Time-resolved Emission Spectra Research Articles

Study of Microheterogeneity in Acetonitrile-Water Binary Mixtures by using Polarity-Resolved Solvation Dynamics.

The solvation dynamics of three coumarin dyes with widely varying polarities were studied in acetonitrile-water (ACN-H2O) mixtures across the entire composition range. At low ACN concentrations [ACN mole fractions (X(ACN))≤0.1], the solvation dynamics are fast (<40 ps), indicating a nearly homogeneous environment. This fast region is followed by a sudden retardation of the average solvation time (230-1120 ps) at higher ACN concentrations (X(ACN)≈0.2), thus indicating the onset of nonideality within the mixture that continues until X(ACN)≈0.8. This nonideality regime (X(ACN)≈0.2-0.8) comprises of multiple dye-dependent anomalous regions. At very high ACN concentrations (X(ACN)≈0.8-1), the ACN-H2O mixtures regain homogeneity, with faster solvation times. The source of the inherent nonideality of the ACN-H2O mixtures is a subject of debate. However, a careful examination of the widths of time-resolved emission spectra shows that the origin of the slow dynamics may be due to the diffusion of polar solvent molecules into the first solvation shell of the excited coumarin dipole.

Laser-induced breakdown spectroscopy using mid-infrared femtosecond pulses

We report on a laser-induced breakdown spectroscopy (LIBS) experiment driven by mid-infrared (2.05-μm) fs pulses, in which time-resolved emission spectra of copper were studied. Ab-initio modeling is consistent with the results of new fs measurements at 2.05 μm and traditional 800-nm fs-LIBS. Ablation by mid-infrared fs pulses results in a plasma with a lower plasma density and temperature compared to fs-LIBS performed at shorter laser wavelength. LIBS driven by mid-infrared fs pulses results in a signal-to-background ratio ∼50% greater and a signal-to-noise ratio ∼40% lower than fs-LIBS at near-infrared laser wavelength.

Efficient exciplex organic light-emitting diodes with a bipolar acceptor

Efficient exciplex organic light-emitting diodes (OLEDs) have been fabricated with an intermixed light-emitting layer comprising electron-rich m-MTDATA or TAPC as a donor and bipolar material 26DCzPPy as an acceptor. Intermolecular charge–transfer interaction between various donor materials with different highest occupied molecular orbital (HOMO) levels and 26DCzPPy are systematically investigated to explore the formation principle of the exciplex. Various techniques like steady state absorption and photoluminescence (PL) spectra, temperature dependent transient PL decay and time-resolved emission spectra (TRES) using time-correlated single-photon counting (TCSPC) principle were adopted to probe the electronic and optical properties of these exciplex systems. Strong exciplex emission was found from the m-MTDATA:26DCzPPy and TAPC:26DCzPPy mixed systems. The devices comprising these two exciplex systems as the emission layer exhibited a promising external quantum efficiency and a high exciton utilization efficiency due to the triplet exciton up-conversion from triplet to singlet states inspired by their small singlet–triplet exchange energy. Our current work contributes to know more about the unique intermolecular charge–transfer property between electron-rich donors and bipolar acceptors, and provides valuable exploration on developing novel high-performance exciplex OLEDs.

Local Dye Concentration and Spectroscopic Properties of Monomer–Aggregate Systems in Hybrid Porous Nanolayers

Analysis of absorption, fluorescence, and time-resolved emission spectra of rhodamine 6G in hybrid porous nanolayers at high dye concentration allowed identification of two fluorescent species: monomers and fluorescent aggregates. The method of determination of mean local concentration of aggregates in matrices is proposed based on the energy transfer process between monomers and fluorescent aggregates. Evaluation of aggregate mean local concentration was carried out by comparing the results of Monte Carlo simulation of forward and reverse nonradiative energy transfer between monomers and aggregates with experimental data of fluorescence decay in which monomer and aggregate concentration were treated as best fit parameters.

