The Fe3O4 NPs were prepared as reported in the literature
[23]. Typically, to prepare Fe3O4 NPs 5.2 g of FeCl3 and 2.0 g
of FeCl2 were successively dissolved in 25 ml of distilled water
containing 0.85 ml of 12.1 mol/L HCl. The resulting solution
was added dropwise into 250 ml of a 1.5 mol/L NaOH solution
under vigorous stirring. The last step generated an instant black
precipitate. The precipitate was isolated in a magnetic field and
the supernatant was removed from the precipitate by decantation.