Size‐Tunable Covalent Organic Framework Nanoparticles for Assembling One‐Dimensional Photonic Crystals With Visual Sensing

Abstract One‐dimensional photonic crystals (1D PCs) emerged as superb sensing platforms due to their high sensitivity to environmental changes. However, effectively translating the microscopic interactions between covalent organic frameworks (COFs) and analytes into macroscopic optical responses via 1D PCs for intuitive detection presents a significant challenge. Here, we propose a stepwise‐induced synthesis strategy that for the first time achieves a size‐controlled synthesis of uniform nanoscale COFs (60–80 nm), leading to a high‐quality COF layer. This advancement enables COF‐based 1D PCs to exhibit diverse color variations and controllable saturation. Notably, the excellent compatibility of the COF layer with inorganic oxides, organic polymers, and metal‐organic frameworks (MOFs) results in these 1D PCs exhibiting bright colors. Crucially, introducing the mesoporous materials enables 1D PCs to achieve deep integration of adsorption and recognition functions for volatile organic compounds (VOCs). The fabricated COF/MOF 1D PC allows differentiation of 12 VOCs and visually detects VOCs at different concentrations (0–80 g m ^{− 3} ) through color changes, with a response time under 1 s. In particular, COF‐based 1D PCs can be transferred to flexible substrates while retaining VOC visual sensing. These attributes highlight the potential of COF‐based 1D PCs for real‐time VOC monitoring in industrial environments.