US scientists discover a new class of iron oxide nanoparticles that, when in solution, change colour in the presence of a magnetic field

A simple magnet can be used to change the colour of iron oxide nanoparticles suspended in water, according to Yadong Yin and colleagues at the University of California, Riverside.1 The discovery could lead to a new class of low-power electronic displays. It also has the potential to be exploited in rewritable electronic paper displays. 

Yin explains that the key was to design the structure of iron oxide nanoparticles, just a few billionths of a metre in diameter, through chemical synthesis so that they self-assemble into three dimensionally ordered crystals floating in the water and held in a magnetic field. Unlike certain other designer nanoparticle materials, such as coated gold particles, iron oxide is cheap, non-toxic and widely available. 

The colours of iron oxide nanocrystals

Source: Yin Laboratory, UCR

The colours of iron oxide nanocrystals

The team found that by changing the strength of the magnetic field applied to their colloidal suspension they were able to change the colour of the iron oxide solution. The magnetic field disturbs the arrangement of the spherical iron oxide particles in solution, altering how light passes through the solution. 

Iron oxide nanoparticles are superparamagnetic, which means they become magnetic only in the presence of an external magnetic field. Ferromagnetic materials, in contrast, such as those used in horseshoe magnets and compass needles become magnetised and stay magnetised, producing their own magnetic field. The applied magnetic field magnetises the nanoparticles, altering the spacing between them as they become attracted to each other. This space then affects the wavelength, and so the colour, of light that is reflected by the aqueous suspension of the particles. 

'By reflecting light, these crystals - also called photonic crystals - show brilliant colours', Yin explains. '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'.