New Research in Invisible Inks

It’s a lot more chemistry than I understand: Invisible inks based on "smart" fluorescent materials have been shining brightly (if only you could see them) in the data-encryption/decryption arena lately…. 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…

It's a lot more chemistry than I understand:

Invisible inks based on "smart" fluorescent materials have been shining brightly (if only you could see them) in the data-encryption/decryption arena lately.... 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...revealing the pattern.... They rendered the pattern invisible again by briefly treating the paper with a polar solvent....

Full paper.

from https://www.schneier.com/blog/