Fluorescence Behavior of Octacalcium Phosphate Incorporating 3,5-Pyridinedicarboxylate Ions in Solutions of Different pH

Abstract

Octacalcium phosphate (OCP) can incorporate dicarboxylate ions into its crystal lattice, which modifies its functionalities. In this study, OCP incorporating 3,5-pyridinedicarboxylate (3,5-PD) ions was synthesized, and its fluorescence behavior in solution over a pH range of 5–9 was examined. Samples were prepared via a wet chemical method through the reaction of phosphoric acid with calcium carbonate in a 3,5-pyridinedicarboxylic acid (3,5-PDA) solution. X-ray diffraction (XRD) analysis revealed an increase in the (100) interplanar spacing from 1.87 to 2.25 nm, confirming the successful incorporation of 3,5-PD ions into the OCP structure. Furthermore, OCP incorporating 3,5-PD ions (3,5-PD-OCP) exhibited the maximum emission at approximately 370 nm under an optimal excitation wavelength of approximately 320 nm in both the powder and slurry states. Notably, neither the optimal excitation wavelength nor the maximum emission wavelength shifted within the pH range of 5–9. The nitrogen atom of the 3,5-PD ions was scarcely protonated in their solution state, and a similar behavior was expected in the incorporated state. Thus, the stable fluorescence wavelength of 3,5-PD-OCP can be attributed to its nitrogen atom remaining nonprotonated within the pH range of 5–9.