All-solid-state sodium-selective electrode based on a calixarene

Nov 1, 1992 - All-solid-state sodium-selective electrode based on a calixarene ionophore in a poly(vinyl chloride) membrane with a polypyrrole solid c...
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Anal. Chem. 1902, 64, 2496-2501

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All-Solid-state Sodium-Selective Electrode Based on a Calixarene Ionophore in a Poly(viny1 chloride) Membrane with a Polypyrrole Solid Contact Aodhmar Cadogan,?Zhiqiang GaoJ Andrzej Lewenstam, and Ari Ivaska* Laboratory of Analytical Chemistry, Abo Akademi University, SF-20500 Turku- Abo, Finland

Dermot Diamond School of Chemical Sciences, Dublin City University, Dublin 9, Ireland

Sodium-selective coated wire electrodes (CWE) and solid contact (SC) electrodes have been constructed and Investigated. The CWE Is based on the appllcatlon of a poly(vlny1 chlorlde) (PVC) membranelncorporatlngthe sodlum-selectlve Ionophore tetraethyl ester of pterl-butylcallx[4]arene onto the surface of a platlnum disk. The SC electrode Is based on the use of a Conducting polymer, polypyrrole (PPy), doped wlth NaBF4as the mediating layer between platlnum and the same PVC membrane as above. Nernstlan responses have been obtained for bothelectrodesand comparableselecthritles to that of a conventional Ion-selective electrode (ISE) based on the same Ionophore. Impedance measurements of the PPy and PVC layers and potentlometrlc studies of the membranes have been used to lnvestlgate the mechanlsm of the charge transfer of the systems.

INTRODUCTION There have been huge developments in ion-selective electrode (ISE) research over the past 30 years, driven in part by the speed and ease of ISE procedures, the wide dynamic range possible, and the relatively low cost of the devices. Nevertheless the traditional barrel configuration can prove cumbersome for some applications, and attempts at miniaturization brought about some new sensing systems, namely solid contact (SC)electrodes’ such as solid crystal membranes, conducting filled polymer electrodes, and in particular coated wire electrodes (CWE). In a CWE an electroactive species is incorporated in a thin polymeric film and coated directly onto a metallic conductor.2 This move to the total elimination of the internal filling solution provides new advantages. Simplicity of design, lower costs, mechanically flexibility, i.e. the electrode can be used horizontally, vertically, or inverted, and the possibility of miniaturization and microfabrication have widened the applications for CWEs, especially in the fields of medicine and biotechnology. The substrate in a CWE is usually platinum wire, but silver, copper, and graphite rods have also been used. The electroactive component is usually embedded in PVC (poly(viny1 chloride)) or PMM (poly(methyl methacrylate)) and the wire dip coated until a bead

* Author to whom correspondence should be addressed.

t Permanent address: School of Chemical Sciences, Dublin City

University, Dublin 9,Ireland. 1 On leave from Henan University, Kaifeng, Henan, People’s Republic of China. (1)Nikolskii, B. P.;Materova, E. A. Ion Sel. Electrode Rev. 1985,7, 3-39. (2) Cattrall, R. W.; Hamilton, I. C. Ion Sel. Electrode Reu. 1984,6, 125-172.

