A wavelength–ratiometric fluoride-sensitive probe based on the quinolinium nucleus and boronic acid moiety

Abstract

We report a new water-soluble fluorescent probe sensitive to aqueous fluoride. The probe shows spectral shifts and intensity changes in the presence of fluoride, in a wavelength ratiometric and colorimetric manner, enabling the detection of fluoride concentrations at visible wavelengths, in the concentration range ≈1–300 mM. The probes response is based on the ability of the boronic acid group to interact with fluoride, changing from the neutral form of the boronic acid group [R–B(OH) 2] to the anionic trifluoro form [R–B–F 3], which is an electron donating group. The presence of an electron-deficient quaternary heterocyclic nitrogen center and a strong electron donating amino group in the 6-position, on the quinolinium backbone, provides for the spectral responses observed upon fluoride binding to the boronic acid group. Interestingly, the presence of the amino group on the 6-position of the quinolinium backbone suppresses the response of the boronic acid containing fluorophore towards monosaccharides, such as glucose and fructose, which are present in biological fluids and food stuffs, potentially allowing for the predominant fluoride sensitivity. In addition to physiological sugars, we discuss the response of aqueous halides as potential interferents, or indeed analytes to be sensed, and show that the new probe responds well to aqueous fluoride in the presence of a high background of other species, such as in a biological cocktail of 50 mM glucose, 50 mM aqueous Cl − and 5 mM fructose.