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 (see diagram).

“The colour of the nanoparticles depends on how close they are to one another,” says …