Anal. Chem. 2001, 73, 514-519
Voltammetric Determination of L-Cysteine at Conductive Diamond Electrodes Nicolae Spa˜taru,† Bulusu V. Sarada, Elena Popa, Donald A. Tryk, and Akira Fujishima*
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656
Boron-doped diamond (BDD) electrodes were used to examine L-cysteine (CySH) oxidation in alkaline media. The results of the voltammetric and polarization measurements showed that at BDD electrodes the overall CySH oxidation reaction is controlled by the initial electrochemical step, i.e., the oxidation of the CyS- electroactive species. The same conclusion was supported by the results of a study of pH effects. Conversely, at glassy carbon (GC) electrodes, the same reaction is controlled by the desorption of the reaction products. These results account for the poor response for CySH determination at GC compared to BDD. It was found that BDD exhibits excellent behavior for CySH determination, clearly outperforming GC. The results demonstrate that measurement of the peak current for CySH oxidation can be used as a basis for simple method for determining CySH in the micromolar concentration range by the use of BDD electrodes. Many electrochemical studies have been devoted to L-cysteine ((R)-2-amino-3-mercaptopropanoic acid), a sulfur-containing R-amino acid, which is one of about 20 amino acids commonly found in natural proteins. L-Cysteine (CySH) has attracted interest in the literature, because this compound (as well as its oxidized form, L-cystine or [R-(R*,R*)]-3,3′-dithiobis(2-aminopropanoic acid)) plays a very important role in living systems. Moreover, the couple L-cystine/L-cysteine is generally used as a model for the role of the disulfide bond and thiol group in proteins in a variety of biological media. Most of the previous studies were performed on mercury electrodes, either with polarography1-3 or by use of hanging Hg drop electrodes.4,5 Platinum and gold electrodes have also been used for the study of CySH oxidation reaction,6-8 but in this case, * Corresponding author: (fax) +81-3-3812-6227; (e-mail)
[email protected]. † On leave from the Institute of Physical Chemistry of the Romanian Academy, 202 Spl. Independentei, 77208 Bucharest, Romania. (1) Kolthoff, I. M.; Barnum, C. J. Am. Chem. Soc. 1940, 62, 3061-3065. (2) Kolthoff, I. M.; Stricks, W.; Tanaka, N. J. Am. Chem. Soc. 1955, 77, 52155218. (3) Christian, G. D.; Knoblock, E. C.; Purdy, W. C. Biochim. Biophys. Acta 1963, 66, 415-419. (4) Stankovich, M. T.; Bard, A. J. J. Electroanal. Chem. 1977, 75, 487-505. (5) Ba˜nica˜, F. G.; Moreira, J. C.; Fogg, A. G. Analyst 1994, 119, 309-318. (6) Davis, D. G.; Bianco, E. J. Electroanal. Chem. 1966, 12, 254-260. (7) Koryta, J.; Pradac, J. J. Electroanal. Chem. 1968, 17, 185-189. (8) Reynaud, J. A.; Maltoy, B.; Canessan, P. J. Electroanal. Chem. 1980, 114, 195-211.
514 Analytical Chemistry, Vol. 73, No. 3, February 1, 2001
surface oxide formation causes complications with analytical applications.9 Bare carbon surfaces show very poor activity for CySH determination,8,10 but promising results were obtained by use of chemical modification of the electrode surface by adsorption of transition metal macrocycles (such as metallophthalocyanines or metalloporphyrins).11,12 Such electrodes have been employed as electrochemical detection elements for thiols following liquid chromatography and capillary electrophoresis separation,13,14 and high sensitivity for CySH determination in the concentration range 1.5 × 10-6-2.0 × 10-4 M was reported.15 Chemically modified carbon paste electrodes16 and bulk-modified carbon electrodes17 have also been used successfully for the quantitative determination of CySH in the concentration ranges 1.0-7.0 mM and 1-12 µM, respectively. Although very promising, the use of chemically modified electrodes as electrochemical detectors for CySH seems to be limited in many cases by the fact that their electrocatalytic activity inevitably decreases with time.18 Conductive diamond represents an electrode material that has attracted great interest, especially in electroanalysis, due to its outstanding electrochemical features: wide potential window in aqueous solutions,19 low background current,20,21 long-term stability of the response,22,23 and low sensitivity to dissolved oxygen.24 In the present paper, we report the results of an investigation of the electrochemical oxidation of L-cysteine at conductive (9) Johll, M. E.; Williams, D. G.; Johnson, D. C. Electroanalysis 