Synthesis and characterization of nanogranular Fe3O4/biomimetic hydroxyapatite

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. Moreover, a Mössbauer spectroscopy investigation on the as-prepared nanoparticles has been carried out between T = 4 K and room temperature; to better distinguish the contribution coming from Fe different lattice sites, low temperature measurements were performed with and without an applied magnetic field of 8 T; the field was applied parallel to the direction of the incoming 14.4 keV -radiation direction. Room temperature Mössbauer spectra show just the presence of a doublet contribution; below 200 K, a sextet contribution appears and it gets narrower as T is decreased. If the sextet is described in terms of a hyperfine fields distribution, that shows three main contributions that shift to higher hyperfine fields values as T approaches 4 K. At 4 K, with the external magnetic field, two sextets can be clearly observed corresponding to Fe in tetrahedral and octahedral sites, showing a slight canting with respect to the field direction. Both the as-prepared and the HA-coated magnetite nanoparticles show a superparamagnetic behaviour 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.