Effect of surface charge of TiO2 particles on hydroxyapatite formation in simulated body fluid

- The low-PO2 TiO2 particles exhibited high HAp-forming ability.
- Ti4+ in TiO2 was reduced to Ti3+ and the oxygen vacancy was introduced.
- TiO2 particles was negatively charged.
- The calcium ions were predominantly adsorbed on the TiO2 particles in the SBF.

TiO2 particles formed by heat treatment in a nitrogen atmosphere with an extremely low oxygen partial pressure exhibited high hydroxyapatite (HAp)-forming ability in simulated body fluid (SBF). X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) indicated that the particles were rutile-type TiO2, which contains hardly any nitrogen. Electron spin resonance (ESR) spectra revealed the reduction of Ti4+ in TiO2 to Ti3+ and the simultaneous introduction of oxygen vacancies to maintain charge neutrality. The enhanced HAp-forming ability of the TiO2 particles was likely related to their negatively charged surface, induced by water adsorption on oxygen vacancies, which resulted in acidic point formation. In contrast, TiO2 particles formed by heat treatment in air showed low HAp-forming ability because their surfaces had almost zero charge.

59