Langmuir 1994,10,912-919
912
Electrochemistry of Cysteine on Low-Index Single-Crystal Gold Electrodes W. Ronald Fawcett,' Milan Fedurco, Zuzana Kov660v6, and Zofia Borkowska Department of Chemistry, University of California, Davis, California 95616 Received July 26, 1993. In Final Form: December 16, 199P The chemisorption and redox properties of cysteine have been studied at Au(100),A u ( l l l ) , and Au(ll0) single-crystalelectrodes in dilute aqueous perchlorate solution using cyclic voltammetry. The appearance of the current peaks on the current potential curves depends on the the relative position of the point of zero charge. The oxidation of cysteine was found to be a two-step process, with the first step involving one electron and the second three electrons. The effects of gold surface reconstruction were pronounced for the Au(100) electrode. Resolution of the current due to gold oxidation and cysteine was realized using a transfer experiment in which an electrode with chemisorbed cysteine was transferred to and cycled in a cell containing no cysteine.
Experimental Section
Introduction
It is a well-known fact that the extent of organic adsorption at a metal electrode depends noticeably on the crystallographic orientation of the exposed crystal face.' The adsorption of several classes of organic compounds has been investigated at gold single-crystalelectrodes using scanning tunneling microscopy (STM), including most recently organothiols (RSH) which form self-assembling monolayers (SAM).2*3The irreversible adsorption of a water-soluble thiol, namely, cysteine, has been studied at polycrystalline gold electrodes in this lab~ratory.~ Cysteine forms a particularly interesting kind of SAM because of ita zwitterionic structure. Moreover, because it is water soluble, one avoids contamination problems which can arise when the SAM is formed in a nonaqueous solution and then studied in water. The preparation of Au single-crystal electrodes and their structure in aqueous solutions have been well establi~hed.l*~ An important feature of these systems is that surface reconstruction occurs under certain conditions.6J This phenomenon should have an effect on the SAM and its permeability and properties as a medium for electron tunneling. The goal of the present study was to investigate the role metal substrate structure plays in the adsorption and redox properties of cysteine at gold. In our previous work4 at polycrystalline gold, the complex details of cysteine oxidation from the adsorbed state and ita effects on water adsorption and gold oxide formation were resolved. In the present paper, a similar analysis is carried out for this system at low-index single-crystal gold electrodes. Abetractpubliehedin Advance ACSAbstracts, February 1,1994. (1) Lipkowski, J.; Stolberg, L. In Adsorption of Molecules at Metal Electrodes:LiDkowski..J..Roee, . . P. N... E&.: . VCH Publishers: New York, 1992;Chaptei4. (2)Kim, Y. T.;McCarley, R. L.; Bard, A. J. J. Phys. Chem. 1992,96, 0
"I...
1410.
(3)Edinger, K.; Golzhaueer, A.; Demota, K.; Woll, Ch; Grunze, M. Langmuir 1993,9,4. (4)Fawcett, W. R.;Fedurco, M.; KovAEovB, Z.; Borkowska, Z. J. Electroanal. Chem., in press. (6) Hamelin, A. In Modern Aspects ofElectrochemistry; Conway, B. E., White, R. E., Bockris, J. O., Ede.; Plenum Preea: New York, 1985;Vol. 16,Chapter 1. (6)(a)Zei, M. S.; Lehmpfuhl, G.; Kolb, D. M. Surf.Sci. 1989,221,23. (b) Roes, P. N.; d'Agostino, A. T. Electrochim. Acta 1992,37,616. (c) Kolb, D.M.; Schneider, J. Surf.Sci. 1985,162,764. (d) Kolb, D. M.; Schneider, J. Electrochim. Acta 1986,3I,929. (e) Ha", U.W.; Kolb, D. M. J. Electroanal. Chem. 1992,332,339.(0Henglein, F.;Kolb, D. M.; Stolberg, L.; Lipkowski, J. Surf.SCL1993,291,326. (7)Hamelin, A. J. Electroanal. Chem. 1992,329,247.
