L.P. Zaino III, W. Xu, D. Han, P.W. Bohn
University of Notre Dame,
Keywords: optofluidics, nanofluidics
Summary:The ultimate goal of analytical electrochemistry is to provide high sensitivity quantitation of electroactive analytes – an objective which is limited by the relatively large electronic background noise when making current measurements. We are investigating a new configuration for ultrasensitive electrochemical measurements which exploits the nanophotonic and nanoelectronic properties of unique single and double-electrode zero mode waveguides. The dual functionality makes it possible to perform single molecule spectroelectrochemical measurements under redox cycling conditions – both when the upper electrode is potential-controlled and self-induced redox cycling of flavoproteins and flavin cofactors. Flavin mononucleotide (FMN) contains an isoalloxazine chromophore which is fluorescent in the oxidized state, while the reduced state, FMNH2, exhibits a substantially lower quantum efficiency, thus permitting the redox state of single FMN molecules to be followed by observing their fluorescence behavior. Because the ~100 zeptoliter volumes of these nanopores dictate very short residence times, evidence for single molecule redox cycling is obtained from the fluorescence dynamics. Freely diffusing species exhibit characteristic behavior in which the probability of observing single reduced molecules increases as the potential is scanned to more negative values. Conversely, single molecule cycling behavior is evidenced by the distribution of on- and off-times, which are altered relative to freely diffusing FMN/FMNH2. Comparisons are made between capture efficiencies with the upper ring electrode floating vs. potential controlled as well as the propensity for the dual ring structure to stabilize the intermediate redox species which are assigned tentatively to semiquinone species.