4612
J . Phys. Chem. 1986, 90, 4612-4617
Electrochemistry at Partially Blocked Carbon-Fiber Microcylinder Electrodes Paul M. Kovach, Mark R. Deakin, and R. Mark Wightman* Department of Chemistry, Indiana University, Bloomington, Indiana 47405 (Received: February 19, 1986)
The voltammetry of several compounds has been examined at the cylindrical surface of carbon fibers in aqueous solution at pH 7.4. The fibers were sealed in glass capillariesand electrochemically oxidized before use. The electrochemical pretreatment previously has been shown to enhance electron-transfer rates. In this work, it is shown via results from chronocoulometry that cations such as ruthenium(II1) hexaammine and cobalt(II1) hexaammine are adsorbed to the oxidized surface. Similar results are found for (3,4-dihydroxyphenyl)ethylamine. In contrast, anions such as ferricyanide, molybdenum(1V) octacyanide, (dihydroxypheny1)acetic acid, and ascorbate show no evidence for adsorption. However, the voltammetric peak current and the chronoamperometric response are much lower than expected for a cylinder with the geometric dimensions of the carbon fiber (5” radius, 500-pm length). The time dependence of the diffusion-controlled electrochemistry does fit to a model for diffusion to a hemicylinder of 0.7-pm radius, while electron microscopy shows that the geometric size of the electrode is unchanged by the electrochemical treatment. Thus, the electrochemical oxidation reduces the area of the electrode that is available for interfacial electron transfer. This observation correlates with reports of the formation of graphitic oxides, an insulator with cation-exchange properties, on the electrode surface following oxidation at extreme potentials.
Introduction The heterogeneity of the surface of carbon electrodes provides a unique surface at which the effect of the density of active sites may be examined by electrochemical techniques. Theoretical treatments of partially active electrode surfaces have been developed by Matsuda and co-w~rkers’-~ and by Amatore et aL4q5 Theory for an array of microelectrodes, which is analogous to a partially active electrode, has been developed by Gileadi and c o - w o r k e r ~ . ~These ~ ~ treatments indicate that the sites function independently as the blockage approaches unity. Since the rate of mass transport to the individual sites is large because of convergent diffusion processes, large current densities can be observed even with small active electrode areas. This report examines the electrochemistry at a carbon-fiber microcylinder electrode which exhibits characteristics of blockage of a significant fraction of surface area. In general, electron-transfer rates at carbon electrodes are sluggish compared to those at other electrode materials. The observed electrochemical response of carbon electrodes has been improved by a variety of treatments.*-I8 Many different physical and chemical properties of the carbon surface have been implicated as responsible for the increase in electrochemical electron-transfer rates. Among these are the orientation of the graphitic lattice (1) Gueshi, T.; Tokuda, K.; Matsuda, H. J . Electroanal. Chem. 1978,89, 247-260. (2) Gueshi, T.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1979. 101, 29-38. (3) Tokuda, K.; Gueshi, T.; Matsuda, H. J . Electroanal. Chem. 1979.102, 41-48. (4) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 146, 37-45. (5) Amatore, C.; Saveant, J. M.; Tessier, D. J . Electroanal. Chem. 1983. 147, 39-51. (6) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J . Electroanal. Chem. 1982, 138, 65-17. (7) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J . Electroanal. Chem. 1984, 161, 247-268. (8) Evans, J. F.; Kuwana, T. Anal. Chem. 1977, 49, 1632-1635. (9) Evans, J. F.; Kuwana, T. Anal. Chem. 1979, 51, 358-365. (10) Kamau, G. N.; Willis, W. S.; Rusling, J. F. Anal. Chem. 1985, 57, 545-551. (1 1) Thornton, D. C.; Corby, K. T.; Spendel, V. A,; Jordan, J.; Robbat Jr., A.; Rutstrom, D. J.; Gross, M.; Ritzler, F. Anal. Chem. 1985, 57, 150-155. (12) Stutts, K. J.; Kovach, P. M.; Kuhr, W. G.; Wightman, R. M. Anal. Chem. 1983, 55, 1632-1634. (13) Hu, I.-F.; Karweik, D. H.; Kuwana, T. J . Electroanal. Chem. 1985, 188, 59-72. (14) Hershenhart, E.; McCreery, R. L.; Knight, R. D. Anal. Chem. 1984, 56, 2256-2257. (15) Weisshaar, D. E.; Kuwana, T. Anal. Chem. 1985, 57, 378-379. (16) Engstrom, R. C. Anal. Chem. 1982, 54, 2310-2314. (17) Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984, 56, 136-141. (18) Diamantis, A. A.; Murphy, . _Jr., W. R.; Meyer, T. J. Inora. Chem 1984, 23, 3230-3234.