0743-7463/94/2410-0912$04.5010
Electrode Materials and Instrumentation. The working electrodematerials Au(lll), Au(100), and Au(ll0) were purchased from Metals Crystals and Oxides, England. The electrode surface preparation, which consisted of polishing a gold rod electrode with different sizes of alumina on a polishing wheel (Buehler), electropolishing in perchloric acid solutions, and flame annealing pretreatment, was the same as described previously.4 The effective surface areas for Au(lll), Au(100), and Au(ll0) were 0.102,0.120, andO.OS~m-~, respectively, as estimated from the charge needed to form a monolayer of gold oxide. The values of the roughness factor for these crystals were determined to be 1.16, 1.02, and 1.13, respectively, from the ratio of the real surface area to the geometric area. The effectiveness of the method used to determine the area was demonstrated earlies on the basis of specific capacity data for polycrystalline gold which agreed very well with resulb obtained in other 1aboratories.QThe glasscleaning procedures, chemical reagents, and water purification were the same as described earlier.' All electrochemical experiments were conducted at room temperature, in an all-glass electrochemical cell containing a calomel electrode (with 0.05 M KC1) and a gold counter electrode, under a nitrogen (99.998%) atmosphere using the instrumentation as previously described? Results Characterization of the Electrode/Solution Interface. Cyclic voltammograms for Au(lOO), Au(lll), and Au(ll0) electrodes in 0.01 M HClO4 are shown in Figure 1. The features seen in the oxidation region have been labeled for ease of reference in the following discussion. Prior to the addition of cysteine to the electrochemical cell, the state of the electrodelsolution interface was checked at two scan rates, namely, 20 and 500 mV s-l, in order to confirm that the state of the surface was the same as observed in previous work.lO-'Z The fingerprint region in which oxide is formed for each crystallographic face (8) Fawcett, W. R.; Fedurco, M.; KovAEovB, Z.; Borkowska, Z. J. Electroanal. Chem., in press. (9)Clavilier,J.; van Huong, N. C. J.Electroanal. Chem. 1977,80,101. (10)Hamelin, A. J. Electroanal. Chem. 1988,265,281. (11)Silva, F.; Sottomayor, M. J.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1990,295,301. (12)Lecoeur, J.;Bellier, J. P.; Koehler, C. Electrochim. Acta 1990,36, 1383.
(9
1994 American Chemical Society
Electrochemistry of Cysteine
Langmuir, Vol. 10, No. 3, 1994 913
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Figure 1. Cyclic voltammograms for Au(100) (a, top left), Au(ll1) (b, top right), and Au(ll0) (c, bottom) in 0.01 M HClOd, at a scan rate of 20 mV 8-1. Anodic peaks denoted as AIA, Bl-2, and C are discussed in the text.
gives a clear indication of the level of impurities at the electrode/solution interface. Continuous cycling in the potential region from -0.5 to +1.3 V after flame pretreatment does not result in any changes in the observed current, indicating that the concentration of contaminants at the electrode/solution interface is very low. It should also be noted that no irreversible gold reconstruction occurs in this potential region.BdThere is a subtle difference in the shape of anodic peaks As and 44 (Figure la) for the oxidation of Au(100), compared to that published by Hamelin.'(' This difference is attributed to the fact that the extent of gold hydroxide formation on energetically different sites in between an overlay-lattice of adsorbed anions on the surface of gold depends on the scan rate, that used in the earlier worklo being 4 times faster than that in the present experiments. The increase in the sweep rate from 20 to 80 mV s-1 in the present experiments resulted in the same current-voltage response as seen earlier.7JOJl
Since ac admittance measurements were not performed in this study, cyclic voltammograms were also recorded at a scan rate of 500 mV s-l. Double-layer charging and the diffuse layer contribution to the capacity can be seen for each single-crystal electrode from cyclic voltammograms recorded after one potential sweep of the polarizable electrodeto +0.6V. In this way, the reconstruction present on the Au(100) electrode was lifted! Further observations related to reconstruction are discussed below. Subsequent sweeps yield the same current profile (see Figures 3a, 6a, and 7a) provided the potential is cycled only in the doublelayer region, that is, from -0.5 to +0.5 V. Values of the potential of zero charge (pzc) determined from these experiments are summarized in Table 1. They compare quite well with those reported recently by Lecoeur et Gold Surface Reconstruction and Potential-Induced Changes in Cysteine Films on Au( 100). Cyclic voltammogramsfor the Au(100) electrode in 0.01 M HClOl in the presence of cysteine are shown in Figures 2-4. Recent
914 Langmuir, Vol. 10, No. 3, 1994
Fawcett et al.