Spatio-temporally resolved electron temperature in argon radio-frequency capacitive discharge at atmospheric pressure

Due to the lack of convincing experimental evidence for electron information, there are still unclearly understood discharge phenomena in atmospheric pressure radio-frequency (rf) capacitive discharge, e.g. the electron heating, discharge structures, and the alpha–gamma mode transition. Thus, to perceive basic and meaningful principles with an unambiguous interpretation, simple and reliable electron diagnostics are required. Since bremsstrahlung emitted through electron-neutral atom interaction depends on electron density (ne) and temperature (Te), their diagnostic is possible. In particular, Te is easily estimated from the ratio of bremsstrahlung emissivities at two different wavelengths or more. In this paper, 2D Te distribution in an argon atmospheric pressure capacitive discharge measured by using a digital camera and optical band pass filters is described. Time-averaged Te in the bulk region obtained by a digital camera is consistent with that measured by an absolutely calibrated spectrometer. In addition, time-resolved emission spectra and the corresponding ne and Te during one rf cycle of the argon capacitive discharge are discussed. The result shows that Te varied from 2.3 to 3.0 eV, while ne did not change significantly.

Time-resolved spectroscopy of Bi3+ centers in Y4Al2O9

Steady-state and time-resolved emission and excitation spectra as well as luminescence decay kinetics are studied at 4.2–400K under excitation in the 3–6eV energy range for Bi3+ ions substituting for Y3+ ions in four inequivalent crystal lattice sites of Y4Al2O9:Bi ceramics. Luminescence characteristics of Bi3+ centers of all the four types are identified and are shown to arise from the radiative decay of the triplet relaxed excited state (RES) of Bi3+ ions. The parameters of the triplet RES, namely, probabilities of the radiative and nonradiative transitions from the metastable and emitting levels as well as the energy distance between these levels, are determined. The influence of the nearest surroundings of Bi3+ ions on the luminescence characteristics and the parameters of the triplet RES of Bi3+ centers is discussed.

Organization and dynamics of tryptophan residues in brain spectrin: novel insight into conformational flexibility.

Brain spectrin enjoys overall structural and sequence similarity with erythroid spectrin, but less is known about its function. We utilized the fluorescence properties of tryptophan residues to monitor their organization and dynamics in brain spectrin. Keeping in mind the functional relevance of hydrophobic binding sites in brain spectrin, we monitored the organization and dynamics of brain spectrin bound to PRODAN. Results from red edge excitation shift (REES) indicate that the organization of tryptophans in brain spectrin is maintained to a considerable extent even after denaturation. These results are supported by acrylamide quenching experiments. To the best of our knowledge, these results constitute the first report of the presence of residual structure in urea-denatured brain spectrin. We further show from REES and time-resolved emission spectra that PRODAN bound to brain spectrin is characterized by motional restriction. These results provide useful information on the differences between erythroid spectrin and brain spectrin.

Metal complexes and time-resolved photoluminescence spectroscopy for sensing applications

This feature article will cover our efforts to sense biologically relevant molecules using photoluminescent metal complexes. Photoluminescent metal complexes possess large Stokes shifts, long lifetimes and tunable photoluminescence maxima. We have developed probes for DNA and RNA detection containing metal complexes of ruthenium and iridium as photoluminescent reporters. Iridium complexes have also been modified to serve as probes for amino acids such as cysteine, homocysteine, and histidine. In addition, probes sensible to the aggregation state of proteins such as amyloid-β and alpha-synuclein were developed, which display changes in photoluminescence (ruthenium dipyridophenazine complexes) or birefringence (ruthenium red). Finally, we will present the detection of solvent vapors using a photoluminescent rhenium complex within a zeolite matrix. The long photoluminescence lifetime of the aforementioned complexes has been synergistically combined with advanced time-resolved methods to enhance the detection scope of these probes. For example techniques such as time-gating have been used to detect oligonucleotides, amino acids and protein aggregation even in highly autofluorescent media. Time-gating allows selecting a time-window in a time-resolved emission spectrum where the long-lived photoluminescence of the probes can be preferentially detected from the short-lived autofluorescence of the medium. In addition, we have used time-resolved photoluminescence spectroscopy to extract the photoluminescence of free histidine from the photoluminescence of histidine-containing proteins. Also, a combination of photoluminescence intensity, maximum and lifetime was used to detect solvent vapors. These examples testify on the advantages of time-resolved photoluminescence spectroscopy for enhancing the detection of analytes using probes with long-lived photoluminescence.

