Direct Electrochemical Measurements Inside a 2000 Angstrom Thick Polymer Film by Scanning Electrochemical Microscopy

An extremely small, conically shaped Pt microelectrode tip (with a radius of 30 nanometers) and the precise positioning capabilities of the scanning electrochemical microscope were used to penetrate a thin (200 nanometers) polymer film and obtain directly the standard potential and kinetic parameters of an electrode reaction within the film. The thickness of the film was determined while it was immersed in and swollen by an electrolyte solution. The film studied was the perfluorosulfonate Nafion containing Os(bpy)$_3^{2+}$ (bpy, 2,2′-bipyridine) cast on an indium tin oxide surface. The steady-state response at the ultra-microelectrode allowed direct determination of the rate constant for heterogeneous electron transfer k° and the diffusion coefficient D without complications caused by transport in the liquid phase, charge exchange at the liquid-polymer interface, and resistive drop.