Diameter-sensitive biocompatibility of anodic TiO₂ nanotubes treated with supercritical CO₂ fluid

This work reports on the diameter-sensitive biocompatibility of anodic TiO₂ nanotubes with different nanotube diameters grown by a self-ordering process and subsequently treated with supercritical CO₂ (ScCO₂) fluid. We find that highly hydrophilic as-grown TiO₂ nanotubes become hydrophobic after the ScCO₂ treatment but can effectively recover their surface wettability under UV light irradiation as a result of photo-oxidation of C-H functional groups formed on the nanotube surface. It is demonstrated that human fibroblast cells show more obvious diameter-specific behavior on the ScCO₂-treated TiO₂ nanotubes than on the as-grown ones in the range of diameters of 15 to 100 nm. This result can be attributed to the removal of disordered Ti(OH)₄ precipitates from the nanotube surface by the ScCO₂ fluid, thus resulting in purer nanotube topography and stronger diameter dependence of cell activity. Furthermore, for the smallest diameter of 15 nm, ScCO₂-treated TiO₂ nanotubes reveal higher biocompatibility than the as-grown sample.