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Magnetic Fe3O4 particles were synthesized based on a modified co-precipitation method [14]. First, deionized water was purged with nitrogen gas for 20 min. FeCl3 (3.89 g) and FeCl2·4H2O (3.18 g) were dissolved in 400 mL deionized water under nitrogen atmosphere with vigorous stirring at 25 °C. NH3·H2O (28% by w/w, 30 mL) was slowly added to the solution. Then, 10 mL polyethylene glycol (PEG 300) was added to the suspension. The mixture was held at 80 °C in a water bath for 1 h with constant stirring and then cooled to room temperature. The black precipitate was aggregated by placing an external magnet on the edge of the flask, and the solution was removed. The black magnetic Fe3O4 NPs were rinsed several times with deionized water and ethanol.Fe3O4/SiO2 core/shell NPs were prepared by the method previously reported [15] and [16]. The magnetic Fe3O4 NPs were dispersed in a water–ethanol solution. In order to obtain silica-coated magnetic NPs with a narrow particle size distribution, an ammonia solution (15 mL) and tetraethyl orthosilicate (2.0 mL) were consecutively added to the reaction mixture, and the reaction was allowed to proceed at 25 °C for 14 h. Then, (3-aminopropyl) triethoxysilane (APS) was added to the reaction mixture, and the mixture was stirred for another 24 h at 40 °C. The silica-coated magnetic NPs grafted with APS were collected by magnetic separation using a magnet and washed several times with deionized water and ethanol. The material obtained was referred to as Fe3O4/SiO2single bondNH2 NP.
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