Laser Activation of Microdisk Electrodes Examined by Fast-Scan Rate

Articles Anal. Chem. 1994,66, 3864-3872

Laser Activation of Microdisk Electrodes Examined Fast-Scan Rate Voltammetry and Digital Simulation Timothy G. Streint and Andrew G. Ewing' Penn State University, 152 Davey Laboratory, University Park, Pennsylvania 16802

Fast-scan rate voltammetry in conjunction with background subtraction techniques has been used to examine the effect of laser activation at carbon fiber microdisk electrodes. The voltammetry observed after laser treatment shows marked changes in electron transfer kinetics and in voltammetric symmetry. These voltammetric parameters are estimated by matching observed voltammetric behavior with digitally simulated voltammograms. The composition and pH of the supporting electrolyte were found to play large roles in the observed voltammetry of catechols after laser activation of the electrode. Voltammetric symmetry of oxidation vs reduction waves is described by the apparent charge transfer coefficient, 8. The asymmetry of fast-scan cyclic voltammograms of dopamine, 4-methylcatechol, and 3,4-dihydroxyphenylacetic acid in a buffer system containing 0.05 M citrate and 0.05 M phosphate is presented. In this buffer system, /3 is approximately 0.5 at pH 2.0 and significantly less than 0.5 at pH 6.0 and 7.0. Interestingly, 0 for dopamine electron transfer at pH 8.0 is above 0.5. Possible explanations for the observed voltammetric asymmetry are presented in terms of electrostatic interactionsand complexationof analyte moleculeswith anionic solution species. In the past decade, the use of carbon electrodes has gained popularity for analytical determinations of electroactive species in complex biological media.'" Although carbon electrodes seem compatible with biological systems, one problem that has been encountered with carbon is that the electrodes often foul, causing decreased electrochemical responses and/or a decrease in the charge transfer kinetics. This, of course, makes accurate analytical measurements difficult to obtain. Consequently, the development of electrode activation schemes has received a significant amount of attention. A large number of electrode activation procedures have been reported for use with carbon-based electrode^,"^^ including electrochemical t r e a t m e n t ~ , ~ - ~ vacuum > l ~ - l ~ heat treatments,*v9the use of radio Present address: Department of Chemistry, Bucknell University, Lewisburg, PA 17837. (1) Wightman, R. M. Science 1988, 240, 415420. (2) Wightman, R. M. Anal. Chem. 1981,53, ll25A-1134A. (3) Adams, R. N. Anal. Chem. 1976, 48, 1126A-1138A. (4) Wightman, R. M.; Deakin, M. R.; Kuhr, W. G.; Stutts, K. J. J . Electrochem. Soc. 1984, 131, 1578-1583. (5) Engstrom, R. C. Anal. Chem. 1982, 54, 2310-2314. (6) Engstrom, R. C.; Stresser, V. A. Anal. Chem. 1984, 56, 136-141. (7) Kepley, L. J.; Bard, A. J. Anal. Chem. 1988, 60, 1459-1467. (8) Stutts, K. J.; Kovach, W. G.; Kuhr, W. G.; Wightman, R. M. Anal. Chem. 1983, 55, 1632-1634.

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Analytical Chemistty, Vol. 66, No. 22, November 15, 1994

frequency plasma," and laser irradiation.12Jg-25 The activation procedures are carried out with the goal of obtaining faster kinetics of electron transfer. Traditionally, electrodes of conventional size have been employed, and the measurement of anodic and cathodic peak separation provides some degree of information concerning the charge transfer kinetics following activation. Recent publications have reported laser activation of smaller electrodes and irradiation of localized spots on large carbon electrode^.^^,^^ Microelectrodes exhibit steady-state voltammetric behavior at conventional scan rates. However, the use of very fast potential scan rate voltammetry at the smaller ele~trodes26.~~ provides peak-shaped voltammetry that can be analyzed using classical method^.^^,^^ A peak-shaped voltammetric response at a small electrode is obtained as a consequence of the scan rate exceeding the rate of molecular diffusion to the microelectrode surface. When fast-scan rate ./ methods are used, capacitive charging currents are often large. Consequently, background subtraction techniques are usually performed on voltammetry obtained using fast potential scan rates to discriminate against electrode charging current

contribution^.^^^^^ (9) Deakin, M. R.; Stutts, K. J.; Wightman, R. M. J . Elecrroanal. Chem. 1985,

182, 113-122. (10) Hu,I.-F.; Kanveik, D. H.; Kuwana, T. J . Electrwnal. Chem. 1985, 188, 59-72. (1 1) Evans, J. F.; Kuwana, T.Anal. Chem. 1979, 51, 358-365. (12) Bowling, R. T.; Packard, R. T.; McCreery, R. L. J. Am. Chem. SOC.1989, I l l , 1217-1223. (13) Gcwirth, A. A.; Bard, A. J. J. Phys. Chem. 1988, 92, 5563-5566. (14) Bowling, R. J.; Mccreery, R. L.; Pharr, C. M.; Engstrom, R. C. Anal. Chem. 1989,61, 2763-2766. (15) Gonon, F. G.; Fombarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1981, 53. 1386-1389. (16) Feng, J.-X.; Brazell, M.; Rcnner, K.; Kasscr, R.; Adams, R. N. Anal. Chem. 1987., 59. - , 1863-1867. ---- --(17) Wan& J.; Tuzhi, P.; Villa, V. J. Elecrraanal. Chem. 1987, 234, 119-131. (18) Chien, J. B.; Saraccno, R. A.; Ewing, A. G. Redox Chemistry andInterfacial

