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Hugh Pickens writes:

Scientific American reports that simply breathing on money could soon reveal if it's the real deal or counterfeit thanks to a photonic crystal ink developed by Ling Bai and Zhongze Gu and colleagues at Southeast University in Nanjing, China that can produce unique color changing patterns on surfaces with an inkjet printer system which would be extremely hard for fraudsters to reproduce. The ink mimics the way Tmesisternus isabellae – a species of longhorn beetle – reversibly switches its color from gold to red according to the humidity in its environment. The color shift is caused by the adsorption of water vapor in their hardened front wings, which alters the thickness and average refractive index of their multi-layered scales.

To emulate this, the team made their photonic crystal ink using mesoporous silica nanoparticles, which have a large surface area and strong vapor adsorption capabilities that can be precisely controlled. The complicated and reversible multi-color shifts of mesoporous CPC patterns are favorable for immediate recognition by naked eyes but hard to copy. "We think the ink's multiple security features may be useful for anti-fraud applications," says Bai, "however we think the technology could be more useful for fabricating multiple functional sensor arrays, which we are now working towards."