Counting Single Redox Molecules in a Nanoscale Electrochemical Cell.

We report the use of a Pt bipolar electrochemical nanocell and fluorescence to detect single redox molecules. A Pt nanocell is formed by depositing a Pt particle at a nanopipet orifice which separates the inside pipet volume from the bulk solution. Highly fluorescent resorufin molecules are generated on the inner Pt surface and optically detected and counted due to unique properties of the nanocell. First, the pipet is horizontally positioned on a microscope allowing one to examine a 6-μm distance from the electrode/solution interface. Second, the resazurin/resorufin molecules are confined inside a 100 nm pipet resulting in a very high signal/background ratio in fluorescence detection. Third, the small pipet size confines the motion of the redox molecules increasing the probability of transient molecular adsorption on the quartz walls. This, along with the longer diffusion distance increases the chance of fluorescence detection. The ability to count single redox molecules allows us to estimate the detection efficiency. This study shows the unique power of fluorescence-based electrochemical detection in studying single redox events. Future use of this method may enable one to study single redox events of conventional nonfluorogenic redox reactions on the outer surface of the bipolar nanoelectrode, such as oxidation of H2 catalyzed by a metal cluster electrocatalyst or a single enzyme.