ch., Eds.; VCH Publishers, Inc.: Weinheim, FRG, 1990; pp 1-73. (4) Cadogan, A.; Gao, Z.; Lewenstam, A.; Ivaska, A.; Diamond, A. Anal. Chem. 1992, 64, 2496-2501. (5) Hulanicki, A.; Michalska, A. Electroanalysis 1995, 7, 692-693. (6) Migdalski, J.; Blaz, T.; Lewenstam, A. Anal. Chim. Acta 1996, 322, 141149. (7) Dong, S.; Sun, Z.; Lu, Z. J. Chem. Soc., Chem. Commun. 1988, 993-995. (8) Dong, S.; Sun, Z.; Lu, Z. Analyst 1988, 113, 1525-1528. (9) Dong, S.; Che, G. Talanta 1991, 38, 111-114. (10) Cadogan, A.; Lewenstam, A.; Ivaska, A. Talanta 1992, 39, 617-620. (11) Hutchins, R. S.; Bachas, R. G. Anal. Chem. 1995, 67, 1654-1660. (12) Lu, Z.; Sun, Z.; Dong, S. Electroanalysis 1989, 1, 271-277. (13) Pei, Q.; Qian, R. Electrochim. Acta 1992, 37, 1075-1081.
4060 Analytical Chemistry, Vol. 69, No. 19, October 1, 1997
from cations.10,14 Predominant cationic responses were observed for PPy synthesized with bulky and immobile doping anions, like indigo carmine14,15 or naphthalenesulfonates,16 or metal-complexing ligands.6 Both the experimental results7-12,14,16 and theoretical17 considerations have indicated that the potentiometric responses may be influenced by the intrinsic film properties. In particular, it has been shown that the rest potential of the polymer electrode may be influenced by anions and/or cations present in the polymer phase.17 It is well-known that the conditions of electrochemical polymerization strongly affect the properties of the deposited layer3 (e.g., morphology, conductivity), but there is only a little evidence of how the conditioning (soaking) processes after polymerization influence the composition of the polymer film. This question is especially important for determining the potentiometric sensitivity of the CP films, whose property is usually recorded for equilibrated CP layers. It is important to notice that, disregarding potentials applied to the polymer during electrodeposition, the conditioned films usually exhibit rather similar open-circuit potentials.8,14 Even a difference of 1 V in an externally applied potential often results in no more than a 100 mV difference in the open-circuit potential readings for the films in the same electrolyte solution. In the case of PPy films, it was observed that, regardless of reduction of the polymer at -0.5 V or oxidation at +0.5 V prior to soaking, the potentials of the PPy electrodes were close to 300 mV after conditioning.14 This is obviously the result of a change in the polymer oxidation level during soaking as a result of the equilibration processes. The influence of the redox couple in the solution during open-circuit conditioning has already been documented.14 The occurrence of a redox reaction between the polymer film and the components of an aqueous potassium chloride solution has also been confirmed by coulometric experiments.18 In this paper, we consider theoretically, and confirm experimentally, the possibility of modeling the potentiometric response of the polymer films by appropriate selection of the conditions during deposition and the postdeposition conditioning of the film formed. A demonstration of the influence of different factors on the final open-circuit performance of the film is made by using (14) Bobacka, J.; Gao, Z.; Ivaska, A.; Lewenstam, A. J. Electroanal. Chem. 1994, 368, 33-41. (15) Gao, Z.; Bobacka, J.; Lewenstam, A.; Ivaska, A. Electrochim. Acta 1994, 39, 755-762. (16) Okada, T.; Hayashi, H.; Hiratani, K.; Sugihara, H.; Koshizaki, N. Analyst 1991, 116, 923-927. (17) Lewenstam, A.; Bobacka, J.; Ivaska, A. J. Electroanal. Chem. 1994, 368, 23-31. (18) Michalska, A.; Maksymiuk, K.; Hulanicki, A. J. Electroanal. Chem. 1995, 392, 63-68. S0003-2700(97)00227-8 CCC: $14.00
