As technology in security printing evolves, counterfeiting always manages to keep up its pace, too. Be it a color-changing ink, a security thread, or a hologram, counterfeiters never fail to mimic features as long as they figure out the recipes.

Researchers led by J. Fraser Stoddart, Northwestern University, Evanston, IL, USA, have made a surprising discovery of chemically responsive materials with tunable solid-state fluorescence, which may provide a better solution to fight against counterfeiting. These materials are heterorotaxanes and made of pyrene and diazaperopyrenium fluorophores inserted in cucurbit[6]uril and γ-cyclodextrin, two macrocylic hosts. As a result of complex aggregation equilibria, their emission profiles are sensitive to the concentrations of all components. Additional binding active species can shift the equilibria further and change the color emitted under UV light, which can be used as an authentication mechanism.

Using these materials as inks, this dynamic and non-linear process enables the encryption and authentication of information with chemical inputs. Since ink formulas are highly customizable, counterfeiters will not be able to decipher the entire equilibria change even if they can mimic the original emission color.

• Tunable solid-state fluorescent materials for supramolecular encryption,
  Xisen Hou, Chenfeng Ke, Carson J. Bruns, Paul R. McGonigal, Roger B. Pettman, J. Fraser Stoddart,
  Nat. Commun. 2015, 6, 6884.
  DOI: 10.1038/ncomms7884