Anal. Chem. 1995,67, 557-560
Dissolution of Gold Electrodes in Alkaline Media Containing Cysteine Anna J. TUd& and Dennis C. Johnson* Department of Chemistry, Iowa State University Am-,
Iowa 5001 1-3111
EQCM results are reported for the voltammetric formation and reduction of surface oxide at a gold electrode in 0.10 M KOH containing cysteine. The data demonstrate that each cycle of oxide formation and subsequent oxide reduction results in loss of a small amount of Au from the electrode surface. This result has significance for applications of pulsed electrochemical detection of organic sulfur compounds in alkaline solutions following their separations by liquid chromatography. There is renewed interest in the anodic reactions of various organic sulfur-containing compounds at gold electrodes.’-7 This interest appears to have redeveloped partly as a result of successful applications of multistep potential waveforms for the pulsed electrochemicaldetection (PED) of numerous sulfur compounds following their separation by liquid chromatography (LC)?-14 Whereas these sulfur compounds are oxidized by electrocatalytic mechanisms requiring simultaneous formation of oxide on the gold electrodes (Au AuOH AuO), the final stable surface oxide (AuO) is i ~ ~ e r t . ’Consequently, ~-~~ detection at constant potential is not acceptable for most sulfur compounds. Success
-
-
(1) Fawcett, W. R; Fedurco, M.; KoviiovA, Z.; Borkowska, Z. J. Electroanal. Chem. 1994,368,265-274. (2) Fawcett, W. R; Fedurco, M.; Kovaiova, Z.; Borkowska, Z. J. Electroanal. Chem. 1994, 368,275-280. (3) Samec, Z.; Malyseva, Zh.; Koryta, J.; Pradhi, J.J. Electroanal. Chem. 1975, 65, 573-586. (4) Sugawara, IC;Tanaka, S.; Taga, M. J. Electroanal. Chem. 1991,316,305314. (5) Reynaud, J. A; Malfoy, B.; Canesson, P. J. Electroanal. Chem. 1980, 114, 195-211. (6) Nuzzo, R G.; Zegarski, B. R; Dubois, L. H. J. Am. Chem. SOC.1987, 109, 733-740. (7) Vandeberg, P. J.; Johnson, D. C. J. Electroanal. Chem. 1993, 362, 129139. (8) Polta, T. Z.; Johnson, D. C. J. Electroanal. Chem. 1986, 209, 159-169. (9) Polta, T. Z.;Johnson, D. C.; Luecke, G. RJ. Electroanal. Chem. 1986,209, 171-181. (10) Welch, L. E.; Lacourse, W. R; Mead, D. A, Jr.; Johnson, D. C.; Hu,T. Anal. Chem. 1989, 61, 555-559. (11) Vandeberg, P. J.; Kawagoe, J. L.; Johnson, D. C. Anal. Chim. Acta 1992, 260, 1-11. (12) Johnson, D. C.; Dobberpuhl, D.; Roberts, R;Vandeberg, P. J.J. Chromatogr. 1993, 640, 79-96. (13) Koprowski, L.; Kirchmann, E.; Welch, L. E. Electroanalysis 1993, 5, 473482. (14) Johnson, D. C.;Lacourse, W. R In CarbohydrateAnalysis: High Peqb”nce Liquid Chromatography and Capillay Electrophoresis; El Rassi, Z., Ed.; Elsevier Publishers B. V.: Amsterdam, in press; Chapter 10. (15) Burke, L. D.; McCarthy, M. M.; Roche, M. B. C.J. Electroanal. Chem. 1984, 167, 291-296. (16) Bruckenstein, S.; Shay, M. ]. Electroanal. Chem. 1985, 188, 131-136. (17) Burke, L. D.; Cunnane, V. J. J. Electroanal. Chem. 1986, 210, 69-94. (18) Burke, L. D.; O’Leary, W. A J. Appl. Electrochem. 1989, 19, 758-767. (19) Vitt, J. E.; h e w , L. A; Johnson, D. C. Electroanalysis 1990,2, 21-30. (20) Burke, L D.; Lee, B. H. ]. Electroanal. Chem. 1992, 330, 637-661. (21) Roberts, R; Johnson, D. C. Electroanalysis 1992, 4, 741-749. (22) Besenhard, J. 0.; Parsons, R; Reeves, R M. ]. Electroanal. Chem. 1979, 96. 57-72. 0003-2700/95/0367-0557$9.00/0 0 1995 American Chemical Society
