Fluorescence coupling for direct imaging of electrocatalytic heterogeneity

Here we report the use of fluorescence microscopy and closed bipolar electrodes to reveal electrochemical and electrocatalytic activity on large electrochemical arrays. We demonstrate fluorescence-enabled electrochemical microscopy (FEEM) as a new electrochemical approach for imaging transient and heterogeneous electrochemical processes. This method uses a bipolar electrode mechanism to directly couple a conventional oxidation reaction, e.g., the oxidation of ferrocene, to a special fluorogenic reduction reaction. The generation of the fluorescent product on the cathodic pole enables one to directly monitor an electrochemical process with optical microscopy. We demonstrate the use of this method on a large electrochemical array containing thousands or more parallel bipolar microelectrodes to enable spatially and temporally resolved electrochemical imaging. We first image molecular transport of a redox analyte in solution using an array containing roughly 1000 carbon fiber ultramicroelectrodes. We then carry out a simple electrocatalysis experiment to show how FEEM can be used for electrocatalyst screening. This new method could prove useful for imaging transient electrochemical events, such as fast exocytosis events on single and networks of neurons, and for parallel, high-throughput screening of new electrocatalysts.