Anal. Chem. 1990, 62, 2424-2429
2424
(34)Seavy, S. S. In Rapid I.llgh Perfwmenoe LlquM chromaiop@y Technques for Monitoring A m h Acids in C W e F W ; Sewer, S. S.. Ed.: Mafcel Dekker: New York. 1967: na 315-318. (35) kyerhotf, M. E. Atmi. chem. ioio. 5z,’i52-1534. (36) mprhotf, M. E.; RoMns, R. ti. AIM!. chem.igoo. 52, 2383-2387. (37)Fratldi, Y. M.; m f f , M. E. Anal. Chem. 1981, 53, 992-997. (38) ProductendDete &&?; R A I Research Corp.: Hauppauge. NY, 1987; n A
(39)k e r , L. J.;
Wice, 6. M.; Kennell, D. J .
Bo/. Chem. 1979,254,
2669-2676.
REC-
for review June 18,1990. Accepted August 7,1990. We gratefully acknowledge the National Science Foundation (Grant EET-8712756) and Mallinckrodt Sensor Systems for the support of these studies.
Detailed in Situ Scanning Tunneling Microscopy of Single Crystal Planes of Gold(111) in Aqueous Solutions Hidetoshi Honbo, Shizuo Sugawara, and Kingo Itaya* Department of Engineering Science, Faculty of Engineering, Tohoku University, Sendai 980, J a p a n
An In situ electrochmlcd swmning tunnelkrg mlcrorcope (ESTM) was applied to a goM(ll1) surface with atomically flat terraces In aqueous perchloric acid Odutkns. The pits, a dngkr layer deep, were formed durlng the reductlon of oxkki tayers with higher owldatlon states. The surface dmudon d gokl atoms In a “pwe” HCIO, SdUHon was qulte slow and comparable to that observed In ultrahigh vacuum (UHV) reported In the prevlous literature. An electrochemical dlsgolution of gold was investigated In a HCIO, solution In the presence of chkrkle Ion. Strongly adsorbed CMOridb ionr, on gold( 111) remarkably enhanced the surface dlffuslon of gold.
INTRODUCTION One of the most exciting fields in electrochemistry seems to be the understanding of the relationship between the structures and properties of the solid-electrolyte interface at an atomic level (1-4). Although electrochemical techniques such as cyclic voltammetry have provided remarkably sensitive tools for characterization of various monolayer processes a t electrode surfaces (1-3),there are not many in situ methods for the determination of the structure of electrode surfaces a t the atomic level within electrochemical environments (4). However, recent efforts using in situ scanning tunneling microscopy (STM) have provided persuasive evidence that the in situ STM is a powerful new method for samples (metals and semiconductors), immersed in aqueous solutions (5-13). Real space imaging a t an atomic level has recently been achieved with graphite (5-7),Au(ll1) (8-101, and Pt(ll1) (11) surfaces in aqueous electrolyte solutions. Wiechers et al. first demonstrated resolution of monatomic Au(ll1) layers using a bulk single crystal (8). However, more recently, Trevor et al. reported the roughening and dissolution accompanying the oxidation and rereduction of the Au(ll1) surface (9). An underpotential deposition of a monolayer of lead has also been investigated on Au(ll1) by Green e t al. (10). The Au substrates used for the latter two studies cited above (9,10) were Au(ll1) films prepared by vacuum evaporation onto freshly cleaved mica. A critical question might be raised whether the surface can be transferred from the vacuum to the electrolyte solution without chemical changes and contaminations, which has been discussed in previous literature (1-41, even though individual clase-packed Au atoms could be imaged on these surfaces in an air environment (14, 15). In our previous study for Pt(ll1)surface, the Pt electrode 0003-2700/90/0362-2424$02.50/0
was annealed in a hydrogen-oxygen flame near loo0 “C for 1 min and then quickly brought into contact with ultrapure water saturated with hydrogen for a final preparation of atomically clean surfaces (11). Based on both electrochemical and in situ STM measurements, we concluded that Pt surfam have been kept clean during the in situ STM observation for a t least several hours (11). In this paper, therefore, we report our in situ STM study of clean Au(ll1) surfaces of single crystal spheres in aqueous perchloric acid solutions. Surface diffusion, pit formation, and electrochemical anodic dissolution are discusaed on Au(111) surfaces. EXPERIMENTAL SECTION Experimental conditions were similar to those described in our previous papers (7,11,15). Single crystals of Au spheres were prepared by the method of Clavilier et al. (16). A molten ball of gold at the end of a gold wire was slowly solidified in a flame of hydrogen and oxygen. Carefully prepared spheres, 2-3 mm in diameter, always consisted of eight facets of Au(ll1) in an octahedral confiiation. The