The moment a water droplet lands, the light fades and a hidden code emerges. A novel composite photonic material whose brightness changes dramatically depending on moisture absorption has been developed by a Korean research team.
A joint research team led by Professor Lee Ji-seok of the Department of Energy and Chemical Engineering and Professor Park Jung-hoon of the Department of Biomedical Engineering at the Ulsan National Institute of Science and Technology (UNIST) announced on the 20th that they have developed a "soft photonic material based on upconversion nanoparticles" that emits light more than seven times more intensely in a dry state than when saturated with moisture.
The breakthrough brightness enhancement stems from the material's unique internal structure. Upconversion nanoparticles, which convert near-infrared light into visible light, are embedded inside a hydrogel dome while trapped in oil droplets. Near-infrared light entering the dome cannot escape immediately, instead scattering among the oil droplets and lingering inside. The longer the light remains trapped, the more opportunities the nanoparticles have to absorb the near-infrared rays, producing visible light seven times brighter than that of conventional materials.
However, when the hydrogel absorbs moisture, the situation changes. Light scattering decreases and the amount of near-infrared light retained inside is reduced, causing the luminescence to weaken sharply. Professor Park Jung-hoon's team successfully demonstrated this "structural near-infrared trapping effect" through optical analysis techniques.
The research team applied this principle to demonstrate innovative water-controlled security and recognition technologies. They first inscribed secret information such as letters or emojis with nanoparticles beneath a glowing hydrogel dome. Under normal (dry) conditions, the information in the lower layer is completely concealed by the dome's strong luminescence, but when water is dropped and the dome's emission subsides, the hidden image finally appears.
Conversely, the team also implemented a QR code that is recognizable only in a dry state. The QR code, created by alternating bright hydrogel dome regions with relatively dark polymer regions, works on the principle that when moisture contacts the dome regions and darkens them, the contrast between the two regions disappears and the code is erased.
The material also features excellent durability and response speed that support its practicality. It reacts instantly within 0.1 seconds upon contact with moisture and visibly darkens within seconds, with brightness variation maintained below 4 percent even after more than 100 cycles of drying and moisture absorption.
"This is a technology that significantly enhances luminescence by designing the path along which light travels inside the hydrogel, without having to complexly modify the upconversion nanoparticles themselves," said Ryu Chae-young, the first author of the study.
"The luminescence color of the nanoparticles and the hydrogel dome patterns can be freely programmed, and the manufacturing process is simple, so the technology can be expanded to various industries such as wearable sensors and displays, in addition to the security technology field," Professor Lee Ji-seok said.
The research, supported by the National Research Foundation of Korea under the Ministry of Science and ICT, was published in the international academic journal Advanced Functional Materials on April 20.
