Fluorescent inks could one day be used in multicolor barcodes to authenticate frequently counterfeited products. Developed by researchers at Northwestern University, the inks are invisible under normal light but reveal themselves under UV illumination. Stamped as barcodes or QR codes on banknotes, whisky bottles, luxury handbags and other expensive items, the inks could be used as marks of authenticity. Consumers would snap photos with their smartphones to find out if the items are real and worth their money. The inks can be made in single colors or as multicolor gradients. An ink’s color depends on the amounts and interaction of three different ingredient molecules, providing a built-in molecular encryption tool. Small changes to the ink’s composition results in a significant color change. "We have introduced a level of complexity not seen before in tools to combat counterfeiters," said Northwestern chemistry professor Fraser Stoddart. "The rather unusual relationship between the composition of the inks and their color makes them ideal for security applications where it's desirable to keep certain information encrypted or to have brand items with unique labels that can be authenticated easily." The inks are formulated by mixing a simple sugar (cyclodextrin) and a competitive binding agent together with an active ingredient (heterorotaxane), whose fluorescent color changes along a spectrum of red to yellow to green depending upon the way the components come together. Sugar concentration controls the fluorescent colors. As more cyclodextrin is added, the ink changes from red to yellow and then green under UV light. This can be reversed by adding another compound that mops up the cyclodextrin. Using an inkjet printer, the researchers printed monochromic barcodes and QR codes on paper that are readable by a smartphone under UV light. As another demonstration of the technology, the research team loaded the three chemical components into an inkjet cartridge and printed one of Vincent Van Gogh’s "Sunflowers" paintings with good color resolution. Like the barcodes and QR codes, the printed image is only visible under UV light. The researchers also discovered that the fluorescent ink is sensitive to the surface to which it is applied. For example, an ink blend that appears as orange on standard copy paper appears as green on newsprint. This observation means that this type of fluorescent ink can be used to identify different papers. “This is a smart technology that allows people to create their own security code by manually setting all the critical parameters,” said graduate student Xisen Hou. “One can imagine that it would be virtually impossible for someone to reproduce the information unless they knew exactly all the parameters.” The researchers also have developed an authentication mechanism to verify the protected information produced by the fluorescent security inks. Simply wiping some wet authentication wipes on top of the fluorescent image causes its colors to change under UV light. "Since the color changing process is dynamic, even if counterfeiters can mimic the initial fluorescent color, they will find it impossible to reproduce the color-changing process," said postdoctoral fellow Chenfeng Ke. Hou and Ke discovered the water-based ink composite while working on another problem. They were trying to prevent fluorophore aggregation by encircling a fluorescent molecule with other ring-shaped molecules, one being cyclodextrin. Unexpectedly, they isolated the compound that is the active ingredient of the inks. They found that the compound’s unusual arrangement of three rings trapped around the fluorescent component affords the unique aggregation behavior that is behind the color-changing inks. "You never know what Mother Nature will give you," Hou said. "It was a real surprise when we first isolated the main component of the inks as an unexpected byproduct. The compound shows a beautiful dark-red fluorescence under UV light, yet when we dissolve it in large amounts of water, the fluorescent color turns green. At that moment, we realized we had discovered something that is quite unique." The research was published in Nature Communications (doi: 10.1038/ncomms7884). For more information, visit www.northwestern.edu.