crystallographic axe8 of the spheres were determined by both X-ray diffraction (11)and laser beam reflection from a Au(ll1) facet as described by Furuya et al. (17, 18). Mechanically exposed Au(ll1) surfaces (denoted by polished Au) with successively finer grades of alumina were annealed in a hydrogen-oxygen flame at ca. 600 OC for 10 min. Both facet and polished Au(ll1) surfaces were investigated. As a final pretreatment for all measurements, the Au electrode was again annealed in a hydrogen-oxygen flame near 600 “C for 30 s and then quickly brought into contact with ultrapure water saturated with hydrogen. The electrochemical cell for STM measurements made by poly(chlorotrifluoroethy1ene)was the same as described previously (11). The Au(ll1) electrode prepared above was transferred to the electrochemical cell. During the transfer, the Au(ll1) surface was protected from contamination by a droplet of ultrapure water on it. Bright Pt wires were used as counter and quasireference electrodes, respectively. All electrochemical potentials are reported with respect to a reversible hydrogen electrode (RHE) in the same electrolyte. Perchloric acid used was Kanto Chem. (super grade) whose content of chloride ion is less than 5 ppm. It is well known that trace chloride ion can substantiallyincrease the amount of soluble Au ions at anodic potential regions (19,20). Therefore, a special effort has been made to prevent contamination of chloride ions in our procedure of the preparation of solutions. The STM unit used for the present study was similar to that described by Hansma et al. (21). A fine mechanical approach is achieved by a 101 reducing lever. We employed a combined STM-bipotentiostat system for the present study in which the electrode potentials of both the tip and the substrate can be independently controlled with respect to a reference electrode 0 1990 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 62. NO. 22. NOVEMBER 15. 1990
2425
a
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15
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-1. STMmagesofaAu(ll1)facetsurfaceinO.l MHCIO,. The electrode potentials of the Au and tip electrodes were 0.85 and 0.8 V vs R M . respectively. The tunnelii current was 2 nA. Scan speeds were 100 nmls (a)and 200 nmls (b). The x . y . and z scales are as Indicated.
(22). The tunneling tip was a Pt wire with a diameter of 15 pm sealed into a soft-glass pipet.
RESULTS AND DISCUSSION Clean Au(ll1) Surface i n a Perchloric Acid Solution. T o ensure the purity of the solution, we employed the characteristic hydrogen adsorption-desorption peaks on a Pt(ll1) in an electrochemical cell containing 0.1 M HCIO, solution. When the hydrogen adsorption-desorption peaks gradually diminished during immersion of the Pt(lll), the cell was again cleaned with hot concentrated H2S0, and refilled with a freshly prepared 0.1 M HCIO, solution 88 described in our previous paper (11). All experiments reported here have been carried out under just such a precaution. Figure 1shows typical STM images of 100 X 100 nm2 (a) and 250 X 250 nm2 (h) regions observed on a Au(ll1) facet in a 0.1 M HCIO, solution. A STM study in air has been reported for a similarly prepared Au facet by Schneir et ai. (23). The electrode potentials of the Au(ll1) and tip electrodes were held a t 0.85 and 0.80 V vs RHE, respectively, where no appreciable current flowed at both electrodes. Several nearly parallel steps are seen in these images which were obtained a t points near the center of the facet. The height of each step in Figure l a is about 0.24 nm in accord with the monatomic step height of 0.235 nm on the Au(ll1) surface. There are a few points where two single monatomic steps merged into a single diatomic step as shown in Figure la. Monatomic, diatomic, and triatomic steps are clearly seen in the STM image of a larger area of 250 x 250 nm2 as shown in Figure lb. Note that we also found pasitions on the Au(ll1) facet where the orientations of steps differ by roughly 60° as expected for a surface with 3-fold symmetry. This regularity has previously been reported for Au(ll1) (8,9,23)and Pt(ll1) ( 2 2 ) surfaces. The terraces shown in Figure 1seem to be absolutely flat, resulting a very stable tunneling current observed during each scan of the x direction. In our previous study, some appre-
Figure 2. Cyclic voltammagrams for a polished Au(l11) elec1rd-s in 0.1 M HCIO,. The sweep rate of the potential was 20 mV1s. The and 1.85 V vs swilching potentials (positiie end) were 1.65 V (-) RHE (---), respectively.
