The cell viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) method, which was based on measuring the activity of living cells via mitochondrial dehydrogenases. Cells were washed with PBS buffer and then incubated with MTT (Sigma) solution (0.5 mg MTT/ml in PBS) at 37 °C for 4 h. After incubation, an aliquot of MTT solubilization solution (10% Triton X-100 plus 0.1 N HCl in anhydrous isopropanol) was added to dissolve the resulted formazan crystals. Afterward, the product was quantified by measuring absorbance at 570 nm. The MTT stock solution (0.5 mg MTT/ml PBS) was stored at 4 °C in dark not longer than 2 weeks and filtered with a 0.22 μm filter prior to use. The cell survival was defined as the ratio of the viability of treated cells and that of non-treated control (neither TiO2 treatment nor UV illumination).