Sample of the gel OTHO
​The OTHO molecule can be varied in many different ways. The picture shows four gels based on the OTHO molecule. The color is due to the incorporation of light absorbed fragments into the basic structure.​​​​
​Photo: Mats Tiborn/ Chalmers

Please identify yourself with your luminous slime

​Using digital solutions to identify and verify documents has become increasingly common, but digital codes can be broken. Researchers at Chalmers propose in an article in the journal Chemical Science an alternative to the digital, a luminescent gel.
What happens if you mix a luminescent molecule, two molecules that react to light and one gel? A hack-proof identification system, says Associate Professor Henrik Sundén at the Department of Chemistry and Chemical Engineering, Chalmers.

The identification technique is based on the ability to state the right color within the well-established CIE diagram. By illuminating the identification gel during a given time and with a given wavelength on the light, the right color is revealed. The color emitted can be controlled in several different ways and is dependent of the composition of the molecules, but also of which wavelengths the gel is illuminated with and for how long. Only the person who has access to the correct lighting sequence and the correct composition of molecules will succeed in giving the correct coordinates in the CIE diagram. In other words, it is impossible to give the correct identification code without the gel. The mixture can potentially form the basis of a hack-proof authentication system.

“Traditional methods for these purposes require complex mathematical algorithms and processing power. By using a gel and light instead, we can achieve similar results with considerably less resources”, says Henrik Sundén.

The technique is based on a gel that contains a kind of luminescent, or fluorescent, molecules, and one or two types of photoresponsive molecules triggered by light exposure. The fluorescent molecule shines naturally in a blue tone, but when its blue light shines on the photoresponsive molecules, they are activated and the mixture begins to shine in another color. The enclosing gel consists of a specially designed molecule with self-healing properties. This allows the identification gel to be reused again and again. Thanks to the complexity of the processes that underlie the color changes of the gel, it is practically impossible to predict them on the basis of a given lighting sequence. It is the same unpredictability that is behind today's digital encryption algorithms.

“Unlike digital solutions, it is not possible to hack molecules. When you identify yourself with today's digital system, you can probably hack the code, but if we disconnect the digital and instead use a gel and a spectrophotometer codes can be created that cannot be cracked digitally”, says Henrik Sundén.

By using the established color chart CIE as the coordinate system, authenticity can be verified. The idea is that both parties in the identification situation agree on a certain composition of the gel. When the identification takes place, the person who is to be identified must receive a number of wavelengths to expose the gel with and time indication for how long. The colors that appear after the correct exposure are plotted into the coordinate system. These provide a non-linear curve that is completely unique to precisely the input conditions. If both parties' results match one another, the identification is approved.

The combination of right lighting sequence and composition and concentration of the gel entails an incalculable number of combinations. In the article, the researchers show a proof of concept. However, much further research remains, but in the future, you may have slime in your pocket instead of digital ID.

Text and image: Mats Tiborn

Published: Fri 22 Mar 2019. Modified: Thu 04 Apr 2019