Scannina Electrochefiical Microscopy Royce C. Engst" and Chrlstlne M. Phaa oepartmenlof Chemistry Universlty of South Dakota Vermillion, SO 57069
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Modern electroanalytical chemistry relies on increasingly complicated interfacial designs to achieve improved selectivity, sensitivity, and adaptability. Chemical modifications, polymer films, composite materials, and microelectrode arrays are examples of schemes in which the electrode-solution interface possesses spatial heterogeneity on a dimensional level ranging from the microscopic to the atomic. The chemical and physical structure of such surfaces can now be characterized with spatial resolutions ranging from the micrometer to the atomic levels using techniques such as optical and electron microscopy, ESCA, Raman microprobe, and scanning tunneling micros-
copy (STM).However, these techniques do not directly provide information about electrochemical actiuity. Knowledge of structure-activity relationships could ultimately lead to optimized designs of electroanalytical devices and increase our ability to interpret data generated with complicated electrode designs. There are many examples of questions related to the spatial distribution of electrochemical activity. For instance, what fraction of a composite electrode surface is in electrochemical communication with the solution? Are electrochemical "hot spots" present on surfaces where events such as nucleat'-- -:cur? Do defects exist in a poly-
0003-2700189/A361-1099/$01.50/0 @ 1989 American Chemical Society
mer film, thereby circumventing the role of the film? What are the spatial relationships between centers of chemical activity and electrochemical activity on modified electrodes? Several techniques are used to spatially resolve electrochemical activity. For example, scanning microreference electrodes ( 1 4 , iontophoretic application of an electroactive probe species ( 4 , 5 ) , and imaging of refractive index gradients (6) provide resolution in the range of tens to hundreds of micrometers. Resolution in the range of visible light microscopy is obtained through visualization of electrodeposited material (7), imaging of electrogenerated chemiluminescence (8,9), and the monitoring or ' lized laser-
in
)tocurrents ( Z 0 , I I ) . Scanning electrochemical microscopy (SECM) is the name applied by Bard and co-workers (12)to the process of using a microelectrode to amperometrically or voltammetrically detect material immediately adjacent to the surface of interest. Because an electrochemical proceaa is involved, the technique is carried out in situ, enabling the study of electrochemical activity with high spatial resolution. The current at the microelectrode is faradaic in nature, in contrast to the tunneling currents measured in STM. Although STM has been applied successfully in situ, measures to eliminate faradaic currents must be taken because these currents can easily swamp out tunneling currents (I3,14). The concept behind what has come to he called SECM was fnst demonstrated in 1986,when microelectrodes were used to amperometrically detect
ANALYTICAL CHEMISTRY, VOL. 61. NO. 19. OCTOBER 1. 1989
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chemical species produced at a specimen electrode (25). The potentials of both electrodes are controlled independently (with respect to some reference electrode) using a bipotentiostat (Figure 1).In the presence of a reversible redox couple, the electrodes are operated in a generator-collector mode. The specimen potential is stepped or scanned to a value that causes the “forward” electrochemical reaction to occur, while the microelectrode potential is held constant at a value that causes the “reverse” reaction to occur. Thus, material generated at the specimen electrode diffuses to the microelectrode tip, generating a microelectrode current. Mapping of electrochemical activity is accomplished by incrementally moving the microelectrode over the specimen surface and repeating the specimen potential waveform at each location. If the microelectrode is positioned over a region of electrocbemical activity, the microelectrode generates a current. Over a region of inactivity, however, no microelectrode current flows, or it arises only after a time delay associated with lateral diffusion of material from the generation site to the microelectrode. The microelectrode probes are made of carbon fibers, either in a beveled disk configuration as is commonly used for in vivo voltammetry (16) or as electrochemicallyetched cylinders with tip diameters of 1-3 pm. Positioning of the microelectrode is controlled in three dimensions with a stepper-motor-driven positioner capable of 1-pm increments. Potassium ferrocyanide is 11WA
