Angle-independent responsive organogel retroreflective structural color film for colorimetric sensing of humidity and organic vapors

Abstract The angle dependence of photonic crystals (PCs) dramatically limits their practical applications in the colorimetrical sensing of humidity and volatile organic compound (VOC) vapors. In addition, it is challenging for inverse opal PCs to colorimetrically distinguish between vapors with similar refractive indices. Different from the mechanism of PC-based sensors, here, we report an angle-independent polyacrylamide (PAAm) organogel structural color film based on the mechanisms of retroreflection, total internal reflection (TIR), and interference with a shape similar to a single-sided “egg waffle”. During the process of responding to humidity and VOC vapors, the color of the film remains angle-independent in the normal angle range of 0° to 45° under coaxial illumination and observation conditions. At the same time, the film can colorimetrically distinguish between vapors with similar refractive indices, such as methanol and ethanol, which is mainly due to the differences in their polarity and solubility parameters. The film shows good stability, reversibility, and selectivity when exposed to vapors. A colorimetric sensor with a new response mechanism is proposed and has the potential to effectively distinguish between vapors with similar refractive indices. Furthermore, this responsive retroreflective structural color film (RRSCF) provides a universal strategy to develop targeted angle-independent structural color sensors by selecting optimized materials.