High density imaging could fight counterfeiting, say scientists

Australian researchers have found a promising anti-counterfeiting measure through high density imaging that uses phosphorus nanocrystals to create fluorescent images.

The team from the University of South Australia (UniSA), the University of Adelaide, and the University of New South Wales have shown that exposing certain inorganic nanocrystals to ultraviolet (UV) light will activate the nanocrystals, essentially turning on their fluorescent properties.

When the nanocrystals are exposed to blue light, the regions that have been activated by the UV light will emit red fluorescence light, according to Dr Nicolas Riesen, a research fellow at UniSA.

“The phosphorous nanocrystals are unique in that they exhibit fluorescence that can be very efficiently switched on or off with a UV light,” says Riesen.

The new method could be used to place small images on banknotes or medical packaging that would be extremely difficult to forge and would be readable with a basic microscope and blue light, according to the researchers.

The fluorescent images have ultra-high-resolution, close to the theoretical upper limit of about 25,000 dots per inch (dpi) or 500 line pairs/mm.

The team had been studying these nanocrystal systems for a variety of applications and Riesen says the small size and resolution of the images presents many potential uses.

“Initially we were studying variants of these nanocrystals for optical data storage, as well as X-ray dental imaging at UNSW in Canberra,” he says.

“We then realised that this platform could be used for creating arbitrary fluorescent images on 2D surfaces. This isn’t limited to bank notes either, for instance you could also use this technique for putting a stamp on different consumer products, for anti-counterfeiting purposes.”

In terms of the material costs, Dr Riesen says resources needed come at a small cost. Expensive chemicals are not required to make the nanocrystals, rather the difficulty comes in their application.

“You would need specialised people and equipment to put those images on bank notes or consumer products, making it very difficult to counterfeit. Someone would have to be highly specialised to know how to do this, however once it’s been set up it’s highly repeatable.”