is formed. Sensors for c a l ~ i u m ,nitrate,5 ~)~ potassium,6r7 chloride? lithium: and perchlorate10 have been developed. Sometimes these electrodes exhibit better selectivities1than more classical sensors with an internal solution, but the standard potential of CWEs is often unstable, varying for one electrode during its lifetime, as well as differing between electrodes of the same type. The apparent success of these systems raised some fundamental questions surrounding the charge conduction mechanism occurring in the membrane phase and at the membrane-substrate interface. In conventional ISEs the reversibility and equilibrium of the transition from ionic to electronic conductivity is ensured by the reversible half cell a t the silver wire in the internal filling solution. Buckll classified coated wire electrodes as “completely blocked” aa the interface between the membrane, i.e. an ionic conductor, and the internal contact, i.e. an electronic conductor, is somehow blocked to a reversible electron or ion transfer. The coupling between the metal and the coating in CWEs can be described as capacitive, and the contact potential is distributed between the capacitors of the two surfaces. These systems cannot provide very reproducible potentials due to the sensitivity of the capacitive coupling to external substances penetrating between the coating and the metal. Cattrall et ala3suggested the presence of an oxygen half cell at the interface caused by the permeation of oxygen gas from the working solution. Many variations have been used while trying to solve the blocked interface problem. Dispersion of a redox couple in the membrane phase or layering of phases containing redox species12have been tried. The use of a hydrogel layer in ISFETs,l3 an epoxy layer loaded with metal,14J5or carbon substrate electrodes16have overcome many of the potential (3)Cattrall, R. W.;Drew, D. M.; Hamilton, I. C. Anal. Chim. Acta 1975. 76. 269-277. (4jCaGrall, R.W.;Drew, D. M. Anal. Chim. Acta 1975,77,9-17. (5) Hulanicki, A.; Lewandowski, R.; Maj, M. Anal. Chim. Acta 1974, 69,409-414. (6) Tamura, H.; Kimura, K.; Shono, T. Anal. Chem. 1982,54,12241227. (7)Trojanowicz, M.; Augustowska, 2.;Matuszewski, W.; Moraczewska, G.;Hulanicki, A. Talanta 1982,29,113-117. (8)James, H.; Carmack, G.; Freiser, H. Anal. Chem. 1972,44,856-857. (9)Xie, R. Y.;Christian, D. D. Anal. Chem. 1986,58, 1806-1810. (10)Alegret, S.;Florido, A. Analyst 1991,116,473-476. (11) Buck, R.P.In Ion Selective Electrodes in Analytical Chemistry; Freiser, H., Ed.; Plenum: New York, 1980; Vol. 1, p 58. (12)Nikolskii, B.P.;Materova, E. A. re/ 1, p 24. (13)van der Vlekkert, H.; Francis, C.; Grisel, A.; de Rooij, N. Analyst 1988,113,1029-1033. (14)Khalil, S. A. H.; Moody, G. J.; Thomas, J. D. R.; Lima, J. L. F. C. Analyst 1986,111, 611-617. (15) Moody, G.J.; Thomas, J. D. R.; Lima, J. L. F. C.; Machado, A. A. S. C. Analyst 1988,113,1023-1027.

0003-2700/92/0364-2496$03.00/0 0 1992 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992

