A research team at UNIST has developed an upconversion nanoparticle-based optical material whose luminescence intensity changes in response to moisture. Utilizing the property that it glows strongly in a dry state and weakens when it absorbs water, this material can be applied to security information display and humidity detection.

Professor Lee Ji-seok's team from the Department of Energy and Chemical Engineering and Professor Park Jung-hoon's team from the Department of Biomedical Engineering at UNIST announced on the 20th that they have developed a soft optical material based on upconversion nanoparticles that shines more than seven times brighter in a dry state than when it is moist.

This material has a structure in which upconversion nanoparticles are placed inside a hydrogel dome. Upconversion nanoparticles have the property of converting near-infrared light into visible light that humans can see when exposed to it. The research team placed the nanoparticles inside an oil droplet and then confined them inside a hydrogel dome to control the path of light.

In this structure, near-infrared light does not escape directly from the hydrogel but is scattered between oil droplets. As the time the light stays inside increases, the opportunity for nanoparticles to absorb near-infrared light increases, and the luminescence intensity increases. Conversely, if moisture is introduced, the scattering effect decreases, and the luminescence weakens.

The research team utilized this property to demonstrate moisture-responsive security technology and QR code recognition technology. When characters and images made of upconversion nanoparticles are placed under a bright hydrogel dome, they remain invisible in a dry state but become visible when they absorb water. The QR code is recognized based on the difference in brightness only in a dry state, and the code disappears when moisture is introduced.

The repeatability of the material was also confirmed. Even after repeating the process of absorbing water and drying it more than 100 times, the change in brightness was less than 4%. The response speed was also fast, with the decrease in luminescence starting within 0.1 seconds after contact with water.

"First author Chaeyoung Ryu explained, 'This is a technology that enhances luminescence by designing a path for light to travel within the hydrogel without complexly altering the upconversion nanoparticles themselves.'"

Professor Lee Ji-seok said, “The luminescence color of the nanoparticles and the hydrogel dome pattern can be controlled, and the manufacturing process is simple, so it can be expanded not only into security technology but also into wearable sensors and displays.”

The results of this study were published in the international academic journal Advanced Functional Materials on April 20 and were conducted with support from the National Research Foundation of Korea under the Ministry of Science and ICT.