Co-substitution of carbonate and fluoride in hydroxyapatite: Effect on substitution type and content

The nanosized hydroxyapatite substituted by fluoride and carbonate ions (CFHA) had been synthesized by aqueous precipitation method. CFHA had been considered as potential bone graft material for orthopedic and dental applications. The objective of this study was to determine the effects of simultaneously incorporated CO^2--₃ and F^-- on the substitution type and content. The morphologies of CFHAs were observed by TEM. The carbonate substitution type and content were characterized by FTIR. The fluoride contents were determined by F-selective electrode. The phase compositions and crystallinity of the samples were investigated by XRD. The fluoride and carbonate contents of CFHA increase with the dopant concentrations nonlinearly. The carbonate substitution has much more obvious effect on morphology compared with the fluoride substitution. The co-existence of CO^2--₃ and F^-- ions can influence the corresponding substitution fraction. The isomorphic substitution of sodium for calcium in the substitution process of CO^2--₃ can improve crystal degree and favor the B-type substitutions. Due to the closeness of the ion radii and equivalent substitution of F^-- and OH^--, F^-- will occupy the OH^-- sites of HA crystals more easily, compelling most of the CO^2--₃ to be located in the B sites.