Electrochemically Induced Conformational Change of Di‐Boronic Acid‐Functionalized Ferrocene for Direct Solid‐State Monitoring of Aqueous Fluoride Ions

While fluoride ions (F−) are abundant across environmental and biological systems, common procedures and potentiometric sensors for quantifying aqueous F− are inefficient, time‐consuming, and suffer from poor pH resiliency and high detection limits. Herein, this work reports a new di‐boronic acid‐functionalized ferrocene (FDBA) molecular receptor for noncovalent F− recognition, toward the development of a solid‐state miniaturized voltammetric fluoride sensor capable of direct and reversible F− detection in drinking water (DW) (pH 6) and community water (pH 7.6–9.1) over the µg L−1–mg L−1 range. The associated sensing mechanism is enabled by the conformational change of FDBA from the open (charge‐repelled) to closed (π‐dimerized) conformation, which is facilitated by the unique linkage of two electron‐accepting phenylboronic acid moieties with the electron‐donating ferrocene moiety using rigid conjugated amide linkers. The square wave voltammetric (SWV) oxidation current response of the FDBA‐based fluoride sensor is spectroscopically investigated, suggesting a combination of electrooxidation‐triggered conformational change of FDBA on a nanocarbon‐modified electrode, F− anion–π interactions, and resulting electron transfer between F− and FDBA. The performance of the voltammetric fluoride sensor is compared to that of a commercial liquid junction‐based fluoride ion‐selective electrode (F‐ISE), and of a solid‐contact (SC) F‐ISE sensor chip, demonstrating significant advantages versus traditional potentiometric F‐ISEs.