Adsorption and Sulfur-Selective Photooxidation of Cysteine on Anatase TiO₂(101)

Understanding how multifunctional amino acids interact with photocatalytic oxide surfaces is essential to controlling their interfacial redox chemistry. Here, we combine scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared reflection absorption spectroscopy (FT-IRRAS), and density functional theory (DFT) calculations to elucidate the adsorption geometry and photooxidation mechanism of cysteine on anatase TiO₂(101). STM reveals a bridging adsorption motif at the surface Ti sites. XPS and FT-IRRAS indicate that adsorption predominantly occurs in configurations where the amino group remains mostly unprotonated, enabling direct coordination of nitrogen to Ti atoms, while sulfur can also independently interact with Ti atoms. DFT calculations confirm two adsorption configurations with comparable stability: a molecular M(N,S) structure, where cysteine is bound to the surface through nitrogen and sulfur, and a partially deprotonated DP_COOH(O,N) structure, where it adsorbs through a carboxylic oxygen and nitrogen. Upon UV irradiation in air, cysteine undergoes highly selective photooxidation at the sulfur site, while the carbon and nitrogen moieties remain largely unaffected. XPS measurements reveal stepwise oxidation of sulfur, progressing from thiol to fully oxidized sulfonic acid (-SO₃H) through distinct intermediate states. DFT calculations rationalize this selectivity by identifying a surface-mediated mechanism in which molecular oxygen activation promotes sequential sulfur oxidation, consistent with the experimentally observed XPS data. These results demonstrate that adsorption geometry and the local coordination environment at oxide interfaces dictate specific photooxidation pathways, providing a molecular-level framework for controlling selective transformations of multifunctional organic molecules on photocatalytic surfaces.