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

We realized the synthesis of a novel nanogranular system consisting of magnetite nanoparticles embedded in biomimetic carbonate hydroxyapatite(HA), for prospective uses in bone tissue engineering. An original two-step method was implemented: in the first step, 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; then,the magnetite nanoparticles are coated witha Ca(OH)2 layer,to induce the growth of HA directly on their surface, by reaction of Ca(OH)2 with HPO4 2−.Two nanogranular samples were collected with magnetite content∼0.8and∼4wt%. The magnetite nanoparticles and the composite material were investigated by x-ray diffraction, Fourier transform infrared spectroscopy and transmission electron microscopy.These analyses provided information on the structure of the nanoparticles(meansize∼6nm)and revealed the presence of surface hydroxylgroups, which promoted the subsequent growth of the HA phase, featuring a nanocrystalline lamellar structure.The magnetic study,by a superconducting quantum interference device magnetometer, has shown that both the as-prepared and the HA-coated magnetite nanoparticles are superparamagnetic at T=300K, but the magnetization relaxation process is dominated by dipolar magnetic interactions of comparable strength.In the three samples, a collective frozen magnetic regime is established belowT∼20K. These 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.