doi:10.1038/nindia.2014.153 Published online 11 November 2014
Researchers have synthesized luminescent nanorods that can be used in security ink1. Such ink could potentially be used to counteract fake currency and fraudulent imitations in the food and pharmaceutical industries.
Governments and private industries lose billions of dollars to counterfeiting activities. One way to counteract forgery is to use security inks, usually made using semiconducting nanoparticles such as cadmium sulfide and cadmium telluride. However, such compounds are toxic and their nanoparticles cannot produce stable, transparent solutions.
To prepare non-toxic nanoparticles that are stable in solution for security inks, the researchers synthesized nanorods by using yttrium oxide and rare-earth elements such as ytterbium and erbium. They assessed the efficacies of the nanorods by dispersing them in a solution of a commercially available polyvinyl chloride gold medium.
Spectroscopic analysis of the nanorod-containing ink by exposing it to laser light of different wavelengths revealed that the ink emitted strongly in the green and red lights. To demonstrate the potential of the ink in counteracting fake currency, the researchers printed various images on black paper using a standard printing technique. When exposed to infrared laser light (wavelength: 980 nm), the images gave off either red or green emission depending on their synthesis temperature, whereas when exposed to ultraviolet light (wavelength: 379 nm) the images produced green emission.
The ink was stable for several hours and remained optically transparent with a high luminescence. “In addition to counteracting counterfeits, these nanorods could also be used as optical biomarkers, displays and sensors,” says Bipin Kumar Gupta, a senior author of the study.
1. Kumar, P. et al. Highly luminescent dual mode rare-earth nanorods assisted multi-stage excitable security ink for anti-counterfeiting applications. J. Mater. Chem. C. (2014) doi: 10.1039/C4TC02065K