Rate Of Back Transfer Research Articles

Unusual Distribution Kinetics of Gadoxetate in Healthy Human Subjects Genotyped for OATP1B1: Application of Population Analysis and a Minimal Physiological-Based Pharmacokinetic Model.

Gadoxetate (Gd-EOB-DTPA) isahepatobiliary-specific contrast agent for magnetic resonance imaging. Using a minimal physiological-based pharmacokinetic (PBPK) model, it has been shown for the first time, that the rapid initial decline of plasma concentration after intravenous injection is the result of an uptake into hepatocytes rather than of a distribution into the extravascular extracellular space. About 50% of the steady-state distribution volume is related to hepatic uptake. The hepatic extraction ratio and hepatic clearance estimated based on the liver model as a part of the PBPK model were in accordance with literature data. The same holds for the predicted time course of the amount of gadoxetate in liver parenchyma. In elucidating the impact of OATP1B1 genotype (*1a/*1a and *15/*15) on the pharmacokinetics of gadoxetate, we found that tissue uptake and back-transfer rates were significantly reduced, whereas the hepatic sinusoidal efflux rate was significantly increased in carriers of the *15/*15 haplotype compared with those of the *1a/*1a (wild type). Themodelispotentiallyuseful for determining hepatic kinetic parameters and distribution properties of drugs.

Eu3+ luminescence properties of Eu- and Mg-codoped AlGaN

We investigated the effect of Mg codoping on luminescence properties of AlGaN:Eu to improve emission through synergy effect between an increase in bandgap by AlGaN and the Mg codoping technique. The luminescence properties of AlGaN:(Eu, Mg) are strongly influenced by the Mg concentration and Al composition. Mg codoping in AlGaN was observed to contribute to increasing photoluminescence (PL) integrated intensity and to improve thermal quenching from 7.3% to 60% while the dominant optical site remained site B (622.3-nm peak) with low excitation cross section. The total concentration of optically activated Eu at 25K was a constant at for either optical site, indicating that Mg codoping did not affect the formation of optical sites. The PL decay times at room temperature (RT) increased with Mg concentration because of suppression of the back-transfer process. For optimized Mg concentration, an increase in the Al composition contributed to the total activated Eu concentration and changed the dominant optical site from A (620.3-nm peak) to B. The activation energy Ea, which is the difference in energy between the 5D0 energy level and the trap level in the host material, was estimated from temperature dependence of PL decay time. The Ea for site A was larger than that for site B, suggesting that the back-transfer rate for site A was less than that for site B.

Interfacial charge transfer events of BODIPY molecules: single molecule spectroelectrochemistry and substrate effects.

We present single molecule fluorescence and spectroelectrochemistry characteristics of 4,4'-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) bearing two carboxylic acid groups at its 2 and 6 positions. Our study shows a heterogeneous half redox potential distribution for the BODIPY molecules embedded in polystyrene film because of the heterogeneity in their charge transfer rates. Single molecules adsorbed onto a TiO2 surface with ordered nanostructures show surprising fluorescence blinking activity with the shortest ON duration time in comparison to bare glass and indium-tin oxide (ITO) surfaces. Single molecule stability tests show longer ON duration time and a stable fluorescence feature when dispersed in polystyrene thin film than molecules exposed to air. Shorter ON times are observed for molecules. In intimate contact with ITO in comparison to glass substrates. Such a decrease in their fluorescence stability or intensity is explained by charge transfer activities from the dye molecules to the metal oxide surface. Electron transfer and back transfer rates are calculated to illustrate the substrate effects by using a well-established model.

