Lowering the Detection Limit of Ion-Selective Plastic Membrane

This method was successfully applied to lower the detection limit of all-solid-state chloride-selective potentiometric sensors containing plastic, pol...
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Anal. Chem. 2003, 75, 4964-4974

Lowering the Detection Limit of Ion-Selective Plastic Membrane Electrodes with Conducting Polymer Solid Contact and Conducting Polymer Potentiometric Sensors Agata Michalska,* Joanna Duman´ska, and Krzysztof Maksymiuk

Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland

The detection limit of conducting polymer (CP) poly(pyrrole)-based potentiometric chloride-sensitive electrodes was lowered over 3 orders of magnitude, applying anodic current of density in the range microamperes per centimeter squared. This effect was attributed to compensation of doping chloride ion leakage from the membrane into adjacent solution layer, caused by selfdischarge of CP. This method was successfully applied to lower the detection limit of all-solid-state chlorideselective potentiometric sensors containing plastic, poly(vinyl chloride) chloride ion-selective membrane, and conducting poly(pyrrole) film applied as ion-to-electron transducer. The control of the leakage of chloride ions from the CP transducer phase to the plastic membrane resulted in linear response of ion-selective electrode shifted down to lower activities. A 2 orders of magnitude lowered detection limit, equal to 4 × 10-7 M, was achieved for current densities corresponding to extended linear range of CP film electrode, i.e., in the range of microamperes per centimeter squared. Experiments with all-solid-state potentiometric sensors were carried out using open-sandwich arrangement (two poly(pyrrole) electrodes coated by the same ion-exchanging membrane): one part of the sensor was working under open circuit conditions, while the second electrode was polarized using anodic current. Due to arrangement used, the lowered detection limit observed for nonpolarized electrode is attributed to modification of ion fluxes in the membrane; it is interfered by neither ohmic drop nor anion depletion layer formation due to anodic current flow. Lowering the detection limit of solvent polymeric membrane electrodes has been given much attention in past few years.1-6 * Corresponding author. E-mail: [email protected]. (1) Sokalski, T.; Ceresa, A.; Zwickl, T.; Pretsch, E. J. Am. Chem. Soc. 1997, 119, 11347-11348. (2) Mathison, S.; Bakker, E. Anal. Chem. 1998, 70, 303-309. (3) Sokalski, T.; Zwickl, T.; Bakker, E.; Pretsch, E. Anal. Chem. 1999, 71, 12041209. (4) Sokalski, T.; Ceresa, A.; Fibbioli, M.; Zwickl, T.; Bakker, E.; Pretsch, E. Anal. Chem. 1999, 71, 1210-1214. (5) Zwickl, T.; Sokalski, T.; Pretsch, E. Electroanalysis 1999, 11, 673-680. (6) Bakker, E.; Pretsch, E. Anal. Chem. 2002, 74, 420A-426A.

