Determination of U(VI) and U(IV) concentrations in aqueous samples containing strong luminescence quenchers using TRLFS

Novel luminescence (LM) spectroscopy-based uranium (U) analysis methods are developed for aqueous samples containing strong LM quenchers. Reducing agents often found in biological samples, such as thiols, ascorbate and Fe(II) ions, are identified as the major strong quencher species. A strategy to selectively oxidize the reducing moieties of the quencher species using monopersulfate (the selected oxidant) is employed to rapidly reduce the LM quenching effects. Without requiring conventional sample pre-processing (ashing) procedures this method improves the limit of detection of U(VI) (~nM levels) and enables rapid and simultaneous determination of U(VI) and U(IV) dissolved in biological samples containing strong quenchers.     

TRLFS study on the speciation of uranium in seepage water and pore water of heavy metal contaminated soil

In situ leaching of uranium ores with sulfuric acid during active uranium mining activity on the Gessenheap has caused longstanding environmental problems of acid mine drainage and elevated concentrations of uranium. To study there remediation measures the test site Gessenwiese, a recultivated former uranium mining heap near Ronnenburg/East Thuringia/Germany, was installed as a part of a research program of the Friedrich-Schiller University Jena to study, among other techniques, the phytoremediation capacity of native and selected plants towards uranium. In the first step the uranium speciation in surface seepage and soil pore waters from Gessenwiese, ranging in pH from 3.2 to 4.0, were studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Both types of water samples showed mono-exponential luminescence decay, indicating the presence of only one major species. The detected emission bands were found at 477.5, 491.8, 513.0, 537.2, 562.3, and 590.7 nm in case of the surface water samples, and were found at 477.2, 493.2, 513.8, 537.0, 562.4, and 590.0 nm in case of the soil water samples. These characteristic peak maxima together with the observed mono-exponential decay indicated that the uranium speciation in the seepage and soil pore waters is dominated by the uranium (VI) sulfate species UO₂SO_4(aq). Due to the presence of luminescence quenchers in the natural water samples the measured luminescence lifetimes of the UO₂SO_4(aq) species of 1.0–2.6 μs were reduced in comparison to pure uranium sulfate solutions, which show a luminescence lifetime of 4.7 μs. These results convincingly show that in the pH range of 3.2–4.0 TRLFS is a suitable and very useful technique to study the uranium speciation in naturally occurring water samples.     

Correction for quenching in fluorimetric determinations using steady state fluorescence

Quenching corrections in fluorimetric estimations are arrived at using time resolved fluorometry, where the reduction in the lifetimes of the fluorophore in the presence of quenchers is a measure of the correction. A novel procedure for the correction of quenching in fluorimetric determinations using steady state fluorescence spectroscopy is described here. The method is based on the variation in the slope of the fluorescence versus concentration plots, as the quencher concentration is changed. As a test case, the procedure for the determination of uranium has been demonstrated in the presence of a number of metal ion quenchers.     

STUDY OF LUMINESCENT FORMS OF THE URANYL ION

Luminescence spectra and luminescence decay kinetics of uranyl sulphate water and uranyl nitrate acetone solutions of different concentrations have been studied. Similar experiments have been done with uranyl sulphate powder under vacuum. It has been experimentally shown that the hydrolysis of uranyl sulphate in water takes place, and under low salt concentrations (0.1-4.0 times 10^-4 M) a luminescence of a basic form of the photoexcited ion with a tentative structure of UO₂OH⁺* has been observed. The luminescence of the acidic form UO⁺* has been observed under higher salt concentrations (1–4 times 10^-2 M) in water and under any salt concentration in acetone. The acidic form has the characteristic emission spectrum possessing vibrational structure. The luminescence concentrational quenching of both photoexcited uranyl forms and exciplex emission have not been observed. The effect of a number of organic quenchers and molecular oxygen on uranyl luminescence has been studied. There is no luminescence quenching by O₂ up to 2 times 10⁶ Pa (20 atm) pressure. The low effectiveness of energy transfer from the photoexcited uranyl forms has been explained in terms of strong steric screening of 5f-uranium (VI) orbital by oxygen atoms and by external filled up uranium electronic shells.     

KINETICS OF LUMINESCENCE QUENCHING IN MICELLAR ASSEMBLIES INCLUDING EXCHANGE OF PROBE and QUENCHER*

The luminescence decay of excited probes in micelles in the presence of a Poisson distribution of quenchers is considered under conditions where both probe and quencher within the lifetime of the excited probe may exchange between the micelles. Analytical and numerical results show that the luminescence decay can be expressed by a generalized self convolution product of a multiexponential decay function.     

