Highly Sensitive CWA Sensing Materials Based on Tetrafluorophenol-Modified s-SWCNTs

High-purity semiconducting single-walled carbon nanotubes (s-SWCNTs) have attracted increasing attention as sensing materials for the detection of chemical warfare agents (CWAs). Their excellent semiconducting properties and extremely large surface areas give them very high sensitivity for detecting chemicals in the vapor phase. However, their practical sensing applications are often limited by poor selectivity due to the nonspecific adsorption of various environmental gases. To address this challenge, we covalently functionalized s-SWCNTs with tetrafluorophenol (TFP), a strong hydrogen-bond donor (pKa = 5.53), via a diazonium reaction using 4-aminotetrafluorophenol (ATFP). The resultant TFP-functionalized s-SWCNTs (TFP-SWCNTs) were characterized by absorption, Raman, and X-ray photoelectron spectroscopy (XPS) and exhibited significantly enhanced performance for sensing dimethyl methylphosphonate (DMMP, a simulant of CWAs). The TFP-SWCNTs had TFP contents ([TFP]/[C]) ranging from 0.012 to 0.315%. Samples with [TFP]/[C] values of 0.045% and 0.093% showed markedly improved DMMP sensing performance, reducing the limit of detection (LOD) from 50 ppb in prior work to 0.01 ppb in nitrogen and 0.9 ppb in dry air. Notably, even in 15% relative humidity (RH) air, the sensors maintained excellent sensitivity, with a 4.5 ppb LOD, promising for field-deployed applications. Moreover, we observed a 10–20-fold increase in sensor response in humid air as DMMP concentration increased from 35.5 to 1204 ppb, compared to a 3–4-fold increase under a similar DMMP concentration change in dry conditions. Computer simulations indicate that this effect is due to the formation of an H2O bridge between DMMP and TFP, which enhances charge transfer (0.060e from DMMP to TFP) compared to direct DMMP–TFP binding (0.049e). In addition, TFP-SWCNTs exhibited strong DMMP selectivity against interfering volatile organic compounds (VOCs) including methanol, acetone, toluene, and N,N′-dimethylformamide (DMF). All these behaviors highlight a strong potential of TFP-SWCNTs for real-world sensing applications.