Fast water-response double-inverse opal films with brilliant structural color

• Double-inverse-opal photonic crystals are used to construct water-response films. • Rapidly responsive time and brilliant structural color are obtained. • The response mechanism achieved by water stimuli is demonstrated. • A highly invisible pattern revealed by water response is designed. • A facile and visual sensor to detect water in an ethanol-rich mixture is proposed. The construction of novel photonic structures renders the possibility for creating stimuli-responsive functional materials. Herein, innovative water-response double-inverse opal (WRDIO) films with fast response time and brilliant structural color are first fabricated by introducing polystyrene microspheres into the inverse-opal backbone. The deprotonation of carboxyl groups generated by sodium carbonate treatment in the inverse-opal backbone inhibits the formation of hydrogen bonds among the polymer chains and the exposed carboxyl anions lead to hydrophilicity, which is conducive to the penetration of water into the internal pores. The response is achieved by inducing the polystyrene spheres within the pores from random distribution to the ordered assembly in the water environment, thus eliminating the scattering disturbance and developing bright structural color. Meanwhile, WRDIO film with a highly invisible pattern revealed by water stimuli is prepared. Furthermore, the content of carboxylic anions in the polymer affects the optical behavior and the increased osmotic pressure caused by high-proportioned carboxylic anions makes the WRDIO film swelled rapidly by water, whereas ethanol cannot. The broad-spectrum shifting (≈92 nm) depends on the composition of the ethanol-water mixture. Thus, the rapidly water-response and easy-to-use film materials with brilliant structural color exhibit potential applications in user-interactive anti-counterfeiting and sensors.