Regular articles

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

Xiaojia He , Sabrieon Sanders , Winfred G. Aker , Yunfeng Lin , Jessica Douglas , Huey-min Hwang


Received March 18, 2015,Revised May 17, 2015, Accepted May 22, 2015, Available online September 01, 2015

Volume 28,2016,Pages 50-60

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 (radical dotOH) and superoxide anions (O2radical dot). Data suggested that O2radical dot 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 radical dotOH and O2radical dot on rutile, but increased O2radical dot 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.

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