Anal. Chem. 1999, 71, 4944-4950
Imaging of Nonuniform Current Density at Microelectrodes by Electrogenerated Chemiluminescence Russell G. Maus, Erin M. McDonald, and R. Mark Wightman*
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290
The chemiluminescence arising from reaction of electrogenerated radical cations of 9,10-diphenylanthracene (DPA) and benzonitrile (solvent) radical anions has been used to image microelectrodes with dimensions in the micrometer range. Experimental conditions including supporting electrolyte, DPA concentration, and excitation frequency were optimized to affect high luminescent intensity. In solutions of high resistance, the light was found to be temporally delayed with respect to the applied potential due to the increased time required to charge the double layer. Spatially nonuniform light at disk- and bandshaped microelectrodes was observed under certain conditions, with the highest intensity occurring at the region of the electrode with highest curvature. The optimum condition for observation of the nonuniform light was with very high electrode currents. Under this condition, the current density approaches that of the primary current distribution, a circumstance where spatially nonuniform potentials occur. This phenomenon was also examined at a conical electrode as a method of reducing the emission area. A submicrometer-size light source was obtained at high frequencies with an electrode that had a significantly larger uninsulated area. The electrogeneration of transient intermediates and their subsequent reaction often results in the production of light in solution near an electrode surface. This phenomenon, referred to as electrogenerated chemiluminescence (ECL),1 can be used to image electrode surfaces and reveal the chemical and physical events that control the electrochemical reactions.2 Engstrom and co-workers used this approach to examine diffusional fluxes at disk electrodes.3,4 By observing the chemiluminescence arising during luminol oxidation, the evolution from uniform flux at short times to the edge-dominated flux at longer times was revealed.5 Recently we showed that ECL provides a useful way to reveal the geometry of irregularly shaped electrodes.6 In that work, electrode (1) Bard, A. J.; Fan, F.-R. F. Acc. Chem. Res. 1996, 29, 572-578. (2) Pantano, P.; Kuhr, W. G. Anal. Chem. 1993, 65, 2452-2458. (3) Engstrom, R. C.; Pharr, C. M.; Koppang, M. D. J. Electroanal. Chem. 1987, 221, 251-255. (4) Engstrom, R. C.; Johnson, K. W.; DesJarlais, S. Anal. Chem. 1987, 59, 670673. (5) Pharr, C. M.; Engstrom, R. C.; Tople, T. K.; Bee, T. K.; Unzelman, P. L. J. Electroanal. Chem. 1990, 278, 119-128. (6) Wightman, R. M.; Curtis, C. L.; Flowers, P. A.; Maus, R. G.; McDonald, E. M. J. Phys. Chem. B 1998, 102, 9991-9996.
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surfaces were imaged with the luminescence arising from the annihilation reaction between the 9,10-diphenylanthracene (DPA) radical cation and the radical anion of the solvent, benzonitrile. The electrode potential was a high-frequency square wave so that the diffusion layer, and its associated light-producing reaction layer, were kept in close proximity to the electrode yielding high resolution. The light emission from ECL also has been used as an imaging source in scanning optical microscopy.7 Central to the use of ECL in imaging applications is the selection of a chemical system that provides high light intensity. An ideal chemical system for such applications would have high efficiency of photons produced per electron and would employ high concentrations of reagents to maximize the emission intensity. Certain ECL reactions that produce light as a result of annihilation reactions of radical anions and cations have been shown to be highly efficient.8,9 For example, the ECL efficiency for the annihilation reaction of DPA radical ions approaches 0.25 in dimethoxyethane solvent with low concentrations of supporting electrolyte.10 Annihilation ECL reactions are advantageous in that the original reagents are regenerated. We have shown that the DPA•+/benzonitrile•- system is particularly useful in imaging applications.6 In this work, we have explored imaging of several microelectrodes with different geometries using the DPA•+/benzonitrile•system. The relatively low time constant of these electrodes allowed the use of high-speed potential pulses ensuring high spatial resolution. As we have previously shown, the images reveal that the generation of reagents is spatially nonuniform.6 