Fluorescence Decay Rate Research Articles

Fluorescence Quenching-Based Evaluation of Glucose Oxidase Composite with Conducting Polymer, Polypyrrole

Glucose oxidase (GOx) composites with conducting polymers (e.g., polypyrrole (Ppy)) are excellent nanobiomaterials suitable for the design of bioelectronic devices such as biosensors and biofuel cells. Here, we address the spectroscopic properties of GOx, flavin adenine dinucleotide (FAD), and composites of these compounds with polypyrrole (Ppy). The exploration of native GOx and FAD solutions confirmed that about 5% of FAD dissociated from GOx during the period of solution preparation, and this fraction remained constant for 1 month. It has been found that the Ppy, which formed composites with FAD and GOx, facilitated the removal of FAD molecules from GOx and twice reduced the fluorescence decay rate. Differences in the FAD and Ppy average fluorescence relaxation times showed that the FAD composite with Ppy and Ppy effectively quenched the FAD fluorescence and FAD could not freely unfold. The intramolecular electron transfer took place between adenine and isoalloxazine moieties over the first 5 ps after ...

Facile synthesis of CdZnS QDs: Effects of different capping agents on the photoluminescence properties

The luminescent semiconductor QDs are important materials because of applications in lasers, sensors, light-emitting diodes, biomedical labeling, biomedical imaging and solar cells. We have systematically studied the influence of five different capping agents on the structural and optical characteristics of CdZnS quantum dots (QDs) synthesized by the simplest aqueous method. The modified QDs were characterized by the UV–visible spectroscopy, X-ray diffraction, steady-state photoluminescence (PL), time-resolved PL, and transmission electron microscopy. We found that band-gap and PL characteristics of CdZnS QDs can be varied by using different capping agents. The CdZnS QDs capped with ethylene diamine (EDA) and mercaptoacetic acid (MAA) showed high PL intensities when compared to other capping agents. The UV–Vis absorption spectra were used to calculate the optical band-gaps: 2.63 eV (MAA), 2.56 eV (MPA), 2.42 eV (CA), 2.64 eV (Glucose) and 2.62 eV (EDA) of five different QDs: The time-resolved PL spectroscopy indicated slowest bi-exponential fluorescence decay rate for the MAA-capped CdZnS (CdZnS-MAA) QDs.

Micellar Self-Assembly and Probe Exchange Dynamics of Triblock Copolymer Poly(ethylene oxide)–Poly(propylene oxide)–Poly(ethylene oxide) PEO57PPO45PEO57

The dynamics of micellar solutions of a triblock poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) copolymer, PEO57PPO45PEO57 where the subscripts indicate the block length, has been analyzed. This copolymer is characterized by a large monodispersity and hydrophilicity if compared to the previously studied commercially available Pluronic block copolymers. Different experimental techniques were used to characterize its micellar self-assembly, elucidating the existence of a concentration-dependent transition to a “pseudo-percolated” state as the temperature increases in which interconnected polymeric micelles are formed. The kinetics of the micellar assembly and disassembly processes and solute exchange dynamics were monitored by stopped-flow time-scan fluorescence measurements using a fluorescent probe, which points to the involvement of several underlying mechanisms other than simple particle collisions: a slow dynamic regime below 45 °C; a transition regime between 45 and 50 °C; and a fast dynamic one above 50 °C. The slow dynamic regime was associated with fragmentation and fusion processes in competition; the fast one was mainly regulated by a fusion-dominant mechanism in which both concentration and temperature play a role; and finally, a linear increase in the fluorescence decay rate with increasing copolymer concentrations indicated that the transition regime (in which the interconnected micelles are formed) starts to be dominated by micellar fusion, whereas fragmentation becomes less likely. It was observed that the kinetic constant for the fusion process, kfusion, increased ca. twofold as the temperature rises in a concentration-dependent manner, whilst that corresponding to the fragmentation process, kfragmentation, increases until ca. 50 °C (close to the transition temperature, Tt), and then, it decreases independently of the micellar concentration. Above the transition temperature, kfragmentation and kfusion are related to two apparent activation energies of similar magnitude but different sign. Finally, the kinetic constants of the micellar fragmentation and fusion processes were influenced by the PEO/PPO ratio when compared to the previously analyzed structure-related commercial Pluronic copolymers.

