Non-Faradaic Capacitive Sensing of Halides at Amine-Rich Electroactive Hybrid Interface

The continuous and selective real-time detection of F⁻ ions is highly desirable owing to its direct significance in both biomedical and environmental applications. Despite considerable advances, achieving sensitive and continuous ion sensing in neutral aqueous medium remains challenging. Herein, we introduce a proof-of-concept electrochemical F- ion sensing strategy through strong interfacial hydrogen bonding (F⁻···H), leveraging non-faradaic capacitance, operating continuously under ambient conditions at pH 7. The efficacy of this approach is demonstrated with an amine-rich electroactive interface, constructed by polyethyleneimine (PEI) on a water-stable MOF (NH₂-UiO-66), serving as a redox-inactive yet receptive substrate. The optimized sensor achieves a detection limit of 0.001 mM (0.18 ppm) for F⁻, attributed to strong F⁻···H-bonding with the cationic amine functional moieties under applied potential, as supported by theoretical calculations. The platform exhibits pronounced selectivity for F⁻ over competing anions (Cl⁻, Br⁻, NO₃⁻, CO₃²⁻, CH₃COO⁻), and retains stability for >15 h (150 cycles) at 30 mM F⁻. Importantly, the methodology enables reliable quantification of F⁻ in real water samples (tap and potable) down to ~1 ppm, underscoring the practical potential of non-faradaic capacitive ion sensing.