Highly Efficient, Chemically Stable, and UV/Blue-Light-Excitable Biluminescent Security Ink to Combat Counterfeiting.

A strategy has been demonstrated to design a biluminescent security ink using Eu(TTA)3Phen (ETP) and fluorescein for protecting the currency and other essential documents, viz., passport, bank check, certificates, etc. against counterfeiting. The biluminescent security ink exhibits strong red and green emission under 367 and 445 nm excitations, respectively. As it is quite challenging to prepare a material that possesses two prominent (green and red) and distinguishable colors upon excitation with two separate light-emitting diode (LED) sources, emitting at different wavelengths, the biluminescent security ink would be hard to counterfeit as compared with the existing luminescent security ink that exhibits single color under UV light exposure. To check its feasibility for security application, the patterns printed out using the biluminescent security ink were kept under a hot and humid atmosphere for 150 days. Also, the ETP and fluorescein fluorophores were exposed to UV light for a prolonged time, which do not show any sign of deterioration in their luminescence intensities. Furthermore, to check their chemical stability, printed patterns were also exposed to chemicals that have potential to wipe out ink, viz., detergent, ethanol, acetone, and sodium hypochlorite (bleach) solution, and it was noticed that it is well stable against these chemicals. Because of the reasons mentioned above and easy availability of 367 and 445 nm LEDs at low cost, authors believe that the application of this biluminescent security ink can trigger the realization of the full potential of this advanced security feature in detecting fake currency.