a44
Anal. Chem. 1986, 58,844-848
Measurements within the Diffusion Layer Using a Microelectrode Probe Royce C. Engstrom,* Michael Weber, Daniel J. Wunder, Robert Burgess,' and Sharon Winquist Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069
Microelectrodes wlth diameters as small as 10 pm were positioned wlthln the diffusion layer of electrodes havlng conventlonal dlmenslons. A blpotentiostat was used to control the potentials of the macroelectrodeand the microelectrode Independently. The product of an electrochemical reactlon at the macroelectrode was detected by the mlcroelectrode after reaching lt by dlffuslon. Movement of the microelectrode wlth a mlcroposltloning device permitted spatial resolution of electrochemlcally generated concentratlons in a dlrection parallel or perpendicular to the macroelectrode surface. Concentration profiles wlthln the diffusion layer were obtalned as close as 5 pm to the macroelectrode surface and with a spatial resolutlon of 2 pm. Model heterogeneous electrodes were mapped to locate and determine the dlmensions of microscopic centers of electrochemical actlvlty. The technlque was also used to detect a short-lived reactlon intermediate wlth a llfetlme In the mllllsecond domain.
Direct observation of concentration profiles within the diffusion layer at an electrode-solution interface requires an analytical procedure with a high degree of spatial resolution, since the thickness of the diffusion layer is ordinarily less than a few hundred micrometers. The spectroelectrochemical techniques developed by McCreery and co-workers, in which a beam of light is made to travel parallel to the electrode surface, have permitted spatial resolution in a direction perpendicular to the electrode surface (1-5). The initial approach from that group employed a very small, movable slit, situated at one edge of the electrode, through which laser light was passed ( 1 , 2 ) . Absorption measurements could be made within a 25-pm window so that concentration profiles of electrochemically generated chromophores could be obtained within the diffusion layer. The same group employed diffractive spectroelectrochemistry to monitor chemical species as close as 5 pm to an electrode surface (3, 4 ) . Recently, a resolution of 1.2 pm has been achieved by imaging a magnified light beam onto a diode array detector ( 5 ) . Other optical methods of lower resolution have also been reported (6, 7). Spatial resolution in the direction parallel to the electrode surface was carried out by Isaacs, who was able to locate centers of corrosion on metals by scanning over their surfaces with a microreference-microauxiliary electrode arrangement. The spatial resolution achieved was of the order of tens of micrometers (8-11). We have reported on the use of iontophoresis for spatial resolution of electrochemical activity on microheterogeneous electrodes (12, 13). In iontophoresis, a micropipet is used to dispense electroactive material onto a microscopic area of an electrode surface. The distribution of electrochemical activity on solid electrode surfaces was mapped with a resolution of 10 Wm, and the technique was applied to the study of graphite-epoxy electrodes. The microelectrode probe technique described in this paper involved using microelectrodes with diameters as small as 10 Present address: University of Iowa School of Medicine, Iowa
City, IA.
pm to detect chemical species residing within the diffusion layer of another electrode of ordinary dimensions. As depicted in Figure 1, the potentials of the microelectrode probe and the macroelectrode under study were controlled independently of one another with a bipotentiostat. The microelectrode tip was positioned within the diffusion layer of the macroelectrode. As the macroelectrode potential attained a value that resulted in an electrochemical reaction, the product created in the reaction was free to diffuse to the microelectrode. The microelectrode was set at a potential at which the product was itself electroactive, so that a Faradaic current resulted at the microelectrode. The experiment has similarities to those carried out at ring-disk electrodes, where the products of the disk reaction are swept out over the ring and electrolyzed. However, in the present experiment mass transfer occurs by diffusion alone, and the distance between the two electrodes is measured in micrometers. The microelectrodes employed were similar to those in routine use in a number of laboratories, the electrochemical behavior of which has been described in the literature (14-27 and references therein). The electrode tips were small enough that they could be placed within micrometers of another electrode surface without the iR drop problems that would result from positioning two ordinary sized electrodes that close. With the use of a micropositioning device, the microelectrodes were moved in a direction either parallel or perpendicular to the macroelectrode surface, providing spatial resolution of concentrations in three dimensions within the diffusion layer. In this report, the results of applying the microelectrode probe technique to the following problems are described detection of product produced by a potential sweep of the macroelectrode, determination of the concentration profile of the product of a potential-step experiment, mapping of electrochemical activity on a heterogeneous electrode surface, and the detection of a short-lived intermediate produced in a reaction at the macroelectrode.