This nonuniformity is shown to be a result of the nonuniform current distribution at the electrode surface that occurs under conditions of high current density. At any conductor, including solutions, the surface charge density is greatest where the surface has greatest curvature. There is a maximal flux at the perimeter of a disk electrode or at the apex of a conical electrode. Evidence for this in electrochemical processes is obtained during electroplating where nonuniform electrodeposition is frequently found. The nonuniform distribution at a disk electrode has been treated in detail by Newman.11 The analyses show that while the electric field in solution at the perimeter of a disk is large, it is much (7) Fan, F. F. R.; Cliffel, D.; Bard, A. J. Anal. Chem. 1998, 70, 2941-2948. (8) Keszthelyi, C. P.; Tokel-Takvoryan, N. E.; Bard, A. J. Anal. Chem. 1975, 47, 249-255. (9) Beldeman, F. E.; Hercules, D. M. J. Phys. Chem. 1979, 83, 2203-2209. (10) Maness, K. M.; Wightman, R. M. J. Electroanal. Chem. 1995, 396, 85-95. (11) Newman, J. J. Electrochem. Soc. 1966, 113, 501-502. 10.1021/ac9905827 CCC: $18.00
© 1999 American Chemical Society Published on Web 10/05/1999
smaller at the center of the disk. As will be shown, the ECL images reveal this to be the case. EXPERIMENTAL SECTION Microscopic Imaging. The imaging system consisted of an inverted stage microscope (Zeiss Axiovert 100 TV, Thornwood, NY) with an electrochemical cell placed over the objective (32×, NA 0.4; 40×, NA 1.3, Zeiss). ECL emission was directed to a Hamamatsu (Bridgewater, NJ) R4632 photomultiplier tube (PMT) operated at -800 V (Bertran Assoc. series 230, Hicksville, NY). The output of the PMT was amplified (EG&G Ortec TV120A, Oak Ridge, TN) and directed to a multichannel scaler (EG&G Ortec T-914). A mirror was used to alternatively direct the ECL to the eyepieces and a B/W, thermoelectrically cooled, digital CCD camera (768 × 512 picture elements, Photometrics Sensys, Tuscon, AZ). Image information from the camera was sent directly to the computer via a data acquisition card. Camera control as well as image analysis was carried out using IPLab software (v. 2.281, Scanalytics, Fairfax, VA). Relative scale within the images was ascertained by acquiring images of a standard objective micrometer. The electrochemical cell was a ∼1.5-in.-diameter glass dish with a microscope cover slip serving as the bottom (25-mm diameter. 0.17-mm thickness). The counter electrode was a silver foil with roughly 50 mm2 surface area. Solutions for electrochemical analysis were deoxygenated by bubbling with nitrogen for a period of at least 30 min prior to transfer to the electrochemical cell. A potential square wave was applied between the microelectrode and reference electrode from a waveform generator (HewlettPackard 8116A, Englewood, CO). A digital oscilloscope (Tektronix TDS 380, Wilsonville, OR) was used to monitor the waveforms. DigiSim finite difference electrochemical simulation software (Bioanalytical Systems, West Lafayette, IN) was used to simulate cyclic voltammograms. Electrode radii were characterized electrochemically from the limiting current during steady-state cyclic voltammetry in acetonitrile containing 1 mM ferrocene (D ) 2.17 × 10-5 cm2/s)12 and 0.1 M tetrabutylammonium hexafluorophosphate (TBAH). Planar Electrodes. Microdisk electrodes13 were constructed by sealing 5-µm-radius Pt microwires (Goodfellow, Cambridge, U.K.) into a soft glass capillary tube (2-mm o.d., 1-mm i.d., Frederick Haer, Bowdoinham, ME). Band electrodes were constructed by sealing thins films of platinum (Goodfellow) between sheets of glass with epoxy.14 The electrodes were then ground on successively finer grits (500, 1000, 1500) of sandpaper to remove excess glass and create a planar surface. Electrode surfaces were then polished with diamond pastes (Buehler, Lake Bluff, IL) suspended in oil. Disk electrodes received a final polish with 0.05-µm alumina (Buehler) suspended in deionized water. Conical Microelectrodes. Conical microelectrodes were prepared from Pt/Ir wires (0.25-mm diameter, Goodfellow, 90/ 10 Pt/Ir) as described by Bard and co-workers.15 The tip was (12) Baur, J. E.; Wightman, R. M. J. Electroanal. Chem. 1991, 305, 73-81. (13) Wightman, R. M.; Wipf, D. O. Voltammetry at Ultramicroelectrodes. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; pp 267-353. (14) Bartelt, J. E.; Drew, S. M.; Wightman, R. M. J. Electrochem. Soc. 1992, 139, 70-74. (15) Gewirth, A. A.; Cranston, D. H.; Bard, A. J. J. Electroanal. Chem. 1989, 261, 477-482.
