*3.4. Vitro Cell Viability Assay*

A standard MTT assay was used to assess the cytotoxicity of the UCNPs, R-TiO2, and UCNPs@R-TiO2. Human Embryonic Kidney 293 cells (HEK293) were selected for the assay. The temperature of the whole incubation process was controlled at 37 ◦C. The HEK293 cells present in a 96-well plate (with 10,000 cells per well) were first cultured for 12 h. After that, the different calculated concentrations of UCNPs, R-TiO2, and UCNPs@R-TiO2 nanomaterials were added to the above plate and the mixture was further incubated for 24 h. Afterwards, 100 μL of MTT solution was added to each well. After incubation for 2 h, the sediment was retained. Then, 100 μL of DMSO (dimethyl sulfoxide) was added into each sample mentioned above and the mixture was shaken for 20 min. The absorbance at 595 nm detected by a microplate reader was used to calculate the cell viability rate.
