Dental enamel-like hydroxyapatite transformed directly from monetite

Up to now, the strategy to fabricate biomimetic dental enamel-like hydroxyapatite (HA) has attracted great interest due to the extremely high degree of architectural organization of HA crystals. In this study, in the absence of surfactants, organic solvent or template-directing agents, a facile method via direct transformation from monetite (CaHPO4) single crystals was developed to synthesize HA with enamel-like hierarchical structures on the nano- to microscale. The suitably shaped CaHPO4 precursor was first synthesized under simultaneous irradiation of microwave and ultrasound. The highly oriented HA bundles, constructed of tiny crystals in a parallel structure, were then fabricated through microwave treatment of the precursor in alkaline aqueous solution. The present study indicated that the formation of the highly oriented hierarchical structure of the obtained HA was due to the intrinsically ordered structure of CaHPO4, the structural similarity between CaHPO4 and HA as well as the ultrafast transformation rate.