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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

First-Principles Characterization of the Elusive I Fluorescent State and the Structural Evolution of Retinal Protonated Schiff Base in Bacteriorhodopsin

The conversion of light energy into work is essential to life on earth. Bacteriorhodopsin (bR), a light-activated proton pump in Archae, has served for many years as a model system for the study of this process in photoactive proteins. Upon absorption of a photon, its chromophore, the retinal protonated Schiff base (RPSB), isomerizes from its native all-trans form to a 13-cis form and pumps a proton out of the cell in a process that is coupled to eventual ATP synthesis. Despite numerous time-resolved spectroscopic studies over the years, the details of the photodynamics of bR on the excited state, particularly the characterization of the I fluorescent state, the time-resolved reaction mechanism, and the role of the counterion cluster of RPSB, remain uncertain. Here, in this work, we use ab initio multiple spawning (AIMS) with spin-restricted ensemble Kohn–Sham (REKS) theory to simulate the nonadiabatic dynamics of the ultrafast photoreaction in bR. The excited state dynamics can be partitioned into three distinct phases: (1) relaxation away from the Franck–Condon region dominated by changes in retinal bond length alternation, (2) dwell time on the excited state in the I fluorescent state featuring an untwisted, bond length inverted RPSB, and (3) rapid torsional evolution to the conical intersection after overcoming a small excited state barrier. We fully characterize the I fluorescent state and the excited state barrier that hinders direct evolution to the conical intersection following photoexcitation. We also find that photoisomerization is accompanied by weakening of the interaction between RPSB and its counterion cluster. However, in contradiction with a recent time-resolved X-ray experiment, hydrogen bond cleavage is not necessary to reproduce the observed photoisomerization dynamics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Systematic Assessment of Retrieval Methods for Canopy Far-Red Solar-Induced Chlorophyll Fluorescence Using High-Frequency Automated Field Spectroscopy

Remote sensing of solar-induced chlorophyll fluorescence (SIF) offers potential to infer photosynthesis across scales and biomes. Many retrieval methods have been developed to estimate top-of-canopy SIF using ground-based spectroscopy. However, inconsistencies among methods may confound interpretation of SIF dynamics, eco-physiological/environmental drivers, and its relationship with photosynthesis. Using high temporal- and spectral resolution ground-based spectroscopy, we aimed to (1) evaluate performance of SIF retrieval methods under diverse sky conditions using continuous field measurements; (2) assess method sensitivity to fluctuating light, reflectance, and fluorescence emission spectra; and (3) inform users for optimal ground-based SIF retrieval. Analysis included field measurements from bi-hemispherical and hemispherical-conical systems and synthetic upwelling radiance constructed from measured downwelling radiance, simulated reflectance, and simulated fluorescence for benchmarking. Fraunhofer-based differential optical absorption spectroscopy (DOAS) and singular vector decomposition (SVD) retrievals exhibit convergent SIF-PAR relationships and diurnal consistency across different sky conditions, while O2A-based spectral fitting method (SFM), SVD, and modified Fraunhofer line discrimination (3FLD) exhibit divergent SIF-PAR relationships across sky conditions. We find that such behavior holds across system configurations, though hemispherical-conical systems diverge less across sky conditions. O 2 A retrieval accuracy, influenced by atmospheric distortion, improves with a narrower fitting window and when training SVD with temporally local spectra. This may impact SIF-photosynthesis relationships interpreted by previous studies using O 2 A-based retrievals with standard (759–767.76 nm) fitting windows. Fraunhofer-based retrievals resist atmospheric impacts but are noisier and more sensitive to assumed SIF spectral shape than O 2 A-based retrievals. We recommend SVD or SFM using reduced fitting window (759.5–761.5 nm) for robust far-red SIF retrievals across sky conditions.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2014

Facile method to stain the bacterial cell surface for super-resolution fluorescence microscopy

A method to fluorescently stain the surfaces of both Gram-negative and Gram-positive bacterial cells compatible with super-resolution fluorescence microscopy is presented. This method utilizes a commercially-available fluorescent probe to label primary amines at the surface of the cell. We demonstrate efficient staining of two bacterial strains, the Gram-negative Shewanella oneidensis MR-1 and the Gram-positive Bacillus subtilis 168. Using structured illumination microscopy and stochastic optical reconstruction microscopy, which require high quantum yield or specialized dyes, we show that this staining method may be used to resolve the bacterial cell surface with sub-diffraction-limited resolution. We further use this method to identify localization patterns of nanomaterials, specifically cadmium selenide quantum dots, following interaction with bacterial cells.

