A ratiometric fluorescence halide sensor based on covalently immobilization of quinine and benzothioxanthene

A new fluorescence chemical sensor for the determination of Cl−, Br− and I− has been successfully demonstrated. The membrane contains two fluorophores that display two largely different response spectra. The first fluorophore, N-(2-methacryloxyethyl) benzo[k,l]thioxanthene-3,4-dicarboximide (MBTD, benzothioxanthene derivative), used as reference was halide-insensitive. The second fluorophore, quinine sulphate, used as sensitive carrier was halide-quenchable. Both were co-polymerized with acrylamide, hydroxyethyl methacrylate and triethylene glycol dimethacrylate onto glass surface. The fluorescence intensity of quinine decreased with increasing of halide concentration due to dynamic quenching, but the fluorescence intensity of MBTD was almost not affected by halide. Under these conditions, the halide-dependent fluorescence intensity of quinine was converted into a ratio of the two fluorescence intensities, which were expressed as a function of halide concentration. Satisfactory reproducibility, reversibility and a short response time were realized. The sensor also showed good selectivity, common ions and some organic species did not interfere with the measurement of halide. With the optimum membrane described, detection limits were of 2.2 × 10−4, 7.5 × 10−5 and 6.2 × 10−5 mol L−1 for Cl−, Br− and I−, respectively. The proposed method was applied to the determination of halide in earth samples. The sensing membrane was found to have a lifetime at least 2 months.