Unraveling the Role of the Multifunctional Groups in the Adsorption of l-Cysteine on Rutile TiO₂(110)

Understanding the interaction between biomolecules and oxide surfaces is essential for advancing technologies in photocatalysis, virus inactivation, and self-cleaning materials. This study investigates the adsorption behavior of l-cysteine on the rutile TiO₂(110) surface using a combined experimental and theoretical approach. By employing X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared reflection absorption spectroscopy (FT-IRRAS), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations, we elucidate the molecular configurations and bonding mechanisms involved in the interaction of cysteine with the TiO₂ surface. The results reveal three distinct adsorption geometries: two bidentate bridging modes involving the carboxylate group and amino group and a configuration involving the interaction of the thiolate group with titanium atoms. Additionally, cysteine molecules form dimers stabilized by disulfide bonds even at low coverage while maintaining a zwitterionic state. Our study highlights, for the first time, the key role of the thiol group in cysteine adsorption on TiO₂, both for surface direct binding and dimer formation. These findings provide new insights into the fundamental principles of biomolecule-semiconductor interactions with important implications for surface-functionalized materials in catalysis and sensing.