Interfacial Redox Engineering of TiO₂ Nanocomposites Using Green Tea-Derived Ligands and Silver

Titanium dioxide (TiO₂) is a widely studied semiconductor whose interfacial redox properties strongly influence its photocatalytic and biological performance. In this work, TiO₂ nanomaterials were surface-functionalized with green tea waste extract (GT) and subsequently impregnated with silver to obtain redox-active nanocomposites with tunable optical and biological properties. HPLC, FTIR, diffuse reflectance spectroscopy (DRS), and density functional theory (DFT) analyses demonstrated the formation of an organic-inorganic interface through adsorption of green tea-derived ligands. DFT calculations revealed complementary interfacial roles of the adsorbed constituents, with epigallocatechin gallate (EGCG) inducing interfacial charge-transfer (ICT) states that enable visible-light absorption, as reflected by the decrease in the apparent optical bandgap from ~3.48 eV for pristine TiO₂ to ~1.83 eV for TiO₂/EGCG. ICP-OES analysis further quantified the Ag loading in TiO₂/GT/Ag at 5.1 wt%. Electron paramagnetic resonance (EPR) experiments demonstrated that GT functionalization shifts the interfacial redox balance toward radical scavenging by suppressing excessive radical generation, whereas silver incorporation partially restores oxidative pathways, particularly under irradiation. These differences in interfacial redox behavior directly translate into distinct biological responses. TiO₂/GT/Ag showed the strongest antimicrobial activity, with visible-light enhancement observed predominantly against Staphylococcus aureus, while TiO₂/GT/Ag reduced H₂O₂-induced oxidative stress in non-malignant cells and promoted intracellular reactive oxygen species generation in cancer cells. These findings demonstrate that engineering the organic-inorganic interface through plant-derived ligands and silver incorporation provides an effective strategy for tuning the interfacial redox properties and light-responsive biological performance of TiO₂-based nanomaterials, thereby expanding their potential for antimicrobial and biomedical applications.