Cells after incubation with TiO2 and UV illumination were characterized by flow cytometry using Annexin-V-FITC and propidium iodide (PI) labeling. The staining solution was prepared by mixing 20 μl Annexin-V-FITC labeling reagent and 20 μl PI into 1 ml Annexin V binding buffer (all from Strong Biotech Corp). Cells released from 35 mm dish by using trypsin-EDTA were mixed with 3× MEM medium, centrifuged, and washed with PBS buffer. The cell pallet was then suspended in 100 μl of the staining solution and the resulting suspension was allowed for incubation for 10–15 min in dark at the room temperature. Finally, the stained cell suspension was added with 0.8 ml PBS buffer and subjected to flow cytometry analysis.