Invisible UV inks devised to help detect counterfeits

Barcodes could be made of difficult-to-reproduce fluorescent inks to allow consumers to snap a photo with their smartphones to identify products that are often fake.

Scientists from Northwestern University in the US have invented inks that can be stamped using an inkjet printer and are invisible under normal light, but visible under UV light. The inks could be printed as either barcodes or QR codes on anything from banknotes to luxury items such as cosmetics or handbags.

“We have introduced a level of complexity not seen before in tools to combat counterfeiters,” said Sir Fraser Stoddart, the senior author of the study. “Our inks are similar to the proprietary formulations of soft drinks. One could approximate their flavour using other ingredients, but it would be impossible to match the flavour exactly without a precise knowledge of the recipe.”

According to the researchers the inks can be made in single colours or as multicolour gradients, depending on the amounts and interaction of three different ‘ingredient’ molecules – one of which is sugar – that provide a built-in molecular encryption tool.

Any attempt to tweak the ink composition would result in significant colour changes, which would make it obvious that the item was counterfeit.

“The rather unusual relationship between the composition of the inks and their colour 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,” Stoddart said.

Manufacturers would have control over the ink’s chemical composition making it impossible to reverse engineer the colour information encoded. Even the ink’s inventors would not be able to do it, scientists said, without detailed knowledge of the encryption settings.

How does it work?

Xisen Hou, a third-year graduate student, and Chenfeng Ke, a postdoctoral fellow, who co-authored the paper, looked into an encryption theory for security printing that led them to devise the invisible inks.

“It was a real surprise when we first isolated the main component of the inks as an unexpected by-product,” Hou said. “The compound shows a beautiful dark-red fluorescence under UV light, yet when we dissolve it in large amounts of water, the fluorescent colour turns green. At that moment, we realised we had discovered something that is quite unique.”

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

Although the sugar itself is colourless, it interacts with the other components of the ink, encapsulating some parts selectively, which prevents the molecules from sticking to one another and causing a change in colour that is difficult to predict. This characteristic could be a great challenge to counterfeiters.

“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.”

Figures from the International Chamber of Commerce showed that counterfeiting leads to $650bn losses per year globally.

A smartphone reads a QR code printed with a fluorescent ink that is only visible under ultraviolet light

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