Synergistic Photothermal Antibacterial Effects of Silver-Coated Upconversion Nanoparticles: Experimental Insights and DFT Calculations

The rise of antibiotic-resistant bacterial infections presents a significant global health challenge, contributing to higher mortality rates and prolonged hospitalizations. A promising solution is the development of localized hyperthermia for bacterial inactivation. In this study, we investigate the fabrication of silver-coated upconversion nanoparticles (UCNP@Ag NPs) that convert near-infrared (NIR) light into localized heat, offering an alternative to traditional antibiotic treatments. UCNP@Ag NPs were successfully synthesized and showed high photothermal conversion efficiency when irradiated with a 980 nm NIR laser. At a concentration of 100 µg/mL, the nanoparticles increased the solution temperature to 58.9°C within 6 minutes under 1.0 W/cm² NIR laser irradiation, which was sufficient to kill the bacteria. The inhibition rates for E. coli and S. aureus were 93.9% and 96.2%, respectively. The core-shell nanostructure exhibited superior antibacterial performance compared to silver nanoparticles (Ag NPs) and non-irradiated groups. Additionally, DFT calculations confirmed that the adsorption energy of Ag clusters on the NaYF₄:Yb,Tm host was thermodynamically favorable (−1.23 eV), ensuring stable integration. The calculated projected density of states (PDOS) and dielectric function analyses showed that Ag doping modifies the electronic structure near the Fermi level, enhancing NIR absorption and localized heating, which were critical for the nanoparticle’s effective antibacterial performance. This work demonstrates a highly efficient approach for antibacterial therapy through photothermal conversion using silver-coated upconversion nanoparticles. The synergy between the thermal and silver effects presents a promising alternative to traditional antibiotics, with potential applications in combating resistant bacterial strain