An indigenous anti-counterfeiting ink

A novel process for manufacturing a new type of security ink that can help prevent counterfeiting currency has been developed by the scientists of New Delhi based CSIR-National Physical Laboratory, a unit of the Council of Scientific and Industrial Research (CSIR). This process can not only be adopted on a large scale without much difficulty, but can also yield an ink that can be used on ordinary paper, says a new study that has been published in Chemistry: A European Journal. "The ink has been developed in response to the 'Make in India' call," says Dr Bipin Gupta, the study's lead author.

Security inks are essential and crucial for printing of currency as they help detect any counterfeits. The first thing anyone does to check the authenticity of a currency note is to place it under a UV lamp. Under this light, one could then clearly see some features that are normally not visible to the naked eye. This is due to a special ink that glows only when it is exposed under a UV light. Although such inks are available, they also need a special paper for the UV marker to be effective. In other words, the paper or surface on which the ink is printed should not glow under UV light. Hence, there is a requirement of special paper.

The ink developed by Bipin's team, however, eliminates this need for special surfaces. "It is printable on all papers and surfaces," reveals Bipin. Not only that, the ink can also be tested under two types of lights: the normal UV lamp and under infrared (IR) light. Infrared light is emitted by TV remotes and by the lamp used to treat muscle sprain. This dual-mode glowing adds to its secure nature. "The ink is formulated from a cost effective dual-mode luminescent composite pigment," says Bipin.

The security ink has been developed by mixing nanorods of rare-earth elements like gadolinium, ytterbium and erbium oxides with light-emitting solids made of zinc and manganese sulphide in a specific polymer-based ink. Bipin's team has judiciously mixed these two together to get a pigment that can be mixed in with PVC gold, which is commonly used in the manufacture of ordinary inks. The rare earth metal nanorods respond to near infrared laser and the phosphors respond to UV light. The sulphides are called phosphors as they glow when high energy light or electron beams hit them. "The composite pigment can be excited with two wavelengths: ultraviolet light (365 nm, UV lamp) and 980 nm with near-infrared laser. In composite pigment, nanorods emit red light upon excitation with a 980 nm laser and phosphor emits yellow light with a 365 nm UV lamp. We have plans to replace NIR laser with NIR LEDs in future to make cost effective excitation source," elaborates Bipin.

The ink designed by Bipin's team shines bright yellow under UV light and intense red when it is shown under infrared light. The composite pigments in the ink are "tunable and more secure," says Bipin. "It means the pigments responding to specific excitation wavelengths and that emit specific wavelengths of light are possible. This makes it extremely difficult to counterfeit," he adds.

In other words, patterns that appear identical on paper or on different currencies may glow differently when exposed to specific frequency of light. Besides making currency secure, these novel inks can also be used in "printing labels of pharmaceuticals or in printing important documents," says Bipin. When the ink hits the market, which may not be too distant, it would be another unique gift from the National Physical Laboratory which gave us that indelible ink to ensure a free franchise.