1997, 9, 13971402. (10) Eggli, R.; Asper, R. Anal. Chim. Acta 1978, 101, 253-259. (11) Zagal, J.; Herrera, P. Electrochim. Acta 1985, 30, 449-454. (12) Wang, Z.; Pang, D. J. Electroanal. Chem. 1990, 283, 349-358. (13) Qi, X.; Baldwin, R. P.; Li, H.; Guarr, T. F. Electroanalysis 1991, 3, 119124. (14) Qi, X.; Baldwin, R. P. Electroanalysis 1994, 6, 353-360. (15) Shi, G. Y.; Lu, J. X.; Xu, F.; Sun, W. L.; Jin, L. T.; Yamamoto, K.; Tao, S. G.; Jin, J. Y. Anal. Chim. Acta 1999, 391, 307-313. (16) Perez, E. F.; Kubota, L. T.; Tanaka, A. A.; Neto, G. D. Electrochim. Acta 1998, 43, 1665-1673. (17) Filanovsky, B. Anal. Chim. Acta 1999, 394, 91-100. (18) Nalini, B.; Narayanan, S. S. Electroanalysis 1998, 10, 779-783. (19) Strojek, J. W.; Granger, M. C.; Dallas, T.; Holtz, M. W.; Swain, G. M. Anal. Chem. 1996, 68, 2031-2037. (20) Jolley, S.; Koppang, M.; Jackson, T.; Swain, G. M. Anal. Chem. 1997, 69, 4099-4107. (21) Yano, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Electrochem. Soc. 1998, 145, 1870-1875. (22) Popa, E.; Notsu, H.; Miwa, T.; Tryk, D. A.; Fujishima, A. Electrochem. Solid State Lett. 1999, 2, 49-51. (23) Rao, T. N.; Yagi, I.; Miwa, T.; Tryk, D. A.; Fujishima, A. Anal. Chem. 1999, 71, 2506-2511. (24) Yano, T.; Popa, E.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Electrochem. Soc. 1999, 146, 1081-1087. 10.1021/ac000220v CCC: $20.00
© 2001 American Chemical Society Published on Web 01/04/2001
diamond electrodes. This compound is a good model for further studies of the determination of other thiols on conductive diamond electrodes. EXPERIMENTAL SECTION Apparatus. The boron-doped polycrystalline diamond coatings were deposited on Si(100) wafers by means of microwave plasmaassisted chemical vapor deposition. The procedure was previously described in detail.21 The measurements were performed using a Hokuto Denko HA-502 potentiostat, a Hokuto Denko HB-111 function generator, and a Riken Denshi x-y recorder. All of the experiments were carried out at a temperature of 20 °C in a glass cell with an working electrode area of 0.07 cm2 and a platinum counter electrode. As a reference, a saturated calomel electrode (SCE) was used. Boron-doped diamond (BDD) was used as the working electrode, and in some cases, glassy carbon (GC) was also used for comparison. Before each set of measurements with the GC electrode, it was polished with alumina suspension (BAS) on fresh polishing cloth (BAS), first 1.0 µm for ∼2 min, 0.1 µm for ∼5 min, and 0.05 µm for ∼5 min, followed by thorough washing with Milli-Q water. Reagents. L-Cysteine hydrochloride monohydrate (Wako) was used without further purification, and all the other substances were analytical-reagent grade. All solutions were prepared using Milli-Q water (Millipore). Procedures. The CySH solutions were prepared immediately before use and were deaerated with purified nitrogen. This procedure was adopted because CySH can be oxidized to cystine by atmospheric oxygen, especially in basic solutions. Measurements under aerated conditions were also performed. Steady-state polarization measurements were carried out by applying increasingly positive potentials in 10-mV steps and then waiting for 3 min for the current to stabilize. To avoid mass transport limitations, the solution was mixed. In most of the experiments, 0.5 M KHCO3 (pH ∼9.0) was used as the supporting electrolyte. In some experiments, the effect of the pH was examined in a series of standard 0.04 M BrittonRobinson solutions, and the pH was adjusted (within the range 5.5-12.1) by adding appropriate amounts of 0.2 M NaOH solution.25 The ionic strength of the above solutions was kept constant (∼0.36 M) by appropriate additions of NaClO4. The pH was measured with a conventional glass electrode. Preliminary flow injection experiments using BDD electrode as an amperometric sensor were carried out in a 0.1 M phosphate buffer solution (pH 7.0) at an applied potential of 0.82 V vs Ag/ AgCl. The detailed procedure of the flow injection experiments has been reported previously.26 RESULTS AND DISCUSSION Cyclic Voltammetry. Figure 1 shows cyclic voltammograms obtained at a potential sweep rate of 5 mV s-1 for a 1 mM CySH + 0.5 M KHCO3 deaerated solution, for both BDD (Figure 1a) (25) Dobos, D. Electrochemical Data: A Handbook for Electrochemists in Industry and Universities; Akademiai Kiado: Budapest, 1975; p 239. (26) Sarada, B. V.; Rao, T. N.; Tryk, D. A.; Fujishima, A. Anal. Chem. 2000, 72, 1632-1638.
