QR codes , those goofy black-and-white arrangements of digital information, could help secure bank notes.
Scientists just showed  that they could print an invisible QR code made of a high-tech ink containing fluorescent nanoparticles, layering it on in a thin, 50 micron layer on a sheet of paper. The printed patch is invisible until it is tickled with a near-infrared laser, at which point the inked QR code glows green or blue. It's then scannable with a smartphone. The ink is stable enough to tough out regular mistreatment in the life of a bank note, such as folding and crumpling, Scientific American  observes.
It took an hour and a half for a single invisible QR code to be printed, but the researchers told New Scientist  that time could be cut down to 15 minutes. Also, the ink itself could be customized with a specific concoction of nanoparticles, and act as an anti-counterfeiting indicator all by itself.
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