Researchers in Finland have created a mineral-based material that hides security markings from view but reveals them under infrared imaging. This material, called davyne, changes color under near-infrared light and could offer a new way to prevent counterfeiting.
The discovery came from research on hackmanite, a photochromic mineral studied by the Intelligent Materials Chemistry Group at the University of Turku. Unlike most color-changing materials, hackmanite reacts outside the visible spectrum. The team recently published their findings after exploring how this unique material creates its hidden color change.
A hidden color response
Hackmanite changes from white to pink or violet when exposed to ultraviolet (UV) light. Its original color comes back after it is exposed to white light or heated to about 212°F (100°C).
In their experiments, researchers found that swapping sodium for calcium changed how hackmanite behaved. Some samples turned yellow, while others showed no visible color change. But the team noticed that these seemingly inactive samples were actually reacting in the near-infrared region.
“The development of this new material began with a thesis project carried out by a student in our research group. The project demonstrated that the color-changing properties of hackmanite are altered when calcium is introduced to the material instead of sodium. Rather than turning pink or violet, the material turned yellow,” revealed Principal Investigator, Professor Mika Lastusaari from the University of Turku.
“We also noticed that some of the material samples did not change color visibly, but similar color changes occurred in the near-infrared region, which is invisible to the human eye.”
Colour change in davyne is visible only in infrared light. The pictures have been taken after UV exposure in white light (light picture) and in near-infrared light under NIR-LED (dark picture). (Image credit: University of Turku)
Davyne emerges during synthesis
The team identified the material that reacts to infrared light as davyne, a mineral from the cancrinite family. It appeared as a by-product when they made calcium hackmanite.
Although the two materials share the same chemical formula, their atomic structures differ. Producing pure davyne initially proved difficult because the chemical equilibrium favored calcium hackmanite.
Researchers worked with materials engineering specialists to refine the production process. They also used machine learning methods developed for materials engineering to optimize the synthesis, eventually producing the purest possible davyne.
Researchers explain the mechanism
The scientists then investigated why davyne changes color. They initially expected a mechanism similar to hackmanite, where an electron moves from a specific ion to a chlorine vacancy. The size of that vacancy influences the resulting color.
“At this stage, however, it was still a mystery to us what causes the color change. We assumed that the mechanism was similar to that of hackmanite, where the color change is caused by the transfer of an electron from a specific ion to a chlorine vacancy in the structure, and the size of this vacancy determines the color the material turns into,” Lastusaari mentioned in the press release.
“However, davyne’s structure is different, and the material has long, empty tunnels instead of the cavities that are typical of hackmanite. At the end, we were able to show that the color-changing mechanism of davyne is similar to hackmanite,”
Potential security applications
The researchers tested whether davyne could work as an invisible security marking. Such markings are already used on banknotes, passports and consumer products to verify authenticity.
The team tested davyne with an inexpensive camera whose color filters had been removed. Its silicon sensor could detect wavelengths suited to the material. The tests showed that the color change became clearly visible under infrared light when the camera was adjusted to davyne’s sensitivity.
“The advantage of davyne over other invisible anti-counterfeit marking materials is that its near-infrared activity can be switched on and off,” said Lastusaari.
“In practice, this means that the marking can be activated so that it is visible for inspection, and then returned to an invisible state,” he added.
The study was published in the journal Angewandte Chemie.