Fluorescence spectroscopic study on complexation of uranium(VI) by glucose - a comparison of room and low temperature measurements

The complexation of uranium(VI) with glucose was studied in the pH range from pH 2 to 6 by time resolved laser-induced fluorescence spectroscopy (TRLFS) at room temperature (RT) and for the first time under cryogenic conditions of 153 K (cryo-TRLFS). A uranyl(VI) glucose complex was spectroscopically identified by cryo-TRLFS measurements at pH 5. At lower pH values only the free uranyl(VI) ion was identified. The study revealed that quenching effects severely influenced the TRLFS measurements conducted at RT and significantly reduced the uranium(VI) fluorescence signal. This decrease in U(VI) fluorescence intensity is usually used to calculate complex formation constants of non-fluorescent uranyl(VI) organic complexes. However, our cryo-TRLFS results clearly showed that the observed decrease in U(VI) fluorescence intensity at RT for the samples at pH 2 to 4 is not attributed to the formation of such a non-fluorescent U(VI) glucose complex, i.e. to static quenching, but related to dynamic quenching of glucose on the uranyl(VI) fluorescence. At higher pH values the formation of uranyl(VI) glucose complexes were suppressed by the formation of uranyl(VI) carbonate species. The detected uranyl(VI) glucose complex was characterized by five emission bands at 499.0, 512.1, 525.2, 541.7, and 559.3 nm. The respective fluorescence lifetime determined at 153 K was 20.9 ± 2.9 µs. The uranyl(VI) glucose complex formation constant was calculated for the first time to be logßI = 0.1 M = 15.35 ± 0.91. Comparing this constant with formation constants of other important environmentally relevant inorganic ligands, in particular carbonate, it became evident that glucose only may influence the transport behaviour of uranium in a very small pH region of about 5. Our cryo-TRLFS investigation opens up new possibilities for the determination of complex formation constants since interfering quenching effects often encounter at RT are suppressed by measurements at cryogenic conditions.