Luminescent QR codes

Nanotechnol. 23, 395201 (2012)

Credit: © 2012 IOP

Upconversion luminescence — emission produced at wavelengths shorter than the excitation light — could have unique applications for security printing. Compared with standard fluorescent inks, which typically emit visible light upon ultraviolet excitation, upconversion inks that involve near-infrared to visible conversion are more difficult to duplicate and therefore offer higher security. Now, using green and blue upconversion inks comprised of Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺ doped β-NaYF₄ nanoparticles, respectively, Jeevan Meruga and colleagues from the USA have printed luminescent quick response (QR) codes: two-dimensional barcodes that can be scanned using most smartphones. They used design software and direct-write aerosol jetting to make the codes and employed a 980 nm continuous-wave laser for illumination. Using oleic acid as the capping agent and a 90:10 solution of toluene and methyl benzoate with poly(methyl methacraylate) as the binding agent for the nanoparticles, the team printed QR codes on paper and transparent tape that were invisible under ambient lighting, but exhibited single- and multicolour upconversion luminescence under near-infrared excitation. They also showed that combining green and blue upconversion inks offers a higher level of security than standard inks, owing to the multiphoton emission processes involved.