Behavior of Biological Molecules; Plenum Press: New York, 1988; pp 417424. (19) Bowling,R.;Packard, R.; McCreery,L. J . Electrochem.Soc. 1988,135,16051606. (20) McCreery, R. L.; Packard, R. T. Anal. Chem. 1989,61, 775A-789A. (21) Poon, M.; McCreery, R. L. Anal. Chem. 1986,58, 2745-2750. (22) Poon, M.; McCreery, R. L. Anal. Chem. 1988, 60, 1725-1730. (23) Poon, M.; McCreery, R. L. Anal. Chem. 1987, 59, 1615-1620. (24) Strein, T. G.; Ewing, A. G. Anal. Chem. 1991, 63, 194-198. (25) Sternitzke, K. D.; McCrcery, R. L. Anal. Chem. 1990, 62, 1339-1344. (26) Wipf, D. 0.; Kristensen, M. R.; Wightman, R. M. Anal. Chem. 1988, 60, 306-310. (27) Wightman, R. M. Science 1988, 240, 415420. (28) Nicholson, R. S.Anal. Chem. 1965, 37, 1351-1355. (29) Nicholson, R. S.;Shah, I. Anal. Chem. 1964, 36, 706-723.

0003-27OOf94/0366-3864$04.50/ 0

0 1994 American Chemical Society

The relative electron transfer rates at carbon, gold, and platinum electrodes have been studied as a function of pH by Deakin et al.9 The electron transfer rates were determined to be pH-dependent at carbon electrodes, but not at metallic electrode surface^.^.^^ The quantitation of charge transfer kinetics at conventional carbon electrodes has been accomplished for the model system Fe(CN)d-I3- lo and for the anodic melanization of cate~holamines.3~Recently, with the aid of digital simulation techniques, fast-scan ratevoltammetry has been used to obtain similar measurements of electron transfer rates at carbon microelectrode^.^^ Laser activation of carbon fiber electrodes has been used to increase the apparent charge transfer rate as determined by waveslope using steady-state~oltammetry.2~ In this paper, a similar activation scheme is characterized by use of fastscan rate (300 V/s) voltammetry. Fast-scan rates provide peak-shaped voltammetry that can be addressed with classical methods of a n a l y ~ i s . ~ The * * ~effects ~ of the laser treatment on the voltammetry have been examined in several different buffer systems over a wide range of pH, using the same buffer systems described earlier.24 In addition, a buffer containing consistent amounts of both phosphate and citrate (0.05 M) has been used in an attempt to determine the mechanism of charge transfer of these catechols after laser treatment of the electrode surface. The voltammetric responses before and after laser activation have been analyzed to obtain information concerning the effects of the laser treatment. First, the separation between the forward and reverse peak potentials of the voltammograms has been used to evaluate the apparent electron transfer kinetics. Second, the dependence of the halfwave potential on solution pH has been determined. Third, the apparent symmetry factor (transfer coefficient), p , has been determined for the electrochemical reactions of three catechols at carbon. The oxidation of catechols is complicated and involves at least four steps with two electron transfers and two proton transfer^.^ We have used simple digital simulations to describe asymetric voltammograms in terms of charge transfer coefficients for the electrochemical reactions of the three catechols.

EXPERIMENTAL SECT1ON Chemicals. Working solutions of dopamine (DA), 4-methylcatechol(4-MC), and 3,4-dihydroxyphenylacetate(DOPAC), all obtained from Sigma Chemical Co., were prepared daily using doubly distilled water. All solutions were purged with nitrogen and maintained under a blanket of nitrogen during analysis to avoid autoxidation of the catechols. Table 1 lists molar concentrations and ionic strengths of the buffer systems used in this study. Throughout this paper, the first set of buffers in Table 1 will be referred to by name (buffer composition) and pH, while the second set of buffers will simply be referred to as “universal buffer” followed by the pH when appropriate. Buffers of varying composition were chosen over a range of pH in which the catechols of interest are reasonably stable with respect to autoxidation. The buffer (30) Howell, J. 0.;Kuhr, W. G.; Ensman, R. E.; Wightman, R. M. J. Electroanal. Chem. 1986, 209, 77-90. (31) Young, T. E.; Babbitt, B. W. J . Org. Chem. 1983,48, 562-566. (32) Wipf, D. 0.;Wehmeyer, K. R.; Wightman, R. M. J. Org. Chem. 1986, 51, 4760-4764.