stability problems in some cases and are improvements on their conventional counterparts. Thick polypyrrole (PPy) films incorporating C1-, BFd-, and Clod- have been shown to produce stable potentiometric responses to a range of anions.17-20 As conducting polymers such as PPy also have well-documented electronic conductivity, we propose that PPy could act as a solidcontact between a sodium-selective PVC membrane containing an ionophore and a metal contact. The ionophore chosen was a calixarene, and the performance of a conventional ISE incorporating this ionophore is already well documented.21.22 EXPERIMENTAL SECTION Reagents. Pyrrole (Py) obtained from Merck was purified by double distillation, stored at low temperature, and protected from light. Poly(viny1chloride) (PVC),2-nitrophenyl octylether (2-NPOE), potassium tetrakisw-chloropheny1)borate (KTpClPB), sodium tetrafluoroborate (NaBF& and tetrahydrofuran (THF) were obtained from Fluka. The sodium-selective ionophore was tetraethyl ester of p-tert-butylcalix[4]arenesynthesized at The Queen’sUniversityBelfast (QUB),N. Ireland. Details of the ligand synthesis have already been published.% Analyticalgrade reagent salts obtained from Merck or Fluka were used without purification and dissolved in distilled, deionized water. Electrode Fabrication and Measurements. APARC Model 273 Potentiostat/Galvanostat with softwareversion 2.00 was used for the electrochemical polymerizations and cyclic voltammetry. The electrochemical cell was a conventional three-electrode system. The working electrode was a platinum (Pt) or glassy carbon (GC) disk (Metrohm)(diameter 3 mm) set in Teflon (0.d. 10 mm) with a carbon rod counter electrode (Metrohm). The reference electrode was an Ag/AgCl saturated KC1 reference electrode (Metrohm). All solutions were degassed by bubbling with high-purity nitrogen, and all electropolymerizations were performed under nitrogen. Electrodes were polished to a mirror finish with 1pm of diamond paste and 0.3 pm of A1203 paste. PPy filmswere prepared by continuous scanning in aqueous solutions of 0.1 M pyrrole and 0.1 M NaBF4 from 0.0 to 1.0 V using a scan rate 20 mV/s for 45 min. Scanning electron microscope (SEM) measurements of several films showed an average dry film thickness of 7-8 pm with a thickening of the film at the outer edge to 10.5 pm. The potentiometric measurements of the PPy films were performed with an Orion Research Expandable Ion Analyser EA 940 in conjunction with an Orion Ag/AgCl reference electrode. A Perkin-Elmer 56 pen recorder was used for the traces. The PVC membrane electrodeswere prepared as follows: the electrode to be coated was clamped in an upright position and a 100-pL portion of the PVC cocktail pipetted on top (see Table I for cocktail compositions). The electrode was then protected from particulate contamination and allowed to dry overnight. This left a thin transparent PVC film on the top of the metal contact. SEM profile measurements of the PVC showed a dry film thickness of 230 pm. The membrane was then soaked in a 0.1 M solution of NaCl for 12 h prior to use. The selectivity coefficients (Kpt) were determined by the separate solution method using 0.1 M chloride solutions of the cations involved. Response time measurements involved the injection of 0.450 mL (16) Midgley, D.; Mulcahy, D. E. Ion Sel. Electrode Reu. 1983,5, 165242. (17) Cadogan, A.; Lewenstam, A.; Ivaska, A. Talanta 1992,39,617620. (18) Ikariyama,Y.; Galiatsatos, C.; Heineman, W.; Yamauchi, S. Sens. Actuators 1987, 12, 455-461. (19) Dong, s.; Sun, 2.;Lu, Z. Analyst 1988,133, 1525-1528. (20) Lu,Z.; Sun, Z.; Dong, s. E~ectronno~ysis 1989, I , 271-277. (21) Cadogan, A,; Diamond, D.; Smyth, M. R.; Deasy, M.; McKervey, M. A.; Harris, S. J. Analyst 1989, 114, 1551-1554. (22) Diamond, D. In Electrochemistry, Sensors and Analysis; Smyth, M. R., Vos,J. G.,Eds.; Analytical Chemistry Symposia Series, Vol. 25; Elsevier: Amsterdam, 1986; pp 155-161. (23) Arnaud-Neu, F.; Collins, E. M.; Deasy, M.; Ferguson, G.; Harris, S. J.; Kaitner, B.; Lough, A. J.; McKervey, M. A.; Marques, E.; Ruhl, B. L.; Schwing-Weill, M. J.; Seward, E. M. J . Am. Chem. SOC.1989, 111, 8681-8691.

Table I. General Composition of the PVC Cocktail Solutions. 5% WIW PVCl PVC2 PVC3 ionophore 0.7 0.7 0.8 0.2 0.1 ion-exchanger: KTpClPB N&F4 0.1 0.1 plasticizer: 2-NPOE 66.1 66.1 66.1 33.0 33.0 PVC 33.0

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PVC4 0.9 66.1 33.0

OThe components were mixed in 1 mL of THF/100 mg of plasticizer. Table 11. Redox Responses of the Electrodes as Slopes of the log ([Fe(III)]/[Fe(II)l) vs Potential Curves. electrode slope, no. substrate mvldecade 1 Pt 58.2 2 PtlPPy 55.6 3 0.7 PtlPPyIPVCl 4 PtlPPyl PVC2 4.4 5 1.3 PtlPPyl PVC3 Pt/PVCl 6 0.8 GC 7 57.6 GCJPPy 8 56.9 9 GCIPPylPVC1 1.6 GCIPVC1 10 0.7 The concentrationratiowas changed from 511to 115. Theoretical response is 59.2 mV/decade at 25 O C . PVC compositions are given in Table I. of a 0.1 M solution of NaCl into 50 mL of lo4 M NaCl, which is close to a 10-fold change in concentration. Although results are shown for the Pt substrate mainly, essentiallythe same results are obtained with GC. Electrochemical Impedance Measurements. Electrochemical impedance measurements were performed in 0.1 M aqueous NaBF4 at room temperature using a HP-9816 computer controlled S-5720Bfrequency response analyzer and a NF-2000 potentiostat/galvanostat with GB-IB interface (NF Circuit Design Block Co, Ltd, Japan). The impedance at 10 discrete frequencies per decade was measured almost immediately after immersing the electrode in the NaBFl solution. The typical amplitude of the sinusoidal voltage signal used was 100 and 200 mV. The impedance spectrum was measured from 60 kHz to 1 mHz at a given dc potential.

