Security details

Counterfeiting is a robust industry – involving banknotes, luxury goods, clothing, cosmetics and electronics. But could next generation security materials one day help to keep counterfeiters at bay? XiaoZhi Lim reports

Shine UV light on a €100 banknote and you will see the familiar circle of EU stars glow red. US dollar bills likewise contain a security strip that fluoresces different colours under UV light. And on a UK passport’s biodata page, intricate patterns barely visible in ambient light suddenly appear when exposed to UV, glowing red and green.

Demand for new materials with covert features remains strong. In 2021, the Bank of England removed 103,000 counterfeit banknotes nominally worth £2.7m from circulation; and before cashless payments took off during the pandemic, the US central bank on average removed $10m of counterfeit notes annually. International trade in counterfeit goods amounted to $464bn in 2019, according to an estimate from the OECD.

Novel security materials are needed in part ‘because the counterfeiters cracked the old technologies,’ says William Cross at the South Dakota School of Mines and Technology. Emerging materials with novel features such as an afterglow or that make it more difficult for security marks to be tampered with could help. And with the advent of smartphone and QR code technologies able to read and verify security marks, members of the general public could soon start to authenticate products for themselves – especially pharmaceuticals, which could be life-threatening if fake.

Invisible infrared

Many fluorescent security inks use UV absorbers that glow in UV light. But Cross and a collaborator, Stanley May at the University of South Dakota, US, hope to expand the light absorption range to infrared light.

Absorbing in the infrared could be a useful feature, says May, by allowing a combination of differently absorbing marks. Infrared absorbers could be used to create security marks next to, or even on top of, a UV-absorbing mark. That way, a printed document could produce one image when exposed to UV, and another when exposed to infrared.

May and his team prepared ‘upconversion’ nanoparticles that can absorb near-infrared light and emit shorter wavelength light, such as visible light. More recently, the group went further to create lanthanide upconversion nanoparticles that not only absorb, but also emit, invisible infrared (RSC Adv., doi: 10.1039/C5RA20785A). Near-infrared is invisible to our eyes and regular smartphone cameras, so an infrared camera is required to view the image. That makes it ‘much more difficult to figure out there even is a mark there,’ Cross says.

Another advantage is that near-infrared light can penetrate polymers, so a security mark does not need to be printed on the product surface. To test this idea, the researchers printed a QR code and coated it with a jet black 0.4mm thick piece of epoxy. On the surface in ambient light, it looks like a piece of black plastic. But under infrared light and to an IR camera, a QR code appears.

‘That’s an added layer of security both in reading the security image, but also in protecting the image from any kind of tampering,’ says May.

Formulating the upconversion nanoparticles into a liquid ink, however, proved a challenge. Inkjet printing inks are normally water-based and use small-molecule dyes instead of particle pigments, to avoid clogging issues. Upconversion nanoparticles are typically prepared with organic solvents resulting in them being stabilised by oleate ligands, explains May, which makes the nanoparticles incompatible with water-based formulations.

Cross and another collaborator Jon Kellar at the South Dakota School of Mines, US, developed a suitable ink formulation for the upconversion nanoparticles (Langmuir, doi: 10.1021/acs.langmuir.7b03415). Using the surfactant sodium dodecyl sulfate, the researchers prepared a nanoemulsion in which the upconversion nanoparticles were contained in nano-sized oil droplets suspended in water. When the researchers filled an empty cartridge for an Epson inkjet printer and periodically printed a test document with the ink, the printouts retained their brightness for seven months. During that time, the researchers also did not experience clogging issues.

Polarised light

Robert Pal and his colleagues at Durham University, UK, specialise in preparing complexes of lanthanide ions with chiral organic molecules that exhibit circularly polarised emission. This light resembles that used in 3D movies – an image rendered in left-handed circularly polarised light, when combined through special glasses with the same image rendered in right-handed circularly polarised light, produces a 3D image. Pal and his colleagues had been synthesising the lanthanide complexes for bioimaging, but not all the complexes are biocompatible so several of them sat on the laboratory shelves. Then, during the pandemic, Pal and his colleagues sat down to think about other uses.

Now, the researchers think that the chiral lanthanide complexes are strong candidates for creating novel fluorescent security inks (Nat. Rev. Chem., doi: 10.1038/s41570-020-00235-4235-4). Lanthanide ions don’t emit light, Pal explains. To make them emissive, they must be complexed with an organic chromophore. These forced emissions are bright, long-lived, and robust, he says. Thanks to the lanthanide ions’ partially filled f-orbitals, their emissions are sharp and line-like, resembling fingerprints on a spectrum, Pal says. And if the organic chromophores are chiral, the lanthanide emissions become circularly polarised, generating hidden features that appear when viewed with a proper light filter and polariser.