EXPERIMENTAL SECTION Apparatus. The bipotentiostat was constructed according to a conventional design (18). The critical component in the bipotentiostat was the operational amplifier used for the microelectrode current-to-voltage converter, since subnanoampere currents were involved. An Analog Devices AD515 (Norwood, MA) was selected, and was designed to have a gain of up to 10' V/ A. The remaining operational amplifiers were Analog Devices Model AD301. The bipotentiostat allowed for setting the microelectrode potential at a constant value, and the macroelectrode potential was driven by an external signal generator. A Bioanalytical Systems Model CV37 voltammograph (West Lafayette, IN) served as a signal generator in most cases, and in some instances a laboratory-build potential-stepping circuit was used. The macroelectrode could therefore be made to undergo a potential sweep (forward and reverse) or a potential step. The electrochemical cell was machined from cast acrylic and designed to fit in the condenser stage of a compound microscope. The macroelectrode under study fit into the cell so that its surface faced upward, visible in the field of view through the microscope. For most experiments, the macroelectrode was prepared from a platinum wire having a diameter of 1 mm, embedded in nonconducting epoxy, and machined flat so that the cross-sectional area of the platinum wire was exposed. The electrode was polished
0003-2700/86/0358-0844$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
845
h
Microelectrode
Bipotentiostat
0
Macroelectrode
Vokammograms (microelectrode current vs. macroelectrode potential) of 5 mM potassium ferrocyanide in 1 M potassium chloride. Negative potential limit of macroelectrode was -0.20 V and positive limit was 0.60 V. Microelectrode potential was constant at -0.20 V. Scan rates were as follows: (a) 2 , (b) IO, (c) 25,and (d) 50 mV/s.
Flgure 1.
Flgure 2.
with successively finer grades of emery paper, followed by alumina slurries down to 0.05 pm (Fisher Scientific). For experiments using a glassy carbon macroelectrode, a short length of Tokai grade 30s glassy carbon rod of 3 mm diameter (International Minerals and Chemical Corp., NY) was used after embedding and polishing in a similar way. For experiments requiring a heterogeneous electrode, an epoxy-impregnated reticulated vitreous carbon electrode was prepared as described earlier (12,19),resulting in an electrode surface with irregularly shaped and sized islands of smooth carbon in an inactive epoxy matrix. Carbon fiber microelectrodeswere prepared from Thornell P-55 grade VSB-32 fibers (Union Carbide Corp., Danbury, CT) having nominal diameters of 10 Fm using fabrication techniques described in the literature (14-16). Platinum microelectrodeswere prepared by heat-sealingwires of 25 pm diameter (Goodfellow Metals, Ltd., Cambridge, England) into glass capillaries by heating on a micropipet puller (David Kopf Instruments, Tujunga, CA). The microelectrodes were held at the end of a steel rod connected to a three-dimensional translational stage with a positioning accuracy of 2 pm (Model MR 50, Klinger Scientific Corp., Jamaica, NY). Positioning of the microelectrode was carried out while viewing the microelectrode tip and macroelectrode surface through the microscope. The reference electrode was a saturated calomel electrode, and all potentials are reported vs. its potential. The auxiliary electrode was a platinum wire. The cell, microscope, and positioning device were housed in a Faraday cage to make possible the measurement of subnanoampere levels of current, and the same components were mounted on a vibrationally isolated table constructed in the laboratory. During potential sweep experiments, data were recorded on an X-Y recorder (Model 2000, Houston Instruments, Austin, TX), where the macroelectrode potential was displayed on the x axis and the output of the microelectrode current-to-voltageconverter was displayed on the y axis. During potential-step experiments, the microelectrode current-to-voltage converter output was displayed directly on a strip-chart recorder when the recorder response time was adequate. When rapid data acquisition was called for, the signal was recorded first on a digital storage oscilloscope (Gould Model OS4100, Cleveland, OH) and subsequently dumped to the strip chart recorder. Reagents. Solutions of potassium ferrocyanide in potassium chloride were prepared