0022-3654/86/2090-4612$01.50/0
at the solution interface, the type, abundance, and interaction of carbon-oxygen surface functionalities, and the “cleanliness” or homogeneity of the carbon surface.’%24 Electrochemical oxidation of the surface of a single carbon fiber microcylinder is particularly effective in the acceleration of electrochemical rates of electron t r a n ~ f e r . ~ ~This . ~ ~treatment is unique because the treated electrode retains its properties for extended periods even in a relatively hostile environment such as the mammalian brain.25 However, the operant mechanisms at these oxidized surfaces have not been determined. Accelerated rates result in a large shift in the observed half-wave potential for many species relative to untreated electrodes; in particular, the overpotential for the oxidation of ascorbic acid is greatly reduced, which allows voltammetric resolution of ascorbate from the catecholamine neutrotransmitters and their metabolite^.^' Previously, we have presented qualitative observations that indicate that these electrodes adsorb cationic species.28 However, rigorous analysis of the voltammetric data was precluded because of the absence of adequate description of the diffusion-controlled behavior at microcylinders. Solutions now exist for cyclic volt a m m e t r ~ and ~ ~ ~c h~r ’~ n o a m p e r o m e t r ywhich ~ ~ predict quasisteady-state behavior at microcylinder electrodes of small radii at relatively long times. Digital simulation of the voltammetry of R u ( N H & ~ +agrees well with that observed experimentally at untreated carbon fiber microcylinder electrodes (Figure 1). In this paper, we report the voltammetric characteristics of some (19) Randin, J. P. In Comprehensive Treatise of Electrochemistry; Bockris, J. O . , Conway, B. E., Yeager, E., White, R. E., Eds.; Plenum: New York, 1981; Vol. 4, Chapter 10. (20) Panzer, R. E.; Elving, P. J. J. Electrochem. SOC.1972, 119, 864-874. (21) Panzer, R. E.; Elving, P. J. Electrochim. Acta 1975, 20, 635-647. (22) Wightman, R. M.; Deakin, M. R.; Kovach, P. M.; Kuhr, W. G.; Stutts, K. J. J . Electrochem. SOC.1984, 131, 1578-1583. (23) Fagan, D. T.; Hu, I.-F.; Kuwana, T. Anal. Chem. 1985, 57, 2759-2763. (24) Deakin, M. R.; Kovach, P. M.; Stutts, K. J., Wightman, R. M. Anal. Chem. 1986, 58, 1474-1480. (25) Gonon, F.; Fombarlet, C. M.; Buda, M. J.; Pujol, J.-F. Anal. Chem. 1981, 53, 1386-1389. (26) Proctor, A,; Sherwood, P. M. A. Carbon 1983, 21, 53-59. (27) Gonon, F.; Buda, M.; Cespuglio, R.; Jouvet, M.; Pujol, J.-F. Brain Res. 1981, 223, 69-80. (28) Kovach, P. M.; Ewing, A. G.; Wilson, R. L.; Wightman, R. M. J . Neurosci. Methods 1984, IO, 215-221. (29) Kovach, P. M.; Caudill, W. L.; Peters, D. G.; Wightman, R. M. J . Electroanal. Chem. 1985, 185, 285-295. (30) Aoki, K.; Honda, K.; Tokuda, K.; Matsuda, H. J . Electroanal. Chem. 1985, 182, 267-279. (31) Amatore, C. A,; Deakin, M. R.; Wightman, R. M. J . Electroanal. Chem. 1986, 206, 23-36. (32) Aoki, K.; Honda, K.; Tokuda, K.; Matsuda, H. J . Electroanal. Chem. 1985, 186, 79-86.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 19, 1986 4613
Carbon-Fiber Microcylinder Electrodes
+0.2
0.0
-0.2
-0.4
-0.6
E ( V v r SSCE)
Figure 1. Voltammetry of Ru(NH,):+ at untreated carbon-fiber microcylinder electrode (pH 7.4): (-), experimental data; ( O ) , simulated voltammogram at cylindrical electrode for reversible system. Conditions: n = 1, D = 5.5 X 10" cm2 s-l, C = 1.03 mM, geometric area = 1.48 X cm2,where r = 5 X cm and I = 0.0470 cm, u = 0.100 V s-l.
highly charged metal complexes, catechols, and ascorbic acid at electrochemically treated carbon-fiber microcylinder electrodes. The characteristics, coupled with diffusional models, demonstrate that this electrode has a greatly reduced active surface area and yet still exhibits large current densities and efficient electrochemical rates of electron transfer.
