Firstly,
to prepare Fe₃O₄ , NH₄ Fe(SO₄) ₂ ·12H₂O (8.35 g) and (NH₄)₂Fe(SO₄)₂·6H₂O(3.40
g) were dissolved in 400mL water at 50 °C to synthesize iron oxides under a N₂ atmosphere. Then, the iron oxides were precipitated with 20 mL NH₃·H₂O
(8 mol/L). The mixed solution was sonicated (200W, 40kHz) for 10 min. After
that, the final mixture was adjusted to pH11. The reaction was allowed to
proceed at 50 °C and stirred for 1 h. The precipitate was separated by an
external magnetic field and washed with double-distilled water to pH7. Next,
1.7mL PVP (25.6 g/L) was added to the mixture that the obtained precipitate was
dispersed in 20 mL water. The mixture was stirred vigorously at room
temperature for 1 day. The Fe₃O₄ microspheres were
gathered by an external magnetic field, which were washed with water and
ethanol, respectively. The product was dried at 25 °C.

In 350mL
ethanol/water (4:1, v/v), 1.4g Fe₃O₄ microspheres were
dispersed and 5 mL NH₃·H₂O was stirred at30 °C for 15
min. Four-milliliter TEOS was added in the dispersion, and the mixture was
stirred at 25 °C for 8 h. The product was magnetically separated and rinsed
three times with ethanol. The obtained Fe₃O₄@SiO₂ was dispersed in 140 mL isopropanol.

To
40-mL Fe₃O₄@SiO₂ solution which was diluted
with 100 mL isopropanol, 0.2 mL APTES was added. The mixture was sonicated (200
W, 40 kHz) for 30 min under a N₂ atmosphere and kept stirring at 70
°C for 6 h to obtain Fe₃O₄@SiO₂-NH₂.
Finally, the products were washed with water and ethanol, respectively, and
dried under vacuum.

In 50
mL of DMF, 0.2g GO was dispersed with the aid of ultrasonication. Then, 0.1g of
NHS and 0.2 g of EDC were added to the dispersion. The mixture was stirred
vigorously for 2 h at room temperature. Next, 0.5g Fe₃O₄@SiO₂-NH₂ was added to the mixture. The mixed solution was stirred overnight at room
temperature. The products were washed with water and air-dried for further use.Composite
materials of graphene oxide (GO) to Fe₃O₄@SiO₂-NH₂ (Fe₃O₄@SiO₂@GO) were prepared .

Fe₃O₄@SiO₂@G was
synthesized by hydrazine reduction of the Fe₃O₄@SiO₂@GO,
and 0.2 g Fe₃O₄@SiO₂@GO was dissolved in 100
mL water. Then, 200 μL hydrazine (85 %) was added to the mixture, and the
reaction was churned for 2h at 95 °C. Finally, the polymers were washed
thoroughly with water and dried at 80 °C.