Regulating the Flexibility of Two-Dimensional Covalent Organic Frameworks for High-Performance Gas Sensing in Noninvasive Monitoring of Trimethylaminuria

Unlike conventional covalent organic frameworks (COFs) with a single structural mode, flexible COFs (FCOFs) containing flexible chain segments have attracted extensive attention for their ability to exploit the advantages of COFs in crystalline structures and application performance. Herein, flexible fabrication-engineered perylene diimide-based COFs were reported as active materials for room-temperature trimethylamine (TMA) detection. Specifically, COFs with distinct flexibility (FAL-COF and RIL-COF) were synthesized through polymerization and linkage conversion with hydrazone bond reduction, enabling efficient gas and charge transport and thus excellent room-temperature gas sensing capability. Notably, FAL-COF with higher flexibility exhibited a 1.28-fold higher gas sensing performance to 4 ppm TMA and achieved a limit of detection of 7 ppb TMA. This enhancement originated from abundant structural transformation behaviors during the framework formation, which created additional active sites via in situ regulation. A mechanistic study employing in situ Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy confirmed that the C−N electronic structure promoted chemically adsorbed oxygen species at the gas−solid interface, altered the charge depletion layer, and played a pivotal role in gas sensing enhancement. Finally, by incorporating self-calibrating logic gates and a wireless transmission module, a flexible and wearable gas sensing system was developed for real-time TMA monitoring in exhaled breath. This work presents a strategy for establishing active sites in situ through rich structural transformation behaviors and highlights the potential of C−N for precise oxygen vacancies modulation in gas sensing while offering a practical application prospect for building a flexible, intelligent electronic wearable warning system in telemedicine diagnosis.