Tunable assembly of hydrophobic and cationic charged Fe3O4/SiO2 colloids.

The hydrophobic and cationic Fe3O4/SiO2 nanoparticles (NPs) were synthesized by coating the Fe3O4 nanoparticles with silica followed by the silanization reaction. The monodisperse nanoparticles were separated under magnetic forces and characterized by FE-SEM and FT-IR. Interestingly, the hydrophobic and cationic NPs were well dispersed in polar solvents (ethanol and propylene carbonate) and a nonpolar solvent (chloroform). The structural colors induced by the assembly of the NPs were changed according to the magnetic forces. The repulsive forces originated from the combination of the cationic charges and hydrophobic groups were strong enough to overcome the magnetic attractive forces between particles. The electrophoretic behavior of the NPs was investigated as well. For preparing devices, the polar and nonpolar solutions of the cationic NPs were injected into the cells with two ITO-coated glasses which were separated by 100 μm. The reversible color changes were observed on the cathode under the electric field suggesting that the Fe3O4/SiO2 nanoparticles were modified with cationic groups.