Density and x-ray emission profile relationships in highly ionized high-Z laser-produced plasmas

We present a benchmark measurement of the electron density profile in the region where the electron density is 1019 cm–3 and where the bulk of extreme ultraviolet (EUV) emission occurs from isotropically expanding spherical high-Z gadolinium plasmas. It was found that, due to opacity effects, the observed EUV emission is mostly produced from an underdense region. We have analyzed time-resolved emission spectra with the aid of atomic structure calculations and find the multiple ion charge states around 18+ during the laser pulse irradiation.

Luminescence and energy transfer phenomena in YVO4 single crystal co-doped with Tm3+ and Eu3+

Detailed spectroscopic investigation of YVO4:5 at% Eu3+, 1 at % Tm3+ single crystal encompassing analysis of absorption, excitation and time resolved emission spectra recorded at 5K and 300K made it possible to determine energies, intensities and bandwidths of transitions within 4fn configurations of trivalent thulium and europium. Transients recorded upon femtosecond pulse excitation made it possible to determine dynamics of population build-up and relaxation of excited states involved in luminescence phenomena. It was found that in YVO4:Eu, Tm system the metastable 5D0 level of Eu3+ and 1G4 level of Tm3+ can be populated by energy transfer from excited VO43− group and respective intra-ion relaxation processes from higher energy levels. Recorded excitation spectra of the 5D0 luminescence of Eu3+ and of the 1G4 luminescence of Tm3+ do not evidence the Eu3+↔Tm3+ energy transfer. Nevertheless, an examination of acquired luminescence decay curves, performed with reference to available information on relaxation dynamics of YVO4:Tm3+ system, reveals that the incorporation of Eu3+ accelerates decays of the 1G4 and 3H4 levels of Tm3+. Spectroscopic parameters characterising the donor–acceptor interaction involved in the 3H4 decay were evaluated applying the Inokuti–Hirayama model. Relevance of donor and acceptor transition peculiarities for energy transfer processes contributing to the 1G4 and 3H4 decays was assessed in the framework of phenomenological model proposed by Kushida and discussed. It was concluded that the intensity ratio of the blue 1G4 luminescence to orange–red 5D0 luminescence; hence the colour of visible emission in UV-excited YVO4:Eu3+, Tm3+ phosphor, may be adjusted by appropriate choice of Eu3+ and Tm3+ concentrations.

Impact of ligand framework on the crystal structures and luminescent properties of Cu(I) and Ag(I) clusters and a coordination polymer derived from thiolate/iodide/dppm ligands.

New homoleptic hexanuclear Ag(I) and heteroleptic trinuclear Cu(I) clusters and a Cu(I) coordination polymer (CP) of the formulas [Ag6(dtc)6] 1, [Cu3I2(dppm)3(dtc)] 2, and [Cu(ttc)I]∞ 3 (dtc = N-methylbenzyl-N-methyl-thiophenedithiocarbamate; dppm = 1,1-bis(diphenylphosphino)methane; and ttc = dimethyltrithiocarbonate) were synthesized and characterized by elemental analysis, IR, UV-vis, (1)H, (13)C, and (31)P NMR spectroscopies, and their structures were elucidated by X-ray crystallography. The complexes show interesting structures and luminescent properties. Complex 1, which is centrosymmetric, contains four short Ag···Ag interactions at 2 × 2.966(1) and 2 × 3.014(1) Å. There are also several Ag···Ag distances of 3.3-3.4 Å. The molecule shows hexagonal orientation with alternating silver and sulfur atoms of the overlapping Ag3S3 hexagons in the front and rear, along the a axis. Complex 2 is a rare trinuclear cluster complex of Cu(I); the Cu···Cu distances are 2.906(2), 3.551(2), and 3.338(2) Å, the foremost representing a substantial intermetallic contact. The Cu3I2P6S2 core is comprised of three fused distorted hexagonal rings with the I1 atom located at the center participating in all three rings. Complex 3 is an iodide-bridged CP with a "staircase"-like arrangement in which the Cu(I) is tetrahedrally surrounded by a sulfur atom from the ttc ligand and three iodine atoms. Unlike 3, which is nonluminescent, 1 and 2 are strongly luminescent in the solid and solution at room temperature. The time-resolved emission spectra reveal a triexponential decay curve and short mean lifetime characteristic of fluorescence behavior. Diffuse reflectance spectroscopy revealed semiconducting behavior with band gaps of 2.12, 3.01, and 2.18 eV for 1-3, respectively.

