Michael V. Mirkin, Department of Chemistry, Queens College - CUNY What is the Upper Limit for the Standard Rate Constant of a Heterogeneous Electron Transfer Reaction? Michael V. Mirkin, Department of Chemistry, Queens College - CUNY Fabrication of recessed nanoelectrodes (A) A polished disk-type nanoelectrode is prepared by heat-pulling of a Pt wire into glass capillary. (B) A nanocavity inside the glass sheath is produced by etching away a thin layer of Pt. (C) Oxidized form of the mediator is reduced at the etched tip and regenerated at the conductive substrate. Electron transfer at nanoelecrodes By using nanoelectrodes to study kinetics of interfacial electron transfer (ET), one can attain very high mass-transfer rate and address a long-standing question: how fast can be an outersphere ET reaction? The kinetics of several fast heterogeneous ET reactions were investigated by steady-state voltammetry at nanoelectrodes and scanning electrochemical microscopy (SECM). A number of experimental curves were obtained at the same nanoelectrode to attain the accuracy and reproducibility similar to those reported previously for micrometer-sized probes. The mass-transfer rate was varied by changing the distance between a nanometer sized Pt tip and a conductive substrate, d. The measured kinetics were somewhat faster than those observed previously at larger electrodes. The fastest rate constant (k° =17.0 ± 0.9 cm/s for Ru(NH3)63+ reduction in 0.1 M KCl) would be hard to measure by other electrochemical techniques. Nanoelectrodes can be polished under videomicroscopic control The shape parameters, a = 84.3 nm and l = 26.1 nm, were determined (A) from steady-state voltammograms obtained before (1) and after etching (2); and (B) by fitting an SECM current-distance curve to the theory (solid line). Experimental (symbols) and theoretical (sold lines) steady-state voltammograms of 1 mM ferrocenemethanol obtained at different tip/substrate separation distances.