Aggregation in nanobundles and the effect of diverse environments on the solution-phase photochemistry and photophysics of -Re(CO)3L+ (L = 1,10-phenanthroline, 2,2'-bipyridine) pendants bonded to poly(4-vinylpyridine)

The UV-vis spectroscopy and photochemical properties of {(vpy-[Re(CO)3(2,2'-bpy)])m(vpy-[Re(CO)3(phen)])n(vpy)p}(CF3SO3)(m+n)}, vpy = 4-vinylpyridine, m = 131, n = 131 or m = 200, n = 150, and m + n + p = 600, were investigated in solution phase. The polymers exist in solution as aggregates of polymer strands with radii as large as approximately 10(2) nm. Given the size of the poly-vpy backbone, the aggregates must contain a large number of strands. The luminescence spectrum exhibits a strong resemblance to the emission spectrum of {(vpy-[Re(CO)3(phen)])200(vpy)400}(CF3SO3)200. The existence of Re(I) chromophores in diverse environments was shown by the intrinsic kinetics of the luminescence, the decay kinetics of the MLCT excited states observed by time resolved-absorption spectroscopy, and the quenching of the luminescence by various quenchers. Redox reactions of the MLCT excited states with the quenchers were responsible for the luminescence quenching. While static quenching resulted when Cu(II) and Fe(III) EDTA complexes were the quenchers, a dynamic quenching resulted with Fe(CN)6(4-) or 2,2',2' '-triethanolamine, TEOA. The photochemical and photophysical properties of the mixed-pendant polymers have been discussed in terms of arrays of MLCT excited states whose energies are determined by the diverse environments of the Re(I) chromophores. Conversions (with and without radiation) of the upper-energy MLCT excited states to the ground state and lower-energy MLCT excited states and the latter excited state to the ground state account for the experimental results.     

Enhancement of the luminescence of europium(III) β-diketonates in the ligand series β-diketones-their unsaturated analogues-copolymers

A change in the luminescence intensity of europium compounds was studied for the series of ligands β-diketones-their unsaturated analogues (monomers)-copolymers with styrene (methyl methacrylate). The optimal ratios of comonomers (1 : 2; 1 : 5) providing the maximum luminescence intensity of europium were determined. Europium (n x 10^-3%) was determined in Ce, Pr, Nd, and Ho oxides, which are strong luminescence quenchers, using copolymers containing trifluoroacetylacetone and benzoylacetone.     

Photosensitized luminescence of singlet oxygen in solutions at 1588 nm

Luminescence of singlet oxygen has been observed at 1588 nm in solutions. Singlet oxygen was photosensitized by photoexcitation of some porphyrins in different solvents. The luminescence lifetimes and the quenching rate constant measurements with different quenchers support the suggestion that the detected luminescence is a result of the electronvibrational transition

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Specific features of luminescence of Q-CdS colloids with different sizes

The specific features of luminescence of colloidal solutions of Q-CdS with particles of different size and the regularities of luminescence quenching by quenchers of various nature were studied. The luminescence spectra of Q-CdS consist of several bands, which are shifted to the long-ware region as the particle size increases. The dependence of the integral quantum yield of luminescence on the particle size has a sharp maximum at a particle diameter of ?23Å. A Stem—Volmer-type equation including the adsorption isotherm of the quencher molecules on the surface of the Q-CdS colloidal particles was used to describe the regularities of luminescence quenching of Q-CdS colloidal solutions. The CdS particle size was found to affect the efficiency of luminescence quenching. The regularities of luminescence quenching depend both on the rate constant of electron transfer to the quencher molecules and on the ability of the quencher molecules to be adsorbed on the surface of the CdS colloidal particle.     

Correction of quenching errors in analytical fluorimetry through use of time resolution

Varying concentrations of quenching agents can cause serious errors in analytical fluorimetry. The origin of these errors is an unexpected change in the quantum efficiency for the observed luminescence. It is known that quantum efficiency can be expressed as the ratio of an observed luminescence decay time to the decay time which would be observed in the absence of quenchers. Because this latter quantity is a constant for any particular fluorophore, quantum efficiency variations can be compensated through measurement of the decay time. For such measurements, the time-correlated single photon technique was employed and measured luminescence values were taken both from averaged photon count rates and from integrated fluorescence decay plots. Division of these values by measured luminescence lifetimes produced values which were independent of quencher concentration. Systems studied were quinine bisulfate quenched with chloride ion and 1-pyrenebutyric acid quenched by iodide.     