© 1997 American Chemical Society
PPy doped with the multivalent, cation-complexing anion, hexacyanoferrate(II) (denoted as X4-), as the model compound. The electrochemical and, in particular, potentiometric data on PPy(X) are limited but encouraging for further research. The accumulation of cations and the release of Fe(CN)64- anions upon the film reduction have been reported earlier.18-25 Moreover, despite the method of electrochemical polymerization, the PPy(X) layer favors the presence of the Fe(CN)64- anions. Therefore, the redistribution of charges within the polymer after deposition has been postulated.26,28 These unique electrochemical properties of the PPy(X) layer offer additional possibilities to model favorable potentiometric properties of CP films. THEORY The chemical composition of a conducting polymer film results from two processes: electrodeposition (electropolymerization) and conditioning (soaking), both of which can be purposely engaged in the modeling of the desirable open-circuit properties of CP films. The main difference between electropolymerization and conditioning is in the driving force. In the case of electropolymerization, it is the external electric potential; for conditioning, it is the chemical potential of the reactants in the solute that drives the reaction. During both the deposition and the soaking processes, the polymer can be oxidized or reduced, doped, undoped, and/or redoped. To clarify the influence of these processes on the film properties, a general theoretical model is presented below. To simplify the formulation of the model, it is accepted that the oxidation level of the polymer is primarily dictated by the conditions during electrodeposition (potential-sweep technique, final potential, composition of the solution) and that the redox and ionic composition of the film can change during the equilibration process by soaking. In practice, the polymer film is equilibrated directly after deposition, and the polymer film during conditioning is in equilibrium with the soaking solution. Equilibration of the Oxidized Polymer. If the monomer is polymerized in the presence of a supporting electrolyte containing anions (polymer doping anions) X4- and cations N+, and the electropolymerization is terminated at a positive potential where the deposited film is oxidized, and then the soaking in the solution containing anions A- (different from X4-), cations N+, and, in addition, a reducing agent RED can be expressed by the following reaction:
[(a + b)PPy+‚kX-(4-)]ox + mN+ + (b + k)RED + zA- f [(b)PPy0‚(a)PPy+‚(m)N+‚(q)X4-‚(z)A-]eq + (k - q)X4- + (b + k)OX (1)
where PPy0 and PPy+ represent the reduced and oxidized polymer backbone, respectively. For simplification, the oxidized polymer (19) Breen, W.; Cassidy, J. F.; Lyons, M. E. G. J. Electroanal. Chem. 1991, 29, 445-450. (20) Dong, S.; Lian, G. J. Electroanal. Chem. 1990, 291, 23-39. (21) Lian, G.; Dong, S. J. Electroanal. Chem. 1989, 260, 127-136. (22) Zinger, B.; Miller, L. L. J. Am. Chem. Soc. 1984, 106, 6861-6863. (23) Miller, L. L.; Zinger, B.; Zhou, Q.-X. J. Am. Chem. Soc. 1987, 109, 22672272. (24) Chen, Ch. Ch.; Wei, Ch.; Rajeshwar, K. Anal. Chem. 1993, 65, 2437-2442. (25) To¨lgyesi, M.; Szu ¨ cs, A.; Visy, Cs.; Nova´k, M. Electrochim. Acta 1995, 40, 1127-1133.
