Adsorption kinetics of L-glutathione on gold and structural changes during self-assembly : an <i>in situ</i> ATR-IR and QCM study

The adsorption of L-glutathione (γ-Glu-Cys-Gly) from ethanol on gold surfaces was studied <i>in situ</i> by both attenuated total reflection infrared (ATR-IR) spectroscopy and using a quartz crystal microbalance (QCM). The molecule is firmly anchored to the gold surface through the thiol group. Different IR signals of adsorbed L-glutathione, notably the amide I and ν(–COOH), show significantly different behavior with time, which reveals that their increase is not related to adsorption (mass uptake) alone. This indicates that structural transformations take place during the formation of the self-assembled monolayer (SAM). In particular, the intensity of the acid signal increases quickly only within the first couple of minutes. The complexity of the self-assembling process is confirmed by QCM measurements, which show fast mass uptake within about 100 s followed by a considerably slower regime. The structural change superimposed on the mass uptake is, based on the <i>in situ</i> time-resolved ATR-IR measurements, assigned to the interaction of the acid group of the Gly moiety with the surface. The latter group is protonated in ethanol but deprotonates upon interaction with the gold surface. The protonation–deprotonation equilibrium is sensitive to external stimuli, such as the presence of dissolved L-glutathione molecules. The interaction of the acid group with the surface and concomitant deprotonation proceeds <i>via</i> two distinguishable steps, the first being a reorientation of the molecule, followed by the deprotonation.