Anal. Chem. 1989, (15) Heineman, W. R.; Burnett, J. N.; Murray, R. W. Anal. Chem. 1968, 40. 1974. (16) Bullock, J. P.; Boyd, D. C.; Mann, K. R. Inwg. Chem. 1987, 26, 3084. (17) DuBois, D. L.; Turner, J. A. J . Am. Chem. Soc. 1982, 704, 4989. (18) Nevln, W. A.; Lever, A. B. P. Anal. Chem. 1988, 60, 727. (19) DuBois, D. L. Inorg. Chem. 1984, 23, 2047. (20) Kadish, K. M.; Mu, X. H.; Lin, X. 0.Electroenalyst 1989, I , 35. (21) Flowers, P. A.; Mamantov, G. Anal. Chem. 1989, 67, 190. (22) b a r , J. L.; Yao, C.-L.; Liu, L.-M.; Capdevielle, F. J.; Korp, J. D.; Albright, T. A.; Kang, S.-K.; Kadish, K. M. Inorg. Chem. 1989, 28, 1254. (23) Lifsey, R. S.Ph.D. Dissertatlon, University of Houston, 1987. (24) Yao, C. L.; Park, K. H.; Khokhar, A. R.; Bear, J. I. Book of Abstracts; 197th National Meeting of the American Chemical Society, Dallas, TX; American Chemical Society: Washington, DC, 1989. INOR 363, (25) Blmths, P. R.; De Haseth, J. A. Fourier Transform Infrared Spectrometry. I n Chemical Analysis; Eiving, P. J., Winefordner, J. D., Eds.;
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Wiiey-Interscience: New York, 1986; Vol 83; Chapter 10. (26) Anderson, R. J.; Griffiths, P. R. Anal. Chem. 1975. 47, 2339. (27) Palma, F. E.; Piotrowki, E. A,; Sundaram, S.;Clevend, F. F. J . Mol. Spectrosc. 1964, 73, 119. (28) Newbound, T. D.; Colsman, M. R.; Miller, M. M.; Wulfsberg, G. P.; Anderson. 0. P.; Strauss, S . H. J. Am. Chem. SOC. 1989, 7 7 , 3762.
RECEIVED for review August 14, 1989. Accepted October 12, 1989. The authors thank the Robert A. Welch Foundation (Grants E-918, J.L.B., and E-680, K.M.K.), the National Science Foundation (Grant CHE-8822881, K.M.K.), and the National Institutes of Health (Grant GM25172, K.M.K.) for financial support.
Highly Stable Voltammetric Measurements of Phenolic Compounds at Poly(3-methylthiophene)-Coated Glassy Carbon Electrodes Joseph Wang* and Ruiliang Li Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003 The determination of phenolic compounds is of great environmental, industrial, and clinical significance. Since most phenols are oxidized at easily accessible potentials, voltammetry and amperometry may serve as highly sensitive tools for their quantification. Unfortunately, the oxidation of phenolic compounds a t solid electrodes produces phenoxy radicals which couple to form a passivating polymeric film on the electrode. A decrease in the response is thus observed upon repetitive scans, the rate of which is concentration dependent (as expected for a dimerization reaction). Consequently, conventional electrodes are usually unsuitable for reliable voltammetric measurements of phenolic compounds. Various strategies have been proposed to address this fouling problem. Anodic polarization in an acidic solution of ferric chloride was employed by Koile and Johnson (1) to remove phenolic films from platinum surfaces. Wang and Lin (2) described a repetitive electrochemical treatment for renewal in situ of glassy carbon electrodes in the presence of phenolic compounds. Laser activation was explored by Poon and McCreery (3)as a means to repeatedly renew solid electrode surfaces in the presence of phenols. Treatment with a flame ( 4 ) was also suggested to restore the surface activity. Such reactivation schemes are often time-consuming and/or require additional (high cost) instrumentation. A more attractive avenue for voltammetric measurements of phenols is to eliminate the passivation problem in the first place (rather than exploring means for surface reactivation). A deliberate modification of electrode surfaces may be very advantageous for this purpose. The objective of the research described in this note is to illustrate the unusual stable response of phenolic compounds at poly(3-methylthiophene) (P3MT) coated electrodes. Electroactive conducting polymers have received a great attention in the modification of electrodes because of potential application for energy storage or electrocatalysis and as electrochromic displays or “ion gate” membranes. Among these, films prepared by electropolymerization of thiophene derivatives have attracted considerable interest (5). In the course of our work on permselective electropolymerized films, we found that P3MT electrodes exhibit excellent resistance to fouling in the presence of high concentrations of phenolic species. The high stability is accompanied by enhanced sensitivity and selectivity. We wish to report these observations in the following sections.
