Nanoparticles' colour-changing ability makes it possible to create self-erasing documents

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London, July 9 (ANI): Scientists have developed a film that exploits the colour-changing ability of nanoparticles, and can make it possible to create documents that wipe themselves clean after they've been read.

Northwestern University researchers in Evanston, Illinois, have revealed that they made the film by coating gold nanoparticles with a layer of hair-like molecules called 4-(11-mercaptoundecanoxy)azobenzene (MUA).

They say that upon being zapped with ultraviolet light, the 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. For instance, gold nanoparticles are red when separated, but become violet, then blue, then colourless as they cluster together," New Scientist magazine quoted lead researcher Bartosz Grzybowski as saying.

The researchers have also shown that this colour-changing ability can be put to good use by dispersing the nanoparticles in a gel, and sandwiching it between plastic sheets to produce a thin, red film.

Grzybowski and his colleagues shone UV light at the film, either through a patterned mask or using a UV pen, and found that they could print a range of images or write words onto the film in just a few seconds.

However, concede the researchers, the colour change is not permanent.

In the absence of UV light, they say, the MUA gradually reverts to its original shape, allowing the nanoparticles to disperse and the images to disappear.

According to them, the image's lifetime depends on the amount of MUA coated onto each nanoparticle.

When the chemists varied this concentration, they found that 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 that the work is interesting, but points to a number of problems that still need to be overcome.

He says that developing higher-resolution films that are colourless in their unwritten state is a priority. (ANI)

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