In situ spectroscopic studies of cysteine adsorbed on silver electrodes




Birnie-Lefcovitch, Simon David Peter

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The study of interfacial processes has long been of interest to scientists. The properties of a material are generally governed by the characteristics of its surface, thus the development of surface specific experimental methods are always of great importance to the scientific community. This thesis presents the results of the spectroelectrochemical characterization of a cysteine-Ag adsorbate-substrate system. The system was probed using two spectroelectrochemical methods. The chiral effect which cysteine has on the electronic structure of the Ag substrate was studied by performing in situ second harmonic generation optical rotatory dispersion (SHG-ORD) experiments. Rotation angles (phi) obtained indicated that the overlayers of adsorbed cysteine molecules imprinted the electronic structure of the Ag with their inherent optical activity. Results also indicate that there are one or more other processes which are contributing to the observed phi values. The second half of this thesis discusses the effect that pH and applied potential have on the adsorption geometry of L-cysteine on polycrystalline Ag as studied by surface enhanced Raman scattering (SERS). Results obtained under neutral and acidic conditions showed that the coadsorption of Cl- plays an important role in the adsorption geometry. At more positive potentials Cl- will be coadsorbed on the Ag surface with cysteine. The Cl- helps to stabilize the adsorbed cysteine via interactions with the protonated amino group. Consequently, as the potential is changed in the cathodic direction the Cl- becomes desorbed from the surface, resulting in changing intensities observed in the SERS spectra. Tracking of which peaks, and consequently vibrational modes, are changing and in which way allowed for a qualitative determination of the adsorption geometry as a function of both pH and potential.