Invisible QR codes to combat counterfeit goods
Invisible codes designed to tackle the issue of fake goods have been created by US researchers.
The Quick Response (QR) codes are more often used in the advertising industry to allow customers access to product information.
The invisible version can be printed onto paper, glass or other materials and is invisible to the naked eye, becoming visible under infrared light.
The code's complexity means they are hard to replicate, researchers said.
Fake goods and banknotes cost governments and industries billions of pounds each year.
QR codes can hold up to 100 times more information than traditional barcodes and are widely used in the advertising industry to give people more information about a certain product.
Usually a square of black and white pixels, the codes can be scanned by a smartphone which links users to a website.
The technique, developed by researchers at the University of South Dakota and South Dakota School of Mines and Technology, is intended as a means of authentication rather than information.
The invisible code can be seen under infrared laser light and can be scanned in the traditional manner, using a code scanner application on a smartphone.
The codes are created from tiny nanoparticles, combined with blue and green fluorescence ink. The nanoparticles absorb photons at a non-visible wavelength but emit them in a visible wavelength, a process known as upconversion.
Created using computer-aided design, the code is then printed onto a surface using an aerosol jet printer.
The process takes about 90 minutes but researchers believe that once the initial code is created, mass printing will take about 15 minutes.
The creators said it also proved robust. To test whether it could withstand the stresses that banknotes regularly undergo, researchers folded paper containing the hidden code 50 times and it still remained readable.
Researchers believe the codes could be printed onto virtually any solid object and will remain "tough to counterfeit".
Jeevan Meruga, lead author of the study, said: "We can also change our parameters to make it even more difficult to counterfeit, such as controlling the intensity of the upconverting light or using inks with a higher weight percentage of nanoparticles.
"We can take the level of security from covert to forensic by simply adding a microscopic message in the QR code, in a different coloured upconverting ink, which then requires a microscope to read the upconverted QR code."
As an added level of security, microscopic or macroscopic letter or symbols can be embedded within the code using different coloured inks.
The research was published in the journal Nanotechnology.