etched in acidic calcium chloride by application of an ac current (initially 500 mA). This procedure produces electrodes with aspect ratio of ∼2. The cones were insulated by translation through molten polyethylene (linear low density, melt index 100, Aldrich, Milwaukee, WI) either manually or by using an inchworm translator (CE-1000 control module, Burleigh, Fishers, NY) at a speed of 150 µm/s.6 The length of the tip that was coated with polyethylene was ∼8 mm. The thickness of the polymer coating ranged from vanishingly thin at the tip to ∼0.5 mm (total outside diameter of the electrode was 1 mm). Flow Cell Apparatus. ECL was characterized in a flow injection analysis system16 with a channel-type electrochemical cell. The floor of the channel was fabricated from epoxy into which was embedded a disk microelectrode and a silver band counter. The ceiling of the channel contained an optical window. A Hamamatsu R5600P PMT operated at -800 V (Bertran Assoc. 230, Hicksville, NY) in conjunction with the EG&G photoncounting apparatus was placed adjacent to this window. The working electrode was connected to a fast current-to-voltage converter built in-house, and the electrochemical signals were recorded with a digital oscilloscope (Tektronix TDS 380, Wilsonville, OR). Flow was at 200 µL/min, and solutions for ECL were introduced to the cell with the loop injector. During ECL generation, the potential waveform was a continuous, 1-kHz square wave. The potential limits of the square wave were at least 200 mV beyond the E1/2 values for each of the redox couples using the values determined from cyclic voltammograms to ensure diffusion control. PMT counts were binned at 100 kHz for each cycle of the square wave, and 1000 collection scans were summed to yield a 1-s total collection. Reagents. Acetonitrile (UV grade, Burdick and Jackson, Muskegon, MI) and benzonitrile (HPLC grade, Aldrich) were used as received. Ferrocene (Aldrich) was sublimed. 9,10-Diphenylanthracene was recrystallized twice from absolute ethanol. Tetrabutylammonium hexafluorophosphate (Fluka) was used as received. DPA-containing solutions were deoxygenated with solventsaturated nitrogen before use. RESULTS AND DISCUSSION Resistance of Benzonitrile Solutions. Previously we showed that benzonitrile has several characteristics that make it desirable as a solvent for ECL.6 These include a relatively high viscosity (1.267 cP) that mitigates against rapid solvent evaporation and a moderate dielectric constant (25) that allows salt dissolution. However, as will be shown, the composition of the solution can have a dramatic effect on the temporal and spatial characteristics of the ECL response. The specific resistance was evaluated by examination of the peak separation in cyclic voltammograms for ferrocene. In these studies, disk electrodes of relatively large diameter were used (r ) 62 µm) to ensure linear diffusion. At moderate scan rates (50 V/s) where the heterogeneous exchange rate constant can be considered sufficiently large that it has little effect on peak separation, the major source of peak separation is ohmic drop.17 Cyclic voltammograms of 1 mM ferrocene in benzonitrile containing varying concentrations of TBAH are shown in Figure (16) Collinson, M. M.; Wightman, R. M. Anal. Chem. 1993, 65, 2576-2582. (17) Wipf, D. O.; Wightman, R. M. Anal. Chem. 1990, 62, 98-102.