EXPERIMENTAL SECTION Apparatus. The 10-mL electrochemical cell (Model MF 1052, Bioanalytical Systems (BAS))was joined to the working electrode, reference electrode (Ag/AgCl) (3 M NaCl) (Model RE-1, BAS), and the platinum wire auxiliary electrode through holes in its Teflon cover. The three electrodes were connected to an EG&G PAR Model 264A voltammetric analyzer, the output of which was displayed on a Houston Instruments X-Y recorder. Flow experiments employed a glassy carbon thin-layer amperometric detector (Model TL-5, BAS) and a 100-pL injection loop. The flow injection system was described previously (6). Reagents. All aqueous solutions were prepared in doubledistilled water. 3-Methylthiophene, acetonitrile (LC grade), m-nitrophenol, p-chlorophenol (Aldrich), acetaminophen, dopamine (Sigma), phenol (Fisher),p-cresol (Kodak), and ascorbic acid (Baker) were used without further purification. The supporting electrolyte was 0.05 M phosphate buffer (pH 7.4). Procedure. Prior to its coating, the glassy carbon electrode was polished with 0.05-pm alumina slurry, rinsed with doubledistilled water, and sonicated in a water bath for 2 min. The electrochemical polymerization was carried out in deaerated acetonitrile solution, containing 0.1 M sodium perchlorate and 0.05 M 3-methylthiophene. For this purpose the potential was cycled three times between 0.0 and 1.7 V (vs Ag/AgCl) at a rate of 20 mV/s; the polymerization was terminated during the third cycle by holding the potential at +0.7 V for 10 min. A film thickness of about 1 pm was estimated. Prior to phenol measurements the modified electrode was pretreated in the phosphate buffer blank solution by repetitively scanning the potential between 0.0 and +0.7 V (10 cycles) until a stable background was obtained.
RESULTS AND DISCUSSION The unusual stability of P3MT-coated electrodes will be illustrated in the presence of several phenolic compounds that exhibit rapid surface fouling at conventional electrodes. Figure 1 compares repetive cycle voltammograms for 2 X M p-cresol obtained a t 50 mV/s a t the P3MT-coated (A) and bare (B) glassy carbon electrodes. An irreversible oxidation process is observed at both electrodes. The inhibitory layer formed at the bare electrode results in disappearance of the peak after the third scan. In contrast, no degraded response is observed for the entire series at the P3MT electrode. Voltammograms of 2 x IO4 M chlorophenol (Figure 2), phenol, or m-nitrophenol (not shown) exhibit similar observations, with complete fouling of the glassy carbon surface within four
0003-2700/89/0361-2809$01.50/0C 1989 American Chemical Society
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Figure 3. Detection peaks for repetttive Injections of a 3 X lo-' M phenol solution at the bare (A) and P3MTcoated (6)electrodes: applied potential, +0.70 V; flow rate, 1.0 mL/min; injections of 100 pL samples. Electrolyte and carrier, 0.05 M phosphate buffer (pH 7.4). Figure 1. Successive (a-f) cyclic voltammograms for 2 X lo-' M p-cresol at PSMT-coated (A) and bare (B) glassy carbon electrodes: scan rate, 50 mV/s; electrolyte, 0.05 M phosphate buffer (pH 7.4).
