Metal organic framework functionalized reflective tapered fibre-optic surface plasmon resonance based refractometer for vapor sensing

Metal-organic frameworks (MOFs), such as ZIF-8, have been extensively studied for their gas adsorption and chemical sensing capabilities owing to their high surface area, porosity, and tunable host-guest interactions. The adsorption of a guest molecule modifies the effective refractive index of ZIF-8 through pore-filling effects, rendering it appropriate for optical sensing. In the present study, we investigate the design of a reflective tip-based tapered fiber optic surface plasmon resonance (FO-SPR) platform functionalized with a ZIF-8 absorbing layer for the detection of chemical vapors. Experimentally measured, analyte-dependent refractive index changes of ZIF-8 are integrated into a Transfer Matrix Method (TMM) based reflective tapered FO-SPR model to systematically evaluate the influence of taper ratio on resonance wavelength, sensitivity, spectral broadening, and figure of merit for different vapor analytes. The analyte-dependent response towards tetrahydrofuran (THF), ethanol, and dimethylformamide (DMF) is analysed in terms of the refractometric modulation characteristics of ZIF-8 using the SPR transduction mechanism. To the best of our knowledge, this study constitutes the first theoretical investigation of MOF pore-filling dynamics within a reflective-tapered FO-SPR configuration. The sensor performance enhances significantly as the taper diameter decreases from 600 to 200 μm with sensitivity increasing from 9.38 to 10.76 nm/Volume Factor for THF and from 4.83 to 5.69 nm/Volume Factor for DMF. The operating range of the sensor is governed by the effective refractive index tunability of the ZIF-8 layer, which varies from 1.355 to 1.421, ensuring conditions necessary for SPR excitation relative to the fiber core. These results establish a fundamental framework for optical refractometric vapour sensing and provide practical design guidelines for MOF-functionalized tapered FO-SPR platforms.