New "Disappearing" Nanoparticle Ink Keeps Messages Cryptic
Remember when, as a kid, you would pen secret messages with "disappearing ink" by writing on paper with lemon juice? A team of researchers at Northwestern have taken the idea just a little bit further, engineering a nanoparticle ink that fades away at a predetermined time, keeping maps or messages away from spying eyes. The scientists used gold and silver nanoparticles -- just 5 nanometers wide -- embedded in a thin organic gel film to create the disappearing medium. Each nanoparticle is covered in molecules that change shape and attract each other in the presence of ultraviolet light. Using an ultraviolet "pen" or a patterned mask to apply light to the surface causes the molecules struck by UV to cluster and change color, allowing a message or image to be penned on the medium. The more light, the stronger the colors, as the particles bond more...