Theoretical Spectroscopic Study of the Conjugate Microcystin-LR-Europium Cryptate

In this work, theoretical tools were used to study spectroscopic properties of the conjugate microcystin-LR-europium cryptate. The Sparkle/AM1 model was applied to predict the geometry of the system and the INDO/S-CIS model was used to calculate the excited state energies. Based on the Judd-Ofelt theory, the intensity parameters were predicted and a theoretical model based on the theory of the 4f-4f transitions was applied to calculate energy transfer and backtransfer rates, radiative and non-radiative decay rates, quantum efficiency and quantum yield. A detailed study of the luminescent properties of the conjugate Microcystin-LR-europium cryptate was carried out. The results show that the theoretical quantum yield of luminescence of 23% is in good agreement with the experimental value published. This fact suggests that this theoretical protocol can be used to design new systems in order to improve their luminescence properties. The results suggest that this luminescent system may be a good conjugate for using in assay ELISA for detection by luminescence of the Microcystin-LR in water.

Ultrafast Electron Transfer and Decay Dynamics in a Small Band Gap Bulk Heterojunction Material

Bulk heterojunction materials comprising bicontinuous networks of electron donor and acceptor components sandwiched between electrodes with different work functions (e.g., indium-tin oxide, ITO, and a lower work-function metal such as Al) have yielded the best results to date for organic polymer-based solar cells. In such bulk heterojunction materials, ultrafast photoinduced electron transfer occurs at the interface between the polymer (donor) and the fullerene (acceptor) and results in efficient charge separation. Provided that the back transfer rate is sufficiently slow, the photo-generated mobile positive and negative carriers can be collected at the electrodes; electrons at the lower work function electrode and holes at the higher work function electrode. Using this approach, power conversion efficiencies of 5 % have been demonstrated in bulk heterojunction materials comprised of poly(3-hexylthiophene), P3HT and the [6,6]phenyl-C61 butyric acid methyl ester fullerene derivative, PCBM. Although the quantum efficiency is high within the absorption spectrum, the band gap of this conjugated polymer is too large (the solar radiation spectrum extends into the infrared). Thus, the quest for higher solar cell efficiencies has focused on smaller band gap polymers designed and synthesized to improve the harvesting of solar radiation. Here we describe the results of time-resolved spectroscopic studies of the small band gap semiconducting copolymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], PCPDTBT, made of alternating electron-rich and electron-deficient units and in bulk heterojunction materials comprising PCPDTBT and PCBM. The molecular structures of PCPDTBT and PCBM are shown in Scheme 1. Because carrier recombination prior to collection at the electrodes would limit the photovoltaic power conversion efficiency, the goal of our stud-

Temperature dependence of Nd 3+ → Yb 3+ energy transfer processes in co-doped oxyfluoride glass ceramics

A study of the Nd 3+ → Yb 3 energy transfer processes in transparent oxyfluoride glass ceramics has been carried out as a function of temperature in the 100–700 K range. This host is a two-phase optical material that consists of a low-phonon energy fluoride nanocrystalline phase embedded in a predominantly aluminosilicate glassy medium and has shown to be an interesting matrix for rare earth ions. Luminescence decay curves of single Nd 3+ and Yb 3+ doped and co-doped samples at different temperatures have been analyzed in order to calculate the energy transfer and backtransfer rates between these ions. Finally, the results have been also investigated to known the phonons involved in the energy transfer processes, concluding at the end that the Nd 3+ → Yb 3+ energy transfer rate takes place by the emission of three phonons with energy around 325 cm −1 and in the other hand, Nd 3+ ← Yb 3+ energy transfer rate has been found to be non-negligible for temperatures over 370 K with the requirement of absorption of phonons.

Temperature dependence of Nd3+↔Yb3+ energy transfer in the YAl3(BO3)4 nonlinear laser crystal

The temperature dependence of the Nd3+→Yb3+ energy-transfer rate in the YAl3(BO3)4 nonlinear laser crystal has been investigated from the analysis of fluorescence decay curves recorded in the 10–600 K range. Three different regimes, independent on the dopant concentration, have been observed in the thermal behavior of the Nd3+→Yb3+ energy-transfer rate. By comparing experimental results with theoretical predictions based on the Dexter model [J. Chem. Phys. 21, 836 (1953)], the origin of these different regimes has been explained. In addition, the influence of temperature and of both Nd3+ and Yb3+ concentrations on the Nd3+←Yb3+ energy back-transfer rate has been also investigated, concluding that it is a migration-assisted energy-transfer process. Finally, the populations of both Nd3+ and Yb3+ metastable states achieved after continuous-wave Nd3+ excitation have been calculated and measured and results have been explained in terms of the thermal behavior of both forward- and back-transfer rates.

