Invisible QR Codes Tackle Counterfeit Bank Notes and Other Security Applications

The combination of the blue and green inks also enabled the researchers to experiment with a variety of characters and symbols in different colors and sizes, varying from microscopic to macroscopic. Embedding these into the QR code further increases the level of security.

Under normal lighting conditions the QR code is invisible, but becomes visible when near infra-red light is passed over it. This process, known as upconversion, involves the absorption of photons by the nanoparticles at a certain wavelength and the subsequent emission of photons at a shorter wavelength.