Table 1. Peak Potentials for Cysteine Oxidation on Gold from 0.01 M HClO4 Au(ll1) Au(100) Au(ll0) poly(Au)
0,090 -0.070
-0.190 -0.200
0.840 0.690 0.730 0.725
1.045 0.940 0.960 0.965
I
-0.470 -0.630 -0.430 -0.530
1.0
S,
p.
s1
work6 has shown in combined in-situ STM and electrochemical experiments that the Au(100) surface consists mainly of 1X 1domains after flame annealing and potential excursion to +0.6 V where lifting of the surface reconstruction occurs. This surface layer is stable at potentials close to the pzc in the presence of dilute HClO4. The flame annealing pretreatment gives results identical to those of surface preparation under ultrahigh vacuum? Peak C (Figure la) has been assigned to the rearrangement of gold surface atoms induced by positive charge densities.1° This peak appears in the absence of cysteine, in the first positively-going cycle recorded immediately after flame annealing, and can be restored in subsequent potential sweeps when the electrode potential is cycled negative of the pzc. The potential-induced reconstruction therefore appears to be reversible under these experimental conditions. Interestingly, the adsorption of an anionic species such as HS04- and Sod2- on Au( 100)from sulfuric acid solutions resulted in the disappearance of peak C, a new peak A being observed at a more positive potential in the first positive sweep after the flame annealing pretreatment. Since the adsorption of cysteine on Au(100) is manifested by very similar current-voltage features in the preoxidation region of gold as described above, the same notation for peaks C and A is used. Peak C (Figure la) transforms to peak A (Figure 2) at concentrations of cysteine as small as 1 X 10-9 M (with mild stirring of the solution). In both cases, in the presence of sulfate anions or cysteine, peak A is found at a potential about 50 mV more positive of peak C, and it disappears in a similar fashion to that reported by Hamelin’O after the second cycle in the potential range from -0.5 to +1.3 V (see Figure 2). The agreement in the potential at which peak A was found and the charge under the peak might indicate a similar arrangement of gold atoms at the electrode surface in the presence of both adsorbates, namely, cysteine and the sulfate anion. The anodic peaks SI,S2, and SS(Figures 2-4) appear on a Au(100) electrode only in the presence of cysteine in perchloric acid solutions. S1 and S2 can be observed on the cyclicvoltammogramfor Au(100)in very dilute cysteine solutions (lessthan 4.0pM RSH). Peak SIis highly specific for the electrooxidation of cysteine. The addition of one more methylene unit (-CH2-) in the RSH molecule (homocysteine) causes the disappearance of peak SI,even though the overall current-potential features in the oxidation of homocysteine remain similar to those for cysteine. Figure 3a shows that the anodic current for peak SIincreases linearlywith the bulk concentration of cysteine and finally overlaps the peak denoted S4 for a cysteine surface coverage larger than approximately 40% (see Figure 3b). The disappearance of peak S1 after extensive washing of the cysteine-modified gold electrode with distilled water and transfer to another cell is probably a result of a change in the cysteine film structure (compare the shaded and unshaded areas in Figure 4a). It is probable that peak SIappears on Au(100) because of the rearrangement of cysteine molecules at the electrode surface at the moment when the cysteine film starts to oxidize, rather than because of reconstruction of the gold surface.
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Figure 2. Cyclic voltammetric curves for the oxidation of 1.2 pM cysteine on Au(100) in 0.01 M HClO, (scan rate 20 mV 8-1). The dotted curvewas recorded immediately after h e annealing, starting from a potential of -0.5 V; the solid curve represents a second potential sweep in the range from -0.5 to +1.3 V. Anodic peaks Sz and Ss correspond to the one- and three-electron oxidation reactions of cysteine in the adsorbed state. Peaks A and SIare capacitative and indicate the reconstruction of gold surface atoms and the conformational changes of the cysteine film at positive charge densities, respectively.