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The primary ions’ leakage from the plastic membrane phase into the sample solution is the origin of the detection limits observed for traditional ion-selective potentiometric sensors with internal solution, being on the order of micromolar at the most. It was demonstrated that successful elimination of the sample-oriented primary ion flux by appropriate choice of the internal solution of constant low activity of the primary ion results in detection limits shifted down to picomolar levels.1-6 Nowadays, for traditional internal solution ion-selective electrodes, the choice of remedies available to eliminate primary ion leakage from the membrane ranges from using ion buffers as an internal solution, e.g., refs 1, 4, and 7 incorporation of ion exchangers able to bind primary ions in the internal solution,8 up to applying a current to compensate ions fluxes across the plasticbased membrane separating the internal solution and the sample.9-11 The possibilities of practical applications of ion-selective electrodes in the range of picomolar activities have arisen.12,13 Since the 1980s, conducting polymer films, especially poly(pyrrole) layers, have received considerable attention as alternative membranes for potentiometry, e.g., refs 14-21. However, if the conducting polymer film is used directly as a membrane of a potentiometric ion sensor, the linear range of responses will usually be restricted to the range from 1 to 10-4 M.14-21 Moreover, both ionic and redox species present in the solution strongly (7) Ceresa, A.; Sokalski, T.; Pretsch, E. J. Electroanal. Chem. 2001, 501, 7076. (8) Qin, W.; Zwickl, T.; Pretsch, E. Anal. Chem. 2000, 72, 3236-3240. (9) Lindner, E.; Gyurcsa´nyi, R.; Buck, R. P. Electroanalysis 1999, 11, 695702. (10) Pergel, E˙ .; Gyurcsa´nyi, R.; Toth, K.; Lindner, E. Anal. Chem. 2001, 73, 4249-4253. (11) Morf, W. E.; Badertscher, M.; Zwickl, T.; de Rooij, N. F.; Prestch, E. J. Electroanal. Chem. 2002, 526, 19-28. (12) Ceresa, A.; Bakker, E.; Hattendorf, B.; Gu ¨ nther, D.; Pretsch, E. Anal. Chem. 2001, 73, 343-351. (13) Ion, A.; Bakker, E.; Pretsch, E. Anal. Chim. Acta 2001, 440, 71-79. (14) Dong, S.; Che, G. Talanta 1991, 38, 111-114. (15) Dong, S.; Sun, Z.; Lu, Z. Analyst 1988, 113, 1525-1528. (16) Okada, T.; Hayashi, H.; Hiratani, K.; Sugihara, H.; Koshizaki, N. Analyst 1991, 116, 923-927. (17) Cadogan, A.; Lewenstam, A.; Ivaska, A. Talanta 1992, 39, 617-620. (18) Hutchins, R.; Bachas, L. Anal. Chem. 1995, 67, 1654-1660. (19) Michalska, A.; Ivaska, A.; Lewenstam, A. Anal. Chem. 1997, 69, 40604065. (20) Migdalski, J.; Błaz˘ , T.; Lewenstam, A. Anal. Chim. Acta 1996, 322, 141149. (21) Migdalski, J.; Błaz˘ , T.; Lewenstam, A. Anal. Chim. Acta 1999, 395, 65-75. 10.1021/ac034335l CCC: $25.00