Figure 1. Cyclic voltammograms recorded in 0.5 M KHCO3 containing 1 mM CySH: initial potential, 0.0 V (SCE); sweep rate, 5 mV s-1. (a) BDD electrode; (b) GC electrode. (Dashed lines represent background curret.)
and GC (Figure 1b). It can be seen that, in the case of the BDD, the voltammogram exhibits a well-defined peak (Ep ∼ 0.57 V), whereas that for GC tends to a plateau. A lower peak current was also noticed in the case of GC (for the same electrode area), in agreement with the poor activity for CySH oxidation previously reported for this material.8,10 No cathodic peaks were observed on the reverse scan within the investigated potential range (-1.3 to +1.1 V). This was not surprising, because it was ascertained that CySH oxidation is an electrochemically irreversible process,27 and there are oxidation products in addition to cystine.8 Figure 2 shows voltammograms recorded during consecutive runs (sweep rate, 20 mV s-1) in a 1 mM CySH solution both for BDD (Figure 2a) and for GC (Figure 2b). The solution was mixed between consecutive runs and was allowed to stand for 1 min without mixing before starting each scan. It can be seen that, unlike BDD, CySH oxidation results in a deactivation of the GC electrode, the reaction being suppressed after the first run. This behavior indicates that the GC surface is blocked by reaction products. The variation of the sweep rate for the BDD electrodes showed that peak currents are linearly proportional to the square root of the sweep rate within the range 5-300 mV s-1. Linear regression statistical analysis (y ) ax) yielded a slope of 27.8 µA s V-1 and R2 ) 0.998. The linearity and the zero intercept suggest that the current is limited by semiinfinite linear diffusion of CySH in the interfacial reaction zone and that rate-limiting adsorption steps and specific surface interactions can be neglected. As shown in Figure 3, the peak current is a nonlinear function of the CySH concentration in the range 0.02-1 mM. From the (27) Ralph, T. R.; Hitchman, M. L.; Millington, J. P.; Walsh, F. C. J. Electroanal. Chem. 1994, 375, 1-15.
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Figure 4. Polarization curves in 0.5 M KHCO3 containing 1 mM CySH: (a) BDD electrode; (b) GC electrode.
references therein):
Figure 2. Cyclic voltammograms in 0.5 M KHCO3 containing 1 mM CySH: initial potential, 0.0 V (SCE); sweep rate, 20 mV s-1. (a) BDD electrode (five consecutive sweeps); (b) GC electrode (the numbers indicate the first, second, and third sweeps).
CySH a CyS- + H+
(2)
CyS- f CyS•ads + e-
(3)
2CyS•ads f CySSCy
(4)
Therefore, establishment of the rate-determining step (rds) could help, at least in part, to understand the electrode behavior. In aqueous solutions, the ionization of CySH depends on pH and can be described as follows:28
Figure 3. Variation of the peak current as a function of CySH concentration: electrolyte, 0.5 M KHCO3; sweep rate, 20 mV s-1.