Oxidation Reactions of Adsorbed Cysteine on Au(100). Cysteine has been shown to be irreversibly adsorbed on polycrystalline gold at potentials positive of -0.6 V in 0.01 M HC104.4 In a similar fashion, the deposition of OH- ions a t Au(100) is blocked on the part of the electrode surface which is occupied by chemisorbed cysteine molecules. This is manifested by a decrease in the current of peaks A1 and A2 with an increase in the bulk concentration of cysteine, and a decrease in the anodic current in the preoxidation region of gold (Figure 3a). The latter is also clearly visible in Figure 4a. The shaded areas of the cyclicvoltammogram represent oxidative desorption of a sulfonated product of cysteine from the electrode after the transfer of the cysteine-modified electrode to a cell containing only 0.01 M HClO4. The ratio between the charge for the oxidation of 1.2 pM cysteine on Au(100) (integrated area under peak SZ)and the charge passed in the potential region from +0.9 to +1.3 V is 1to 3 (Figure 4a). The total charges for the oxidation of a monolayer of cysteine, Q, on Au(100) and on a polycrystalline gold are rather similar (see Table 2) and can be determined from the integrated area under the solid curve (Figure 4b) after subtraction of the charge for gold oxide formation. It was shown in earlier work4 that the charge due to Au oxidation is independent of cysteine surface coverage,and can be determined from the charge passed to reduce the gold oxide on the negatively going sweep. This is true because the cysteine is oxidized and the oxidation products are quantitatively desorbed from the electrode in a positively going sweep to +1.3 V. A second cycle in the potential region from -0.5 to +1.3 V gives a cyclic voltammogram similar to that for Au(100) in the absence of RSH (curve b). This shows that the electrode surface was not contaminated by transfer to the second cell. The peak C seen in Figure l a is not clearly visible on the second cycle but appears clearly after additional cycling.
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* Langmuir, Vol. 10, No. 3, 1994 915
Electrochemistry of Cysteine
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Figure 3. (a, top) Cyclic voltammograms for the oxidation of cysteine on Au(100) in 0.01 M HClOd for various bulk concentrations: (a) 0.2, (b) 0.4, (c) 0.6, (d) 0.8, (e) 1.2, (0 2.0pM. The dotted curve is for a solution with no added cysteine (scan rate 20 mV 8-1). The solid curve in the potential region from -0.5 to +0.5 Vat a scan rate of 500 mV s-1 shows double-layer charging for Au(100) in the absence of cysteine. (b, bottom) As in (a) but for 4.0 (g) and 30.0 (h) NM cysteine. The dotted curve shows results of a transfer experiment after accumulation from 100.0 pM cysteine solution at +0.3 V for 5 min, transfer to a solution containing0.01 M HClO,, and scanningfrom -0.1 to +1.3 V (scan rate 20 mV 9-1).
In order to determine the number of electrons in the overall reaction for cysteine oxidation on gold from the adsorbed state, estimated in our previous work to be four, we investigated the dependence of the peak potentials E, for SZand S3 on solution pH (2 < pH < 6). The slopes of plots of E, against pH (Figure 5) for 1.2 pM RSH were found to be -0.059 and -0.053 V/pH unit, respectively, for
Figure 4. (a,top) Difference between cyclic voltammograms for the oxidation of 1.2 pM cysteine from the bulk of the solution (solid curve) and in B transfer experiment (shaded area). The cysteine was accumulated at the electrode surface during a potential scan from -0.5 to +1.3 V and returning to -0.5 V. The electrode was washed with water and transferred to a solution of 0.01 M HC104 and cysteine oxidatively removed from the electrode surface (scan rate 20 mV 8-1). (b, bottom) Oxidative desorption of a monolayer of cysteine from Au(100) in 0.01 M HC104 after performing a transfer experiment (curve a). Experimental conditions are the same as indicated in Figure 3b. Curve b corresponds to the second scan in the potential range from -0.5 to +1.3 V (scan rate 20 mV 8-1).
values of pH less than 5. This implies near-Nernstian behavior for both waves, suggesting that the numbers of electrons and protons in each step of the reaction mechanism are the same. Similar values of the total charge for cysteine oxidation on Au(100) and polycrystalline gold (see Table 2) indicate a multistep electron transfer at peak SB.The oxidation of cysteine with less than four electrons would result in a very small projected geometric area for RSH which seems to be improbable for the model of
916 Langmuir, Vol. 10, No.3, 1994
Fawcett et al.
