PNNL

Abstract

Detection of fluoride in water samples is critical in many chemical, environmental, and biological processes that often rely on costly analytical instrumentation. This has motivated the design of chemical sensors that can be applied in the field where conventional instrumentation may be time-consuming or inaccessible. Molecular, water-soluble fluoride sensors are rare, with most fluoride sensors requiring organic co-solvents. 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), for aqueous fluoride sensing. This species exhibits an intense blue fluorescence upon excitation with near-UV light that is quenched in the presence of NaF (100 mM acetate buffer, pH = 5) under pure aqueous conditions. The equilibrium constant (K_eq=1.26 ±0.00184 ×10^8 M^(-2)) and free energy (?G_rxn^°=-11.0 kcal mol^(-1)) under the conditions used for fluoride sensing were determined, indicating a highly favorable reaction with F- in water. Zr-F bond formation and precipitation of the protonated flavonolate ligand were found to be the key driving forces behind this reaction, inhibiting the reversible binding of the ligand and leading to fluorescence quenching. Finally, we developed paper-based sensing strips by immobilizing the fluoride sensor in a gel for qualitative fluoride sensing in water.