© 2003 American Chemical Society Published on Web 08/30/2003

interfere with potential values recorded, e.g., refs 22-27. Thus, it seems that the most promising way of applying conducting polymer films in the construction of potentiometric sensors is to use them as a solid-contact phasesion-to-electron transducer, placed between the plastic poly(vinyl chloride) (PVC)-based membrane and the electron-conducting substrate.28 In this way, the interferences related to the properties of the synthetic metal phase, especially coupled electron and ion conductivity, are not only eliminated but even turned to the benefit of the sensor obtained.28-32 The elimination of the internal solution enables work at virtually any position and at conditions in which evaporation of the internal solution can occur.33 In fact, among other materials34-36 conducting polymers, such as poly(pyrrole)29-32 poly(thiophene),37,38 poly(aniline),39,40 and poly(3,4-ethylenedioxythiophene),41,42 have been successfully applied for this purpose. Thus, the obtained conducting polymer-based all-solid-state sensors were characterized with potential stability equivalent to that of internal solution electrodes.29-31,43 The linear range of responses and selectivity values were comparable with those obtained for traditional ionselective electrodes.28-31 However, for obvious reasons, the method of tailoring ion fluxes and thus lowering the detection limit for this class of potentiometric sensors is not readily available, still being one of the great challenges.6 Recent reports point out the possibility of binding calcium ions at the transducer-conducting polymer, poly(3-methylthiophene) surface functionalized with complexing ligand;44 thus, the superNernstian behavior of the calcium-selective electrode was induced. However, as the amount of complexing agent present on the (22) Hulanicki, A.; Michalska, A.; Lewenstam, A. Electroanalysis 1994, 6, 604605. (23) Bobacka, J.; Gao. Z.; Ivaska, A.; Lewenstam, A. J. Electroanal. Chem. 1994, 368, 33-42. (24) Michalska, A.; Maksymiuk, K.; Hulanicki, A. J. Electroanal. Chem. 1995, 392, 63-68. (25) Michalska, A.; Lewenstam, A. Anal. Chim. Acta 2000, 406, 159-169. (26) Michalska, A.; Nadrzycka, U.; Maksymiuk, K. Electrochim. Acta 2001, 46, 4113-4123. (27) Michalska, A.; Nadrzycka, U.; Maksymiuk, K. Fresenius J. Anal. Chem. 2001, 371, 35-38. (28) Cadogan, A.; Gao, Z.; Lewenstam, A.; Ivaska, A.; Diamond, D. Anal. Chem. 1992, 64, 2496-2501. (29) Hulanicki, A.; Michalska, A.; Lewenstam, A. Analyst 1994, 119, 2417-2420. (30) Michalska, A.; Hulanicki, A.; Lewenstam, A. Microchem. J. 1997, 57, 5964. (31) Hulanicki, A.; Michalska, A. Electroanalysis 1995, 7, 692-693. (32) Zielin´ska, R.; Mulik, E.; Michalska, A.; Achmatowicz, S.; Maj-Z˙ urawska, M. Anal. Chim. Acta 2002, 451, 243-249. (33) Nikolskii, B. P.; Materowa, E. A. Ion-Sel. Electrode Rev. 1985, 7, 3-39. (34) Cattrall, R. W.; Freiser, H. Anal. Chem. 1971, 43, 1905-1906. (35) Fibbioli, M.; Bandyopadhyay, Liu, S.; Echegoyen, L.; Enger, O.; Diederich, F.; Bu ¨ hlmann, P.; Pretsch, E. J. Chem. Soc., Chem. Commun. 2000, 339340. (36) Fibbioli, M.; Bandyopadhyay, Liu, S.; Echegoyen, L.; Enger, O.; Diederich, F.; Gingery, D.; Bu ¨ hlmann, P.; Persson, H.; Suter, U. W.; Pretsch, E. Chem. Mater. 2002, 14, 1721-1729. (37) Bobacka, J.; Lewenstam, A.; Ivaska, A. Talanta 1993, 40, 1437-1441. (38) Sjo ¨berg, P.; Bobacka, J.; Lewenstam; A.; Ivaska, A. Electroanalysis 1999, 11, 821-824. (39) Lindfors, T.; Ivaska, A. Anal. Chim. Acta 1999, 400, 101-110. (40) Lindfors, T.; Ivaska, A. Anal. Chim. Acta 2000, 404, 111-119. (41) Va´zquez, M.; Bobacka, J.; Ivaska, A.; Lewenstam, A. Sens. Actuators, B 2002, 82, 7-13. (42) Bobacka, J.; Alaviuhkola, T.; Hietapelto, V.; Koskinen, H.; Lewenstam, A.; La¨msa¨, M.; Pursiainen, J.; Ivaska, A. Talanta 2002, 58, 341-349. (43) Bobacka, J. Anal. Chem. 1999, 71, 4932-4937. (44) Michalska, A.; Konopka, A.; Maj-Z˙ urawska, M. Anal. Chem. 2003, 75, 141144.

