Abstract
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.
Highlights
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 with incorporated pyromellitate ions was synthesised by the hydrolysis of dicalcium phosphate dihydrate (DCPD; CaHPO4·2H2O) in an acetate buffer containing pyromellitic acid
The advantages of this synthetic method are the pH stability and relatively low calcium (Ca) ion concentration during the synthesis process. Both the crystalline phases of calcium phosphates synthesised by wet processes, and the dissociation states of pyromellitate ions strongly depend on the pH of the reaction solution
Summary
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. Previous studies suggested the presence of a similar layered structure of OCP that incorporated a type of tricarboxylate ion (citrate ion) in bone minerals[24]. The process of carboxylate ion-incorporation in OCPs is similar to the intercalation of inorganic layered compounds, the two processes differ greatly in terms of guest selectivity. As the phthalic acid dimer has a similar structure to that of pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid, the molecular structure of which is shown as Fig. 1c), the incorporation of pyromellitate ions into OCP could be realised, despite pyromellitic acid being a tetracarboxylic acid
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