*2.3. Cytotoxicity Assessment*

We evaluated the potential cytotoxicity to cells (HEK 293) of the as-prepared nanocomposites via MTT assay. There was about 80%, 87%, and 98% viability of HEK293 cells upon the treatment of the UCNPs, R-TiO2, and UCNPs@R-TiO2 materials at a concentration of 12 μg/mL, indicating low cytotoxicity to mammalian cells (Figure 6). It was noted that the cell viability was largely decreased to around 68% when the UCNP concentration

was up to 50 μg/mL. By contrast, R-TiO2 showed almost no toxicity to HEK 293 cells, because the HEK 293 cells still manifested a high survival rate (more than 91%) even under a high concentration (100 μg/mL) condition. Notably, the cell viability of the UCNPs@R-TiO2 treatment was improved, possibly due to the low toxicity of R-TiO2. About 80% of cells survived under a high concentration (40 μg/mL) of UCNPs@R-TiO2 incubation, demonstrating its potential for bactericide material application [35].

**Figure 6.** Cytotoxicity of UCNPs (**a**), R-TiO2 (**b**), and UCNPs@R-TiO2 nanocomposite (**c**) in HEK 293 cells.