surface of the contact phase was limited, the observed effect disappeared with time. In this paper, we present a different approach to the problem of lowering the detection limit of conducting polymer-based ionselective electrodes. The considerations and experimental results presented herein take into account the recently discussed role of spontaneous charging/discharging processes of poly(pyrrole) in aqueous solution,45 which are accompanied by ion fluxes resulting from dopant ion exchange with an adjacent solution layer. These phenomena can also affect the properties of all-solidstate sensors with conducting polymer applied as an ion-to-electron transducer; unfortunately, this effect was not considered earlier. Taking into account the advantage of the electron and ion conductivity of a solid-contact layer, the ion fluxes through the outer plastic PVC-based ion-selective membrane were adjusted by means of current control. This method, to our best knowledge, has not been reported earlier to affect the detection limit of allsolid-state conducting polymer-based ion-selective electrodes. Experimental results were obtained for the relatively welldefined model system chosenspoly(pyrrole) film doped with chloride ions14,31 and plastic, poly(vinyl chloride)-based, chlorideselective electrode.31,32 The responses of both conducting polymer film membrane electrode and all-solid-state arrangement with poly(pyrrole) layer applied as ion-to-electron transducer were tested. To enable the parallel, undisturbed potentiometric measurement and the current control of ion fluxes, a twin-contact electrode (open-sandwich type) was used. EXPERIMENTAL SECTION Reagents. Distilled pyrrole was stored in a refrigerator and prior to its use was purified by passing through an alumina gel (Alumina Fluka for chromatography type 507C, neutral) homemade minicolumn. Tetrahydrofuran (THF) (Merck, Darmstadt, Germany) was freshly distilled. PVC, 2-nitrophenyl octyl ether (o-NPOE), and tridodecylmethylammonium chloride (MTTDA) were from Fluka (Buchs, Switzerland). Doubly distilled and freshly deionized water (resistance 18.2 MΩ cm, Milli-Qplus, Millipore) was used throughout this work. Used salts were of analytical grade (POCh, Gliwice, Poland). Apparatus. In the potentiometric experiments, a multichannel data acquisition setup and software (Lawson Labs. Inc., 3217 Phoenixville Pike, Malvern, PA 19355) was used. In other electrochemical measurements, a galvanostat-potentiostat CH Instruments model 660A (Austin, TX) was used. The pumps systems 700 Dosino and 711 Liquino (Metrohm, Herisau, Switzerland) were used to obtain sequential dilutions of calibrating solutions. An Ag/AgCl in KCl gel (1 M) reference electrode (Detektor, Raszyn, Poland) with liquid junction filled with 0.1 M KF was used. Stable potential readings were taken (within (0.2 mV), and the recorded potential values were corrected for the liquid junction potential calculated according to the Henderson approximation. Potentiostatic polarization, cyclic voltammetry, galvanostatic experiments, electrochemical impedance spectroscopy (EIS), and electrochemical quartz crystal microbalance measurements (EQCM) were done in the conventional cell with the Pt counter (45) Duman´ska, J.; Maksymiuk, K. Electroanalysis 2001, 13, 567-573.

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Figure 1. Scheme of the twin-contact open-sandwich electrode used. (A) Top view of the substrate electrode covered with conducting polymer; (B) top view of the conducting polymer and subsequently plastic membrane-covered electrode; (C) cross section of obtained all-solid-state chloride selective electrode: (1) glassy carbon substrate, (2) insulator electrode body, (3) electrical connection, (4) conducting polymer layers, and (5) plastic membrane phase.

electrode and reference electrode described above. Electrochemical impedance spectroscopy data were collected within the frequency range from 1 Hz to 100 kHz, using an amplitude of 10 mV. The EQCM apparatus used in the studies is based on electronic design described in ref 46 and was constructed in Institute of Physical Chemistry (Warsaw).47 Working/Indicator Electrodes Used. Glassy carbon disk electrodes of area 0.07 cm2 were used in experiments with poly(pyrrole) membrane electrodes (unmodified with plastic membrane). For other experiments, when simultaneous potentiometric measurements, with a nonpolarized or polarized electrode (according to terminology given in ref 48) were done, the twin-contact electrode was used.49 This electrode, Figure 1, consists of two glassy carbon semicircles, each of 0.07 cm2 surface area, separated with a 1-mm insulator gap; each of the semicircles is equipped with its own electrical connection. Glassy carbon electrodes were polished with 0.3-µm Al2O3. A mirror smooth polishing was avoided to prevent peeling off the relatively thick polymer film. The conventional internal solution electrodes tested in parallel were prepared with Philips IS 561 electrode bodies (Mo¨ller Glasbla¨serei, Zu¨rich, Switzerland). The internal solution used was 0.1 M KCl. (46) Stoeckel, W.; Schuhmacher, R. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 345349. (47) Koh, W.; Kutner, W.; Jones, M. T.; Kadish, K. M. Electroanalysis 1993, 5, 209-214. (48) Delahay, P. New Instrumental Methods in Electrochemistry; Interscience Publishers: New York, 1954. (49) Błasiak, M.; Golimowski, J.; Maksymiuk, K. Pol. J. Chem. 2001, 75, 17191728.