Table 1. Llst of Buffer Sydems Used In Thk Work. buffer system PH concn (M)

citrate/phosphate citrate/phosphate citrate/phosphate phosphate phosphate phosphate phosphate phthalate

UB UB UB UB UB UB

2.7 4.0 7.4 6.0 7.0 7.4 8.0 4.3 2.0 2.7 4.0 6.0 7.0 8.0

0.0858/0.0284 0.0671/0.769 0.0091/0.181 0.100 0.100 0.100 0.050 0.050 0.050/0.050 0.050/0.050 0.050/0.050 0.050/0.050 0.050/0.050 0.050/0.050

a The top set of buffers is identical to those used in ref 24, and the second set (universal buffer; UB)is a more controlledbuffer, which consists of 50 mM citrate and 50 mM phosphate.

solutions of varying composition (top segment of Table 1) were prepared as described p r e v i o u ~ l y . ~ ~The J ~ universal buffer solutions were prepared by dilution and pH adjustment of stock solution that was 0.1 M NaH2P04, and 0.1 M citric acid at a pH of 2.5. All pH adjustments were accomplished by the addition of 1.O M HCl (for the pH 2.0 solution) or 1.O M NaOH (for all other solutions), and each dilution resulted in a solution that was 0.05 M in both citrate and phosphate. Monobasic sodium phosphate monohydrate and dibasic sodium phosphate heptahydrate were purchased from J. T. Baker, Inc. All other chemicals were obtained from Fisher Scientific. All chemicals were reagent grade and used without further purification. Electrodes. The electrodes used in this work were carbon fiber microdisk electrodes constructed as previously rep ~ r t e d . ~Briefly, ~ , ~ ~an 11-pm carbon fiber (Amoco Performance Products, Inc.) was aspirated into a glass capillary that was pulled down around the fiber using a vertical pipet puller (Harvard Apparatus), sealed with epoxy (Epotek 301), and filled with gallium for electrical contact. Apparatus. Electrochemical experiments were performed using an E1400 potentiostat (Ensman Instrumentation; Bloomington, IN) in the two-electrode mode. A sodiumsaturated calomel counter/reference electrode (SSCE) completed the circuit. Fast potential scan rate voltammetry was accomplished by triggering the E1400 potentiostat to apply a triangular wave (-0.4 to +1.0 V vs SSCE at 300 V/s), and the resulting current response was monitored with a Labmaster interface (Scientific Solutions, Solon, OH) and an IBM personal computer. The software for data acquisition was locally written. The apparatus used for laser treatment of microelectrodes has been described previou~ly.~~ The commercial stock-pulsed nitrogen laser (VSL-337ND) used in this work was obtained from Laser Science Incorporated (Cambridge, MA). Experimental Protocol. Acquisition of Voltammetry. For each combination of analyte and buffer system, two sets of voltammograms were obtained at carbon fiber microelectrodes. (33) Perrin, D. D.; Dempsey, B. Buffers forpHand Metal Ion Control; John Wiley & Sons, Inc.: New York, 1974. (34) Dayton, M. A,; Brown, J. C.; Stutts, K.J.; Wightman, R. M. Anal. Chem. 1980, 52, 946-950.

Analytical Chemistty, Vol. $6,No. 22, November 15, 1994

3865

la lo0

m 60

60

40

0 -20

-4a -60 -0

BId I

-IW -120

-120 -0,4

-0.1

-110

0.2

0.4

0.1

0.1

Potential (V vs SSCE)

-0.4

-0.1

0

0.2

0.4

0.6

0.1


Figure 1. Background-corrected voltammograms for dopamine in citrate/phosphate buffer at pH 7.4 before and after laser treatment. Scan rate: 300 V/s. Key: (0) before laser treatment; (+) after laser treatment.

Flguro 2. Experimental and digitally simulated voltammograms. The experimental data Is for dopamine voltammetry at a laser activated electrode. Key: (-), experimentaldata: (0)digitalslmulatlon.Simulatlon parameters: 9 = 0.01, (? = 0.50.

The first set of voltammetric responses were backgroundcorrected voltammograms obtained in buffer solution to which no analyte had been added. The second set of data were obtained after a standard addition of the analyte was made to the buffer solution, making the analyte concentration 0.1 mM. Each data set consisted of five voltammetric scans, each lasting ca. 9 ms, and a 1000-ms delay between each subsequent scan. For the laser-treated samples, all experiments were carried out at a laser pulse frequency of 20 Hz, and each pulse had a duration of 3 ns. The laser beam was reflected through a prism, focused with a 150-mm focal length lens, and passed through the bottom of the electrochemical cell before striking the electrode. The electrochemical cell and all optics were constructed of fused silica. The laser spot size was approximately 100 pm. The pulsed laser was turned on for the duration of each five scan experiment, assuring that a pulse of the radiation struck the electrode surface within 41 ms of the initiation of the scan. Occasionally, a pulse of laser radiation would strike the electrode during a scan and give rise to a sharp spike in the voltammetric response of the electrode. These situations were not considered in the quantitation of the kinetic and thermodynamic parameters presented in this paper. Background subtraction was facilitated by the useof Lotus 123 commercial spreadsheet software. Simulated Voltammetry. The digital simulation of voltammetry was based on the Feldberg finite difference s i m ~ l a t i o nand , ~ ~the particular computer software employed was originally written by Corrigan and Evans.36 These simulations assume a two-electron transfer that occurs in a single, discrete step and do not consider any complications associated with adsorption onto the electrode surface.

