UNIVERSITY OF SOUTH DAKOTA & SOUTH DAKOTA SCHOOL OF MINES AND TECHNOLOGY, SOUTH DAKOTA, USA. The use of an invisible QR (Quick Response) Code on virtually any solid object like printed documents and paper bills will immensely reduce counterfeiting. This concern costs billions of dollars in private industries and governments worlwide.

There will always be banknote forgers all over the world. The ultimate challenge is to come up with a security feature in the printing process of most printed documents. Researchers from the University of South Dakota and South Dakota School of Mines and Technology capitalized on the functionalities of QR codes.

They developed an invisible QR code that will provide security to printed documents. It is made of nanoparticles that were combined with blue and green fluorescence ink. A computer-aided design is employed and it is printed using an aerosol jet printer.

The process of upconversion is being utilized. This is when the photons are absorbed by the nanoparticles at a particular wavelength and the eventual emission of photons at a shorter wavelength. Normal lighting conditions will make the QR code invisible. They will only become visible once it is illuminated by an infra-red light. The code can be read by any smartphone.

The printed QR code used only mechanically and chemically stable nanoparticles that endured the strains on paper. The researchers folded the paper fifty times and the QR code was still readable. They also printed on flexible plastic film and glass. Since the QR code is invisible, it did not cause any problem with the physical appearance of the commercial goods.

Counterfeiting QR codes is a very difficult task. Parameters can be adjusted such as utilizing inks with a higher weight percentage of nanoparticles or controlling the intensity of the upconverting light. Also, a microscopic message can be embedded in the QR code using a different upconverting colored ink that can only be read with the use of a microscope. This innovation can absolutely tighten security features and authenticate any solid object that can save billions of dollars worldwide.