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.

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.