Ambient, parts-per-billion formic acid capture and quantification with a covalent-organic-framework–coated quartz crystal microbalance

Covalent organic frameworks (COFs) represent a distinct class of porous crystalline materials with significant potential for advanced applications. However, their use in detecting hazardous organic acid gases continues to be challenging. In this study, two benzidine-based COFs (COF-OH and COF-OCH₃) were synthesized via solvothermal reactions; both exhibit remarkable crystallinity, superior thermal stability, and large surface areas. These COFs were deposited as thin films onto gold-coated quartz crystal microbalance (QCM) chips to fabricate sensors that selectively detect formic acid vapor. The COF-OH–coated sensor exhibited a BET surface area of 290 m² g⁻¹, a pore width of 2.20 nm, and a pore volume of 0.054 cm³ g⁻¹, favoring rapid gas diffusion and adsorption. It achieved a high sensitivity of 46.48 Hz ppm⁻¹, a limit of detection of 277 ppb, and a fast response within 60–90 s to 21.2 ppm of formic acid vapor under a nitrogen atmosphere. The device also demonstrated excellent reproducibility (RSD ≈ 4.3 %) and long-term stability over 60 days. Selectivity studies confirmed that the COF-OH QCM sensor exhibited excellent selectivity, with approximately 49 % discrimination toward formic acid at 53 ppm, demonstrating superior discrimination capability against various common volatile organic compounds. This performance arises from directional hydrogen bonds between the COF-OH hydroxyl sites and formic acid's carboxyl groups, as supported by systematic characterization and DFT calculations. These results establish the COF-OH QCM sensor as a robust platform for monitoring trace formic acid vapor.