Synthesis and fluorescence properties of octacalcium phosphate with incorporated pyridinedicarboxylate ions

Octacalcium phosphate (OCP) has a layered structure and various carboxylate ions can be incorporated into its structure as interlayer ions. In recent studies, aromatic carboxylate ions have been incorporated into OCP to impart fluorescence properties. In such fluorescent OCP materials, the incorporated carboxylate ions can be regarded as ‘crystal defect cores’. Although fluorescent OCPs with incorporated benzenedicarboxylate ions have been reported, the synthesis and fluorescence of OCPs with incorporated pyridinedicarboxlate ions have not yet been investigated. In this study, our aim was to synthesise OCP with incorporated 2,5-pyridinedicarboxylate ions and evaluate its fluorescence behaviour. Samples were synthesised by reacting calcium carbonate and phosphoric acid in 2,5-pyridinedicarboxylic acid solution. When the 2,5-pyridinedicarboxylic acid concentration in the solution was low, plate-shaped crystals of OCP with incorporated 2,5-pyridinedicarboxylate ions were formed. As the 2,5-pyridinedicarboxylic acid concentration increased, OCP formation was suppressed, and a poorly soluble salt composed of calcium and 2,5-pyridinedicarboxylate ions was formed in its place. The (100) interplanar spacing of OCP was increased from 1.87 to 2.26 nm by the incorporation of 2,5-pyridinedicarboxylate ions as interlayer ions. The incorporation of 2,5-pyridinedicarboxylate ions was confirmed by Fourier-transform infrared spectroscopy. OCP with incorporated 2,5-pyridinedicarboxylate ions exhibited fluorescence with an emission wavelength of 430 nm upon excitation at 320 nm. The results of this study should prove useful for the design and development of fluorescent OCP materials.