Complexation of europium(III) with the zwitterionic form of amino acids studied with UV/vis and time-resolved laser-induced fluorescence spectroscopy

The complex formation of europium(III) with the zwitterionic form of amino acids (alanine, phenylalanine and threonine) has been studied in aqueous solution. Measurements were performed at I = 0.1 M (NaCl/NaClO4), room temperature and trace metal concentrations in the range of pH 2 to 8 using UV/vis and time-resolved laser-induced fluorescence spectroscopy. While complexation leads to a significant luminescence enhancement in the emission spectrum of the metal ion, absorption in the UV/vis spectrum of the amino acid (AA) decreases. As zwitterionic species (AAH) all three ligands form weak complexes with 1:1 stoichiometry and a general formulae of EuAAH3+ with the metal. The complex stability constants were determined to be log K ~ 1 for all complexes, indicating the negligible contribution of the amino acid side chain to the complex formation reaction.