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Electro-fluorescence characterization of oxidative and reductive desorpton{sic} of thiol monolayers Musgrove, Amanda

Abstract

The electro-fluorescence microscopy technique was used to characterize the oxidative and reductive desorption behavior of fluorescent-tagged thiol self-assembled monolayers on a polycrystalline gold electrode. Effects of pH and thiol alkyl chain length on desorption potential and fluorescence behavior were studied. The use of an alternating 'base-step' potential was established in order to eliminate uncertainty in capacitance measurements arising from chargetransfer reactions (H2 evolution or gold oxidation) occurring at more extreme potential values. The potential of monolayer desorption, here defined as the potential at which the capacitance of the electrode interface begins to change rapidly with potential, was found to be affected by solution pH. Electrolyte solutions with a higher pH required a lower potential to desorb the thiol monolayers. An increase in fluorescence intensity at the electrode surface accompanied the change in capacitance, confirming that the thiol molecules are migrating away from the electrode. Evidence of oxidative readsorption following reduction of the monolayer was also observed, both by the associated decrease in capacitance and a dramatic decrease in fluorescence intensity. Fluorescence intensity during oxidative desorption was found to be linked to the pH of the environment surrounding the electrode surface. Lower fluorescence intensity was associated with a decrease in solution pH, and was also observed at the step (oxidizing) potential. The decrease in fluorescence at the step potential is believed to be linked to a local decrease in electrolyte pH at the electrode surface due to the oxidation of gold. The oxidation products are expected to be less soluble in acidic media, causing aggregation and self-quenching of the BODIPY fluorophore used. On return to the base potential, the local pH was returned to the bulk solution pH, and fluorescence was restored. At all pH values studied, a more extreme potential was required to desorb the thiol with the longer alkyl chain (16 carbon atoms) than the shorter-chain thiol studied (10 carbon atoms).

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