Want invisible ink? Just put water in your inkjet

Agent 86 from Get Smart would have a field day with this new paper developed by Chinese researchers. A cheap and recyclable way of keeping secrets, they’ve worked out how to print confidential documents on it using plain water as invisible ink.

Anything printed with water on the specially coated paper is invisible to the naked eye and can be revealed only under ultraviolet lighting at a particular frequency.

What’s more, the protected information can be erased by briefly heating the document with a blow dryer, and the blank document can then be reused in the same way at least 30 times.

Qiang Zhao and colleagues, from the Nanjing University of Posts and Telecommunications, created two prototypes of their new paper: one that uses plain water for everyday low-level security, and another that uses additional chemicals for even higher levels of secrecy.

They write in the latest issue of the journal Matter that their work “could be considered a major step forward toward rewritable and multi-level security printing”.

Using fluorescent security inks that are only visible under ultraviolet light is one of the most popular ways to make printed documents secure today.

However, this approach has known weaknesses, the biggest being that fluorescent inks are easily identified and thus provide insufficient security for important military and economic information. Also, the inks cannot be erased and are environmentally unfriendly.

So, Zhao and his team approached the problem from a different angle. Instead of focusing on the ink, they worked on the paper.

By coating the paper with a layer of an oxidised form of the common chemical element manganese, they were able to exploit that material’s luminescent response to water.

Using a commercially available inkjet printer, but with pure water as ink, the team printed images and text which were invisible unless viewed under ultraviolet light at 254 nanometers.

That was a good start: an everyday invisible ink made from water, with reusable paper costing less than $0.002 per printed page.

But the researchers wanted to take things further by creating a much more secure version of their product, with possible military and government applications.

So, they filled a printer cartridge with a solution of manganese halide salts. When these come into contact with the manganese layer of the paper, they change its ions and photochemical properties in a precise and controllable fashion.

This refinement enabled the team to configure the material’s emission lifetime – the average time a molecule stays in its excited state before emitting a photon.

Documents printed this way are no longer visible under UV lighting. Anyone wanting to view the contents needs to use a special method called Photoluminescence Lifetime Imaging to retrieve the data.

Different emission lifetimes can be applied to various parts of the document to provide multi-level security.

“This information decryption method possesses an extremely high security level and might be promising in military and economic domains,” the study authors write.

“Data leakage has become a global problem with terrible consequences, including outbreaks of war and severe economic and social issues. Efficient information encryption and decryption methods are urgently needed.

“Although electronic media have become indispensable in our daily lives, paper is still the most widespread medium for information storage, and many important documents are still paper based. Consequently, there is an increasing demand for paper document security.”

Next, the researchers believe their methods could be tested with elements other than manganese to create even more advanced, secure printing options. 

cREDIT: NANJING UNIVERSITY OF POSTS AND TELECOMMUNICATIONS