Highly selective luminescence determination of terbium at the sub-ppb level with sulfonylcalix[4]arene-p-tetrasulfonate

The energy-transfer luminescence of complexes of Tb³⁺ with calix- (CAS), thiacalix- (TCAS), and sulfonylcalix[4]arene-p-tetrasulfonates (SO₂CAS), in which four p-phenolsulfonates are jointed by -CH₂-, -S-, and -SO₂-, respectively, was applied to an ultratrace determination of the Tb³⁺ ion. Based on the complexation behavior of each calix ligand, a determination procedure was established. A time-resolved measurement was conveniently employed to separate the background fluorescence from the luminescence of the calix-Tb³⁺ complexes. An improved sensitivity was attained by sulfur-bridged calixes, TCAS and SO₂CAS, as compared to CAS, owing to the photophysical properties of the Tb³⁺-complexes. The detection limits for Tb³⁺ ion by CAS, TCAS, and SO₂CAS ligands were estimated to be 8.2 × 10^-10 mol dm^-3 (131 ppt), 2.0 × 10^-10 mol dm^-3 (32 ppt), and 2.3 × 10^-10 mol dm^-3 (37 ppt) at S/N = 3, respectively. In terms of the selectivity, the effect of diverse coexisting ions on the luminescence intensity of the Tb³⁺ complexes was studied. The luminescence of the TCAS complex was interfered by the presence of a 5-fold amount of other lanthanide ions, while that of the SO₂CAS complex was more tolerant for those metal ions, allowing the presence of as much as 50 to 100-fold amounts. Also, 10 to 1500-fold amounts of other common ions were tolerated by using SO₂CAS. Thus, SO₂CAS was proved to be a practical reagent for determining the Tb³⁺ ion at the sub-ppb level.