Enhanced bioactivity of self-organized ZrO2 nanotube layer by annealing and UV irradiation

Superhydrophilic ZrO2 nanotube layer was prepared by anodic oxidation of commercial pure Zr in aqueous solutions containing 1 M (NH4)2SO4 and 0.15 M NH4F. The effect of annealing and ultraviolet (UV) irradiation treatment on the microstructure, water contact angle and bioactivity of the ZrO2 nanotube layer was investigated. The as-anodized nanotube layer consists of cubic and amorphous ZrO2, no apatite crystals are deposited on its surface even after immersion in simulated body fluids (SBF) for 30 days, exhibiting weak apatite-inducing ability. After annealing at 450 °C for 3 h, the nanotube layer is composed of cubic and monoclinic ZrO2, and its apatite-forming ability is significantly enhanced because of its lattice structure matching that of apatite, apatite can be induced after immersion in SBF for 15 days. UV irradiation of the ZrO2 nanotube layers does not alter their surface morphologies and phase components, however, can improve the bioactivity only when the ZrO2 nanotube layer is well crystallized. The enhanced bioactivity by UV irradiation is thought to result from the abundant basic ZrOH groups on the crystallized ZrO2 nanotube layer. Annealing and UV irradiation treatment do not alter the superhydrophilic nature of the ZrO2 nanotubes.