after laser treatment have been background corrected and are plotted at the same current scale. Laser treatment of the carbon fiber surface greatly enhances both the peak currents and the apparent charge transfer rate (decreases the peak separation). The increase in peak current for laser-activated electrodes appears to result from an increase in either the electrode surface area or at least the area active for electron transfer. This is also manifested as an increase in the background current of about the same magnitude following laser treatment. However, following repeated scans at slowscan rates (10 or more scans at 100-500 V/s), the resultant voltammetry is again similar to that obtained prior to laser treatment. Thus, the physical size of the electrode surface does not appear to be changing significantly. In addition, the voltammetry obtained following laser treatment is often asymmetric, indicating changes in the apparent transfer coefficient, 8. Apparently, the asymmetric voltammetry is an indication of differences in the formation of the activated complex, the intermediate point at which electron transfer has a 50% chance of occurring. This paper will address the effects of laser treatment on the kinetics of charge transfer, the thermodynamic halfwave potential, and the symmetry factor, p . Digital Simulation of Cyclic Voltammetry. The voltammogram shown in Figure 1 was obtained after laser treatment of the electrode and has been superimposed on a simulated cyclic voltammogram in Figure 2. The simulation was accomplished using a well-known theory, originally developed by Nicholson and Shain,28929 and put in the form of a digital simulation program. Digital simulation is a powerful method to aid in the interpretation of electrochemical data.32 Here, the simulations are used to accurately determine the halfwave potentials and the apparent transfer coefficients for the electrochemical reactions of several catechols. The key parameters for adjustment when using digital simulations are $,a dimensionless kinetic parameter relating Up to thecharge transfer rate constant, and p, the charge transfer coefficient. The dimensionless parameter $, which has an inverse logarithmic dependence on the peak separation for nonreversible systems, is a function of both the charge transfer rate

RESULTS AND DISCUSSION Effect of Laser Treatment on Fast Scan Rate Voltammetry. Representative voltammetric scans illustrating the general effect of laser treatment on fast-scan rate voltammetry are shown in Figure 1. The voltammograms obtained before and (35) Feldberg, S.In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Decker: New York, 1969; Vol. 3. (36) Corrigan, D. A,; Evans, D.H.J . Electroanal, Chem. 1980, 106, 287-304.

3866 Analytical Chemistry, Vol. 66,No. 22, November 15, 1994

constant, kO, and the scan rate, u. For an initial scan in the anodic direction, J/ varies in accord with the following expre~sion:3~

0A

ai

J, = kQ(Do,/D,d)8 (nnFvDrd/RT)-'I2

0

-0.1

where Do, and Drd are the diffusion coefficients of the oxidized and reduced form, respectively, n is the number of electrons transferred, F is the Faraday constant, u is the potential scan rate, R is the gas constant, and Tis the absolute temperature. In this paper, the assumption was made that Do, = Dred = 6 X 10" cm s-l. Thus, 0falls out, and the equation simplifies to

The transfer coefficient, 8, is a result of the symmetry of the energy barrier between the oxidized and reduced species. The value of /3 and the shape of the energy curve are dependent on the mechanism of formation of the activated complex from which the electron transfer can occur. Asymmetry in the energy profile causes asymmetric cyclicvoltammetry as shown in Figure 3. The voltammograms shown in Figure 3 were the result of digital simulations when /3 was set equal to 0.25, 0.50, and 0.75, respectively. These simulations do not include background or capacitive currents. Only peak and halfwave potentials can be accurately compared between simulated voltammograms, and those obtained in solution as experimental background and capacitive currents vary widely between different electrodes. Peak Separationsafter Laser Treatment. General. Table 2 summarizes the variation of oxidation and reduction peak potentials for cyclic voltammetry in the eight buffer systems previously described, and Table 3 summarizes the average peak separations for the universal citrate/phosphate buffer system. Thevalues of AEpbefore laser treatment ranged from 600 mV to more than 1400 mV (not able to observe) and are not tabulated. However, the AE, values observed following laser treatment are considerably lower, indicating a dramatic increase in the rate of electron transfer. The tabulated results for the values of AE, after laser treatment do not lead to any significant relationship between the charge transfer kinetics and the pH. Apparently, the laser treatment results in a renewed surface, capable of faster electron transfer, as predicted earlier by McCreery and c o - w ~ r k e r s . ~ ~ - ~ ~ In general, the AE, values reported in Table 3 are higher than those in Table 2 . This is most likely attributed to the fact that two distinctly different batches of electrodes were used for these two experiments. In addition, theconcentrations of the buffer constituents are varied in the first set of buffers, but constant in the universal buffer system (see Table 1). Therefore, only comparisons within a given set of buffers are made in this paper. Dopamine. The value of AEp for cyclic voltammetry of dopamine at laser-activated carbon fiber electrodes varies (37) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley and Sons: New York, 1980.

42

a4

14

t

IC

IR

421

"

-700

" -"ma "

" -m " " -'la

100

m

5m

700

Potential (V vs SSCE) Figure 3. Digital simulations of vokammetv with different transfer coefficients. (A) 8 = 0.25, (6) 0 = 0.50, (C) = 0.75. J, = 0.005.

substantially as a function of pH in the citrate/phosphate buffers (Table 2 ) . Unfortunately, the concentration of the two buffer constituents also changes, so the origin of the faster kinetics observed at higher pH can not be attributed to a single parameter. The phosphate buffers show decreasing peak separation with increasing phosphate ionic strength (Table 2 ) . This suggests that the phosphate ion plays at least a partial role in the charge transfer mechanism for dopamine. Table 3 illustrates no significant change in AE,as a function of pH when the citrate and phosphate concentrations are held constant. Thus, it appears that the composition of the buffer and not a pH-dependent electrode surface causes the observed pH-dependent kinetics shown in Table 2 for oxidation and reduction of dopamine at laser-activated carbon electrodes. 4-Methylcatechol. A similar trend of decreasing AE, with increasing pH is not observed for the redox chemistry of 4-MC (Table 2). Thus, the phenomenon observed above for dopamine appears most likely a charge-dependent interaction with phosphate. However, Table 3 shows a marked change in the peak separation for 4-MC between pH 6 and 7 . The third pKa for citrate falls at pH 6.4, so it appears that the Analytical Chemistry, Vol. 66, No. 22, November 15, 1994

