Invisible inks to help foil counterfeiters

Details of the fluorescent inks, which are prepared from simple and inexpensive commodity chemicals, will be published 22 April by the journal Nature Communications.

Stoddart’s research team, led by Xisen Hou and Chenfeng Ke, stumbled across the water-based ink composite serendipitously. A series of rigorous follow-up investigations unraveled the mechanism of the unique behavior of the inks and led the scientists to propose an encryption theory for security printing.

Hou, a third-year graduate student, and Ke, a postdoctoral fellow, are co-first authors of the paper.

The researchers developed an encryption and authentication security system combined with inkjet-printing technology. In the study, they demonstrated both a monochromic barcode and QR code printed on paper from an inkjet printer. The information, invisible under natural light, can be read on 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 Vincent Van Gogh’s “Sunflowers” painting with good color resolution. Like the barcodes and QR codes, the printed image is only visible under UV light.

The inks are formulated by mixing a simple sugar (cyclodextrin) and a competitive binding agent together with an active ingredient (a molecule known as heterorotaxane) whose fluorescent color changes along a spectrum of red to yellow to green, depending upon the way the components come together. An infinite number of combinations can be defined easily.

Although the sugar itself is colorless, it interacts with the other components of the ink, encapsulating some parts selectively, thus preventing the molecules from sticking to one another and causing a change in color that is difficult to predict. This characteristic presents a formidable challenge to counterfeiters.

Hou and Ke 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 fluorescent colors can be tuned easily by adding the sugar dissolved in water. As more cyclodextrin is added, the fluorescent color changes from red to yellow and then green, giving a wide range of beautiful colors. The fluorescent color can be reversed, by adding another compound that mops up the cyclodextrin.

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,” Hou said. “One can imagine that it would be virtually impossible for someone to reproduce the information unless they knew exactly all the parameters.”

The release notes that the researchers also have developed an authentication mechanism to verify the protected information produced by the fluorescent security inks. Simply by 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,” Ke emphasized.

— Read more in Xisen Hou et al., “Tunable solid-state fluorescent materials for supramolecular encryption,” Nature Communications 6, Article number: 6884 (22 April 2015) (doi:10.1038/ncomms7884)