LaMer-model-based synthesis method for fine particles of octacalcium phosphate and related functional compounds†

Octacalcium phosphate (OCP) is a fascinating calcium phosphate material with a layered structure that can incorporate various guest molecules, particularly dicarboxylate ions. However, the solution-phase synthesis of OCP fine particles is challenging. In this study, we developed a new precipitation method based on the LaMer model for synthesising fine particles of plain OCP and OCP with incorporated isophthalate and succinate ions. Highly concentrated aqueous solutions and appropriate reaction conditions yielded particle sizes of 100–200 nm for plain OCP and 200–300 nm for OCP with incorporated dicarboxylate ions. The obtained particle sizes were significantly smaller than those of OCPs synthesised using conventional methods. The incorporation of isophthalate and succinate ions into OCP was confirmed using various methods, particularly X-ray diffraction, which revealed expansion of the (100) interplanar spacing. During the synthesis of OCP with incorporated isophthalate ions, hydrolysis of the OCP phase was minimised when the initial pH range of the isophthalic acid solution was 5.2–5.3. Interestingly, OCP with incorporated isophthalate ions exhibited fluorescence (excitation and emission at 325 and 390 nm, respectively), demonstrating the potential of these fine particles as novel biofriendly fluorescent probes. This novel LaMer-model-based synthesis for preparing fine particles of OCP and OCP with incorporated functional carboxylate ions can contribute to the design and development of next-generation biofriendly imaging probes and other functional calcium phosphate materials, such as for use in biosensing and drug delivery applications.