Table 2. Adrorption Parameten for Cyrteine on Gold from 0.01
chargefor monolayer oxidation (4 e), pC cm-2 eurface concentration, molecules X 10-15 mol cm-* x 109 area per molecule, n m 2
M HCIO4
Au(ll1) Au(100) Au(ll0) poly(Au) 750 850 1020 900 1.17 1.95 0.085
1.33 2.21 0.075
1.59 2.65 0.063
I_
1.41 2.34 0.071
cysteine constructed on the basis of crystallographicdatal3 The six-electron oxidation of cysteine to cysteic acid can also be excluded, since the numbers of protons and electrons in the second wave would differ, which in turn would result in a smaller slopeof the peak potential against pH for the S3 plot than experimentally observed. On the basis of the results presented here and their similarity to results for polycrystalline gold,” it is proposed that the faradaic peak SZcorresponds to the one-electron oxidation of cysteine and subsequent chemical reaction with a water molecule, which makes this process completely irreversible. Peak S3correspondsto the further three-electron oxidation of a surface-bonded intermediate and the desorption of cysteinesulfinic acid from the electrode surface. The anodic peak S4 is observed at a cysteine surface coverage approaching a monolayer and involves bulk water in the surface oxidation of cysteine; therefore, this peak appears at more positive potentials than peak S3. Uvdal and co-workers14 reported that the structure of cysteine films deposited on polycrystallinegold from acidic solutions (pH < 7) does not depend on pH. The Epvalues for cysteine oxidation shift significantly toward more negative potentials with a decrease in the pH (1< pH < 6), but at the same time no significant change in the intensity or the shape peaks Sz,S3, and S1 is observed. This behavior shows that the structure of the cysteine film on Au(100) is rather independent of pH in acidic solutions. Oxidation Reactions of Adsorbed Cysteine on Au(ll1). Cyclic voltammograms for the oxidation of cysteine on Au(ll1) in 0.01 M HC104 show some new features compared to those observed for Au(100). The anodic current corresponding to the two-step oxidation of cysteine from the adsorbed state on Au(ll1) is superimposed on the current for the oxidation of gold to gold oxide (Figure 6a, dotted curve). The dotted curve h (Figure 6b) correspondsto cysteine monolayer oxidationrecorded after transfer to a cell without cysteine, and cyclic voltammogram g (Figure 6b) shows the oxidation of RSH from the bulk of the solution. The mixed oxidation current, namely, from the adsorbed state and from the bulk of the solution, is much more pronounced in Figure 6c,d, where new faradaic peaks are observed in the presence of 100 p M or higher RSH concentrations in the bulk of the solution. The peak Do,(b) corresponds to the oxidation of cysteine through the monolayer of chemisorbed cysteine. The oxidationof cysteine stillproceeds in the negatively-going scan after reversal of the potential at +1.3 V and gives an anodic current denoted as Dox(bl);this is due to the diffusion of cysteine to the electrode surface from the bulk of the solution. The oxidative desorption of cysteine in 0.01 M HC104, after the transfer of Au(ll1) to a cell containing only HC104, gives a cyclic voltammogram in which both of these peaks are absent (dotted curves in Figure 6b-d). The charge correspondingto each oxidation ~~~
1.o
~
(13) Herding, M.M.;Long, H. A. Acta Crystallogr., Sect. E 1968,24, 1096. (14)Uvdd, K.;Bodo, P.;Liedberg, B. J. Colloid Interface Sci. 1992, 149, 162.