ciable structures with a height less than 0.1 nm were seen on the terraces of Pt(ll1) even under the precaution described above (22). Stronger tendency of Pt surfaces to adsorb various impurities might cause such disordered structures on the Pt(ll1) surface ( 2 2 ) . However, the very flat nature of the Au(ll1) facet shown in Figure 1strongly encouraged us to try to observe individual Au atoms in electrochemical circumstances. Although the observation of individual close-packed Au atoms has recently been achieved in our laboratory on the Au(ll1) facet in air (25) as that reported in a previous literature (14),a similar STM image has not yet clearly been obtained in aqueous solutions. Nevertheless, it might be suggested from the quality of the terraces shown in Figure 1 and the observation of Au atoms in air that the Au(ll1) surface has a 1 X 1 structure (close-packed) in aqueous solutions. Dagostino and Ross have confirmed that an UHV reconstructed surface of the Au(ll1) crystal was transformed to a (1 X 1) structure upon electrolyte contact (24). Electrochemical Oxidation and Reduction of Au(ll1). Trevor et al. have recently shown that pits of a single layer deep were formed after an oxidation-reduction cycle using a Au film on mica (9).They simply compared STM images obtained at a potential of 0.7 V vs a normal hydrogen electrode both before and after an oxidation-reduction cycle of the electrochemical formation of oxide layers. However, it was not addressed whether the pits were formed during the formation of oxide layers or the rereduction of these oxide layers. It has been pointed out in our previous study for Pt(ll1) that disordered structures on the Pt(ll1)terraces were formed after the rereduction of the PtO layer, not after the formation of the oxide layer (12). Our ESTM has an advantage in that the electrode potentials of the tip and Au electrodes are simultaneously controlled with respect to an independent reference electrodes as previously described (22). Therefore it is now possible to acquire STM images with ESTM during the potential cycle of the Au(ll1) electrode in order to elucidate a mechanism of the formation of pits. Figure 2 shows cyclic voltammograms on a polished Au(ll1) surface obtained in a 0.1 M HCIO, solution. The first main oxidation peak appeared at 1.35 V vs RHE, accompanying two
2426
ANALYTICAL CHEMISTRY, VOL. 62. NO. 22. NOVEMBER 15. 1990
small waves as a shoulder. The oxygen evolution reaction at potentials more positive than 1.75 V follows the second main peak at 1.55 V. The peak position of the reduction waves was shifted to the positive direction with an increase in the positive end of the potential scan as indicated in a broken line. The overall shape of the voltammograms shown in Figure 2 is almost identical in detail with that reported previously (25). A similar voltammogram has also been reported for a Au(ll1) film (9). The two main anodic peaks that appeared a t 1.35 and 1.55 V correspond to the oxidation of the first layer of the Au(lll), according to the following reactions (25): Au
-
+ H20
Au'OH
+ H+ + e+ H+ + e-
Au'OH
Au"0
2
a
1 nm
100
(1) (2)
The cathodic peak at ca. 1.2 V is due to the backward reactions of eqs 1 and 2. In addition of these reactions, it is known that Au can he dissolved during the anodic and cathodic parts of a potential cycle which is strongly affected by the concentration of chloride ion even under M (19). We found in the present study that contaminations of chloride ion should he minimized in order to prevent the anodic dissolution of Au during the anodic part of a potential cycle. Under these conditions, we acquired sets of STM images of one region of a Au(ll1) facet with changing the electrode potential of Au, while the potential of the tip was held a t 0.8 V vs RHE. F i i 3a shows an image obtained a t 0.85 V. Sets of the monatomic steps and very fiat terraces have consistently been observed at potentials in the double layer region as in the case shown in Figure 1. After the STM image of Figure 3a was taken, the electrode potential was scanned from 0.85 to 1.45 V vs RHE where nearly a monolayer of AuOH can he expected to form on the Au(ll1) surface. It is interestingly found that spiky irregular structures, like islands, can he clearly seen on the terraces whose heights were in an order of the monatomic height or less. The observation shown in Figure 3b could suggest that the Au'OH layer is not absolutely uniform over the (111)terrace in an atomic scale. However, these islands completely disappeared when the electrode potential