Invisible fluorescent ink opens new frontier in fight against counterfeiting

Counterfeiting gives brand owners major headaches. Companies lose sales and governments lose tax income. Resulting costs to businesses of counterfeit and pirated products add up to as much as US$650 billion a year worldwide, according to the International Chamber of Commerce.

100 Euro bill under UV light.European Central Bank, CC BY

Scientists and engineers have developed many techniques in the fight against counterfeiters. You might have one example in your pocket right now – the banknotes we use almost every day are produced using special paper, with watermarks, holograms, glossy strips and many other security features. When we hand a large-denomination bill to a cashier, they usually look at it under an ultraviolet lamp to check whether it’s genuine or fake. Under this light, one can see a color image that’s not visible in plain sunlight. Lights that glow under a UV lamp are said to be fluorescent or luminescent. Similar fluorescent tags on our driver’s licenses and passports are also designed to glow under ultraviolet light.

Although these fluorescent materials have been implemented widely in order to protect high-value merchandise, government documents and banknotes, they have a weakness: once their recipes are familiar to counterfeiters, they can be mimicked rather easily.

Now we’ve invented next-generation fluorescent inks that will present a formidable challenge to counterfeiters. Images printed from these new inks display colors that depend on a type of built-in “molecular encryption” and become visible only when viewed under ultraviolet light. Each user can select his own ink recipes, so even we the inventors will not be able to mimic the protected fluorescent tag.

Representation of a heterorotaxane.Stoddart Group, CC BY-NC-ND

Creating the ink

This new ink has its roots in a serendipitous discovery we made when trying to make a fluorescent molecule that would contain some ring-shaped molecules. Unexpectedly, we isolated a compound – known as heterorotaxane – which has become our invisible ink’s active ingredient.

a) Van Gogh’s Sunflower printed using our b) fluorescent ink under ultraviolet light and c) sunlight.CC BY-NC-ND

On its own, the heterorotaxane glows dark-red under ultraviolet light. But its unusual arrangement of molecules can be interrupted by adding a sugar, namely cyclodextrin, which is derived from cornstarch. Depending on how much cyclodextrin we add and how it interacts with the heterorotaxane, we can adjust our ink to give different fluorescent colors along a spectrum of red to yellow to green.

On a molecular level, the colorless heterorotaxane interacts with the other components of the ink. It selectively encapsulates some parts and prevents other molecules from sticking to one another – ultimately causing a change in color that is somewhat difficult to predict. This is a level of complexity not seen before in anti-counterfeiting tools.

Our inks are similar to the proprietary formulations of soft drinks. One could approximate their flavor using other ingredients, but it would be impossible to match the flavor exactly without a precise knowledge of the recipe.

Fluorescent inks change color under ultraviolet light when printed on different paperstocks.Stoddart Group, CC BY-NC-ND

Not only that, the fluorescent ink is also sensitive to the surface on which it’s applied. For example, an ink blend that appears orange on standard copy paper appears as green on newspaper. This phenomenon means that this new type of fluorescent ink can be used to identify different papers.

Encrypting and authenticating

Think about encryption processes in computer science. Cryptography algorithms protect the original information and transform the data into a set of “random” information that gets decoded by a recipient.

Our fluorescent inks work in a similar way. The “molecular encryption” process involves picking a set of color-changing agents and playing around with their relative proportions. Users can set their parameters, thus generating thousands of color combinations via different settings. The individual secret ink recipe can be printed out via ordinary ink jet printer onto a label or other tag. It’s impossible to reverse engineer the process without comprehensive knowledge of the encryption settings.

Fluorescent security ink can have a specific – and unfakeable – fingerprint.Stoddart Group, CC BY-NC-ND

That’s how the information is encoded in the fluorescent dye. But it also has to be verified in some way in order to validate an object as legitimate or counterfeit.

We’ve developed an authentication mechanism that can verify the information within a preexisting image printed using the fluorescent inks. One simply sprays or wipes an authentication indicator over the printed image. While inks with different formulations may appear to be the same color, they will respond very differently when an authentication indicator molecule, such as cyclodextrin, is applied. There’s a large library of authentication indicators that can result in different color changes. This authentication mechanism is a result of the complex molecular interactions among the ink ingredients, so that even if a counterfeiter is able to mimic the original fluorescent color, it will be nigh impossible to replicate the color change during the authentication process.

Legit or phony? Get out your authenticating wipes and UV light.Simon A, CC BY-NC-ND

So here’s how it works from start to finish. A luxury manufacturer would pick a secret setting for its proprietary ink. The company would print out a tag for each handbag, for instance, using the ink invisible under normal light. Then each boutique owner or end consumer that buys the products can view the printed tag under UV light to make sure it matches up with the color they’re expecting. And they can also wipe an authentication swab over it to confirm that the changes that come from that particular combination of authenticating molecules and printed ink are identical to what the manufacturer has told them they should see. Counterfeiters will be out of luck since essentially this process can’t be mimicked.

Fraser Stoddart is affiliated with Northwestern University.

Chenfeng Ke is affiliated with Northwestern University.

Xisen Hou does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.