Experimental Section Reagents. Transition-metal complexes K3[Fe(CN),] (Mallinckrodt), [C0(NH3),]Cl3 (Kodak), [Ru(NH3),]cl3, and [Ru(NH3)5Cl]C12(Johnson-Matthey) were used as received. K4[ M o ( C N ) ~ ] . ~ Hwas ~ O synthesized following reported procedures.33 Dopamine (DA), (3,4-dihydroxyphenyl)acetic acid (DOPAC), 4-methylcatechol (4-MC), and ascorbic acid (AA) were obtained from Aldrich or Sigma and used as received, except 4-MC which was recrystallized from toluene. All other chemicals were reagent grade and used as received. Except where noted, all species were studied in phosphate buffer solution (0.1 M phosphate) adjusted to the proper pH with phosphoric acid. Solutions were prepared with doubly distilled, deionized water and purged thoroughly with nitrogen or argon before use. Electrodes. Microcylinder electrodes were constructed with the use of carbon fibers (Thornel VSB-32, Union Carbide). Individual fibers (radius 5 m) were aspirated into glass capillaries which were then pulled to a fine taper with a pipet puller (Narishige Scientific Instrument Co., Model PF-2, Tokyo, Japan). The fiber was sealed in the pulled glass capillary by carefully placing a drop of epoxy (Epon 828 mixed with 14% mphenylenediamine and cured at 70 OC; Miller-Stephenson, Danbury, CT) at the end of the glass. This was accomplished by placing another pulled capillary, filled with epoxy, in close proximity to the electrode, so that the epoxy enters the electrode capillary without coating the carbon fiber that extends beyond the glass. Epon 828 epoxy was used since it is widely used in carbon-fiber composite materials because of its bonding strength and inertness to chemical attack. Electrodes were trimmed with a scalpel so tht approximately 500 pm of carbon fiber extended beyond the end of the capillary. The exact length of the protruding fiber was measured with an optical microscope. Before electro(33) van de Poel, J.; Neumann, H. M. Inorg. Synth. 1968, 2 2 , 53-56. (34) Shoup, D.;Szabo, A. J . Electroanal. Chem. 1984, 260, 1-17.
chemical treatment, the geometric area of the electrode was verified voltammetrically by the reduction of R u ( N H ~ ) ~ Only +.~~ electrodes whose optically and voltammetrically determined areas agreed to 6% were used. Electrical contact to the fiber was made by filling the capillary with mercury and inserting a short wire into the capillary. Electron micrographs of the fiber surface were obtained with a Cambridge Instruments, Ltd., Stereoscan 250 MK 11. Apparatus. Potential control was provided by a polarographic analyzer (Princeton Applied Research Corp., Model 174A, Princeton, NJ). Chronoamperometric and chronocoulometric experiments and data analysis were performed with an IBM PC equipped with a Tecmar Labmaster interface (Tecmar, Inc., Solon, OH). Potential waveforms for electrochemical treatment of the carbon fiber electrodes were applied with the polarographic analyzer and an external function generator (Wavetek Inc., Model 182A, San Diego, CA). A 25-mL vial served as the electrochemical cell and was isolated on a vibration-free platform. Platinum wire served as the auxiliary electrode. Potentials are reported vs. the sodium-saturated calomel electrode (SSCE). Electrochemical Treatment. Electrochemical treatment of carbon fiber microcylinder electrodes was similar to that described p r e v i o u ~ l y . ~The ~ electrochemical treatment was applied in phosphate buffer (pH 7.4) and consisted of a triangular wave (70 Hz, 0 to +3.0 V vs. SSCE reference) for 20 s, followed by a constant potential of +1.5 V for 20 s. The current-potential curves for the triangular wave treatment were monitored with an oscilloscope. The electrode was then potentiostated at 0.0 V vs. SSCE for a period of 30-60 min in buffer solution. The latter equilibration period is necessary for a more stable response. Simulation and Analysis Methods. Cyclic voltammetry was used to estimate the active area of the electrode. For the case of cylindrical diffusion a standard curve of log (p), the cylindrical factor, vs. log (*), the dimensionless peak current, has been generated by numerical ~imulation.~'In this work we have used the following definitions
0=~[RTD/~FU~]'/~ and
9' = i,/nFCDl where i, is the peak current of the voltammogram, D is the diffusion coefficient, C is the bulk concentration, u is the scan rate, r and 1 are the radius and length of the cylinder, respectively, and the other terms have the usual electrochemical significance. We have recomputed the working curve accordingly. Thus, the abscissa of the standard curve is a function of r and the ordinate is a function of I, where log
