Mesoscopic concentration fluctuations in a fluidic nanocavity detected by redox cycling.

We show that a nanofluidic device consisting of a solution-filled cavity bounded by two closely spaced parallel electrodes can amplify the electrical current from redox molecules inside the cavity by a factor of approximately 400 through redox cycling. The noise spectral density of this signal agrees quantitatively with the calculated fluctuations in the number of molecules inside the cavity due to their diffusive motion. We demonstrate sensitivity to fluctuations of as few as approximately 70 molecules.