STUDIES OF L-DOPA AND RELATED-COMPOUNDS ADSORBED FROM AQUEOUS-SOLUTIONS AT PT(100) AND PT(111) - ELECTRON ENERGY-LOSS SPECTROSCOPY, AUGER-SPECTROSCOPY, AND ELECTROCHEMISTRY

Recent chemisorption studies at annealed polycrystalline Pt electrodes have demonstrated that aromatic and cyclohexadienyl compounds form oriented adsorbed layers. Numerous factors affect orientation and mode of attachment, including adsorbate molecular structure, adsorbate concentration, electrode potential, adsorbate chirality, temperature, etc. The present work employed well-defined Pt(111) and Pt(100) surfaces rather than electrochemically cycled polycrystalline Pt. Vibrational spectra of the adsorbed layers were obtained by means of high-resolution electron energy-loss spectroscopy (EELS). Surface molecular packing densities were measured by means of Auger electron spectroscopy rather than by thin-layer coulometry, thus extending the range of applicability to adsorbates which do not display definitive electrochemical reactivity and to concentrations outside the range of adsorption measurements with thin-layer electrodes. The present studies investigated amino acids L-DOPA (DOPA), L-tyrosine (TYR), L-cysteine (CYS), L-phenylalanine (PHE), and L-alanine (ALA) and the related compounds dopamine (DA), catechol (CT), and (3,4-dihydroxyphenyl)acetic acid (DOPAC). Packing densities, nature of the adsorbed species, and mode of attachment to the surface are described.