Secrets shown in a good light

Scientists in France have created paper that can carry secret messages. In visible light, the paper is indistinguishable from regular paper and users can read, write or erase messages using three different wavelengths of UV light.

The functionalised paper, made by François-Xavier Felpin from the University of Nantes, and colleagues, contains coumarin molecules attached to the paper’s cellulose fibres. Exposing the paper to UV light with a wavelength of 340nm causes coumarin to react and create cyclobutane dimers. These dimers are invisible under visible light, but fluoresce under a UV lamp. Users write information onto the paper using a stencil to produce dark and bright areas. As the dimerisation is reversible, exposing the paper to a shorter wavelength of UV light erases the message so the paper is ready for a new message.

The advantage of Felpin’s paper is that unlike traditional marking systems the message is both undetectable in visible light, and cannot be washed away with solvents. This gives it potential for use in anti-counterfeiting and tracking applications.