backbone is represented by the polaron PPy+ only. The stoichiometric coefficients a, b, m, k, q, and z, a partial charge , and the symbol “‚” are used to cover all the possible compositions of CP. In reaction 1, a > 0, k > 0, 0 e e 1, k > q, and, by virtue of mass and charge balance, (4 - )k - 4q ) b - m + z. Subscripts ox and eq denote the oxidized and equilibrated polymer, respectively; barred symbols refer to the polymer phase. One-electron oxidation of RED to OX, and an ability of the RED species to reduce the oxidized polymer layer, i.e. [(a+b)PPy+‚kX-(4-)]ox, are assumed. Reaction 1 shows that the reduction of the polymer backbone by the solution components (e.g., water) decreases the number of positive charges within the polymer and leads to consecutive dopant expulsion and/or cation uptake, depending on the nature of the doping anion (X). In a case of the polymer doped with bulky anions that are immobile within the polymer film (k ≈ q), cation uptake is expected in the course of equilibration. Consequently, oxidative polymerization of PPy doped with bulky, immobile anions will favor the cationic responses of p-doped polymer. The complexing properties of the doping anion (e.g., as in the case of hexacyanoferrate(II)) are expected to enhance the cation incorporation both in the polymerization step and during conditioning. During the equilibration process, the anion exchange between the polymer layer containing immobilized dopants and the solution is expected to be negligible (z f 0), and, therefore, reaction 1 can be rewritten in the following form:
[(a + b)PPy+‚kX-(4-)]ox + mN+ + (b + k)RED f [(b)PPy0‚(a)PPy+‚(m)N+‚(q)X4-]eq + (k - q)X4- + (b + k)OX (2) where (k - q) is close to zero. Equilibration of the Reduced Polymer. Assuming that, after electrochemical deposition of the film, the polymer layer was reduced, i.e., that the deposition was completed at a potential more negative than the polymer reduction potential, the reaction that takes place during conditioning can be written as follows:
[(b + c)PPy0‚(a - c)PPy+‚(m + n)N+‚(p)X4-]red + cOX + zA- f [(b)PPy0‚(a)PPy+‚(m)N+‚(q)X4-‚(z)A-]eq + (p - q)X4- + nN+ + cRED (3) where c, n, and p are the stoichiometric coefficients, and all other symbols have the same meaning as in reaction 1. By virtue of mass and charge balance, 4p - 4q ) z + n - c. The barred symbols refer to the polymer phase, and subscripts red and eq denote partially reduced and equilibrated polymer, respectively. One-electron reduction of OX and an ability of the OX species to oxidize the reduced polymer layer, i.e. [(b+c)PPy0‚(a-c)PPy+‚(m+n)N+‚(p)X4-]red, are assumed. (26) Zago´rska, M.; Pron, A.; Lefrant, S.; Bernier, P. Synth. Met. 1987, 18, 4348. (27) Przyluski, J.; Zago´rska, M.; Pron, A.; Kucharski, Z.; Suwalski, J. J. Phys. Chem. Solids 1987, 48, 635-640. (28) Daroux, M.; Gerdes, H.; Scherson, D.; Eldrige, J.; Kordesch, M. E.; Hoffman, R. W. The Electrochemical Society Extended Abstracts; San Francisco, CA, May 8-13, 1983; Vol. 83-1, Abstarct 548, p 829.
Analytical Chemistry, Vol. 69, No. 19, October 1, 1997
4061
As can be seen in reaction 3, a primarily reduced polymer can be oxidized by the components of the soaking solution (e.g., oxygen) to reach the equilibrium potential. The oxidation of the polymer backbone results in the incorporation of anions and/or expulsion of cations. Both cationic and anionic potentiometric sensitivity of a polymer layer equilibrated according to reaction 3 can be expected. Which of the responses is predominant primarily depends on the molar ratio of cation to anion within the polymer film and the individual mobility of the ions. To get the complete picture of the problem, two different cases have to be considered: (i) deposition resulting in partially reduced film with dopants that can be expelled and (ii) deposition resulting in partially reduced polymer with immobile dopants. (In both of these cases, also anion exchange between polymer and solution can be observed, but, due to the rather low content of dopants in the reduced layer, as in case i, or due to the immobility of the doping anions in case ii, this process is assumed to be negligible, (p - q) f 0.) Let us discuss these two cases in more detail. (i) In the case of the polymer doped with mobile anions, the reduction results in the expulsion of dopants from the polymer structure (p < z). The reduced polymer is expected to contain only small amounts of the doping salt NX. During oxidative equilibration, positive charges created inside the PPy are compensated for by anions from the conditioning solution, A(redoping the polymer), and/or by the expulsion of cations N+. For these polymer films, anionic responses are expected, in accordance with reaction 3. Assuming that the dopant is not oxidized during equilibration, reaction 3 can be rewritten in the following form:
