kodak aq composite polymeric films

269

J. Electroanal. Chem, 310 (1991) 269-279

Elsevier Sequoia S.A., Lausanne

Electrochemical behavior of polypyrrole/ composite polymeric films

Kodak AQ

Joseph Wang *, Zhisheng Sun and Ziling Lu Department

of Chemistry, New Mexico State University, Las Cruces, NM 88003 (USA)

(Received 10 December 1990; in revised form 4 February 1991)

Abstract

The anodic polymerization of pyrrole (P) onto glassy carbon in an aqueous solution of the Kodak-AQ poly(ester sulfonic acid) polyelectrolyte gives a PP/AQ composite film. While incorporated as charge compensators during the anodic growth of PP, the entangled AQ- chains cannot easily diffuse out upon reduction. The composite layer, resulting from such unique use of AQ ionomers (as electrolyte and dopant) possesses the features of both its conducting polymer and cation exchanger components. These include effective loading of hydrophobic cations, potential switch effect or permselective response. For example, the uptake of Ru(bpy)$+ by the AQ anion, residing in the conducting polymer, is facilitated by an electrochemical event (reduction of the film to PP’/AQ- ). Similarly, the redox switchable PP component offers electrochemical control of the release of loaded cations. These and other attractive properties of PP/AQ composite layers are explored by cyclic voltammetry, chronocoulometry, potentiometry and flow injection amperometry.

INTRODUCTION

The considerable current interest in conducting polymers has led to a number of important applications [l-3]. Special attention is currently being given to the preparation of new conducting electrode coatings based on composite polymers [4]. It is expected that this strategy will improve the behavior of conducting polymers and will result in polymers designed for specific applications. Diaz and coworkers [5] described the preparation of poly (vinyl chloride) (PVC)/polypyrrole (PP) composite membranes, by electropolymerizing PP inside a PVC film on the electrode surface. Buttry’s and Hirai’s groups [6,7] prepared poly(aniline) (PA)/Nafion @ and PP/Nafion composites by electropolymerization within precast Nafion layers, while Penner and Martin [8] illustrated the advantages of Nafion-impregnated Goretex membranes. Recent attention has focused on the preparation of charge

*To

correspondence should be addressed.

0022-0728/91/$03.50

0 1991 - Elsevier Sequoia S.A.

270

balance composites, via the electropolymerization of cationic conducting polymers in the presence of anionic polymers [9,10]. Large polyelectrolyte anions, such as poly(styrenesulfonate) and poly(vinylsulfate) have thus been employed in cormection with PP. In this report we describe a new polypyrrole/poly(ester sulfonic acid) composite coating, prepared by the electropolymerization and doping of PP with a poly(ester sulfonic acid) ionomer, Eastman Kodak’s AQ-55D polymer. Recent work has illustrated the unique transport, antifouling and ion-exchange properties of singledomain AQ polymeric films [ll-131. Our interest is to combine the attractive behavior of AQ polymers with the features of PP films. For this purpose, the large AQ anion is used a dopant, and the resulting PP/AQ composite coating (on the glassy carbon surface) exhibits properties superior to those of the two components alone. In particular, the PP/AQ offers unique coupling of cation-exchange properties, potential switch effect and permselective transport. These properties and advantages are illustrated in the following sections. EXPERIMENTAL

Apparatus

Electrochemical experiments were performed with an EG&G PAR Model 264A Voltammetric Analyzer, in connection with an EGtG PAR Model RE 0073 X-Y recorder for cyclic voltammetry (CV) or a Houston Omniscribe strip-chart recorder for flow injection experiments. All potentials were measured vs. the Ag/AgCl reference electrode (Model RE-1, Bioanalytical Systems (BAS)). A 10 ml voltammetric cell (Model VC-2 (BAS)) was used for CV experiments. The flow injection system consisted of a carrier reservoir, a Rheodyne Model 7010 injection valve (20 ~1 sample loop), interconnecting Teflon tubing and a glassy carbon thin layer detector (Model Tl-5, BAS). Flow of the carrier solution was maintained by gravity at a rate of 1.0 ml/min. Chronocoulometry was performed with the BAS 1OOA Electrochemical Analyzer. Open-circuit potentials were measured with a digital multimeter. Reagents