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AT-cut quartz crystals (Phelps Electronics) of 5 MHz nominal resonant frequency and 14 mm in diameter were used as masssensitive oscillators. Poly(pyrrole) Films. All poly(pyrrole) films were electrodeposited potentiostatically at 0.9 V. The aqueous monomer solution used was 0.2 M pyrrole and 0.1 M KCl to yield poly(pyrrole) doped with chloride anions, PPyCl. The polymerization charge was 1.43 C cm-2 or 70 mC cm-2 for EQCM measurements. Thus, the obtained conducting polymer films were thoroughly rinsed with deionized water, transferred to 0.1 M KCl solution, and polarized under conditions of cyclic voltammetry (within the range from -0.5 to 0.5 V, scan rate 50 mV s-1; three scans were applied); then the potential +0.2 V was applied to the film for 90 s in the same solution. The poly(pyrrole) films were either conditioned overnight in 0.1 M KCl solution and used for potentiometric experiments or directly modified with plastic PVC-based membrane to obtain the all-solid-state sensor. The polymer films used in EQCM experiments were only conditioned by linear potential sweeping (25 mV s-1, in the range from -0.8 to 0.5 V, for 5 min), in 1 M KCl solution. Ion-Selective Membranes. The composition of chlorideselective membranes used was as follows (m/m): 35.5% PVC, 52.3% o-NPOE, 12.2% MTTDA. A total of 200 mg of membrane components was dissolved in 2 mL of THF. Preparation of All-Solid-State Twin-Contact Electrodes. The twin-contact electrode with PPyCl membranes electrodeposited separately on each glassy carbon semicircle was placed in up-side down position. The top of the electrode was covered with 20 µL of THF solution, and immediately the THF solution of plastic PVC-based membrane components was pipetted on the top of both semicircles modified with conducting polymer (Figure 1). The amount of membrane component solution applied was 40 µL. After overnight evaporation of the membrane solvent, the all-solid-state chloride selective electrodes thus obtained were conditioned overnight in 0.1 M KCl solution. Twin-Contact Coated Wire Electrode. The coated wire type twin-contact electrode was prepared by a consecutive application of THF and THF-based plastic membrane component solution on the top of the up-side down twin-contact glassy carbon electrode. The amounts of THF and membrane solution were as described above, as was the conditioning procedure. Classical Internal Solution Electrodes. A 0.2-mL sample of the solution of membrane components was cast into the 0.8-cmdiameter glass ring; after evaporation of the solvent, the membrane was cut off and placed in the Philips body. The single ion activities were calculated according to DebyeHu¨ckel theory.50 All experiments were performed at ambient temperature (23 °C). THEORETICAL SECTION In the following considerations it was assumed, for simplicity, that (i) the conducting polymer film is an anion-exchanging permselective membrane (no mobile cations present), (ii) there are no interferences resulting from the presence of solution anions other than dopant and from solution pH changes, and (iii) the only redox species affecting the conducting polymer oxidation state is dissolved oxygen. (50) Meier, P. C. Anal. Chim. Acta 1982, 136, 363-368.

Figure 2. Schematic representation of the effect of spontaneous processes occurring at the polymer/solution interface and their effect on the detection limit (DL) of the potentiometric sensor. X- is the mobile anion; subscripts sol and poly refer to the solution or polymer phase, respectively. For concentration vs distance plots, the dotted lines (b. c) correspond to the open circuit conditions, whereas the solid line (in all cases) represents the actual concentration profile.