was swept hack to 0.85 V. The very flat terraces have repeatedly re-formed during such a limited potential cycling, indicating that the anodic dissolution did not involve both the oxidation and rereduction processes of eq 1. When the electrode potential of Au was further scanned to 1.65 V vs RHE, the spiky irregular islands observed a t 1.45 V were suppressed, leaving small corrugations with heights less than 0.1 nm on the terraees as shown in Figure 3c. This observation suggests that the AuO layer is much more compact and uniform than that of the AuOH layer. The small corrugations shown in Figure 3c were again abolished and the smooth terraces were consistently seen in subsequent images after scanning hack to 0.85 V. No pit could he clearly seen in this limited potential cycle. The above result is well in accordance with that reported by Trevor et al. (9). After the image shown in Figure 3c was acquired, the potential was scanned from 1.65 to 1.85 V and hack to 1.65 V at 20 mV/s. Note that cycling to 1.85 V caused the oxygen evolution a t Au(ll1) as shown in Figure 2 hut did not cause a serious interference in the tunneling current. The tunneling tip could be continuously scanned over the surface during such a potential excunion. We found almost the same STM image as shown in Figure 3c even after the potential cycling to 1.85 V. The above result strongly suggests that the anodic dissolution of Au has not taken place or is a very minor process in a "pure" HCIOl solution under the potential region examined here. However, pits of a single layer deep are now clearly seen after the reduction of the oxide layers as shown in Figure 3d. This STM image was taken after potential cycling to 1.85
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Flgure 3. STM images of a 100 X 100 nm2 region of a AN11 1) facet obtained at 0.85 V (a). 1.45 V (b), and 1.65 V vs RHE (c). Part d was acquired a1 0.8 V immediately after a potential cycling to 1.85 V vs RHE at a scan rate of 20 mVh. The electrde potential of #a tip was 0.8 V vs RHE. The tunneling current was 2 nA. Scan speed was 200 nmls.
V and hack to 0.85 V vs RHE. I t can now he concluded that the pits on the terraces are formed during the electrochemical reduction of oxide layers formed a t potentials more positive than ca. 1.65 V vs RHE. The anodic behavior of Au has been extensively investigated by many workers using various electrochemical techniques (ZS). Bruckenstein et al. suggested that some oxides in higher oxidation states such as Au,O, present a t the Au surface and
ANALYTICAL CHEMISTRY,
15. 1990
-
2427
Flgurs 5. Cyclic voltammograms fa a polished AM11 1) elecbode h 0.1 MHCK),solutionswim(---)andwlthout(-)l X 104MHCI. The sweep rate of the potential was 20 mV/s.
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VOL. 62. NO. 22. NOVEMBER
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Au(II1) ions are dissolved into solutions upon the electrochemical reduction of the oxide layers (Z9,27).Our observation shown in Figure 3 seems to be very consistent with the previous electrochemical study. On the basis of the above discussion, one might expect that the pits could be an indicator of the exact location of the Au,O, presented at the surface before the reduction of the oxide layer. Unfortunately, it is not clear to see such a correspondence between two STM images shown in Figure 3, park c and d, respectively. It is obvious that studies must be carried out to reveal further
detailed structures of the oxide layers in an atomic scale. It is noteworthy that the shape and location of each monatomic step line after the potential excursion remained at almost the same position as observed before the potential excursion. We did not observe large protruding features in the location of steps in a "pure" HCIO, solution as reported in the previous literature (9). Although the appearance of new terraces was rarely observed in a HCIO, solution used for the present study, large protruding features were found in solutions containing chloride ions, which will be shown in a latter section. Surface Diffusion of Au. Figure 4 shows another set of experiments obtained on a different region where two monatomic step lines were crossed near the center of the images. This crossing point was employed as a marker for acquiring images at the same area on the sample. The electrode PO tential of Au was scanned from 0.85 to 1.85 V vs RHE and then hack to 0.85 V at a scan rate of 20 mV/s. Figure 4a shows a top view image of a 100 X 100 nm2 region obtained i m m e diately after the potential cycling. About 2 min was required for each image. The pits in various sizes on the terraces are more easily seen in Figure 4a. As discussed above, the feature in the shape of step lines was almost the same as that observed before the potential cycling. The diameters of these pits are in a range of 1-5 nm. The smaller pits disappeared at first and changes in the shape of step lines become gradually apparent as indicated in Figure 3, parts h and e, which were acquired after 30 and 60 min of the potential cycle, respectively. We notice that the rate of disaooearance of smaller D i t s and chanee of
