(Left panel) convex lense printing under sunlight on cellulose filter paper coated with fluorescent material; (right panel) corresponding printed images under ambient light and UV light. © Panda, T. et al.

Researchers have prepared a light-emitting organic material that can be used to print patterns, write documents and even encode secret codes on a filter paper using just sunlight1. Sunlight can also erase the printing and writing, visible only under ultraviolet light.

Plant-derived paper is widely used for conventional writing and printing. This generates solid waste and destroys forests, a natural reservoir that absorbs greenhouse gases, known to contribute to global warming.

Scientists from CSIR-National Institute for Interdisciplinary Science & Technology (NIIST) in Thiruvananthapuram, University of Calcutta in Kolkata, India, and New York University Abu Dhabi in United Arab Emirates synthesised the light-emitting material in a one-step process using organic solvent dimethyl sulfoxide.

The researchers, led by Manas Panda, then printed complex designs on a filter paper using a stencil under sunlight. Exposure to sunlight also erased the designs, rendering them invisible to the naked eye. The colour of the printed designs changed from green to orange-red, making them visible only under ultraviolet light.

Using the same process, they also printed flowers, alphabets and secret codes on the filter paper. Since documents can be printed and written using sunlight and gentle heat, it is possible to develop a hot air blow pen with a micrometre-sized circular tip in which hot air acts as ink.

This method of printing and self-erasing can be adopted to protect data in the military communication and financial sectors, Panda says.