Colour-Changing Material Enables Invisible Security Markings

Researchers in Finland have developed a new colour-changing material that responds in the near-infrared region of the electromagnetic spectrum. Called davyne, the mineral-based material remains visually unchanged to the human eye, but its response can be detected using a spectrometer or infrared-sensitive camera. This combination could support discreet security features in banknotes, passports, packaging and consumer products.

From Hackmanite To Davyne

The discovery emerged from research into hackmanite, a natural mineral that can also be produced synthetically. Hackmanite is photochromic: exposure to ultraviolet light changes its colour from white to pink or violet. White light or heating the material to approximately 100°C reverses this effect.

Researchers at the University of Turku discovered that replacing sodium with calcium altered these colour-changing properties. Some samples turned yellow, while others showed no visible change. Further investigation revealed that the apparently inactive samples responded to light in the near-infrared region.

The yellow material was identified as calcium hackmanite. The infrared-active material was davyne, a member of the cancrinite mineral family that initially formed as a by-product during synthesis. Although calcium hackmanite and davyne have the same chemical formula, their atomic structures differ.

Optimising The Material

Producing pure davyne proved challenging because the chemical equilibrium tended to favour calcium hackmanite. The research team therefore optimised both the composition and manufacturing process in collaboration with materials engineers. Machine-learning methods helped the researchers achieve a highly pure form of davyne.

They also investigated the mechanism behind its response. In hackmanite, an electron moves from a specific ion to a chlorine vacancy within the structure. The size of this vacancy influences the resulting colour. Davyne contains long, empty tunnels rather than hackmanite’s characteristic cavities, but the researchers established that its colour-changing mechanism is similar.

Invisible But Detectable

According to the researchers, davyne is the only known colour-changing material whose transformation remains invisible to the human eye. Its near-infrared response can nevertheless be detected using a relatively inexpensive modified camera with its colour filters removed.

An important characteristic is that this infrared activity can be switched on and off. A security marking could therefore be activated for inspection and subsequently returned to an invisible state. This may provide an additional authentication layer for official documents, branded goods and packaging without affecting their visible design.

The research involved the University of Turku, Aalto University, the National Institute of Chemical Physics and Biophysics in Estonia, and the University of Lyon. The study was published in Angewandte Chemie.

Source: University of Turku
Photo: Ibrahim Boran