A Nanoporous Covalent Organic Framework Film-Based Optical Waveguide Sensor for H2S Gas Detection

Nanoporous covalent organic frameworks (COFs) exhibit exceptional potential as sensitive materials for gas sensors due to their film-forming capabilities and tunable host–guest interactions. This study addresses the challenge of selectively detecting H2S gas by developing an optical waveguide gas sensor (OWGS) utilizing [H4bptc-(TAPT)3]n-COF films (Here, H4bptc refers to biphenyl- 3,3′,5,5′-tetracarboxylic acid, and TAPT represents 2,4,6-tris(4-aminophenyl)-1,3,5-triazine). These films were fabricated by immobilizing COFs on TiO2 substrates via a solvothermal reaction, followed by surface optimization using a layer-by-layer assembly method. This assembly method induced a structural transformation in the [H4bptc-(TAPT)3]n-COF films, progressing from densely layered structures to a graphene-like architecture and eventually forming more intricate configurations. Among these, the 3-layered [H4bptc-(TAPT)3]n-COF film demonstrates a graphene-like structure and achieved rapid (<2 s) and selective response to H2S gas, with notable refractive index changes upon proton transfer. The Density-functional theory (DFT) calculations revealed the highest binding energy between the triazine ring in [H4bptc-(TAPT)3]n-COF and H2S molecules. The sensor exhibited excellent selectivity, a broad detection range (100 ppm-1 ppb), outstanding reproducibility, moisture resistance, and an ultralow detection limit of 1.07 ppb at room temperature. Additionally, the H2S adsorption process was determined as endothermic, with an adsorption capacity of 10.98 ng·cm–2 at 293 K.