Solution Chemistry of Europium(III) Aqua Ion at Micromolar Concentrations as Probed by Direct Excitation Luminescence Spectroscopy

Abstract

Dedicated to Jean-Claude Bünzli on the occasion of his 65th birthday.Direct excitation europium(III) luminescence spectroscopy is used to study the speciation of aqueous europium(III) ions at micromolar concentrations and near neutral pH. The pH and concentration dependence of the europium(III) 7 F 0 ! 5 D 0 excitation peak is consistent with the formation of both mononuclear and dinuclear europium(III) hydroxide complexes at pH 6.5. Luminescence intensity and lifetime quenching studies in the presence of Nd III at pH 5.0 and 6.5 support the formation of a dinuclear complex at pH 6.5. Steady state excitation and time-resolved luminescence spectroscopy are consistent with the formation of innersphere nitrate and fluoride complexes, but outersphere perchlorate and chloride complexes at pH 6.5 and 5.0.Introduction. -The aqueous chemistry of trivalent lanthanide ions (Ln III ) has been studied over several decades, dating from the pioneering studies compiled by Baes and Mesmer [1] and extending to more recent studies using lanthanide luminescence and Xray scattering techniques [2 -6]. These studies show that lanthanide ions undergo H 2 O ligand hydrolysis and aggregation to form mononuclear and multinuclear hydroxide complexes. Other aspects of solution chemistry of interest include the interaction of various simple anions of supporting electrolytes with lanthanide ions. Luminescence and UV/VIS absorbance spectroscopy studies show that most simple anions form outersphere complexes with Ln III ions even at molar concentrations of the anion [7 -10]. However, all of these studies use high concentrations of lanthanide ions ranging from molar to millimolar. In addition, most of these studies using photoluminescence spectroscopy do not control pH. Our interest in complexation of micromolar concentrations of lanthanide ions to biopolymers in aqueous conditions has led us to reexamine Eu III aqueous solution chemistry by using a new MOPO (master oscillator power oscillator)/laser system, for direct excitation lanthanide photoluminescence [11]. The instrument is tunable over a broad wavelength range and facilitates direct excitation luminescence studies on Eu III complexes at as low as nanomolar concentrations in aqueous solutions.Previous studies have focused on time-resolved luminescence spectroscopy to obtain information about the coordination sphere of Eu III [7] [9] [10] [12]. Such work relies on the quenching of excited state Eu III by OH oscillators of both inner and outersphere H 2 O molecules. The recovered luminescence lifetimes in these experiments are not always easily interpreted, in part because outersphere and innersphere ligand quenching parameters are not well-defined for highly hydrated lanthanides.

Luminescence Resonance Energy Transfer in Heterodinuclear LnIII Complexes for Sensing Biologically Relevant Anions

“…Such outersphere complexes of Eu III complexes have been shown to form with other monoanions such as phosphate diesters through hydrogen bonding to pendent groups and bound waters. [68][69][70] Formation of an outersphere complex does not perturb the excitation spectrum or change the q number.…”

“…[11,72] Fluoride displaces a single water molecule on each Eu III center to form a 2:1 complex. Fluoride has been shown to bind strongly to the Eu III aqua ion [68] as the only halide that forms innersphere complexes. Speciation diagrams are shown in Figure S2.…”

Solid-phase luminescence determination of tetracycline in bottled water using chemically modified silica

“…It is known [15] that, in the solution, the luminescence intensity of europium(III) compounds considerably increases in the formation of mixed-ligand complexes in the presence of donor-active organic ligands, such as hydroxycarbonic acids [15], cationic and nonionic surfactants [15,16,26], hydrogen peroxide [15], and also some inorganic salts [38]. The luminescence intensity of the Eu-TC system increased in presence citrate ions (Fig.…”

Circularly Polarized Luminescence of Curium: A New Characterization of the 5f Actinide Complexes

“…This red shift of 8–10 nm is much larger than those observed for Ln(III) complexes and free cations (0–3 nm). 34, 35 For comparison, aqueous Cm(III) has reported emission maxima at 593.8 nm. 1, 32, 36 The nephelauxetic effect seen in the luminescence spectra of a range has been previously explained (see Table 4 of reference 1).…”