l-Phenylalanine adsorption on Pt: electrochemical impedance spectroscopy and quartz crystal nanobalance studies

The interfacial behaviour of l-phenylalanine (Phe) at a Pt surface was studied over the temperature range 273–353 K in a phosphate buffer solution at pH 7.0, using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and at 298 K using electrochemical quartz crystal nanobalance (EQCN) measurements. It was shown that the surface charge density and corresponding charge transfer resistance is directly proportional to the amount of adsorbed Phe (surface concentration), indicating that adsorption at positive potentials is accompanied by the transfer of charge from the carboxylate group on the amino acid. The adsorption process for Phe was described with the Langmuir adsorption isotherm, which revealed a high affinity of the amino acid towards adsorption onto a Pt surface as shown by the calculated Gibbs energies of adsorption (−35 to −45 kJ mol−1 over the temperature range from 273 to 353 K). Although the adsorption process was found to be slightly exothermic (ΔHads=−13±1 kJ mol−1 (CV) and −11±2 kJ mol−1 (EIS)) corresponding to physisorption, the large positive entropy of adsorption (TΔSads=25 kJ mol−1 (CV) and 31 kJ mol−1 (EIS)) is the main driving force for the adsorption process. At positive potentials the adsorption of Phe is a competitive process with the surface oxidation. This was verified at 298 K by EQCN measurements where the mass changes calculated from the recorded frequency changes corresponded to the adsorption and reduction of the Oads species in the presence of Phe.