Hydrophobic COFs Promote NH₃ Sensing Response of Pyridine-Based Polymers Under High Humidity

Humidity severely limited the practical application of sensors in real atmospheric conditions with varying RHs. Herein, pyridine-based polymers coupled with hydrophobic covalent organic frameworks (COFs) exhibited enhanced NH3 sensing responses under higher RHs. Poly(pyridine-2,5-diyl) (PPy) demonstrated excellent NH3 sensor with a −87% response value and a 8 s/248 s response/recovery time to 100 ppm of NH3, with a 16.4 ppb NH3 LOD at room temperature (20 °C) under 20% RH. However, the response values decreased quickly as the air RH increased. Two hydrophobic COFs, composed of benzene or difluorobenzene building blocks, were used as water-repelling layers to prevent H2O reach sensing layer, thus improving the NH3 sensing response of pyridine-based polymers. A maximum 3.6-fold improvement was observed in the P2MeOPy sensor covered with the COF1 layer under a 73% RH atmosphere. The micropores in the COFs caused gaseous H2O liquefied at lower RHs, according to the Kelvin equation. A large amount of liquid H2O, due to mutual repulsion with the COF layer, make it difficult for the water to penetrate the COF layer and reach the sensing layer. In general, the hydrophobic COF layer prohibited H2O from reaching the sensing layer, limited H2O adsorption on pyridine-based polymers, and enhanced their NH3 sensing response.