3867

Table 2. Average Peak Separation for Oxidation and Reductlon Peaks for Dopamine, 4-Methyicstechoi, and 3,4-Dihydroxyphenylacetate after Laser Actlvatlon of Carbon Fiber Electrodes in Eight Different Buffer Systems. analyte buffer- system N U P (VI PH

DA DA DA DA DA DA DA DA 4-MC 4-MC 4-MC 4-MC 4-MC 4-MC 4-MC 4-MC DOPAC DOPAC DOPAC DOPAC DOPAC DOPAC DOPAC DOPAC

citrate/phosphate citrate/phosphate citrate/phosphate phosphate phosphate phosphate phosphate phthalate citrate/phosphate citrate/phosphate citrate/phosphate phosphate phosphate phosphate phosphate phthalate citrate/phosphate citrate/phosphate citrate/phosphate phosphate phosphate phosphate phosphate phthalate

2.7 4.0 7.4 6.0 7.0 7.4 8.0 4.3 2.7 4.0 7.4 6.0 7.0 7.4 8.0 4.3 2.7 4.0 7.4 6.0 7.0 7.4 8.0 4.3

0.480 f 0.006 0.390 f 0.007 0.232 f 0.004 0.377 f 0.052 0.283 f 0.014 0.223 f 0.009 0.200 f 0.012 0.427 f 0.004 0.467 f 0.007 0.433 f 0.055 0.390 f 0.01 1 0.597 f 0.034 0.547 f 0.082 0.463 f 0.068 0.510 f 0.076 0.380 f 0.060 0.421 f 0.083 0.487 f 0.061 0.360 f 0.071 0.553 f 0.061 0.547 f 0.089 0.466 f 0.084 0.537 f 0.109 0.523 f 0.1 14

3 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 2 3 3 3 3 2

Error values are standard error of the mean. Table 3. Average Peak Separation for Oxidation and Reduction Peaks for Dopamine, 4-Methylcatechol, and 3,4-Dihydroxyphenyiacetate after Laser Activation of Carbon Fiber Electrodes in Universal Buffer. analyte PH U P (V) N

DA DA DA DA DA DA 4-MC 4-MC 4-MC 4-MC 4-MC 4-MC DOPAC DOPAC DOPAC DOPAC DOPAC DOPAC

2.0 2.7 4.0

6.0 7.O 8.0 2.0 2.7 4.0 6.0 7.0 8.0 2.0 2.7 4.0 6.0 7.0 8.0

0.600 f 0.079 0.680 f 0.012 0.647 f 0.037 0.860 f 0.087 0.660 f 0.1 11 0.733 f 0.124 1.187 f 0.177 1.013 f 0.144 1.330 f 0.250 1.050 f 0.010 0.525 f 0.005 0.575 f 0.025 0.880 f 0.107 0.880 f 0.064 0.973 f 0.252 0.993 f 0.210 1.033 f 0.177 1.113 f 0.144

2 3 3 3 3 4 3 3 2 2 2 2 2 3 3 3 3 3

Error values are standard error of the mean.

activated complex for 4-MC electrochemistry is influenced by the charge associated with the citrate ion. The interaction could possibly be explained by hydrogen bonding between the 4-MC and the citrate. 3,4-Dihydroxyphenylacetate.Examination of the charge transfer kinetics for DOPAC should be extremely interesting since the molecule is essentially identical to dopamine at the electrophore but has an oppositely charged side chain. Unfortunately, the AE,, values for DOPAC are extremely inconsistent. The large errors associated with the measurements of AE, preclude the postulation of any dependence on buffer composition and pH. This inconsistency might in itself 3868

Analytical Chemistry, Vol. 66, No. 22, November 15, 1994

I

I

I

I

I

I

I

I

~

I

I

I

BUFFERS

+I

Phosphate -0 Citrate

a -1

1-

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+

. .

----l+-1-4---2--1--3

-0

-1

Phthalate -0

- 1 . 1-

-2 I

I

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I

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ANALYTES DA

4-MC MPAC

.

t1

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cltO-1-

-1-

0

0 -1-

-1-1

-2 1-

1-1--

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I

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1

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1

1

4

5

6

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8

9

I O 1 1 1 1

,

I

PH Figure 4. Illustration of the charges associated with ail species in solutions for analysis as a function of pH. Table 4. Average Halfwave Potential for Oxidation and Reduction of Dopamine, 4-Methylcatechoi, and 3,4-Dihydroxyphenyiacetate after Laser Actlvatlon of Carbon Flber Electrodes in Eight Different Buffer Systems. analyte buffer system pH E112 (V vs SCCE) N


citrate/phosphate citrate/phosphate citratejphosphate phosphate phosphate phosphate phosphate phthalate citrate/phosphate citrate/phosphate citrate/phosphate phosphate phosphate phosphate phosphate phthalate citrate/phosphate citrate/phosphate citrate/phosphate phosphate phosphate phosphate phosphate phthalate

