Solvent-Induced Structural Modulation in Nanoscale Covalent Organic Frameworks Enables High-Performance NO₂ Sensing

Covalent organic frameworks (COFs) offer exceptional structural and functional tunability, enabling performance optimization in separation, catalysis, and sensing applications. Here, we report a solvent-induced strategy for inducing structural disorder in porphyrin-based COF nanoparticles (nanoCOFs) synthesized in aqueous acetic acid, where the porphyrin units are protonated during the synthesis. Subsequent post-treatment with polar organic solvents induces deprotonation of the porphyrin units and effectively reduces the crystallinity of the nanoCOFs. The decreased crystallinity arises from solvent insertion between COF layers, which disrupts the ordered interlayer stacking and promotes structural disorder. Systematic control experiments demonstrated that the protonation-deprotonation behaviors of porphyrin units, the strength of interlayer interactions, and solvent properties (e.g., size and polarity) are key factors in regulating the framework structure and degree of structural disorder. Despite the significantly decreased surface area and reduced crystallinity, the solvent-treated nanoCOF-366, with its less compactly stacked layers, exposes more accessible porphyrin sites to guest molecules. As a result, they exhibit enhanced NO₂ uptake and superior chemiresistive sensing performance, achieving an ultrahigh record response of 1083.0 to 10 ppm NO₂ and an ultralow limit of detection of 0.21 ppb. This work presents an effective strategy for structural regulation of COFs toward advanced gas sensing applications.