Bacterial↗
DOE OSTI2012

Geometrical factor correction in grazing incident x-ray fluorescence experiment

The geometrical factor in the grazing incident x-ray fluorescence analysis is an important angle-dependent term, which can have a great effect on the measured data. In this paper, the effects of the geometrical factor on the florescence yield have been demonstrated. A formula is presented to estimate the geometrical factor, which includes the experimental parameters of the beam and setup. The validity of this formula is proven by the good agreement between the calculated fluorescence yields with the experimental results in grazing incident x-ray fluorescence analysis.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Grazing-incidence x-ray fluorescence analysis for non-destructive determination of In and Ga depth profiles in Cu(In,Ga)Se{sub 2} absorber films

Development of highly efficient thin film solar cells involves band gap engineering by tuning their elemental composition with depth. Here we show that grazing incidence X-ray fluorescence (GIXRF) analysis using monochromatic synchrotron radiation and well-characterized instrumentation is suitable for a non-destructive and reference-free analysis of compositional depth profiles in thin films. Variation of the incidence angle provides quantitative access to the in-depth distribution of the elements, which are retrieved from measured fluorescence intensities by modeling parameterized gradients and fitting calculated to measured fluorescence intensities. Our results show that double Ga gradients in Cu(In{sub 1−x},Ga{sub x})Se{sub 2} can be resolved by GIXRF.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Integrated ultrasonic particle positioning and low excitation light fluorescence imaging

A compact hybrid system has been developed to position and detect fluorescent micro-particles by combining a Single Photon Avalanche Diode (SPAD) imager with an acoustic manipulator. The detector comprises a SPAD array, light-emitting diode (LED), lenses, and optical filters. The acoustic device is formed of multiple transducers surrounding an octagonal cavity. By stimulating pairs of transducers simultaneously, an acoustic landscape is created causing fluorescent micro-particles to agglomerate into lines. The fluorescent pattern is excited by a low power LED and detected by the SPAD imager. Our technique combines particle manipulation and visualization in a compact, low power, portable setup.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Inhibition of beta-amyloid aggregation by fluorescent dye labels

The fluorescence decay of beta-amyloid's (Aβ) intrinsic fluorophore tyrosine has been used for sensing the oligomer formation of dye-labelled Aβ monomers and the results compared with previously studied oligomerization of the non-labelled Aβ peptides. It has been demonstrated that two different sized, covalently bound probes 7-diethylaminocoumarin-3-carbonyl and Hilyte Fluor 488 (HLF), alter the rate and character of oligomerization to different extents. The ability of HLF to inhibit formation of highly ordered structures containing beta-sheets was also shown. The implications of our findings for using fluorescence methods in amyloidosis research are discussed and the advantages of this auto-fluorescence approach highlighted.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Fluorescence imaging of viscous materials in the ultraviolet-visible wavelength range

This paper presents an approach of an innovative measurement principle for the quality control of viscous materials during a manufacturing process based on fluorescence imaging. The main contribution to the state of the art provided by this measurement system is that three equal fluorescence images of a static or moving viscous object are available in different optical paths. The independent images are obtained by two beam splitters which are connected in series. Based on these images, it is possible to evaluate each image separately. In our case, three optical bandpass filters with different center wavelengths of 405 nm, 420 nm, and 440 nm were used to filter the separate fluorescence images. The developed system is useable for the detection of impurities in the micrometer range. Further, incorrect mixing ratios of particular components and wrong single components in the viscous materials can be detected with the setup. Moreover, it is possible to realize static and dynamic measurements. In this case the maximum speed of the objects was 0.2 m/s for the dynamic measurements. Advantages of this measurement setup are the universality due to the use of optical standard components, the small dimension and the opportunity to integrate it easily into ongoing processes. In addition, the measurement system works on a non-contact basis. Thus, the expense for maintenance is at a very low level compared to currently available measurement setups for the investigated application. Furthermore, the setup provides for the first time a simultaneous analysis of more than one component and the detection of impurities concerning their nature and size in a manufacturing process.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Franck-Condon factors perturbed by damped harmonic oscillators: Solvent enhanced X {sup 1}A{sub g} ↔ A{sup 1}B{sub 1u} absorption and fluorescence spectra of perylene

