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.