The systemsconducting polymer, e.g., poly(pyrrole)/electrolyte solution, is quite complicated. Thus, different processes of diverse rate should be taken into account. The ion exchange occurring at the polymer/solution interface is usually a relatively fast process for partially oxidized poly(pyrrole) and a Donnantype equilibrium prevails. However, this fast process is accompanied by much slower charging/discharging reactions. The conducting polymer film in contact with electrolyte after cathodic polarization can be spontaneously oxidized (charged), e.g., by dissolved oxygen.51 Alternatively, the oxidized polymer film, just after anodic polarization, can undergo spontaneous discharge45 resulting from polymer deprotonation.52 Both processes are accompanied by ion flows across the polymer/solution interface, Figure 2a. If the polymer film is an anion-exchanging matrix in contact with solution containing the same anion as dopant, the charging of the polymer film, i.e., oxidation of the polymer, connected with creation of extra positive charge, is accompanied by incorporation of anions from the solution to the polymer phase. On the other hand, the discharge is related to release of the doping anions accompanied by H+ ion expulsion from the polymer phase to the solution. The polymer open circuit potential is dependent on both (ion-exchange and redox processes), and thus, a potential drift can be observed. The open circuit potential continuously reflects the ion-exchange equilibrium (Nernst relation) corresponding to the actual (but (51) Pfluger, P.; Kronubi, M.; Street, G. B.; Weiser, G. J. Chem. Phys. 1983, 78, 3212-3218. (52) Krivan, E.; Visy, C.; Kankare, J. J. Phys. Chem. B 2003, 107, 1302-1308.

changing in time) redox state of the polymer. If charging/ discharging processes are sufficiently slow and do not affect significantly the oxidation state of the polymer, stable potentiometric responses can be recorded. For high or moderate electrolyte activity, the effect of anion leakage does not significantly affect the activity; the anion activities near the polymer surface and in the bulk can be regarded as equal. For this range of ion activities in solution, a linear dependence of the polymer open circuit potential on the logarithm of electrolyte activity is obtained. However, for a low-activity range, a different situation is observed. Directly after immersion of the electrode in the dilute electrolyte solution, the process of polymer discharge dominates (the rate of charging is very low due to limited anion activity) leading to release of electrolyte ions previously accumulated in the polymer film (in course of polymerization or conditioning). Therefore, the anion activity at the polymer/solution interface increases and a stationary state is established, with anion activity close the polymer surface higher than that in the bulk. This local activity increase affects the linear range of the potentiometric responses obtained for conducting polymer films, similar to the way the leakage of primary ions affects the detection limits of ion-selective electrodes.1-6 Thus, the detection limit of conducting polymer film electrodes is dependent on the rate of the spontaneous oxidation/reduction process. For poly(pyrrole) layers, the detection limit observed is within the range 10-5-10-4 M. The spontaneous process of polymer charging/discharging can be compensated by a current applied to the polymer-coated Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

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electrode. In this manner, the leakage of dopant anionssthe local ion activity fluctuationssare also controlled. The current polarization applied should be small enough not to noticeably affect the polymer oxidation state in typical experiment time, but only to compensate the spontaneous process. Ideal Case (Absence of Spontaneous Charging/Discharging). The minimal value of the applied current resulting in change of the potential of a CP-modified electrode, compared to open circuit conditions in dilute solutions, can be a useful criterion to characterize the system. It can be applied to distinguish whether the measured anion activity in solution, indicated by the open circuit potentiometric experiment, is equal to bulk activity or if it results from the leakage due to polymer discharge. In an ideal case, in the absence of spontaneous charging/discharging processes, applying a constant anodic current results in decreased anion concentration at the polymer/solution interface due to the polymer oxidation. The anion transport from the bulk can be described by formula, derived from the Nernst-Planck equation and Poisson equation for electroneutrality condition in solution:

∂2c(x,t) ∂c(x,t) ∂φ ∂c(x,t) + zFu )D 2 ∂t ∂x ∂x ∂x

(1)

dE/dt ) i/CL

where t is time, x is the distance from the polymer surface, c is anion concentration, D is diffusion coefficient of anions in solution, F is Faraday’s constant, u is anion mobility, z is anion charge, and ∂φ/∂x is potential gradient, a sum of diffusion and ohmic potential drop gradient. Considering the process in KCl solution, the diffusion potential is close to 0, while for low-polarizing currents (range of nA or lower), the ohmic potential drop is negligibly small as well. Therefore, the second part of the sum in eq 1 can be omitted. Taking into account boundary conditions (x ) 0 relates to the polymer/solution interface)

t ) 0,

x ) 0,

c ) c0

t > 0,

x f ∞,

c f c0

D(∂c(x,t)/∂x)x)0 ) i/FA where c0 is bulk anion concentration, A is electrode surface area, and i is applied current; the equation can be solved to obtain53

