Quick Response (QR) codes—those grainy, black-and-white squares increasingly found on advertisements and packaging—can quickly deliver encoded data to mobile gadgets, whether the info is a Web address for a promotional video or details about a package's shipment. Could this same technology effectively barcode currency, to crack down on counterfeiters?

A team of researchers from the South Dakota School of Mines and Technology and the University of South Dakota says it has developed an invisible QR code consisting of printed nanoscale phosphor particles that could serve as an enhanced security watermark. The researchers made the invisible codes using a computer-aided design file, which they then printed using an aerosol jet printer.

The codes are invisible in ambient light, but glow green and blue when a near-infrared laser is used to excite the printed phosphors (see video), a phenomenon known as upconversion luminescence. Once illuminated by the laser, the codes can be read using any standard QR reader on any mobile device. The researchers experimented with a combination of characters and symbols, using phosphors that show up green and blue to vary complexity and security. The codes can be printed on paper, plastic or glass, so their use could extend beyond protecting currency.

QR codes can hold 100 times more information than conventional barcodes, according to the researchers, whose work appears Wednesday in IOP Publishing's journal Nanotechnology. The nanoparticle-infused ink is both chemically and mechanically stable, so once the ink dries on a piece of paper the code could still be read even if that paper is crumpled or folded several times.

The researchers are already looking to enhance the security of their code even further. One approach will be embedding a microscopic message within the QR code that shows up as a different color under the laser--creating a code within a code that could only be seen under a microscope.

Image courtesy of South Dakota School of Mines and Technology/Nanotechnology