Zn and Ag Co-doped Anti-microbial TiO₂ Coatings on Ti by Micro-arc Oxidation

Micro-porous TiO₂ coatings co-doped with Zn²⁺ and Ag nanoparticles were fabricated on Ti by micro-arc oxidation (MAO) for 0.5, 1.5, 2 and 4 min, respectively. The evolutions of morphology and phase component of the coating as a function of processing time were investigated. The microstructure of the 2 min treated coating was further observed by transmission electron microscopy to explore the coating formation mechanism. The amounts of Ag and Zn released from the 2 min treated coating were measured and the antibacterial properties of the coatings against Staphylococcus aureus (S. aureus) were also investigated. The obtained results showed that with prolonged MAO time, the contents of Ag and Zn on the coating surfaces increased. All the coatings were micro-porous with pore diameters of 1–4 µm; however, some pores were blocked by deposits on the 4 min treated coating. The 2 min treated coating was composed of amorphous TiO₂, anatase, rutile, ZnO, Zn₂TiO₄ and homogenously distributed Ag nanoparticles. After immersion, Zn²⁺, Ag⁺, Ti²⁺ and Ca²⁺ were released from the coating and with the immersion time prolonged, the accumulated concentrations of these ions increased. After immersion for 36 weeks, the accumulated Zn²⁺ and Ag⁺ concentrations were 6.88 and 0.684 ppm, respectively, which are higher than the minimal inhibitory concentration but much lower than the cytotoxic concentration. Compared with polished Ti control, the coatings co-doped with Zn²⁺ and Ag nanoparticles significantly inhibited the adhesions of S. aureus and reduced the amounts of planktonic bacteria in culture medium, indicating that the Zn and Ag co-doped TiO₂ could be a bio-adaptable coating for long-lasting anti-microbial performance.