The Nd-nanocluster coupling strength and its effect in excitation/de-excitation of Nd3+ luminescence in Nd-doped silicon-rich silicon oxide

The Nd-nanocluster Si (nc-Si) coupling strength and its effect in excitation/de-excitation of Nd3+ luminescence in Nd-doped silicon-rich silicon oxide (SRSO) is investigated. Nd-doped SRSO thin films, which consist of nc-Si embedded inside a SiO2 matrix, were prepared by electron-cyclotron-resonance plasma-enhanced chemical vapor deposition of SiH4 and O2 with cosputtering of Nd and subsequent anneal at 950 °C. Efficient Nd3+ luminescence with moderate temperature quenching is observed. Based on an analysis of the temperature dependence of Nd3+ luminescence lifetime, we find a coupling strength between nc-Si and Nd that is strong enough to result in efficient excitation of Nd3+ via quantum-confined excitons, while weak enough to result in a small back-transfer rate is identified as the key to Nd3+ luminescence.

The Nd-nanocluster Coupling Strength and its Effect in Excitation/de-excitation of Nd3+Luminescence in Nd-doped Silicon-rich Silicon Oxide

AbstractThe Nd-nanocluster Si (nc-Si) coupling strength and its effect in excitation/de-excitation of Nd3+luminescence in Nd-doped silicon-rich silicon oxide (SRSO) is investigated. Nd-doped SRSO thin films, which consist of nc-Si embedded inside a SiO2matrix, were prepared by electron-cyclotron-resonance plasma enhanced chemical vapor deposition (ECR-PECVD) of SiH4and O2with co-sputtering of Nd and subsequent anneal at 950 °C. Efficient Nd3+luminescence with moderate temperature quenching is observed. Based on the temperature dependence of Nd3+luminescence lifetime, a coupling strength between nc-Si and Nd that is strong enough to result in efficient excitation of Nd3+via quantum confined excitons while weak enough to result in a small back-transfer rate is identified as the key to Nd3+luminescence.

Back transfer in the 4S 3/2 state of Cs 2NaEr xY 1− xCl 6: extension of the shell model

The 4S 3/2 state of Er 3+ in Cs 2NaEr x Y 1− x Cl 6 undergoes a cross relaxation above 100 K which involves a transition from the 4S 3/2 to the 4I 9/2 and 4I 3/2 states at the donor (D) and the excitation of the acceptor (A) to the 4I 13/2, 4I 9/2 states. Analysis of experimental decay curves from the 4S 3/2 state for x=0.001−1 using the standard shell model for energy transfer in the elpasolite lattice has not been successful. In this system the energies of the crystal field levels means that the rate of back transfer from the acceptor to the donor is comparable with or greater than the cross relaxation rate, and therefore must be included explicitly. The effect that inclusion of the back transfer terms has on the decay of the 4S 3/2 state is examined numerically. We have simulated decay curves for the 4S 3/2 state within the shell model with inclusion of the back transfer terms. These simulated curves are in good agreement with the experimental decay curves.

2E relaxation of Cr(III) complexes in rigid and fluid media

2E decays have been recorded for a group of Cr(III) complexes with varying 4T 2 2E separations in solvents with independently measured relaxation times. Thermal quenching that occurs when solvent motion is slow relative to the excited state decay time leads to non-exponential decay. Comparison of the 2E lifetimes and estimated deviations from exponentiality in a wide range of complex-solvent combinations as a function of temperature suggests a model in which 2 E ⇒ 4 T 2 back-transfer is the dominant decay pathway in most complexes. The back-transfer rate is increased by solvent motion.