standpoint of CySH determination, the most important concentration range lies below 0.2 mM. Under these circumstances, the peak current is linearly proportional to the CySH concentration. Nevertheless, the shape of the curve in Figure 3 suggests that the response of the BDD electrode is appropriate for quantitative CySH determination even in the millimolar concentration range. Such a wide concentration range of analytically useful response was also reported for CySH determination by chemically modified electrodes.16 Polarization Studies. The electrochemical oxidation of CySH can be written as
and it is widely accepted that, excepting mercury electrodes, this reaction occurs by the following mechanism (see ref 27 and 516
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Taking into account the pK value for reaction 2 (pK2 ) 8.37), it is obvious that under our experimental conditions (KHCO3 0.5 M, pH ∼9) this step does not play an important role, because the equilibrium lies far to the right, and most of the cysteine is already in the CyS- form. In the elucidation of the mechanism from the rds of a multistep reaction, the Tafel slope, b, plays a prominent role. This parameter can be calculated as b ) 2.303RT/(RF) in which R (the chargetransfer coefficient) varies according to the rds. Usually, the relationships used as guidelines for interpreting kinetic parameters are based upon limiting behavior, that is, when the coverage by adsorbed intermediates (θ) is either zero or unity.29 In our case, when the rds is reaction 3, R ) 0.5 and b ) 118 mV decade-1 for both θ f 0 and θ f 1; if reaction 4 is the rds, R ) 2 and b ) 29 mV decade-1 if θ f 0, and b f ∞ if θ f 1. Figure 4 shows the polarization curves for both BDD (Figure 4, curve a) and GC (Figure 4, curve b) in 0.5 M KHCO3 containing 1 mM CySH. In the case of the BDD, two Tafel slopes can be seen: one of ∼38 mV decade-1 in the low-potential range and a second one of ∼130 mV decade-1 in the higher potential range. This could indicate that, in the low-potential range, the desorption of CyS• radicals is the rds (with low surface coverage), whereas (28) Yakobke, H. D.; Jeschkert, H. Amino Acids, Peptides and Proteins: an Introduction; Akademie Verlag: Berlin, 1977; p 25. (29) Tilak, B. V.; Chen, C. P. J. Appl. Electrochem. 1993, 23, 631-640.
Figure 5. Variation of the current at constant potential (0.6 V vs SCE) for a BDD electrode as a function of CySH concentration: electrolyte, 0.5 M KHCO3.
at higher potentials, the electrochemical reaction 3 becomes the rds. GC exhibits a high Tafel slope (∼250 mV decade-1) under the same conditions, suggesting that in this case reaction 4 is the rds, and the surface coverage by intermediates is rather high. Alternatively, it may indicate that there is a potential drop within an adsorbed layer. Such a high value of the Tafel slope supports the lack of activity for CySH oxidation previously reported for GC.30 The polarization measurements also showed that at the BDD electrode the current due to CySH oxidation is strongly enhanced by mixing the solution. For example, at a constant potential of 0.6 V, mixing the solution results in an increase of the current density from 87.3 to 210 µA cm-2 . In contrast, under the same experimental conditions, GC exhibits only a small increase of the current density (from 38.6 to 43 µA cm-2). This is a further indication that at the GC the process is kinetically controlled. Figure 5 shows the variation of the current at constant potential (0.6 V) as a function of CySH concentration for the BDD electrode. A reaction order (r ) d log i/d log C) of ∼0.8 was calculated. Fractional electrochemical reaction orders for CySH at gold electrodes have been reported in the literature and were ascribed to a change in the amino acid adsorption with coverage.31 Although the reasons for the fractional reaction order remain ambiguous, the value of 0.8 also indicates that in the case of the BDD the electrochemical reaction 3 is the rds, because, if reaction 4 is ratedetermining, then r ) 2. The results of cyclic voltammetric and polarization measurements seem to indicate that during the CySH oxidation, unlike BDD, the active sites of the GC surface are blocked by adsorption of reaction products. Elucidating the nature of these products is not an easy task and was beyond the scope of the present work. We shall note, however, that in many cases cysteic acid (CySO3H) was also found as an oxidation product of CySH.6,8,32 The presence of this compound could explain to some extent the different behavior of BDD and GC, because it was previously shown that the functional group SO3H strongly enhances adsorption at GC, whereas this is not the case for BDD.33 Effect of pH. The mechanism assumed for CySH oxidation (reactions 2-4) suggests a pH dependence of the overall process. (30) Tong, J.; Nie, M.; Li, H. J. Electroanal. Chem. 1997, 433, 121-126. (31) Safranov, A. Yu.; Tarasevich, M.; Bogdanovskaya, V. A.; Chernyak, A. S. J. Electroanal. Chem. 1986, 202, 147-167. (32) Zagal, J.; Fierro, C.; Rozas, R. J. Electroanal. Chem. 1981, 119, 403-408. (33) Xu, J.; Chen, Q.; Swain, G. M. Anal. Chem. 1998, 70, 3146-3154.
