Electrochemical Behavior of Nanoelectrode Ensembles in the Ionic Liquid [BMIm][BF₄]

Abstract

The electrochemical behavior of nanoelectrode ensembles (NEEs), prepared by electroless plating of Au using microporous polycarbonate membranes as template, is tested in the ionic liquid [BMIm][BF₄]. The accessible potential window is significantly wider in [BMIm][BF₄] than in water, extending approximately, for 3.4 V vs. 1 V, respectively. The voltammetric behavior at NEEs of two redox probes, namely butyl viologen (BV²⁺) and (ferrocenylmethyl) trimethylammonium (FA⁺) are examined at different scan rates. In both cases, at scan rates higher than 200 mV/s sigmoidally shaped voltammograms typical of a pure radial diffusion regime are observed. At lower scan rates the voltammograms are peak shaped, as expected for total overlap diffusion conditions. This is the first time that the pure radial regime is obtained with NEEs made using commercially available polycarbonate templates, since in water solution only the total overlap regime is typically observed. This is explained as a consequence of the high viscosity of [BMIm][BF₄] which reflects in lowering of diffusion coefficients and smaller thickness of diffusion layers, for the same time scale, with respect to water solutions, but also the fact that the nanoelectrodes are slightly recessed helps in observing the pure radial regime. In order to make operative the pure radial condition it is indeed required that the thickness of diffusion layer at individual nanoelectrodes be smaller than the hemi-distance between neighboring nanoelectrodes. Examining the analytical performances achievable with NEEs in [BMIm][BF₄], it is shown that a limit is given by the decreased diffusion coefficients. Detection limits at NEEs in the ionic liquid are indeed higher than those obtained in water solutions. This notwithstanding, detections limits at NEEs in [BMIm][BF₄] are always improved with respect to those at conventional electrodes.