-.n r, 1
2
3
4
5
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7
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Figure 5. Peak potential Epplotted against the solution pH for the one (51)and three (Sa)electron oxidation reactions of cysteine on Au(100) in 0.01 M HC104, as indicated. The straight lines were drawn by least squares considering only the data obtained at a pH of lese than 5.
process, whether it is surface or diffusion controlled, can be easily separated in this way. Since the E, value for this oxidation reaction is much more negative than for the bulk oxidation of cysteine at peaks S4 and SS, and oxidation does not take place from the adsorbed state, it is concluded that the oxidation product at this potential is the disulfide (cystine)rather than a sulfonated oxidation product. The bulk electrolysis of cysteine on a gold electrode in the same potential region supports this conclusi~n.~~ The bulk oxidation of cysteine which occurs at more positive potentials was not studied. Oxidation Reactions of Adsorbed Cysteine on Au( 110). Figure 7 shows the cyclic voltammograms for the oxidation of cysteine on Au(ll0) in 0.01 M perchloric acid. Current-potential characteristics for the surfacecontrolled cysteine oxidation on this crystallographicface are very similarto those on a polycrystallinegold electrode. There is again a peak in the preoxidation region of gold, denoted peak S2,and another peak, Sa, at more positive potentials (Figure 7a), where a surface-bonded sulfur compound is further oxidized and desorbed from the electrode surface. The dotted curve in Figure 7b was recorded after the accumulation of cysteine on Au(ll0) from 100.0 p M RSH solution at +0.2 V for 5 min and subsequent oxidative desorption of cysteine in another cell containing only 0.01 M HC104. The adsorption characteristics for cysteine on Au(ll0) are presented in Table 2. The large current at peaks S3 and S4 and superimposed oxidation current from the bulk of the solution on the current of cysteine oxidation from the adsorbed state are shown in Figure 7b. The peaks Dox(b), Dox(bl),and Dd(s) appear similarly to those on Au(ll1) electrodes described above. One-Electron Redox Reactions of Cysteine on Gold at Negative Potentials. There are two types of anodic current for the oxidation of cysteine on Au(ll1) (Figure 6c,d), namely, that controlled by the diffusion from the bulk (denoted by the symbol b), and the surface-controlled oxidation of cysteine (denoted as 8). The one-electron reduction of cysteine from the adsorbed state takes place in the peak Dd(s) and is observed also with a transferred electrode in the f i t negatively-going scan, for example, from +0.3 to -0.6 V (Figure 6d). It should be noted that no peak D,(s) is observed in the absence of cysteine in the bulk of the solution after the transfer experiment (Figure 6d, dotted curve), but Do&) disappears in the positivelygoing cycle since cysteine diffuses into the bulk of the (15) Davis, D.
G.;Bianco, E. J . ElectroaMI. Chem. 1966, 12, 2S4.
Electrochemistry of Cysteine
Langmuir, Vol. 10, No.3, 1994 917
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Figure 6. (a, top left) Cyclic voltammograms for the oxidation of cysteine on Au(ll1) in 0.01 M HClOr for various bulk concentrations (scan rate 20 mV 8-11: (a) 0.4, (b), 0.6, (c) 0.8, (d) 1.2 gM RSH. The dotted curve was obtained in the absence of cysteine. T h e double-layer region charging curve (e) was recorded at 500 mV 8-1 with no cysteine add+ (b, top right) Ae in (a) but for 2.0 (e), 10.0 (0, and 30.0 (9) gM cysteine. The result of a transfer experiment with a subsequent oudative desorption of a cysteine monolayer as described in Figure 3b ie shown by the dotted curve. (c, d, bottom) Cyclic voltammograms for the oxidation of 100.0 gM cysteine on Au(ll1) in 0.01 M HClO, (scan rate 20 mV 8-9. Anodic peake D d b ) and D,(bd correspond to the bulk oxidation of cysteine through ,.& represents reductive desorption of part of the cysteine the monolayer of chemisorbed cysteine on the electrode surface. Peak D monolayer from the electrode surface (pinhole formation), and D,(s) is the formation of a cysteine monolayer. T h e dotted curve in (d) represents reductive desorption of a part of the cyateine monolayer (40%) after transfer to a cell containing only 0.01 M HClO,.
