The ever-increasing sophistication of the counterfeit trade is a growing economic problem, and when applied to pharmaceuticals, dangerous to human health. More covert strategies are required to combat the trade and US researchers have developed a potentially vital tool in the battle against counterfeiting.

A group at the University of Massachusetts, Amherst, led by Vincent Rotello, incorporated gold nanoparticles into ink by straightforward inkjet printing. This 'barcode' can be detected in an ambient and non-destructive manner by laser desorption ionisation mass spectrometry imaging - a method to determine the spatial distribution of particles based on their mass. More conventional methods, such as chromatography, require sample destruction for analysis, and non-destructive analytical techniques usually do not give specific chemical information.

By incorporating ligands of varying alkyl chain length (and thus mass) into the nanoparticle, the area can be selectively scanned for a particular mass. Combining the different nanoparticles enables a complex signature to be printed that is complete only when all the nanoparticles are scanned for. As group member Richard Vachet explains: 'The materials themselves are not easy to make and the imaging is sophisticated enough that anyone wanting to counterfeit may not have the ability to do that.'

Graham Cooks, an expert in the area at Purdue University, US, is positive about the work and comments that it 'utilises a capability of mass spectrometry that hasn't been utilised. The experiment is non-destructive and high quality'.

While being pleased with the results, Vachet says that imprinting the nanoparticle ink onto fibrous materials, such as those used in currency and packaging materials, remains a challenge. These will be the next steps for the team.

The team also foresees further applications, not just in anti-counterfeiting, but also, for example, in the pharmaceutical industry. Using biologically inert gold for the nanoparticle signatures means that they can be imprinted onto a drug and scanned for verification without damage to human health or destruction of the valuable sample.

Helen Gray

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