2.7 4.0 7.4 6.0 7.0 7.4 8.0 4.3 2.7 4.0 7.4 6.0 7.0 7.4 8.0 4.3 2.1 4.0 7.4 6.0 7.0 7.4 8.0 4.3

0.377 f 0.004 0.300 f 0.000 0.310 f 0.007 0.143 f 0.022 0.120 f 0.010 0.117 f 0.007 0.095 f 0.004 0.090 f 0.006 0.323 f 0.004 0.253 f 0.009 0.273 f 0.009 0.070 f 0.012 -0.003 f 0.022 -0.003 f 0.014 -0.007 f 0.006 -0.020 f 0.010 0.390 f 0.006 0.297 f 0.004 0.340 f 0.030 0.133 f 0.009 0.040 f 0.01 5 0.043 f 0.013 0.055 f 0.020 0.013 f 0.057

3 2 2 3 3

3 3 3 3 2

3 2


be indicative of an interesting charge transfer mechanism; however, this is not easily evaluated. Halfwave Potentials. Because of the complexity of the various buffer systems and analytes presented in this paper, a plot of the ionic charges associated with the relevant chemical species as a function of pH is given in Figure 4. Tables 4 and 5 summarize the halfwave potentials observed in the eight buffer systems examined earlier and in theuniversal buffers, respectively. Plots of the halfwave potential as a function of pH in the 50 mM universal buffer system are shown in Figure 5. Figure 5A illustrates the dependence of dopamine redox behavior on the pH at laser-treated carbon fiber surfaces. From pH 2 to pH 7, the dependence is linear with a slope of 4 . 0 6 6 and an intercept of 0.55 (r = -0.9999). At pH 8, a positive deviation from the linear behavior is observed. Just above pH 7, phosphate becomes doubly anionic

~~

Table 5. Average HaHwave Potential for Oxldatlon and Reduction of Dopamlne, +Methyicatechol, and 3,4-Dihydroxyphenylacetate after Laser Activation of Carbon Fiber Electrodes in Universal Buffer* anal yte PH E112 (V vs SSCE) N

DA DA DA DA DA DA 4-MC 4-MC 4-MC 4-MC 4-MC 4-MC DOPAC DOPAC DOPAC DOPAC DOPAC DOPAC (1

2.0 2.7 4.0 6.0 7.0 8.0

0.420 f 0.000 0.377 f 0.004 0.293 f 0.004 0.160 f 0.006 0.090 f 0.010 0.077 f 0.003 0.363 f 0.009 0.323 f 0.004 0.260 f 0.007 0.130 f 0.000 0.065 f 0.000

2 3 3 3 3 4

2.0 2.7 4.0 6.0 7.0 8.0

-0.050 f 0.000

3 3 2 2 2 2

2.0 2.7 4.0 6.0 7.0 8.0

0.460 f 0.026 0.397 f 0.009 0.303 f 0.012 0.113 f 0.017 0.047 f 0.009 -0.023 f 0.007

2 3 3 3 3 3

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=

A

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0.35

H

0.25i I

0.15

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0.35

32

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m

4.05


(Figure 4). Unfortunately, the buffering capacity of phosphate does not allow experiments at pH values above 8, and dopamine is not stable at these high pH values. Hence, it is not entirely clear that the observed value of E112 at pH 8 is in fact due to the change in phosphate charge; however, this remains a distinct possibility. The analogous plot for 4-methylcatechol is shown in Figure 5B. The points from pH 2 to pH 8 form a straight line with a slope of -0.066 and an intercept of 0.51 ( r = -0.992). The data for DOPAC are shown in Figure 5C. The points from pH 2 to pH 8 result in a best fit straight line with a slope of -0.087 and an intercept of 0.66 (r = -0.993). It appears that two distinct regions are observed for the E l p of DOPAC. From pH 2 to pH 4 the slope is -0.078 and the intercept is 0.61 ( r = -0.998) and from pH 6 to pH 8 the slope is -0.068 and the intercept is 0.52 ( r = -0.9999). However, the change in slope between pH ranges is not statistically significant relative to the error in the measurements. Interestingly, DOPAC is neutral over the first pH range and anionic over the latter pH range (pKa for DOPAC = 4.5). Hence, a small change in the slope of Ell2 vs pH might result from a change in the analyte charge, but our data are not conclusive. It appears that the thermodynamic halfwave potential is dependent on the charge of the phosphate ion for cationic DA, but not for anionic DOPAC. These data appear to suggest that a complex is formed between the negatively charged phosphate ions and DA in solution prior to electron transfer. Apparent Transfer Coefficient, j3. General. The apparent transfer coefficient was found to be the most interesting parameter when examining the effects of laser treatment on the non-steady-state voltammetry of the catechols. As the oxidation of catechols is complicated, involving multistep electron transfer^,^ the physical significance of the charge transfer coefficient is not totally clear. However, we have used the apparent charge transfer coefficient to describe the asymmetric voltammetry obtained experimentally. Two examples of the voltammetry of catechols obtained after laser treatment of the electrode surface with p values significantly differing from 0.5 are shown in Figure 6. In general, the

I

,

2

,

,

I

,

6

4

8

PH Figure5. Halfwavepotentialplotted as a function of pH in the unlversal 4methylcatechol. cltratelphosphatebuffer system. (A) Dopamine, (9) (C) 3,edihydroxyphenyiacetate. Error bars are standard error of the mean.

