Angle-dependent and liquid-responsive photonic crystals for anti-counterfeiting and optical sensing

More and more attention has recently been attracted to structural color due to its eco-friendly nature and excellent color fastness. As a type of structural color material, the construction of photonic crystals (PCs) has become well-established. However, some problems still limit their applicability, such as low color saturation and complicated processing. Herein, a multi-step dropwise addition of precursors was adopted to achieve the controllable preparation of monodispersed nano-silica. The gravity deposition assembly method was then used to construct quick-drying SiO₂ PCs with heterochromatic characteristics at different angles. Furthermore, the structural orderliness was improved through heat treatment after self-assembly. If the SiO₂ suspension was left to stand for 72 hours to remove the supernatant, followed by gravity deposition self-assembly of the SiO₂ sediment, it was observed that the SiO₂ PCs assembled at the bottom of the dish exhibited bright, angle-dependent colors. The SiO₂ PCs assembled on the dish wall were liquid-responsive, demonstrating good reversibility and recyclability. Therefore, a simple and rapid method for the preparation of angle-dependent and liquid-responsive SiO₂ PCs was provided. This approach will broaden the applications of PCs in areas such as anti-counterfeiting, holographic displays, information encryption, liquid/optical sensing, smart packaging, and functional coatings.