Hyperspectral, Time-Resolved, Fluorescence Imaging System for Large Sample Sizes: Part II. Detection of Fecal Contamination on Spinach Leaves

Abstract. To reduce the risk of foodborne illnesses resulting from fecal contamination in produce fields, a hyperspectral, line-scan, laser-induced fluorescence imaging system was developed with the goal of eventually incorporating the imaging system into a pre-harvest detection apparatus for fecal contamination. The imaging system includes an intensified, gated camera, a spectral adapter, a 355 nm pulsed laser, and laser expansion optics that produce a line-illumination profile. To validate and test the system, spinach leaves inoculated with dilutions of bovine fecal material were imaged repeatedly using a predefined set of imaging parameters. These images were used to evaluate methods for detecting fecal contamination. Methods investigated included ratio, edge, threshold, and slope detection. Differences in the magnitude of averaged intensities for the spectral range of 450-500 nm for regions within fecal contamination sites and in nearby uncontaminated surface areas suggested that the 450-500 nm waveband would be a good region for use in detection tests. Validation tests that used threshold or slope detection, the 450-500 nm waveband, and that took advantage of the slower fluorescence decay rates of fecal contamination sites relative to uncontaminated surface areas showed almost 100% detection of 1:2, 1:10, and 1:100 dilution sites and over 70% detection of 1:200 dilution sites with essentially zero false positives. These results suggest that the imaging system has potential for development of a commercially viable apparatus for pre-harvest detection of fecal contamination in produce fields and for detection of fecal contamination of leafy green vegetables in general. Keywords: Fecal detection, Fluorescence imaging, Food safety, Machine vision, Spinach, Time-resolved imaging.

The photoacidity of phenol chloro benzoate cyanine picolinium salt photoacid in alkanols

Abstract Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) rate to methanol, ethanol and propanol of a new photoacid, the chloro benzoate phenol cyanine picolinium salt (CBCyP). We found that the ESPT rate constants for methanol, ethanol and propanol are about 3 × 1012 s−1, 2 × 1012 s−1 and 1.2 × 1012 s−1 respectively, whereas for water it is 6 × 1012 s−1. The photoacid pKa* in water is about pKa ∼ −7. The kinetic isotope effect as measured from the fluorescence decay rate of the protonated form is 1.5 and 1.25 for methanol and ethanol respectively, whereas in water it is 1.7. We suggest that a nonradiative process takes place and reduces the measured kinetic isotope effect in both methanol and ethanol.

Surface Charges on CdSe-Dot/CdS-Rod Nanocrystals: Measuring and Modeling the Diffusion of Exciton-Fluorescence Rates and Energies.

By performing spectroscopic single-particle measurements at cryogenic temperatures over the course of hours, we study both the spectral diffusion as well as the diffusion of the decay rates of the fluorescence emission of core/shell CdSe/CdS dot/rod nanoparticles. A special analysis of the measurements allows for a correlation of data for single neutral excitons only, undisturbed by the possible emission of other excitonic complexes. We find a nearly linear dependency of the fluorescence decay rate on the emission energy. The experimental data are compared to self-consistent model calculations within the effective-mass approximation, in which migrating point charges set onto the surface of the nanoparticles have been assumed to cause the temporal changes of optical properties. These calculations reveal a nearly linear relationship between the squared electron-hole wave function overlap, which is linked to the experimentally determined fluorescence rate, and the exciton emission energy. Within our model, single migrating surface charges are not sufficient to fully explain the measured rather broad ranges of emission rates and energies, while two-and in particular negative-surface charges close to the core of the DR induce large enough shifts. Importantly, for our nanoparticle system, the surface charges more strongly affect the hole wave function than the electron wave function and both wave functions are still localized within the dot-like core of the nanoparticle, showing that the type-I character of the band alignment between core and shell is preserved.

