Electrochemistry in small drops and vials

Exploration of electrochemical behavior in small volume containers requires fabrication methods capable of defining micrometer size structures from a suitable substrate. Initially, Bowyer et al. (88) sandwiched silver and platinum foil between layers of Tefzel film and glass to form an electrochemical cell with auxiliary, reference and working band electrodes, the smallest of which measured 4 pm thick. This electrochemical cell was used to investigate the differential pulse, normal pulse, and cyclic voltammetric behavior of ferro-cence in aqueous solutions with volumes ranging from 2 mL to 50 nL. 

A second method of droplet formation involves modifying the surface of the working electrode with a small droplet of the solution to be examined. This method has the advantage that the droplet does not have to contain any deliberately added supporting electrolyte as the droplet-coated electrode is inserted in an immiscible conductive solution to perform the analysis. The electrochemistry of electrode immobilized droplets has been thoroughly reviewed by Banks et al. (99). 

The examination of the voltanunetric response at carbon fiber UMEs as a function of miCTOvial volume was undertaken by Clark et al. (89). The experimental volume was varied from about 4 nL to as little as 1 pL. For a 5 pm diameter electrode, the volammograms of ferrocenecarboxylic acid did not vary as a function of sample volume. The shape of the voltammograms was sigmoidal which is expected for disk microelectrodes scanned at slow rates (1-1000 mV/sec) under steady-state conditions (102). The half wave potential measured in the microvials was identical to that of ferrocenecarboxylic acid in bulk solution and the current value also matched the expected value. In addition, Clark et al. performed voltammetry with a 1 pm diameter flame etched carbon fiber electrode in a 1 pL vial however, no deviation from bulk solution behavior was apparent. 

Peak shaped voltammograms at slow scan rates only appeared in smaller vials (16 pL or less) and can be explained by comparison of diffusion in bulk solution vs. diffusion in microvials. In microvials the diffusion profile to the electrode is altered due to the vial boundaries. Thus, depletion of electroactive molecules near small electrodes can be achieved in the smallest vials. In bulk solution the amount of molecules reduced at a microelectrode is negligible compared to the bulk analyte concentration. But, in a 16-pL vial the data show that 21% of the total analyte in solution is oxidized by a 5-pm electrode scanned at 0.1 V/sec. These data suggest that bulk electrolysis in microvials can be easily implemented to determine the total amount of analyte present in a vial. 

Experiments in microvials also reveal an increase in current on the reverse wave for slow scan rates. After ruling out any effect from interactions between the analytes and the microvial surface, a diffusion-based explanation was formulated. By determining the concentration dependence of the ratio of reduction currents in microvials and bulk solution 

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