Cyclodextrin-, UV-, and high pH-induced fluorescence enhancement of the pesticide azinphos-methyl: Applications to its trace analysis

Abstract

Various methods for the enhancement of the fluorescence of the pesticide azinphos-methyl are reported. The addition of hydroxypropyl-β-cyclodextrin was found to moderately enhance the fluorescence by a factor of three, presumably via inclusion of the pesticide into the cyclodextrin cavity. This complexation was found to occur with an association constant of 690 ± 140 M–1. This enhancement was found to be too low to be useful in a fluorescence-based analytical method for this pesticide. Exposure of azinphos-methyl to UV light was also found to result in enhanced fluorescence, with much larger enhancement than in the case of addition of cyclodextrins. This enhancement was presumed to occur via photolysis of azinphos-methyl to a more fluorescent photoproduct. However, the difficulty of providing a constant UV dosage prevents its useful application as a fluorescence-based analytical method. Finally, it was found that base hydrolysis of azinphos-methyl resulted in a very large fluorescence enhancement. Although this enhancement was previously reported in the literature, it was found that contrary to the published reports, the hydrolysis occurred rapidly at room temperature, with no need for carrying out the reaction at increased temperature. Fluorescence enhancements of a factor of 300 were obtained by simply adding the appropriate amount of an aqueous NaOH solution to the aqueous azinphos-methyl sample. This procedure was used as the basis of a simple fluorescence-based analytical method for azinphos-methyl in water, with excellent linear calibration curves obtained down to concentrations of 5 ppb.Key words: fluorescence spectroscopy, pesticides, azinphos-methyl, fluorescence enhancement, host–guest inclusion.