A Nanoporous CovalentOrganic Framework Film-BasedOptical 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]<i>_n</i>-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]<i>_n</i>-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]<i>_n</i>-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]<i>_n</i>-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.