Damped harmonic oscillators are utilized to calculate Franck-Condon factors within displaced harmonic oscillator approximation. This is practically done by scaling unperturbed Hessian matrix that represents local modes of force constants for molecule in gaseous phase, and then by diagonalizing perturbed Hessian matrix it results in direct modification of Huang–Rhys factors which represent normal modes of solute molecule perturbed by solvent environment. Scaling parameters are empirically introduced for simulating absorption and fluorescence spectra of an isolated solute molecule in solution. The present method is especially useful for simulating vibronic spectra of polycyclic aromatic hydrocarbon molecules in which hydrogen atom vibrations in solution can be scaled equally, namely the same scaling factor being applied to all hydrogen atoms in polycyclic aromatic hydrocarbons. The present method is demonstrated in simulating solvent enhanced X {sup 1}A{sub g} ↔ A{sup 1}B{sub 1u} absorption and fluorescence spectra of perylene (medium-sized polycyclic aromatic hydrocarbon) in benzene solution. It is found that one of six active normal modes v{sub 10} is actually responsible to the solvent enhancement of spectra observed in experiment. Simulations from all functionals (TD) B3LYP, (TD) B3LYP35, (TD) B3LYP50, and (TD) B3LYP100 draw the same conclusion. Hence, the present method is able to adequately reproduce experimental absorption and fluorescence spectra in both gas and solution phases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Density relaxation and particle motion characteristics in a non-ionic deep eutectic solvent (acetamide + urea): Time-resolved fluorescence measurements and all-atom molecular dynamics simulations

Temperature dependent relaxation dynamics, particle motion characteristics, and heterogeneity aspects of deep eutectic solvents (DESs) made of acetamide (CH{sub 3}CONH{sub 2}) and urea (NH{sub 2}CONH{sub 2}) have been investigated by employing time-resolved fluorescence measurements and all-atom molecular dynamics simulations. Three different compositions (f) for the mixture [fCH{sub 3}CONH{sub 2} + (1 − f)NH{sub 2}CONH{sub 2}] have been studied in a temperature range of 328-353 K which is ∼120-145 K above the measured glass transition temperatures (∼207 K) of these DESs but much lower than the individual melting temperature of either of the constituents. Steady state fluorescence emission measurements using probe solutes with sharply different lifetimes do not indicate any dependence on excitation wavelength in these metastable molten systems. Time-resolved fluorescence anisotropy measurements reveal near-hydrodynamic coupling between medium viscosity and rotation of a dissolved dipolar solute. Stokes shift dynamics have been found to be too fast to be detected by the time-resolution (∼70 ps) employed, suggesting extremely rapid medium polarization relaxation. All-atom simulations reveal Gaussian distribution for particle displacements and van Hove correlations, and significant overlap between non-Gaussian (α{sub 2}) and new non-Gaussian (γ) heterogeneity parameters. In addition, no stretched exponential relaxations have been detected in the simulated wavenumber dependent acetamide dynamic structure factors. All these results are in sharp contrast to earlier observations for ionic deep eutectics with acetamide [Guchhait et al., J. Chem. Phys. 140, 104514 (2014)] and suggest a fundamental difference in interaction and dynamics between ionic and non-ionic deep eutectic solvent systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Fluorescently tuned nitrogen-doped carbon dots from carbon source with different content of carboxyl groups

