Electrochemistry in microscopic domains: a model system to interpret voltammetry measurements in restricted biological spaces

The motivation of this work is to provide a simple and robust model system to help quantitatively interpret electrochemical measurements made in restricted biological domains, such as in or near single biological cells, or in microscopic tissue cavities an example being dopamine release measurements in brain. Microscopic aqueous sample droplets of nano- to picoliter volumes were formed on the bottom of a polystyrene dish under heptane which was presaturated with distilled water to keep the droplets stable. The electrochemical cell consisted of a bevelled carbon fiber microdisk working electrode and a reference electrode with a miniature junction. This latter was made of a diffusional microburet (DMB), previously used for diffusional reagent delivery into similar size droplets, containing 0.1 M KCl and an inserted chloridized silver wire. Both electrodes had a nominal tip diameter of 7.5 μm. 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 microelectrode behavior in microscopic domains as compared to bulk volume. These results must be taken into account to conduct and interpret voltammetry measurements in restricted biological spaces correctly. The methodology developed has, however, more general analytical and physicochemical applications as well