Counterfeit cash could be detected with human breath

A study by Chinese researchers has found a new and relatively cheap way of discovering fake money: just blow on it.

This is thanks to a beetle-inspired ink that reversibly changes colour in response to humidity, and human breath would be enough to spot a fake.

This is done by using a photonic crystal ink developed by the Chinese researchers that can produce unique colour-changing patterns on surfaces with an inkjet printer system, which would be extremely hard for fraudsters to reproduce.

Most of the research over the past 25 years has used expensive high precision tools developed by the microelectronics industry to create their desirable properties.

But inkjet printing of photonic crystals onto a surface is cheaper. Previous efforts have struggled to integrate responsive photonic crystal inks into such a system.

Now, Ling Bai and Zhongze Gu and colleagues at Southeast University in Nanjing, China, have developed a photonic crystal ink that mimics the way Tmesisternus isabellae – a species of longhorn beetle – reversibly switches its colour from gold to red according to the humidity in its environment.

This is a much cheaper and quicker way of detecting the counterfeit notes.

Bai says: “This technology allows for high resolution, high precision, fast speed, large-scale preparation of photonic crystal patterns. Patterning of photonic crystals is critical for the realisation of photonic crystal displays and for designing the special functions of photonic crystal optical devices.”

Speaking to Chemistry World, Thomas Krauss, who investigates photonic crystals at the University of York, said: “It is particularly refreshing to see simple techniques such as inkjet printing being used to such great effect.

“Being able to create highly functional photonic crystals by low-cost techniques is important for commercial applications, and I believe that the type of flexible and scalable technology demonstrated here has a great future.”