Figure 6. Variation of the half-peak potential for CySH (1 mM concentration) oxidation as a function of the pH: sweep rate, 10 mV s-1.
Indeed, decreasing pH results in a shift of the CySH oxidation peak toward more positive potentials. Furthermore, at pH values from 6.5 to 5.5, the voltammetric response tends toward a sigmoidal-shaped (plateaulike) curve. This could indicate mass transport control, but in this case, the most likely explanation is that the higher proton concentrations shift the equilibrium in reaction 2 to the left, and the supply of CyS- becomes a limiting factor, i.e., there is a rate-limiting preceding chemical reaction (CE mechanism). Figure 6 shows the variation of the half-peak potential for CySH oxidation at a BDD electrode as a function of pH. It can be seen that, at pH values higher than ∼8.6, the half-peak potential is rather constant, indicating that in alkaline media the process is not pHdependent. In neutral and acidic media, the half-peak potential shifts linearly toward more positive potentials with a slope dEp/2/ dpH of ∼80 mV. At pH values lower than ∼4, CySH oxidation is obscured by oxygen evolution, and reliable data for Ep/2 could not be obtained for a 1 mM CySH concentration. Data from Figure 6 support the conclusion that, in the case of CySH oxidation at BDD electrodes, the ionization of CySH (reaction 2) precedes the electrochemical step, and the electrochemically active species are CyS- ions. Furthermore, the pH value of 8.6 (Figure 6) correlates quite well with the pK value of the dissociation of the proton from the thiol group in CySH (pK2 ) 8.37). It should be noticed that the value of 80 mV for dEp/2/dpH does not correspond strictly to the overall equilibrium:
2CySH a CySSCy + 2H+ + 2e-
(5)
A possible reason for this behavior could be the more positive potential required for CySH oxidation in neutral and acidic media, because in this potential range, it is likely that CySSCy oxidation also takes place.6,7 This reaction will obviously perturb the overall equilibrium. Nevertheless, this is an issue that remains to be addressed. The study of the pH effect suggests that, in the case of CySH oxidation on BDD electrodes, alkaline media are suitable for analytical purposes ensuring well-shaped and pH-independent voltammetric curves. Analytical Performance Characteristics. Calibration data in the micromolar concentration range shown in Table 1 are the results of six, nine, and five measurements, for the concentration ranges 1-10, 10-100, and 100-200 µM, respectively. Accordingly, Analytical Chemistry, Vol. 73, No. 3, February 1, 2001
517
Table 1. Parameters of the Calibration Graph (I ) aCCySH) for CySH Determinationa concn range (µM)
scan rate (mV s-1)
sensitivity (a) (µA/µM)
50 20 20
0.020 0.012 0.013
1-10 10-100 100-200 a
Electrolyte, 0.5 M KHCO3.
Table 2. Reproducibility of the Oxidation Peak Current at Various CySH Concentrations CySH concn (µM)
scan rate (mV s-1)
peak currenta (µA)
RSDb of peak current (%)
7 40 120
50 20 20
0.14 0.49 1.54
3.5 1.4 0.6
a
Average of five runs. b Relative standard deviation.
the peak current is proportional to the concentration of CySH below 200 µM. The limit of detection was 0.9 µM (S/N ) 3), and the sensitivity of the method can be adjusted to some extent by changing the sweep rate. The zero value of the intercept for the calibration plot enables the determination of CySH by the standard addition method. This is an advantage when compared to some methods involving mercury electrodes, in which case the calibration plot crosses the abscissa, presumably owing to the interference of trace heavy metals in the supporting electrolyte.34 The tests for reproducibility also included the oxygen purging step, because CySH can be oxidized by atmospheric oxygen, particularly in basic solutions. The test consisted of running linear sweep voltammetry curves for five aliquots of the same solution containing CySH in a 0.5 M KHCO3 solution. The test was run for three different CySH concentrations, and the results are summarized in Table 2. It appears that the reproducibility is satisfactory for the investigated concentration range. It was also found that the presence of dissolved oxygen is not a major source of variability, although the possibility exists (at least in principle) for the CySH to be chemically oxidized.
