Riboflavin and flavin mononucleotide adsorption on Ag (111) electrodes and their interaction with L-cysteine investigated by electrochemical and non-linear optical methods

The electrochemical behaviour and non-linear optical properties of riboflavin (RF), flavine mononucleotide (FMN), and L-cysteine on Ag (1 1 1) single crystal electrodes in a non-adsorbing electrolyte have been investigated for the first time. Two adsorption peaks were observed in the cyclic voltammograms (CVs) in 0.01 M KClO(4) solutions containing RF at -320 mV and -420 mV vs. saturated calomel electrode. Under the same conditions, the CVs from solutions containing FMN showed two adsorption peaks at -450 mV and -550 mV. This indicated that the RF and FMN have different adsorption properties, this finding being more noticeable at potentials positive of the pzc, where FMN behaviour resembles that of thiols on silver. The use of an electrochemical flow cell allowed flavin injection after in situ formation of a stable L-cysteine adlayer on the electrode surface. The structure of the flavin adlayer is potential-independent in the double-layer region and has C3v symmetry, which is not altered in the presence of L-cysteine. The L-cysteine injection removed the previously adsorbed RF but not FMN. Debye relaxation was observed for flavin molecules, with relaxation times strongly dependent on the presence of L-cysteine, which in turn, has a more complicated response with time when injected to a bare electrode. The general effectiveness of combination of electrochemical and second harmonic generation measurements (CV-SHG method) for obtaining information on a well-defined interface is demonstrated. (C) 2011 Elsevier B.V. All rights reserved.