from reagent grade chemicals used without further purification. Water was purified by distillation followed by passage through a cartridge system (Sybron/Barnstead Nanopure, Boston, MA). Nicotinamide adenine dinucleotide was obtained from Sigma Chemical Co. (St. Louis, MO), and was prepared in 0.10 M reagent grade sodium pyrophosphate and adjusted to pH 8.0 with hydrochloric acid. Procedure. Upon assembly of the apparatus prior to an experiment, a cyclic voltammogram of 1mM potassium ferrocyanide in 1 M potassium chloride was ordinarily taken directly at the microelectrode to ensure its proper functioning. The microelectrode potential was then set at the desired value, and it was moved into position over the macroelectrode surface. Final positioning could usually be accomplished by noting when the microelectrode made contact with the surface of the macroelectrode and then moving the microelectrode away just a few micrometers. The potential of the macroelectrodewas then scanned or stepped from its initial value to some predetermined limit and back. The current through the microelectrode was monitored as a function of time or macroelectrode potential, depending on the experiment.
RESULTS Response to Potential Sweep. In a solution of 5.0 mM potassium ferrocyanide and 1.0 M potassium chloride, the potential of the macroelectrode was scanned from -0.20 to 0.60 V and back, and the current through the microelectrode was monitored while its potential was held constant a t -0.20 V. A series of voltammograms (microelectrode current vs. macroelectrode potential) are shown in Figure 2. Referring to trace a, as the macroelectrode potential reached a value at which ferrocyanide was oxidized to ferricyanide, the microelectrode current increased in the cathodic direction, indicating that ferricyanide produced at the macroelectrode surface had diffused to the microelectrode and was being reduced. The microelectrode current reached a plateau when the macroelectrode potential resulted in a mass-transfer limited reaction. The negative-going scan generated the same response as the positive-going scan, since the rate of ferricyanide production is dependent on the macroelectrode potential and not the direction of potential change. Traces b, c, and d of Figure 2 show that as the potential scan rate of the macroelectrode was increased, a hysteresis developed in the voltammograms. The same effect was observed as the distance between the macroelectrode and microelectrode was increased. The hysteresis can be explained by considering that a t higher scan rates, or at greater interelectrode distances, the macroelectrode potential will have changed a significant amount during the time needed for ferricyanide to diffuse to the microelectrode. An additional effect of increasing interelectrode distance was a decrease in the amplitude of the reponse, since the concentration of ferricyanide available to the microelectrode is smaller the greater the distance away from the macroelectrode. The signal-to-background ratio at the microelectrode was found to be enhanced compared to that at the macroelectrode, and even compared to that at a microelectrode subjected to conventional cyclic voltammetry. Figure 3 shows cyclic voltammograms taken of 0.10 mM potassium ferrocyanide at a 3-mm diameter glassy carbon electrode (trace a), at a carbon fiber microelectrode operated conventionally (its own potential scanned, trace b), and at the microelectrode with a constant potential situated in the diffusion layer of a macroelectrode (trace c). In the first two traces, the Faradaic current resulted from the direct oxidation of ferrocyanide and was superimposed on the charging currents and other background currents that result from a constantly changing potential. As has been adequately described, the microelectrode produces a greater signal-to-background ratio due to radial diffusion (14-16,20). However, when the microelectrode potential was held constant at -0.20 V and its tip positioned within the diffusion layer of a macroelectrode, as in trace c, the Faradaic current resulted from the reduction of ferricyanide and was superimposed on a much lower background current. This same phenomenon has been capitalized upon for analytical purposes in the technique of anodic stripping voltammetry with collection at
Schematic illustration of the microelectrode probe technique. Only the two working electrodes are shown for clarity.
