Synthesis of Fe3O4@SiO2 microsphere

Synthesis of Fe3O4@SiO2 microspheres
The Fe3O4 nanoparticles were synthesized by the solvothermal method [38]. The Fe3O4@SiO2 core–shell microspheres were prepared according to the previously reported method with some modification [22] and [38]. In brief, freshly prepared Fe3O4 particles (1.0 g) were treated with 0.1 M HCl aqueous solution by ultrasonication for 10 min, and then washed with ultrapure water. Then, the microspheres were re-dispersed in a mixture of ethanol (320 mL), ultrapure water (80 mL) and NH3·H2O solution (5.0 mL, 25–28%, w/w). At last, TEOS (0.5 mL) was added into the above dispersion system after ultrasonic vibration for 10 min. The reaction was allowed to proceed under mechanical stirring for 6 h. The resulting particles were collected with a magnet, and washed with ethanol and ultrapure water for several times.