*3.6. Antibacterial Properties*

The Gram-negative bacterium *E. coli* K-12 was used to investigate the in vitro antibacterial abilities of the UCNPs, R-TiO2, and UCNPs@R-TiO2. An amount of 900 μL of *E. coli* suspension (~10<sup>6</sup> CFU/mL) and 100 μL of these nanomaterials at different concentrations were mixed. The resulting concentrations were 20, 30, 40, and 50 μg/mL, respectively. After incubation at 37 ◦C for 2 h, the obtained bacterial suspension was diluted by a factor of 103. The resulting bacterial samples were irradiated for 20 min by a 980 nm NIR light (1 W). Afterwards, 100 μL of the above suspension was spread on the Luria-Bertani medium and hatched at 37 ◦C for 16 h. In the end, the antibacterial abilities were assessed on the LB agar plates using the colony counting method. Simultaneously, instead of the added nanocomposites, an isotonic saline solution was added into the *E. coli* suspension as a blank control.
