Thermochemical Properties of a Water-Soluble Zr(IV)-Flavonolate Complex for Fluoride Sensing

Detection of fluoride in water samples is critical for monitoring many chemical, environmental, and biological processes, however, these measurements often rely on costly instrumentation. This situation has motivated the design of simple to use, fieldable fluoride sensors. Previously, a fluorometric sensor based on (EDTA)Zr(H₂O)₂ (EDTA = ethylenediamine-N,N,N',N'-tetraacetate) and 3-hydroxyflavone (FlvH) was reported for fluoride detection in ethanol-H₂O mixtures. Herein, we report the synthesis and characterization of the previously proposed Zr(IV)-flavonolate complex as the dimethylammonium salt, [(EDTA)Zr(Flv)][NMe₂H₂] (Flv = 3-hydroxyflavonolate), and detail its utility for aqueous fluoride sensing. This species exhibits an intense blue fluorescence upon excitation with near-UV light (λ_ex = 390 nm; λ_em = 460 nm) that is quenched in the presence of NaF (100 mM acetate buffer, pH = 5) with good sensitivity (LOD = 20.9 nM; [F^-] from 10^-8 to 10^-4 M) under neat aqueous conditions. The equilibrium constant (K_eq = 1.26 ± 0.00184 × 10⁸ M^-2) and free energy (ΔG_rxn^°= -11.0 kcal mol^-1) were determined under the conditions used for fluoride sensing, indicating a highly favorable reaction with F^- in water. Finally, we developed paper-based sensing strips for qualitative fluoride sensing by immobilizing the sensor in a deposited gel enabling good selectivity and reduced interference due to slower ion diffusion.