Surface-mounted metal-organic framework for the adsorption and sensing of monoaromatic pollutants in water

The increase in environmental pollution from industrial and anthropogenic activities, particularly toxic and persistent pollutants, is becoming an increasing threat to our access to clean water. While laboratory-based analytical techniques offer high sensitivity, their cost and time requirements limit spatial and temporal resolution for effective environmental monitoring. This highlights the critical need for technically advanced, yet simple-to-operate sensors suitable for field deployment. Specifically, the detection of water-dissolved pollutants is challenging due to the high propensity of water molecules to interact with sensor materials, obstructing the detection mechanism. One class of highly water-soluble pollutants are benzene, toluene, ethylbenzene and xylene isomers (collectively referred to as BTEX). These compounds are prevalent specifically in fossil fuels and are therefore often found in areas surrounding processing and storage facilities. Due to their relatively high water solubility, they have a high propensity to migrate and transport to the groundwater. Porous metal-organic frameworks have shown promise for separation technology, including as analyte-receptors in adsorption-based sensing. We hypothesized that a hydrophobic MOF with pore dimensions similar to the BTEX molecules would selectively partition these analytes from water. In our study, we have specifically investigated UHMOF-100, a material previously shown to be highly water-repellent with narrow pores hypothesized to selectively adsorb non-polar compounds. Bulk adsorption experiments confirmed the ability of UHMOF-100 to rapidly adsorb BTEX from water, demonstrating high mass capacities (up to 402 mg g⁻¹) influenced by a complex interplay of water solubility, molecular size, and guest–host interactions. Building upon this, we developed a method for fabricating robust UHMOF-100 thin films on commercial quartz crystal microbalance (QCM) resonators using a layer-by-layer deposition technique. The sensor morphology, crystallite size and density have been characterized using a combination of imaging, spectroscopic and diffraction techniques. The functionalized QCM sensors successfully detected individual BTEX species spiked in water within the concentration range of 0-50 mg L¹. Quantifiable responses were observed at concentrations as low as 5 mg L⁻¹, with sensitivities ranging from 2.04 to 4.59 Hz / mg L⁻¹. The sensors showed low cross-sensitivity towards more polar environmental contaminants such as phenol and benzoic acid, and a limited response to naphthalene, validating a degree of selective interaction with the target BTEX molecules. Furthermore, the UHMOF-100 films demonstrated both chemical stability in water and mechanical robustness under continuous flow conditions over extended measurement periods. This work presents, to our knowledge, the first example of a MOF-based QCM sensor for the detection of BTEX in water, demonstrating the potential of suitably designed porous materials for addressing challenging aqueous sensing applications.