Invisible security feature with nanotechnology QR codes
(Nanowerk Spotlight) Quick Response Codes, or QR codes for short, are two-dimensional matrix codes that can hold 100 times more data than a traditional barcode. QR codes have rapidly gained popularity and are now very commonly used in various products, because of fast readability and large storage capacity. Applying the concept of QR codes to security printing applications – think banknotes – researchers have now developed an invisible QR code made from nanoparticles.
A team of researchers from the University of South Dakota and South Dakota School of Mines and Technology have now applied upconverting inks to print QR codes on paper and transparent tape, using an aerosol jet direct writing machine. They produced QR codes with embedded security characters using blue and green upconverting inks. These codes are invisible to the naked eye but produce single- and multi-color upconversion luminescence images under near-infrared (NIR) excitation which can be read and decoded with an unmodified smart phone.
a) Regular QR code of SDSM&T printed on paper by standard methods. b) The covert luminescent upconversion image of the same QR code printed using an aerosol jet with the letters ‘U’, ‘S’, and ‘D’ (University of South Dakota) written in the upper-left, upper-right, and lower-left corners, respectively. (Reprinted with permission from IOP Publishing)
Upconversion luminescence refers to emission produced at wavelengths shorter than the excitation light. In the case of NIR-to-visible upconversion, visible emission is produced using NIR excitation. This process involves the absorption of photons by the nanoparticles at a certain wavelength and the subsequent emission of photons at a shorter wavelength.
As the team explains, "there are several potential advantages to the use of upconversion 'inks' in security printing as opposed to standard fluorescent inks, which typically produce visible emission upon UV excitation: First, both the upconversion inks and NIR excitation sources (or ‘readers’) are intrinsically more difficult to duplicate than standard fluorescent dyes and UV sources. Also, upconversion patterns can be written on highly fluorescent surfaces, since the NIR reader will not excite standard downconversion luminescence. Moreover, upconversion inks can be formulated to produce the correct color rendering only under specific excitation power densities, making it even more difficult to duplicate the correct read conditions."
This means that a QR code made with such an will add an upconversion luminescence technique increases the level of security over existing counterfeiting methods as the complexity of the production process makes it very difficult to replicate.
The team used a combination of blue and green upconversion fluorescence ink in combination with lanthanide doped β-NaYF4 nanoparticles with approx. 60 nm in size. Using this ink, they employed a commercial aerosol jet printer to print a QR code on paper. Under normal lighting conditions the QR code is invisible but becomes visible when near infra-red light is passed over it.
The nanoparticles that were used to print the QR code are both chemically and mechanically stable meaning they could withstand the stresses and strains of being placed on paper. To prove this, the researchers printed the QR code onto a piece of paper and then randomly folded it fifty times; the code was still readable.
In addition to being printed on paper, the QR code has also been printed on glass and a flexible plastic film, demonstrating its applicability to a wide variety of solid commercial goods. The fact that the QR code is invisible is also beneficial as it would not interfere with the physical appearance of the goods.
According to Jeevan Meruga, first author of the paper, the QR code is tough to counterfeit. "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," he says. "We can take the level of security from covert to forensic by simply adding a microscopic message in the QR code, in a different colored upconverting ink, which then requires a microscope to read the upconverted QR code."
In this video, Jeevan Meruga explains the development process for the QR codes.