Kinetics, Assembling, and Conformation Control of L-Cysteine Adsorption on Pt Investigated by in situ FTIR Spectroscopy and QCM-D.

A quartz crystal microbalance method with dissipation (QCM-D) and attenuated total reflection Fourier-transform infrared (ATR-FTIRS) spectroscopy were used to study the adsorption of L-cysteine (L-Cys) on Pt. Through QCM-D, it was possible to verify that the viscoelastic properties of the adsorbed species play an important role in the adsorption, rendering Sauerbrey's equation inapplicable. The modelling of QCM-D data exposed two different processes for the adsorption reaction. The first one had an activation time and is fast, whereas the second is slow. These processes were also resolved by ATR-FTIRS and identified to be water and anion adsorption preceded by L-Cys adsorption. Both techniques reveal that the degree of surface coverage is pH dependent. Spectroscopic data indicate that the conformation of L-Cys(ads) changes with pH and that the structures do not fully agree with those proposed in literature for other metallic surfaces. The assembling of the adsorbed monolayer appeared to be very fast, and it was not possible to determine or quantify this kinetics. The conformation is also controlled by applied potential, and the anion adsorption and interfacial water depends on the conformation of the adsorbed molecules.