2i t FA Dπ

1/2

( )

(2)

(c0 - c(0,t))

(3)

c(0,t) ) c0 -

interface (e.g., c(0,t) ) 0.8c0). Assuming c0 ) 10-5 M (i.e., the concentration value near the usually observed detection limit of CP electrodes), D (for Cl-)54 ) 2 × 10-5 cm2 s-1, A ) 0.07 cm2, and t ) 100 s, the anodic current evaluated from eq 3 is 5 × 10-9 A, i.e., in the range of single nanoamperes (current density is below 0.1 µA cm-2). This current range is in good accordance with previously reported current values needed to affect the potentiometric response of PVC-based ion-selective electrodes,9-11 where the charging/discharging processes are absent. Applied current, within the nanoampere range, can ultimately result in conducting polymer film oxidation, although it can be shown by simple calculation that for considered poly(pyrole) layers even after 5 h of continuous polarization the change in the polymer oxidation state would be smaller than 1%. Considering the value of the uncompensated solution resistance in 10-5 M electrolyte (for KCl solution near 5 × 105 Ω, as obtained in this work) and the above evaluated current, the ohmic drop is only ∼0.3 mV, confirming the assumption of its low significance. Additionally, the flowing current can result in a potential drift, expressed by the formula43

(4)

where CL is the low-frequency capacitance of the polymer film. Assuming current applied equal to 5 nA and CL ∼ 1 mF (a typical value for oxidized CP), the potential drift evaluated from eq 4 is lower than 0.3 mV min-1; i.e., a reasonably stable potential can be recorded for the galvanostatically polarized electrode. Real Case (Presence of Spontaneous Charging/Discharging). The above considerations set the lowest current value yielding in change of electrolyte concentration at the interface. However, in the presence of spontaneous dedoping, different results should be obtained; the current needed to change the measured potential (in comparison to open circuit conditions) should be higher than that obtained from eq 3. The polymer discharge process, formally expressed as a current, idisch, can be compensated when anodic current, i, is applied. Thus, the leackage of the anions from the polymer phase is prevented, Figure 2b, a stationary state is established with

icharg + i ) idisch

(5)

Using this equation, the minimal polarizing current necessary to change the potential measured in a dilute solution, in the absence of spontaneous charging/discharging processes, can be calculated. Let us assume that this potential change results from a significant change of the anion concentration at the polymer/solution

where icharg ) vcharg/nFA and idisch ) vdisch/nFA, ν is the flux of anions across the polymer/solution interface, related to the process denoted in subscript, and n is the number of exchanged electrons. Taking into account results of the discussion presented above (current necessary to change the potential in the absence of self-discharge-caused anion leakage is close to 0.1 µA cm-2), it should be underlined that only the influence of currents, icharg and idisch, higher than 0.1 µA cm-2 can be detected under these conditions. If the potentials of the polarized electrode as described above are plotted against the logarithm of activity of anions in the solution, the dependence obtained will be different from that

(53) Galus, Z. Fundamentals of Electrochemical Analysis; Ellis Horwood Ltd.: Chichester, 1994.

