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Electro-fluorescence characterization of insoluble surfactants adsorbed on solid electrodes Shepherd, Jeffrey L.


The development of an in situ technique for characterizing water-insoluble organic molecules adsorbed onto electrode surfaces was successfully accomplished. The developed technique is termed electro-fluorescence microscopy and was created through the union of epi-fluorescence microscopy and electrochemical methods. Initial investigations were conducted on a Au(III) electrode with adsorbed 1-octadecanol containing a small amount of a fluorescent dye molecule. Octadecanol adsorbed onto Au(III) was chosen as the initial starting point for the developed electro-fluorescence technique since a potential controlled adsorption/desorption process of these molecules to/from the electrode has been extensively studied. The system was then extended to monitor the behaviour of oleyl alcohol adsorbed on the Au(III) electrode. Investigations of mixed monolayers containing various concentrations of oleyl alcohol and octadecanol were also characterized with the developed technique. Because octadecanol and oleyl alcohol do not have a fluorescent moiety in their chemical make up, the surfactants were mixed with a small amounts of two fluorescent dye molecules which had the same alkyl chain length as the alcohols. These dye molecules efficiently mixed with the lipid-like alcohols and faithfully reported on the potential controlled behaviour of the surfactants. Fluorescence imaging revealed that the alcohol molecules desorbed from the electrode in a heterogeneous and aggregated morphology which did not diffuse from the metal with desorption time. The process was found to be repeatable for a given layer and reproducible on successive investigations. To extend the mechanism of the reductive desorption of self-assembled thiols from noble metals, the developed technique was also used to monitor the reductive desorption of a thiol containing a fluorescent moiety. In this investigation the electrode was a polycrystalline Au bead having a variety of surface features. The thiol was observed to selectively desorb from the various surface regions at certain values of potential.

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