in applications of multistep PED wave forms results from periodic reduction of the surface oxide back to clean Au, followed by a positive potential step to a value for oxide formation with concomitant detection of the sulfur compounds. There can be a significant contribution to the total faradaic signal obtained for numerous sulfur compounds at gold electrodes as the result of oxidative desorption of molecules adsorbed on the oxide-free Au surface prior to the detection step in the PED wave forms. Therefore, we were motivated to apply an electrochemical quartz crystal microbalance (EQCM)27-30to voltammetric studies of the adsorption/desorption phenomena for cysteine at gold electrodes in hope of quantitating the extent of adsorption. In a previous EQCM results for cysteine in 0.10 M HClOI were compared with data obtained by using a rotated ring-disk electrode W E ) . It was apparent from those results that EQCM data cannot be used as the sole basis for quantitative evaluation of adsorption/desorption phenomena when significant changes in surface hydration occur concurrently. Here, we report briefly on EQCM results obtained during voltammetric formation and reduction of surface oxide at gold electrodes in 0.10 M NaOH. These data demonstrate that dissolution of Au occurs simultaneously with reduction of the oxide in the presence of cysteine. This result can have significance for prolonged application of PED wave forms to sulfur compounds in alkaline media. EXPERIMENTAL SECTION
Chemicals. All chemicals were analytical grade and were used as received without further purification. Stock solutions of Lqsteine (Aldrich, Milwaukee, WI) were prepared in deionized water and kept refrigerated not longer than 2 weeks. The measurements were performed in 0.10 M KOH (Fisher Scientitic, Fair Lawn, NJ). All solutions were diluted fresh before each measurement. Apparatus. The EQCM was constructed according to a published design,32and methods for cleaning and calibrating the EQCM electrodes have been Change in resonant frequency (4t;Hz) in the EQCM is a function of change in surface mass (Am, g) of the quartz crystal, as indicated by33334 (23) Burke, L. D.; Cunnane, V. J.; Lee, B. H. J. Electrochem. SOC.1992, 139, 399-406. (24) Strbac, S.; Hamelin, A; Adzic, R R J. Electround. Chem. 1993, 362, 4753. (25) Polewska, W.; Vitus,C. M.; Ocko, B. M.; Adzic, R R]. Electroanal. Chem. 1994, 364, 265-269. (26) Burke, L. D.; O’Sullivan,J. F.Electrochim. Acta 1992,37, 585-594. (27) Guilbault, G. G.; Jordan, J. M. CRCCnt. Rev.Ana1. Chem. 1988,19,1-28. (28) Schumacher, R Angew. Chem., Int. Ed. Engl. 1990,29,329-343. (29) Ward, M. D.; Buttry, D. A Science 1 9 9 0 , 2 4 9 , 1000-1007. (30) Buttry, D. A; Ward, M. D. Chem. Rev. 1992,92, 1355-1379. (31) Tiidos, A J.; Vandeberg, P. J.; Johnson, D. C. Anal. Chem. 1995,67,552556. (32) Ostrom, G. S.; Buttry, D. A]. Electroanal. Chem. 1 9 8 8 , 2 5 6 , 411-431.
Analytical Chemistty, Vol. 67,No. 3,Februaty 1, 1995 557
Current (ma)
A 0.6 r
I
-
1
-:::1 0.2
b
, \
a
,
,
,
-0.4 - 2 . 0 -1.5 -1.0 - 0 . 5
,
,
,
0.0
0.5
1.0 Potential
(V)
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential (VI
Figure 1. Current-potential (A) and frequency-potential (B) response curves for a single cyclic scan originating at -1.50 V in 0.10 KOH. Scan rate, 100 mV s-'. Curves: (a, solid line) residual and (b, dashed line) 0.20 mM cysteine.