Aptamer adaptive binding assessed by stilbene photoisomerization towards regenerating aptasensors

Fluorescent aptasensors are reliant on static fluorescence intensity measurements, which suffer drawbacks such as background interference and laborious separation procedures. A unique aptasensor based on the photochrome aptamer switch assay (PHASA) has been developed that is independent of background fluorescence, requires no analyte separation, and allows rapid quantification within seconds. Malachite green aptamer (MGA) conjugated with a water-soluble stilbene on the MGA 3′ C38 terminus was chosen for building the proof-of-concept aptasensor. In the presence of malachite green and tetramethylrosamine ligands, the rate of the stilbene fluorescence decay was found to be linearly dependent on the ligand concentration. Molecular dynamic simulation suggests hydrogen bonding between stilbene sulfonates and neighboring nucleotides is the primary mechanism responsible for rate changes in stilbene photoisomerization. Analysis of the apparent fluorescence decay rate (kapp) versus analyte concentration gives a limit of detection (LOD) of 2μM for MG and 0.6μM for TMR. This aptasensor design opens up a new sensing mode, which is promising for rapid development of SELEX generated molecular recognition elements.

Protolytic behavior of axially coordinated hydroxy groups of Tin(IV) porphyrins as promising molecular catalysts for water oxidation

A series of four Tin(IV)porphyrins with different substituents on meso-phenyl groups, trans-dihydroxy-5,10,15,20-tetra(p-tolyl)porphyrinatetin(IV) (Sn(IV)TTP(OH)2), trans-dihydroxy-tetra(2,4,6-trimethyl)phenylporphyrinatetin(IV) (Sn(IV)TMP(OH)2), trans-dihydroxy-5,10,15,20-tetra(p-carboxyphenyl)porphyrinatetin(IV) (Sn(IV)TCPP), and trans-dihydroxy-5,10,15,20-tetra(N-methyl-4′-pyridiniumyl)porphyrinatetin(IV) (Sn(IV)TMPyP(OH)2) have been prepared and characterized focusing on their protolytic behavior of the axial ligands, hydroxy groups, both in the ground and excites states as well as their electrochemical oxidation behavior. The two axially ligating hydroxy groups on each Tin(IV)-porphyrins exhibit protonations and deprotpnations in five steps. The pKa value of each protolytic equilibrium in the ground states was spectrophotometrically determined, while fluorescence decay time profiles indicate that the protolytic processes are sufficiently slower than the intrinsic fluorescence decay rates to lead to non-protolysis in the excited states. The oxidation peak potentials were determined by cyclic voltammetric analysis. Pourbaix diagram of each protolytic species of Sn(IV)TMPyP in water indicated that the one-electron oxidation of fully deprotonated Sn(IV)TMPyP(O−)2 is not a proton-coupled process, while the doubly hydroxy coordinated species Sn(IV)TMPyP(OH)2 has a proton-coupled one. Tin(IV)-porphyrins exhibit efficient catalytic currents on the first oxidation wave in their cyclic voltammetry and the saturated catalytic currents were high enough in a wide range of pH to be promising as water oxidation catalysts. This is the first detailed study on Tin(IV)-porphyrins to examine water oxidation ability of the one-electron oxidized form against twenty kinds of species (five protolytic species for each of the four Tin(IV)-porphyrins).

Patterned multilayer metamaterial for fast and efficient photon collection from dipolar emitters.

Solid-state quantum emitters are prime candidates for the realization of fast, on-demand single-photon sources. The improvement in photon emission rate and collection efficiency for point-like emitters can be achieved by using a near-field coupling to nanophotonic structures. Plasmonic metamaterials with hyperbolic dispersion have previously been demonstrated to significantly increase the fluorescence decay rates from dipolar emitters due to a large broadband density of plasmonic modes supported by such metamaterials. However, the emission coupled to the plasmonic modes must then be outcoupled into the far field before it succumbs to ohmic losses. We propose a nano-grooved hyperbolic metamaterial that improves the collection efficiency by several times compared to a conventional planar lamellar hyperbolic metamaterial. Our approach can be utilized to achieve broadband enhancement of emission for diverse types of quantum emitters.

Superradiance from Two Dimensional Brick-Wall Aggregates of Dye Molecules: The Role of Size and Shape for the Temperature Dependence.

Aggregates of interacting molecules can exhibit electronically excited states that are coherently delocalized over many molecules. This can lead to a strong enhancement of the fluorescence decay rate which is referred to as superradiance (SR). To date, the temperature dependence of SR is described by a 1/T law. Using an epitaxial dye layer and a Frenkel-exciton based model we provide both experimental and theoretical evidence that significant deviations from the 1/T behavior can occur for brick-wall-type aggregates of finite size leading even to a maximum of the SR at finite temperature. This is due to the presence of low energy excitations of weak or zero transition strength. These findings are relevant for designing light-emitting molecular materials.

