Anal. Chem. 1980, 52,345-346
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CORRESPONDENCE Polymer Film Chemically Modified Electrode as a Potentiometric Sensor Sir: In the past few years there has been an explosion of research activity in the area of chemically modified electrodes. This area of research has attracted such interest because of potential applications to electrocatalysis, electrosynthesis, and photosensitization. Relatively few applications have employed chemically modified electrodes as potentiometric sensors (1-4). Most of the methods that have been used t o modify electrodes have involved covalent attachment to the electrode surface. In addition to covalent attachment, polymeric films have been recently used to modify electrodes. These polymeric films have been formed by casting the film on an electrode surface (5-12), using radio-frequency plasmas (13), and electropolymerization (14-1 7). This paper reports the first use of a polymer film chemically modified electrode as a potentiometric sensor. Ordinarily, electrochemically initiated polymer film formation on an electrode surface is avoided by electrochemists whenever possible, and is considered as electrode “poisoning”. This type of electrode “poisoning” has been utilized in a constructive fashion to make a potentiometric sensing electrode. EXPERIMENTAL The electrochemical oxidation of 1,2-diaminobenzene (0phenylenediamine) produces an insulating film which completely coats a platinum electrode surface (18, 19). The poly(l,2-diaminobenzene) coated platinum electrode exhibits a nearly Nernstian response with changes in pH. All electrochemical experiments were performed with a POtentiostat of conventional design with a three-electrode system. A platinum wire sealed in glass was used as the working electrode, a commercial saturated calomel electrode (SCE) was used as the reference electrode,and a platinum mesh was used as the auxiliary electrode. The electrodes were not isolated from each other. The platinum working electrode was electrochemically conditioned before each experiment using the method described by Adams (20). The supporting electrolyte was 0.1 M phosphate buffer solution (pH 7.0) in triply distilled water. The concentration of the 1,2-diaminobenzenewas 5 mM. All electrolysis solutions were deaerated for 15 min with high purity grade nitrogen prior to the initiation of the experiment and a nitrogen atmosphere was maintained over the solution during the course of the experiment. The 1,2-diaminobenzene(practicalgrade, Aldrich Chemical Co.) was purified by recrystallizing from dichloromethane three times using ordinary techniques, and was decolorized with activated carbon (2g/L) during the first recrystallization. The final product had pure white plate-like crystals with a melting point range of 100.3-101.8 “C. The literature value for the melting point range of pure 1,2-&aminobenzeneis 102-103 “C (21). All other chemicals were reagent grade and were used as obtained from the manufacturer. The potential measurements were carried out using a Fisher 525 digital pH meter and a commercial SCE. A series of ClarkLubs buffer solutions were used for the various pH solutions. This series of buffers uses one of three different systems to obtain the desired pH: phthalic acid/potassium hydrogen phthalate, potassium dihydrogen phosphate/dipotassium hydrogen phosphate, and boric acid/sodium borate.
RESULTS AND DISCUSSION Figure 1 shows a typical cyclic voltammogram for the oxidation of 1,2-diaminobenzene in a phosphate buffer (pH 7.0) 0003-2700/80/0352-0345$01 .OO/O
solution. The scan rate was 50 mV/s and the peak potential (E,) occurs a t +0.51 V. vs. a SCE reference electrode. The oxidation wave is totally irreversible in nature. On successive scans without cleaning the electrode, the peak current dropped significantly with each scan until ultimately no current flowed. This behavior is indicative of a polymeric film coating the electrode and blocking the access of 1,2-diaminobenzene to the electrode surface. 1,2-Diaminobenzene forms polymeric films on electrochemical oxidation a t nearly all pH ranges (22), and it is very difficult to derive much information from electrochemical techniques. Previous studies have shown that electrochemical oxidations of this type form a monocation radical as the initial electrolysis product, and this product is then very often involved in a follow-up chemical reaction such as hydrolysis or polymerization (23,24). The exact nature of the polymeric film is not known; its insolubility in a variety of solvents makes characterization of the structure extremely difficult (19). However, studies with similar compounds (2,3-diaminopyridine) indicate that the initial oxidation forms a cation radical, which eventually forms an amine-linked polymer (25). Although there is no direct evidence, the circumstantial evidence indicates that there probably are a t least some amine linkages in poly(1,2-diaminobenzene). Figure 2 shows the potentiometric response of the poly(1,2-diaminobenzene) coated platinum electrode over a p H range of 4 to 10. This response is representative of three different electrodes, with individual electrodes having a response reproducibility of approximately &5’?