From water marks to colored threads, governments are constantly adding new features to paper money to stay one step ahead of counterfeiters. Now a longhorn beetle has inspired yet another way to foil cash fraud, as well as to produce colorful, changing billboards and art displays.
In the journal ACS Nano, researchers report a new kind of ink that mimics the beetle's color-shifting ability in a way that would be long-lasting and difficult to copy.
Zhongze Gu, Zhuoying Xie, Chunwei Yuan and colleagues explain that some U.S. bills have color-changing features to help thwart attempts by counterfeiters to make fake money. But these features based on the chemical structural changes of dyes, pigments or polymers tend to fade when exposed to light and air. Researchers have been developing a new set of color-changing materials known as colloidal photonic crystals that are bleach resistant. The methods that use these crystals remain expensive, however. Inkjet printing is a fast, precise and low-cost alternative, but until now, researchers had not developed the right inks for making such color-changing and complex patterns. For inspiration, Gu's team turned to Tmesisternus isabellae, a longhorn beetle that can shift from gold to red and back again, depending on the humidity.
The researchers designed an ink that they can finely tune to change color, for example, from bright green to yellow or red when exposed to ethanol vapors. It can also return to its original color. The ink is also durable, resistant to bleaching when exposed to light and can be applied to hard or flexible surfaces.