Photosensitized TiO2 nanomaterials for visible-light antimicrobial photodynamic inactivation

Abstract Photodynamic inactivation (PDI) can exploit molecular photosensitizer photochemistry and semiconductor-mediated redox processes, while biological activity depends on how these mechanisms operate at the material–microbe interface. Colloidal nanocrystalline TiO 2 (qTiO 2 ) was surface-sensitized with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP), 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (THPP), its zinc(II) complex (ZnTHPP), or hypericin (Hyp). The free porphyrins were efficient singlet oxygen sensitizers, with Φ Δ values of 0.7, 0.5, and 0.4 for TCPP, THPP, and ZnTHPP, respectively. Association with qTiO 2 quenched porphyrin fluorescence and extended photocurrent generation into the visible region, supporting photoinduced electron transfer at the sensitizer–semiconductor interface. Measurements with APF, DHE, and SOSG showed that the resulting materials generated both oxygen-centered radicals and singlet oxygen, providing access to Type I and Type II photochemistry under visible-light excitation. THPP@qTiO 2 showed strong contributions from both pathways. Photochemical activity did not directly predict antibacterial efficacy. Under 420 ± 20 nm irradiation (10 J cm −2 ), THPP@qTiO 2 and ZnTHPP@qTiO 2 showed strong photodynamic activity against Staphylococcus aureus , reducing viable counts by 5.7 and 5.6 log 10 units, respectively, whereas TCPP@qTiO 2 was only weakly active. Klebsiella pneumoniae was unaffected without potentiation. Addition of 100 mM KI further enhanced S. aureus killing, with TCPP@qTiO 2 , THPP@qTiO 2 , and ZnTHPP@qTiO 2 reducing viable counts to the detection limit. KI also rendered TCPP@qTiO 2 highly active against K. pneumoniae , reducing viability to the detection limit (> 5-log reduction). Hyp@qTiO 2 aggregated under bacterial assay conditions but showed increased antimicrobial activity after KI addition. Antibacterial efficacy therefore reflects the combined influence of molecular and semiconductor photochemistry, interfacial and colloidal properties, bacterial-envelope accessibility, and iodide-mediated secondary chemistry. Key points Interfacial electron transfer enables radical photochemistry while singlet oxygen generation is retained. Cell-free ROS generation does not predict antibacterial efficacy of sensitized qTiO2. KI potentiation renders TCPP@qTiO2 highly active against Gram-negative K. pneumoniae.