Effect of borate, fluoride and strontium ions on biomimetic nucleation of calcium phosphate studied using solid-state nuclear magnetic resonance and X-ray diffraction

Abstract

Objectives: Apatite minerals can have various anions and cations in their crystal structure in addition to phosphate ion (PO₄³⁻) and calcium ion (Ca²⁺). The aim of this study is to investigate effects of the borate, fluoride and strontium ions on biomimetic nucleation of calcium phosphate.

Methods: Nano-crystalline hydroxyapatite (H-Ap) was obtained from a supersaturated buffered solution containing 4.12 mM HPO₄^2- and 5.88 mM Ca²⁺ (H-Ap). Four additives were used in solid solution methods: (i) 0.588 mM F^- (F-Ap), (ii) 5.88 mM Sr²⁺ (Sr-Ap), (iii) 4.12 mM BO₃^3- (BO3-Ap), and (iv) a surface pre-reacted glass ionomer (S-PRG) filler eluate that contained 0.17 mM Sr²⁺, 0.588 mM F^-, 11.1 mM BO₃^3-- (SPRG-Ap). Apatite crystallization was investigated using a solid-state magic-angle spinning NMR spectroscopy and X-ray diffraction (XRD) with the Rietveld analysis.

Results: A 2D ¹H-³¹P heteronuclear-correlation NMR showed F^- ion incorporation in the apatite structure of the F-Ap and SPRG-Ap. The peaks on the ³¹P axis of the F-Ap, Sr-Ap, and BO₃-Ap were different from that of the H-Ap, and the full width at half maximum increased in the following order: H-Ap∼F-Ap∼BO₃-Ap< SPRG-Ap< Sr-Ap, suggesting the incorporation of the F^-, Sr²⁺ and BO₃^3-. The incorporation of F and BO₃ was further confirmed by ¹⁹F and ¹¹B NMR. The XRD revealed that Sr²⁺ was preferentially incorporated into the CaII site.

Significance: The F^-, Sr²⁺ and BO₃^3-ions might be involved in modifying the crystallization of apatite precipitation, producing a variety of apatite. S-PRG filler that release these ions may have an effect on remineralization, i.e., the reformation of apatite lost due to caries.

Keywords: Apatite; Boron; Fluoride; Heteronuclear; Solid-state nuclear magnetic resonance; Strontium; Surface pre-reacted glass ionomer filler; X-ray diffraction.