A new method for making invisible ink visible, demonstrated in Nature Communications this week, could have applications in the protection of confidential information.

Invisible ink is made to luminesce on demand and turned off when desired, offering encryption and decryption of confidential information. Some materials change their luminescent properties in response to chemicals, light or heat, but those developed to date are not truly invisible in their off state - a vital property for information encryption.

Liang Li and colleagues show that colourless lead-based compounds can be reversibly transformed into luminescent materials by adding a simple chemical trigger. The materials are inkjet-printed onto parchment paper and the resulting text and pattern are hidden under both visible and UV light. The addition of a salt causes the material to alter its chemical composition, changing to luminesce brightly under a UV lamp. Reapplying the same salt switches the material back to its original state, providing multiple information encryption and decryption cycles.

The authors note that the toxic effects of lead-based materials need to be considered, but suggest that it might be possible to design lead-free alternatives. They propose that their strategy could aid the production of invisible inks for security protection applications.