Detection of uranyl ion via fluorescence quenching and photochemical oxidation of calcein

This paper reports the detection of uranyl ion via fluorescence quenching of a dye molecule followed by the selective photocatalysis of the dye molecule by excited state uranyl. We have found that selectivity can be obtained when calcein, a highly fluorescent fluorescein-type dye, is quenched by complex formation with uranyl in solution at low concentrations. Following the quenching by uranyl, the entire sample is excited at a wavelength that is strongly absorbed by uranyl at 425 nm. This produces the excited state uranyl ion that has a large oxidation potential and photocatalytically decarboxylates the dye, breaking the dye/metal bond. The photocatalysis product is highly fluorescent and produces an increase in the fluorescence signal. The detection limit for uranyl via quenching is 60 nM and via photocatalysis is 40 nM. Interfering metal ions such as iron and chromate have little effect on the amount of fluorescence regenerated since their absorbance bands, modes of quenching, and photochemistry are different from uranyl.