solution upon its reductive desorption. The integrated area of peak Dd(s) or D,(s) does not change with a further increase in the bulk concentration of RSH (higher than 100 rM).The peak Dr&) corresponds to the reductive desorption of cysteine from the gold surface. Our experiments in cysteine solutions at pH 6.16 (0.06 M KClOI) have shown that the peak Dd(s) is shifted negatively to -0.63 V and at the same time another cathodic peak appears at -1.4 V. The latter peak is hidden in a solution of pH 2.09 because of hydrogen evolution on the gold electrode. The charge passed in the f i t peak Dd(s)
corresponds to the desorption of approximately 40% of the cysteine monolayer (one-electron reduction), the rest of the surface thiolate being desorbed at more negative potentials in a wave which is partially obscured by hydrogen evolution at -1.5 V. The pinholes created at the electrode surface in the negatively-going sweep at peak Dd(s) are then refilled with cysteine from the bulk in the positively-going potential scan at peak D,,(s) (Figure6d, solid line). The charge passed in the peak Dd(e) on Au(ll0) is significantly smaller than that passed on a Au(ll1) at the same bulk concentration of cysteine (100
Fawcett et al.
918 Langmuir, Vol. 10, No.3, 1994
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Figure 7. (a, top left) Current-voltage response to Au(ll0) in 0.01 M HClOd with increasing bulk concentration of cysteine (scan rate 20 mV 8-1): (a) 0.2, (b) 0.4, (c) 0.8, (d) 1.2 cM. The dotted curve was obtained in the absence cysteine. The double-layer charging curve (e) was recorded at a scan rate of 600 mV s-1 without cysteine. (b, top right) As in (a) but for 1.4 (d) and 30.0 (e) pM cysteine. The dotted curve (0corresponds to the oxidative desorption of a cysteine monolayer from the electrode surface in a transfer experiment as described in Figure 3b. (c, bottom) Electrooxidation of 100.0 cM cysteine on Au(ll0) in 0.01 M HClOr (scan rate 20 mV 8-1). p M RSH). The peak potentials for Au-SR reduction in the peak Dd(s) are summarized in Table 1. Preliminary experimentsin thislaboratory have shown that such partial reductive desorption of a thiol from the monolayer film in the double-layer region not only is typical for cysteine but also occurs in the presence of other sulfur compounds including glutathione reduced, homocysteine, cysteinylg-
lycine, and N-acetylcysteamine. This phenomenon will be discussed in detail in a future paper. Effect of Crystallographic Orientation on the Redox Reactions of Cysteine at Gold. Table 1 shows that there is a significant effect of crystallographic orientation on the peak potential at which cysteine undergoes oxidation or reduction reactions. The Epvalue
Electrochemistry of Cysteine
for the surface-controlled oxidation of 1.2 pM cysteine in peak S2 is more positive on Au(ll1) than on Au(100) and follows approximately the shift in the pzc. Peak Ss or Au(100) is shifted 105 mV in the same direction. Interestingly, the E, for the peaks S2 and SSon Au(ll0) and on a polycrystalline gold do not differ greatly, just as their pzc values. The charge passed in the peak Drd(s) varied in the following order: polycrystalline Au < Au(ll0) < Au(100) < Au(ll1). The potentialfor reductivedesorption of cysteine Drd on gold electrodes in 1 X 10-4 M RSH takes place in the potential region from -0.43 to -0.65 V, but no clear dependence on the crystallographic orientation of gold is observed. The one-electron reductive desorption of cysteine from the monolayer film might be connected with reconstruction of the electrode surface at negative potentials since it takes place to a different extent and at different potentials depending on crystallographic orientation. This desorption is also observed in the presence of a large number of water-solubleorganosulfur compounds including N-acetylcysteamine and N-cysteinylglycine in the same potential range. Camillone et al.lShave shown using low-energy helium diffraction that Au(lll), Au(llO), and Au(100) faces reconstruct in the presence of chemisorbed docosyl mercaptan, forming (d3Xd3)R30° domains on the surface. In the case of Au(ll0) these domains have ~ ( 2 x 2structure. ) For Au(100), reconstruction is more complex with four types of surface domains. Similarly, it has been demonstrated”J8 that, in the absence of