Complete parameterization of temporally and spectrally resolved laser induced fluorescence data with applications in bio-photonics

We present a set of spectrally and temporally resolved clinical fluorescence data—with two separate excitation wavelengths—that was recorded in vivo. We demonstrate that data in the spectral and temporal domains are in certain ways coupled and provide a method for integrated and effective parameterization of spectrally and temporally resolved fluorescence (i.e., time-resolved emission spectra). This parameterization is based on linear algebra, matrix formulation and system identification. We demonstrate how to empirically extract single exponentially decaying components and provide rectified emission spectra without prior knowledge. We investigate the potential for improved cancer diagnostics according to the reduced parameters along the various domains. In this case, in terms of cancer diagnostics, we were unable to identify any benefits of simultaneously measuring both the temporal and spectral properties of the observed fluorescence. However, we note that this may be explained by an important experimental bias present in many studies of optical cancer diagnostics, namely, that, in general, suspected lesions always differ visually from the neighboring healthy tissue.

The Role of Structural Dynamics in Determining the Prion Strain Diversity

Prion proteins exhibit alternate structural states and are associated with a number of devastating transmissible diseases. Recent discoveries have revealed the emerging functional roles of prions in a wide range of organisms. For instance, the self-perpetuating conformational change coupled with amyloid formation of a yeast prion protein, Sup35p, a translational termination factor in yeast, is responsible for novel [PSI+] prion phenotypes in Saccharomyces cerevisiae. The 253-residue NM domain of Sup35p is an intrinsically disordered segment and is sufficient for [PSI+] prion initiation and propagation. The NM amyloid recapitulates one of the most spectacular phenomena of prions, namely, the strain-diversity. Earlier it has been shown that two well-defined strains of [PSI+] can be created in vitro. The molecular origin of these strains is postulated to involve diverse, yet related, conformational states and supramolecular packing of proteins within the amyloid fibrils. However, the precise structural and dynamical variations between the prion strains and their distinct physiological impacts remain elusive. To elucidate the structural origins of the prion strains, we took advantage of the fact that NM is devoid of tryptophan and created 19 single tryptophan mutants encompassing the entire length of NM. After establishing that these mutants behave similar to wild-type, we recorded a number of steady-state and dynamic fluorescence readouts that revealed the residue-specific dynamics and supramolecular packing within the amyloids responsible for different strains. The structural differences in two prion strains provided unique molecular insight into the differential binding of Hsp104 that is known to govern the strain propagation. The strain-diversity was further elucidated using time-resolved emission spectra that provided intriguing insights into the water relaxation dynamics within the amyloid architecture. Taken together, our results provide important biophysical clues in discerning the prion strain-diversity in a residue-specific manner.

Lipid Dynamics of Cardiolipin/DMPC and Cardiolipin/DOPC in Nanodiscs

Cardiolipin is a negatively charged phospholipid that comprises roughly 20% of the lipids in the inner mitochondrial membrane. The dynamics of this membrane can influence interactions between cytochrome c and cardiolipin, which have been shown to initiate the intrinsic pathway of apoptosis. This investigation studies how the dynamics of a model membrane system of varying composition will behave in the presence and absence of cardiolipin. Nanodiscs are composed of varying concentrations of 18:1 cardiolipin and DOPC or DMPC, doped with either NBD-PE or TMA-DPH. (18:1 cardiolipin: 1′,3′-bis[1,2-dioleoyl-sn-glycero-3-phospho]-sn-glycerol DMPC: 1,2-dimyristoyl-sn-glycero-3-phosphocholine DOPC: (Δ9-Cis) PC 1,2-dioleoyl-sn-glycero-3-phosphocholine NBD-PE: 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl) TMA-DPH: trimethylammonium diphenylhexatriene). Time-Resolved Emission Spectra (TRES) of NBD-PE doped nanodiscs are used to report on head group relaxation and Time-Resolved Fluorescence Anisotropy of TMA-DPH doped nanodiscs are used to report on acyl chain motions. These methods are combined to provide insight into how cardiolipin affects lipid membrane dynamics.

