Development of Zn/Nb-Bimetallic Organic Framework Film Optical Waveguides for the Study of Improved Optical Gas-Sensing Specificity

Gas adsorption selectivity improvement of a metal–organic framework (MOF)-based sensor presents a significant challenge. This study addresses the enhancement of gas adsorption selectivity in MOFs by introducing bimetallic centers. The bimetallic zinc/niobium MOF (Zn/Nb–NDI–BMOF) was synthesized through the solvothermal coordination of Zn and Nb metal sources with the 1,4,5,8-naphthalene tetracarboxydiimide (NDI) organic ligand. Zn/Nb–NDI–BMOF formed a hexagonal-platelet structure with a high refractive index and optical transparency after 5 h of solvothermal growth. In comparison to Zn–NDI–MOF and Nb–NDI–MOF, the developed Zn/Nb–NDI–BMOF film displayed a unique response to H 2 S in the presence of SO 2, NO 2, CO 2, and HCl gases when incorporated into an optical waveguide sensor (OWGS). The enhancement in the gas-sensing selectivity of Zn/Nb–NDI–BMOF can be attributed to the bimetallic synergistic effect, which alters the framework structure and pore sizes and consequently modifies the optical configuration and activity. Upon exposure to H 2 S, it is susceptible to proton transfer between H 2 S and Zn/Nb–NDI–BMOF. Features such as a fast response (response time less than 2 s), a wide detection range (100 ppm to 1 ppb), good reproducibility, a low detection limit (0.69 ppb), thermal stability, and humidity resistance of the Zn/Nb–NDI–BMOF film OWGS for H 2 S make it a promising candidate for potential applications in environmental monitoring and food freshness assessment.