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Control of Catechol and Hydroquinone Electron-Transfer Kinetics on Native and Modified Glassy Carbon Electrodes

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Control of Catechol and Hydroquinone Electron-Transfer Kinetics on Native and Modified Glassy Carbon Electrodes Anal. Chem. 1999, 71, 4594-4602 Control of Catechol and Hydroquinone Electron-Transfer Kinetics on Native and Modified Glassy Carbon Electrodes Stacy Hunt DuVall and Richard L. McCreery* Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210 The electrochemical oxidation of dopamine, 4-methylcat- heterogeneous electron transfer between solid electrodes and echol, dihydroxyphenylacetic acid, dihydroxyphenyl eth- catechols or quinones. Such studies are of fundamental importance ylene glycol, and hydroquinone was examined on several to the electrochemical detection of catecholamines as well as to native and modified glassy carbon (GC) surfaces. Treat- the broader questions about quinone redox chemistry. Notable ment of polished GC with pyridine yielded small ∆Ep examples from the literature are the detailed examination of values for cyclic voltammetry of all systems studied, quinone reduction on platinum in aqueous buffers,7 the oxidation implying fast electron-transfer kinetics. Changes in surface of catechols on carbon paste,2,3 and the dramatic effects of oxide coverage had little effect on kinetics, nor did the adsorbates on quinone electrochemistry on platinum and iridium charge of the catechol species or the solution pH. Small electrodes.5,6,8 These experiments provide an excellent description + - ∆Ep values correlated with catechol adsorption, and of the elementary steps comprising the 2 H ,2e reduction of surface pretreatments that decreased adsorption also an o-orp-quinone to the corresponding hydroquinone and account increased ∆Ep. Electron transfer from catechols was for much of the pH dependence of the observed redox potential profoundly inhibited by a monolayer of nitrophenyl or and kinetics. However, the dependence of quinone redox kinetics (trifluoromethyl)phenyl (TFMP) groups on the GC surface, on the state of the electrode surface is much less well understood, so that voltammetric waves were not observed. The ∆Ep particularly for widely used electroanalytical sensors based on increased monotonically with surface coverage of TFMP glassy carbon (GC) or carbon fibers. groups. The results indicate that catechol adsorption to Many surface treatments of both Pt and carbon electrodes have GC is required for fast electron transfer for the redox demonstrated that the electrode surface has a profound effect on 3+/2+ systems studied. Unlike Ru(NH3)6 , chlorpromazine, catechol voltammetry, with apparently minor changes to the methyl viologen, and several others, electron tunneling surface causing large changes in electrode kinetics. For example, through monolayer films was not observed for the cat- anodization of GC greatly decreases ∆Ep for dopamine (DA) and echols. The results are not consistent with an electron- increases its adsorption.9-12 However, laser activation also greatly 13,14 transfer mechanism involving proton transfer or electro- decreases ∆Ep, with no apparent surface oxidation. Fast DA static interactions between the catechols and surface sites kinetics may be observed on carbon surfaces with low or high on the GC surface. The vital role of adsorption in the oxygen/carbon (O/C) ratio, with or without anodization, and in electron-transfer process is currently under study but certain cases on heavily modified electrodes (e.g., Nafion coated).14 appears to involve changes in the inner-sphere reorgani- Many hypotheses for the pronounced sensitivity of DA kinetics zation energy. to surface condition have been proposed, including ionic effects related to surface oxides,11 proton transfer from oxide sites Quinone/hydroquinone redox systems have been studied accompanying electron transfer,10 and electrocatalysis by adsorp- extensively for several reasons, not the least of which is their tion to the Pt or carbon surface.1 For the case of clean Pt biological importance.1 Catecholamine neurotransmitters, quinone- electrodes, irreversible adsorption of catechols and hydroquinones based metabolic cofactors, and the fundamental electron-transfer properties of quinones have stimulated extensive examinations (7) Laviron, E. J. Electroanal. Chem. 1984, 164, 213. (8) Temesghen, W.; Jeng, J.-J.; Carrasquillo, A., Jr.; Soriaga, M. P. Langmuir of quinone redox chemistry, in both aqueous and nonaqueous 1994, 10, 3929. environments.2-9 Of particular relevance here are studies of (9) Allred, C. A.; McCreery, R. L. Anal. Chem. 1992, 64, 448. (10) Cabaniss, G. E.; Diamantis, A. A.; Murphy, W. R.; Linton, R. W.; Meyer, T. (1) Chambers, J. Q. In The Chemistry of Quinonoid Compounds; Patai, S., J. J. Am. Chem. Soc. 1985, 107, 1845-1854. Rappaport, Z., Eds.; Wiley and Sons: New York, 1988; Vol. II, p 719. (11) McCreery, R. L. Carbon Electrodes: Structural Effects on Electron Transfer (2) Cabaniss, G. E.; Diamantis, A. A.; Murphy, W. R.; Linton, R. W.; Meyer, T. Kinetics. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, J. J. Am. Chem. Soc. 1985, 107, 1845-1854. 