(54) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions; Butterworths: London, 1959.

and then

i)

4968

FA Dπ 2 t

1/2

( )

Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

recorded under open-circuit conditionssthe detection limit of the electrode with conducting polymer film will be shifted to lower activities. However, if the applied anodic current exactly compensates the discharge process (i ) idisch), the resulting process is continuous oxidation of the polymer by oxygen present in the solution, represented by icharg ultimately resulting in potential increase. Depending on the rate of the process, it can lead to depletion of anions at the polymer surface. This effect appears if the rate of the spontaneous charge-transfer process is high in comparison to the rate of anions transport in the solution:53

kt1/2 > 5D1/2

(6)

where k is the “apparent” rate constant (cm s-1) of polymer oxidation (oxygen reduction at the polymer). Assuming, as above, t ) 100 s and D (O2)55 ) 2.1 × 10-5 cm2 s-1, one obtains k > 2.3 × 10-3 cm s-1. For polymers characterized with this k value, an exact compensation of the spontaneous discharge will lead to decrease of anion concentration at the interface, observed as a deviation from the linear dependence E versus log a, resulting in a “super-Nernstian” slope. The same holds for the overcompensation of the anion flow into sample solution. In this case, electrochemical oxidation of the polymer will take place, enhancing the super-Nernstian slope of potential versus log a relation, due to anion uptake. Therefore, the optimal anodic current, resulting in a lowest detection limit, will correspond to almost total compensation of the discharging process (eq 5 fulfilled). On the other hand, the cathodic current applied to the polymer will enhance the discharge process (Figure 2c). As a result, the electrolyte layer at the polymer surface will be enriched with the dopant anions and the detection limit of conducting polymer potential versus log a line is expected to be shifted to the higher activities. Again, the effect is expected to be dependent on the magnitude of cathodic current applied. For a higher current, a shorter linear range of responses is predicted, compared to those usually obtained under open circuit conditions. The above-mentioned processes will also affect the polymer when used as a solid-contact layer between electrically conducting metal (or glassy carbon) substrate and ionically conductive plasticbased ion-selective membrane, especially if the system contains the same anion both in the contact and in the plastic membrane phase. The conducting polymer film, due to the permeability of the ion-selective membrane layer to electrolyte ions, water, and oxygen, is expected to undergo processes similar to when it is in direct contact with solution. Therefore, the conducting polymer, from the point of view of lowering the detection limit, can be regarded as an analogue of the inner solution of the ion-selective electrode of relatively high electrolyte activity, in the sense of availability of dopant anions at the contact side of the plastic ionselective membrane. As a result, the detection limit of this type of all-solid-state ion-selective electrode is expected and is observed, e.g., refs 28-32, to be close to the value obtained for the traditional plastic membrane separating inner solution from the sample phase. For obvious reasons, to lower the detection limit of the above-mentioned all-solid-state potentiometric system, the same (55) Jakobs, R. C. M.; Janssen, L. J. J.; Barendrecht, E. Electrochim. Acta 1985, 30, 1085-1091.

Figure 3. Time dependence of the open circuit potential and frequency using the EQCM method for poly(pyrrole) film in 1 M KCl solution, after polarization at E ) 0.5 V for 3 min.

remedies as used for traditional internal solution arrangements cannot be used. Moreover, precise control of the leakage of the ions from the all-solid-state sensor system cannot be done in a manner similar to that used for any other ion-selective electrodes with internal solution.1-8 To our best knowledge, a suitable method has not been reported yet. Application of current compensating the self-discharge of conducting polymer solid contact layer offers an attractive possibility to lower the detection limit of the all-solidstate ion-selective electrode with plastic membrane and conducting polymer contact. It should be stressed that the above-described method of lowering the detection limit of the all-solid-state conducting polymer-based contact and PVC-based membranes is relatively simple. The required behavior of the sensor ranging from superNernstian, through lowered detection limit, to a shortened range of linear responses can be obtained for literally the same sensor, depending on the absolute value of the current applied. RESULTS AND DISCUSSION Poly(pyrrole) Films. The spontaneous process of oxidized poly(pyrrole) discharge, resulting in open circuit potential decrease, was followed using EQCM method (Figure 3). Directly after polarization of the polymer layer at 0.5 V, a potential decrease accompanied with a frequency increase was recorded. The latter was attributed to polymer mass decrease due to doping anion release into the solution, as a result of spontaneous discharge. As discussed above, this process can contribute to increase of anion activity at the interface under open circuit conditions. Similar effects, but of different magnitude, can be observed even though the polymer is not prepolarized electrochemically but left in solution. These relatively small effects are pronounced if sufficiently dilute (