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This article will self-erase in 10 seconds. At least it would if it had been written on a film that exploits the colour-changing ability of nanoparticles. The technology could make it possible to create documents that wipe themselves clean after they've been read.

A team at Northwestern University in Evanston, Illinois, coated gold nanoparticles with a layer of hair-like molecules called 4-(11-mercaptoundecanoxy) azobenzene or MUA. When zapped with ultraviolet light, these filaments change their shape and charge distribution, causing the nanoparticles to congregate together and change colour.

"The colour of the nanoparticles depends on how close they are to one another," says lead researcher Bartosz Grzybowski. "For instance, gold nanoparticles are red when separated, but become violet, then blue, then colourless as they cluster together."

To put this colour-changing ability to good use, the team dispersed the nanoparticles in a gel and sandwiched it between plastic sheets to produce a thin, red film. When Grzybowski and his colleagues shone UV light at the film, either through a patterned mask or using a UV pen, they found they could print a range of images or write words onto the film in just a few seconds (Angewandte Chemie, DOI: link).

The colour change is not permanent, however. In the absence of UV light, the MUA gradually reverts to its original shape, allowing the nanoparticles to disperse and the images to disappear.

The image's lifetime depends on the amount of MUA coated onto each nanoparticle. By varying this concentration, the chemists found they could control how long the images stayed visible, from hours to days. Exposing the films to intense visible light or mild heat erased the images even faster, in a matter of seconds.

Masahiro Irie, an expert on light-responsive materials at Rikkyo University in Tokyo, Japan, says the work is interesting but points to a number of problems that still need to be overcome. Developing higher-resolution films that are colourless in their unwritten state is a priority, he says.