Pendant Micro-droplet Evaporation Fabricates Fiber-optic MOF Gas Sensor in Seconds

The development of photonic-based gas sensors using metal-organic frameworks (MOFs) and other microporous solids is often a multistep, complex process, typically involving MOF synthesis, purification, and attachment of microcrystals to an optical fiber end face. This study introduces a one-step method that integrates MOF synthesis and sensor head fabrication directly onto the fiber end face, forming an extrinsic Fabry-Perot interferometer (EFPI) with a thin film of MOF microcrystals. The resulting film, only 3-10-μm-thick, enhances sensor response by enabling rapid gas detection within seconds. Utilizing a pendant micro-droplet evaporation technique, this method forms a microporous MOF layer <i>in situ</i>, allowing unreacted molecular components to act as an adhesive that secures the MOF crystallites to the optical fiber, potentially also contributing to the film's adsorption properties. This process, demonstrated with the HKUST-1 MOF as a model system, optimizes both the fabrication speed and sensor response times by reducing the film formation process to 24 s under nitrogen and 90 s in ambient conditions. <i>In situ</i> Raman spectroscopy, X-ray diffraction (XRD), thermogravimetry (TGA), and energy dispersive X-ray spectroscopy (EDS) were used to validate the composition of the sensor head, confirming the presence of MOF crystallites as the primary sensing component within the EFPI film and characterizing additional film components that may enhance stability, selectivity, and response. This bottom-up approach holds significant promise for the scalable production of fiber-optic sensors that leverage MOF's gas adsorption properties.