Mediated Charge Transfer at Nanoelectrodes: A New Approach to Electrochemical Reactivity Mapping and Nanosensing.

Scanning electrochemical microscopy (SECM) is a powerful tool for mapping surface reactivity. Electrochemical mapping of electrocatalytic processes at the nanoscale is, however, challenging because the surface of a nanoelectrode tip is easily fouled by impurities and/or deactivated by products and intermediates of innersphere surface reactions. To overcome this difficulty, we introduce new types of SECM nanotips based on bimolecular electron transfer between the dissolved electroactive species and a redox mediator attached to the surface of a carbon nanoelectrode. A tris(2,2'-bipyridine)ruthenium complex, Ru(bpy)3, that undergoes reversible oxidation/reduction reactions at both positive and negative potentials was used to prepare the SECM nanoprobes for mapping a wide range of electrocatalytic processes through oxidation of H2, reduction of O2, and both oxidation and reduction of H2O2 at the tip. In addition to high-resolution reactivity mapping and localized kinetic measurements, chemically modified nanoelectrodes can serve as nanosensors for a number of important analytes such as reactive oxygen and nitrogen species and neurotransmitters.