Uranyl fluorescence lifetime in nanoporous silica gel: The influence of pore size, pH, and water

Abstract

The fluorescence lifetime of uranyl adsorbed within nanoporous silica gel was measured as a function of pore size at two different pH values and both wet and dry. It was determined that for uranyl adsorbed within pores larger than 4 nm, the lifetime is relatively independent of pore size, whereas below 4 nm, the lifetime increases with decreasing pore size. A blue shift in the emission spectra was observed at the smallest pore size (2.2 nm) and is believed to be caused by quantum confinement. The lifetime was found to be longer at a neutral pH than in an acidic pH, and this is caused by the formation of a uranyl hydroxyl complex at higher pH values. The presence of water within the pores is found to increase the fluorescence lifetime at all pore sizes and pH values studied in this paper; this is caused by the formation of a uranyl silicate bond in the absence of water. An understanding of the parameters that influence the fluorescence lifetime of uranyl within silica gel is important for the development of more sensitive detection methods.