present in solution at millimolar concentrations. Spatial resolution of electrochemical activity is demonstrated over specimen electrodes prepared as platinum arrays and over heterogeneous electrodes prepared from reticulated vitreous carbon. Both one- and twodimensional “maps” of electrochemical activity are created with a spatial resolution of approximately 20 pm. The technique is also used to spatially resolve concentrations in the direction perpendicular to the specimen electrode surface, permitting the characterization of transient species residing in the diffusion layer of the specimen electrode (17). Resolution in the perpendicular direction is on the order of 23 pn, and a temporal resolution of 20 ms is obtained. The shape of experimentally determined concentration profiles is used to distinguish between two p s i b l e mechanisms for the chemical decomposition of the product of the specimen electrode reaction. Bard and eo-workers (18,19) recently increased the versatility of SECM by employing modes of operation other than the simple generator-collector scheme described above. For example, instead of holding the microelectrode potential constant, its potential is scanned while the specimen electrode is generating, allowing for voltammetric characterization of species in the diffusion layer. Another mode of operation involves application of a sinusoidal waveform to the generator electrode while monitoring the concentration of electrogenerated species with the microelectrode. In this latter case,
ANALYTICAL CHEMISTRY, VOL. 61. NO. 19, OCTOBER 1. 1989
the microelectrode current is a damped sinusoid whose amplitude decreases with increasing distance from the specimen. An especially interesting approach to SECM is the “feedback mode” (19), in which the microelectrode directly electrolyzes a species present in bulk solution and does not serve as the collector of something generated at the specimen. The specimen is not operated as a generator; therefore, it does not have to be connected to a power source. In fact, the specimen need not he a conductor at all. When the distance between the two electrodes is large compared with the size of the microelectrode diffusion layer, the microelectrode current is unaffected by the specimen regardless of whether the specimen is conductive or nonconductive. However, when the microelectrode is close enough so that the specimen is within the diffusion layer of the microelectrode, the microelectrode response is affected in a way that depends on the nature of the specimen. If the specimen is conductive and held at the appropriate potential, the microelectrode current is enhanced because of recycling of electroactive materials between the two electrodes. If the specimen is nonconductive, the microelectrcde current is diminished, because the suhstrate blocks diffusion of electroactive material from bulk solution to the microelectrode. Figure 2, taken from the work of Kwak and Bard (20). illustrates the feedback mode over a conductive specimen. Scans are taken over a section of
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ANALYTICAL CHEMISTRY, VC
gold minigrid, where the grid spacing is 25 pm and the width of the individual goldlinesis7.6pm. When theplatinum microelectrode is located over a grid l i e , recycling of ferrocyanide/ferricyanide provides an increased microelectrode current compared with the current obtained when the microelectrode is over a space in the grid. Figure 3 illustrates scanning over a nonconductive specimen, in this case a 50-pm glass fiber resting on a glass slide. As the 5-pm-radius platinum microelectrode traverses the region of the glass fiber, interference with diffusion of ferrocyanide to the microelectrode leads to decreased current (20).Results from digital simulation (19) show that for a conductive substrate, constructive feedback becomes significant when the microelectrode is closer than approximately 3-5 times ita own radius. When the substrate is nonconductive, interference with diffusion to the microelectrode can be detected at even greater interelectrode distances. SECM is being applied tostudy nonelectrochemical aspects of surfaces as well. Wang et al. (21) have characterized the distribution of biological activity on modified carbon paste electrodes by probing their surfaces with a microelectrode. In this example, the specimen acta as a biochemical generator ~ rather than an electrochemical generator. For example, a carbon paste surface modified with banana tissue converts dopamine to dopamine quinone through the action of polyphenol oxidase. The quinone is 'monitored voltammetrically at the microelectrode, which is rastered incrementally over the surface. Two-dimensional mapa of the biological activity on the carbonpaste surface are obtained. Published work on SECM to date bas demonstrated spatial resolution in the micrometer range, a level far from that of STM even when the latter is carried out in situ. STM relies on currents that become negligible over distances measured in angstroms,whereas SECM relies on the diffusion of chemical species to the microelectrode tip, a process that can occur over much larger distances. In the steady-state feedback mode of SECM, resolution depends on the size, geometry, and stabiliw of the probe tip. In the generator-collector mode, time of measurement becomes important as well; resolution is improved if the microelectrode current is read at short times after the specimen begins generating (22), before the generated species diffuses away from ita site of origin. Yet short-time measurements become difficult because of capacitive I, NO. 19.
OCTOBER 1. 1989
F l @ ~5. n Scans over a 50-pm glass fiber obtained in the feedback mode. cunmtIs d b n i n l M 0" (he I l k a imataen- wlth dimsion to mS pmbe oaw. I n t e r s b bode distance &ueaaa tmmmpm (be).(Mlpaed hom Refmncu 20).
