Phosphate-dependent morphological evolution of hydroxyapatite and implication for biomineralisation

• Hydroxyapatite (HAP) is formed at different phosphate concentrations.
• HAP morphology and architecture depend prominently on phosphate concentrations.
• Biological genetic and physicochemical factors may cooperate in biomineralization.

Hydroxyapatite (HAP) with various morphologies was prepared, in the absence of biological or organic molecules, through an ammonia gas diffusion method at room temperature. Contrary to the common consensus that crystal morphology control of biominerals is generally achieved by biological or organic molecules, our results suggest that PO43 − may also play a crucial role in the special morphogenesis of hydroxyapatite. The morphology, structure and composition of the obtained products were characterised by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). The FESEM and TEM analyses demonstrate that at a given concentration of Ca2 +, increasing PO43 − concentration leads to the formation of hydroxyapatite with various morphologies ranging from porous flower-like spheres, hollow bur-like spheres to solid bur-like spheres. If the PO43 − concentration remains constant, however, the porous flower-like spheres are always obtained at different concentrations of Ca2 +. For all concentrations of PO43 −, a series of time-resolved experiments reveal that the initial precipitate is always unstable amorphous calcium phosphate (ACP), and that the generation of the different morphologies originates from the dissolution of amorphous calcium phosphate, followed by the crystallisation and self-assembly of hydroxyapatite. Possible mechanisms are proposed for the formation of HAP with the different shapes and architectures. The dependence of HAP morphology on phosphate concentration suggests that, in biomineralisation, biological genetic and physicochemical factors can cooperatively influence the formation of hydroxyapatite with unusual morphologies and hierarchical structures.

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