The invisible ink that can be made visible, then invisible, and visible again

A team from Shanghai Jiao Tong University has developed an ink that can be made visible and invisible repeatedly.

The university announced on Thursday that the process will help to keep secrets safer. 

The research achievement was published in the online magazine Nature Communications.

Smart fluorescent materials have been attracting increasing interest for security protection of information or documents that people don't want others know about.

Though they are invisible in ordinary conditions, they are not secure as they can easily become visible under UV light.

The ink developed with lead ions by the team led by Li Liang, a professor at Shanghai Jiao Tong University, is proving a better choice, the university said. 

The ink is usually invisible, but can be read under UV light when compounds called halide salts are added to cause a chemical reaction and produce nanocrystals that are fluorescent.

The salts can then be flushed out with solvents such as methanol to render the ink invisible again.

The process could also be used in anti-counterfeit measures on documents, such as banknotes, said researchers.