Credit: © 2009 WILEY-VCH

Images and messages that automatically erase themselves after a set period of time have long been a popular feature of spy films. Although this idea is already used for toy 'water pens', a more practical method could help alleviate society's reliance on paper and benefit the environment by reducing waste and the demands of recycling.

Now, researchers at Northwestern University, USA, have shown that such a rewritable medium can be achieved using thin, flexible organogel films that contain embedded metallic nanoparticle 'ink' (Angew. Chem. Int. Ed. 48, 7035–7039; 2009). Rafal Klajn and co-workers used inks containing either gold or silver nanoparticles coated with mixed self-assembled monolayers of dodecylamine and photoswitchable azobenzene-terminated thiol, 4-(11-mercaptoundecanoxy) azobenzene. When exposed to UV light, the trans-azobenzene groups coating the nanoparticles undergo isomerization to form cis-azobenzene, which has a large dipole moment. As a result, the nanoparticles aggregate into so-called supraspherical assemblies (clusters), and thus become visible. The colour of the assemblies depends on the duration of UV irradiation.

For example, as exposure time increases, gold-nanoparticle inks evolve gradually from red to pale blue, and silver-nanoparticle inks change from yellow to violet. By moving a 'light pen' (of intensity 10 mW cm−2) over a film at 3 mm s−1, one can write an image. Remarkably, multicolour images can be created using only a single nanoparticle ink by varying the irradiation dose over different regions of the film. Typical exposure times depend on the colour required and are in the range of 0.8–10 s, using 365-nm light at an intensity of 10 mW cm−2.

The images fade with time in the absence of UV light as the nanoparticle assemblies gradually break up, a process that typically takes a few hours but can be accelerated by heating the film. Interestingly, the optical response of these carefully engineered films does not change for at least 300 rewrite cycles.

One idea is that such rewritable 'paper' based on these new films could be useful for storing sensitive data, or for temporary information such as self-expiring bus tickets.