Hidden dynamics of noble-metal-bound thiol monolayers revealed by SERS-monitored entropy-driven exchange of cysteine isotopologues

Vibrational spectroscopy coupled with isotopic labeling provides many insights into dynamic processes within various molecular systems. Here, a newfound utility of surface-enhanced Raman scattering (SERS) spectroscopy as a tool to study noble metal-anchored thiol monolayers is demonstrated for a pair of L-cysteine isotopologues competing to bind the surface of silver nanoparticles (AgNPs). According to our DFT calculations, SERS spectra of L-Cys could be sensitive to 12C/13C and 14N/15N isotopic substitutions, which has been experimentally confirmed for the pair of L-Cys isotopologues: Cys-cabn (all 12C/14N) and Cys-CABN (all 13C/15N). In the AgNP-anchored state, the two isotopologues reveal distinct Raman shift values (1577 cm-1 / 1633 cm-1) of the band assigned to C=O stretching. This characteristic SERS feature has been subsequently employed to probe various exchange scenarios between AgNP-bound and free L-Cys molecules. As the exchange involves two spectrally distinct but chemically identical molecules, the process is exclusively entropy-driven ultimately leading to the equilibrium state in which Cys-cabn/Cys-CABN concentration ratios in the Ag surface-bound layers and the bulk solution are identical. In a system containing AgNP-L-Cys and an excess of free L-Cys molecules, the exchange energy barrier limits the overall kinetics. Although the SERS-monitored rate of progression toward the equilibrium state under ambient conditions (25 °C) is negligible, a very steep acceleration of the exchange is observed at 50 °C. While the temperature-induced transition is very abrupt, it is still reversible with cooling. We argue that the dramatic acceleration of the dynamics of the L-Cys exchange between free and AgNP-bound molecules may be rationalized as a collective phase transition to an excited and reaction-prone state. Our work highlights unexplored potential of SERS spectroscopy coupled to isotopic exchange as a tool to study obscured dynamic phenomena within metal-anchored adsorbate layers.