‘You have two kinds of fingerprints, one left-handed and one right-handed,’ which can be an extra layer of security, says Pal. But to use the lanthanide complexes and their unique emissions, one needs to detect circularly polarised light easily. Typically, this is a large laboratory microscope – a big obstacle to using circularly polarised light in security inks. ‘The ultimate game would be to use a compound as a security marker or a barcode,’ Pal says. ‘But also, you cannot just walk around with 50,000Ib, fridge-size equipment saying that’s the one to detect it.’

Meanwhile, the 3D glasses used at cinemas may not be sensitive enough to distinguish between the right-handed and left-handed emissions from lanthanide complexes. To this end, Pal and his colleagues are developing a device small enough to be handheld while retaining sufficient performance. They are also working to incorporate the best lanthanide complex candidates into polymeric substrates for use in printing banknotes.

An afterglow

Phosphorescence – seen in glow-in-the-dark materials – can also be helpful for security inks. But phosphorescence is not as common as fluorescence, typically seen in materials containing heavy metals or at ultracold temperatures, says Xiaogang Liu at the Singapore University of Technology and Design. That’s because phosphorescence requires an ‘intersystem crossing’: when excited electrons change their spin states from singlet to triplet.

Instead of trying to design individual organic molecules that exhibit phosphorescence, Liu turned to two-component systems that combine a host molecule with a guest molecule. Such host-guest interactions could enhance the intersystem crossing by allowing charge to transfer between host and guest molecules. As a bonus, each host-guest pair could be kept secret, leading to ‘a unique ink that others cannot copy,’ Liu says.

Liu and his team developed a model to predict and identify pairs of organic host and guest molecules that exhibit room temperature phosphorescence (Angew. Chem. Int. Ed., doi: 10.1002/anie.202200546). Several promising pairs were then synthesised and tested. Five of these host-guest combinations had long-lasting afterglows, between 445 and 1669 milliseconds.

[Security tags on pharma-ceuticals packaging] are highly vulnerable. The best way to protect medicines is to do the authentication at the dose level.
Young Kim Weldon School of Biomedical Engineering, Purdue University, US.

For use in security writing, a paper material could be coated in the host molecule and written on with a solution containing the guest molecule, or vice versa, Liu explains. Liu’s team also demonstrated how to layer two host-guest combinations for extra security. The researchers first prepared a piece of paper coated in a host molecule, then wrote the acronyms ‘SUTD’ and ‘FRG’ with two solutions containing different guest molecules. The acronyms initially glowed brightly in UV. When the researchers turned off the light, the ‘SUTD’ text glowed yellow and faded within one second, while the ‘FRG’ text glowed green and persisted longer.

To check the phosphorescent ink, Liu’s undergraduate students produced a viewing box containing UV light and a sample stage. A smartphone is placed face down with its camera over a window in the box’s lid to capture the phosphorescent glow and its decay. This decay is intrinsic to a host-guest pair’s afterglow, Liu explains, so a computer program could be used to monitor the afterglow’s lifetime and verify a security ink. Moreover, the host-guest combinations can come from thousands of existing chemicals, Liu says, generating a vast number of security ink recipes.

Viewing sample in prototype outdoors

Edible tags

Pharmaceuticals are especially vulnerable for counterfeiting. Security tags such as barcodes or holograms on pharma packaging could be tampered with or replaced with counterfeits.

Instead, ‘the best way to protect medicines is to do the authentication at the dose level,’ says Young Kim at Purdue University, US.

That means tagging pharmaceuticals directly using edible fluorescent tags. For this, Kim and his colleagues turned to silk proteins, already generally regarded as safe by the US Food and Drug Administration (ACS Cent. Sci., doi: 10.1021/acscentsci.1c01233).

To make fluorescent silk proteins, Kim and his collaborators engineered silkworms by injecting vectors coding for the production of green, cyan and far-red fluorescent proteins into silkworm embryos. Then, the silkworms spin fluorescent silk, which the researchers collected and combined with non-fluorescent silk proteins to produce tiny, multi-layered physical tags resembling QR codes.

The physical tags can be imprinted directly on a pill or placed inside a liquid. To check the proteins are compatible with high alcohol liquid formulations, Kim’s team left an edible silk protein tag submerged in a bottle of Scotch whisky for a year, finding that it stayed intact. This encouraged the researchers to explore using the edible tags for anti-counterfeiting in alcoholic spirits, Kim says.

The group also developed an authentication process for the edible tags. A custom smartphone app first acquires images of the fluorescent tag. Then, the app converts the images to a digitised key that locates the pharmaceutical in an online database, prompting the app to open a webpage with product and manufacturing data such as dosage strength, manufacturing date, and lot number. This process is compatible with current methods of finding and authenticating products in large inventories, says Kim.

The researchers are now working to scale up synthesis and obtain regulatory approvals for the fluorescent proteins from transgenic silkworms. Kim also hopes to create information-storing implants from the fluorescent silk protein tags. Such tags could then be embedded under the skin, similar to a radiofrequency implant called Verichip, to retrieve health information like vaccination status, he says.