Secret letter writing gets material boost
doi:10.1038/nindia.2009.361 Published online 4 January 2010
Do you want to write a secret letter that is transparent in daylight, can not be photocopied, and is visible only when you shine light of a specific wavelength? Researchers in the photosciences & photonics division of the National Institute for Interdisciplinary Science and Technology (NIIST) -- formerly Regional Research Laboratory -- in Thiruvananthapuram have shown it can be done.
Their approach is "a novel strategy for creating thermally writable, non-copyable, and erasable fluorescent images useful for secret documentation by using a fluorescent gelator entrapped in a polystyrene film." Ayyappanpillai Ajayaghosh who led the team says they have applied for a patent for the invention.
Central to this concept of secret writing are the light emitting (luminescent) molecules commonly known as OPVs (oligo-p-phenylenevinylenes) and their "self assembly," says Ajayaghosh. "At appropriate conditions these molecules emit light, the color of which can be tuned by modifying the interaction between the molecules," he told Nature India. Once the OPV molecules are introduced into styrene (a precursor of polystyrene), the weak interactions between the molecules cause them to self-organize forming a gel comprising of nanometer to micrometer size supramolecular architectures.
Subsequently, the OPV entrapped styrene can be polymerised by a photochemical method to prepare a polystyrene matrix in which the OPV molecules are self-assembled. These self-assembled structures emit a specific color, in this case green, he said. If heat is applied at specific points, the OPV molecules at those points get detached from the self-assembly and emit blue color characteristic of the OPVs when they are free.
This property of OPV molecules shifting their emission from green to blue is exploited by the researchers to make secret writing possible. In actual practice, a polymer film entrapping the OPVs is used instead of normal paper and, instead of the ink pen, one writes "thermally" with a soldering pen -- like the one used by technicians to solder electronic components.
"The film is normally greenish but wherever I write it turns blue due to heat from the soldering pen, and wherever I do not write remains green," Ajayaghosh explained. But the written images cannot be read with naked eye. To be made visible one should excite the film with light of appropriate wavelength, the scientist said.
The mechanism of the rewritable imaging involves the reversible self-assembly of OPVs in the polystyrene film. When heated above a certain temperature, the film softens, and the thermal energy induces breakage of the initially formed OPV self-assembly (with green emission) into individual molecules with strong blue emission. Upon exposure to chloroform vapors, the film allows the re-assembly of the OPV molecules, regenerating the green emission. "Since chloroform is a good solvent of polystyrene, the polymer film is softened by the solvent vapors, which allows the OPV molecules to recreate the self-assembly," the scientists report.
"The most striking advantage of fluorescence imaging is the prevention of photocopying and invisibility of the image under daylight," Ajayaghosh said. This is so because, the material being fluorescent, photocopying the film only results in black background, he said. Invisible inks are standard items in the toolkit of spies but Ajayaghosh says the material used in the new approach "is superior since we can erase the image at will and reuse the film." The image could be erased by exposing the film to chloroform vapor for one minute, he said. Experiments by the scientists showed that "the writing and erasing could be done several times without much loss of the intensity of the emission –- a property attributed to the reversible self assembly of OPV molecules in the polystyrene matrix.
Ajayaghosh said the research belonged to the discipline of supramolecular chemistry encompassing the broad area of advanced materials that his group is working on. Other members of his team are Sampath Srinivasan, Palathingal Babu and Sankarapillai Mahesh.
- Srinivasan, S. et al.Reversible Self-Assembly of Entrapped Fluorescent Gelators in Polymerized Styrene Gel Matrix: Erasable Thermal Imaging via Recreation of Supramolecular Architectures. J. Am. Chem. Soc. 131, 15122–15123 (2009)