Effect of Condensed Media on the Luminescent Characteristics of Lanthanide Complexes

The effect condensed media based on water-soluble polymers (WSPs) have on the luminescence of Eu³⁺ and Tb³⁺ complexes with derivatives of benzoic acid and β-diketones is investigated. The effect of WSPs is found to depend on the ratio between the triplet energy levels of polymers and ligands. The most intense luminescence is observed in WSP films. The possibility of a proportional increase in the intensity of Ln³⁺ luminescence in WSP solutions over a wider range of concentrations than in an aqueous solution, and of a considerable increase in Ln³⁺ luminescence in WSP solutions and films in the presence of d- and f-element quenchers, is established.     

Diffusion-controlled luminescence quenching in metal-organic frameworks

Diffusion-controlled luminescence quenching of a phosphorescent metal-organic framework built from the Ru(bpy)(3)(2+)-derived bridging ligand (MOF-1) was studied using a series of amines of different sizes as quenchers. The dynamics of amine diffusion into solvent-filled MOF-1 channels was probed by modeling time-dependent luminescence quenching data, which provide quantitative diffusion coefficients for the amine quenchers. Triethylamine, tripropylamine, and tributylamine were found to follow Fickian diffusion with a diffusivity of (1.1 ± 0.2) × 10(-13), (4.8 ± 1.2) × 10(-14), and (4.0 ± 0.4) × 10(-14) m(2)/s, respectively. Diisopropylethylamine (DIPEA), on the other hand, was found to be too large to enter the MOF channels. Despite its size, 4-MeOPhNPh(2) can enter the MOF channels via a slow, complicated framework/guest intercalation process to result in extensive framework distortion as revealed by powder X-ray diffraction. This work represents the first quantitative study of the dynamics of molecular diffusion into solvent-filled MOF channels. Such quantitative information on molecular diffusion in MOFs is of fundamental importance to many of their potential applications (e.g., heterogeneous catalysis).     

The luminescence of uranime-activated yttrium tungstate (Y2WO6)

The luminescence of uranium in Y₂WO₆ can be excited with long wavelength ultraviolet radiation at temperatures below room temperature. The emission consists of a fairly narrow band in the orange part of the spectrum. Its characteristics are comparable with the luminescence of uranium in MgWO₄ and the scheelites MWO₄ (M = Ca, Sr, Ba). It is possible to relate the thermal quenching temperature of the luminescence to spectral data. Energy transfer from the tungstate group to the uranate group is observed, but its efficiency is not very high compared with other systems.     

Novel fusion method for direct determination of uranium in ilmenite,rutile, columbite,tantalite, and xenotime minerals by laser induced fluorimetry

A simple, rapid, effective and eco-friendly decomposition method is developed for the determination of uranium (U) by laser induced fluorimetry (LIF). The salts of sodium di-hydrogen phosphate (NaH₂PO₄) and di-sodium hydrogen phosphate (Na₂HPO₄) were used in the ratio of 1:1 (phosphate flux) for the decomposition and dissolution of refractory, non silicate minerals like ilmenite, rutile, columbite, tantalite, and xenotime. The effect of associated matrix elements (Ti, Fe, Nb, Ta, Mn and Y present in the sample) on quenching of uranyl fluorescence was studied. The flux used for the sample decomposition has several advantages. In the reported sample decomposition methods, α-hydroxy acids are used as complexing agents to prevent hydrolysis and to get clear and stable solution. This solution can not be directly used for U determination by LIF as α-hydroxy acids quench uranyl fluorescence, hence separation is required. In the present method no such separation is required. The flux itself acts as fluorescence enhancing reagent and buffer (maintaining the optimum pH of 7.1 ± 0.1). The fused melt of the flux mixture, when disintegrated in water, gives clear and stable solution and has high tolerance for most of inorganic quenchers compared to reported phosphate buffers. Also just by dilution (due to high sensitivity of LIF), the concentration of quenchers could be brought down well within the tolerance limit. The accuracy and precision of the method was evaluated by analyzing Certified Reference Materials (IGS-33 and IGS-34 of Institute of Geological Sciences, UK) and Synthetic Minerals. The accuracy of the data is further evaluated by comparing with standard decomposition methods. The results are well within the experimental error. The RSD of the method is ±10% (n = 6) at 10 ppm level for Ilmenite and for other minerals the RSD of the method is ±5% (n = 6) at 50 ppm level. The method is being routinely applied to various refractory samples received from Rare Metal and Rare Earth Investigations for determination of uranium by laser fluorimetry.     

