Synthesis and characterization of Fe3O4@SiO2 core-shell magnetic nanoparticles.

Magnetic nanoparticles, such as magnetite (Fe3O4 NPs), have gained considerable attention due to their high potential application in medicine, chemistry, material engineering and biomedicine. However, these materials are susceptible to slow oxidation by atmospheric oxygen, as well as being relatively unstable and poorly dispersed in aqueous media. Core-shell structures represent attractive alternatives to overcome such disadvantages by combining core magnetic properties with shell stability and dispersibility, usually composed of polymers, metal nanoparticles or silica. This work reports the synthesis of Fe3O4 NPs by precipitation of ferrous hydroxide followed by partial oxidation to magnetite. Subsequent, core-shell sphere structures (Fe3O4@SiO2) were obtained through the Stöber method. Both materials, Fe3O4 NPs and Fe3O4@SiO2, were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and Infrared spectroscopy. The SEM images reveal the formation of Fe3O4 nanoparticles with octahedral morphology (Fig. 1A), while the silica-coated nanoparticles adopted spherical shape (Fig. 1B). The diffraction peaks correspond to Fe3O4 planes (220), (311), (222), (400), (422), (511), (440), (620), (533), (622) and (444) (Fig. 1C). Besides the magnetite diffraction pattern, the Fe3O4@SiO2 exhibits a wide band corresponding to amorphous silica in the range of 20-27. The FT-IR spectra exhibit the Si-O-Si asymmetric stretching vibration at 1020-1100 cm-1. Finally, the magnetic susceptibilities were measured; the saturation magnetization values of Fe3O4@SiO2 (12,5 emu/g) are much lower than that of Fe3O4 NPs (81,0 emu/g), probably because of the thick shells coated on magnetic nanoparticles.