Surviving fraction of treated and untreated HeLa cells. (a) Surviving fraction of HeLa cells as a function of the concentration of TiO2 nanoparticles. HeLa cells were treated with 50, 100, 150, and 200 μg/mL TiO2, respectively, in the dark. The surviving fraction of untreated cells (control group) was set as 100%. (b) The photokilling effects of pure and N-TiO2 with different concentrations under visible irradiation. The inset is the transmittance spectrum of the combination of a 400 nm longpass filter and an IR cutoff filter used to acquire the visible-light irradiation from a Xe lamp.