Lab on optical fiber: surface nano-functionalization for real-time monitoring of VOC adsorption/desorption in metal-organic frameworks

Metal-organic framework (MOF) nanomaterials are emerging porous coordinative polymers with large surface area and high porosity. Their application scenarios highly depend on adsorption/desorption dynamics of guest molecules in the framework. For representative ZIF-8 with framework flexibility, the study of molecule transportation in the pore channels of ZIF-8 will address the ambiguity of unclear application scenarios. In this study, the integration of lab-on-fiber technology and nanotechnology are demonstrated for real-time monitoring of adsorption/desorption dynamics of heterocyclic volatile compounds (VOCs) with kinetic diameters larger than the window aperture of ZIF-8. The in-line fiber interferometer with cascaded long-period gratings is used to monitor the real-time refractive index change of VOC adsorption/desorption. The structure-effect relationship between guest VOCs and framework flexibility is analyzed. It shows that the adsorption dynamics is highly related to the molecular geometry and kinetic diameter. The framework flexibility results in the trapping of guest VOCs toluene, pyridine, and tetrahydrofuran in the frameworks. The methanol adsorption/desorption is an effective strategy for the fast desorption of trapped residual VOCs in the framework. Finally, we conceptually demonstrated the real-time monitoring of trace toluene enrichment using ZIF-8 for indoor air purification. This study paves the way for the in-depth understanding of framework flexibility for MOF’s application.