Fluorescence Confocal Laser Scanning Microscopy as a Probe of pH Gradients in Electrode Reactions and Surface Activity

Abstract

The application of fluorescence confocal laser scanning microscopy (CLSM) to quantify three-dimensional pH gradients near electrode surfaces is described. The methodology utilizes a trace quantity of a fluorescent dye, fluorescein, in solution, which fluoresces strongly above pH 6.5, to map the pH adjacent to various ultramicroelectrodes undergoing electrochemical processes that lead to pH changes. The experimental fluorescence profiles, determined by CLSM, have been compared to models by solving the underlying mass transport equations, including the effect of natural convection, using the finite element method. The methodology has been validated through studies of the galvanostatic reduction of water at both disk and ring ultramicroelectrodes. The fluorescence profiles were found to be highly sensitive to both the initial bulk solution pH and applied current in a predictable fashion. The potentiostatic reduction of oxygen has been investigated at 25- and 10-μm-diameter platinum electrodes to confirm the effective number of electrons transferred in the reaction. Finally, the application of this methodology to observe defects in microelectrode arrays, particularly those that cannot be seen by optical microscopy, is described.