Upconversion Nanoparticle-Assisted Payload Delivery from TiO₂ under Near-Infrared Light Irradiation for Bacterial Inactivation

The low penetration depth of UV light in mammalian tissue is the critical limitation for the use of TiO 2 -based photocatalysis in biomedical applications. In this work, we develop an effective near-infrared (NIR)-active photocatalytic platform that consists of a shell structure of upconversion nanocrystals decorated on a core of Au/dark-TiO 2 . The heart of this system is the strong photocatalytic activity in the visible region enabled by the gold surface-plasmon resonance on dark TiO 2 (D-TiO 2 ). Simulation and experiment demonstrate for an optimized Au/D-TiO 2 combination a highly enhanced light absorption in the visible range. Using ampicillin sodium (AMP) as model drug, we exemplify the effective use of this principle by demonstrating a NIR light-triggered photocatalytic payload release. Importantly, the photocatalytically generated reactive oxygen species can effectively inactivate AMP-resistant bacteria strains, thus maintaining an antibacterial effect even after all drug is released. Overall, we anticipate that the here-introduced NIR-light-active photocatalytic cascade can considerably widen TiO 2 -based photocatalysis and its applications into the infrared range.