Anal. Chem. 1985, 57, 545-551 Bright, F. V.; McGown, L. 8 . Talanfa, in press. Lakowicz, J. R.; Baiter, A. Biophys. Chern. 1982, 16, 223. Lakowicz. J. R.: Baiter, A. Biophys. Chern. 1982, 15, 353. Lakowicz, J. R.; Cherek. H. J . Biochem. Biophys. Methods 1981, 5 , 19. Jameson. D. M.; Gratton, E.; Hall, R. D. Appl. Specfrosc. Rev. 1984, 20,55. Demas, J. N. "Excited State Lifetime Measurements"; Academic Press: New York, 1983. Dovichi, N. J.; Martin, J. C.; Jett, J. H.; Trkula, M.; Keller, R. A. Anal. Chem. 1984, 56, 348. Miller, J. N. Trends Anal. Chem. 1981, 1. 31. Cline Love, L. J.; Skrilec, M.: Habarta, J. G. Anal. Chem. 1980, 5 2 , 754. Kalvanasundaram. K. Coord. Chem. Rev. 1982. 46. 159. D e k w , D., Ed. "ARRL Electronics Data Book"; American Radio Relay League: Newington, CT, 1976: p 95. Seymour, E.. Ithaco Co., private communications. Demas, J. N. J Chern. Ed. 1976, 53, 657. Snavely, B. B. "Dye Lasers"; Schafer. F. P., Ed.; Springer-Verlag: New York, 1973. Fendler. J. H.; Fendler, E. J. "Catalysis in Micellar and Macromolecular Systems"; Academic Press: New York, 1975.
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RECEIVED for review June 22, 1984. Accepted October 25, 1984. J.N.D. thanks the University of Virginia for the award of a Sesquicentennial Fellowship which helped make his sabbatical at LANL possible. We gratefully acknowledge and the National Science suppol.t Of the Department Of Foundation (NSF 82-06279).
Electrochemical and Electron Spectroscopic Studies of Highly Polished Glassy Carbon Electrodes Geoffrey N. Kamau, William S. Willis, and James F. Rusling*
Department of Chemistry (U-60) and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06268
Preparation of glassy carbon electrodes by high-speed polishing with successively smaller particle slze sllicon carblde, dlamond paste, and y-alumlna, and ultrasonic cleaning, ylelded reproduclble actlvatlon for anodic oxidations of ferrocyanide, ferrocene, ascorbate, qulnones, and catharanthlne. Electron spectroscopy showed that the highly pollshed electrodes had a higher oqygen content in the outer 20-30 nm than In the bulk material or In unactlvated electrodes. A major portlon of the oxygen Is probably assoclated with phenollc-like groups. For simple electron traders, creation of a favorable charge density at the eiectrwie Is an Important factor In the actlvatlon, but other nonspecif/c Interactions may also be Involved. For protoncoupled reactions, such as those of ascorbate and dopamine, SpecHic Interactions of reactants with catalytlc groups created on the surface may play a slgnlflcant role In electrqde actlvatlon. Response of the electrodes degraded wlth time.
Because of a low residual current over a range of about &1
V (vs. SCE) in aqueous media ( I , 2 ) , and a more extended range in organic solvents ( I , 31, glassy carbon has been used extensively as a working electrode for a variety of electrochemical applications. A serious difficulty with glassy carbon is the extreme sensitivity of the apparent rate of many redox reactions to the surface state of the electrode. In short, the quantitative results obtained for certain redox couples depend intimately on the method of preparing the electrode surface immediately before analysis, as well as on the electrode's prior history. For these reasons, there has been an extensive and continuing interest in developing reproducible methods of "activating" the glassy carbon surface. The goal of such activation is the increase of the apparent rate of the electrode process, thereby decreasing overpotential and peak width and improving sensitivity and resolution. Some success has been
reported (4-9) with methods involving polishing of the surface with abrasives of micrometer or submicrometer particle size, followed by electrochemical oxidation, sometimes followed by reduction. However, these techniques cause an increase in residual current (5, 12) and often require different electrochemical pretreatments for different redox couples (4,8). It would be advantageous, therefore, to have a general activation procedure which avoided electrochemicalpretreatment. There have been several previous ipdications (5, 7 , I O , I I ) that careful attention to electrode polishing and cleaning could yield an electrode of very good activity. One of the most impressive examples described to date is the procedure of Jordan and Robbat (10, 1 I ) , which involves high-speed metallographic polishing with successively smaller particle size silicon carbide, diamond paste, and finally alumina. This procedure yielded reversible cyclic voltammograms for the reduction of ferricyanide ion in 1 M KCl a t scan rates up to 1 V s-l and was successfully used for a quantitative study of the electrooxidation of benzothiophenes. In other examples, manual polishing on a glass slide with S i c followed by alumina yielded an electrode activated toward oxidation of ferrocyanide to an extent equivalent to electrochemical pretreatment ( 5 ) . Polishing with l-pm diamond paste followed by extensive refluxing in toluene provided a 300 mV decrease in overpotential for the oxidation of ascorbate (7). The method of Jordan and Robbat appeared attractive since it was easily done with commercial metallographic polishing apparatus and materials, and, after the initial extensive preparation, required only brief repolishing between experiments. To our knowledge, its only reported quantitative tests had been with ferricyanide in aqueous media and benzothiophenes in acetonitrile (11). However, as mentioned above, different electrochemicalprocedures are required for activating glassy carbon toward different redox couples (4,8),and some redox reactions in organic solvents are relatively insensitive to changes in the nature of the electrode surface (6, 13). We concluded that a wider-ranging quantitative study of the
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highly polished electrodes was warranted. Thus, because of our interest in a general method for activating glassy carbon for biologically important redox reactions, we have investigated a modification of the Jordan-Robbat approach, including ultrasonic cleaning steps (14). In this paper, we report on the use of this procedure to prepare glassy carbon for electrochemical experiments on a variety of redox couples in organic and aqueous media. Furthermore, we have correlated electrochemical and electron spectroscopic results to obtain a better understanding of the factors governing the electrode activation process.
