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The competition between REE, alkaline earth and d-transition metals for organic matter binding sites is still an open field of research; particularly, the mechanisms governing these phenomena need to be characterized in more detail. In this study, we examine spectroscopically the mechanisms of competitive binding of Eu(III)/Cu(II) and Eu(III)/ Ca(II) pair to Gorleben humic acid (HA), as previously proposed in the framework of the NICA-Donnan model. The evolution of time-resolved laser induced luminescence spectra of humic-complexed Eu(Ill) showed two strikingly different environments for a comparable bound proportion for Cu(II) and Ca(II). Cu(II) seems to compete more effectively with Eu(III) inducing its release into the Donnan phase, and into the bulk solution as free Eu3+. This is evidenced both by the shapes of the spectra and by the decrease in the luminescence decay times. In contrast with that, Ca(II) induces a modification of the HA structure, which enhances the luminescence of humic-bound Eu(III), and causes a minor modification of the chemical environment of the complexed rare earth ion.
Europium ions were introduced in SiO2 and MCM-41 via two different pathways: (1) grafting the europium complexes with two alkoxide structures, 3-(2-imidazolin-1-yl)-propyl-triethoxysilane (IPTES) and aminopropyltrimethoxysilane (APTMS), and (2) functionalization of the SiO2 support with silicon 4- carboxylbutyltriethoxide followed by subsequent addition of the europium ions. The new materials were characterized using nitrogen adsorption isotherms at -196 degrees C, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, Fourier transform infrared, NMR, DR-UV-vis, steady-state emission and excitation, and time-resolved photoluminescence spectroscopy. Spectral changes found in the time-resolved photoluminscence spectra strongly point to the distribution of europium ions on a range of environments in both SiO2 and MCM-41 supports. The average europium photoluminescence lifetimes decrease within the order: Eu3+-IPTES/SiO2 (550 mu s) > Eu3+-APTMS/SiO2 (425 mu s) > Eu3+-APTMS/MCM-41 (370 mu s) > Eu3+-IPTES/MCM-41 (320 mu s) > Eu3+-CABES/SiO2 (240 mu s). The photoluminescence quantum efficiency has the largest value, of 22%, for Eu3+-IPTES/SiO2, while the most reduced value, of 9%, was measured for Eu3+-CABES/SiO2.
Species-related luminescence-structure relationships in europium-exchanged mesoporous material
(2009)
Europium exchanged into a mesoporous material (Zeotile-1) was extensively characterized with respect to the Si/ Al ratio and surface silylation by using time-resolved emission spectroscopy. Qualitative as well as quantitative details of the europium species-related luminescence-structure relationships were obtained and discussed such as the decay associated spectra, local distortion and structure of the bonding environment, crystal-field strength, radiative relaxation rates, and the quantum efficiency. Thus, two europium species were found in the parent as well as in the silylated materials: one species located on the internal surface and the second inside the 2-2.5 nm pores. The species located on the internal surface is characterized by photoluminescence decay times of 105 mu s <tau < 125 mu s, an asymmetry value R of 0.6 < R < 0.8, and a quantum efficiency of 2%-2.5%. Upon silylation, the photoluminescence decay times, the asymmetry values, and the quantum efficiency were increased to 160 mu s <tau < 180 mu s, 1.7 < R < 2.1, and similar to 4%, respectively. Following silylation, the number of water molecules is reduced in the first coordination shell of the europium species located on the internal surface from eight to nine to about five. On the other hand, the europium species located inside the pores showed a much longer photoluminescence decay time (460 mu s <tau < 560 mu s) and a much higher asymmetry ratio (5 < R < 6.5). The related photoluminescence efficiency was 26%-30%. An average of one up to two water molecules in the first coordination shell of the europium species located inside the pores was estimated for both parent and silylated materials.
Terbium-exchanged ZSM-5, MOR and (H)BEA zeolites were silylated with phenyl-, vinyl- and hexadecyl trimethoxysilanes via a post-synthesis grafting. All samples were investigated by means of PXRD, FT-IR, TGA, physical adsorption, DR-UV-Vis and time-resolved photoluminescence spectroscopy. From the comparison of the photoluminescence decays of terbium-exchanged in parent (non-silylated) and silylated zeolites, it resulted that the silylation efficiency of the various alkoxysilanes is determined by the type of zeolite and follows the sequences: phenyl > vinyl > hexadecyl > parent for ZSM-5, hexadecyl a parts per thousand phenyl a parts per thousand vinyl > parent for MOR and hexadecyl > phenyl a parts per thousand vinyl > a parts per thousand parent for BEA zeolites, respectively.
