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

OBJECTIVES

Apatite minerals can have various anions and cations in their crystal structure in addition to phosphate ion (PO₄³⁻) and calcium ion (Ca). 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 HPO and 5.88 mM Ca (H-Ap). Four additives were used in solid solution methods: (i) 0.588 mM F (F-Ap), (ii) 5.88 mM Sr (Sr-Ap), (iii) 4.12 mM BO (BO3-Ap), and (iv) a surface pre-reacted glass ionomer (S-PRG) filler eluate that contained 0.17 mM Sr, 0.588 mM F, 11.1 mM BO (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 H-P heteronuclear-correlation NMR showed F ion incorporation in the apatite structure of the F-Ap and SPRG-Ap. The peaks on the P axis of the F-Ap, Sr-Ap, and BO-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∼BO-Ap< SPRG-Ap< Sr-Ap, suggesting the incorporation of the F, Sr and BO. The incorporation of F and BO was further confirmed by F and B NMR. The XRD revealed that Sr was preferentially incorporated into the CaII site.

SIGNIFICANCE

The F, Sr and BOions 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.