International. From watermarks to colored threads, governments are constantly adding new features to banknotes to stay one step ahead of counterfeiters. Now, a longhorned beetle has inspired another way to thwart fraud, produce colors, change posters and art exhibitions. In the journal ACS Nano, researchers report a new type of ink that simulates the beetle's ability to change color in a way that would be durable and difficult to copy.
Zhongze Gu, Zhuoying Xie, Chunwei Yuan and colleagues explain that some U.S. banknotes have color-changing features to help thwart counterfeiters' attempts to make fake money. But these characteristics as a function of the structural chemical changes of dyes, pigments or polymers tend to disappear when exposed to light and air.
Researchers have been developing a new set of color-changing materials, known as colloidal photonic crystals that are resistant to bleaching. However, the methods using these crystals are still expensive. Inkjet printing is a low-cost, fast and accurate alternative; but until now, researchers had not developed the right inks to make such a transition of complex colors and patterns. In search of inspiration, Gu's team turned to Isabellae Tmesisternus, a long-horned beetle that can go from gold to red and vice versa, depending on humidity.
The team of scientists designed an ink that can be finely tuned 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 discoloration when exposed to light and can be applied to hard or flexible surfaces.