1991; Vol. 17, pp 221-374. (3) Deakin, M. R.; Wightman, R. M. J. Electroanal. Chem. 1986, 206, 167- (12) McCreery, R. L. Carbon Electrode Surface Chemistry: Optimization of 177. Bioanalytical Performance. In Voltammetric Methods in Brain Systems; (4) Jaegfeldt, H.; Kuwana, T.; Johansson, G. J. Am. Chem. Soc. 1983, 105, 1805. Boulton, A. A., Baker, G. B., Adams, R. N., Eds.; Humana Press: Totowa, (5) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. Soc. 1982, 104, 2735. NJ, 1995; pp 1-26. (6) Soriaga, M. P.; Wilson, P. H.; Hubbard, A. T.; Benton, C. S. J. Electroanal. (13) Poon, M.; McCreery, R. L. Anal. Chem. 1986, 58, 2745. Chem. 1984, 142, 317. (14) Strein, T.; Ewing, A. G. Anal. Chem. 1994, 66, 3864. 4594 Analytical Chemistry, Vol. 71, No. 20, October 15, 1999 10.1021/ac990399d CCC: $18.00 © 1999 American Chemical Society Published on Web 09/09/1999 occurs, and this chemisorbed layer does not undergo reversible previously17 involving adsorption of MB at high concentration (10 2e- transfer to the corresponding quinone. However, chemically mM) and then transfer to catechol solution. In the second, MB reversible electron transfer was observed on the irreversibly and catechol were present simultaneously, as will be discussed adsorbed organic layer.5,6 For iridium surfaces, an oxide layer was in more detail later. Chemisorption of nitrobenzene and (trifluo- found to inhibit the H2Q/Q electron transfer, but the inhibition romethyl)benzene was accomplished by the procedure of SaveÂant was reversed by small amounts of chemisorbed sulfur or iodide.8 et al.21 Diazonium salt solutions (1 mM) were reduced in 0.1 M For either carbon or metal electrodes the oxidation of catechol tetrabutylammonium tetrafluoroborate in acetonitrile by voltam- to quinone involves 2 e- and usually 2 H+, so adsorbed species metric scans from +0.1 to -1.4 V vs Ag/Ag+ at 200 mV/s until could affect one or more of four possible steps.2,7 the reduction peak was absent (3 scans for nitrophenyl and 5-10 We reported previously on a procedure for diagnosing surface scans for trifluoromethyl phenyl derivatives). effects on electrode kinetics, for the case of GC electrodes in Surfaces with low oxide coverage (as judged by O/C ratio from aqueous solution.16,17 The approach involved specific modifications XPS) were prepared by anaerobic polishing in cyclohexane/ to GC surfaces, followed by assessment of their effects on alumina slurries on bare glass plates.16 The cyclohexane was electrode kinetics for selected redox systems. Of note is the saturated with argon for 15-20 min before mixing with dry observation that several common redox systems are insensitive alumina. An alternative method for preparing low-oxide sur- 3+/2+ 3,4,22 to surface modification (e.g., Ru(NH3)6 , methyl viologen) and faces was vacuum heat treatment (VHT). Briefly, the GC the observed rates are controlled by electron tunneling.16,18 In sample was heated (∼800 °C) by resistive heating with a tantalum contrast, several redox reactions (e.g., Fe3+/2+) are catalyzed by heating stub. After 30 min at ∼800 °C in UHV, the sample was certain surface groups and are very dependent on surface cooled and an XPS spectrum was acquired. Then the sample was preparation.17,19 The approach has been applied to a variety of quickly transferred in air to the electrochemical cell for voltam- inorganic16 and organic18 redox systems, but with the exception metry. of ascorbic acid, none of these involved protons or multielectron Cyclic voltammetry was carried out using a BAS 100/W transfer. In the current work, several catechols and hydroquinone electrochemical workstation. The reference electrode (Bioana- were examined with the same approach, not only to extend our lytical Systems Inc., West Lafayette, IN) used for aqueous work understanding of carbon electrode behavior but also to provide was a Ag/AgCl (3 M NaCl), and a Ag/Ag+ reference was used new insights into quinone electron-transfer mechanisms. for solutions in acetonitrile. A platinum wire was used as the counter electrode. Each voltammogram was recorded on a freshly EXPERIMENTAL SECTION prepared surface. Survey and regional XPS spectra were acquired Electrodes used include commercial GC electrodes from with a VG Scientific Escalab MKII spectrometer with either Al or Bioanalytical Systems, Inc. (West Lafayette, IN) and disks cut from Mg anode. Grams32 (Galactic) software was used to calculate peak Tokai GC-20 plates. The disks were mounted in a homemade areas. Instrumental sensitivity factors were applied when calculat- Teflon electrode holder after polishing or modification. All ing atomic ratios. electrodes were polished before any surface treatments in 1-, 0.3-, Catechol solutions were 1 mM unless otherwise noted and and 0.05-µm alumina powders (Buehler, Lake Bluff, IL) slurried were prepared in either 0.1 M H2SO4 or 0.1 M PBS (0.1 M with Nanopure water (Barnstead Nanopure System, Dubuque, IA) phosphate buffer with 0.1 M NaCl added). Dopamine and D,L-3,4- on Microcloth polishing cloth (Buehler, Lake Bluff, IL) according dihydroxyphenyl ethylene glycol (DOPEG) were used as received 16,17,20 to previously published procedures.
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