Effects of material morphology on the phototoxicity of nano-TiO₂ to bacteria

Nanostructured titania (nano-TiO₂) is produced in diverse shapes, but it remains largely unknown how tuning the morphology of nano-TiO₂ may alter its toxicity. Herein, we show that material morphology plays a critical role in regulating the phototoxicity of nano-TiO₂ to bacteria. Low-dimensional nano-TiO₂, including nanotubes, nanorods, and nanosheets, were synthesized hydrothermally, and their effects on the bacterial viability of Escherichia coli and Aeromonas hydrophila were compared to spherical nanostructures (anatase nanospheres and P25). Results reveal that TiO₂ nanotubes and nanosheets are less phototoxic than their rod- and sphere-shape counterparts under simulated solar irradiation. None of the tested nano-TiO₂ shows toxicity in the dark. In contrast to their diminished phototoxicity, however, TiO₂ nanotubes and nanosheets exhibit comparable or even higher photoactivity than other nanostructures. Observations by scanning transmission electron microscopy suggest that material morphology influences nano-TiO₂ phototoxicity by governing how nano-TiO₂ particles align at the bacterial cell surface. Overall, when comparing materials with different morphologies and dimensionality, nano-TiO₂ phototoxicity is not a simple function of photocatalytic reactivity or ROS production. Instead, we propose that the evaluation of nano-TiO₂ phototoxicity encompasses a three-pronged approach, involving the intrinsic photoactivity, aggregation of nano-TiO ₂, and the nano-TiO₂/bacteria surface interactions.