Controllable synthesis of uniform large-sized spherical covalent organic frameworks for facile sample pretreatment and as naked-eye indicator

The successful application of covalent organic frameworks (COFs) depends on not only their unique chemical structures but also their morphology, size, and architecture. Spherical COFs (SCOFs) are attracted special attention due to the superiority of spherical materials in many applications. However, the synthesis of uniform large-sized SCOFs remains a challenge. Herein, by carefully optimizing the synthesis of a heteropore COF, we find that solvent type and catalyst concentration play important roles in determining the morphology and size of COFs, and eventually achieve the controllable synthesis of large SCOFs with uniform sizes ranging from 200 μm to 5 mm. The obtained SCOFs keep the dual-pore feature of the heteropore COF and show good stability and high crystallinity. To exhibit the superior application potential of SCOFs, the SCOFs with a size range of 200-300 μm were demonstrated to be promising solid-phase extraction (SPE) fillers. As-prepared SCOFs-packed SPE column could effectively remove ≥99% phytochrome matrix from 6 different vegetable samples in 10 s, accompanied by 72.56-112.37% recoveries of 33 chemical hazards with different physicochemical properties, thus showing greatly promising application prospects in sample pretreatment of nontargeted food safety analysis. By utilizing acid/base-adjusted reversible color change, millimeter-sized SCOFs were developed as an easy-to-operate and reusable naked-eye indicator of acids.