Multicolour nanocrystal barcodes for anti-counterfeiting

4 July 2014. NUS scientists have developed upconversion nanosized single crystal rods with micrometer lengths and colour bands for use as barcodes.

Researchers have long wished to be able to make very tiny colourful beads which they can then use to generate unique microscopic tags for genuine products to thwart counterfeiters. It is relatively easy to make very tiny colourful beads, such as of quantum dots or phosphor nanocrystals. However the challenge is to be able to make them with sufficient complexity such as different combinations of colour bands that will stump potential counterfeiters. Now a research team led by Professor LIU Xiaogang from the Department of Chemistry in NUS has developed multi-colour upconversion luminescent nanocrystal rods which can fulfill this objective (see Figure). The nanocrystal rods are based on sodium yettrium fluoride. A lanthanide activator, such as Er or Tm, is doped into the nanocrystal rod during its growth, and then a different lanthanide activated is doped into the crystal during a subsequent end-on growth. As a result different colour bands which become visible when illuminated with near-infrared light appear in the nanocrystal rod. This enables facile multicolor tuning within a single nanocrystal, which was not possible previously in conventional conversion nanoparticles. The nanocrystal rod length is be chosen to be just larger than 0.5 micrometer, so that the combination colour bands can be seen in an optical microscope. ”The colorful barcodes have excellent photo-stability, even under an intense laser excitation,” says Prof LIU. ”The making of security inks using these nanocrystal barcodes is inexpensive and may likely cost only $0.1 /mL, due to the use of wet chemistry.”

Image shows the fluorescence image from three single-colour “spacers”, and six dual-colour nanocrystal barcodes generated from an end-on epitaxial growth on the “spacers”. (Photo credit: LIU Xiaogang)

References

1. Zhang Y, Zhang L, Deng R, Tian J, Zong Y, Jin D, Liu X. “Multicolor Barcoding in a Single Upconversion Crystal”. Journal of the American Chemical Society. 136 (2014) 4893.
2. Wang F, Liu X. “Multicolor Tuning of Lanthanide-Doped Nanoparticles by Single Wavelength Excitation”. Accounts of Chemical Research. 47 (2014) 1378.