equations describing the kinetics of an oxidation and a reduction reaction are k,, = k0eB n f l E - P )

where k,, and kr4 are the rate constants for the oxidation and reduction reactions at potential E , respectively, ko is the standard rate constant, 0 is the transfer coefficient, a = 1 0, n is the number of electrons transferred in the redox reaction, and Eo’ is the formal potential of the redox couple. Thus, change in the transfer coefficient is indicated by a dramatic change in the relative oxidation and reduction rates, affecting the voltammetric wave shape. Changes in the transfer coefficient are often associated with different mechanistic steps in the electron transfer process. The apparent transfer coefficient for the electrochemistry of catechols at laser-activated carbon fibers is highly dependent on the analyte charge, the solution pH, the buffer constituents, and their ionic charges, indicating that all these parameters play an important role in the mechanism of charge transfer. The 0 values observed for the three catechols in the eight Analytical Chemistty, Vol. 66,No. 22, November 75, 1994

3869

Table 6. Average Transfer Codfkknt 8, for Oxidation and Reduction of Dopamine, 4-Methyicatechoi, and 3,4-Dlhydroxyphyiacetate afler Laser Actlvation of Carbon Fiber Electrodes In Ehht Different Buffer Systems'

analyte

buffe; system

PH

B

N


citrate/phosphate citrate/phosphate citrate/phosphate phosphate phosphate phosphate phosphate phthalate citrate/phosphate citrate/phosphate citrate/phosphate phosphate phosphate phosphate phosphate phthalate citrate/phosphate citrate/phosphate citrate/phosphate phosphate phosphate phosphate phosphate phthalate

2.7 4.0 7.4 6.0 7.0 7.4 8.0 4.3 2.7 4.0 7.4 6.0 7.0 7.4 8.0 4.3 2.7 4.0 7.4 6.0 7.0 7.4 8.0 4.3

0.050 f 0.00 0.48 f 0.02 0.45 f 0.00 0.38 f 0.02 0.43 f 0.02 0.57 f 0.03 0.67 f 0.03 0.50 f 0.00 0.52 f 0.02 0.48 f 0.02 0.27 f 0.02 0.32 f 0.02 0.27 f 0.02 0.32 f 0.02 0.33 f 0.02 0.55 f 0.00 0.52 f 0.02 0.50 f 0.00 0.32 f 0.02 0.33 f 0.03 0.34 f 0.02 0.32 f 0.02 0.32 f 0.02 0.68 f 0.02

3 2 2 3 3 3 3 3

-70


-30


Table 7- Average Transfer Coenlcknt, 8, for Oxidation and Reduction of Dopamine, CMethyicatechol, and 3,CDihydroxyphenyiacetate after Laser Activation of Carbon Fiber Electrodes In Universal Buffer'

Figure 6. Subtractedvoltammograms obtained after laser treatment of the electrode which have 0 values significantly differing from 0.50. (A) Dopamine in phosphate buffer at pH 8.0 (0 = 0.70); (e) 4-methylcatechol in citratelphosphate buffer at pH 7.0 (@= 0.30). Scan rate = 300 V l s . 0 values are evaluated by matching digital simulations to actual voltammograms.

buffer systems previously considered are listed in Table 6 , and those for the 50 mM universal buffer series are given in Table 7. Dopamine. The voltammograms shown in Figure 7 illustrate the response at activated carbon electrodes for dopamine oxidation and reduction at pH 2,4,6, and 8. A plot of the average /3 value for dopamine electrochemistry as a function of pH in the 50 mM universal buffer system is shown in Figure 8. There appear to be two distinct shifts in @. The first, occurring between pH 4 and pH 6 , corresponding to the region in which citrate becomes doubly anionic. The second, more dramatic shift is between pH 7 and pH 8, the region in which phosphate becomes doubly anionic. Since dopamine is cationic over the entire range of pH, these shifts in @ appear to be the result of different activated complex mechanisms involving the two anionic buffer constituents. When citrate becomes doubly anionic between pH 4 and pH 6 , the 0 value decreases sharply to ca. 0.35, indicating that the kinetics of the re-reduction are significantly faster than the oxidation kinetics. The second shift in 0 is observed between pH 7 and pH 8, the region in which phosphate becomes doubly anionic. The same trend is seen between pH 7 and pH 8 in Table 6 for buffers containing only phosphate, suggesting 3870

3 3 3 3 3 3 3 3 3 3 2 3 3 3 3 2

Analytical Chemlstty, Vol. 66, No. 22, November 15, 1994

a

analyte

PH

B

N

DA DA DA DA DA DA 4-MC 4-MC 4-MC 4-MC 4-MC 4-MC DOPAC DOPAC DOPAC DOPAC DOPAC DOPAC

2.0 2.7 4.0 6.0 7.0 8.0 2.0 2.7 4.0 6.0 7.0 8.0 2.0 2.7 4.0 6.0 7.0 8.0

0.49 f 0.01 0.45 t 0.01 0.43 f 0.02 0.35 f 0.00 0.36 f 0.01 0.57 f 0.01 0.50 f 0.00 0.52 f 0.02 0.48 f 0.02 0.48 f 0.02 0.40 f 0.00 0.40 f 0.00 0.55 f 0.00 0.52 f 0.02 0.52 k 0.01 0.38 f 0.02 0.35 f 0.00 0.37 f 0.02

2 3 3 3 3 4 3 3 2 2 2 2 2 3 3 3 3 3


that this second shift is phosphate charge dependent. These data are consistent with those discussed earlier in this paper, indicating a strong role for the phosphate ion in the electron transfer reactions of dopamine at carbon electrodes. 4-Methylcatechol. A plot of the average p value for 4-methylcatechol electrochemistry as a function of pH in the 50 mM universal buffer system in shown in Figure 9. The only significant change in the transfer coefficient occurs between pH 4 and pH 7, the range in which citrate becomes doubly and then triply anionic. Apparently, citrate influences the formation of the activated complex for 4-MC at carbon, and the change in the charge of citrate affects 0.

