Synthesis of Stabilized Fe3O4 NPs

The Fe₃O₄ NPs were synthesized by a thermal decomposition method (Tang et al., 2016). Firstly, sodium oleate and ferric chloride were dissolved with the mole ratio of 3:1 in a mixture containing hexane, distilled water and ethanol. Then the as-obtained complex was heated at 70°C for 4 h followed by washed and dried in vacuum for 24 h. After that, we dissolved oleate acid (3.1 g) and iron oleate complex (20 g) in 1-octadecene with evenly stirring at ambient temperature. Next, the solution was slowly heated to 250°C and maintained for 1 h. Afterward, the mixture continued to be heated to 320°C and maintained the temperature for another 45 min. The reactants were finally cooled to ambient temperature and ethanol (500 ml) was added in company. After centrifugation at 7500 rpm for 10 min, the precipitation of Fe₃O₄ nanocrystals was obtained and washed three times using ethanol and hexane mixture solution (v:v, 1:1).