A f = -CfAm/A
(1)
where Cf(Hz cm2 g-l) is a constant and A (cm2) is the area of the oscillating surface. For our EQCM, Cf= 5.7 x lo7 Hz cmz g-1. Potential control and data acquisition were achieved using an IBM-compatible PC/AT computer, equipped with a DT2801-A interface. RESULTS AND DISCUSSION
Figure 1contains typical current-potential (A) and frequencypotential (B) curves obtained in 0.10 M KOH for the absence (curve a) and presence (curve b) of 0.20 mM cysteine. The main features of the residual voltammetric response (Figure lA,curve a) are (i) anodic formation of surface oxide for E > -0.2 V (positive scan), (ii) onset of O2evolution for E > -0.6 V (positive scan), and (iii) cathodic reduction of surface oxide in the region -0.2 to -0.5 V (negative scan). Additional features observed in the presence of cysteine (Figure lA,curve b) are (i) an anodic peak for oxidation of cysteine simultaneously with formation of surface oxide (positive scan) and (ii) small, pseudoreversible adsorption/desorption peaks in the region between -1.4 and -0.6 V. (33) h e w , L.A.; Gordon, J. S.; Hsiao, Y.-L.; Johnson, D. C.; Buttry, D. k J. Electrochem. SOC.1990, 137, 3071-3078. (34) Gordon, J. S.; Johnson, D. C. J. Electroanul. Chem. 1994, 365, 267-274.
558 Analytical Chemistry, Vol. 67,No. 3, February 1, 1995
The adsorption/desorption peaks between -1.4 and -0.6 V exhibit very little sensitivity to an increase in cysteine concentration above 0.01 mM, in agreement with results for acidic media.31 Our tentative explanation of this observation is that this concentration corresponds approximately to the formation of a complete monolayer of adsorbed cysteine. The large anodic peak at -0.4 V (positive scan) is sensitive to cysteine concentration over a large range, which makes this anodic response suitable for detection of cysteine in PED; however, care must be taken in selection of the detection potential in PED waveforms, because the peak potential is observed to shift slightly to more positive values as concentration is increased. The cysteine oxidation occurs by an irreversible reaction, and a reduction peak for the product is not observed during the negative scan. We conclude, albeit without proof, that the product is the correspondingsulfonic acid derivative of cysteine. The cathodic peak for oxide reduction (negative scan) in the presence of cysteine is nearly identical with that observed for the pure supporting electrolyte. Figure 1B containsf-E curves recorded simultaneously with the i-E curves in Figure 1A. The residualf-E response curve (curve a) shows very little change during the initial portion of the positive scan (-1.5 to ca. -0.4 V). With continued positive scan, a gradual frequency decrease (Le., mass increase) occurs in the region from -0.7 to 0.2 V, corresponding to the slight increase in anodic current in the i-E curve (Figure lA,curve a). This process is believed to correspond to formation of a fractional monolayer of hydrous oxide with an increase in water adsorption, perhaps via a H-bonding mechanism to the hydrous oxide.18-22 The frequency continues to decrease during formation of surface oxide in the region -0.2-0.7 V (positive scan). Scan reversal at 0.7 V results in stabilization of the frequency corresponding to the region of zero current, -0.6-0.2 V (negative scan). Upon reduction of surface oxide in the region from 0.2 to -0.3 V, a frequency increase occurs as a result of the loss of surface oxygen and adsorbed water. The frequency remains nearly constant in the region from -0.2 to -1.5 V (negative scan). For the presence of cysteine (Figure lB, curve b), a rapid decrease in frequency is observed during the initial portion of the positive scan in the region from -1.4 to -0.6 V as a result of the adsorption of cysteine, which is associated with the small anodic peak in the i-E response (Figure lA,curve b). A more gradual decrease in frequency is observed during continued scan through the region from -0.6 to 0.2 V, perhaps as a result of formation of some hydrous oxide with concomitant adsorption of water. Of significance in curve b is the rather abrupt frequency increase (i.e., mass decrease) in the region -0.3-0.5 V (positive scan), corresponding to oxide formation with concomitant oxidative desorption of adsorbed cysteine. The slight frequency decrease at E > 0.5 V is indicative of continued oxide formation, following oxidative desorption of cysteine, with concomitant adsorption of HzO. Following scan reversal at 0.7 V, the frequency remains constant throughout the region from 0.7 to -0.2 V. The frequency increases abruptly in the region from 0.2 to -0.2 V (negative scan) as a result of the reduction of the surface oxide with concomitant loss of adsorbed HzO. Of greatest significance in curve b is the observation that the frequency observed at the termination of the cylic scan does not equal the frequency at the start of the scan. This hysteresis indicates the possibility of dissolution of a small portion of the gold electrode surface.
