Electrochemistry in microscopic domains. 1. The electrochemical cell and its voltammetric and amperometric response

Microscopic aqueous sample droplets of nano- and picoliter volumes were formed on the bottom of a polystyrene dish under water-saturated heptane. The electrochemical cell consisted of a beveled carbon fiber microdisk working electrode and a reference electrode with a miniature junction, both inserted into the studied droplet. Both electrodes had a nominal tip diameter of 7.5 microns. Cyclic voltammetry and chronoamperometry in droplets of 3.3 mM ruthenium hexaammine trichloride in 0.1 M KCl solution were performed with this system. The experiments revealed for the first time major deviations in both voltammetric and amperometric microelectrode behavior in picoliter domains as compared to nanoliter volume and bulk solution. The concept and criteria of electrochemical microscopicity of volume is discussed. This work also provides a simple and robust experimental model system to verify electrochemical experiments in restricted domains such as in or near single biological cells or in microscopic tissue cavities. The methodology developed has, however, more general analytical and physicochemical applications as well.