Effect of Size and Crystalline Phase of TiO₂ Nanoparticles on Photocatalytic Inactivation of Escherichia coli.

Abstract

We studied the size dependent toxicity of TiO₂ nanoparticles (TiO₂ NPs; 5-50 nm) of the anatase and rutile crystalline phases (including the mixture of anatase and rutile) against the model organism Escherichia coli. All the TiO₂ NPs were characterized and their photocatalytic inactivation of E. coli was studied under the solar simulated light irradiation and dark conditions. In addition, the mechanism of toxicity was studied by measurement of reactive oxygen species (ROS), an indicator of oxidative stress. Rutile TiO₂ NPs (TiO₂-R-30 nm) of 30 nm showed the highest photocatalytic activity against E. coli (LC50 of 14.11 mg/L), followed by rutile TiO₂ NPs (TiO₂-R-50 nm) (LC50 of 35.96 mg/L). The anatase and rutile mixture of 20 nm size produced LC50 of 17.12 and 27.26 mg/L for A80%-R20% and A20%-R80% respectively, whereas none of the anatase TiO₂ NPs with various sizes (5 nm, 15 nm and 30 nm) showed any toxicity against E. coli. The results indicate that the rutile had higher photocatalytic activity than anatase and the toxicity is size dependent, while the mixture of anatase and rutile had the median toxicity. Hydroxyl radical formation is the major ROS causing oxidative stress in E. coli, the primary mechanism of toxicity.