Electrochemical and FTIR studies of L-phenylalanine adsorption at the Au(111) electrode

The adsorption of L-phenylalanine (Phe) at the Au(111) electrode surface ha s been studied using electrochemical techniques and subtractively normalize d interfacial Fourier transform infrared (SNIFTIR) techniques. The electroc hemical measurements of cyclic voltammetry, differential capacity and chron ocoulometry were used to determine Gibbs energies of adsorption and the ref erence (E-1) and sample (E-2) potentials to be used in the spectroscopic me asurements. The vibrational spectra have been used to determine: (i) the or ientation of the molecule at the surface as a function of potential; (ii) t he dependence of the band intensity on the surface coverage; (iii) the char acter of surface coordination, and (iv) the oxidation of adsorbed Phe molec ules at positive potentials. The adsorption of Phe is characterized by Delt aG values ranging from - 18 to - 37 kJ mol(-1) that are characteristic for a weak chemisorption of small aromatic molecules. The electrochemical and S NIFTIR measurements indicated that adsorbed Phe molecules change orientatio n as a function of applied potential. At the negatively charged surface Phe is predominantly adsorbed in the neutral form of the amino acid. At potent ials positive to the pzc, adsorption occurs predominantly in the zwitterion ic form with the -COO- group directed towards the surface and the ammonium group towards the solution. At more positive potentials electrocatalytic ox idation of Phe occurs and is marked by the appearance of the CO2 asymmetric stretch band in the FTIR spectrum. Thus, relative to pzc, Phe is weakly ch emisorbed at negative potentials, changes orientation at potentials close t o the pzc and is oxidized at positive potentials. (C) 2001 Elsevier Science B.V. All rights reserved.