( p ) = log ( ~ [ R T D / ~ F U ]-~log / ~ () r )
and log (*') = log (i,/nFCD) - log ( I ) Values of log ( r ) and log ( I ) were obtained by overlaying an experimental plot of i,/nFCD vs. 2[RTD/nFv]'12 onto the standard curve. The current at a hemicylinder is half of that of a cylinder of equal radius and length. Application of the equations for a hemicylinder to model a band geometry is s t r a i g h t f o r ~ a r dwith ~~ w = w , where w is the equivalent width of the band. Sites of a hemicylinder or band geometry on the electrode surface were assumed as individual elements of finite length in the absence of diffusional overlap; thus, the length determined in the analysis represents the summation of contributions from individual bands. The geometric parameters obtained from these analyses were used to calculate chronoamperometric current transients and simulate cyclic voltammograms for the respective diffusional conditions for comparison to the data. Simulation of voltammograms at microcylinder electrodes was performed on an IBM PC. The simulation program for resolution of the diffusion equation was based on a finite-difference technique with an expanding grid using the hopscotch a l g ~ r i t h m . ~Simulations ~.~~ were
4614
The Journal of Physical Chemistry, Vol. 90, No. 19, 1986
Kovach et al.
performed at calculation intervals necessary to assure less than 0.1% relative error. Chronoamperograms were evaluated with equations for the current response for diffusion to cylindrical or hemicylinder (band) electrode^.^^,^^ Experimental data were
AA
background corrected for residual current before analysis. Diffusion coefficients for the catechols and ascorbate in neutral phosphate media were obtained from the literature,35whereas those ~ - X 10” cm2 for Fe(CN),3- (7.2 X IO” cmz s-l), M O ( C N ) ~(6.4 s-l), Ru(NH,),~+(5.5 X cm2 s-I), and C O ( N H ~ ) , ~(5.9 + X cm2 s-l) were determined from the limiting current at microdisk electrodes under steady-state condition^.^^ The diffusion coefficient for Fe(CN),3- (7.7 X IO” cm2 s-’) in neutral phosphate with 1 M KCI was obtained in this manner also.
i
...... ....,
(35) Gerhardt, G.; Adams. R. N. Anal. Chem. 1982, 5 4 , 2618-2620. (36) Howell. J. 0.;Wightman, R. M. Anal. Chem. 1984, 56, 524-529. (37) Falat, L.; Cheng, H.-Y. Anal. Chem. 1982, 5 4 , 2108-2111. (38) Falat, L.; Cheng, H.-Y. J. Electroanal. Chem. 1983, 157, 393-397. (39) Ono, S.; Takagi, M.: Wasa. T. Bull. Chem. SOC.Jpn. 1958, 31, 356-364. (40) Perone, S . P.; Kretlow, W . J. Anal. Chem. 1966, 38, 1761-1763. (41) Wehmeyer, K. R.; Wightman, R . M. Anal. Chem. 1985. 57. 1989-1993
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Results Properties of Electrochemically Treated Microcylinder Electrodes. Gonon et al. have shown that activation of carbon-fiber microcylinder electrodes, as assessed by the decrease in overpotential for the oxidation of ascorbate, is optimal at an anodic potential limit of +3.0 V.25 At potential limits greater than +3.6 V the electrode is passivated; potential limits less than +2.7 V do not result in activation for ascorbate. We have replicated these results and find that the maximum current density during treatment of the fiber is 2-3 A cm-* at +3.0 V. Electrochemical oxidation of the carbon fiber occurs at high current densities which are possible because of the small surface area. Gonon et al. also reported that the activation procedure was independent of media. In contrast, Cheng reported that the presence of chloride in solution is necessary for activation of 200-pm radius graphite-epoxy disk^.