vestigated in UHV a t roan temperature ( 2 8 , reporting a similar rate of diffusion as observed in the present study. However, surface diffusion is strongly enhanced by the presence of chloride ions as already mentioned in the previous literature (9). Anodic Dissolution of Au i n Chloride Containing Sclutions. Figure 5 shows cyclic voltammograms of Au(ll1) in the Dresence and the absence of chloride ions. The oxidation; of Au occurred at more positive potentials and then merged intoasingle wavewith an inereax in theconcentration
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ANALYTICAL CHEMISTRY, VOL. 62. NO. 22. NOVEMBER
15. 1990
of chloride ions. The reduction peak of the oxide layers also shifted anodically in the presence of chloride ions. These results seem to he fairly consistent with that reported for a polycrystalline Au electrode hy Bruckenstein et al.. suggesting that chloride ions adsorb strongly on the Au(ll1) surface (19, 27). It has also been shown by their ring-disk electrode study that strongly adsorbed chloride ions substantially increase the amount of soluble Au species produced during both the anodic potential scan and the reduction of the oxide layer (19). As discussed in the previous section, the formation of pits during the reduction of the oxide layer might he strongly affected by the presence of chloride ion. However, it seems to he more important to understand the dissolution of Au during the anodic potential scan as because this process takes place prior to the reduction of the oxide layers. In this section, therefore, we are mainly concerned about an electrochemical process occurring a t the foot of the oxidation wave, commencing at ca. 1.45 V vs RHE in a 0.1 M HCIO, and 1.0 X lo-' M HCI solution. Figure 6 shows a set of STM images of one area obtained in the above solution. Figure fia shows a STM image obtained a t 0.85 V vs RHE, revealing atomically flat terraces and monatomic steps, just like the images obtained in a 0.1 M HCIO, solution as shown in Figure 1. These observations continued in this area for 30 min, resulting in exactly the m e images as shown in Figure fia. This result suggests that a steady-state surface structure can be achieved in this medium. After the image of Figure fia WBS taken, the electrode potential was cycled to 1.55 V at a scan rate of 10 mV/s which is about 100 mV more positive than that of the onset potential (ea.1.45 V) of the oxidation wave as shown in Figure 5. The Au(ll1) surface was continuously observed with ESTM during the potential cycling as described above. We found large increases in the density of the monatomic step and heavily protruding features in step lines BS won as the anodic current commenced during the positive scan to 1.45 V. Figure fib was acquired at 0.85 V immediately after the potential cycle. It is now very clear that Au simply dissolved anodically at the foot of the oxidation wave, leaving a large number of monatomic steps. However, a steady STM image could be seen in the m e area after 15-20 min as shown in Figure fic. The surface rapidly smoothed during the above Deriod of time due to the surface diffusion of Au.'The rate oE the diffusion is obviously faster in the solution containing chloride ions than that in a "pure" 0.1 M HCIO, solution as discussed before. After the STM image shown in Figure 6c was acquired, the observation continued in the same area for a further 30 min, resulting in a similar image with minor changes. The result in Figure 6 shows straightforwardly a surface restructuring process from a steady state to the other induced by a perturbation of the anodic dissolution. It is very interesting to compare the STM images of parts a and c of Figure 6. which seem to be somewhat similar. The shape of the highest terrace at the upper left corner remains similar in shape, hut an additional terrace with monatomic depth is seen in the middle of the image of Figure 6c. As expected, however, completely