White and black anodic TiO2 nanotubes: Comparison of biological effects in A549 and SH-SY5Y cells

In this work, the cytotoxicity of black TiO2 nanotubes in comparison to their white counterparts is exploited for the first time. To provide a comprehensive overview, the toxicity is also assessed in comparison to multi-walled carbon nanotubes (MWCNTs) using two different cell lines. The TiO2 nanotube layers were prepared by anodic oxidation of Ti foils and investigated in the as-prepared white form, and in the black form, achieved by reduction via comproportionation reaction of Ti and TiO2. Additionally, a part of the black TiO2 nanotube layers was coated with a thin TiO2 coating with a nominal thickness of ~0.275 nm by Atomic Layer Deposition (ALD). Pulmonary A549 cells and neural SH-SY5Y cells were exposed to a wide range of TiO2 nanotube concentrations (1–500 μg/mL) for 24 and 48 h. Then, the cytotoxicity was evaluated using cell viability and glutathione assessments. Essentially, no reduction in cell viability was observed after incubating the cells with TiO2 nanotubes up to 100 μg/mL, while MWCNTs decreased the cell viability at a similar concentration significantly. In addition, ALD TiO2 coating on the black TiO2 nanotubes caused disappearance of mild toxic effects of white and black TiO2 nanotubes due to the shading of carbon and fluorine species incorporated within the TiO2 nanotube walls.