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 H₂S in the presence of SO₂, NO₂, CO₂, 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 H₂S, it is susceptible to proton transfer between H₂S 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 H₂S make it a promising candidate for potential applications in environmental monitoring and food freshness assessment.