Sensitive Detection of NAD+/NADP+ via Strong Coupling Fluorescence from Silver Nanoclusters

The pyridine nucleotides (NAD+, NADP+) are the major coenzymes participate in multiple redox processes in living cells. Both NAD+ and NADP+ are not fluorescent and almost structurally identical, so it is difficult to directly distinguish NAD+ or NADP+ via optical methods (such as fluorescence and Raman spectroscopy). We report here a sensitive probe of NAD+/NADP+ based on fluorescent silver nanoclusters with dual emission band. The silver nanoclusters with an initial fluorescent emission peak at 410 nm were synthesized by etching large size silver nanoparticles. With the addition of NAD+/NADP+ solution, due to the strong coupling (charge-transfer) between silver nanoclusters and ligands (NAD+/NADP+), a new fluorescence emission peak of the silver nanoclusters was found and raised at 550 nm and the fluorescence intensity was dependent on the ratio between NAD+ and NADP+. The time-resolved fluorescence decay (at 550 nm) of silver nanoclusters showed a single-exponential decay lifetime of 3.9 ns caused by the strong coupling between silver nanoclusters and ligands (NAD+/NADP+). Meanwhile, the 410 nm emission was also selectively enhanced by the different ratio of NAD+ /NADP+ molecules. The intensity ratio of fluorescence emission at 410nm and 550nm may be useful to monitor the levels of NAD+ /NADP+ in aqueous solutions, cellular extracts and living cells. Candidate mechanisms and the analysis of time resolved emission spectra will be discussed.

Study of the time-resolved emission spectra of the ejected plume generated by ultrashort laser ablation of graphite

In this paper ultrashort laser pulses with different fluences (18 J/cm2-115 J/cm2) and pulse widths (50 fs-4 ps) are employed to ablate highly oriented pyrolytic graphite in vacuum (4×10-4 Pa). By recording the time-resolved emission spectra of the ablated plume, the ultrafast time evolution of the ablation process is investigated. The Swan bands of C2 radicals, the spectral band near 416 nm which may be assigned to the electronic transition from 1Σu+ to X1Σg+ of C15 clusters, and the emission continuum ranging from 370-700 nm are observed. From the recorded time-resolved emission spectra of the ablated plume, it is seen that at larger time delays only the emission continuum is observed. The decay process of the emission continuum of the plume generated by 50 fs, 115 J/cm2 laser pulses can be divided into a fast decreasing stage (before 20 ns time delay) and a slow decreasing stage (after 20 ns time delay), indicating that the emission continuum may come from two different compositions. During the fast decreasing process, the bremsstrahlung of the ablation-generated carbon plasma contributes to the major part of the continuum; while during the slow decreasing process, the thermal radiation of carbon clusters generated at a later stage of ablation mainly contributes to the continuum. In addition, the existence time of the continuum generated by 50 fs laser pulses increases with the decrease of laser fluence, indicating that laser pulses with lower fluences can generate more carbon clusters at later stages of ablation. It is also found that for the 50 fs pulses, when the laser fluence increases at the early stage of ablation, the quantities of carbon plasma and excited C2 radicals in the plume increase significantly, but the quantity of excited C15 radicals with larger mass only increases slightly. Therefore the laser fluence has a great impact on the concentrations of different compositions in the ejected plume, implying that different material removal mechanisms exist for ablation induced by laser pulses with different laser fluences. Finally, pulse width plays an important role in the time evolution manner of the emission continuum. As the laser pulse width increases, the two-stage decay process of the emission continuum gradually changes into one-stage process, indicating that the existence time intervals of carbon plasma and carbon clusters overlap each other for longer laser pulse width. And the whole evolution process of the emission continuum induced by 4 ps laser pulses is much slower than that induced by 50 fs laser pulses. Longer laser pulse width also causes the decrease of the spectral intensity of C2 radicals, and thus higher laser intensity favors the generation of excited C2 radicals.