846
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986 I.
a
d
0
Figure 3. Voltammograms of 0.10 mM potassium ferrocyanide in 1.0 M potassium chloride at (a)glassy carbon electrode with 3 mm diam-
eter, (b) carbon fiber microelectrode with 10 pm diameter, (c)carbon fiber of 10 pm diameter at constant potential and positioned within diffusion layer of glassy carbon electrode. Scan rate was 100 mV/s in all cases. Potential settings are as in Figure 2.
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Time, seconds
Experimental current responses (data points) and theoretical concentration behavior (solid lines) after a potential step of the macroelectrode. Distance between macroelectrode and microelectrode Is (a) 10, (b) 30, and (c) 50 pm. Theoretical values were calculated from eq 1 in text. For experimental data, macroelectrode potential was stepped from -0.20 to 0.60 V while microelectrode potential was constant at -0.20 V. Solution conditions are as in Figure 2. Flgure 4.
the ring-disk electrode (21,22)or at dual tubular electrodes (23). Metal ions produced anodically at the disk or upstream electrode are reduced at the constant-potential ring or downstream electrode. Response to Potential Step. Current was monitored through the microelectrode as a function of time as the macroelectrode potential was stepped from -0.20 to 0.60 V in the presence of potassium ferrocyanide. The results, shown as individual data points in Figure 4,indicate that as the distance between the two electrodes was increased, the microelectrode current rose to a smaller value, since the concentration of ferricyanide should be smaller as distance from the macroelectrode increases. In addition, a time delay in the current response became evident at greater interelectrode distances, representing the time needed for ferricyanide produced at the macroelectrode to reach the microelectrode. These two observations are predicted from the theory of chronoamperometry, which yields the following relationship for the product concentration, C(x, t), as a function of time, t, and distance, x , from the macroelectrode surface (24):
In eq 1, C* is the bulk concentration of the reactant (in this case ferrocyanide) and D is the diffusion coefficient of ferricyanide, which was taken from the literature (25). The behavior of eq 1is shown in Figure 4 as solid lines. A more quantitative comparison of theoretical and observed results will be described below, but inspection of Figure 4 shows that the general shape, relative amplitude, and time delay of the potential-step results are predictable.
Distance, urn
Figure 5. Concentration profiles of ferricyanidein the diffusion layer at times of (a) 40, (b) 60, (c) 120, (d) 500, (e) 1000, and (f) 1500 ms. Potential conditions are as in Figure 4. Solid lines are theoretical responses calculated from eq 1; circles are experimental responses.
Direct observation of concentration profiles within the diffusion layer was performed by measuring the microelectrode current at a fixed time following the potential step, with the micropipet positioned at various distances from the macroelectrode surface. Figure 5 shows results of that experiment for several different times. The solid lines are the theoretical predictions based on the use of eq 1,and the circles are experimental values. For the experimental values, the data point acquired with the microelectrode in its closest position to the macroelectrode surface was taken to have a relative response of 1,and the remaining responses were normalized to the first. The agreement with theory is quite good at times as short as 40 ms. At least three factors contribute to the disagreement that does exist. (1)The data taken at the smallest interelectrode distance were assigned a distance of zero, while in reality the microelectrode tip was some distance away from the macroelectrode surface. That distance is not directly determinable, but the discrepancy between theory and experiment indicates it is no more than approximately 5 pm. That amount could be added on to the distances assigned to each of the experimental points, having the effect of shifting the experimental points toward higher distances. The minimum distance varies from one microelectrode to the next, especially when carbon fiber microelectrodes are used, since cutting the microelectrode tip during fabrication frequently produces a jagged tip. (2) The microelectrode is not an ideal probe, but it consumes ferricyanide while measuring its concentration and likely interferes with diffusion of ferrocyanide toward the macroelectrode. We are in the process of investigating the extent to which the concentration profile is altered by the presence of the microelectrode using digital simulation. (3) At short distances the accuracy of the micropositioner, rated by the manufacturer at 2 pm, becomes significant. Considering these uncertainties, we conclude that concentration profiles can be determined in the diffusion layer with a resolution of approximately 2 wm in a direction normal to the macroelectrode surface and with an absolute error in distance of 5 pm or less. We have not attempted to assign absolute values of concentration to the currents measured at the microelectrode, since calibration of the microelectrodes in bulk solution probably does not apply to the measurements made under the geometry of the microprobe experiment. Mapping of Electrode Surface Activity. In this experiment, the microelectrode was moved in a plane parallel to the surface of a heterogeneous electrode. The electrode in this case was either an array of three 100-pm platinum microelectrodes or an epoxy-impregnated reticulated vitreous carbon electrode, the preparation and electrochemical properties of which have been described earlier (12, 19). The
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
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Figure 8. Response to potential steps over an array of three 100 pm diameter platinum microelectrodes with microelectrode probe moved in one dimension. Potential conditions are as in Figure 4. Data points are taken at approximately 20 pm intervals.