RESULTS AND DISCUSSION In order to envisage the possible mechanism for the use of conducting polymers as a solid contact it was necessary to investigate the ionic and the electronic (or redox) sensitivity of the PPy layers and of the PVC membrane electrodes. BFdwas chosen as the doping ion for the electropolymerization of pyrrole because members of the class of borates have previously been used in PVC membrane systems. In view of the Na+selectivityof the calixarene ionophore and the possible effects of the cation in the doping process we chose the sodium salt for the preparation of PPy films. Redox Responses. The redox sensitivity of the different electrode assemblies was checked by measuring the potential of the redox couple Fe(II)/Fe(III) in various ratios with a constant ionic background of 0.1 M NaCl and then plotting the potential vs log ([Fe(III)I/[Fe(II)l). The concentration ratio was changed from 5/1 to 1/5. The results are given in Table 11. An instantaneous Nernstian response was observed at bare Pt and GC electrodes and at the PPy-coated Pt and GC electrodes. In this way the polypyrrole layer can be seen to be functioning as an extension of the electronically conducting metal or glassy carbon contact. All electrodes coated with PVC showed no redox response as would be

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ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992 Table 111. Limit of Detection (LOD), Potentiometric Selectivity Coefficients (Kijm), and Slopes of PVC-Based Solid-state Electrodes. log K p t electrode slope,

POTENTIAL (mV) 450

no.

400

LOD,M

1 X 1Oa 3 X lW6 5 3 x 10-5 6 1 X 1Oa conventionalU 4 X 1V

3 4 350

mV/decade j = K + j = & +

-2.7 -2.1 -2.7 -2.7 -2.7

58.7 58.1 56.7 58.1 56.4

-3.4 -3.3 -3.0 -3.3 -2.6

jrLi+

-3.4 -3.5 -3.2 -3.6 -2.7

0 Electrode compositions as in Table II. Values for a conventional liquid-filled ISE are also included.

300

250

2 00

150

-5

-4

-3

-2

-1

0

iog[anionI Figwo 1. Calibration curves and sbpes (9mVldecade of PPy films to sodlum salts: (1) ideal wRh arbitrary P (S= -59.2), (2) BF4- (S= -43.2), (3) Ci-(S = -51.4), (4) IW3- ( S I -54.7), (5) C W 4 - ( S I -5t.2), and (6) SCN- (S= -56.5). Polvpyrrole fkn formed by contlnuoue scanning between 0.0 and 1.O V with 20 mV/s scan rate for 45 mln in 0.1 M NaBF4 and 0.1 M pyrrole.

expected as there is no mechanism present for electron transport in the PVC. Hence the PVC layer can protect the inner Pt, GC, or PPy redox-sensitive layers from the effects of strong oxidizing or reducing agents in solutione.g. electrodes 3-6,9, and 10, Table 11. The readings for the bare Pt and Pt/PPy-coated electrodesshowed the same absolute potential values within 1mV, with the calibration curve lying in the 100-250-mVrange. In contrast, the absolute potential range for the PVC-coated electrodeswas much higher, between 360 and 600 mV, and varied between the different PVC membrane coatings.

Ionic Responses. The response of the PPy layers to sodium salts of different electrolytss are shown in Figure 1. PPy films show a near Nernstian response to chloride, perchlorate, nitrate, and thiocyanate with a slightly lower response to tetrafluoroborate. In addition to its electronically conducting nature the polypyrrole layer shows an ability to exchange ions, in this case anions, between the adjacent solution and the polymer matrix. There is a definite stable responseto anions,although the response is fairly nonselective. Three ionophore-basedion-selective PVC membranes with different ion-exchangercompositionswere examined for their performance as sodium-selective membranes incorporating as the ion-exchange: (1)KTpClPB only (PVCl), (2) KTpClPB and NaBF4 (PVCS), and (3) NaBF4 only (PVC3), see Table I. The response characteristics for the PVC-coated electrodes are outlined in Table 111. We also determined the same data for the conventional ISE based on the same caliiarene. These data are also included for comparison in Table III. All the electrodesshowed good Nernstian responses to sodium ions with selectivity generally in the order Na+ > K+ > Cs+ > Li+. As the results in Table I11 suggest, the selectivity is higher at the solid contact (SC) electrodes compared to the conventional liquid filliig solution electrode determined by the separate solution method in 0.1 M chloride