Quantitatively Understanding Plant Signaling: Novel Theoretical-Experimental Approaches.

With the need to respond to and integrate a multitude of external and internal stimuli, plant signaling is highly complex, exhibiting signaling component redundancy and high interconnectedness between individual pathways. We review here novel theoretical-experimental approaches in manipulating plant signaling towards the goal of a comprehensive understanding and targeted quantitative control of plant processes. We highlight approaches taken in the field of synthetic biology used in other systems and discuss their applicability in plants. Finally, we introduce existing tools for the quantitative analysis and monitoring of plant signaling and the integration of experimentally obtained quantitative data into mathematical models. Incorporating principles of synthetic biology into plant sciences more widely will lead this field forward in both fundamental and applied research.

Fluorescence decay of naphthalene studied in an electrostatic storage ring, the Mini-Ring

The cooling of naphthalene cations (C10H8)+ has been studied in a compact electrostatic ion storage ring, the Mini-Ring. A nano second laser pulse of 532nm (2.33eV) was used to probe the internal energy distribution every millisecond during the storage time up to 5ms. The evolution of the internal energy distribution of the stored ions was simulated with a model taking into account the dissociation and the radiative decay processes. Calculated decay curves were fitted to the corresponding laser induced neutral decays. For a laser power of 200μJ/pulse, a good agreement between experiment and modeling was found using an initial Gaussian energy distribution centered to 5.9eV and a fluorescence decay rate varying from 200 to 300s−1 in the energy range from 6 to 7eV. This fast decay was attributed to the delayed Poincaré fluorescence process.

Magnesium as a Novel UV Plasmonic Material for Fluorescence Decay Rate Engineering in Free Solution

We report modification of the ultraviolet (UV) fluorescence decay rate of p-terphenyl dye molecules by magnesium (Mg) nanoapertures in free solution, in comparison with aluminum (Al). Mg nanoapertures exhibit a lifetime reduction of up to ∼7.2×, which to our knowledge is the largest lifetime reduction of a UV fluorescence dye reported so far in the literature. In comparison, Al nanoapertures exhibit a lifetime reduction of ∼5.3×, exceeding the previously reported value ∼3.5× due to smaller aperture size employed here. Simulation results reveal large plasmon resonance frequency shifts between Mg and Al nanoapertures, contributing to different lifetime reductions, where average lifetime reductions of Mg nanoapertures are greater than those of Al nanoapertures with diameters smaller than 40 nm. The dependence of count rate per molecule (CRM) on aperture size and aperture undercut is also reported, revealing that CRM increases with increasing undercut.

Development of Far-Red and Ultraviolet Digital Frequency-Domain Flow Cytometry Systems

Flow cytometry is a powerful single cell analysis technique that uses fluorescence and light scatter to characterize differences (i.e., phenotype, genotype, morphology, etc.) in large cell populations. Digital excited state lifetime measurements with flow cytometry have been largely underutilized; the measurement of fluorescence decay rates by molecules in or on the surface of cells is challenging to accurately measure when the samples are moving rapidly through the path of a laser excitation source. In past work, we have shown that time-resolved cytometry can provide accurate measurements of the fluorescence lifetime of organic fluorophores at standard cytometric throughputs (∼1000 cells/s). Herein, we advance our digital laser modulation and signal processing approaches to develop cytometry systems that operate across a broader spectral range. We optimized a far-red and ultraviolet (UV) cytometry instruments and present fluorescence microsphere results to determine detection sensitivity. The expansion of time-resolved systems across a wider color spectrum is important for many new applications including the development of near-infrared fluorescent proteins and the detection of UV-excitable autofluorescence in single cells.

Spatial correlations of the spontaneous decay rate as a probe of dense and correlated disordered materials

We study theoretically and numerically a new kind of spatial correlation for waves in disordered media. We define CΓ as the correlation function of the fluorescent decay rate of an emitter at two different positions inside the medium. We show that the amplitude and the width of CΓ provide decoupled information on the structural correlation of the disordered medium and on the local environment of the emitter. This result may stimulate the emergence of new imaging and sensing modalities in complex media.

