Abstract
We investigated the structure and acid resistance of dissolution–precipitation reaction products obtained through fluoride treatment of β-tricalcium phosphate (TCP) particles using two types of fluoride-containing acetic acid (Ac)-sodium acetate (AcNa) buffer solutions with or without added pyrophosphate (PP) ions. X-ray diffraction and nuclear magnetic resonance spectroscopic analyses confirmed that fluoride promotes dissolution–precipitation reactions on the surface of β-TCP in fluoride treatment solutions, resulting in the formation of fluorine-substituted hydroxyapatite (F-HAp) and calcium fluoride (CaF2). In this dissolution–precipitation reaction in acidic solutions with high fluoride concentrations, hydrolysis of β-TCP and precipitation of spherical CaF2 particles occurred, resulting in the formation of raspberry-like aggregates covering the surface of the β-TCP. Subsequently, rod-shaped F-HAp particles formed in the gaps between CaF2 particles and at the interface between CaF2 particles and β-TCP particles. The addition of PP ions in the fluoride treatment solution inhibited the hydrolysis reaction of β-TCP, and higher PP concentrations suppressed the formation of F-HAp and CaF2. Results of the acid resistance evaluation indicated that dissolution–precipitation products of β-TCP, which was composed of a core–shell-type structure with F-HAp core and CaF2 shell, had a decreased initial dissolution rate in Ac-AcNa buffer solutions.
