Scientists at the CSIR-National Physical Laboratory (CSIR-NPL), New Delhi, have invented a unique security ink that emits two different colours on exposure to ultraviolet light – a feature that could help prevent counterfeiting of currency notes.
Counterfeiting is one of the major issues that has grown into a global threat, including the duplication of currency notes, passports, etc. causing major losses in the economy of a nation. Special or security inks are used in the printing of currency notes, passports, etc. to ensure security and prevent forgery, counterfeiting or tampering. Security inks play a significant role in detecting counterfeiting attempts by illegal printing.
Cashiers in banks check currency notes under a special kind of light bulb before accepting the notes. The currency notes are generally printed with special inks which glow with different colours under the ultraviolet illumination helping detect the fake currency. However, over time scamsters have started copying the currency notes.
To prevent such instances, scientists at the CSIR-National Physical Laboratory (CSIR-NPL), New Delhi, have invented a unique security ink with advanced features which emits two different colours under the exposure of ultraviolet light. The advanced security ink may help in combating the issue of counterfeiting of notes, passports, pharmaceuticals, etc. According to the study published in the Journal of Materials Chemistry C, the novel strategy is based on a combination of an unexplored concept of fluorescence and phosphorescence phenomenon.
The lead scientist of the research work Dr Bipin Kumar Gupta, Senior Scientist, NPL, says, “This would be the first report of a security ink that emits at two very different wavelengths under single wavelength UV exposure.” The ink glows red (611 nm) under UV exposure (254 nm) and green (532 nm) when UV exposure is off. The red emission is due to fluorescence and green emission is due to phosphorescence. Unlike the other materials, the engineering elegance of the ink is that the excitation spectrum of one colour does not cover the spectrum of the other. The green pigment does not quench the red pigment under UV exposure.
In the synthesis process, sodium yttrium fluoride was doped with europium, a rare earth element for preparing the red pigment; and for green pigment europium and dysprosium were doped in aluminium strontium oxide. It was prepared via the hydrothermal method and subsequently annealed at 400 °C. Annealing is important for proper mixing of the red and green pigments otherwise it would separate during the printing of the ink. For commercial application, the feasibility of the advanced security ink was also tested wherein printed patterns were exposed to different atmospheric conditions like hot, cold, and humid for six months.
Dr Gupta added, “A novel strategy has been devised to develop a single excitable dual emissive luminescent pigment for the formulation of unclonable security ink. The new advanced security features of the ink have been demonstrated successfully and make it highly suitable for the printing of the valuable products, viz. currency, passports, pharmaceuticals, etc. for their protection against duplicity.”
The luminescence printing based method is considered as one of the best approaches to combat counterfeiting due to the low cost of production at a commercial scale, being eco-friendly and hard to counterfeit.
The NPL scientists mention that they have synthesised the single excitable dual emissive luminescent pigment in a single batch which can be further scaled up as desired by any industry.