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 H 2 S gas by developing an optical waveguide gas sensor (OWGS) utilizing [H 4 bptc-(TAPT) 3 ] n -COF films (Here, H 4 bptc 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 TiO 2 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 [H 4 bptc-(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 [H 4 bptc-(TAPT) 3 ] n -COF film demonstrates a graphene-like structure and achieved rapid (<2 s) and selective response to H 2 S 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 [H 4 bptc-(TAPT) 3 ] n -COF and H 2 S 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 H 2 S adsorption process was determined as endothermic, with an adsorption capacity of 10.98 ng·cm –2 at 293 K.