Annealing-Induced Antibacterial Activity in TiO₂ under Ambient Light

We demonstrate that annealing at 850 °C in the presence of Cu universally results in robust antibacterial activity under ambient illumination for TiO₂ nanoparticles, different from those annealed in a quartz crucible without metal or in the presence of Ti. Resulting robust antibacterial activity occurred after annealing regardless of the initial properties and crystal structure of the starting samples (two anatase, one rutile, and P25). A clear difference in the powder color from white to gray and a pure rutile crystal structure is observed after annealing in all of the samples. ESR measurements, however, reveal obvious differences in the defects present in the samples annealed under different conditions. Strong antibacterial activity is observed under ambient illumination for samples annealed in the presence of Cu, despite the lower activity for photocatalytic degradation of common dyes such as methylene blue after annealing. Antibacterial activity could not be attributed to the presence of Cu (no activity in the dark) or to the ROS production (none detected under ambient illumination). This indicates that other mechanisms, such as direct charge transfer involving defect levels induced by annealing in the presence of copper, may play a role in the observed antibacterial activity