Apatite Coating of Iron Oxide Nanoparticles by Alternate Addition of Calcium and Phosphate Solutions: A Calcium and Carboxylate (Ca-COO) Complex-Mediated Apatite Deposition

Apatite is a biocompatible material widely used to encapsulate iron-oxide nanoparticles (IONPs) for biomedical applications, such as drug-delivery or fluorescent probe agent. Apatite-coated IONPs are commonly fabricated by initially incubating carboxylate-functionalized IONPs in calcium solution and directly adding phosphate solution to initiate apatite precipitation ( direct - addition method ). Apatite precipitation took place not only on IONPs surface but also in the bulk solution, resulting in apatite-IONPs mixture instead of coated structure. In this study, robust apatite-coated IONPs structure were aimed by modifying steps in direct - addition method . Initially, carboxylate-functionalized IONPs were incubated in calcium solution, physically separated from the incubating calcium solution by external magnet, and then separately reacted with phosphate solution to induce apatite deposition ( alternate - addition method ). Fourier-transform infrared (FTIR) analysis showed that a calcium solution at a concentration of 0.8 mol/L was required to initiate the formation of the calcium-carboxylate (Ca-COO) complex. The formation of non-stoichiometric apatite was confirmed for IONPs with Ca-COO complex, as evidenced by X-ray diffraction and FTIR analysis. The alternate - addition method produced apatite coating in the form of flake-like structures, which also exhibited strong adhesion to IONPs surface. In contrast, direct - addition method mainly produced agglomerate of apatite particles that weakly associated with IONPs. Both of apatite-coating methods did not alter the magnetic properties of IONPs. The simple modification of reaction steps in the widely used apatite-coating method was demonstrated to be beneficial in producing robust apatite-coated IONPs structure.