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Well-defined silica-magnetite core-shell nanospheres were prepared via a modified sol–gel method. Sphere-like magnetite aggregates were obtained as cores of the final nanospheres by assembling in the presence of Tween 20. Characterization by transmission electron microscopy (TEM) showed spherical morphology of the nanospheres with controlled silica shell thickness from 9 to 30 nm, depending on the amount of tetraethoxysilane (TEOS) used. The nanospheres contained up to 41.7 wt% magnetite with a saturation magnetization of 21.8 emu/g. Up to 35 μg/mg of the model biomolecule streptavidin (SA) could be bound covalently to the hydrophilic silica nanospheres.

Monodispersed, hydrophilic, superparamagnetic magnetic nanospheres with a high fraction of magnetite were synthesized by combining modified miniemulsion/emulsion polymerization and sol-gel technique for the first time. The surface of the nanospheres was coated by a silica layer with controlled thickness. Transmission electron microscopy experimental results showed well-proportioned, equal-sized, magnetite/polystyrene (Fe3O4/PS) nanospheres with a thin silica shell. Based on the TGA data, the fraction of magnetite in the Fe3O4/PS nanospheres core was estimated to be 80 wt %. Magnetization measurements indicated that the superparamagnetic nature of the nanospheres had high saturation magnetization of 40 emu/g at 300 K. The procedures of the novel synthesis are described in detail. Also discussed are the mechanisms of the novel combined miniemulsion/emulsion polymerization processes.