-F

so

t 30-

D ED-

-30""'"""'*

-0s

-oa

-0.1

ai

OJ

0.s

""'

a7

09



Figure 7. Background-subtracted voltammograms for the oxidation and reduction of dopamine at a laseractivated carbon fiber electrode in the universal citrate/phosphate buffer system. (A) pH 2.0; (B) pH 4.0; (C) pH 6.0; (D) pH 8.0. Scan rate 300 V/s.

1

.. 0.56 0.54

::I

0.46

0.4

p

0.S

0.56

x 0.34

P

3

2

6

4

8

PH

PH

Figure 8. Graph of p as a function of pH for dopamine electrochemistry in the universal c1traWphosphate system. Error bars represent the standard error of the mean (SEM).

Flgure Q. Graph of @ as a function of pH for 4methylcatechol electrochemistry In the universal citrate/phosphatesystem. Error bars represent the standard error of the mean (SEM).

3,4-Dihydroxyphenylacetate.A plot of the average /3 value for 3,4-dihydroxyphenylacetateelectrochemistry as a function of pH in the 50 mM universal buffer system in shown in Figure 10. The distinct change in /3 between pH 4 and pH 6 is similar to that observed with the neutral 4-MC. However, the pKa for DOPAC also lies in this region, so both citrate and DOPAC itself are deprotonated above pH 6.0. Above pH 4.0, the p values for DOPAC are lower on average than those obtained for the neutral analyte, suggesting that

electrostatic interactions may also affect p for DOPAC electrochemistry. Charge Transfer Mechanism at Laser-Activated Carbon Electrodes. At laser-treated carbon electrode surfaces, citrate buffers can apparently complex with all of the catechols, resulting in voltammetry that is asymmetric toward reduction (B < 0.5). This effect is pronounced at elevated pH where citrate is more highly anionic. This may or may not be the Analytical Chemistty, Vol. 66, No. 22, November 15, 1994

3071

U 0.51

: I, 0.5

,

,

,

0.38

I

0.36 0.14

2

4

6

8

PH Flgure 10. Graph of /3 as a function of pH for 3,Mihydroxyphenylacetate electrochemistry in the universal ckrate/phosphate system. Error bars represent the standard error of the mean (SEM).

case for DOPAC, which changes charge at around pH 4.5. Otherwise, as the citrate charge becomes more negative (increased pH), the oxidation reactions are more difficult. Since citrate affects the symmetry of both DA and 4-MC voltammetry, the interaction between citrate and the catechols is most likely hydrogen bonding or some form of pH dependent interaction. In addition, there may also be a dual interaction that exists in which the citrate is associated with both the analyte and the electrode surface. Phosphate is an important player in the oxidation of DA at activated carbon electrodes (see Tables 6 and 7). This interaction is not totally understood at present. However, since phosphate appears to interact selectively with DA, this interaction probably takes place at the amine side chain. It should be noted that the effects of laser activation might include an electrode surface having cracks and fissures leading to an increased effective surface area. This activated surface might adsorb catechols more effectively than an inactivated

3072

Analytical Chemistry, Vol. 66, No. 22, November 15, 1994

carbon surface. Certain aspects of the shapes of the voltammograms obtained for catechols under these conditions might reflect this adsorption. This would not explain the asymmetry observed between oxidation and reduction waves for catechol voltammetry and the differences at varied pH values. However, if it is found that adsorption is an important aspect of the voltammetric asymetry observed, the simulations and the subsequent analysis might be in error. Given the above caveat, it is clear from the data presented here that the charge on the analyte, and the buffer type, and pH are important in determining the charge transfer characteristics and the charge transfer coefficient following laser activation of carbon electrodes. Hence, the improved overall charge transfer characteristics following laser activation might, in some cases, involve a significant change in the actual mechanism of electron transfer between the carbon surface and the catechol analytes tested. The exact nature of this change in reaction mechanism is not clear and cannot be detailed from thedata presented here. Thechanges in apparent charge transfer coefficient with different buffer pH values and buffer compositions suggest that buffer ions asymmetrically affect at least part of the multistep oxidation of catechols at laser-activated carbon fiber electrodes.

ACKNOWLEDGMENT This work was supported by the Office of Naval Research and the National Science Foundation. T.G.S. acknowledges support from the American Chemical Society Analytical Division Fellowship sponsored by the Proctor & Gamble Company. A.G.E. is a Camille and Henry Dreyfus Teacher Scholar. The authors wish to thank Dennis Evans, Richard McCreery, and Christie Allred for helpful discussions and for providing software for the simulations presented in this paper. Recehred for review January 10, 1994. Accepted August 10, 1994.' .Abstract published in Aduonce ACS Abstracts, 0ctobe.r 1, 1994.

Laser Activation of Microdisk Electrodes Examined by Fast-Scan Rate

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