A Flexible Paper-Based Microdischarge Array Device: A Novel Route to Cost-Effective and Simple Setup Microplasma Generation Devices

A low-cost and simple-arranged paper-based microplasma array device made by tape and Xerox paper was presented. Uniform array of discharge can be sustained in Ar, He, and ambient air atmosphere. Stable discharge can be sustained for at least 55 min when plasma is ignited in Ar ambient, based on the examination of time-resolved optical emission spectra. Throughout the conditions investigated, the power consumed by the device was well below 0.3 W. We also demonstrated that stable and uniform discharge can be generated when the device was bent to a radius of curvature of 2.5 mm.

Silicon sensitisation using light harvesting layers

Langmuir–Blodgett monolayers of a cyanine dye mixed with stearic acid were deposited on glass and silicon substrates with spacer layers of pure stearic acid monolayers or silicon dioxide films deposited by PECVD. By using the time correlated single photon counting technique, time resolved emission spectra (TRES) and decay curves were measured to characterise the dependence of energy transfer rate on the separation between the dye monolayer and the silicon surface and also for the dye concentrations in the monolayers. We observe interlayer energy transfer between monomers, dimers and higher aggregates present in the monolayer deposited on glass but also competing directly with energy transfer to silicon at close distances. We find that the fluorescence lifetime of the dye monolayer is significantly shortened when deposited close to the silicon surface signifying efficient energy transfer. The dissipation of the excitation energy near silicon is explained using the classical theory developed for metals and a deviation is observed for monolayers deposited at distances close to the silicon surface.

Ultrafast torsional dynamics of Thioflavin-T in an anionic cyclodextrin cavity

Using sub-picosecond fluorescence upconversion spectroscopy, the effect of supramolecular confinement on the ultrafast excited state torsional dynamics of an amyloid fibril sensing dye, Thioflavin-T (ThT), is investigated in a novel cyclodextrin derivative, Sulphobutylether β-cyclodextrin (SBE-β-CD). The encapsulation of ThT inside the nanocavity of SBE-β-CD results in a significant increase in the emission intensity and the excited state lifetime of ThT when compared with native β-CD. Detailed analysis of the time-resolved emission spectra (TRES) shows that the time dependent changes in the integrated area, mean frequency and the width of the emission spectra are appreciably slower as compared with bulk water and native β-CD. These features suggest a significant confinement effect on the ultrafast torsional dynamics of ThT. The ionic strength of the medium significantly affects the complexation of ThT with SBE-β-CD, which indicates that the interaction between the host and the guest is predominantly electrostatic in nature. The results further suggest that the hydrophobic interaction in SBE-β-CD, although minor in contribution in the present case, is comparatively stronger than the native β-CD.

Influence of cooking on the levels of bioactive compounds in Purple Majesty potato observed via chemical and spectroscopic means

Tubers rich in phytochemicals can exhibit a potential health benefit. This work aims at studying the relative effect of different domestic cooking techniques by monitoring the level of total phenolic compounds (TP), total anthocyanins (TA) and anti-oxidant activity (AOA) on a variety of pigmented potatoes. Raw purple potatoes are a good source of anthocyanins (219mg/kgFW) and the level of these compounds increased using different cooking techniques, with the exception of baking. However, the levels of phenolic compounds (originally 209mgGAE/100gFW) decreased in the cooked potatoes. Although potatoes contain different antioxidants in this work the antioxidant activity seems to be related to the levels of phenolic compounds present in the pigmented potato. The fact that some of the compounds present fluoresce enabled both steady state and time-resolved fluorescence techniques to be assessed as a non destructive means of monitoring. This elucidated the presence of different components (via spectral deconvolution and time-resolved emission spectra). Their relative contribution to the fluorescence emission was found to be affected by the different cooking process, with a longer wavelength emission appearing to relate to reflect the presence of anthocyanins.