:L 20

8

time

Flguro 2. Response of the electrode 3 from Tabk I1 to In)ectlon of 0.450 mL of 0.1 M NaCl Into 50 mL of lo4 M NaCi which is practically equivalent to a lO-fold change In Na+ concentratlon.

so1ution.u The observed higher selectivitymay be explained by the asymmetry of the half-cell. The interface PPy/ISEPVC is blocked for sodium ions, and therefore the transport in the bulk membrane is affected coursing divergence in selectivity. The fact that coated wire electrodes may possess better selectivity has been reported earlier.112 The dynamic response of all the electrodes was very fast, of the order of a few seconds, see Figure 2. The three PVC compositions studied all performed well, showing that BF4- can as effectively as phenylborate derivative be used as the ion-exchange site in the membrane, i.e. exogenous charge carrier in the membrane. The limit of detection (LOD) of CWE and SC electrodes are not as low as LOD of the conventional electrode membrane. The success of both systems,however, raised two questions. Firstly, from the theoretical point of view, what is the basis of the charge transfer of the Pt/PPy/ISE-PVC which allows the PPy to act as a solid contact. Secondly,from the analytical point of view, what are the differences in analytical performance between the CWE system and the SC electrode system. Impedance Response. To investigate the charge-transfer mechanism we studied the impedance spectra of five systems in 0.1 M NaBFd: (a) Pt/PVC4, (b) Pt/PVC2, (c) Pt/PPy/ PVC4, (d) Pt/PPy/PVCl, and (e) Pt/PPy/PVC2. The impedance spectra are measured at a constant potential polarization of 200 mV at which potential the PPy is in the oxidized (doped)state, i.e. the PPy membrane is electronically conductive. The spectra observed are shown in Figure 3. As can be seen, all of the systems possess a high-frequency semicircle which is due to bulk properties of the PVC membrane and the diameter of which equals the bulk resistance.n (24)Cadogan, A. Ph.D. Thesis, Dublin City University, Ireland, 1992. (25)Nieman, T. A.; Horvai, G. A d . Chim. Acta lSSS,l70,369-363. (26) Armetrong, R. D.; Lockhart, J. C.; Todd, M.Electrochim. Acto 1986,31,591-594.

ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992

I

(a)

PtlpVC4

(b)

2499

PWVC2 500

-

300.

loo0

400,

Z', kQ

Z',k n

Z , kQ

PtlPPyPVCl

(di 15~1

i

(e)

2ml

PtlPPylPVC2

100

50-1

I

. m

I

Z',kR

Z', kQ

Flgwe 9. Impedance spectra of the (a)PtIPVC4, (b) Pt/PVC2, (c)Pt/PPy/PVC4, (d) Pt/PPy/PVCl, and (e) Pt/PPy/PVC2 electrodes in 0.1 M NaBF, soiutlon. All electrodes are polarlred to +200 mV and the amplitude of the ac perturbation Is 200 mV. Z'and Z"are the real and the imaglnary parts of the Impedance, respectively.

At low frequencies, f < 1 Hz, Warburg impedance is observed in the cases b, d, and e where the PVC membrane contains NaBF4 and/or KTpClPB as exogenous charge carriers. Addition of KTpClPB to neutral carrier PVC membranes is known to decrease the bulk resistance of the membrane.25 This effect can also be seen in Figure 3 where the resistance of the membrane decreases from 900 kQ of Pt/PVC4 to 200 kQ of PUPVCP.. By introducing the electronically and ionically conducting PPy layer between Pt and PVC4, the resistance was decreased to 400 kQ. This change is far larger than what would be expected from the uncertainty in making films of the same thickness. The PPy layer is also found to decrease the resistance from 200 kQ for Pt/PVCB to 70 k0 for Pt/PPy/PVCl and PtIPPyIPVC2. Because our studies are performed in a nonsymmetrical cell, a complete interpretation of the impedance responses of our systems is even more difficult than the responses of the normal ion-selective PVC membranes containing uncharged carriers.Z"Z8 However it is clear that the addition of a layer of conducting PPy as a solid contact between Pt and ionselective PVC significantly lowers the charge-transfer resistance and facilitates the transition from ionic to electronic conductivity across the interfaces, i.e. ionic charge transfer at the PVC/solution interface to electronic charge transfer at the Pt/PPy interface. Stability. As can be seen in Table I11 both the CWEbased system electrode 6 and the SC system electrodes 3-5 showed basically the same behavior with respect to slope, selectivity, and LOD. Typically the performance of many CWEs is characterized by irreproducible potentials, noise, and overshoot of potential. These problems are usually more apparent in the first few days of use. Figure 4 shows the potential of electrodes 3 and 6 over the first 10 days of use. The CWE syatem,electrode 6, exhibits largejumps in potential

