Crystallographic Facet-Induced Toxicological Responses by Faceted Titanium Dioxide Nanocrystals

Toxicological responses of nanomaterials have been closely correlated to their physicochemical properties, and establishment of a property-activity relationship of nanomaterials is favorable for a deep understanding of the nanomaterials' toxicity mechanism, prospectively predicting nanomaterials' potential hazards and rationally designing safer nanomaterials. Faceted nanomaterials usually exhibit more versatile and effective performance than spherical nanomaterials due to their selectively exposed crystallographic facets with high densities of unsaturated atoms. These facets have high surface reactivity, capable of eliciting strong interactions with biological systems. Few studies paid attention to the toxic behaviors of faceted nanomaterials in terms of their distinctive facets. In the present study, the toxicological role of the crystallographic facets of TiO₂ nanomaterials was investigated, and the precise property-activity relationship was exploited to clearly understand the toxicity of faceted nanomaterials. A series of faceted TiO₂ nanocrystals with the morphology of truncated octahedral bipyramids were prepared to expose different percentages of {101} and {001} facets on the surface. Density functional theory calculation revealed that {101} facets could only molecularly absorb water molecules while {001} facets due to their surface-unsaturated Ti atoms could dissociate the absorbed water molecules to generate hydroxyl radicals. Biophysical assessments corroborated the increased production of hydroxyl radicals on the {001} facets compared to {101} facets, which endowed {001} facets with strong hemolytic activity and elicited severe toxicities. A series of increased oxidative stress toxicological responses, including cellular ROS production, heme oxygenase-1 expression, cellular GSH depletion, and mitochondrial dysfunctions, were triggered by faceted TiO₂ nanocrystals with progressively increased {001} percentages, demonstrating the toxicological roles of {001} facets.