Last updated December 21, 2017 at 3:26 pm
Have you colluded with a foreign power and are now wishing you’d hidden the evidence? Maybe you have some awkward citizenship papers you need to make disappear? Unfortunately a new invisible ink announced this week may have come just a touch too late.
Research published in Nature Communications has described a new lead-based invisible ink which is able to be printed using an ink-jet printer. The printed text is completely undetectable under visible and ultra-violet light, but reveals itself when combined with a salt, with the chemical reaction causing the text to luminesce under the same UV light unable to detect it before. Reapplying the same salt switches off the ink, reverting it back to its original undetectable chemical state.
Invisible inks aren’t just the domain of fictional spy thrillers – many exist outside the world of Ian Fleming. Made to reveal themselves on demand, they offer encryption and decryption of confidential information. However until now, invisible ink hasn’t been exactly as advertised. They either haven’t been truly invisible in their “hidden” state, or investigators have been able to reveal the information using just light, heat or a small amount of acid. And that’s where the new formulation has its advantage, not only is it absolutely invisible before activation, it requires the specific salt to make it glow.
The new ink uses metal-organic frameworks, or MOFs. This relatively new class of chemical structures are essentially porous chemical scaffolds which can be tailored to have a number of properties. In the case of the ink, the addition of a specific salt converts the lead-based MOFs into luminescent nanocrystals which glow under UV light. Reapplying the salt destroys the nanocrystals and quenches the luminescence. According to the researchers the salt can be added multiple times to switch on or off the nanocrystals, allowing repeated decryption/encryption of the information.
There are some drawbacks to the new invisible ink, however. Foremost is the lead contained in the MOFs, which is toxic to humans. However the researchers do anticipate there may be a formulation which has equal efficacy without requiring the lead.
There is also a question around the wisdom of creating an invisible ink, then publishing the details publicly. Surely the people who you might want to keep the information from will now be aware of the method to reveal your messages?
Image from Li et al, Nature Communications