Conjugated chromophore near the quantum-confined cadmium sulfide cluster: quenched photoluminescence and enhanced two-photon absorption

Quenching effect of the photoluminescence of 1,2,4,5-tetrakis(4-pyridylvinyl)benzene in the presence of CdS colloids stabilized by inverse micelles was observed. The observed regularities of luminescence quenching by the quenchers of different size were studied. An increase in the two-photon absorption cross section (6.5 times higher) and in the two-photon-induced fluorescence intensity was observed for the composite solution when pumped by 740-nm laser irradiation. The results are in accord with theoretical prediction of enhancement of third-order optical nonlinearity of quantum-confined semiconductor.     

A titrimetric method for the sequential determination of thorium and uranium

A method for the sequential determination of thorium and uranium has been developed. In the sample solution containing thorium and uranium, thorium is first determined by complexometric titration with ethylenediaminetetraacetic acid (EDTA) and then in the same solution uranium is determined by redox titration employing potentiometry. As EDTA interferes in uranium determination giving positive bias, it is destroyed by fuming with HClO₄ prior to the determination of uranium. A precision and accuracy of better than ±0.15% is obtained for thorium at 10mg level and uranium ranging from 5 mg to 20 mg in the aliquot.     

Synthesis of asymmetrically substituted cyclen-based ligands for the controlled sensitisation of lanthanides

A series of unsymmetrical cyclen-based ligands incorporating an antenna and a quencher have been prepared for the complexation of the visible- (Eu, Tb) and near IR-emitting (Nd, Yb) lanthanides. Eu and Tb were sensitised with coumarin 2, and Nd and Yb with rhodamine. Both antennae were paired with nucleoside (uridine and adenosine) quenchers. The interaction between the quencher and the antenna can be regulated by the addition of the complementary base or DNA to the complexes, resulting in changes in the lanthanide luminescence intensity and lifetime.     

Europium tetracycline as a luminescent probe for nucleoside phosphates and its application to the determination of kinase activity

The determination of enzyme activities and the screening of enzyme regulators is a major task in clinical chemistry and drug development. A broad variety of enzymatic reactions is associated with the consumption of adenosine triphosphate (ATP), including, in particular, phosphorylation reactions catalyzed by kinases, formation of adenosine cyclic monophosphate (cAMP) by adenylate cyclases, and ATP decomposition by ATPase. We have studied the effect of a series of adenosine (ATP, ADP, AMP, cAMP) and guanosine (GTP, GDP) phosphoric esters, and of pyrophosphate (PP) on the fluorescence emission of the europium tetracycline (EuTC) complex. We found that these compounds have strongly different quenching effects on the luminescence emission of EuTC. The triphosphates ATP and GTP behave as strong quenchers in reducing the fluorescence intensity of EuTC to 25 % of its initial value by formation of a ternary 1:1:1 complex. All other phosphate esters showed a weak quenching effect only. The applicability of this fluorescent probe to the determination of the activity of phosphorylation enzymes is demonstrated by means of creatine kinase as a model for non-membrane-bound kinases. In contrast to other methods, this approach does not require the use of radioactively labeled ATP substrates, additional enzymes, or of rather complex immunoassays.     

Spectrophotometric method for the direct determination of uranium in carbonate solution

Summary A spectrophotometric method for direct determination of uranium in carbonate solution has been developed. No extraction or decomposition steps are necessary. The highly sensitive (4-2-pyridylazo) resorcinol, as well as 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol, producing very stable uranium(VI) complexes, were used for its spectrophotometric determination.The usefulness of the proposed methods was examined by determination of uranium in real samples from a wet-process phosphoric acid estimation of uranium.     

Laser techniques in the study of drug photochemistry

This review describes the application of different laser time-resolved techniques, such as time-resolved fluorescence, phosphorescence and laser flash photolysis, to elucidate the mechanism of drug photodegradation. The assignment of transient species based on their luminescence or absorption spectra and based on their reactivity to various quenchers is illustrated. The mathematical expressions used in fitting measured transient decays over time are also discussed. Practical situations found in the literature and relevant to drug photodegradation illustrate different examples. The importance of laser power, concentration and absorbance of the ground state, as well as other parameters intrinsic to laser techniques are discussed.