Covalent-Organic-Framework-Modified Quartz Crystal Microbalance Sensor for Selective Detection of Hazardous Formic Acid

Covalent organic frameworks (COFs) are a novel family of porous crystalline materials utilized in various advanced applications. However, applying COFs as a hazardous organic acid gas sensor is substantial but still challenging. Herein, a phenylenediamine-based covalent organic framework (TPDA-TPB COF) featuring excellent crystallinity, ultrastable thermal stability, and high surface area was successfully constructed. Then, the TPDA-TPB COF-modified quartz crystal microbalance (QCM) sensor is fabricated by immobilizing the TPDA-TPB COF thin film on the gold-QCM chip. The fabricated TPDA-TPB COF-modified QCM sensor demonstrates a rapid response, excellent reproducibility, high selectivity, and sensitivity to formic gas, arising from hydrogen-bonding interactions between formic acid and the outermost layer of the TPDA-TPB COF, as determined by extensive analysis and density functional theory calculations. The basic sites of the TPDA-TPB COF, which are numerous due to its high nitrogen content, and the carboxylic acid groups present in formic acid exhibit efficient interactions. The sensitivity of the TPDA-TPB COF-modified QCM sensor was found to be 7.75 Hz ppm–1 at standard room temperature and pressure conditions, with a limit of detection (LOD) of formic acid down to 1.18 ppm, which is significantly below the workplace olfactory threshold limit of 5.0 ppm established by the Occupational Safety and Health Administration. The TPDA-TPB COF-modified QCM sensor exhibits remarkable detecting capabilities, making it highly attractive for detecting organic acid vapors in diverse applications that require superior performance.

This solid was then subjected to filtration and underwent many rounds of washing using acetone and tetrahydrofuran (THF) in a sequential manner.Achieved a 91% yield of TPDA-TPB COF using vacuum drying at 120 °C for 24 hours.

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Table S2.Fractional atomic coordinates for the unit cell of TPDA-TPB COF with AA-stacking.

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Table S3.Analytical characteristic parameters for sensing activity of the TPDA-TPB COF towards vaporized formic acid using the QCM sensor technique.

Figure S9 .
Figure S9.BET linear and linear fitting plots derived from N2 sorption of TPDA-TPB COF.

Figure S11 .
Figure S11.The mass-normalized frequency variations recorded by the TPDA-TPB COF -modified QCM sensor following being subjected to a formic acid (53 ppm) under different degrees of humidity.
of formic acid injections.

Figure S12 .
Figure S12.FTIR spectra of TPDA-TPB before and after 2 months of the formic acid detection.

Table S4 .
Summary of hazardous formic acid vapor sensing performances of various porous materials.