What happens to the thiolates created by reductively desorbing SAMs? An in situ study using fluorescence microscopy and electrochemistry.

In situ examination of the reductive desorption process for Au microelectrodes modified with a thiol self-assembled monolayer (SAM) using fluorescence microscopy enabled the study of the fate of the desorbed thiolate species. The Bodipy labeled alkyl-thiol SAM, when adsorbed, is not fluorescent due to quenching by the Au surface. Once reductively desorbed, the thiolate molecules fluoresce and their direction and speed are monitored. At moderately negative reduction potentials, the thiolate species hemispherically diffuse away from the microelectrode. Also observed is the influence of a closely positioned counter electrode on the direction of the desorbed thiolate movement. As the potential becomes more negative, the molecules move in an upward direction, with a speed that depends on the amount of dissolved H(2) produced by water reduction. Shown is that this motion is controlled, in large part, by the change in the electrolyte density near the electrode due to dissolved H(2). These results should help in explaining the extent of readsorption at oxidative potentials observed in cyclic voltammetry (CV) reductive desorption measurements, as well as improving the general understanding of the SAM removal process by reductive desorption. The electrogenerated H(2) was also shown to be able to reductively remove the thiol SAM from the Pt/Ir particles that decorate the microelectrode glass sheath.