POTENTIAL ( m V )

650

1

6oo-

/

1

550i f- 500

1 0

2

4

6

8

10

12

TIME ( d ) Figure 4. Stability study. Response of (1)electrode 6 and (2)electrode 3 to 0.1 M NaCi solutron over 10 days.

both in the positive and negative direction. The SC system, electrode 3, showed more stable potential readings, f8 mV, over the period studied. The potential of CWEs is also known to be dependent on the partial pressure of oxygen.a*m Figure 5 shows the effect of Nz on the potential of bare Pt and Pt/ PPy. The electrodes were kept in open stirred solutions of

(27) Horvai,G.;GrBf,E.;T6th,K.;Pungor,E.;Buck,R.P.Anal. Chem. (29) Maj-Zurawska, M.; Hulanicki, A. Anal. Chim. Acta 1982, 136, 1986,58, 2735-2740. 395-398. (28) Tbth,K.;GrBf,E.;Horvai,G.;Pungor,E.;Buck,R.P.Anal.Chem. (30)Schindler, J.G.;Stork,G.;Struth,H.-J.;Schmid, W.;Karaschinski, 1986,58, 2741-2744. K.-D. Fresenius' 2. Anal. Chem. 1979,295, 248-251.

ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992

2500

(i) Na+ exchange at the PVC/analyte solution interface:

Na+(aq) + C(m) + Na+C(m) where (aq) is the aqueous phase, (m) represents the PVC membrane phase, and C is the ionophore. (ii) Transport of the charged complex within the bulk PVC layer:

>;-

____

.-

T

-x

~

-

~

Ld

nr

b-

Figure 5. Effect of nitrogen on the rest potentlai of (1)Pt and (2) Pt/PpV in 0.1 M NaBF,. Electrodes were held In an open stirred solution for 48 h, then purged with nltrogen as marked.

Conventional ISE (a) f%

ref soluhon

Solid contact ISE PI

PPy

membrane

arialyfe

(b) membrarie

analyle

Flgure 6. Schematic diagram of the charge-transfer processes in (a) a conventlonai liquid-filled ISE and (b) a PPy solid contact electrode: C = ionophore, A- = doplng ion, @ = oxldired polypyrroie moieties, X- = counterion.

0.1 M NaBF4 for several days and then purged with nitrogen. The bare platinum shows a large dependence on the oxygen content of the solution, the potential decreasing rapidly as the cell is switched to nitrogen as would be expected. The Pt/PPy is more stable to oxygen effectswith no obvious change after the switch to nitrogen. The coated wire electrode, Pt/ PVC2, showed a potential change of approximately -30 mV under the same experimental conditions, indicating the capability of oxygen to penetrate through the PVC membrane to the Pt surface. This strongly suggests that the extra stability of the solid contact electrode arises from the stability of the PPy substrate to the oxygen content of the sample solution. Charge Transfer at t h e Interfaces. On the basis of our results in this work and the well-known mechanisms in PVCbased ion-selective electrodes, we show in Figure 6 the chargetransfer processes to occur at the different interfaces both in the conventional liquid-fiied ISE (a)and the PPy solid contact electrode (b). In the conventional ISE the overall charge transport involves the following steps.

