Cells were seeded onto a 24-well multiplate (8 × 104/well) for culture for one day. After washing with PBS buffer, cells were incubated with a dispersion of TiO2 in MEM medium for 24 h or folic acid-modified TiO2 for 30 min for cellular uptake. After a wash, cells were mixed with 0.5 ml PBS and irradiated with UV at a distance of 1 cm from the 100 W long-wave ultraviolet lamp (Blak-Ray model B 100AP; UVP, Upland, CA) for 5 min, 15 min or 30 min. Cells were then subjected to viability assay.