Assessing the effects of surface-bound humic acid on the phototoxicity of anatase and rutile TiO2 nanoparticles in vitro

In this study, the cytotoxicity of two different crystal phases of TiO2 nanoparticles, with surface modification by humic acid (HA), to Escherichia coli, was assessed. The physicochemical properties of TiO2 nanoparticles were thoroughly characterized. Three different initial concentrations, namely 50, 100, and 200 ppm, of HA were used for synthesis of HA coated TiO2 nanoparticles (denoted as A/RHA50, A/RHA100, and A/RHA200, respectively). Results indicate that rutile (LC50 (concentration that causes 50% mortality compared the control group) = 6.5) was more toxic than anatase (LC50 = 278.8) under simulated sunlight (SSL) irradiation, possibly due to an extremely narrow band gap. It is noted that HA coating increased the toxicity of anatase, but decreased that of rutile. Additionally, AHA50 and RHA50 had the biggest differences compared to uncoated anatase and rutile with LC50 of 201.9 and 21.6, respectively. We then investigated the formation of reactive oxygen species (ROS) by TiO2 nanoparticles in terms of hydroxyl radicals (OH) and superoxide anions (O2−). Data suggested that O2− was the main ROS that accounted for the higher toxicity of rutile upon SSL irradiation. We also observed that HA coating decreased the generation of OH and O2− on rutile, but increased O2− formation on anatase. Results from TEM analysis also indicated that HA coated rutile tended to be attached to the surface of E. coli more than anatase.