Images printed in a lead-based ink become visible when salts are added. C. Zhang et al./Nat. Commun. (CC BY 4.0)

Chemistry

‘Invisible’ ink can be made visible, then invisible, and back again

Ink based on lead ions might keep secrets safer.

A lead-based ink can be used to print invisible messages that become legible when decrypted with a chemical trigger.

Conventional ‘invisible’ inks are not very secure, because they are easily read under ultraviolet (UV) light. A team led by Liang Li at Shanghai Jiao Tong University in China developed an ink made of lead ions linked by organic molecules to form a crystalline material. Ordinarily, the ink scatters almost no light. But when compounds called halide salts are added, they pervade the ink’s crystalline structure, forming nanocrystals that fluoresce under UV light. The salts can be flushed out with solvents such as methanol to render the ink invisible again.

Logos and illustrations printed with this ink could be made visible and invisible 20 times. If concerns about the ink’s toxicity can be overcome, the method could be used in anti-counterfeit measures on documents such as banknotes.