In the absence of light the clusters fall apart. How quickly they fall apart, erasing the writing, depends on the amount of gluelike particles on them.

You can write in different colors depending on how much light you put in--more UV light makes the particles form tighter clusters, which have a different color than looser clusters. The researchers were also able to write two images, one over the other, on the same film. All the nanoparticles do not get used to write the first image and can be used for the second image.

"The concept of using photostimulated reversible aggregation of gold or silver particles for self-erasing images is quite interesting and new," says Masahiro Irie, a chemistry professor at Rikkyo University in Tokyo who studies photochromic molecules. However, he believes that photochromic molecules might be better for practical self-erasing systems. Images or text written with the new inks might not have a high resolution because they require clusters of nanoparticles. Plus, the unwritten film is colored because of the nanoparticles, and it would be more desirable to have a colorless or white original film, he says.

But the flexibility and control that the new material offers makes it attractive. It is easy to control the speed of writing and erasure, as well as the color, Grzybowski says. He adds that the technology has drawn interest from a United Kingdom-based security firm.