Fluorine-Driven Polychromatic Colorimetric Screening of PFAS via Iodide-Mediated Gold Nanorod Etching on a Dual-Ligand Cu-MOF: A “No Aggregation, Low Inhibition” Sensing Paradigm

Developing hue-tunable polychromatic colorimetric sensors for perfluoroalkyl and polyfluoroalkyl substance (PFAS) detection offers significant advantages over conventional monochromatic systems, particularly in terms of improving visual readability and on-site applicability. Herein, inspired by classical iodometric titration and guided by theoretical precalculations, fluorinated tetrafluoroterephthalic acid (TFBDC) ligands were rationally incorporated into the HKUST-1 framework to construct a dual-ligand Cu-based metal-organic framework (Cu-BTC/TFBDC). Coupling this metal-organic framework (MOF) with an iodide-mediated gold nanorod (Au NR) etching system enables the first polychromatic colorimetric sensor for general PFAS screening (using perfluorooctanoic acid (PFOA) as a model target), allowing reliable quantification in three representative real water samples. In this design, Cu-BTC/TFBDC intrinsically couples efficient iodide oxidation with fluorine-driven analyte enrichment within a single framework, thereby eliminating the need for external oxidants and circumventing the limitations of conventional I^-/Au NR-based systems for PFAS sensing. The resulting platform exhibits concentration-dependent longitudinal localized surface plasmon resonance (LSPR) shifts of Au NRs over a wide detection range of 0.1-30 μM with a low limit of detection (LOD) of 13.1 nM, outperforming most existing fluorescent and colorimetric PFAS sensors. Mechanistic investigations reveal that PFAS molecules undergo surface-induced self-aggregation on Cu-BTC/TFBDC driven by intermolecular F-F interactions. This aggregation effectively blocks active Cu²⁺ sites, suppresses iodide oxidation, and inhibits Au NR etching, thereby amplifying the sensing response. This previously unrecognized "no aggregation, low inhibition" mechanism underpins the exceptional class specificity of the platform and provides new insights into fluorine-driven interfacial regulation in MOF-based colorimetric sensing. Overall, this work establishes a versatile material design strategy and a generalizable mechanistic framework for developing next-generation polychromatic sensors targeting persistent environmental contaminants.