Eu(III) luminescence spectroscopy, both in the steady-state and the time-resolved mode, is an appropriate technique to study the properties of complexes between heavy metal ions and humic substances (HS), which play a key role in the distribution of metal species in the environment. Unfortunately, room temperature luminescence spectra of Eu(III) complexes with aromatic and aliphatic carboxylic acids - model compounds of HS binding sites - are too broad to fully exploit their potential analytical information content. It is shown that under cryogenic conditions fluorescence-line- narrowing (FLN) is achieved, and the highly resolved spectra provide detailed information on the complexes. Ten model ligands were investigated. Total luminescence spectra (TLS) were recorded, using the D-5(0) -> F-7(0) transition for excitation and the D-5(0) -> F-7(1) and D-5(0) -> F-7(2) transitions for emission. The energy of the excitation transition depends on the ligand involved and the structure and composition of the complex. For most ligands, discontinuities in the high-resolution TLS indicated that more species, i.e. distinct complex structures, coexisted in the sample. Selective excitation was performed to measure the species-associated luminescence decay times tau. The latter strongly depend on nearby OH oscillators from coordinating water molecules or ligand hydroxyl groups. Furthermore, the asymmetry ratios r, defined as the intensity ratio of the D-5(0) -> F-7(2) and D-5(0) -> F-7(1) transitions, were calculated and the variation of the excitation energy E-exc with the splitting of the F-7(1) triplet (Delta E) was determined. which yielded the crystal field strength parameter N-nu(B-2q), as well as the crystal field parameters B-20 and B-22. An in-depth analysis of the results is presented, providing detailed information on the number of coexisting complexes, their stoichiometry, the number of water molecules in the first coordination sphere and their geometry (symmetry point group).
Cyanine dyes have become widely used fluorescence labels in clinical and biological chemistry. In particular, cyanine dyes with excitation wavelengths lambda(ex) > 600 nm are often used in biological applications. However, aggregation behavior and matrix effects on cyanine fluorescence are not fully understood yet and interfere with the data interpretation. In this study, we analyzed the spectroscopic characteristics of a model system consisting of the biotinylated cyanine dyes DY-635 and DY-647 and their streptavidin conjugates. On the basis of the spectroscopic data, the interaction processes between cyanine dye molecules and proteins are discussed. Binding to streptavidin had a significant influence on both fluorescence and anisotropy decays of the cyanine dyes investigated. In particular, the fluorescence anisotropy was significantly altered, making it a promising detection parameter for bioanalytical applications in connection with the cyanine dyes used in the present study. In order to evaluate the time-resolved anisotropy, the introduction of a sophisticated kinetic model was required to describe the contributions from different fluorescing species properly. The rotational motion of streptavidin-bound dyes was analyzed using the associated anisotropy model, which allowed discrimination between contributions from different microenvironments. The anisotropy decay times increased by a factor of up to 20 due to protein binding.
Deuteration effects on the vibronic structure of the emission and excitation spectra of triangular [ 4] phenylene (D-3h [4]phenylene) were studied using laser-excited Shpol'skii spectroscopy (LESS) in an octane matrix at 4.2 K. For correct assignment of the vibrational modes, the experimental results were compared with calculated frequencies (B3LYP/6-31G*). CH vibrations were identified by their characteristic isotopic shifts in the spectra of deuterated triangular [4]phenylenes. Two CC stretching modes, at 100 cm(-1) and 1176 cm(-1), suitable as probes for bond strength changes in the excited state, were identified. The isotope effect on the internal conversion rates of triangular [4] phenylene was evaluated from measurements of temperature dependent lifetime. Isotope dependency and the magnitude of the internal conversion rates indicate that internal conversion in triangular [4] phenylene is most likely induced by CH vibrations. The results obtained by LESS and lifetime measurements were compared with PM3 PECI calculations of the excited state structure. The theoretical results and the relation between ground and excited state vibration energies of the 1176 cm(-1) probe vibration indicate a reduction of bond alternation of the central cyclohexatriene ring in the excited state
To determine whether Forster resonance energy transfer (FRET) measurements can provide quantitative distance information in single-molecule fluorescence experiments on polypeptides, we measured FRET efficiency distributions for donor and acceptor dyes attached to the ends of freely diffusing polyproline molecules of various lengths. The observed mean FRET efficiencies agree with those determined from ensemble lifetime measurements but differ considerably from the values expected from Forster theory, with polyproline treated as a rigid rod. At donor-acceptor distances much less than the Forster radius R-o, the observed efficiencies are lower than predicted, whereas at distances comparable to and greater than R-0, they are much higher. Two possible contributions to the former are incomplete orientational averaging during the donor lifetime and, because of the large size of the dyes, breakdown of the point-dipole approximation assumed in Forster theory. End-to-end distance distributions and correlation times obtained from Langevin molecular dynamics simulations suggest that the differences for the longer polyproline peptides can be explained by chain bending, which considerably shortens the donor-acceptor distances
A series of terbium- and europium-exchanged microporous-mesoporous zeolite Socony Mobil Five (MFI)-type materials such as Zeotile-1 and Zeogrid with varying Si/Al ratios was investigated using FTIR, PXRD, adsorption- desorption isotherms of N-2 at 77 K and time-resolved luminescence spectroscopy. Silylation of the lanthanides-exchanged Zeotile-1 and Zeogrid with hexadecyl trimethoxysilanes via post-synthesis grafting was also studied. The results showed that the lanthanide's photoluminescence spectra and decays were modified due to silylation. The different silylation effects in Zeotile-1 and Zeogrid were correlated with the textural properties of the investigated materials. (C) 2007 Elsevier B.V. All rights reserved.
Intrinsic fluorescence quenching of humic substances (HS) and the sensitization of Ln(3+) luminescence (Ln3+ Tb3+, Eu3+) in HS complexes were investigated. Both measurements yielded complementary information on the complexation of metals by HS. Large differences between fulvic acids(FA)and humic acids (HA) were found. From time-resolved luminescence measurements it is concluded that a combination of energy transfer and energy back transfer between HS and Ln(3+) is responsible for the observed luminescence decay characteristics. In the case of Eu3+, an additional participation of charge-transfer states is suggested. A new concept for the evaluation of the sensitized luminescence decays of Ln(3+) was adapted