General Method for the Determination of Stability Constants of Lanthanide Ion Chelates by Ligand−Ligand Competition: Laser-Excited Eu3+ Luminescence Excitation Spectroscopy

Abstract

A highly sensitive, convenient, direct spectroscopic method for the measurement of stability constants of Eu3+ ion complexes of multidentate ligands is described. Eu3+ ion complexation is monitored by means of 7F0 → 5D0 excitation spectroscopy. The method involves the competition against one another, of two ligands, one (L‘) of known and one (L) of unknown stability constant for its Eu3+ complex. With laser excitation at a particular wavelength (in the 578−581 nm range), the excitation intensity of EuL and/or EuL‘ is measured by either a time-resolved method, if the excited state lifetimes of EuL and EuL‘ are sufficiently different, or in a non-time-resolved manner, if the emission intensities (λem = 614 nm) of EuL and EuL‘ are significantly different at the exciting wavelength. These data lead to the determination of relative conditional stability constants (Krel = [EuL‘][L]t/[EuL][L‘]t) of the two ligands which, with knowledge of the protonation constants of the ligands, can be used to calculate the thermodynamic formation constant of the fully deprotonated ligand, L. Using ethylenediaminetetraacetic acid (edta) as the reference ligand, formation constants for the 1:1 complexes of Eu3+ with N-(2-hydroxyethyl)ethylenediaminetriacetic acid (hedta), diethylenetriaminepentaacetic acid (dtpa), and 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (do3a) were determined, all in good to excellent agreement with their literature values. The method was also applied to the Eu3+−dtpa-dien [1,4,7-tris(carboxymethyl)-9,17-dioxo-1,4,7,10,13,16-hexaazacyclooctadecane] system, where the species EuH(dtpa-dien)+, Eu(dtpa-dien), and Eu(OH)(dtpa-dien)¯ exist in different pH regions. The respective formation constants are log KEuL = 17.2 ± 0.1, log KEuHL = 14.2 ± 0.2, and, for the reaction EuL + OH- = Eu(OH)L-, log KOH = 5.7 ± 0.2 (pKa = 8.3 ± 0.2). Advantages of the present method include high sensitivity (1−10 μM concentrations of Eu3+ and ligand), small sample volumes (1 mL or less), and the ability to detect directly and characterize the species present in solution under particular pH conditions using excitation spectroscopy and lifetime measurements.