[(b + c)PPy0‚(a - c)PPy+‚(m + n)N+‚(p)X4-]red + cOX + zA- f [(b)PPy0‚(a)PPy+‚(m)N+‚(p)X4-‚(z)A-]eq + nN+ + cRED (4) (ii) In the case of the polymer layers doped with immobile dopants, cation uptake is observed during reduction.17,18 In the course of the oxidative equilibration, the expulsion of cations from the polymer (n > 0) is expected. Although cations are released from the PPy layer, their content in the equilibrated polymer layer is expected to be still great enough to ensure cationic potentiometric response (during equilibration, the polymer is only partially oxidized). If the doping anion is not oxidized in equilibration, reaction 3 can be rewritten as
[(b + c)PPy0‚(a - c)PPy+‚(m + n)N+‚(p)X4-]red + cOX f [(b)PPy0‚(a)PPy+‚(m)N+‚(p)X4-]eq + +nN+ + cRED (5) It should be noted that, for thick poly(pyrrole) films doped with hexacyanoferrate(II), both the expulsion and retention of dopants, accompanied with cation incorporation in the course of reduction, have been reported.19,20 It is, therefore, expected that the potentiometric responses of these types of films are influenced by the composition of the monomer solution and/or electrolysis time, i.e., the final composition, structure, and thickness of the polymer film. 4062 Analytical Chemistry, Vol. 69, No. 19, October 1, 1997
Table 1. Potentiometric Sensitivity of PPy Film Electrodesa monomer solution composition (pyrrole, potassium hexacyanoferrate(II), mol L-1) A (0.1, 0.1) B (0.46, 0.1) C (0.46, 0.5) D (0.1, 0.5)
slopes in KCl after soaking (mV decade-1) CSCV POTS 21.4 -40.2 -33.4 35.6
44.9 46.9 33.6 47.0
a CSCV and POTS denote continuous scanning cyclic voltammetry and potentiostatic polymerization, respectively.
Finally, it should be reiterated that the model presented herein formally allows for the same composition of the polymer after equilibration is completed, regardless of whether the film submitted for soaking has been in reduced or oxidized form (reactions 1 and 3). The model thus predicts the convergence of the rest potentials of the polymer films during soaking to a hypothetical “common” value, which may be attained from the positive side in the case of initially oxidized films or from the negative side in the case of initially reduced films. EXPERIMENTAL SECTION Materials and Methods. All the reagents used were of analytical grade. The electrochemical polymerization of pyrrole was conducted in a conventional three-electrode cell under nitrogen atmosphere. All the reported potentials refer to the silver/silver chloride reference electrode with a bridge containing saturated KCl and 1 mol L-1 Na2SO4 in the voltammetric and potentiometric experiments, respectively. A platinum disk of area 0.07 cm2 was used as the working electrode. Before each polymerization, the surface of the working electrode was polished with 1 µm aluminum oxide powder to a mirror-smooth surface and washed with water in an ultrasonic bath. A glassy carbon rod was used as the auxiliary electrode. The voltammetric experiments were performed with an EG&G Princeton Applied Research potentiostat/galvanostat, Model 273, software version 2.00. The potentiometric measurements were made with a Metrohm potentiometer, Model E576. The elemental analysis of the film composition by EDAX was performed with a Princeton Gamma Tech IMIX apparatus coupled with a thin-window detector. The EDAX was applied to assess qualitatively the influence of soaking on the composition of the film. For this reason, and to ensure the same film thickness, the measurements were performed for the same batch of film samples before and after soaking with constant count time for each element. In all EDAX experiments, the background level was equal to 25 counts at 6 keV. Polymerization and Pretreatment Conditions of the Poly(pyrrole) Films. The polymers were deposited either by continuous scanning cyclic voltammetry (CVCS) or by potentiostatic polymerization (POTS). In film deposition by CSCV, linear sweep within the potential range from -0.5 to 1.0 V, with scan rate 20 mV s-1, was used. Usually, five scans were applied, and the deposition of the film was terminated at -0.5 V. In the case of deposition by the POTS method, +1.0 V for 90 s was applied. The poly(pyrrole) films were deposited from aqueous solution containing pyrrole monomer and potassium hexacyanoferrate(II) at different concentrations, as given in Table 1. Deposition was
