Top-secret maps and messages that fade away to keep unwanted eyes from seeing them could be made with a new nanoparticle ink. Researchers at Northwestern University, led by chemical and biological engineering professor Bartosz Grzybowski, have used gold and silver nanoparticles embedded in a thin, flexible organic gel film to make the new type of self-erasing medium.

Shining ultraviolet light on the film through a patterned mask or moving an ultraviolet "pen" over it records an image on the film. In visible light, the image slowly vanishes. Writing on the medium takes a few tens of milliseconds, but the researchers can speed up the process by using brighter light. They can also tweak the nanoparticles to control how quickly the images disappear, anywhere from hours to a few days. The images vanish in a few seconds when they are exposed to bright light or heat.

The film can be erased and rewritten hundreds of times without any change in quality. It can be bent and twisted.

The technology, described in an online Angewandte Chemie paper, would be ideal for making secure messages, Grzybowski says. He also envisions self-expiring bus and train tickets. "It self-erases and there's no way of tracing it back," Grzybowski says. "Also this material self-erases when exposed to intense light, so putting it on a copier is not possible."

There have been previous reports of self-erasing media. In 2006, Xerox announced a paper that erases itself in 16 to 24 hours. These materials use photochromic molecules that rearrange their internal chemical structure when exposed to light, which changes their color. Typically, these molecules can only switch between two colors and they lose their ability to switch after a few cycles. Besides, says Grzybowski, the molecules are not bright so you need a large number to see any color change. "You have to put a kilogram of this into paper before you see something," he says.

Grzybowski and his colleagues make the self-erasing ink with 5-nanometer-wide gold or silver particles. They attach on the nanoparticles' surface molecules that change shape under ultraviolet (UV) light and attract each other. "They're like a molecular glue that you can regulate using light," he says. The unwritten films are red if they contain gold particles and yellow if they contain silver. The films can also be made of other colors, ranging from red to blue, by choosing nanoparticles of a different size. Particles exposed to light form clusters of a different color--the red film changes to blue and yellow changes to violet.