Xiangquan Liu, Zhicong Zhang, Lingya Peng, Jinglun Yang, Yan Jiang, Rongrong Huang, Yan Luo, Dongxu Xue, Sanyuan Ding, Daqiang Yuan, Xiaoyan Liu, Liping Ding, Yu Fang. Angew. Chem. Int. Ed. 2025, e20736. DOI: 10.1002/anie.202520736

Nitrogen dioxide (NO₂) is associated with acid rain, smog, greenhouse effects, and human respiratory diseases. Currently, NO₂ monitoring mainly relies on ultraviolet-visible absorption spectroscopy. While the method meets sensitivity requirements, the instruments are costly and huge, preventing in situ online monitoring. Consequently, there is an urgent need for new approaches that enable real-time detection of low NO₂ concentrations.

In recent years, sensors that are simple in structure, small in size, and capable of rapid on-site NO₂ detection have made significant progress. For example, some groups have developed room-temperature electrochemical sensors with high sensitivity, albeit relying on expensive noble metal electrodes, which raises manufacturing costs and compromises long-term stability. Chemoresistive sensors offer low cost and easy fabrication but suffer from poor selectivity and high operating temperatures. Recently, porous materials such as MOFs, COFs, and carbon nanotubes have been used to construct NO₂ resistive sensors, which show high sensitivity and selectivity at room temperature. However, their response recovery times typically span tens of seconds to minutes. Therefore, developing compact, structurally simple, portable sensors capable of rapidly detecting NO₂ remains challenging.

Figure 1. Schematic representation of the β-ketoenamine linked ETTA-TP COF membrane and its fluorescence response to NO₂.

Film-based fluorescence sensors have attracted broad attention due to their high sensitivity, design flexibility, relatively simple hardware, and high selectivity. COFs are crystalline porous organic materials characterized by high surface area, tunable pore sizes, and excellent porosity. β-ketoenamine-linked COFs, with highly conjugated backbones, rich porosity, electron-rich characteristics, and strong chemical stability, are particularly promising. Interfacial polymerization enables the fabrication of COF thin films conducive to mass transport and sensor integration.

Figure 2. Photograph and characterization of the ETTA-TP COF membrane.

Recently, our team prepared a β-ketoenamine-linked fluorescence covalent organic framework (COF) membrane via interfacial confined condensation reactions between 4-aminostyrene (ETTA) and trimesic aldehyde (TP) to form the ETTA-TP COF. The membrane was characterized by FTIR, XRD, SEM, and TEM, confirming successful synthesis. Based on its NO₂ responsive, reversible fluorescence quenching behavior, we developed a NO₂ film-based fluorescence sensor. The sensor exhibits ultra-fast response/recovery times (1.5 s / 2.0 s), excellent selectivity (able to exclude 16 potential interferents), a low detection limit (0.1 ppm), and a wide detection range (0.1-50 ppm). It maintained good stability over 5,000 consecutive tests and enables in situ online monitoring of NO₂ from automotive exhaust and incineration processes, highlighting strong application potential.

Figure 3. NO₂ sensing performance of the ETTA-TP COF membrane.

The superior NO₂ response arises from the square-grid topology of the ETTA-TP COF membrane, its high porosity with small pore size, and a surface area up to 753 m² g⁻¹; the pore size (~0.6 nm) facilitates gas molecule diffusion and interaction with active sites via confinement effects. Infrared, XPS, EPR, and theoretical calculations indicate a photoinduced electron transfer between NO₂ and the COF membrane. Control experiments with thin films and small molecules show that the carbonyl groups serve as universal NO₂ binding sites for the fluorescence response. Theoretical calculations suggest electrostatic interactions between surface carbonyls and NO₂ drive the photoinduced electron transfer that results in fluorescence quenching. This work provides a new approach for developing portable gas sensors with fast response, high sensitivity, and in situ online monitoring capability for NO₂.

Figure 4. Investigation of the fluorescence quenching mechanism of the ETTA-TP COF membrane to NO₂.

This study develops a new method to the design of ultra-sensitive film-based fluorescent sensors (FFSs).

First Author: Dr. Liu Xiangquan, Shaanxi Normal University

Correspondence Authors: Prof. Fang Yu, Assoc. Prof. Liu Xiaoyan, Prof. Ding Liping, Shaanxi Normal University

Full Text Link: https://doi.org/10.1002/ange.202520736