Halide sensing using the SPQ molecule

The fluorescent and halide sensitive 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) molecule has been synthesised and incorporated into a hydrophilic copolymer to produce a halide sensor film. The sensor film typically swells in aqueous media allowing dye fluorescence to be quenched by the diffusion of aqueous halide ions at high pH. The steady-state Stern–Volmer constants, KSV, for the sensor film towards chloride, bromide and iodide (pH 10) are 24, 65 and 123 mol−1 dm3, less than half the pH 10 solution values of 79, 144 and 236 mol−1 dm3, respectively. The reduction in KSV values of the immobilised dye is discussed and are compared to KSV values for SPQ immobilised in a hydrophobic copolymer. Fluorescence lifetime data, obtained using the single photon counting technique, of SPQ sensor films is also presented. The sensor films are reversibly capable of determining aqueous iodide, bromide and chloride with ≈1.5, 2.5 and 5% accuracy, respectively, at concentrations around 0.001 mol dm3, where chloride concentrations in blood, sweat and some photographic processes are typically in the range 0.1–0.001 mol dm3. The hydrophilic sensor films 90% response time to molar halide solution was typically 30 s. The sensor films were found to have a shelf life in excess of 2 years.