Self-assembly and field-responsive optical diffractions of superparamagnetic colloids.

Superparamagnetic Fe(3)O(4) colloids with highly charged surfaces have been assembled into ordered structures in water in response to external magnetic fields. The colloids form chainlike structures with regular interparticle spacings of a few hundred nanometers along the direction of the external field so that the system strongly diffracts visible light. The balance between attractive (in this case, magnetic) and repulsive (electrostatic) forces dictates interparticle spacing and therefore optical properties. By changing the relative strength of these two forces, one can tune the peak diffraction wavelength over the entire visible spectrum. We were able to optimize the diffraction intensity and the tuning range through studying their dependence on variables such as the size distribution and concentration of the Fe(3)O(4) colloids or ionic strength of the solutions. The fast, reversible response and the feasibility for miniaturization impart these photonic materials great potential in applications such as optoelectronic devices, sensors, and color displays.