UCNP@ZnO:Co/Ag Composites: A Dual-Action Platform for Antibacterial Photodynamic Therapy via Synergistic Silver Ion and Reactive Oxygen Species Release

Antibacterial photodynamic therapy (aPDT) based on ZnO-coated lanthanide-doped upconversion nanoparticles (UCNP@ZnO) is a promising alternative for treating infections caused by antibacterial-resistant bacteria. By doping the ZnO shell of UCNP@ZnO materials with cobalt and silver, the latter in the form of Ag nanoparticles on the UCNP@ZnO:Co surface, we enhance reactive oxygen species (ROS) generation and stimulate Ag ion release. We attribute this to a narrowed band gap and minimized electron-hole pair recombination. This approach is in distinct contrast to reported strategies that use organic photosensitizers or molecular drug-loaded systems. When these improved UCNP@ZnO:Co/Ag materials are activated by near-infrared (NIR) light (980 nm) they demonstrated strong antibacterial efficacy (>99.9% bacterial kill) against Staphylococcus aureus small colony variant (S. aureus JB1-SCV) and the parental (6850) strain. It is proposed that Ag NPs loaded onto the UCNPs@ZnO:Co surface act as a cocatalyst and, in combination with a localized surface plasmon resonance (LSPR), enhance the photogeneration of ROS by ZnO:Co shells (>2 or >5-fold ROS enhancement compared with undoped materials). Irradiation also results in the release of Ag ions (>2-fold Ag release under NIR irradiation) and a more pronounced antibacterial response. In comparison, control experiments with the two bacteria strains in dark conditions showed no antibacterial activity. This unique, synergistic mode of action promises efficient treatment of resistant or hard-to-treat bacterial strains in topical applications.