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Figure 2. Successive (a-1) cyclic voltammograms for 2 X lo-' M chlorophenol at P3MT-coated (A) and bare (8)glassy carbon electrodes. Other conditions are given In Figure 1. scans and a highly stable response at the P3MT surface. The voltammograms of Figure 2A are a part of a series of 140 repetitive runs, that yielded a reproducible chlorophenol peak. The excellent behavior after numerous voltammetric scans implies negligible accumulation of reaction products. Such a high state of surface cleanliness is attributed to the use of a different substrate material (conducting polymer rather than glassy carbon) a t which the phenoxy products do not deposit, and thus do not block the surface. It is possible that the microporous structure of the film (discussed in the following section) results in steric hindrance to the dimerization of the phenoxy radicals. The high stability implies that repetitive surface renewal schemes are no longer necessary when voltammetry of phenolic compounds is concerned. The observation that the P3MT electrode yields a highly stable response in the presence of phenolic compounds can greatly benefit the detection of such compounds by liquid chromatography or flow injection analysis. Amperometric detection has been widely used for monitoring phenols in
at PSMTcoated and bare, glassy carbon electrodes (soHd and dotted lines, respectively): scan rate, 10 mV/s; electrolyte 0.05 M phosphate buffer. environmental, indugtrial, clinical, and food samples. Similar flow measurements have been explored in electrochemical immunoassays (based on the conversion of phenyl phosphate to a detectable phenol) (7). Under flow injection or liquid chromatography conditions, the fouling problem is not as severe (as in voltammetric, batch, experiments), because of the small amount of product that is electrogenerated. This is, in particular, the case for work a t low concentrations (lo4 to lod M). Electrode fouling, however, is evidenced at higher concentrations. For example, for a series of 15 repetitive flow injections of samples containing 3 X lo4 M phenol, an appreciable (37%) loss of electrode activity is observed (Figure 3A). In contrast, a highly stable phenol detection peak is observed when the P3MT detector is used (Figure 3B). Notice, however, that while both electrodes exhibit a fast response time, the recovery time is slower at the P3MT detector (probably due to trapping of the analyte within the surface micropores). Another observation that accrues from the surface alteration is the overall appearance of the voltammogram. Figure 4
ANALYTICAL CHEMISTRY, VOL. 61, NO. 24, DECEMBER 15 1989
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Figure 5. Differential pulse voltammograms for acetaminophen solutions of increasing concentration, (2.5-7.5) X lod M (a-c) at bare (A) and P3MTcoated (6) electrodes: scan rate, 10 mV/s; amplitude, 25 mV; electrolyte, 0.05 M phosphate buffer (pH 7.4).
compares cyclic volt ammo gram^ for 2 X lo-' M acetaminophen recorded at 10 mV/s at P3MT and ordinary glassy carbon electrodes (solid and dotted lines, respectively). Well-defined symmetrical peaks are observed at the coated electrode; the value of AE, is only 34 mV. Such response is consistent with a thin-layer electrochemical behavior and is attributed to solute depletion within the micropores of the P3MT. The large surface area provided by the three-dimensional porous structure greatly increases the faradaic current (m the ordinary electrode). Analogous changes in the nonfaradaic residual current are also observed. Note, for example, the marked increase in the cyclic voltammetric background current envelope (e.g., Figures 1and 2) that reflects the corresponding change in the double layer capacitance. Such changes were observed also in analyte-free electrolyte solutions (not shown). Adsorptive accumulation of acetaminophen may also account for the shape of the voltammogram shown in Figure 4. Analytical advantages accrue from the use of pulse voltammetric procedures aimed at compensating the high background currents. For example, Figure 5 illustrates differential pulse voltammograms, recorded at bare (A) and P3MT (B)electrodes, for successive standard additions of acetaminophen ((2.5-7.5) X lob M (a-c)). Significantly larger oxidation peaks are observed at the coated electrode. (Note the different current scales.) The effective correction of the charging current contribution results in similar (flat) base lines. The three peaks of Figure 5 are a part of eight concentration increments, up to 2 X 10"' M. At both electrodes, linearity prevailed over the lower (
