Preparation of Fe3O4@SiO2 Spheres.

Silica was formed to coat Fe₃O₄ nanoparticles via a reverse microemulsion method (49). Five hundred milligram of Igepal CO-520 was added into 10 mL cyclohexane and sonicated for 10 min. Then, 4 ml Fe₃O₄ solution (2 mg/mL in cyclohexane) was added to the above solution under ultrasound for 30 min at room temperature. Subsequently, 2 mL NH₄OH (25 to 28%) was added to the above solution, and 2 mL TEOS was added drop by drop. After stirring at room temperature for 10 h, the resulting Fe₃O₄@SiO₂ core-shell spheres were collected after adding ethanol and centrifuging. After washing with ethanol for three times, Fe₃O₄@SiO₂ core-shell nanoparticles were dispersed in ethanol for further use.