Assembly of magnetically tunable photonic crystals in nonpolar solvents.

We have successfully assembled superparamagnetic colloids into ordered structures with magnetically tunable photonic properties in nonpolar solvents by establishing long-range electrostatic repulsive forces using charge control agents. Reverse micelles resulted from the introduction of charge control agents such as AOT molecules can enhance the charge separation on the surfaces of n-octadecyltrimethoxysilane modified Fe(3)O(4)@SiO(2) particles. The significantly improved long-range electrostatic repulsion can counterbalance the magnetically induced attraction and therefore allow ordering of superparamagnetic colloids in nonpolar solvents. This system possesses a fast and fully reversible optical response to the external magnetic fields, long-term stability in performance, and good diffraction intensity.