coupling of generator currents to the microelectrode. As the specimen electrode potential is stepped, an induced current spike at the microelectrode prevents measurements before a few milliseconds, during which time the generated species will have diffused over micrometer distances. Decreasing the capacitive coupling of the microelectrode as well as decreasing tip size should improve resolution into the aubmicrometer range. An additional advantage of the feedback mode over the generator-colledor mode is that capacitive coupling is not operative when the specimen is not conducting. In addition to using SECM as a char-
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acterization tool, the faradaic process occurring between the t i p and a substrate has been used in microfabrication. T h e potential difference between tip and substrate can result in the electrochemical deposition of metal on the substrate surface. Bard and co-workers have created silver structures of micrometer dimensions by reducing silver ions at the surface of a Nafion matrix (23),and they have photoelectrochemically etched GaAs surfaces (24). As it evolves, SECM should become a valuable technique for characterization of surface activity. Certainly, one of its most important capabilities is to describe the location of activity on a surface, allowing correlation with known structural features. Although there are interesting problems in the micrometer domain, the resolution of SECM must still be improved considerably if we are t o study many of the problems relevant t o electroanalytical chemistry and to complement available structural information. In addition t o simply identifying the location of activity, SECM should be able t o provide information about microscopically local electron transfer kinetics, rates of mass transfer through microscopic
channels, the fate of chemical species involved in interfacial reactions, and the kinetics of adsorption-desorption Drocesses.
References (1) Isaacs, H. S.; Kissel, G. J. Electroehem. SOC.1972,119,1628. (2) Isaacs, H. S.; Kendig, M. W. Corrosion 1980,36,269. (3) Isaaes, H. S. Localized Corrosion-Cause of Metal Failure, ASTM STP 1972,516, 158. (4) Engstrom, R. C. Anal Chem. 1984, 56,
RW.
(SfEngstrom, R. C.; Weber, M.; Werth, J. Anal. Chem. 1985,57,844. (6) Kragt, H. J.; Earl, D. J.; White, H. S.; Smyrl, W. H. Abstracts of Papers; 22nd Great Lakes Regional Meeting of the
American Chemical Society,Duluth, M N American Chemical Society: Washington, DC, 1989; Abstract 16. ( 7 ) Rubinstein, I. J . Appl. Electrochem.
, .
1983.23.889.
(8) Engstrom, R. C.; Johnson, K. W.; DesJarlais, S. Anal. Chem. 1987,59,670. (9) Engstrom, R. C.; Pharr, C. M.; Koppang, M. D. J. Electroanol. Chem. 1987,
(13) Heben, M. J.; Penner, R. M.; Lewis,
N. S.; Dovek, M. M.; Quate, C. F. Appl. Phys. Lett. 1989,54,1421. (14)Itaya,K.; ~ ~ ~ E. i tsurf, e , sei. 1988, PO? r . m 7 -"_,I"".. (15) Engstrom, R. C.; Weber, M.; Wunder, D. J.; Burgess, R.; Winquist, S. Anal. Chem. 1986,58,844. (16) Kelly, R. S.; Wightman, R. M. Anal. Chim. Acto 1986,267.79. (17) Engstrom, R. C.; Meaney, T.; Tople, R.; Wightman, R. M. Anal. Chem. 1987, 59, 2005. (18) Bard, A. J.; Fan, F. F.; Hwak, J.; Lev, 0. Anal. Chem. 1989,61,132. (19) Kwak, J.; Bard, A. J. Anal. Chem. 1989.61,1221. (20) Kwak, J.; Bard, A. J. Anal. Chem. 1989,61,1794. (21) Wang, J.; Wu,L-H.;Li, R.New Mexico
State University, personal commnnication, 1989. (22) Tople, R., Honors Thesis, University of South Dakota, 1989. (23) Craston, D. H.; Lin, C. W.; Bard, A. J. J. Electrochem. Sac. 1988,135,785. (24) Lin, C. W.; Fan, F-R. R.; Bard, A. J. J. Electrochem. SOC.1987,234,1038.
221,251. (10) Butler. M. A. J . Electrochem. Soe. 1983,130,2358. (11) Butler, M. A. J . Electrochem. SOC. 1984.131,2185. (12) Liu,H.Y.;Fan,R.R.;Lin,C.W.;Bard, A. J. J.Am. Chem. SOC.1986,108,3838.
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Royce C. Engstrom is professor and chairman of the Department of Chemistry at the Uniuersity of South Dakota. He received his B.S. degree in chemistry f r o m the Uniuersity of Nebraska at Omaha in 1975 and his PhD. in chemistry f r o m the Uniuersity of Wisconsin-Madison in 1979. His research interests are in electroanalysis, microanalytical chemistry, and t h e relationship between electrode surface stmcture and electrochemical activity.
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1104A * ANALYTICAL CHEMISTRY, VOL. 61, NO. 19, OCTOBER 1, 1989
Christine M. Pharr is a research associate at the Uniuersity of South Dakota. She received her B.A. degree from
Mount Marty College (SouthDakota) in 1979 and her M.A. degree from the University of South Dakota in 1989. Her research interests are in electrochemistry and biochemistry. S h e has applied luminescence imaging to the s t u d y of electrode processes.