Table 2. Results of the EDAX Determination of Film Composition number of counts polymer layer film
potassium
chloride
iron
CSCV A after soaking CSCV B unsoaked CSCV B after soaking CSCV C unsoaked CSCV C after soaking POTS C unsoaked POTS C after soaking CSCV D after soaking
60 240 background 340 50 100 230 100
background background 150 background 180 background background background
background 100 100 150 110 240 230 background
always conducted under nitrogen atmosphere, and the solution was continuously stirred. Immediately after disconnection of the external potential, the polymer was transferred to the conditioning solution. Before the potentiometric measurements and EDAX analysis, the poly(pyrrole) films were soaked (conditioned) for 15-20 h in 0.1 mol L-1 KCl unless otherwise specified. A stable potential with a drift of less than 0.5 mV min-1 was recorded after 3 min in most cases. This contrasts with unsoaked films, for which any rational criteria of potentiometric signal stability could not be applied. RESULTS AND DISCUSSION The potentiometric signals of the poly(pyrrole) films after soaking were recorded in potassium chloride in the concentration range from 1 to 10-5 mol L-1. The slopes of the linear parts of the calibration plot, calculated for the concentration range from 0.1 to 10-3 mol L-1, are given in Table 1. The results of the EDAX analysis of the molecular composition of the poly(pyrrole) films prior to and after soaking are presented in Table 2. As can be seen from Table 1, the potentiometric sensitivity of the films, measured after soaking, depends on the monomer solution composition and the electrochemical method of deposition used. On the other hand, the results presented in Table 2 indicate that film composition can change in the process of equilibration imposed by soaking. In view of Tables 1 and 2, it is, therefore, evident that the potentiometric sensitivity is clearly combined with the composition of the films after soaking. This conclusion thus provides a possibility of more elaborate interpretation of the influence of soaking on the potentiometric properties of CP films. Equilibration of the Oxidized Poly(pyrrole) Film. By virtue of the model presented above, it can be expected that the poly(pyrrole) films prepared by oxidative polymerization (POTS) would undergo reductive equilibration during soaking, in accordance with reaction 2. It could be anticipated that concurrent reduction of both the polymer units and the doping anions, combined with the inclusion of mobile cations, would result in a descending value of the open-circuit potential.24,25 Indeed, rather similar final rest potential values, about 100 mV, were observed after conditioning in 0.1 mol L-1 KCl for all POTSdeposited layers, even if the polymerization potential for this technique was set to +1000 mV. These rest potentials indicate that hexacyanoferrate ions should not have been expelled from the film, and, thus, they are relatively immobile inside the polymer matrix. This assertion is in accordance with the results of Miller et al.23 These authors showed that, for the expulsion of hexacyanoferrate dopant from poly(pyrrole) matrix formed with a total charge of 2 mC cm-2, a
potential more negative than -0.4 V vs SCE is needed. In all cases, the effective charge consumed for POTS polymerization (from the total polymerization charge, the charge consumed for the oxidation of Fe(CN)64- ions, assessed in a separate experiment, was subtracted) was greater than 2 mC cm-2. For the polymer films obtained from monomer solutions B and C, this was 11.3 and 9.0 C cm-2, and for the films prepared from monomer solutions A and D, 3.3 and 2.2 C cm-2, respectively. Since the PPy layers after equilibration were characterized by more positive potential values, hexacyanoferrate ions in POTS-polymerized layers ought to be treated as immobile dopants (k ≈ q). The validity of this hypothesis was confirmed by the EDAX studies. The results of the analysis of the POTS C film, as given in Table 2, confirm that the dopant content (measured as number of counts for iron) was practically unaffected by soaking. Conditioning, however, increased the incorporation of potassium into the film. The