Colourful colloids

A simple mixture of iron oxide, a polymer and water can take on any colour simply by applying a magnetic field, US researchers report.

Team leader Yadong Yin of the University of California, Riverside, said that the photonic crystals, closely packed arrays of magnetic colloids, could provide a low-cost route to materials for display screens.

When light passes through a photonic crystal only certain wavelengths related to the spacing between the particles can be reflected. However, if a quick and simple way to alter the spacing, and hence control the reflected light, can be found then displays could be made with only one colour-emitting component instead of three - one each for red, green and blue.

To tackle this, Yin and colleagues created particles of superparamagnetic Fe₃O₄ coated with a charged polymer, polyacrylic acid, and suspended them in water. In a magnetic field the particles, around 100-200 nm across, assembled into colloidal photonic crystals that appeared brightly coloured in ambient light.

Magnetic trade off

This spontaneous assembly happens thanks to a trade-off between the repulsion of the charged surfaces and magnetic forces acting on the iron oxide cores. Yin’s team found they could affect the spacing in the crystals by simply moving the magnet closer or further away. As they did this they were able to tune the colour through the whole visible spectrum. The same effect can be seen by varying the field using an electromagnet, said Yin, whose results are published in Angewandte Chemie¹. It takes about 200 milliseconds to change colour, so it isn’t fast enough for video but could be used in static images such as signs or electronic paper.

Using superparamagnetic particles has been done before² by Sandy Asher at the University of Pittsburgh, US, but Yin’s particles are more responsive to the magnetic field. Yin said this gives his particles the edge. ’The major advantages are the tenability, which covers the entire visible spectrum, and the instant optical response to the magnetic fields’ explains Yin, who also points to the low toxicity and cost of his system.

Stability warnings

But independent researchers contacted by Chemistry World warned of drawbacks that could hinder progress of this material. Roy Sambles, professor of experimental physics at the University of Exeter, UK, said Yin’s paper ’is a very nice study’ but added ’it is by no means easy to see how individual pixels could be switched.’ Sambles is also concerned about the thickness of material required for strong colours and the stability of the structure: ’Will the nanoparticles start to align in groups and clump together?’ he wondered.

Nanochemist  Geoffrey Ozin at the   University of Toronto, Canada, who has developed a similar, voltage-controlled ’photonic ink,’ shared the concerns over stability, and said that it would be a challenge to process a fluid material like this into a device that can cover a large area. Also having to constantly apply the field to hold the colour may make a power-hungry device, he said.

Yin has put his colloids in glass capillaries of a few hundred micrometers in diameter and still observed the color changing effect. ’It seems that the miniaturized system has much higher sensitivity, which means that it can operate at much weaker magnetic fields’ he said. ’There are many ways to miniatuarise the systems such as lithographic patterning and microencapsulation. Technically there should be no problem to do that’ he told Chemistry World.

Tom Westgate

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