distribution of electrochemical activity on the electrode surfaces was determined by using the potential-step experiment described in the previous section as a function of microelectrode position. With ferricyanide present in solution at 5.0 mM, the macroelectrode was again stepped from -0.20 to 0.60 V while the microelectrode was held constant at -0.20 V. The steady-state current attained by the microelectrode after the potential step was taken as a measure of the electrochemical activity of the macroelectrode in the immediate vicinity of the microelectrode tip. Figure 6 shows a single pass of the microelectrode in a straight line of 20 pm increments over the platinum array. The microelectrode response to the potential step increased when the microelectrode tip was in the vicinity of a platinum region and decreased when over the inactive epoxy matrix in which the platinum electrodes were embedded. Base-line resolution of the sites of electrochemical activity was achieved, with the locations and dimensions of the individual sites readily apparent. The difference in maximum response over the three regions may be due to variation in the interelectrode distance as the microelectrode is moved over the array surface. A two-dimensional mapping experiment of a single active region of the RVC-epoxy electrode is shown in Figure 7, where each square represents a single data point taken at 20-pm increments in both the x and y direction. The responses are shown in digitized form, where responses greater than half the maximum response are shaded and responses lower than half the maximum are unshaded. The microelectrode probe technique is capable of defining the size and shape of individual active regions with a resolution of 20 pm. The level of spatial resolution exhibited by the microelectrode probe technique is similar to that achieved by our earlier experiments based on iontophoresis, in which electroactive material was dispensed onto an electrode surface from a micropipet (12, 13). The microelectrode probe technique has the advantage of being applicable to uncharged as well as charged electroactive species, whereas the electrophoretic ejection of material from the micropipet used in iontophoresis requires that the electroactive species be charged. It is anticipated that the spatial resolution 6f the microelectrode probe technique will be improved with the use of even smaller electrodes, such as those that have been prepared with platinum wires having diameters of 0.6 pm (26), an area that we are currently investigating. Detection of Short-Lived Reaction Intermediates. Since the distance between the macroelectrode and the microelectrode is so small, there exists the possibility of detecting a product of the macroeIectrode reaction having a lifetime equal to or greater than the time needed to diffuse to the microelectrode. As an illustration of detecting a short-lived reaction intermediate, the reduction of nicotinamide adenine dinucleotide (NAD) was studied. The electrochemical reduction of the oxidized form, NAD+,has been studied in detail (27, 28) and is thought to proceed via two one-electron
Flgure 7. Response to potential steps over a region of RVC-epoxy electrode with microelectrode moved in two dimensions. Potential conditions are as in Figure 4. Each square represents a single data point taken at 20 pm intervals. Responses greater than half the maximum responses are shaded.
