Synthesis and characterization of magnetic nanogranular Fe3O4/biomimetic hydroxyapatite for potential applications in nanomedicine

A major concern about the biomedical application of magnetic nanoparticles is their biocompatibility. A possible solution is coating them with hydroxyapatite (HA) [Ca5(PO4)3OH], which is the inorganic component of biological hard tissues, e.g. bone and teeth. This approach appears especially appealing for uses in the field of bone tissue engineering. We have synthetized a novel nanogranular system, consisting of magnetite nanoparticles embedded in biomimetic carbonate HA, through an original two-step method: i) magnetite nanoparticles are prepared by refluxing an aqueous solution of Fe(SO4) and Fe2(SO4)3 in an excess of Tetrabutilammonium hydroxide acting as surfactant; ii) the nanoparticles are coated with a Ca(OH)2 layer, to induce the growth of HA directly on their surface, by reaction of Ca(OH)2 with HPO42-. Two samples have been collected with magnetite content ~ 0.8 wt. % and ~ 4 wt.%. The magnetite nanoparticles and the nanogranular material have been investigated by X-ray Diffraction, Fourier Transform Infrared Spectroscopy and Transmission Electron Microscopy. These analyses have provided structural information on the as-prepared nanoparticles (mean size ~ 6 nm) and revealed the presence of surface hydroxyl groups, which promote the growth of the HA phase featuring a nanocrystalline lamellar structure. Hysteresis loops (temperature range 5-300 K), thermal and time dependence of the magnetization under different magnetic fields and field dependence of the remanence have been measured by SQUID magnetometer. Both the as-prepared and the HA-coated magnetite nanoparticles are superparamagnetic at T=300 K. However, the magnetization relaxation process is affected by dipolar magnetic interactions of comparable strength in the three samples, also inducing the onset of a collective frozen magnetic regime below T~20 K. The results indicate that the magnetite nanoparticles tend to form agglomerates in the as-prepared state, which are not substantially altered by the HA growth, coherently with the creation of electrostatic hydrogen bonds among the surface hydroxyl groups.