The colour of iron oxide particles suspended in a solution can be changed by altering the strength of an external magnetic field, researchers at the University of California, Riverside, reported Wednesday.

The research could be used to improve the quality and size of electronic display screens or to change the colour of words or images on screens, the nanotechnologists said. It could also have telecommunications uses in sensors and lasers.

A solution of iron oxide in water changes colour under a magnetic field. The strength of the field increases from left to right.
(Yin Laboratory, University of California, Riverside)

"What should make the technology commercially attractive is that iron oxide is cheap, non-toxic and available in plenty,” said Yadong Yin, research leader and an assistant professor of chemistry at Riverside.

Inexpensive colour displays could be made by forming millions of small pixels using the crystals, the researchers said. A magnetic field could be used to assign each pixel a different colour.

Changing the external magnetic field alters the arrangement of the tiny iron oxide particles in the solution, which in turn changes the way light passes through or is deflected by the solution, the researchers said in a news release.

The key was the structure of iron oxide nanoparticles, said Yin.

The particles were chemically synthesized so they self-assembled into crystals uniformly distributed throughout the liquid.

"By reflecting light, these crystals … show brilliant colors," Yin said. The researchers compared the effect to adjusting the colour of a TV screen image.

Superparamagnetic property is key

Iron oxide nanoparticles are superparamagnetic, which means they become magnetic only when in an external magnetic field.

The researchers used that property to control the spacing between the particles. The spacing determines the wavelength of light that the crystals reflect, which in turn controls the colour.

The results were reported in Angewandte Chemie International Edition’s online edition Wednesday.