different images were observed when the total amount of anodic dissolution was increased. Finally, it is noteworthy that the anodic dissolution is prohibited by the formation of the oxide layer even in a solution containing chloride ions. When the electrode potential was scanned to 1.7 V vs RHE at a scan rate of 50 mV/s and held at this value, randomly oriented monatomic and diatomic height islands were found on the terraces. The diameter of these islands was quite small and in the range of 2-5 nm. No appreciable change in STM images has been observed for at least 20 min as long as the electrode potential was held at 1.7 V. The above result suggests that the anodic dissolution of
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Flgure 8. STM lop views of a 250 X 250 nm' region of a Au(1 11) facet in 0.1 M HCD, coniahing 1 X lo4 M HCI at 0.85 V vs R M : (a) before the potential cycle lo 1.55 V at a scan rate of 50 mV1s: (b) immediiatety after: (c)20 min later. The tip potential and ihe lllnnebng cunent were Um same as indicated in Figure 3. Scan speed was 200
nmls
Au during the anodic potential scan was inhibited by the formation of the oxide layer. The small islands observed on the terraces at 1.7 V should be a result of the anodic disso lution at potentials near the foot of the oxidation wave as discussed above. The STM images observed a t 1.7 V seem to have useful information of an initial stage of the anodic dissolution occurring during the potential scan. It was very difficult to see the initial structure change of anodic dissolution in such an experimental procedure as shown in Figure 6, due
Anal. Chem. 1990, 62, 2429-2436
to faster surface diffusion of Au in the presence of chloride. A more detailed study is now under investigation. €&&try NO.Au, 7440-57-5;HC10~,7601-90-3; Cl-, 16887-00-6; HCl, 7647-01-0; AuOH, 12256-43-8;A u ~ O 1303-58-8. ~,
LITERATURE CITED (1) Hubbard, A. T. Chem. Rev. 1988. 88, 633-656. (2) Ross, P. N.; Wagner, F. T. Advances In Electrochemistry and Electrochefnbl Engineering; Gerlscher, H., Ed.: Wlley: New York, 1984; VOl. 13, pp 69-112. (3) Kdb, D. M. Z . Phys. Chem. (Munlch)1987, 154, 179-199. (4) Yeager, E. J . Electrochem. Soc. 1981, 128, 16OC-171C. (5) Sonnenfeld, R.; Hansma, P. K. Sclence 1988, 232, 211-213. (6) GewMh, A. A.; Bard, A. J. J . Phys. Chem. 1988, 92, 5563-5566. (7) Itaya. K.; Sugawara. S.; Hlgaki. K. J . Phys. Chem. 1988, 92, 6714-6718. (8) Wiechers, J.; Twomey, T.; Koib, D. M.; Behm, R. J. J . Electroanal. Chem. Interfacial Electrochem. 1988, 248, 451-460. (9) Trevor, D. J.; Chidsey, C. E. D.; Loiacono, D. N. Phys. Rev. Len. 1989, 82, 929-932. (10) Green, M. P.: Hanson, K. J.; Scherson, D. A.; Xing, X.; Richter, M.; Ross, P. N.: Can, R.; Lindau, I. J . Phys. Chem. 1989, 93, 218 1-2 184. (11) Itaya. K.; Sugawara, S.; Sashikata, K.; Furuya, N. J . Vac. Scl. Techn d . A 1990, 8 . 515-519. (12) Sonnenfetd, R.; Schneir, J.; Drake, 6.; Hansma, P. K.; Aspnes, D. E. Awl. Php. Len. 1987, 50, 1742-1744. (13) Tomita, E.; Matsuda, N.; Itaya, K. J . Vac. Scl. Techno/.,A 1990, 8 , 534-538. (14) Hallmark, V. M.; Chiang, S.; Rabok, J. F.; Swaien, J. D.; Wilson, R. J. Phys. Rev. Len. 1987, 59, 2879-2882.
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(15) SasMkata, K.; Honbo, H.; Furuya. N.; Itaya, K. Bull. Chem. Soc.Jpn., in press. (16) Clavilier, J.; Armand, D.; Wu, 6 . L. J . € k t f o ~ n a l Chem. . Interfacial Electrochem. 1982, 735, 153-166. (17) Motoo, S.; Furuya, N. J . Electroanal. Chem. IntetfaciaIElectrochem. 1984, 167, 309-315. (18) Motoo, S.; Furuya, N. Ber. Bunsen-Ges. Phys. Chem. 1987, 91. 457-461. (19) Cadle, S. H.; Bruckenstein. S. J . Efectroanal. Chem. InterfaclalEktrochem. 1973. 48, 325-331. (20) Rand, D. A. J.; Woods, R. J . Electroanal. Chem. InterfacialEfectrochem. 1972. 3 5 , 209-218. Bracker, C. E. Science 1988, (21) Hansma. P. K.; Eiings, V. B.; Marti, 0.; 242. 209-216. (22) Itaya, K.; Tomita, E. Surf. Sci. 1988, 207, L507-L512. (23) Schneir, J.; Sonnenfeld, R.; Marti, 0.; Hansma, P. K.; Demuth, J. E.; Hamers. R. J. J . Appl. Phys. 1988, 63, 717-721. (24) D'agostino, A. T.; Ross, P. N. Surf. Sci. 1987, 185, 88-104. (25) Angerstein-Kozbwska, H.; Conway, B. E.; Hamelln, H.;Stolcoviciu, L. J . Electroanal. Chem. Interfacial Electrochem. 1987, 228,429-453. (26) Hamelin, A. Modern Aspect of EIectrochem&fry; Bockris, J. O'M., Conway. B. E., Eds.; Butterworths: London, 1985; Vol. 16, pp 1-101. (27) Cadie, S. H.; Bruckenstein, S. Anal. Chem. 1974. 46, 16-20. (28) Jakievic, R. C.; Elie, L. Phys. Rev. Len. 1988. 60, 120-123.