A
1
C u r re n t ( m A)
Current (mA)
o.6 0.4 -
A
o'6
0.4
-
0.2
-
0.0
-
-0.2
-
END 0.2
-
0.0
-
-0.2
-
-0.4 L -2.0
START
c
-1.5
t
-1.0
I
-0.5
1
,
I
0.0
0.5
1.0
-0.4 1
-2.0
-1.5
-1.0
-0.5
15
B
,
I
0.5
1.0
Potentioi (v)
Potential (V) Freq. ( H z )
0.0
Freq. (Hz)
100
10
90
5
80
B
START c 0
-2.0
-1.5
-1.0
-0.5
0.0
0.5
70 -2.0
1.0
Potential (V) Figure 2. Current-potential (A) and frequency-potential (B) response curves for a single cyclic scan originating from 0.70 V after electrode passivation at 0.70 V (ca. 5 min). Solution, 0.20 mM cysteine in 0.10 M KOH; scan rate, 100 mV s-I. The apparent dissolution of the gold surface was investigated further. Figure 2 contains i-E (A) and f-E (B) curves recorded for 0.20 mM cysteine in 0.10 M KOH during a single cyclic scan, originating at 0.70 V, following anodization of the gold surface at 0.70 V for -5 min. No anodic current is observed in the region from 0.70 to -0.10 V (negative scan), which is evidence that the oxidwovered surface is inert with respect to oxidation of cysteine. Thereafter, during the negative scan, cathodic peaks are observed for reduction of surface oxide (from -0.2 to -0.3 V) and desorption of cysteine (from -0.6 to -1.4 V). Following scan reversal at -1.5 V, weak anodic peaks are observed for cysteine adsorption (from -1.4 to -0.6 V), and a large peak is obtained for oxidation of cysteine (0.2-0.7 V). The frequency response curve F i e 2B) follows a pattern similar to that shown in F i i e 1(curve b) . A large hysteresis is observed between the frequency values at the start and the end of the cyclic scan (4= 6.3 Hz, A m = -3.9 x g). Based on the assumption that the surface states of the gold electrode are equivalent at the start and end of the cyclic scan (Le., approximately one monolayer of AuO), this value of Am corresponds approximately to the loss of 25%of one monolayer of Au atoms, assuming a surface roughness of unity. Figure 3 demonstrates the cumulative effects on f-E response of four consecutive cyclic scans originating at -1.5 V. Clearly, the values of frequency observed at the scan limits are shifted to higher values with each consecutive cyclic scan. This can be explained only on the basis of the loss of a small quantity of Au
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential (V) Figure 3. Current-potential (A) and frequency-potential (8) response curves for four consecutivecyclic scans originating at -1.5 V in 0.10 M KOH containing 0.20 mM cysteine. Scan rate, 100 mV S-1.
within each cyclic scan in the presence of cysteine. No comparable shift was observed for multiple scans in the absence of cysteine. The i-E curves (Figure 3 4 are similar for the four consecutive cyclic scans, indicating identical activity of the electrode surface for cysteine detection within each cycle. The total frequency shift (4f) for each consecutive cycle is -2.5 Hz. g, Le., -10% of one This corresponds to Am % -1.5 x monolayer of Au atoms per cycle, assuming a surface roughness factor of unity. Further investigation was designed to determine the potential region in which loss of Au occurs in the presence of cysteine. Figure 4 contains i-E (A) and f-E (B) curves for five consecutive cyclic scans within the limits -1.5 and -0.3 V for 0.10 M KOH containing 0.10 mM cysteine. Within these scan h i t s , the gold surface remains in a reduced state. The existence of multiple adsorption (positive scan) and desorption (negative scan) peaks for cysteine is readily apparent in Figure 4A. The occurrence of multiple peaks is evidence for differences in binding energies at the various adsorption sites that exist at the roughened Au ~urface.3~ The f - E curves in Figure 4B are identical for the five cyclic scans, and there is no evidence for loss of Au atoms. Figure 5 contains i-E (A) and f-E (B) curves for multiple cyclic scans withim the limits 0.3 and 0.7 V for the same solution used in Figure (35) Walczak, M. M.; Alves, C. A; Lamp, B. D.;Porter, M. D. Lungmuir, submitted.
