Facile Fabrication of Carbon Ultramicro- to Nanoelectrode Arrays with Tunable Voltammetric Response

The voltammetric response for nano- to micrometer-sized electrode arrays are represented by two major regimes: a sigmoidal shaped i–v response for arrays acting as individual electrodes in parallel and peak-shaped i–v response for arrays acting as an ensemble in concert. Here, we present a facile and versatile technique to fabricate ultramicro- to nanoelectrode arrays using atomic layer deposition of insulating Al2O3 on conductive carbon films masked by 1.54, 11, or 90-μm-diameter polystyrene spheres (PSS). The ratio between the interelectrode distance and the electrode radii of the electrode arrays is a predictable function of the PSS radius used in fabrication, resulting in electrode arrays with a tunable voltammetric response. Arrays are characterized utilizing cyclic voltammetry and electrochemical impedance spectroscopy, which provides the critical scan rate, νcrit, the scan rate at which the radial diffusion layers of the individual electrodes overlap and appear as a single linear diffusion layer. Thus, below νcrit, the electrode has a peak-shaped i–v response associated with semi-infinite linear diffusion, whereas above this critical scan rate, the i–v response is sigmoidal as a result of hemispherical radial diffusion. Results indicate that the critical scan rates are 6.6, 1.0, and 0.01 V/s for the 1.54, 11, and 90 μm PSS prepared electrode arrays, respectively.