RECEIVED for review May 14, 1990. Accepted July 18, 1990. This work was supported by Ministry of Education, Science and Culture, Grant-in-Aid for Research No. 63850160 and 480540125498, and a foundation of Nippon Soda Industrial Corporation.
Cyanide Detection Using a Substrate-Regenerating, Peroxidase-Based Biosensor Mark H. Smit and Anthony E. G. Cam* Centre for Biotechnology, Imperial College of Science, Technology, and Medicine, London SW7 2AZ, United Kingdom
An enzyme-based, dual working electrode system Is described for the sendng of cyanlde. Horseradish peroxidase (HRP) Is Incorporated as the senslng element. A contlnuous monltorln~d oxklattve actMty by the enzyme resuJts through the generatkn and rageneratbn of substrates at the electrode surfaces. Thus, HRP Is oxldlzed by hydrogen peroxide generated from dbolved oxygen, at the prbnary electrode, and then reduced through the secondary electrode by mediated electron transfer using ferrocene as a carrler. Ferrocene regeneratlon at thls electrode Is proportlonal to the lntrlnslc actlv#y of HRP. The dynamics of the system are Investigated by uslng a rotatlng rlng-dlsk electrode. The enzyme Is Immoblflzed to provlde better control over its catalytlc actlvlty and to Increase the llfetlme of the biosensor. Cyankle lnhlbIthof current can be modeled by reverdh blndng khetks. Detection of cyanlde is possible In submicromolar (ppb) concentrations, wlth a hall maxknal response at 2 wM. The response tkne for detectlon of Introduced cyanlde Is within 1 8. The sensor can be operated between 5 and 40 OC, and cyanide inhlbl#on Is unaffected by pH changes between 5 and 8. The sensor Is reprocludble for cyanlde determlnatlon and Is stabk for over 6 months.
INTRODUCTION The catalytic activity of enzymes not only is very specific for their substrates but also can be very sensitive to the presence of specific inhibitors. The amplifying nature of 0003-2700/90/0362-2429$02.50/0
enzyme activity coupled to the sensitivity of amperometric devices provides us with the possibility of developing sensitive and specific biosensors for the detection of toxins at very low concentrations. The sensitivity of the device would increase with the toxicity of the substance. A variety of enzymebased amperometric systems have resulted from recent developments in methods for monitoring enzyme activity by electrochemical means. Transduction methods for the determination of enzyme activity are usually based on the electrochemical measurement of substrates or producti, either natural mediators, like oxygen and hydrogen peroxide (I),or artificial mediators, such as the ferrocenes (2). The design of enzyme electrodes is such that the current or potential measured is proportional to the reaction of the substrate with the enzyme. The magnitude of the signal is controlled by the rate-determining step involved in this reaction (3). Many sensors have been described that rely on high enzyme loading; their response is dependent on the transport of substrate to the catalytic site and is thus sensitive to measuring variations in the concentration of substrate. Sensors based on enzyme inhibition, however, rely on the measured response being proportional to the amount of catalytically active enzyme present. These sensors are therefore sensitive to any perturbation of enzyme activity. The theory and application of enzyme inhibition for the detection of substances has recently been addressed (4). This present report describes the development of a sensitive enzyme-based amperometric sensor for the detection and determination of cyanide, using a very stable enzyme, horseradish peroxidase (HRP), and incorporating substrate-gen0 1990 American Chemical Society