potassium content in the film, measured as the number of counts, was more than 2 times higher in the conditioned film than in the unconditioned one. As could be expected for the polymer films containing immobile dopant (reaction 2), redoping by the ions in the solution was negligible, and, thus, chloride ions could not be found inside the POTS C layer, neither before nor after conditioning. The results presented prove that equilibration of the PPy(X) films prepared by oxidative polymerization can be described by reaction 2. Regardless of the monomer solution composition, all layers were characterized by cationic potentiometric responses (Table 1), which is in good agreement with the prediction of the model. However, it should be noted that the potentiometric slopes for films prepared by the POTS technique are slightly lower than the theoretical Nernstian values. This difference indicates a minor parasitic influence of anions, e.g., chlorides. It is interesting to note that, in contradiction to some previous reports,26,27 the analysis of the polymer films studied in our work indicates a rather high content of potassium. The high content of potassium supports the pertinent suggestion by some other authors28 that potassium ions are inserted into poly(pyrrole) in the form of [KFe(CN)6]-(4-). In more general terms, it can be concluded that the potentiostatic polymerization of pyrrole, combined with doping by immobile anions such as Fe(CN)64-, results in cationic potentiometric sensitivity, which is due to the presence of mobile metal ions in the polymer film. The conditioning process after polymerization can lead to an increased content of metal ion, with resulting enhancement of cationic sensitivity. Equilibration of the Reduced Poly(pyrrole) Films. Reduced poly(pyrrole) films prepared by continuous scanning cyclic voltammetry (CSCV), stopped at -0.5 V, can be expected to equilibrate according to reactions 4 or 5, depending on the mobility of dopants inside the polymer film. It was observed that, after equilibration in 0.1 mol L-1 KCl, all films prepared by CSCV exhibited similar rest potential values, ∼150 mV. Conditioning results in over a 600 mV increase in polymer potential, indicating that, during equilibration, the films prepared by the CSCV technique are oxidized by the solution components, e.g., oxygen. During CSCV polymerization, each freshly formed PPy layer is reduced in the reverse scan. The reduction results also in the partial expulsion of the doping Fe(CN)64- ions and/or incorporation of the potassium ions. Redoping with hexacyanoferrate(II) ions in each subsequent oxidative part of the scan might be Analytical Chemistry, Vol. 69, No. 19, October 1, 1997
4063
difficult due to steric hindrances and the competitive polymerization of each new layer, and, according to some authors, the removal of the hexacyanoferrate(II) ions from the PPy(X) may also depend on the thickness of the polymer film.23,24 On the other hand, the formation of a new polymer layer should not affect the expulsion of highly mobile potassium ions. Two cases will be discussed separately: (a) films prepared by CSCV from the solutions containing 0.46 mol L-1 pyrrole, i.e., B and C, and (b) films prepared by CSCV from monomer solution containing 0.1 mol L-1 pyrrole, i.e., A and D. The main difference in these two cases is the thickness of the deposited poly(pyrrole) film. From the more concentrated pyrrole solution, thicker films, accompanied by significantly higher deposition currents during cycling, and correspondingly higher counts for iron (Table 2), were obtained. Although CSCV B and C films were definitively thicker than the A and D films, they were obviously thinner than the films prepared by the POTS technique, i.e., POTS B and POTS C. These two cases should reflect different mobility of doping hexacyanoferrate(II) ions inside the film. For the thicker poly(pyrrole) films obtained from more concentrated pyrrole solutions B and C (case a), less negative potential is needed to expel the dopant from the polymer structure.23,24 Therefore, it was expected that, in the course of polymerization, the dopants would be partially expelled from the PPy structure and that those films should equilibrate according to reaction 4. To confirm this expectation, the compositions of poly(pyrrole) CSCV B and CSCV C films, before and after conditioning, were examined by EDAX. As expected, for unconditioned layers, both potassium and iron signals were recorded (Table 2). After soaking, the CSCV C polymer film contained mainly iron, chloride, and traces of potassium, whereas the CSCV B film contained only iron and chloride, and the potassium signal has decreased to the background level. Thus, during conditioning, the amount of potassium in both films decreased significantly, and the chloride content increased. The incorporation of chloride ions and partial or total expulsion of potassium during soaking is predicted by reaction 4. The iron content in the CSCV B and C films was only slightly affected during soaking. This indicates, as expected, that only minor anion exchange is observed during polymer equilibration. The polymer films prepared from monomer solutions B and C, containing a higher pyrrole concentration, exhibited anionic potentiometric sensitivity, as can be seen in Table 1. The potentiometric slopes observed were smaller than the theoretical slopes, indicating some contribution from the cations. The slope of the CSCV C film deviates more from the theoretical value than the slope of the CSCV B film. The CSCV C film was prepared from a solution of higher potassium concentration, and, therefore, also the potassium content in the CSCV C film is higher than that in the CSCV B film, as can be seen in Table 2. These results are in good agreement with the theory,17 which allows for the mixed potentiometric response for p-doped polymer films. The extent of the mixed response depends on the relative content of the anions and cations inside the polymer membrane.
4064
Analytical Chemistry, Vol. 69, No. 19, October 1, 1997
The thinner films, i.e., CSCV A and CSCV D (case b), prepared from monomer solutions containing 0.1 mol L-1 pyrrole, are expected to release doping anions at an even more negative potential than the thicker PPy films, as described above.23,24 Therefore, under polymerization conditions, hexacyanoferrate ions may be regarded as immobilized dopants. The equilibration of the CSCV A and CSCV D films is described by reaction 5. The equilibrated layers should contain potassium ions and exhibit cationic potentiometric responses. Indeed, the EDAX analysis of the equilibrated PPy(X) layers indicates clear signals of potassium (Table 2), and a predominant cationic potentiometric sensitivity was observed for both CSCV A and CSCV D films (Table 1). The iron signals were on the background level, which should be attributed to a rather low content of the dopants due to thin films. The difference between the potentiometric slopes of the cationsensitive poly(pyrrole) films prepared by CSCV and POTS, observed in the case of A and D monomer solutions, results from different thicknesses of the membranes. The POTS layers are thicker and contain more exchangeable potassium ions. Slopes closer to the theoretical value are obtained for CSCV and POTS films prepared from the monomer solution D, which contains a higher concentration of potassium hexacyanoferrate(II). Experiments conducted with PPy(X) films deposited by continuous scanning cyclic voltammetry indicated that both the composition and the potentiometric sensitivity of the film depend on the properties of the polymer film before soaking, i.e., on the polymerization conditions and on the conditioning process. Hence, both conditioning and polymerization can be employed to model the desired potentiometric properties of poly(pyrrole) layers. CONCLUSIONS The theoretical and experimental findings presented above indicate that the composition and resulting potentiometric sensitivity of poly(pyrrole) films are affected both in the course of film deposition and during soaking. These two steps of the film preparation may equally well be used to obtain desired potentiometric characteristics of a polymer film. The conditions of electrodeposition allow the regulation of the desired film thickness, the introduction of proper doping ions, and the adjustment of the oxidation level, morphology, and effective ion mobilities in the film; through soaking, the desired change of the chemical composition of the film and enrichment of the content of the mobile ions can be achieved. Thus, the overall quality of the film can be modeled to obtain the most beneficial operational functionality of the polymer-based electrode. ACKNOWLEDGMENT We thank Dr. J. Bobacka and Mr. N. Kimberley for valuable comments and discussions. Received for review February 27, 1997. Accepted July 11, 1997.X AC970227L X
Abstract published in Advance ACS Abstracts, August 15, 1997.
