The synthesis process of GO-Fe3O4 nanocomposite is schematically illustrated in Figure 1. Twenty milligrams of each of graphene oxide sample (B-GO and N-GO) was exfoliated in 60 mL H2O by ultrasonication to produce a homogeneous graphene oxide water-based suspension. Then, the carboxylic groups on the graphene oxide surface were activated with 8 mg of N-hydroxysuccinimide (NHS) and 10 mg of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC). The surface of iron oxide (20 mg) was modified with oleic acid (3 mL) and then stirred and sonicated for 1 h. The mixture of modified iron oxide and graphene oxide was stirred for 48 h. In the next step, the mixture was filtered by a polycarbonate membrane and washed several times with water and ethanol. Finally, the obtained product was dried for 12 h at 100°C. The sample with broad size distribution of graphene oxide flakes is named B-GO-Fe3O4, and the sample with narrow size distribution is named N-GO-Fe3O4