Luminescence spectroscopy characterization of uranium(VI) at low temperature

Cryogenic techniques in laser spectroscopy are currently used in scanning tunnelling microscopy (STM) and single molecule spectroscopy. Recently such cryogenic devices have also been adapted to time resolved laser-induced fluorescence spectroscopy (TRLFS) systems applied to uranium. We used TRLFS at low temperatures (cryo-TRLFS) as an experimental technique to investigate the uranium(VI) complexation with a variety of organic ligands (glucose, citric acid, oxalic acid). For the first time, luminescence properties were determined for a uranium(VI) glucose complex [1], uranium(VI) citrate [2] and uranium(VI) oxalate [2] species by cryo-TRLFS at 153 K. The emission signals are bathochromic shifted in comparison to the emission maxima of the uncomplexed uranyl(VI) cation. Using the spectroscopic data, the corresponding complex formation constants were calculated. This is a great improvement towards TRLFS measurements at room temperature, where emission signals of the complex species are not detectable. Our TRLFS investigation at low temperature opens up new possibilities for the determination of complex formation constants since interfering quenching effects often encountered at room temperature condition are suppressed. In addition we used cryo-TRLFS technique for studying uranium(VI) speciation in natural samples. The uranium(VI) speciation in urine and in mineral water [3] could be thus investigated. Samples with an uranium concentration of < 0.1 μg/L could be analyzed unproblematically. The complexation of uranium(VI) with carbonate in commercial mineral waters could be verified. In urine, it was found that the uranium(VI) speciation strongly depends on the pH, with the latter varied between 5 and 7 as observed in human samples. Up to pH value of 6 the uranium(VI) speciation is defined mostly by organic ligands. At higher pH the uranium(VI) speciation should be dominated by inorganic phosphate complexes or carbonate complexes. The fluorescence spectroscopic analysis showed a mixture of complexation by citrate and phosphate at lower pH (< 6) and at higher pH (> 6) an dominance of complexation by carbonate. This results are in a very good correspondence with model calculation of the uranium(VI) speciation in urine.