Pyrrole, Ru(bpy)l* (bpy = 2,2’-bipyridine) (Aldrich), ascorbic acid (Baker), uric acid, catechol and dopamine (Sigma), and the Eastman AQ-55D polymer (28% dispersion, Eastman Kodak Co.) were used as received. The 1: 4 diluted AQ polymer solution was used as supporting electrolyte during the pyrrole polymerization, while an aqueous 0.05 M phosphate buffer (pH 7.4) solution was used as electrolyte during the characterization of the coated electrodes. Electrode coating procedure

Prior to its modification the glassy carbon surface was polished with a 0.05 pm alumina slurry for 2 min, rinsed with doubly-distilled water and sonicated in a water bath for 5 min.

271

PP/AQ films were prepared in the 1: 4 diluted AQ solution containing 7-14 mM pyrrole (and without any other electrolyte). Electropolymerization was effected by cycling the potential (several times, see text) between 0.4 and 1.1 V at a rate of 50 mV/s. AQ-coated electrodes were prepared by syringing 5 ~1 of a 1: 20 (v/v) AQ-55D: methanol solution on the electrode and its surrounding, followed by evaporation of the solvent. All experiments (including the electrode modification) were performed at room temperature. RESULTS AND DISCUSSION

Electropolymerization The preparation

process

of PP/AQ composite films can be carried out by anodic polymerization of pyrrole on the glassy carbon electrode in an aqueous solution of the AQ polyelectrolyte. Figure 1 compares cyclic voltammograms for 7 mM pyrrole in 0.1 M KC1 (A) and AQ (B) solutions. It is clear that in both cases the electrolyte

I

I

1.0

-0.2 Potential

/

V

Fig. 1. Cyclic voltammograms (50 mV/s 20 cycles) for 7 mM pyrrole in 0.1 M KC1 (A) and 1: 4 diluted AQ 550 (B) solutions. Scan rate, 50 mV/s. The first scan is designated as 1.

272

constituents act also as dopants (charge compensators to the PP cation produced), and that the oxidation of pyrrole can occur in the AQ solution (with no other added electrolyte). Notice, however, the different nature of the polymerization process in these solutions. Using the AQ polyelectrolyte, the current decreases rapidly with continued scanning, as compared with the gradual increase observed (and common) with PP formation in potassium chloride media. Such behavior is attributed to the low mobility of the large polymer anions (molar mass of 1500 g) versus that of the smaller chloride species. An oxidation reaction was not observed in the absence of pyrrole in the AQ solution (not shown). Overall, the polymerization process can be expressed as: (AQ-Na’),

+ n P -+ (P’AQ-),

+ n Na++ n e-

(I)

Because of the slow motion of the large AQ anion and the entrapment of its sulfonates in the PP, the AQ anion is not removed from the composite film upon reduction. In addition, the decreased film porosity results in reduced mobility of the

a b

C

50

~_IA

d

B

I

I

0.8

-0.2 Potential

/ V

Fig. 2. Cyclic voltammograms of PP (A) and PP/AQ (B) electrodes in 0.05 M KCl/l M HCl solution, recorded at different scan rates: 20 (a), 50 (b), 100 (c) and 200 (d) mV/s. Polymerization, as in Fig. 1.

273

chloride anion in the composite. Hence, currents associated with the doping/ undoping process are significantly smaller at the PP/AQ film compared to those observed at the single domain PP layer (e.g. Fig. 2). As will be illustrated below, such behavior of the PP/AQ composite film greatly facilitates its utility for electrochemically controlled binding and release of cations. Potential control of the ion-exchange characteristics The ability of single-domain AQ films to “collect” and retain hydrophobic cations has been documented [11,13]. These loading properties can be manipulated by incorporating the AQ anionomer within the PP matrix. Figure 3 shows repetitive

0.1 Potential

/V

Fig. 3. Cyclic voltammograms for 1 X 1O-4 M Ru(bpy)<+ recorded continuously at PP/AQ (A, B) and AQ (C) coated electrodes. Electrolyte, 0.05 M phosphate buffer @H 7.4). Electropolymerization as in Fig. lB, with 4 (A) and 12 (B) cycles in the presence of 14 mM pyrrole in the AQ solution.