^',^^ This inconsistency may be a result of iR drop at the larger electrode since our results indicate that activation of carbon fibers is equivalent with or without chloride in the media. Voltammetry at pH 3.0 indicates that the background response of the electrode is unstable with potential cycling; however, a stable background is observed at pH 7.4. The capacitance per unit geometric area of the carbon fiber surface was estimated from the voltammetric current at +0.200 V in pH 7.4 buffer. The capacitance of untreated electrodes is 16 f 4 p F c m 2 and of electrochemically treated electrodes is 86 & 6 pF ( n = 8). The increase in capacitance correlates with the increased surface area of the treated carbon-fiber electrode observed with electron microscopy. A rough, striated surface is observed, and cracks are visible in the striations which run parallel to the longitudinal axis of the fiber. The width of these cracks is in the range of 0.1-1 .O pm. A large amount of surface charging is present during electron microscopy despite sputter-coating the samples with gold. Voltammerry of Ascorbic Acid. At electrochemically treated electrodes, the half-wave potential for the oxidation of ascorbate is shifted to more negative potentials by more than 400 mV. The position of the oxidation wave is similar to that observed at mercury electrode^.^^-^^ A reverse wave is not observed because the product is rapidly hydrated. However, the voltammogram is less peak shaped than predicted at a cylindrical electrode of 5-pm radius and is of lower current amplitude (Figure 2 ) . A plot of potential vs. log ((i - il)/i) gives a slope of 42 mV, indicative of a quasi-reversible wave for a two-electron process. No evidence of adsorbed ascorbate is observed when the electrode is rinsed and transferred to buffer solution. Chronoamperograms of AA were obtained with a 1.O-s step duration (Figure 3). and the results were compared to those prediced for diffusion-controlled behavior at a cylinder. The chronoamperometric current is lower than predicted for the
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E ( V vs SSCE) Figure 2. Voltammetry of catechols and ascorbate at treated carbon-fiber microcylinder electrode (pH 7.4): (-), experimental data; simulated voltammogram at cylindrical electrode for revesible system. Conditions: n = 2, D values are given in text, C = 0.20 mM DA, 4-MC, and DOPAC, 1.0 m M AA, geometric area = 1.35 X cm2, radius = 5 X cm, G = 0.100 V SKI (e-),
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Figure 3. Chronoamperometry of DA, DOPAC, and AA at treated carbon-fiber microcylinder electrode (pH 7.4): (-) experimental data for (a) DA, (b) DOPAC, and (c) AA; (0, d) calculated current transient from eq 4 (ref 29). Conditions: n = 2, D = 6.0 X IOT6 cm2 s-’, C = 0.2 mM, geometric area = 1.35 X cm2, radius = 5 X cm, initial potential = -0.200 V, step potential = +0.200 V, step time = 1.00 s.
geometric area of the electrode, particularly at short times. Voltammetry of Catechols. Voltammograms of DA, DOPAC, and 4-MC obtained at electrochemically treated carbon-fiber microcylinder electrodes also show increased electrochemical reversibility relative to untreated microcylinder electrodes. At intermediate scan rates (100 mV s-l), peak-shaped voltammograms are obtained (Figure 2), but the peak curents are less than predicted for diffusion-controlled behavior for DOPAC and 4-MC, whereas that for DA is greater. The voltammetric waves of
The Journal of Physical Chemistry, Vol. 90, No. 19, I986 4615
Carbon-Fiber Microcylinder Electrodes
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tw Figure 5. Chronoamperometry of transition-metal species at treated carbon-fiber microcylinder electrode (pH 7.4): (-) experimental data for (a) Ru(NH3),"+ (step from +0.200 to -0.500 V vs. SSCE), (b) Co(NH3),"+(step from +0.200 to -0.500 V vs. SSCE), (c) Mo(CN),4(oxidation,step from +0.300 to +0.700 V vs. SSCE); (0,d) calculated
current transient from eq 4 (ref 29) for cylindrical geometry. Chronoamperogram was obtained at electrode after voltammetry. Conditions: n = 1, D = 5.9 X IO" cm2 s-I, C = 1.0 mM, geometric area = 1.45 X lo4 cm2. I
I
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Figure 4. Voltammetry of transition-metal complexes at treated car-
bon-fiber microcylinder electrode (pH 7.4): (-), experimental data; (-), simulated voltammogram at cylindrical electrode for reversible system. Conditions: n = 1, D values are given in text, C = 1.0 mM, geometric area = 1.45 x IO4 cm2,u = 0.100 V s-I. DOPAC and 4-MC may have a slight adsorption component because, after transfer to buffer solution, small voltammetric waves for these species are observed. In contrast, dopamine shows the strongest effect of adsorption. More than 5 min must elapse before the wave obtained in buffer solution decreases to less than half the magnitude obtained from the solution with dopamine. Chronoamperograms for DOPAC at a modified electrode were of less current than predicted and similar in amplitude to those observed for ascorbate (Figure 3). The slower decay