Octacalcium phosphate (OCP), which has a layered structure composed of an apatitic layer and a hydrated layer, is a precursor of hydroxyapatite (HAp). Although the topotactic transformation of OCP to HAp is a well-known phenomenon, its mechanism remains unclear. In this study, to clarify the role of the OCP hydrated layer in the mentioned transformation, we investigated the behavior of the transformation of OCP with hydrated layers of different thicknesses to HAp under hydrothermal conditions. We used three types of samples: plain OCP (Pure-OCP), OCP with incorporated succinate ions (Suc-OCP), and OCP with incorporated suberate ions (Sub-OCP). We found that all three OCP types transformed topotactically into HAp. The order of reactivity was Sub-OCP > Suc-OCP > Pure-OCP, which corresponds to the thickness of the hydrated layer. Since the transformation involves a compositional change, our finding implies that the hydrated layer acts as a diffusion (mass transfer) passage for dicarboxylate ions and hydrogen phosphate ions located within it, which move outside the crystal during the topotactic transformation. Our results thus contribute to a better understanding of the topotactic transformation mechanism of OCPs to HAp.

Octacalcium phosphate (OCP; Ca₈(HPO₄)₂(PO₄)₄ ∙ 5H₂O) is a precursor of hydroxyapatite found in human bones and teeth, and is among the inorganic substances critical for hard tissue formation and regeneration in the human body. OCP has a layered structure and can incorporate carboxylate ions into its interlayers. However, studies involving the incorporation of tetracarboxylic and multivalent (pentavalent and above) carboxylic acids into OCP have not yet been reported. In this study, we investigate the incorporation of pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid), a type of tetracarboxylic acid, into OCP. We established that pyromellitate ions could be incorporated into OCP by a wet chemical method using an acetate buffer solution containing pyromellitic acid. The derived OCP showed a brilliant blue emission under UV light owing to the incorporated pyromellitate ions. Incorporation of a carboxylic acid into OCP imparted new functions, which could enable the development of novel functional materials for biomedical applications.