274

cyclic volt~o~~s illustrating the uptake of Ru(bpy)$+ at PP/AQ (A, B) and AQ (C) coated electrodes. The partition of the cationic species into these films is indicated from the gradual increase in the peak currents. Notice, however, the different accumulation behavior exhibited by the different coatings. For example, the larger steady-state currents at the composite-coated electrodes indicate larger ion-exchange capacities. The increased thickness of the PP/AQ layer (and associated AQ dopant) results in a larger quantity of the bound complex (A vs. B). The rate of binding is also affected by the film preparation conditions. The thicker and denser PP/AQ film (B) exhibits a slower attainment of equilibrium (as expected from the slower diffusion associated with the reduced porosity). When the Ru(bpy):2-loaded electrode was transferred to a blank solution, the majority of the incorporated cation was retained by the film. For example, a retention ratio, I&/F0 [14], of 0.71 was obtained after accumulation from a 1 X lop4 M Ru(bpy)z+ solution. Electrochemical controlled binding and release of chemicals is of great interest in various areas [9,15,16]. The charge balance properties of PP/AQ films allow

I

1

I

I

,

1.2

0.6

1.2

0.6

Potential

/

V

Fig. 4. Cyclic voltammograms of AQ (A) and PP/AQ (B) coated electrodes after 5 min accumulation in the presence of 1 X 10e4 M Ru(bpy):+ at 0.1(a) and 1.0 (b) V. Electropolymerization (for B), 6 cycles in the presence of 14 mM pyrrole snd 1: 4 diluted AQ solution. Other conditions, as in Fig. 3.

215

convenient potentiostatic control of the binding of cationic species. Figures 4 and 5 illustrate the potential effect upon the uptake and retention of Ru(bpy):+ , respectively. For example, while the collection of Ru(bpy):+ at the AQ-coated electrode is not affected by the accumulation potential (0.1 vs. 1.0 V, Fig. 4A (a vs. b)), a larger quantity of the complex is accumulated when the PP/AQ film is held at 0.1 V, i.e. reduced to PP’/AQ- (Fig. 4B): PP+/AQ-

+*I

PP’/AQ-/M+

(2)

Similarly, the release of the cationic species from the fihn could be facilitated by reoxidizing it in the blank solution. For example, Figure 5 shows cyclic voltammograms of Ru(bpy)z+-loaded electrodes, after different “releasing times” at + 1.0 V. No apparent change of the voltammetric peaks is observed at the AQ-coated electrode (A). In contrast, the bound cation is nearly completely discharged from the PP/AQ film following the five-mm period (B (b)). Overall, in accordance with eqn. (2), the voltammetric data of Figs. 4 and 5 illustrate that the redox switchable PP can load and discharge cations (in an electrochemical event), while accommodating the AQ counter anion permanently.

I

0.6

1.2

Potential

/

0.6 V

Fig. 5. Cyclic voltammograms of AQ (A) and PP/AQ (B) coated electrodes, “loaded” with Ru(bpy)$+, and polarized at 1.0 V for 0 (a) and 5 (b) min in the blank (phosphate buffer) solution. Loading conditions, as in Fig.3.

01 0

I

I

1

I

I 30

I

I

I

L

I 60

Time / s

Fig. 6. Double potential step chronocoulometry for AQ (A) and PP/AQ (B) coated electrodes, after being loaded with Ru(bpy)z+ (from a 1 x 10e4 M solution) and transferred to a phosphate buffer solution.

