Imaging of Oxygen Evolution and Oxide Formation Using Quinine Fluorescence

Quinine was used as a fluorescent acid−base indicator to image electrode reactions that consume hydroxide ions. The fluorescence intensity was used as a measure of the microscopically local current at the electrode surface during O2 evolution and oxide formation. Highest resolution images were obtained 300−600 ms after application of the potential in an unbuffered (pH 10) 0.5 mM quinine solution. The increase in fluorescence intensity observed at later times was offset by a decrease in spatial resolution. Imaging anodic reactions using quinine fluorescence is complementary to the previously established method for imaging cathodic processes using fluorescein fluorescence. Similarly, this new method should be able to provide information about microscopically local reaction rates and concentration profiles at electrode surfaces for a variety of anodic reactions.