(Na’C)’ += (Na’C)’’ where (’) is the external side and (”) is the internal side of the PVC membrane. (iii) Release of ions at the internal PVC/reference solution interface: (Na+C)”(m)== C(m) + Na+(aq) (iv) Coupling of the ionic and electronic conductivity via the redox couple Ag/Ag+ of the internal reference electrode: AgCl + e- Ag + C1(v) Electron flow within the metal. In the PPy contact system, Figure 6b, the fiist two processes are identical to (i) and (ii) above. This is followed by (vi) Exchange of ions at the PPy/PVC interface. (vii) Electron transport within the PPy. (viii) Electron transfer at the Pt/PPy interface. (ix) Electron flow within the metal. Of these processes (vi) is the most important process concerning the stability of the solid contact electrode. As can be seen in Figure 3d,e, Pt/PPy/PVCl and Pt/PPy/ PVC2 show similar impedance behavior. Although we have found that the exogenous carrier KTpClPB in ISE-PVC is unable, contrary to BF4-, to dope PPy, a stable electrical contact is still maintained. Our results in Figure 1strongly suggest that PPy behaves preferentially as an anion exchanger although some cation exchange cannot completely be discounted. In the case of Pt/PPy/PVC2 and Pt/PPy/PVC3 the electrical contact between the PPy and PVC phases is accomplishedby direct and reversibletransfer of BF4-between the two phases. In the case of Pt/PPy/PVCl the electrical contact is obviously due to an ion exchange at the PPy/PVC interface. The impedance behavior of Pt/PVC4 indicates that although BF4- is not added to the PVC cocktail an electrical contact still exists between the PPy and PVC phases. It is feasible that BF4- will diffuse from the PPy into the PVC/THF solution and subseuqently into the PVC plasticizer during the PVC curing and conditioning period of the electrode. This kind of diffusion of the dopant into the PVC phase might also take place when the Pt/PPy/PVCl electrode was prepared. It is important to emphasize that irrespective of any particular mechanism of ion contact at the PPy/PVC interface a constant bulk concentration of BF4- in PPy and TpClPB- in PVC is providing a sufficient driving force to control a local equilibrium at this interface and to keep there a constant gradient of electrical potential.31 The galvanic potential at this interface is obviously not influenced by a change of oxygen concentration in the analyte as shown in Figure 5. Measurement of the open circuit potential of PPy layers prior to covering with PVC indicated that the polymer rests at a stable potential where it is electronically conductive. In our work PPy is oxidized to a degree where it contains a rather high concentration of the dopant BF4-. The Oz/Nz studies in Figure 5 showed that 02 permeation through PVC is less likely to have an effect on the electrode potential as the Pt/PPy interface is largely insensitive to the effects of 0 2 . (31)Lewenstam, A.; Sokalski,T.;Hulanicki, A. Anal. Chem. 1987,59, 1539-1544.

ANALYTICAL CHEMISTRY, VOL. 64, NO. 21, NOVEMBER 1, 1992

These observations lead us to conclude that the Pt/PPy acts as a stable pseudoreference electrode with a constant bulk concentration of the dopant BF4- in contact with the ionically conducting PVC phase having a constant concentration of the exogenous charge carrier ion TpClPB- and/or BF4-. In our work advantage is taken of the ionic and electronic conductor properties of PPy to bridge a purely electronic conductor, Pt, and a purely ionic conductor, ISE-PVC, together. At the Pt/PPy interface the contact is electronic, and at the PPy/ISE-PVC interface it is ionic so in this way both interfaces become unblocked to some degree.

CONCLUSIONS The electrode incorporating polypyrrole as a solid contact between platinum and an ion-selective PVC membrane showed improved performance over an electrode without the polypyrrole layer. From the studies of the redox and potentiometric responses of the polypyrrole and PVC membranes and from the impedance measurements of various films and membranes, we conclude that the polypyrrole is capable of both ionic and electronic charge transfer. We suggest the polypyrrole could provide a mediating layer having

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both ionic and electronic charge transfer and so in theory could act as a suitable transition layer for blocked electrodes such as coated wire electrodes. Analytically the system containing the PPy/PVC solid contact was found to have improved stability characteristics. The kind of electrode construction studied in this work lends itself well to the possibility of miniaturization by the use of a thin platinum wire instead of the disk and also to the substitution of less expensive metal contacts such as copper or silver. Our work is continuing in this area.

ACKNOWLEDGMENT We are grateful to M. Anthony McKervey, Department of Chemistry, The Queen’s University, Belfast, N. Ireland, for providing the calixarene compounds used in this work. A.C. thanks the Ministry of Education of Finland and Z.G. the Foundation for Development of Technology in Finland for their scholarships.

RECEIVED for review October 28, 1991. Accepted July 20, 1992.