Direct Immobilization of Folic Acid Molecules on Hydroxyapatite Nanoparticles with Substitution and Coordination Phenomena.

We successfully synthesized folic acid (FA) immobilized hydroxyapatite (HA) nanoparticles without using a mediative reagent (e.g., silane coupling agent), and the immobilization states were evaluated and discussed. The HA nanoparticles with higher biocompatibility have two different planes, namely, - and -planes. These plane surfaces are rich in phosphate groups (P-site) and Ca ions (C-site), respectively. We suggested that during the synthesis of the HA nanoparticles, the P-site substitution and C-site coordination with the addition of organic molecules containing -COO ions can occur. Thus, it is possible to simultaneously immobilize two molecules to one HA nanoparticle. In this study, we successfully synthesized FA-immobilized HA nanoparticles by P-site substitution and C-site coordination reactions, which were named as substitution type and coordination type. In the substitution type, when FA was reacted with HA during the nucleation stage, the PO ions of HA decreased as the FA ratio of coverage surface area increased, and the crystalline phase was changed significantly from the Ca deficient HA to the carbonated HA phase. Accordingly, it was indicated that FA was immobilized on HA by the P-site substitution. In the coordination type, since FA was reacted with HA after the completion of crystal growth, the crystalline phase was changed slightly as the FA ratio of coverage surface area increased, indicating that FA was immobilized on HA by the C-site coordination. From the above, we controlled the FA immobilization states on the HA nanoparticles by the P-site substitution and the C-site coordination through the FA addition timing in the synthesis. Since the -COO ions in FA could be selectively substituted with the P-site in HA, it is possible to directly coordinate the foreign organic molecules to the Ca ions in HA. Therefore, the immobilization technique of this study is expected to achieve two different drug molecules with diagnosis and therapy functions (i.e., theranostics) on one nanoparticle.