A breakthrough by nanotechnologists at the University of California, Riverside could pave the way for rewritable electronic paper and ink that can change colour electromagnetically.

The boffins demonstrated a way to control the colour of very small particles of iron oxide suspended in water simply by applying an external magnetic field.

When the strength of the magnetic field is changed, it alters the arrangement of the spherical iron oxide particles in solution, thereby modifying how light falling on the particles passes through or is deflected by the solution.

"The key is to design the structure of iron oxide nano-particles through chemical synthesis so that these nano-particles self-assemble into three-dimensionally ordered colloidal crystals in a magnetic field," said Yadong Yin, an assistant professor of chemistry who led the research.

"By reflecting light, these photonic crystals show brilliant colours. Ours is the first report of a photonic crystal that is fully tuneable in the visible range of the electromagnetic spectrum from violet light to red light."

Yin explained that a photonic crystal controls the flow of light (photons) and works like a semiconductor for light. The spacing of the nano-particles dictates the wavelength of light that a photonic crystal reflects.

Iron oxide nano-particles are 'superparamagnetic' in that they turn magnetic only in the presence of an external magnetic field.

In contrast, 'ferromagnetic' materials become magnetised in a magnetic field and retain their magnetism when the field is removed.

The researchers used the superparamagnetic property of iron oxide particles to tune the spacing between nano-particles, and therefore the wavelength of the light reflection, or the colour of the colloidal crystals, by changing the strength of the external magnetic field.

The study results will appear in Angewandte Chemie International Edition's online edition today.