&0 The p H response is nearly Nernstian with a slope of 53 mV and a linear correlation coefficient of 0.991. It is believed that the p H response is probably due to the protonation of amine linkages in the polymer. The degradation of potential response a t p H 4 and 5 could be due to saturation of protonation sites on the polymer. If the points a t pH 4 and 5 are omitted, the linear correlation coefficient of the potential response improves to 0.999 with a slope of 61 mV. The stability of the poly(l,2-diaminobenzene)film electrode was tested by cyclic voltammetry after soaking in buffer solutions of the following pH: l, 4-10, 13. After remaining in each buffer solution for approximately 15 min, the poly(l,2diaminobenzene) electrode was placed into a solution that was identical t o the solution (5 mM 1,2-diaminobenzene; 0.1 M, p H 7 , phosphate buffer) that was used to form the film, and the voltage was scanned from 0 to 0.8 V. If there was any indication of an oxidation wave due to the 1,2-diaminobenzene, the film was considered to be unstable a t that pH. However, if there was no oxidation wave due to 1,2-diaminobenzene, the film was intact and considered stable a t that pH. The poly(l,2-diaminobenzene)film was stable a t p H 4-10 and unstable a t p H 1 and 13. The potentiometric testing was therefore limited to a p H range of 4 to 10. In addition to the p H response, the poly( 1,2-diaminobenzene) platinum electrode response to Co(I1) was tested over a concentration range of t o lo4 M. Co(I1) forms tetrahedral and octahedral complexes with amines (26). Although it was expected that the stereochemical limitations imposed I
0 1980 American Chemical Society
346
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980 f
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Flgure 1. Cyclic voltammogram for the oxidation of 5 mM 1,2diaminobenzene at a platinum electrode in pH 7.0 phosphate buffer at a scan rate of 50 mV/s. 1. 2. and 3 refer to the first. second. and third scan, respectively
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Flgure 2. Potential response of the poly(1,2diaminobenzene)coated platinum electrode, vs. an SCE reference electrode, with changes in pH from 4 to 10
by the polymer surface would not favor complexation, one could not rule out the possibility of a thick polymer film with available complexation sites. The electrode, however, showed no change in response as the Co(I1) concentration was varied. Electrode filming also occurs with the electrochemical oxidation of 3,4-diaminobenzoic acid. An attempt was made to test the potentiometric response of this type of an electrode, but the film formed on the platinum electrode proved to be unstable, to some degree, in all the pH solutions tested. Therefore, unambiguous results could not be obtained in determining the potentiometric response of this electrode with changes in pH. The value of this study is not so much the pH response of the poly(l,2-diaminobenzene)electrode, but the fact that polymer film chemically modified electrodes can be used as potentiometric sensors. Various polymer film chemically
modified electrodes have been shown to interact with metal ions (5-12). By changing the nature of the polymeric film, its selectivity for various species could also be changed. A variety of potentiometric sensors could be constructed from chemically modified electrodes by employing different polymeric films or attaching various species to the electrode surface. In fact, polymer film chemically modified electrodes that are formed by casting the film on the electrode surface (5-12) would be similar to Freiser's coated wire potentiometric sensor (27). Research is continuing in our Laboratories on further applications of chemically modified electrodes as potentiometric sensors.
LITERATURE CITED Yamanto, N.; Nagasawa, Y.; Shvto, S.;Sawai, M.; Sudo,T.; Tsubomwa. H. Chem. Lett. 1978, 245. Yamamoto, N.; Nagasawa, Y.; Sawai, M.; Sudo, T.; Tsubomura, H. J. Immunol. Methods 1878, 22, 309. Laithinen, H. A.; Hseu, T. M. Anal. Chem. 1979, 51, 1550. Ianniello, R. M.; Yacynych, A. M., submitted for publlcation in Anal. Chem . Oyama, N.; Anson, F. C. J. Am. Chem. SOC. 1979, 101, 739. Oyama, N.; Anson, F. C. J. Am. Chem. SOC. 1979, 101, 3450. Merz, A.; Bard, A. J. J. Am. Chem. SOC.1978, 100, 3222. Itaya, K . ; Bard, A. J. Anal. Chem. 1978, 50, 1487. Miller, L. L.; Van De Mark, M. R. J . Am. Chem. SOC. 1978, 100, 639. Van De Mark,M. R.; Miller, L. L. J. Am. Chem. Soc. 1978, 100, 3223. Miller, L. L.; Van De Mark, M. R. J . Electroanal. Chem. 1978, 88, 437. Kaufman, F. 6.; Engler, E. M. J. Am. Chem. SOC. 1978, 101, 547. Nowak, R.; Schuitz, F. A.; Umana, M.; Abruna, H.; Murray, R. W. J. Nectroanal. Chem. 1978, 9 4 , 219. Landrum, H. L.; Salmon, R. T.; Hawkridge, F. M. J. Am. Chem. Soc. 1977, 99, 3154. Stargardt, J. F.; Hawkridge. F. M.; Landrum, H. L. Anal. Chem. 1878, 5 0 , 930. Pham, M. C.; Lacaze, P. C.; Dubois, J. E. J. Elwtroanal. Chem. 1978, 86, 147. Pham, M. C.; Dubois, J. E.; Lacaze, P. C. J. €/echoanal. Chem. 1979, 9 9 , 331. Yacynych, A. M.. R . D . Thesis, University of Cincinnati, Cincinnatl, Ohio, 1975. Yacynych, A. M.; Mark, H. B.. Jr. J. Nectrochem. SOC. 1978, 123, 1346. Adams, R. N. "Electrochemistry at Solid Electrodes"; Marcel Dekker: New York, 1969; pp 206-208. "Dictionary of Organic Compounds", Vol. 4; Oxford University Press: London, 1965; p 2684. Ref. 20, p 360. Ref. 20, p 305. Lee, H. Y.; Adams, R. N. Anal. Chem. 1882, 34, 1587. Desideri, P. G.;Helmler, D.; Lepri, L. J. Nectroanal. Chem. 1978, 88, 407. Cotton, F. A.; Wilkinson, G. "Advanced Inorganic Chemistry", 2nd ed.; Interscience: New York, 1966; p 866. Cattrall, R. W.; Freiser, H. Anal. Chem. 1971, 43, 1905.
William R. Heineman* Department of Chemistry University of Cincinnati Cincinnati, Ohio 45221
Henry J. Wieck Alexander M. Yacynych* Department of Chemistry Rutgers, The State University of New Jersey New Brunswick, New Jersey 08903 RECEIVED for review August 31, 1979. Accepted November 8, 1979. A.M.Y. thanks Rutgers Research Council, Biomedical Research Support Grants, and Merck Company Foundation through Merck Grants for Faculty Development Program for partial support of this project. W.R.H. acknowledges support from the National Science Foundation.
