Anal. Chem. 2001, 73, 930-938
A Positionable Microcell for Electrochemistry and Scanning Electrochemical Microscopy in Subnanoliter Volumes Thomas W. Spaine† and John E. Baur*
Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160
Positionable voltammetric cells with tip diameters of 100 µm on a side at tip-substrate separations of 1-2 µm without appreciable tilt. Microdroplets of mediator solution were placed onto the substrate using a micropipet attached to a Picospritzer II (General Valve Co., Fairfield, NJ). All experiments involving these microdroplets were carried out in a constant-humidity chamber consisting of a Teflon cell and a Plexiglas cover with an access hole (Figure 2). The substrate electrode was pressed into the bottom of the cell and elevated above the cell floor. Water was added to the chamber and allowed to soak absorbent paper lining the interior of the cell via an access hole. The water level was always (25) Wipf, D. O.; Ge, F.; Spaine, T. W.; Baur, J. E. Anal. Chem. 2000, 72, 49214927.
Figure 3. Cyclic voltammograms (v ) 0.10 V s-1) in bulk solutions of 1.0 mM Ru(NH3)63+ in pH 7 phosphate buffer recorded using a microcell. (A) Voltammograms recorded with an external (dashed line) and an internal (solid line) reference electrode. (B) Voltammograms from (A) plotted on a normalized potential axis. (C) Voltammograms recorded during immersion in 1.0 mM Ru(NH3)63+ over 24 h at a microcell with an agar salt bridge at the tip of the internal reference opening. (D) Voltammograms recorded during immersion in 1.0 mM Ru(NH3)63+ for 24 h at a microcell with no salt bridge. The voltammograms in (C) and (D) were recorded using the internal reference electrode.
maintained below the substrate electrode surface. Before the microdroplet was added, the atmosphere inside the chamber was allowed to equilibrate for at least 15 min. In experiments where microdroplets were formed on the glass substrate, the glass was first silanized by application of a drop of chlorotrimethylsilane before the substrate was enclosed in the humidity chamber. A BAS 100B voltammetric analyzer (Bioanalytical Systems) was used to record cyclic voltammograms, to control the potential during electrodeposition of iridium oxide onto the carbon fiber substrate, and to make measurements of cell resistance. All voltammetric measurements with the external commercial reference electrode (Bioanalytical Systems) were made in a threeelectrode cell. All voltammetric measurements with the internal reference electrode were made in a two-electrode cell. Reagents. Reagents were used as received from commercial sources. House water was initially purified by reverse osmosis and ion exchange and then was further purified using a NanoPure system (Barnstead Thermolyne, Dubuque, IA). Iridium oxide deposition solutions were prepared from Na3IrCl6 or Na2IrCl6 (Aldrich) as described previously.26 RESULTS AND DISCUSSION Microcell Characterization. Figure 3A compares voltammograms of 1.0 mM Ru(NH3)63+ recorded at a microcell using the
internal reference electrode (two-electrode cell) and using an external commercial Ag/AgCl reference electrode (three-electrode cell). The most notable difference between the two voltammograms is the positive shift in E1/2 when the internal reference electrode is used. This shift does not arise from cell resistance, as one would expect the E1/2 for a reduction to become more negative with a large ohmic potential.27 Rather, the shift is a result of the type of wire used to make the reference electrodes. Silver wires small enough to fit into the 0.25-mm-diameter channels of the theta glass quickly broke under the stress of repeated connection and disconnection of the potentiostat leads. Attempts to attach silver wire to copper hookup wires with silver epoxy always resulted in a joint too large to fit within the theta glass channel and/or too weak to be inserted a sufficient distance. Silvercoated copper wire-wrap wire was found to have the necessary mechanical and electrochemical properties to make a satisfactory reference electrode. These wires produced reference electrodes with stable equilibrium potentials 100 mV more positive than a commercial Ag/AgCl reference (Table 1), consistent with the potential shift observed in the voltammograms of Figure 3. Further evidence of the similarity in voltammetric response between the internal and external reference electrodes can be seen in Figure 3B. Here the two voltammograms from Figure 3A are plotted on a normalized potential scale. It is clear from the nearly
(26) Baur, J. E.; Spaine, T. W. J. Electroanal. Chem. 1998, 443, 208-216.
(27) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980; p 230.
Analytical Chemistry, Vol. 73, No. 5, March 1, 2001
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Table 1. Half-Wave Potentials (E1/2) and Cell Resistances (Rcell) for the Microcellsa
internal reference external reference difference
E1/2 (V)
Rcell (MΩ)
-99 ( 14 -203 ( 4 104 ( 9
2.7 ( 0.4 2.0 (0.1 0.7 ( 0.2
a Results are the average and standard deviation of five measurements with four microcells.
identical shapes of the voltammograms that there is no appreciable increase in cell resistance when the internal reference electrode is used. To confirm that these cell resistances were not an important factor when the internal reference electrode was used, the cell resistances for the microcell were compared to those measured using an external reference electrode (Table 1). Although the microcells on average have a higher resistance, the difference of 0.7 MΩ will result in a difference in ohmic potential of
