Tiny Tags To Fight Fakes

Pharmaceuticals, electronics, and currency all have this in common: Counterfeiters make their ill-gotten gains by faking them. Patrick S. Doyle and coworkers at MIT have come up with a way to thwart counterfeiters by creating microparticle tags that could help verify the authenticity of goods (Nat. Mater. 2014, DOI: 10.1038/nmat3938). Each tag is about 200 μm long and made of colored stripes that glow when illuminated with near-infrared light (shown). Doyle’s team makes the particles using stop-flow lithography, a technique in which multiple streams of liquid monomer flow parallel to one another. Pulses of ultraviolet light polymerize the monomers, creating solid striped tags. The stripes’ colors arise from NaYF₄ nanocrystals that have been doped with rare-earth elements, such as ytterbium, gadolinium, erbium, and thulium. Although the tags are invisible to the naked eye, they’re easy to read. Just shine a near-IR laser pointer on the tags and they can be visualized using a smartphone equipped with a magnifying lens. Doyle’s team showed that the tags can be readily incorporated into currency, credit cards, artwork, and pharmaceutical blister packs.