Investigation of Eu(III) Binding Sites on Datura Innoxia Using Eu(III) Luminescence

Abstract

A pulsed tunable dye laser has been used to obtain excitation spectra and fluorescence decay curves of solid Eu(III)-Datura innoxia and from a series of Eu(III)-containing complexes. Carboxyl and sulfate groups have been demonstrated to be the dominant functional groups for forming binding sites on the cell wall of Datura innoxia at high (≥4) and low (≤3) pH conditions, respectively. The excitation spectra associated with the ⁷F₀ → ⁵D₀ electronic transition of Eu(III) luminescence have been used to provide a measure of the electronic structure factors contributing to the interaction between Eu(III) ions and the binding sites on the cell wall of D. innoxia. The noticeably broadened and asymmetric excitation spectra obtained at high pH conditions are ascribed to multiple binding sites. The corresponding lifetime decay curves exhibited a bi-exponential decay. A pK of 4.5 was determined for the binding of Eu(III) to the cell wall at pH ≥4. Kinetic and thermodynamic studies were also undertaken.