transfers, with the formation of an intermediate free radical, NAD., which rapidly dimerizes NAD+
NAD-
e, H+
NADH
'1,( NAD),
The first reduction step has been observed at potentials somewhat negative of -1.0 V, depending on the electrode material and the solution conditions. A return wave, representing the oxidation of NAD., is ordinarily not observable due to the rapid dimerization. The half-life of the free radical has been estimated at less than 1 ms (29),and the rate constant has been given as 2.2 X lo6 M-'s-l (27). The second reduction step occurs only when its rate exceeds that of the dimerization reaction. A second cathodic wave has been reported at mercury electrodes (27)but has not been observed at solid electrodes. For our purposes, only the first reduction step followed by the dimerization was of interest, with the intent being to observe the oxidation of the free radical at a microelectrode positioned in the diffusion layer of another electrode, before dimerization completely consumed the free radical. In a solution of 1.0 mM NAD+ in 0.1 M sodium pyrophosphate at pH 8.0, the potential of a glassy carbon macroelectrode was scanned from -0.50 to -1.35 V and back. A platinum microelectrode of 25 pm diameter was positioned as close as possible to the glassy carbon surface and its potential set at 0.0 V. Figure 8, trace a, shows an ordinary cyclic voltammogram of the NAD+ reduction at the glassy carbon macroelectrode, showing a peak at -1.20 V for the reduction of NAD+ to NAD.. As expected, no evidence of the return peak was observed. Trace b shows the platinum microelectrode current plotted vs. the glass carbon potential. Because of the high current sensitivity used, a sloping base line indicates imperfect electrical isolation of the two electrodes. However, when the macroelectrode potential reached a value where NAD. was produced, the microelectrode current showed a peak in the anodic direction indicating the conversion of NAD. back to NAD+. In the absence of NAD+,no such peak was observed. The anodic peak could not be due to the oxidation of the dimer, since it is oxidizable only at quite positive potentials (27) and would be observable in an ordinary cyclic voltammogram, which it was not in the present case. The NAD- observed in this experiment must have had a lifetime of at least several milliseconds to be detected by the microelectrode. Considering the high sensitivity needed to observe the signal, most of the NAD. had probably already been
848
Anal. Chem. 1988, 58,848-852
preparing microelectrodes is greatly appreciated. Registry No. NAD', 865-05-4; NAD., 50958-71-9; Pt, 744006-4; K,Fe(CN)G,13943-58-3;C, 7440-44-0.
LITERATURE CITED
V Flgure 8. Cyclic voltammograms of 1 mM NAD' in 0.1 M sodium pyrophosphate, pH 8.0. Voltammograms taken (a) directly at a 3 mm diameter glassy carbon electrode and (b) at a 25 pm diameter platinum electrode set at 0.0 V positioned in diffusion layer of the glassy carbon electrode. Positive potential limit for scans was -0.50 V; negative limit was -1.35 V.
converted to the dimer, indicating a half-life substantially shorter than a few milliseconds. With our present capabilities concerning microelectrode construction and positioning, a reaction intermediate would need to have a lifetime in the millisecond domain to be detected. Admittedly, other electrochemical techniques are capable of detecting intermediates with shorter lifetimes than that. Efforts are under way to decrease the interelectrode distance further so that species with shorter lifetimes can be observed. Because of the second-order dependence of diffusional time on distance, decreasing the interelectrode distance by a modest amount could result in a significant payoff in the detection of short-lived species.
ACKNOWLEDGMENT The carbon fibers were a gift from Gary Mabbott of Colby College. The help of Mark Wightman and his students in
