Go with the glow: Beat counterfeiting

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  • Published: May 15, 2016
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: Go with the glow: Beat counterfeiting

Inked

A material that emits light in three different ways at room temperature could be used as an anticounterfeiting system in money and branded products (Credit: Lin et al/Angewandte Chemie)

An ink material that can generate light through three different mechanisms could be used as an anti-counterfeiting measure for money as well as branded products, pharmaceuticals and electronic components, according to new research from China. The carbon nanodot ink luminesces via three different modes, the team says.

There are many valuable commodities that are branded but counterfeiters are always stepping one beyond manufacturers with the technology to imitate goods and circumvent anti-counterfeiting measures. Those who develop protective measures must seek out the latest materials science to find new ways to make the forgers' job that bit tougher. Fluorescent tags were a strong defence against the imitators for a while but conventional chemistry has already been circumvented in some instances and it is always only a matter of time before all such measures will be counter by the fraudsters.

Luminescence

Writing in the journal Angewandte Chemie, the team has now turned to a new and exceptional area of anti-counterfeit materials, a nanodot ink that has properties that would be very difficult to replicate and so could defend brands against the imitators for an extended period of time. "Counterfeiting is a growing problem that challenges companies, governments, and customers around the world. Thus, anti-counterfeiting is in high demand for protecting important and valuable items that should not be replicated, such as brands, luxury items, banknotes, tickets, and certificates," Hengwei Lin at the Ningbo Institute of Materials Technology & Engineering of Chinese Academy of Sciences, the University of Chongqing, and Southeast University in Nanjing and colleagues report.

The material the team has developed emits light in three different ways at room temperature, is a first in science. The researchers used a quick and easy method to make carbon nanodots from m-phenylenediamine. They could then disperse the nanodots in water using polyvinyl alcohol and dispense them as an ink from a gel pen onto a banknote and a document as a proof of principle. After drying, the result is a colourless, transparent film of carbon nanodots in a polyvinyl alcohol matrix. Although the film looks colourless under ordinary light, it can be triggered to glow. First irradiation with an ultraviolet lamp at a wavelength of 365 nanometres causes it to emit a blue light through photoluminescence. The same ultraviolet irradiation then leads to a green afterglow that persists for several seconds once the lamp is switched off, this is room temperature phosphorescence. The third mode is made manifest through irradiation with an infrared femtosecond pulse laser at 800 nm, which induces a blue-green glow through two-photon luminescence.

Triple protection

The team explains that photoluminescence is a phenomenon that is widely observed. Irradiation with UV light excites the electrons in the substance into a higher energy level and as they drop back down to their ground state, some of their excitation energy is re-emitted as visible light.

Two-photon luminescence (up-conversion photoluminescence) is a far less common phenomenon wherein two photons are absorbed simultaneously in this case in the infrared range and the electron jumps to a higher level. From this higher level, it can then return by a direct route back to the ground state by emitting light of a shorter wavelength in the visible range.

Phosphorescence at room temperature is an even rarer occurrence. It involves a delay in the release of the absorbed energy because the process is forbidden by quantum mechanics and essentially involves electronic transitions the probability of which is very low. The researchers realised that it is the nitrogen-containing groups on the surface of their carbon nanodots that are critical to the phosphorescence that they observed. However, they also explain that the embedding of the carbon nanodots in the polyvinyl alcohol matrix is also important, because it inhibits intramolecular motion that would otherwise quench the phosphorescence process.

Luminescence printing has already been used in anti-counterfeiting measures because it has advantages such as easy handling, high-throughput, and facile design. The team's triple-mode variation on this theme could be a more secure format without adding any more complexity to the application of the inks. The system has demonstrable potential as an advanced anti-counterfeiting ink allowing rare triple-mode optical authentication, the team reports. "Such a unique feature would be very difficult to replicate," they conclude.

Related Links

Angew Chem Int Edn Engl 2016, online: "Triple-Mode Emission of Carbon Dots: Applications for Advanced Anti-Counterfeiting"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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