Analytical Chemistry, Vol. 67, No. 3, February 1, 1995
559
C u r r e n t ( m A) 0'05
A
i
0.00
-
-0.05
-
A
2 3-10 -0.10
Freq. (Hz) lo
.
t
I
0.1 0.2
I
0.3
I
I
,
0.4
0.5
0.6
I
I
0.7 0.8 P o t e n t i a l (V)
B
T
I
-1,6
'
1
1
-1.4
-1.2
-1.0
I
B
I
1
-0.8 -0.6 -0.4 -0.2
response curves for consecutive cyclic scans originating at -1.5 V in 0.10 M KOH containing 0.10 mM cysteine. Scan rate, 100 mV
s-' .
4. The first scan originates with a reduced electrode surface at 0.25 V, and therefore, a large anodic signal is obtained resulting from formation of surface oxide and oxidation of cysteine. Following scan reversal at 0.7 V, the current quickly decays to zero in the manner discussed for Figure lA (curve b). For the negative scan l i t of 0.25 V, the surface oxide is not reduced, and therefore, the currents observed for the remaining scans (210) are very small because the oxide-covered surface (AuO) does not support oxidation of cysteine. Thef-E curves are shown in Figure 5B for cycles 2-6. These curves are identical, except for small frequency fluctuations that correspond to noise in the EQCM. On the basis of these results, we conclude the loss of Au occurs simultaneously with reduction of surface oxide during the negative scan in the region 0.2 to -0.3 V. This conclusion is consistent with the large positive shift in frequency observed in Figure 2B for the preanodized electrode (Af= +6.3 Hz). CONCLUSIONS The loss of submonolayer quantities of gold during reduction of surface oxide is concluded to correspond to a dissolution mechanism resulting from formation of strong coordination bonds between cysteine and AuO formed as an intermediate product in the cathodic reduction of AuO to Au. The dissolution of small amounts of gold electrode surfaces has been reported during reduction of surface oxide in acidid6336 and alkaline37media. (36) Cadle, S. H.; Bruckenstein, S. Anal. Cketn. 1974, 46, 16-20. (37) Thacker, R; Hoare, J. P. Electrockem. Technol. 1964,2, 61-64
560 Analytical Chemistry, Vol. 67,No. 3, February 1, 1995
t
45
-
40
5
-
0.2
Potential (V)
Figure 4. Current-potential (A) and frequency-potential (8)
55 50
2-6
0.3
0.4
0.5
0.6
0.7 0 . 8 Potential
(v)
Figure 5. Current-potential (A) and frequency-potential (B) response curves for consecutive cyclic scans originating at 0.3 V in 0.10 M KOH containing 0.10 mM cysteine. Scan rate, 100 mV s-l.
Based on the average value of Af w 2.5 Hz observed for consecutive cyclic scans in Figure 3, we calculate Am = -1.5 x 10-8 g per voltammetric cycle. The total mass of the vapordeposited gold electrodes used in this study is typically 4.6 x lo-* g. Therefore, we estimate that -30 000 voltammetric cycles are required to achieve total dissolution of the electrode in the presence of cysteine. Whereas there undoubtedly is no adverse effect of the dissolution for traditional voltammetric research at gold electrodes whose surfaces are subjected to mechanical polishing, there can be a sign&ant consequence in the continuous applications of pulsed potential-time wave forms applied for determinations of organic sulfur compounds, especially thiols, separated by liquid chromatography. In these applications, the electrodes often are left unattended for several years. ACKNOWLEWMENT Helpful comments by Marc D. Porter are acknowledged. This research was supported by the National Science Foundation under Contract CHE-9215963. Received for review August 8, 1994. Accepted November 18, 1994.@ AC940784X @Abstractpublished in Advance ACS Abstracts, January 1,1995.