adsorption, a potential excursion negative of the pzc (more than 200 mV) causes long-range diffusional transport of an additional 24 5% of the gold atoms from the bulk of the metal, creating 1 X 3 and 1 X 4 domains on the surface of both Au(ll0) and Au(100). In the latter case, the pzc value can be obtained by measuring the dependence of the differential capacity on potential in dilute electrolyte solutions; therefore, charge densities can be determined at the potential where the lifting of the reconstruction occurs.10 The behavior of cysteine in the double-layer region is similar on singlecrystal gold to that found on polycrystalline gold.8 In this respect, it is not possible to determine the pzc in the presence of cysteine using the usual electrochemical techniques. The charging current in the presence of the monolayer is significantly reduced from the value in its absence (by approximately 40 pF cm-2). It is reasonable to expect that the different arrangement of gold atoms on the reconstructed electrode surface and therefore the different charge density on the metal electrode will affect the peak potential where the reductive desorption of cysteine takes place. Unfortunately, the fact that the charge density and the number of gold atoms on the (16) Camillone, N., 111; Chidsey, C. E. D.; Liu, G.; Scoles, G. J. Chem. Phys. 1993,98,4234. (17) Gao, X.; Hamelin, A.; Weaver,M. J.Phy8. Reu.Lett. 1991,67,618. (18) Gao, X.; Hamelin, A.; Weaver, M. J . Phys.Reu. B 1991,44,10983.
Langmuir, Vol. 10, No. 3, 1994 919
reconstructed surface are not known makes any direct comparison of the phenomena connectedwith the cysteine reductive desorption at the negative potentials on different crystal faces very difficult. The extent of reductive desorption of cysteine of Au(ll0) and Au(100) is much smaller than on Au(ll1) where reconstruction to the (d3Xd3)R30° structure occur^.^^^^^^ Discussion The present work qualitatively and quantitatively describes the rather complicatedcatalytic processes which occur on gold single-crystal electrodes in the presence of the water-soluble amino acid cysteine. The inhibition of deposition of hydroxyl ions on gold has been clearly demonstrated. Previous work has also shown that these monolayers provided effective blocks to the adsorption of anions such as chloride.‘ The surface concentration of chemisorbed cysteine molecules on all Au electrodes is significantly higher than that previously reported for n-alkanethiols.ls21 However, it should be noted that the earlier experimentsls21 made use of reductive desorption in alkaline solutions to determine surface coverage. This technique is not possible in the case of cysteine which decomposes at high pH. Experiments reported in this work on gold single-crystal electrodes further support our previous conclusions about the mechanism of the surfacecontrolled multiple-electron oxidation of cysteine. We suggest that the high surface coverage of cysteine is mainly due to electrostatic interactions between the ionized carboxyl and amine groups in the monolayer fibs. It should also be realized that the degree of ordering of watersoluble thiols as well as the number of adsorbate molecules per unit mesh of gold might also depend on the potential applied at the electrode/solution interface. For example, the differential capacity-potential profile for a monolayer film of 2-mercaptoethanesulfonicacid differs significantly depending on whether the compound is deposited by selfassembly or at a constant potential such as +0.3 V w the calomel electrode.= New faradaic processes have been observed on Au singlecrystal electrodes in the double-layer region which have been assigned to pinhole formation. This process is controlled by the charge density on reconstructed singlecrystal faces and is most pronounced on Au(ll1). Further studies to elucidate the properties of these systems are currently in progress. Acknowledgment. The financial support of the Office of Naval Research is gratefully acknowledged. (19) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991,310, 336. (20) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. 5.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7,2687. (21) Weisehaar, D. E.; Lamp, B. D.;Porter, M. J. Am. Chem. Soc. 1992, 114,5860. (22) Fawcett, W. R.; Fedurco, M.; KovBEovB, 2.Unpublished reaulta.