The anodic release of bound cations can be studied also by chronocoulometry. For example, Fig. 6 illustrates a double potential step chronocoulometric experiment for AQ (A) and PP/AQ (B) coated electrodes which have been loaded with Ru(bpy):+ and soaked in phosphate buffer solution. The experiment was performed with steps between 0.4 to 1.2 V (forward) and 1.2 to 0.4 V (backward). The significantly larger charge measured at the composite electrode in the forward step, compared to that of the reversed one, indicates significant release of Ru(bpy):+ from the film into the solution at 1.2 V. The different time profiles in these steps also indicate diffusional contribution (during the forward one, from the depleted cation). In comparison, charges measured at the single-domain AQ electrode in both steps are significantly smaller. The potential switch effect of the PP/AQ-coated electrode for Ru(bpy):+ can also be monitored by measuring the open-circuit potential of the film in the presence of the incorporated ion. It was documented earlier [17] that PP-coated electrodes exhibit a potentiometric response characteristic of the dopant ion. Such open-circuit potential should be affected by the loading and releasing of ions in the film, through the charge distribution at the polymer-solution interface. Figure 7 shows the dependence of the open-circuit potential of AQ (a) and PP/AQ (b) coated electrodes upon the applied voltage (in a 1 x 10m2 M Ru(bpy):+ solution). The open-circuit potential was measured after polarization at different voltages for 2 rnin, followed by keeping the circuit open for 2 min. The potentiometric response

271

I

- 0.2

0.6 E/V

Fig. 7. Effect of polarization voltage upon the open-circuit potentiometric response of the AQ (a) and PP/AQ (b) coated electrodes. Solution, 1 X 10e2 M Ru(bpy):+ (no added electrolyte).

of the PP/AQ-coated electrode is affected strongly by the applied voltage. Such a positive shift of the measured potential with increasing applied voltage, reflects the controlled uptake of the cation. In the absence of such a potential switch effect, the potentiometric response of the single-domain AQ-coated electrode is nearly independent of the applied voltage. Permselective

response

Permselectivity is another attractive property of PP/AQ films, which holds promise for sensing applications. The permselective response of single-domain PP and AQ coatings has been documented [11,12,18]. The PP/AQ composite couples the size and charge exclusion properties of its components and offers a controllable and discriminative access toward the surface. Flow injection measurements of biologically important compounds (at pH 7.4) are used to illustrate this behavior (Fig. 8). Among the four flow amperometric electrodes tested, the PP/AQ-coated one (D) yielded the best permselective response toward the cationic neurotransmitter dopamine (d). Note the effective discrimination against the anionic ascorbic and uric acids (a, b), and the attenuation of the neutral catechol peak (C). Analogous cyclic voltammetric experiments yielded similar observations (not shown).

I

4,

2.5

min

B

Ill_ d

C

b

a

d

b

C

D

a

C

-- 1 d uu ci

C

b

JL ci

a

C

Lt b

a

Time

Fig. 8. Flow injection peaks for 1 x lop4 M uric acid (a), ascorbic acid (b), catechol (c) and dopamine (d) at the bare electrode (A) and PP (B), AQ (C) and PP/AQ (D) coated electrodes. Applied potential, + 0.8 V. Flow rate, 1 ml/min. Carrier and electrolyte, 0.05 M phosphate buffer (pH 7.4). Coating procedure (B, D), as in Fig. 1, with one potential cycle.

Studies of brain chemistry should benefit greatly from such permselective behavior of PP/AQ coatings. The permeability of the composite film may be manipulated via control of the polymerization conditions (to meet specific sensing needs). In conclusion, the experiments described above indicate that significant advantages can be achieved by anodic polymerization of pyrrole on glassy carbon in an aqueous solution containing the AQ polyelectrolyte. The resulting composite electrode coating exhibits properties superior to those of the two components alone, resulting in several potential applications. Particularly attractive is the prospect of its use as a controlled-release device for hydrophobic cationic species.

279 ACKNOWLEDGEMENT

This work was supported (Petroleum Research Fund).

by a grant form the American

Chemical

Society

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