of the current transient for DOPAC can be attributed to the slight amount of adsorption observed for this catechol. In contrast, the currents observed for chronoamperograms of DA were larger than predicted for diffusion control. When plotted as charge vs. the resultant chronocoulograms of DA at a modified electrode do not approach diffusion control for a step time of 1.O s. The excess charge over that expected for diffusion control indicates a surface coverage of DA of greater than 1.6 X mol cm-2 (based on geometric area, n = 2). Voltammetry of Anionic Transition-Metal Complexes. Voltammograms of highly charged anions Fe(CN)63- and Mo(CN)8e at electrochemically treated carbon-fiber microcylinder electrodes show sigmoidal voltammograms at slow to intermediate scan rates (Figure 4). The magnitude of the limiting current obtained from the voltammograms a t p H 7.4 is less than would be predicted for diffusion control to the geometric area of the electrode. Analysis of the rising portion of the voltammetric wave (20 mV s-l) indicates reversible electrochemical kinetics (slope 59 mV) for Fe(CN),3-. Voltammetry at scan rates from 0.020 to 100 V s-I indicate a transition from sigmoidal to peak-shaped voltammograms. Adsorption of these species is not evident in voltammograms obtained after transfer to buffer solution. Chronoamperometry of Fe(CN)63- and MO(CN)~"at treated microcylinder electrodes gives currents which are less than predicted for diffusion control a t both low and high concentrations of electroactive species. Voltammetry of Cationic Transition-Metal Complexes. Voltammograms of the reduction of the highly charged cations
Ru(NH,):+ and Co(NH,):+ exhibit significant differences from voltammograms of the anions examined at electrochemically treated carbon-fiber microcylinder electrodes. Voltammetry of these cations at slow to intermediate scan rates shows distorted peak-shaped voltammograms relative to those predicted by simulation (Figure 4). The cathodic peak current is much greater than predicted for these species. The redox couple CO(NH3)63+/2+ is chemically irreversible on the voltammetric time scale. Therefore, no reverse wave is observed.42 The Ru(NH,):+/~+ redox couple is substitutionally inert on the voltammetric time scale and should exhibit a reverse wave; however, only a very small reverse wave is observed on the initial scan. The amplitude of this wave increases with each scan until it is approximately equal in magnitude to the cathodic wave. An oxidation wave for Ru(NH3)63+is observed at +0.8 V at treated carbon-fiber microcylinders which is not observed at untreated fibers. This result is in accord with that obtained by Meyer and co-workers at treated glassy carbon.43 Adsorption of the hexaammines at the treated electrodes is dramatically evident in voltammetric scans in buffer solution after exposure to these compounds. Voltammetric waves due to Ru(NH3):+ are observed for more than 15 min after rinse and transfer to buffer solution. Chronoamperometry of Co(NH3)6)+and Ru(NH,):+ at treated microcylinder electrodes yields currents which are greater than predicted (Figure 5). Chronocoulometric curves do not approach diffusion control for Ru(NH&'+ and indicate that the surface mol concentration of adsorbed species is greater than 4.6 X cm-2 (based on the geometric area, n = 1). Analysis of the Active Area of the Treated Electrode. Observation of current less than that predicted for the voltammetry and chronoamperometry of nonadsorbing species indicates that the active electrode area is less than the geometric area after electrochemical treatment. Ferricyanide was chosen to evaluate the active area of the electrode since it does not show adsorptive behavior at this electrode and its redox characteristics are well documented. Cyclic voltammetry at fast scan rates and chronoamperometry at short step times were used to examine these characteristics on a time scale where contribution from linear (42) Maki, N.; Tanaka, N. In Encyclopedia of the Electrochemistry of the Elements; Bard, A. J., Lund, H., Eds.; Marcel Dekker: New York, 1975; Vol. 111, Chapter 2, pp 154-158. (43) Cabaniss, G . E.; Diamantis, A. A,; Murphy Jr., W. R.; Linton, R. W.; Meyer, T. J. J . Am. Chem. SOC.1985, 107, 1845-1853. (44) Dayton, M. A.; Brown,J. C; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1980,52, 946-950.
4616 The Journal of Physical Chemistry, Vol. 90, No. 19, 1986
Kovach et al.