In this study, fluorescent nitrogen-doped carbon dots (NCDs) were tuned via varying the sources with different number of carboxyl groups. Owing to the interaction between amino and carboxyl, more amino groups conjugate the surface of the NCDs by the source with more carboxyl groups. Fluorescent NCDs were tuned via varying the sources with different content of carboxyl groups. Correspondingly, the nitrogen content, fluorescence quantum yields and lifetime of NCDs increases with the content of carboxyl groups from the source. Furthermore, cytotoxicity assay and cell imaging test indicate that the resultant NCDs possess low cytotoxicity and excellent biocompatibility.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Neutron, fluorescence, and optical imaging: An in situ combination of complementary techniques

An apparatus which enables the simultaneous combination of three complementary imaging techniques, optical imaging, fluorescence imaging, and neutron radiography, is presented. While each individual technique can provide information on certain aspects of the sample and their time evolution, a combination of the three techniques in one setup provides a more complete and consistent data set. The setup can be used in transmission and reflection modes and thus with optically transparent as well as opaque samples. Its capabilities are illustrated with two examples. A polymer hydrogel represents a transparent sample and the diffusion of fluorescent particles into and through this polymer matrix is followed. In reflection mode, the absorption of solvent by a nile red-functionalized mesoporous silica powder and the corresponding change in fluorescent signal are studied.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Role of excited state solvent fluctuations on time-dependent fluorescence Stokes shift

We explore the connection between the solvation dynamics of a chromophore upon photon excitation and equilibrium fluctuations of the solvent. Using molecular dynamics simulations, fluorescence Stokes shift for the tryptophan in Staphylococcus nuclease was examined using both nonequilibrium calculations and linear response theory. When the perturbed and unperturbed surfaces exhibit different solvent equilibrium fluctuations, the linear response approach on the former surface shows agreement with the nonequilibrium process. This agreement is excellent when the perturbed surface exhibits Gaussian statistics and qualitative in the case of an isomerization induced non-Gaussian statistics. However, the linear response theory on the unperturbed surface breaks down even in the presence of Gaussian fluctuations. Experiments also provide evidence of the connection between the excited state solvent fluctuations and the total fluorescence shift. These observations indicate that the equilibrium statistics on the excited state surface characterize the relaxation dynamics of the fluorescence Stokes shift. Our studies specifically analyze the Gaussian fluctuations of the solvent in the complex protein environment and further confirm the role of solvent fluctuations on the excited state surface. The results are consistent with previous investigations, found in the literature, of solutes dissolved in liquids.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

High performance blue-emitting organic light-emitting diodes from thermally activated delayed fluorescence: A guest/host ratio study

Emitters displaying thermally activated delayed fluorescence (TADF) can drive highly efficient organic light-emitting diodes (OLEDs). Such emitters are usually incorporated into the emissive layer (EML) at a relatively low concentration (<20 wt. %) with respect to a host material, a guest-host method inherited from conventional fluorescent or phosphorescent OLEDs. In this work, detailed studies on OLEDs, in which the concentration of oBFCzTrz (a blue-emitter displaying TADF) in a DPEPO host was varied from 8 to 100 wt. %, reveals that oBFCzTrz displays ambipolar transport and limited fluorescence-quenching due to aggregation. For instance, a neat film of oBFCzTrz maintains a photoluminescence quantum yield of 82%. We determine that OLEDs with an EML having an oBFCzTrz concentration of 50 wt. % in DPEPO yield a maximum external quantum efficiency (EQE) of 25.5% with an EQE roll-off of 10% in the range from 10 to 1000 cd/m 2 and Commission Internationale de l'Eclairage color coordinates of (0.20, 0.44). OLEDs with smaller or larger oBFCzTrz concentrations display smaller maximum EQE values. OLEDs with a neat oBFCzTrz EML display a maximum EQE of 14.0%. Finally, time-resolved electroluminescent decay studies, and analysis using a simple model, reveal significant differences in transport, trapping, and recombination in these devices.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Non-Kasha fluorescence of pyrene emerges from a dynamic equilibrium between excited states

