Octacalcium phosphate with incorporated carboxylate ions: a review

Octacalcium phosphate (OCP) belongs to a family of calcium phosphate compounds. OCP has unique crystal-chemical properties; among calcium phosphate compounds, only OCP can incorporate carboxylate ions into its crystal lattice. An OCP with incorporated carboxylate ions is called an OCP carboxylate (OCPC). OCPCs are investigated for applications in novel adsorbents, electrochemical devices, and biomaterials. Several wet methods are available for the synthesis of OCPCs, and the characteristics and advantages of each method are explained. Representative characterization methods, i.e. X-ray diffraction and Fourier transform infrared spectroscopy, used for the detection of carboxylate ion incorporation into the OCP interlayers are explained. Various carboxylic acids can be incorporated into OCP, and these types of carboxylic acid are presented with reference to the latest research results. The incorporation of carboxylate ions into OCP represents a modification of the OCP crystal at the molecular level and can impart various functions. Challenging physicochemical and biomaterial applications of OCPCs are thus introduced, although they are still in the research phase. Finally, future perspectives and challenges for OCPC research are described.