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Figure 6. Noramalized peak current vs. scan rate for reduction of Fe(CN)?- at treated carbon-fiber microcylinder electrode: (0) experimental peak current; (-), simulated peak current by method of Amatore et aL3' for hemicylinder, where r = 0.7 X IO4 cm and I = 0.0400 cm; (- - -), simulated peak current by method of Amatore et aL3' for a cylinder of the original geometric dimensions, where r = 5.0 X IO4 cm and I = 0.0350 cm. Conditions: n = 1, D = 7.7 X IO" cm2s-', C = 10.08 mM in phosphate buffer, pH 7.4, with 1 M KCI. 50 V d
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Figure 8. Chronoamperometry of Fe(CN)?- at treated carbon-fiber microcylinder electrode. (a) (-), experimental data; (0) calculated current transient with eq 4 (ref 29) for hemicylinder where r = 0.7 X lo4 cm, A = 8.80 X 10" cm2. (b) (-), experimental data; (0)calculated current transient with eq 4 (ref 29) for hemicylinder, where r = 0.7 X cm, A = 8.80 X 10" cm2;(- - -), calculated current transient with eq 4 (ref 29) for cylinder of original geometric dimensions, where r = 5.0 X lo4 cm, A = 1.10 X lod cm2. Conditions: n = 1, D = 7.7 X 10" cm2 s-I, C = 10.08 mM in phosphate buffer, pH 7.4, with 1 M KCI, initial potential = +0.500 V, step potential = 0.000 V, step time = 0.250 S.
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Figure 7. Voltammetry of Fe(CN):-
at treated electrode: (-), experimental data; (0)simulated voltammogram for hemicylinder electrode. Conditions: n = 1, D = 7.7 X 10" cmz s-l, C = 10.08 mM in phosphate buffer, pH 7.4, with 1 M KCI, electrochemically determined area = 8.80 X 10" cm2,where r = 0.7 X lo4 cm.
diffusion should be significant for a cylinder of 5-pm radius. The results of the analysis of one electrode are presented in detail below. Similar results were found at two other electrodes analyzed in this manner, and this type of behavior was observed qualitatively at more than 100 electrodes. The case of cylindrical diffusion to a cylinder or band was considered because of the gross geometry of the electrode and the striations observed in the electron micrographs of the surface. The measured voltammetric peak currents were fit to the standard curve for cylindrical diffusion (Figure 6). A fit to the geometric radius ( 5 Mm) was not observed. The best fit indicated an effective radius of 0.7 pm and an area of 8.80 X 10" cm2. Voltammograms of Fe(CN)," simulated with these parameters and a heterogeneous
rate constant k o = 0.2 cm s-l (approximately that found at activated glassy carbon, ref 13) agree very well with data obtained at 0.10(t100 V s-l (Figure 7). Similarly, the chronoamperometric transient calculated from these parameters agrees well with the data (Figure 8). To evaluate whether the electrochemistry is occurring at microscopic sites with the geometry of a disk, the chronoamperometric data measured at short times were evaluated vs. t'12. The ratio of the slope to the intercept gives a term which is approximately equal to the radius of such an isolated disk.44 However, the number of disks required by this model exceeds the physical dimensions of the electrode. The active area of the electrode from the cylindrical analysis is approximately 7.3% of the original geometric area. Discussion The voltammetric properties of carbon-fiber microcylinder electrodes are drastically altered by electrochemical treatment. As with other methods for the activation of carbon, the apparent electron-transfer rates for many compounds are accelerated. However, several distinct features are immediately apparent from the voltammetric curves. The peak and limiting currents observed for anions such as ascorbate, DOPAC, ferricyanide, and octacyanomolybdate(1V) are less than predicted for the geometric area. In contrast, significantly larger currents are observed for the electroactive cations dopamine, ruthenium(II1) hexaammine, and
Carbon-Fiber Microcylinder Electrodes cobalt(II1) hexaammine. Chronocoulometric data, as well as the data after transfer to buffer solutions, indicate that the cationic species are adsorbed. These features are a direct result of the electrochemical treatmentZSand can be correlated with the known physical and chemical properties of oxidized carbon fiber^.^^^^^ Surface properties of carbon fibers have been studied extensively because of their commercial importance in composite materials.* Many of these studies have compared oxidized and untreated surfaces of these fibers, and a large increase in the amount of covalently bound oxygen is found by XPS.26v47Electrochemical oxidation of carbon fibers at 3.0 V vs. S C E in neutral aqueous electrolyte can result in as much as 50% of the surface carbon bound to oxygen present as singly and doubly bonded oxygen, as In addition to the surface well as acidic or ester f~nctionalities.“~~~ oxide skin, cracking of the surface is also observed,50 in accord with our observations by electron microscopy. Formation of cracks is accompanied by the evolution of gases (C02 at acidic pH and O2at basic pH) during ~xidation!