Pyrene fluorescence after a high-energy electronic excitation exhibits a prominent band shoulder not present after excitation at low energies. The standard assignment of this shoulder as a non-Kasha emission from the second-excited state (S 2 ) has been recently questioned. To elucidate this issue, we simulated the fluorescence of pyrene using two different theoretical approaches based on vertical convolution and nonadiabatic dynamics with nuclear ensembles. To conduct the necessary nonadiabatic dynamics simulations with high-lying electronic states and deal with fluorescence timescales of about 100 ns of this large molecule, we developed new computational protocols. The results from both approaches confirm that the band shoulder is, in fact, due to S 2 emission. We show that the non-Kasha behavior is a dynamic-equilibrium effect not caused by a metastable S 2 minimum. However, it requires considerable vibrational energy, which can only be achieved in collisionless regimes after transitions into highly excited states. This strict condition explains why the S 2 emission was not observed in some experiments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Ultrafast fluorescent decay induced by metal-mediated dipole–dipole interaction in two-dimensional molecular aggregates

Two-dimensional molecular aggregate (2DMA), a thin sheet of strongly interacting dipole molecules self-assembled at close distance on an ordered lattice, is a fascinating fluorescent material. It is distinctively different from the conventional (single or colloidal) dye molecules and quantum dots. Here, in this paper, we verify that when a 2DMA is placed at a nanometric distance from a metallic substrate, the strong and coherent interaction between the dipoles inside the 2DMA dominates its fluorescent decay at a picosecond timescale. Our streak-camera lifetime measurement and interacting lattice–dipole calculation reveal that the metal-mediated dipole–dipole interaction shortens the fluorescent lifetime to about one-half and increases the energy dissipation rate by 10 times that expected from the noninteracting single-dipole picture. In conclusion, our finding can enrich our understanding of nanoscale energy transfer in molecular excitonic systems and may designate a unique direction for developing fast and efficient optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Synthesis of two fluorescent GTP$\gamma$S molecules and their biological relevance

Fluorescent GTP analogues are utilized for an assortment of nucleic acid and protein characterization studies. Non-hydrolysable analogues such as GTPγS offer the advantage of keeping proteins in a GTP-bound conformation due to their resistance to hydrolysis into GDP. Two novel fluorescent GTPγS molecules were developed by linking fluorescein and tetramethylrhodamine to the γ-thiophosphate of GTPγS. Chemical and biological analysis of these two compounds revealed their successful synthesis and ability to bind to the nucleotide-binding site of tubulin. These two new fluorescent non-hydrolysable nucleotides offer new possibilities for biophysical and biochemical characterization of GTP-binding proteins.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2012

A QUANTUM BAND MODEL OF THE {nu}{sub 3} FUNDAMENTAL OF METHANOL (CH{sub 3}OH) AND ITS APPLICATION TO FLUORESCENCE SPECTRA OF COMETS

Methanol (CH{sub 3}OH) radiates efficiently at infrared wavelengths, dominating the C-H stretching region in comets, yet inadequate quantum-mechanical models have imposed limits on the practical use of its emission spectra. Accordingly, we constructed a new line-by-line model for the {nu}{sub 3} fundamental band of methanol at 2844 cm{sup -1} (3.52 {mu}m) and applied it to interpret cometary fluorescence spectra. The new model permits accurate synthesis of line-by-line spectra for a wide range of rotational temperatures, ranging from 10 K to more than 400 K. We validated the model by comparing simulations of CH{sub 3}OH fluorescent emission with measured spectra of three comets (C/2001 A2 LINEAR, C/2004 Q2 Machholz and 8P/Tuttle) acquired with high-resolution infrared spectrometers at high-altitude sites. The new model accurately describes the complex emission spectrum of the {nu}{sub 3} band, providing distinct rotational temperatures and production rates at greatly improved confidence levels compared with results derived from earlier fluorescence models. The new model reconciles production rates measured at infrared and radio wavelengths in C/2001 A2 (LINEAR). Methanol can now be quantified with unprecedented precision and accuracy in astrophysical sources through high-dispersion spectroscopy at infrared wavelengths.

79 ASTRONOMY AND ASTROPHYSICS↗