Stability of the Inner Polyaniline Solid Contact Layer in All-Solid-State K + -Selective Electrodes Based on Plasticized Poly(vinyl chloride) Tom Lindfors* and Ari Ivaska Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, A ° bo Akademi University, Biskopsgatan 8, 20500 A ° bo/Turku, Finland A simple and powerful method based on UV-visible spectroscopy is presented for studying the stability of the inner electrically conducting polyaniline (PANI) solid contact (SC) layer in all-solid-state ion-selective electrodes (ISE). The influence of the plasticized poly(vinyl chloride) (PVC) membrane (ISM) composition and the pH of the sample solution on the stability of the solid contact is reported. PANI is used as a model compound in this study, but the method presented is universal and can be applied to different types and combinations of SCs and ISMs. It provides a tool for finding the best combination of conducting polymer and ISM for solid contact ISEs. PANI is deposited electrochemically either on glassy carbon or quartz glass covered with a thin layer of tin oxide, and a K + -selective ISM is deposited on top of the PANI layer. The short-term stability of the inner PANI layer is good for all membrane types in buffer solutions with pH 2 , 6 , and 9 , indicating that the outer plasticized PVC membrane hinders the emeraldine salt -emeraldine base transition of the highly pH sensitive PANI layer. The solid contact K + -selective electrodes studied showed a Nernstian response of 5 8 .2 ( 0 .1 mV/ log a K . Significant differences are observed in the long-term stability of the inner PANI layer between the different membrane types. This indicates that water uptake of the PVC membrane and its permeability to OH - ions are critical parameters affecting the stability of the PANI layer. The solid contact electrodes based on PANI may require a composition of the PVC membrane different from those typically used in conventional ISEs with an inner solution. The solid contact (SC) all-solid-state ion-selective electrode (ISE) concept with an electrically conducting polymer (CP) as the ion-to-electron transducer between the ion-selective membrane (ISM) and the metal substrate was introduced a decade ago and is now studied by many research groups working with ISEs. 1 The CP layer converts the ionic conduction to an electronic signal due to its reversible oxidation-reduction reaction and has the same function as the Ag|AgCl wire in conventional ISEs. The redox reaction of the CP is accompanied by insertion/ expulsion of anions/ cations/ electrons depending on the composition of the CP layer. The CPs cannot directly be used as such as membrane materials in ISEs due to their nonselective properties 1 even though there are some exceptions. 2-4 They are therefore suitable as sublayer (solid contact) materials that are not in direct contact with the sample solution. It is very important that the CP solid contact is stable and chemically inert when the ISM is contacted with different sample solutions. The long-term stability of the ISE can be affected, for example, by water uptake of the ISM and its chemical stability. Changes in the sample solution composition are also reflected at the ISM|CP interface, which may undergo chemical changes and therefore affect the ISE response (poten- tial). It has recently been reported that a thin aqueous layer is formed between the ISM and a Au substrate, causing a drift in the electrode potential, which can be avoided by depositing a lipophilic self-assembled monolayer on the Au surface. 5 The most common solid contact materials are polypyrrole (PPy), 1,6-19 polyaniline (PANI), 20-27 poly(3,4-ethylenedioxythiophene) * Corresponding author: ( e-mail) Tom.Lindfors@ abo.fi; (fax) +358-2- 2154479. (1) Cadogan, A.; Gao, Z.; Lewenstam, A.; Ivaska, A.; Diamond, D. Anal. Chem. 1992 , 64, 2496-2501. (2) Lindfors, T.; Ivaska, A. Anal. Chim. Acta 2000 , 404, 111-119. (3) Lindfors, T.; Ivaska, A. Anal. Chim. Acta 2001 , 437, 171-182. (4) Lindfors, T.; Ivaska, A. J. Electroanal. Chem. 2002 , 531, 43-52. (5) Fibbioli, M.; Morf, W. E.; Badertscher, M.; de Rooij, N. F.; Pretsch, E. Electroanalysis 2000 , 12, 1286-1292. (6) Hulanicki, A.; Michalska, A. Electroanalysis 1995 , 7, 692-693. (7) Michalska, A.; Hulanicki, A.; Lewenstam, A. Analyst 1994 , 119, 2417-2420. (8) Momma, T.; Komaba, S.; Yamamoto, M.; Osaka, T.; Yamauchi, S. Sens. Actuators, B 1995 , 24-25, 724-728. (9) Momma, T.; Yamamoto, M.; Komaba, S.; Osaka, T. J. Electroanal. Chem. 1996 , 407, 91-96. (10) Michalska, A.; Hulanicki, A.; Lewenstam, A. Microchem. J. 1997 , 57, 59- 64. (11) Gyurcsa ´nyi, R.; Nyba ¨ ck, A.-S.; To ´th, K.; Nagy, G.; Ivaska, A. Analyst 1998 , 123, 1339-1344. (12) Komaba, S.; Arakawa, J.; Seyama, M.; Osaka, T.; Satoh, I.; Nakamura, S. Talanta 1998 , 1293-1297. (13) Blaz, T.; Migdalski, J.; Lewenstam, A. Talanta 2000 , 52, 319-328. (14) Quan, D. P.; Quang, C. X.; Hai, D. D.; Hanh, N. T. H.; Viet, P. H. Anal. Sci. 2001 , 17 (Suppl.), a25-a28. (15) Kova ´cs, B.; Cso ´ka, B.; Nagy, G.; Ivaska, A. Anal. Chim. Acta 2001 , 437, 67-76. (16) Kaden, H.; Jahn, H.; Berthold, M.; Ju ¨ ttner, K.; Mangold, K.-M.; Scha ¨ fer, S. Chem. Eng. Technol. 2001 , 24, 1120-1124. (17) Zielin ´ska, R.; Mulik, E.; Michalska, A., Achmatowicz, S.; Maj-Z ˙ urawska, M. Anal. Chim. Acta 2002 , 243-249. (18) Pandley, P. C.; Singh, G.; Srivastava, P. K. Electroanalysis 2002 , 14, 427- 432. Anal. Chem. 2004, 76, 4387-4394 10.1021/ac049439q CCC: $27.50 © 2004 American Chemical Society Analytical Chemistry, Vol. 76, No. 15, August 1, 2004 4387 Published on Web 06/30/2004