Combined Toxicity of TiO2 Nanospherical Particles and TiO2 Nanotubes to Two Microalgae with Different Morphology

The joint activity of multiple engineered nanoparticles (ENPs) has attracted much attention in recent years. Many previous studies have focused on the combined toxicity of different ENPs with nanostructures of the same dimension. However, the mixture toxicity of multiple ENPs with different dimensions is much less understood. Herein, we investigated the toxicity of the binary mixture of TiO₂ nanospherical particles (NPs) and TiO₂ nanotubes (NTs) to two freshwater algae with different morphology, namely, <i>Scenedesmus obliquus</i> and <i>Chlorella pyrenoidosa</i>. The physicochemical properties, dispersion stability, and the generation of reactive oxygen species (ROS) were determined in the single and binary systems. Classical approaches to assessing mixture toxicity were applied to evaluate and predict the toxicity of the binary mixtures. The results show that the combined toxicity of TiO₂ NPs and NTs to <i>S. obliquus</i> was between the single toxicity of TiO₂ NTs and NPs, while the combined toxicity to <i>C. pyrenoidosa</i> was higher than their single toxicity. Moreover, the toxicity of the binary mixtures to <i>C. pyrenoidosa</i> was higher than that to <i>S. obliquus.</i> A toxic unit assessment showed that the effects of TiO₂ NPs and NTs were additive to the algae. The combined toxicity to <i>S. obliquus</i> and <i>C. pyrenoidosa</i> can be effectively predicted by the concentration addition model and the independent action model, respectively. The mechanism of the toxicity caused by the binary mixtures of TiO₂ NPs and NTs may be associated with the dispersion stability of the nanoparticles in aquatic media and the ROS-induced oxidative stress effects. Our results may offer a new insight into evaluating and predicting the combined toxicological effects of ENPs with different dimensions and of probing the mechanisms involved in their joint toxicity.