Multi-Optical-State Hydrogel Film with a Nanometer-Sized Inverse Quasi-Amorphous Array for Information Security

Amid the proliferation of counterfeit and shoddy products, it is imperative to develop efficient and reliable encryption and anticounterfeiting technologies. Here, we present a two-level information security system with encryption and anticounterfeiting functions that can exhibit multioptical states. A phase transition material of P(N-isopropylacrylamide) (PNIPAM) film with a nanometer-sized inverse quasi-amorphous array (PIQA) is prepared using a sacrificial templating method. In the dry state, the PIQA film is highly transparent due to the collapse of internal pores, effectively concealing any preset information. When immersed in water, the nanometer-sized quasi-amorphous array of the PIQA film rapidly recovers, producing vibrant structural colors and revealing encrypted information. By simple increase of the temperature, the PIQA film undergoes a vibrant color change. This occurs because as the temperature increases, the PIQA film undergoes a phase transition, with its volume first expanding and then contracting. Correspondingly, the reflection peak wavelength of the PIQA film undergoes a red shift, followed by a blue shift. When the temperature exceeds 35 °C, phase separation occurs in the PIQA film, resulting in a white color. This feature for multiple color transformations solely through temperature increase serves as an authentication for accurate information. The reversible switching of multioptical states demonstrates great potential and advantages in the field of information security.