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Figure 1. Cyclic voltammograms of 1 mM ferrocene in benzonitrile recorded with varying concentrations of TBAH supporting electrolyte. (Solid line, raw data; dotted line, simulation.)
1. Superimposed on the experimental data are finite-difference simulations of the cyclic voltammograms which account for the ohmic drop of the solutions. At the lowest concentration of supporting electrolyte evaluated, 1 mM (equal to the ferrocene concentration), the apparent specific resistance was 13 kΩ‚cm. Note, however, this value is an underestimate of the resistivity of the electrolyte because ferrocenium generated during the electrolysis will decrease the resistance. The specific resistances with 0.1, 0.025, and 0.01 M electrolyte were 860, 3000, and 4100 Ω‚cm, respectively. The first value is in reasonable agreement with the value (480 Ω‚cm) obtained in 0.1 M tetrabutylammonium perchlorate.18 The nonlinearity of the resistance values with electrolyte concentration indicates incomplete dissociation of the electrolyte. Temporal Characteristics of the ECL Response. The effect of the cell time constant on the temporal response of the ECL in 5 mM DPA is shown in Figure 2. In these experiments, the light generation reaction is between the DPA radical cation and the benzonitrile radical anion (DPA•+/benzonitrile•-).6 As we have shown previously, although this reaction generates a higher intensity than a DPA•+/DPA•- system, light is seen on the positive step only. The electrode time constant prevents the electrode potential from achieving the desired value until several tens of microseconds after its application. This results in a delayed formation of the desired electrochemically generated radical and a delay in light emission.19 This is true even though the potential limits were adjusted to partially overcome the ohmic drop. (18) Kadish, K. M.; Ding, J. Q.; Malinski, T. Anal. Chem. 1984, 56, 1741-1744.
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Figure 2. Temporal ECL emission profile during 1-kHz potential pulses (50% duty cycle potential square wave, negative step first) sufficient to generate DPA radical cation and benzonitrile radical anion at a 2.5-µm-radius Au disk electrode measured in the flow injection apparatus. Solution consisted of 5 mM DPA in benzonitrile with varying concentrations of supporting electrolyte (TBAH). The traces were recorded with a neutral density filter (OD ) 2) between the electrode and PMT. The counts are in 1-µs bins and summed for 1000 potential cycles. Inset shows time-averaged counts per second (corrected for the presence of the neutral density filter) for solutions containing different concentrations of supporting electrolyte (N ) 4). The applied potential profile is shown as an unreferenced dashed line at the top. (Concentrations on graph refer to TBAH.)
Supporting electrolyte concentrations of 0.1, 0.05, 0.01, and 0.005 M yielded delay times for light emission of 22, 50, 116, and 148 µs, respectively, for light intensity to reach 10% of maximum. The intensity measured during one complete cycle, 1.3 × 107 photons/s, varied little with electrolyte concentration (Figure 2, inset). Maximal light output during the first 250 ms after the time delay, however, was significantly increased as the supporting electrolyte concentration was diminished, contrary to the predictions of finite-difference simulations.19 Annihilation ECL tends to be more efficient with low electrolyte in solvents of low dielectric constant because the encounter complex formed between electrogenerated ions has a greater lifetime facilitating the lightproducing electron exchange.10 Only at the lowest concentation of supporting electrolyte is the maximal light intensity decreased. Under these conditions, the slow rise time of the electrode and the consequent removal of benzonitrile radical cation by its electrooxidation negates the increased efficiency of the reaction. Spatial Nonuniformity of the ECL Response. ECL responses at disk electrodes as a function of the applied square wave frequency were recorded with the imaging system and microscope combination. The exposure time for imaging was always at least 500 ms or at least 10× greater than the period of the excitation waveform, ensuring that the observed signal was a time-averaged ECL response. At low frequencies (