The stability for the cysteine response was examined for 4 days without observing any fouling of the diamond electrode. It is also worth noting that, during continuous daily experiments for over one month, negligible changes in the electrochemical performance were observed for the same BDD electrode. The accuracy of the procedure was assessed by analyzing spiked samples by the method of standard addition. The method of multiple standard additions was first applied for a synthetic sample prepared with distilled water. The experiments were performed in air-saturated 0.5 M KHCO3 solution at a sweep rate of 50 mV s-1 within the potential range 0.00-0.95 V. The I(µA) vs ∆C(µM) regression equation (for three additions of 2.5 µM each) was I ) 0.0184 ∆C + 0.186, with R2 ) 0.9934. The calculated concentration (10.1 µM) agrees satisfactorily with the expected one (10.0 µM). The single standard addition method, which minimizes sample manipulation, was also tested by means of an analysis of an airsaturated tap water sample spiked with CySH. The measurements were carried out as above except for the sweep rate, 20 mV s-1. The procedure was repeated with the same sample after adding standard CySH solution (∆C ) 15 µM). The result was 39.3 µM, in fair accord with the expected value (40.0 µM). The performance of BDD as an electrode material for CySH determination was also assessed by flow injection measurements. Preliminary experiments showed a sensitivity of 12 nA µM-1 (within the concentration range 0.1-100 µM), an experimental detection limit of 100 nM, and a theoretical detection limit of 21 nM (S/N ) 3). Table 3 compares the analytical figures of merit of the diamond electrode for CySH determination to those for other electrode materials. For cases in which we are able to make a reliable comparison with published results, it appears that the present detection limits are somewhat lower than those previously reported for flow injection. Detailed results will be published elsewhere. CONCLUSIONS The unique electrochemical features of conductive diamond prompted us to investigate the possibility of using BDD as an electrode material for the analytical determination of CySH. Voltammetric and polarization studies showed that the CySH oxidation reaction takes place at the BDD electrode by a mechanism involving the dissociation of the proton from the thiol
Table 3. Analytical Figures of Merit for Several Electrode Materials for CySH Determination electrode CME (GC)
method
dynamic range (µM)
limit of detection (µM)
sensitivity (nA µM-1)
liquid chromatography with amperometric detection flow injection amperometry
1.5-200
1
0.085-58
10.3
bulk-modified carbon CME (carbon paste) GC tungsten
amperometry chronoamperometry voltammetry (catalytic processes) HPLC with amperometric detection
1-12 1000-7000 50-5000 2-62
0.085 (S/N ) 2) 0.2
0.045
Au/Hg
capillary zone electrophoresis with amperometric detection voltammetry flow injection
0.1-100
0.4 (S/N ) 3) 0.058 (S/N ) 2) 0.9 0.021 (S/N ) 3)
BDD
a
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This work.
Analytical Chemistry, Vol. 73, No. 3, February 1, 2001
1-200 0.1-100
RSD (%)
ref
4.5
15
4.2 (at 5 µM)
35
22.2 0.233
24000 12-20 12
17 16 30 36 4.2 (at 5 µM) 3.5
37 a
group, followed by the electrochemical oxidation of the CySspecies. The study of the effect of pH supported the conclusion that the electroactive species is the CyS- ion. In the case of BDD, the electrochemical oxidation reaction is the rds, whereas at GC the overall CySH oxidation reaction is controlled by the desorption of reaction products. For that reason BDD clearly outperforms GC as an electrode material for CySH determination. Based upon the voltammetric peak for CySH oxidation, a method is proposed for the CySH determination in the micromolar concentration range. The sensitivity of the method is comparable to those involving chemically modified electrodes15-17 or catalytic processes.30 In addition, the use of BDD electrodes results in a simple analytical procedure, because no chemical modification is
required. The inactivation of the electrode with time (which is common when chemically modified electrodes are used) is also avoided due to the long-term stability of the BDD electrode response.22,23 Further experiments are in progress in order to improve the detection limit and to extend the method to other thiolic compounds.
(34) Florence, T. M. J. Electroanal. Chem. 1979, 97, 237-255. (35) Li, H.; Li, T.; Wang, E. Talanta 1995, 42, 885-890. (36) Hidayat, A.; Hibbert, D. B.; Alexander, P. W. J. Chromatogr., B 1997, 693, 139-146. (37) Jin, W.; Wang, Y. J. Chromatogr., A 1997, 769, 307-314.
Received for review November 1, 2000.
ACKNOWLEDGMENT This research was supported by the Japan Society for the Promotion of Science (JSPS) Research for the Future Program (Exploratory Research on Novel Materials and Substances for Next-Generation Industries).
February
18,
2000.
Accepted
AC000220V
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