(1) Pruiksma, R.; McCreery, R. L. Anal. Chem. 1979, 57,2253. (2) Pruiksma, R.; McCreery, R. L. Anal. Chem. 1981, 53,202. (3) Rossi, P.; McCurdy, C. W.; McCreery, R. L. J. Am. Chem. SOC.1981, 703,2524. (4) Rossi, P.; McCreery, R . L. J. Nectroanal. Chem. 1983, 757,47. (5) Jan, C.; McCreery, R. L.; Gamble, F. T. Anal. Chem. 1985, 57,1764. (6) McLarnon, F. R.; Muller, R. H.; Tobias, C. W. J Electrochem. SOC. 1982, 729, 2201. (7) Awadura, Y.; Kondo, Y. J . Electrochem. Soc. 1976, 723, 1184. (8) Isaacs, H. S.;Kissei, G. J. Nectrochem. SOC.1972, 779, 1628. (9) Isaacs, H. S.;Kendig, M. W. Corroslon (Houston) 1980, 36,269. (10) Isaacs, H. S. Localized Corros.-Cause of Metal Failure 1972, 576, 158. (11) Isaacs, H. S.; Vyas, B. Nectrochem. Corros. Test. 727. (12) Engstrom, R. C. Anal. Chem. 1984, 56,890. (13) Engstrom, R. C.; Weber, M. W.; Werth, J. Anal. Chem. 1985. (14) Wightman, R. M. Anal. Chem. 1981, 53, 1125A. (15) Dayton, M. A.; Brown, J. C.; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1980, 82, 846. (16) Ewing, A. G.; Dayton, M. A.; Wightman, R. M. Anal. Chem. 1981, 53, 1842. (17) Hepel, T.; Osteryoung, J. J. Phys. Chem. 1982, 8 6 , 1406. (16) Bard, A. J.; Faulkner, L. "Electrochemical Methods"; Wiiey: New York, 1980; p 566. (19) Sleszynski, N.; Osteryoung, J.; Carter, M. Anal. Chem. 1984, 56, 130. (20) Welsshaar, D. E.; Tallman, D. E. Anal. Chem. 1983, 55, 1146. (21) Johnson, D. C.; Allen, R. 6.Talanta 1973, 20,305. (22) Laser, D.; Ariel, M. J . Nectroanal. Chem. 1974, 49, 123. (23) Schieffer, G. W.; Blaedel, W. J. Anal. Chem. 1977, 49, 49. (24) Bard, A. J.; Faulkner, L. "Electrochemical Methods": Wllev: New York, 1980; p 180. (25) Adams, R. N. "Electrochemistry at Solid Electrodes"; Marcel Dekker: New York, 1969; p 219. (26) Fleischmann, M.; Lasserri, F.; Roblnson, J.; Swan, D. J . Electroanal. Chem. 1984, 777,97. (27) Schmakel, C. 0.; Santhanam, K. S. V.; Eiving, P. J. J. Am. Chem. SOC. 1975, 75,5083. (28) Burnett, J. N.; Underwood, A. L. Blochemisfry 1965, 4 , 2060. (29) Wilson, A. M.; Epple, D. 0.Biochemistry 1988, 5,3170.
RECEIVED for review August 9,1985. Accepted November 22, 1985. This work was supported in part by the National Science Foundation, Grant CHE-8411000.
Electrocatalytic Response of Cobalt Phthalocyanine Chemically Modified Electrodes toward Oxalic Acid and a-Keto Acids Leone1 M. Santos and Richard P. Baldwin* Department of Chemistry, University of Louisville, Louisville, Kentucky 40292 Oxalic acid and several a-keto acids (pyruvic, phenylpyruvic, a-ketobutyric, a-ketoglutaric, and a-ketolsocaprolc) exhibited electrocataiytlc behavior at chemically modified carbon paste electrodes contalnlng cobalt phthaiocyanlne Incorporated Into the paste mixture. All underwent oxldation at unmodified carbon electrodes at potentlais more posltlve than +1.2 V vs. Ag/AgCI but gave substantlal anodlc peaks between +0.75 and 0.90 V at the cobalt phthalocyanine contalnlng surface. When used In LCEC, the modified electrodes permitted detection of the aclds at correspondingly lower potentlais than at conventlonal electrodes. As a result, quantltatlon in urine samples was possible with no sample treatment other than dllutlon and partlculate filtratlon. The detection llmlt for oxalic acid at +0.75 V was 0.3 pmol injected and less than 1 nmol for the a-keto aclds.
Over the past decade, electrochemical measurement tech-
niques have achieved wide acceptance as a sensitive and selective approach for the detection of numerous compounds following high-performance liquid chromatography. For example, liquid chromatography with electrochemical detection (LCEC) now represents a powerful method for the determination of phenols and catechols, aromatic amines, thiols, and nitro compounds in complex physiological and environmental samples (1,2). However, because the principal requirement of the technique is that the analyte of interest undergoes oxidation or reduction at a comparatively low potential, many important but difficult-to-electrolyzespecies are not accessible to LCEC. A particular problem arises for compounds whose electrode reactions involve slow electron transfer kinetics and therefore occur at an appreciable rate only at potentials drastically exceeding their thermodynamic redox potentials. For these compounds, LCEC detection usually cannot provide optimum sensitivity and selectivity and, in extreme cases, can provide no usable quantitation at all.
0003-2700/86/0358-0848$01.50/00 1986 American Chemical Society
