Growth and application of paired gold electrode junctions: evidence for nitrosonium phosphate during nitric oxide oxidation

A bipotentiostatic gold electrodeposition process is developed to grow gold junctions between two adjacent 100 full diameter platinum disc electrodes. Gold is electrodeposited simultaneously on both electrodes with in automated termination mechanism close to short-circuit conditions. Gap junctions (average gap width ca. 4 mu m) are obtained reproducibly and the behavior of the resulting generator-collector electrode system is investigated for two relevant redox systems. First, the chemically reversible oxidation of 1,1'-ferrocenedimethanol in aqueous 0.1 M KCI is studied. Well-defined feedback currents across the electrode junction in generator-collector mode are recorded down to sub-micromolar analyte concentration. Electrochemically reversible voltammetric responses suggest fast heterogeneous electron transfer and this allows further gap geometry analysis. Second, the (apparently) chemically irreversible oxidation of nitric oxide in 0.1 M phosphate buffer solution (pH 7) at gold electrodes is re-investigated and, perhaps surprisingly, generator-collector feedback currents are observed for a solution phase intermediate. here tentatively assigned to nitrosonium phosphate, NO+H2PO4-. The life time of this intermediate, ca. 10 ms, is surprisingly log, given a typical decay time for free NO+ in water of only nanoseconds. The results are consistent with an estimated nitrosonium phosphate association equilibrium constant, K approximate to 10(7) mol(-1) dm(3). Without further optimization of the electrode junction gap geometry, the determination of nitric oxide down to ca. 10 mu M concentration is achieved. The benefits of smaller junctions and potential analytical applications of paired nanojunction electrodes are discussed.