Electrochemistry of individual molecules in zeptoliter volumes

Electrochemical experiments were carried out in a nanometer-sized cylindrical thin layer cell (TLC) formed by etching the surface of a disk-type platinum nanoelectrode (5- to 150-nm radius). Using high frequency ac voltage, the surface of such an electrode was etched to remove a very thin (> or = 1-nm-thick) layer of Pt. The resulting zeptoliter-scale cavity inside the glass sheath was filled with aqueous solution containing redox species, and the etched electrode was immersed in a dry (no external solution) pool of mercury to produce a TLC. Several approaches based on steady-state voltammetry and scanning electrochemical microscopy (SECM) were developed to independently evaluate the electrode radius and the etched volume. The number of redox molecules in the TLC could be varied between one and a few hundred by changing its volume and solution concentration. In this way, the transition between a random and deterministic number of trapped molecules was observed. High quality steady-state voltammograms of > or = 1 molecules were obtained for different neutral and charged redox species and different concentrations of supporting electrolyte. The analysis of such voltammograms yields information about mass transfer, adsorption, electron transfer kinetics, and double-layer effects on the nanoscale.