EXPERIMENTAL SECTION Chemicals. Ascorbic acid (Baker Chemical Co.), p-hydroquinone (Baker Chemicals Co.), 1,2,3,4-tetrahydrocarbazole (Aldrich Chemical Co., %'%), dopamine (Aldrich), and vinblastine sulfate (Sigma Chemical Co.) were used as received. Catechol (Eastman) was crystallized from absolute methanol before use. Catharanthine hydrochloride and vindoline were gifts from J. M. Bobbitt, University of Connecticut. Ferrocene was obtained and purified as described previously (15). Water with a specific resistance greater than 10 MR cm, obtained by passing distilled water through a Sybron/Bamstead NANOpure water purification system, was used for ultrasonic cleaning and to prepare hydroquinone, dopamine, and ascorbic acid solutions. The latter were prepared immediately before use to avoid decomposition. Ordinary distilled water was used for all other applications. Acetonitrile was Matheson, Coleman and Bell (glass distilled) used as received. All other chemicals were reagent grade. Electrochemical Apparatus. A Bioanalytical Systems BAS-100 Electrochemical Analyzer was used for cyclic voltammetry (CV), differential-pulse voltammetry (DPV), and chronocoulometry. Results were output to an Epson FX-80 printer or a Houston DMP-40 digital plotter. A three-electrode Metrohm cell with a platinum wire counterelectrode was used for all electrochemicalexperiments. A PARC saturated calomel electrode (SCE) with a vycor tip, separated from the sample compartment by a salt-bridge containing supporting electrolyte and terminating in a medium porosity glass frit, was the reference. Cell resistance, as measured by the BAS-100, was fully compensated in all voltammetric experiments. All work was done at the ambient temperature of the laboratory (23 f 2 "C). Electrode Polishing Procedure. Glassy carbon rods were obtained from Normar Industries and Electrosynthesis Co. (manufactured by Le Carbone). Construction of the planar glassy carbon electrodes hm been described previously (15). Geometric areas of the completed electrodes were 0.088 and 0.071 cm2 for the Normar and Le Carbone materials, respectively. All grinding and polishing steps were done on a Buehler metallographic polishing wheel operated a t 1140 rpm. The rough-cut glassy carbon (5 mm long), was press fitted into its Teflon collar (3 mm thick) (15),and lightly sanded successively with Buehler Carbimet silicon carbide paper of grit numbers 240,400, and 600. A stream of distilled water was directed a t the electrode to eliminate frictional heating. The 240 grit Sic paper was used to grind the glassy carbon flush with the Teflon sleeve. The electrode was hand held perpendicular to the wheel and rotated 4.5" at intervals of several seconds. A V-motion toward and away from the center of the polishing wheel was maintained during each grinding step of approximately 2 min, yielding successively smoother surfaces after each step. After the Sic grinding is done upon initial preparation of the electrode, it is generally not repeated. The next series of operations involved polishing with Metadi diamond pastes of 6-, 1-,and 0.25-pm particle sizes. The sequential steps were done on the polishing wheel on a Buehler Microcloth (No. 40-7208) maintaining firm pressure, the V-shpaed motion, and 45" rotations as above, during each step of 2 min. A different microcloth was used for each size of diamond paste. After each polishing step, the paste and debris accumulated on the glassy carbon surface were removed by ultrasonication for 1 min in a beaker of absolute methanol (spectrograde). The electrode was rinsed with pure water and polished on the polishing wheel on separate Buehler billiard cloths (no. 40-7308) with 0.3-pm alumina for 2 min, followed by 1min ultrasonication in pure water, followed by a final polish with 0.05-pm alumina (Buehler Micropolish
y-alumina). The electrode was cooled with water while polishing with alumina. The final step was ultrasonic cleaning for 0.5-1.0 min in a beaker of pure water, followed by washing with water. At this point the glassy carbon had a shiny, mirrorlike appearance. Water forms beands on the surface but methanol appears to wet it uniformly. Before each day's experiments, steps beginning with 6-pm diamond paste were repeated twice. Following the final ultrasonication, the electrode was transferred to the cell containing the analyte solution, from which oxygen had been removed by bubbling with purified nitrogen (151, and equilibrated for 1 min at the initial potential of the experiment, generally between h0.3 V vs. SCE. For replicate electrochemical experiments, only polishing with 0.3 and 0.05 pm alumina and ultrasonic cleaning was used, except when the electrode had been exposed to high (>0.1 mM) concentrations of film-forming species, such as dopamine. Potentials applied to the electrodes were kept between -0.3 and 1.0 V vs. SCE to minimize electrochemicaltransformation of the surface. The above procedure differs from that described by Robbat (11) as follows: (i) we omitted the 320 grit S i c polish, (ii) we included the ultrasonic cleaning steps in place of cleaning with detergent, (iii) our diamond paste was of smaller particle size than the 30-, 6-, and 1-pm pastes used previously, (iv) we introduced a step involving 0.3-pm alumina prior to the final step, and (v) our final step was with 0.05-pm y-alumina followed by ultrasonic cleaning whereas the previously reported final step specified only
