Fighting forgeries, whether bogus banknotes or high-priced handbag knockoffs, has become a complex chemical and engineering arms race. Once a counterfeiter figures out how to mimic a security feature, such as a fluorescent tag, that feature essentially loses its value. Northwestern University chemists led by J. Fraser Stoddart have come up with novel security inks based on supramolecular chemistry that they believe will be especially difficult to duplicate (Nat. Commun. 2015, DOI: 10.1038/ncomms7884). The inks are based on a heterorotaxane molecule—a linear moiety threaded through two or more different cyclic moieties. The heterorotaxane is pale yellow under natural light but glows dark red in ultraviolet light. Adding cyclodextrin to the compound changes the fluorescent hue across a rainbow of colors, depending on how much cyclodextrin is added. The resulting array of inks can be printed in myriad images on the authentic item and verified with a UV lamp and a smartphone. What’s more, the images can also be authenticated with a library of indicators that cause the inks to change color. So even if a counterfeiter is able to mimic the original fluorescent colors, the authentication color changes would be nearly impossible to fake.