~Furthermore, electrochemical oxidation of carbon fibers leads to disruption of the graphitic lattice and decreased conductance. The surface graphitic oxides are known to have cation-exchange capacity.51J2 Intercalation of ions can occur during electrochemical oxidation, but this is unlikely in phosphate b ~ f f e r s . ’ ~ , ~ ~ The differential capacitance of the electrochemically treated carbon fiber is indicative of the substantial changes in the surface properties that result from this treatment. Electrochemical treatment increases the capacitance fivefold based on the geometric area. The capacitance of the treated surface is even greater than that found at edge-oriented graphite.I9 The “excess” capacitance correlates with the increased microscopic surface area of the fiber caused by surface cracking during o ~ i d a t i o n . ~ ~ - ~ * , ~ ~ Although the capacitance data suggest that the total microscopic area of the electrode is increased, the observed diffusion-controlled electrochemical behavior of anionic species, such as Fe(CN),3-, which do not show adsorptive behavior at this electrode, demonstrates a much smaller area is available for interfacial electron (45) Theodoridou, E.; Resenhard, J. 0.;Fritz, H. P. J. Electroanal. Chem. 1981, 122,67-71. (46) Besenhard, J. 0.;Fritz, H. P. Angew. Chem., Int. Ed. Engl. 1983,22, 950-975. (47) Takahapi. T.: Ishitani. A. Carbon 1984. 22. 43-46. (48j Kozlow;G, C.; Sherwdod, P. M. A. J . Chem. SOL.,Faraday Trans. I 1984,80, 2099-2107. (49) Kozlowski, C.; S h e r w d , P. M. A. J . Chem. Soc., Faraday Trans. 1 1985,81, 2745-2756. (50) S h e r w d , P. M. A. Chem. Soc. Rev. 1985, 14, 1-44. (51) Mattson, J. S.; Mark, Jr., H. B. Activated Carbon: Surface Chemistry and Adsorption from Solution; Marcel Dekker: New York, 1971. (52) Lowde, D. R.; Williams, J. 0.;A t t w d , P. A.; Bird, R. J.; McNicol, B. D.;Short, R. T. J . Chem. Soc., Faraday Trans. I 1979, 75,2312-2324. (53) Randin, J.-P.;Yeager, E. J . Electroanal. Chem. 1975, 58,313-322.
The Journal of Physical Chemistry, Vol. 90, No. 19, I986 4617
transfer. Chronoamperometric and cyclic voltammetric data fit the model for diffusion to a hemicylinder (or band of equivalent dimensions), but the effective radius or width of the band determined by this analysis is significantly less than the original radius of the carbon fiber. It is difficult to determine whether the cracks in the fiber surface represent the active bands described by the analysis, but the physical dimensions of these cracks are similar to those calculated from the model. Diffusional interaction between active sites may occur to a small degree; however, complete diffusional overlap of sites is not observed since a current response proportional to the geometric area of the original electrode is never attained. Even though the active area of the electrode is small, a large rate of electrolysis is observed because the individual sites are subjected to an enhanced rate of mass transport. The reduction in the electrochemically active area may arise from formation of insulating surface oxides during electrochemical treatment. The surface excesses of adsorbed hexaammines and dopamine greatly exceed that for monolayer coverage based on the original geometric area. While the total surface area maybe increased following treatment by a factor of 5-10, this still does not account for the surface excesses observed. The formation of a layer of graphitic oxide with cation-exchange capabilities would explain this data. Surface diffusion of adsorbed species is suggested because of the slow return to diffusion-controlled behavior in chronoamperometric step experiments. Additional evidence is provided by the persistence of voltammetric waves in buffer solution after rinse and transfer from hexaammine or catechol solution. Similar adsorption of DA has been reported recently at carbon fiber electrodes which had been electrolyzed at +2.5 V vs. SCE.54 Electrochemical oxidation of carbon-fiber microcylinder electrodes yields an electrode with voltammetric properties that are unique. A combination of structural features contributes to the observed electrochemistry. Generation of surface oxides appears to block a significant portion of the electrode and provides for the adsortion of large surface excesses. Cracks and pits in the fiber contribute also to structural deformation. Despite these features, quasi-reversible or reversible electrochemical kinetics are observed for all species examined. Furthermore, the deformation of the surface appears to be an advantage when making measurements in environments such as the mammalian brain. Stability of the electrode response over long periods suggests that activated sites are protected from passivation on the oxidized carbon fiber. Acknowledgment. This research was supported by the National Institutes of Health (R01 NS15841). (54) Sujaritvanichpong, S.; Aoki, K.; Tokuda, K.; Matsuda, H . J . Electroanal. Chem. 1986, 198, 195-203.
