| Summary: | 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). === Science, Faculty of === Chemistry, Department of === Graduate
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