In a boost to the fight against the menace of counterfeiting, two researchers at the Hyderabad campus of the Birla Institute of Technology and Science have developed an ink that changes colour when exposed to acid vapours and reverts to its original colour when exposed to base vapours.

The ink contains small amounts of what are called fluorophore particles made from a chemical called Âmono-carbazole-linked anthranyl p-conjugates. Fluorophore particles are invisible under normal light but light up with a yellow colour when exposed to a commercial ultraviolet light torch. The ink is found to be stable under ambient conditions and up to a temperature of 300 degrees C.

ÂWe tested our yellow coloured fluorophore along with many other conventional yellow fluorophores used in many applications such as currency. However, upon exposure to acid vapour, the colour change (yellow to green) happens only for our fluorophores. Therefore, even if the counterfeiters use any fake yellow fluorophores, the Âacid key will quickly reveal the original, said Dr Manab Chakravarty, lead researcher, while speaking to India Science Wire.

The ink was tested on plastic, paper, currency notes and leaves. It was found to be highly effective on paper and currency. 

The team demonstrated the efficacy of the ink on currency note by making a small mark using a quill (pigeon feather). When exposed to acid vapours, the mark changed colour from yellow to green. The pattern could be retrieved again by placing the note over base vapours, indicating the recoverability of the design.

Dr Chakravarty said the novelty of the ink lies in its ability to change its colour quickly, thereby providing a better way of detecting the original from the fake.  Further, it was synthesised adopting simple organic methods using readily accessible laboratory chemicals and at room temperature.ÂWe were able to eliminate use of costly chemicals and complex procedures that are normally adopted to make anti-counterfeiting inksÂ, he added. 

The team has planned to take the research forward by transferring the ink on to an adsorbent material of a marker pen.

Manab Chakravarty conducted the study with his student Banchhanidhi Prusti. The results were published in the journal ACS Omega (India Science Wire)