Four- and Five-Component Syntheses and Photophysical Properties of Emission Solvatochromic 3-Aminovinylquinoxalines

3-Aminovinylquinoxalines are readily accessible from (hetero)aryl glyoxylic acids or heterocyclic π-nucleophiles by consecutive four- and five-component syntheses in the sense of an activation-alkynylation-cyclocondensation-addition sequence or glyoxylation-alkynylation-cyclocondensation-addition sequence in good yields. The title compounds are highly fluorescent with pronounced emission solvatochromicity and protochromic fluorescence quenching. Time-resolved fluorescence spectroscopy furnishes radiative and nonradiative fluorescence decay rates in various solvent polarities. The electronic structure is corroborated by DFT and TD-DFT calculations rationalizing the observed spectroscopic effects.

Optimizing Cr3+ concentration and evaluating energy transfer from Cr3+ to Nd3+ in Cr, Nd:GGG nanocrystals prepared by sol-gel method

Nanopowder of Cr:GGG and nanopowder of Cr, Nd:GGG with different concentrations of Cr3+ ranging from 0.1 at.% to 1.5 at.% were synthesized by the sol-gel method using acetic acid and ethylene glycol. Thermal gravimetric analysis and differential scanning calorimetry (TGA-DSC), X-ray diffraction (XRD) and photoluminescence spectroscopy were used to characterize the powder. The crystallite size was about 58 nm when treated at 1000 °C for 2 h. Cr3+ photoluminescence spectrum in GGG showed a broad band emission around 730 nm. The intensity of this band decreased when co-doped with Nd, indicating an efficient energy transfer from Cr3+ to Nd3+. Photoluminescence intensity of Nd in Cr, Nd:GGG at 1.06 μm showed that the optimum concentration of Cr3+ was about 1 at.% (more or less) for 1 at.% Nd3+. This result was also confirmed by chromium fluorescence decay rate analysis. Energy transfer efficiency was found to be about 84% for 1 at.% concentration of each chromium and neodymium.

Fluctuations of the electromagnetic local density of states as a probe for structural phase switching

We study the statistics of the fluorescence decay rates for single quantum emitters embedded in a scattering medium undergoing a phase transition. Under certain circumstances, the structural properties of the scattering medium explore a regime in which the system dynamically switches between two different phases. While in that regime the light scattering properties of both phases are hardly distinguishable, we demonstrate that the lifetime statistics of single emitters with low diffusivity is clearly dependent on the dynamical state in which the medium evolves. Hence, lifetime statistics provides clear signatures of phase switching in systems where light scattering does not.

Superradiance in a Large and Dilute Cloud of Cold Atoms in the Linear-Optics Regime.

Superradiance has been extensively studied in the 1970s and 1980s in the regime of superfluorescence, where a large number of atoms are initially excited. Cooperative scattering in the linear-optics regime, or "single-photon superradiance," has been investigated much more recently, and superradiant decay has also been predicted, even for a spherical sample of large extent and low density, where the distance between atoms is much larger than the wavelength. Here, we demonstrate this effect experimentally by directly measuring the decay rate of the off-axis fluorescence of a large and dilute cloud of cold rubidium atoms after the sudden switch off of a low-intensity laser driving the atomic transition. We show that, at large detuning, the decay rate increases with the on-resonance optical depth. In contrast to forward scattering, the superradiant decay of off-axis fluorescence is suppressed near resonance due to attenuation and multiple-scattering effects.

Enhanced Radiative Decay Rate of Confined Green Fluorescent Protein in Polyvinylpyrrolidone-Based Nanofiber

Green fluorescent protein (GFP) molecules are encapsulated by polyvinylpyrrolidone material in the form of nanofibers to study their diameter dependence of the fluorescence decay rate. Fluorescence dynamics of the confined GFP is governed by the Purcell effect. It is demonstrated that the electrospun nanofibers are quite controllable geometries and are suitable local photonic environments for exploring such effects. The chromophore of GFP, responsible for the intense green fluorescence, is attached to the α helix and perfectly surrounded by an 11-stranded β-barrel cylinder. It is clearly observed that the molecular structures of the confined GFP protein molecules are well protected and are able to maintain their fluorescence properties.