Making invisible inks more invisible

Invisible inks based on “smart” fluorescent materials have been shining brightly (if only you could see them) in the data-encryption/decryption arena lately. Researchers have debuted various organic dyes, nanocrystals, and other materials to help keep confidential information secure and to develop anticounterfeiting strategies. But some of the materials are costly or difficult to prepare, and many of these inks remain somewhat visible when illuminated with ambient or ultraviolet light. Liang Li and coworkers at Shanghai Jiao Tong University may have come up with a way to get around those problems. The team prepared a colorless solution of an inexpensive lead-based metal-organic framework (MOF) compound and used it in an ink-jet printer to create completely invisible patterns on paper. Then they exposed the paper to a methylammonium bromide decryption solution, thereby converting the starting material to luminescent, MOF-encased methylammonium lead tribromide nanocrystals, which shine brightly under UV light, revealing the pattern (Nat. Commun. 2017, DOI: 10.1038/s41467-017-01248-2). They rendered the pattern invisible again by briefly treating the paper with a polar solvent, which destroys the structure of the MOF-encased crystals and quenches their luminescence. Now, the team is working to develop tin-based or other lead-free starting materials and procedures for using the ink repeatedly.