On the Excitation Mechanism of Erbium and Ytterbium in the Quaternary Compounds InGaAsP

AbstractAn efficient transfer of excitonic energy to rare earth (RE) ions is crucial for possible optoelectronic applications of RE doped semiconductors. In order to investigate the energy transfer mechanism to RE ions after optical above bandgap excitation we studied the intensity of the 4I13/2 →4 4I15/2-transition of Er3+ (1.54μm) and the one of the 2F5/2 →2F7/2 -transition of Yb3+ (lμm) (loped into InGaAsP-layers lattice matched to InP. By varying the composition of the quaternary compoun ds, the bandgap energy together with the RE bound exciton energy were tuned relative to the RE excitation energy, and the effect on the RE luminescence intensity was observed. The results can be interpreted by stating a) that the energy transfer to the RE proceeds via the RE bound exciton, and b) that the intensity of the RE luminescence is essentially determined by the rate of back transfer of the RE excitation energy ERE to the bound exciton (with excitation energy Ebe). In this back transfer the energy of the excited RE ion plus the energy of 0, 1, 2 ... L0-phonons (energy Elo) is used to reexcite the bound exciton, instead of being emitted as an RE luminescence photon. For compositions where Ebe = ERE + n. Elo (n = 0, 1, 2...) we have a maximum of back transfer and correspondingly a, minimum in RE luminescence. In between the intensity has a, maximum.

Interfacial transport processes in the reversed micellar extraction of proteins

Interfacial mass transfer coefficients for the transfer of the proteins α-chymotrypsin and cytochrome c between a bulk aqueous and a reversed micellar phase were measured using a stirred diffusion cell. The strong dependence of interfacial forward transport kinetics on pH and salt concentration indicates the dominant role that charge interactions play in this system. This role is elucidated theoretically by determining the electrostatic interactions between a protein particle and the bulk interface, and the degree to which that interface will deform in response to these interactions. Back transfer rates were found to be three orders of magnitude slower than those for forward transfer. The dependence of these rates on aqueous pH suggests that coalescence of the protein-filled micelle with the bulk interface dominates the desolubilization kinetics.

Analytical estimates of turbulent MHD transport coefficients

Turbulent transfer rates from small-scale MHD excitations to large-scale Fourier modes are calculated algebraically using the method of Biskamp and Welter (1983). Three cases are considered: two-dimensional Navier-Stokes flows, two-dimensional incompressible MHD, and the weakly three-dimensional Strauss equations. In all cases, an initially large spectral gap, between the small-scale and large-scale excitations is assumed, and attention focuses on the initial values of the back-transfer rates. The sign of the transfer is determined by the sign of an analytically calculable eddy viscosity and/or anomalous resistivity. The authors confirm the results of Biskamp and Welter for the case of two-dimensional MHD, but find some differences for the case of the Strauss equations. It is argued that the Strauss equations may not exhibit an inverse cascade phenomenon for the spatially periodic case unless their initial spectra are such that the behaviour is essentially that of two-dimensional MHD.

Time development of the spectral redistribution in disordered systems with a Lorentzian transfer rate: Theoretical models and a Monte Carlo study

The time-dependent spectral redistribution in fluorescence line narrowing in an inhomogeneously broadened system is investigated by means of simple theoretical models and Monte Carlo calculations. The two-site phonon-assisted energy transfer rate is assumed to be site-symmetric and a Lorentzian function of energy mismatch. For the Monte Carlo calculations initial excitations are made at the wing, shoulder, and center of the inhomogeneously broadened Gaussian band. For the wing excitation it is argued that, because of the above Lorentzian factor, the intensity in the initial site of excitation decays with a much smaller back-transfer rate than for the center excitation, simplifying the theoretical treatment. This effect is demonstrated by the Monte Carlo study. A refinement of this model is developed for a general situation, and its predictions are compared with the Monte Carlo results.