Counterfeiters date back to at least the 13th Century, when watermarks were invented to authenticate original documents. Ever since then, printers and forgers have been in an arms race to out-do eachother. Now new and simple technology may give printers the upper hand.

A team of researchers from the South Dakota School of

Mines and Technology and the University of South Dakota have found a way to print invisible quick response codes onto documents. More frequently referred to as QR codes, these black barcode-like stamps contain digital information in the form of square dots arranged in a square pattern. The codes are commonly used in advertising and may contain a URL to another website or other useful information that can be accessed by scanning it with a camera from a smartphone.

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Instead of making the QR codes with black ink printed on a white background, the researchers found a way to make the codes with invisible ink that's still visible to a smartphone camera.

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The researchers started with ink made with

nanometer-sized particles that glow under a laser light. But the way the ink fluoresces is different than expected. Normally, fluorescent ink emits light of a longer wavelength. For example, shine ultraviolet light onto one of those blacklight posters and visible light colors are produced.

But in this case, the nanoparticle ink produces wavelengths that are shorter. Near-infrared light shined onto the ink produces bright blue or green colors. These types of fluorescent

inks are a lot harder for forgers to reproduce. A

smartphone can read it and know if the document is authentic or not. The laser light reader doesn't have to be a part of the phone either,

but can be a separate device that's linked wirelessly to a smartphone's


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A third element in this QR code enhances its security. The team devised a way to embed microscopic image into the code, something forger would have to produce. Without the microscopic image, a close examination would show that the QR code was a fake.

The ink was printed on ordinary paper and the QR codes held

up to being folded and unfolded 50 times. Jon Kellar, a professor of materials and metallurgical engineering at the South Dakota School of Mines, told Discovery News that the ink can even work with a desktop printer. And because the code can be printed on plastic or even glass, manufacturers could use it to authenticate other items besides documents -–

perhaps as a way of differentiating a real Rolex from a knock-off.

The work appears in the Sept. 12 issue of the journal Nanotechnology.

Credit: South Dakota School of Mines and Technology/Nanotechnology