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.