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Monday, 23 September 2019
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Raw materials & technologies, Technologies

Invisible inks could help foil counterfeiters of all kinds

Thursday, 23 April 2015

Real or counterfeit? Scientists have invented sophisticated fluorescent inks that one day could be used as multicoloured barcodes for consumers to authenticate products that are often counterfeited.

In the study, the researchers demonstrated both a monochromic barcode and QR code printed on paper from an inkjet printer. Source: Shawn Hempel/Fotolia
In the study, the researchers demonstrated both a monochromic barcode and QR code printed on paper from an inkjet printer. Source: Shawn Hempel/Fot...

Snap a photo with your smartphone, and it will tell you if the item is real and worth your money.

Counterfeiting is big business

Counterfeiting is very big business worldwide, with EUR 605 billion per year lost globally, according to the International Chamber of Commerce. The new fluorescent inks give manufacturers and consumers an authentication tool that would be very difficult for counterfeiters to mimic.

These inks, which can be printed using an inkjet printer, are invisible under normal light but visible under ultraviolet light. The inks could be stamped as barcodes or QR codes on anything from banknotes and bottles of whisky to luxury handbags and expensive cosmetics, providing proof of authenticity.

Controlling the colour of the ink

A key advantage is the control one has over the colour of the ink; the inks can be made in single colours or as multicolour gradients. An ink's colour depends on the amounts and interaction of three different "ingredient" molecules, providing a built-in "molecular encryption" tool. (One of the ingredients is a sugar.) Even a tiny tweak to the ink's composition results in a significant colour change.

"We have introduced a level of complexity not seen before in tools to combat counterfeiters," said Sir Fraser Stoddart from Northwestern University, 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 flavor exactly without a precise knowledge of the recipe."

"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.

Information can be read on a smartphone

With a manufacturer controlling the ink's "recipe," or chemical composition, counterfeiters would find it virtually impossible to reverse engineer the colour information encoded in the printed barcodes, QR codes or trademarks. Even the inks' inventors would not be able to reverse engineer the process without a detailed knowledge of the encryption settings.

Stoddart's research team 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.

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.

Image is only visible 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 colour 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 colour 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.

Preventing fluorophore aggregation

Although the sugar itself is colourless, 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 colour that is difficult to predict. This characteristic presents a formidable challenge to counterfeiters.

The researchers 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 behaviour that is behind the colour-changing inks.

We had discovered something that is unique

"You never know what Mother Nature will give you," a researcher 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 colour turns green. At that moment, we realised we had discovered something that is quite unique."

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

Fluorescent ink is sensitive to the surface

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 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 colours to change under UV light. "Since the colour changing process is dynamic, even if counterfeiters can mimic the initial fluorescent colour, they will find it impossible to reproduce the colour-changing process," the researcher emphasised.

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