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.