Characterization of electrochemical responses in picoliter volumes

Cyclic voltammetry is conducted in picoliter microvials to investigate the influence of small volumes on the electrochemical response. Electrochemical experiments using a standard reduction-oxidation couple, ferrocene-carboxylic acid, have been performed in volumes as small as 1 pt. Peak-shaped voltammetry and an increase in the current on the reverse wave of the cyclic voltammogram are observed in the voltammetric response when ultrasmall volumes (16 pL or less) are used. This deviation from bulk microelectrode behavior is observed only at slower scan rates in the smaller microvials. The origins of the voltammetric behavior in the small-volume experiments have been probed by varying experimental conditions such as scan rate, vial size, and concentration. The voltammetric properties have been shown to be dependent on all three parameters. The peak-shaped voltammetry and increases in the current on the reverse wave are attributed to depletion of oxidizable species on the forward scan and reduction of the analyte restricted at the electrode surface, respectively. A physical model based on restriction of analyte in these well-defined microenvironments is proposed to explain the differences in current compared to that predicted by microelectrode theory in bulk solutions.