Studies of L-DOPA and related compounds adsorbed from aqueous solutions at platinum(100) and platinum(111): electron energy-loss spectroscopy, Auger spectroscopy, and electrochemistry

Abstract

Adsorption of L-DOPA and a series of other amino acids and related compounds from aqueous solutions at well-defined Pt(lll) and Pt(100) single-crystal surfaces has been studied: 3-(3,4-dihydroxyphenyl)-Lalanine (DOPA), L-tyrosine (TYR), L-cysteine (CYS), L-phenylalanine (PHE), L-alanine (ALA), dopamine (DA), catechol (CT), and (3,4-dihydroxyphenyl)acetic acid (DOPAC). Packing densities (moles adsorbed per unit area) were measured for each compound by means of quantitative Auger spectroscopy; two independent Auger spectroscopic methods were employed: one based upon measurement of elemental Auger signals from the adsorbed layer and the other upon measurement of the attenuation of the Pt Auger signal by the adsorbed layer. Packing densities of DOPA, TYR, DA, CT, and DOPAC indicate adsorption with the aromatic ring attached parallel to the surface. In contrast, CYS is adsorbed through the sulfur atom, while ALA and PHE are attached to the surface through their amino acid moieties. Vibrational spectra of the adsorbed layer formed from each of these compounds were obtained by means of electron energy-loss spectroscopy (EELS) and were compared with the infrared spectra of the parent compounds in KBr. The EELS (adsorbed layer) and IR (solid compound) spectra are similar except that the hydroxyl hydrogens of DOPA, TYR, DA, CT, and DOPAC as well as the carboxyl hydrogens of PHE and DOPAC are removed as a result of the adsorption process. EELS bands of polar groups such as O-H are not broadened to the same extent as in the IR spectra of the solid compounds, evidently due to less intermolecular bonding among such groups at the surface. LEED experiments indicate that CT forms a (3X3) adlattice at Pt(lll), while each of the other compounds forms an oriented but otherwise structurally disordered layer at Pt(lll) and Pt(100). The packing density of DOPA, CYS, and DOPAC is higher at Pt(100) than at Pt(lll) due to differences in molecular orientation. Single-crystal surface structure, adsorbate orientation, and mode of surface bonding exert a profound influence on the product distribution of electrocatalytic oxidation of these well-characterized adsorbed organic intermediates, on the basis of cyclic voltammetry and potential-step chronocoulometry experiments. This study has also revealed that evacuation does not alter the composition or electrochemical properties of the chemisorbed layer formed from solution.

Enantioselective recognition at mesoporous chiral metal surfaces

“…Indeed DOPA is known to adsorb on platinum4647, and this could eventually lead to supramolecular diastereomeric interactions with DOPA molecules in solution, leading in fine to chiral discrimination. Such a mechanism could be excluded as shown in Supplementary Figs 2–7).…”

Studies on Self-Assembly Monolayers of Cysteine on Gold by XPS, QCM, and Electrochemical Techniques

“…As cysteine is more stable and cheaper compared to other commonly used selfassembly reagents including cystamine and some mercaptan compounds, cysteine is also an often-used SAMs reagent, however, little work has been dedicated to the description of the native properties of SAMs of cysteine [12]. So in the present study, we attempted to make some detailed studies on the adsorption kinetics of a cysteine modified gold electrode, and therefore estimate the time required to form saturated SAMs.…”

Molecular dynamics simulation of a DOPA/ST monolayer on the Au(111) surface

“…10,12 Therefore, the configuration of the catechol groups has been studied. According to the statistics of distances between the aromatic rings and the gold surface (Fig.…”

“…7 While for the adhesion on gold, the main mechanism proposed is due to the p-electron system. 9,10 Moreover, hydrogen-bonding between 3,4-dihydroxy-L-phenylalanine molecules is also of possible importance for the formation of a strongly adhering layer. 9 The Langmuir-Blodgett monolayer has received considerable attention as a functional surface for various applications.…”