Conformational Equilibrium of Cysteamine on Silver Metal Surface Studied by Surface‐Enhanced Raman Scattering

ABSTRACT In this study, we utilized surface‐enhanced Raman scattering (SERS) to examine the conformational changes of cysteamine on silver bumpy nanoshell (AgNS) in various solvents, including water, ethanol (EtOH), isopropyl alcohol (IPA), and acetonitrile (ACN). It was found that the interaction between cysteamine and solvent molecules could affect the conformational equilibrium of cysteamine on the AgNS surface, particularly the ratio between its trans and gauche conformations. Using the SERS bands at 630 and 720 cm −1 assigned to S–C stretching modes of the gauche and trans conformations, respectively, and calculating the SERS intensity ratio of these bands (I trans /I gauche ), we found that water stabilized the gauche conformation because of the strong hydrogen‐bonding interactions with cysteamine, while solvents such as ACN favored the trans conformation. SERS measurements over time further indicated that cysteamine reached conformational equilibrium on AgNS slowly within hours in a single solvent environment. In mixed water/ACN solvents, the gauche conformation became increasingly dominant with higher water fractions. Conversely, introducing ACN into water‐stabilized AgNS dispersion did not reverse the conformational shift, suggesting that the gauche to trans transition was limited by higher activation barrier due to irreversible stabilization of Ag–N bond in the gauche form and local solvent environment around surface‐bound cysteamine. Conformational equilibrium of cysteamine on AgNS could be quantified using the degree of mixing (γ), a parameter reflecting the distribution and interaction of cysteamine in mixed solvent. This study shows that SERS is effective for monitoring solvent‐mediated conformational changes in surface‐bound cysteamine. This approach might help us optimize solvent selection for applications that leverage the reactive amine group of cysteamine on metal surfaces.