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

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.