Slow Lifetime Research Articles

A steady-state and time-resolved fluorescence study on liposome-calf thymus DNA interaction: probed by an anticancer drug ellipticine

Interaction of calf thymus DNA (CT-DNA) with two liposomes made of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and its impact on an encapsulated anticancer drug ellipticine have been studied by steady-state and time-resolved fluorescence spectroscopy. Addition of DNA to the ellipticine impregnated liposomes causes quenching in the fluorescence intensity as DNA induces subsequent hydration and rupture of the bilayers leading to the leakage of drug molecules. We have found higher quenching in DPPC than that in DMPC liposomes at both below and above their respective phase transition temperature. Since DPPC remains less prehydrated due to its higher phase transition temperature than DMPC, therefore DNA induced hydration is higher in DPPC and this results in higher quenching. The time-resolved studies revealed that both the fast and slow lifetime components of ellipticine decrease significantly with addition of DNA due to DNA induced hydration and rupture of bilayers. Above the phase transition temperature, no such change in lifetime components takes place as bilayers already remain prehydrated and further DNA induced hydration does not occur. It is revealed that with addition of DNA to liposomes at both the temperatures, the amplitude of the fast component increases and that of slow component decreases which implies that the drug molecules are migrating to the aqueous phase, leaving the liposome hydrophobic core. The extent of migration of drug molecules is much more pronounced in DPPC compared to that in DMPC.

Poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] and poly[6-fluoro-3-(pyridin-2-yl)-3,4-dihydro-2H-benzoxazine] based polymer electrolyte membranes for fuel cells at elevated temperature

Cross-linked copolymer membranes were prepared by casting poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (PBI) solutions containing 6-fluoro-3-(pyridin-2-yl)-3,4-dihydro-2H-benzoxazine (pF) in N,N-dimethylacetamide, with subsequent step-wise heating to 220 °C. The phosphoric acid (PA) content, proton conductivity, and cell performance were found to increase with the increase in benzoxazine content in these membranes. The proton conductivities of the cross-linked copolymer membranes reached 0.09 S cm−1 at 150 °C under anhydrous conditions. Membrane-electrode assemblies (MEAs), prepared using the cross-linked copolymer membranes, showed larger operating voltage, 0.71 V at 0.2 A cm−2, than those from commercially available PBI membranes. Additionally, the MEAs were durable until 1,077 cycles, with slow lifetime loss rate, −0.14 mV h−1 on in situ accelerated lifetime mode (load cycling test).

Advances in ultrafast time resolved fluorescence physics for cancer detection in optical biopsy

We discuss the use of time resolved fluorescence spectroscopy to extract fundamental kinetic information on molecular species in tissues. The temporal profiles reveal the lifetime and amplitudes associated with key active molecules distinguishing the local spectral environment of tissues. The femtosecond laser pulses at 310 nm excite the tissue. The emission profile at 340 nm from tryptophan is non-exponential due to the micro-environment. The slow and fast amplitudes and lifetimes of emission profiles reveal that cancer and normal states can be distinguished. Time resolved optical methods offer a new cancer diagnostic modality for the medical community.

Monitoring Photosensitizer Uptake Using Two Photon Fluorescence Lifetime Imaging Microscopy

Photodynamic Therapy (PDT) provides an opportunity for treatment of various invasive tumors by the use of a cancer targeting photosensitizing agent and light of specific wavelengths. However, real-time monitoring of drug localization is desirable because the induction of the phototoxic effect relies on interplay between the dosage of localized drug and light. Fluorescence emission in PDT may be used to monitor the uptake process but fluorescence intensity is subject to variability due to scattering and absorption; the addition of fluorescence lifetime may be beneficial to probe site-specific drug-molecular interactions and cell damage. We investigated the fluorescence lifetime changes of Photofrin® at various intracellular components in the Mat-LyLu (MLL) cell line. The fluorescence decays were analyzed using a bi-exponential model, followed by segmentation analysis of lifetime parameters. When Photofrin® was localized at the cell membrane, the slow lifetime component was found to be significantly shorter (4.3 ± 0.5 ns) compared to those at other locations (cytoplasm: 7.3 ± 0.3 ns; mitochondria: 7.0 ± 0.2 ns, p < 0.05).

Photoluminescence dynamics of organic molecule-passivated Si nanoclusters

The size-selected phenyl-passivated Si nanoclusters with the mean diameters of 1.5 and 1.3 nm have been prepared in the solution route. The intense PL observed in Si nanoclusters, in which the band gap energies increase up to approximately 4 eV, originates from electron-hole pair recombination. From time-resolved PL, two kinds of exponential PL decay components with fast and slow lifetime were observed. The identical decays with slow lifetime will be derived from the surface states of Si nanoclusters passivated by phenyl molecules. The size-dependent zero-phonon assisted optical transitions by quantum size effects occur at a high rate of sub-nanosecond timescale.

Time‐resolved photoluminescence of Al‐rich AlGaN/AlN quantum wells under selective excitation

AbstractWe performed time‐resolved photoluminescence (TRPL) measurements for high‐quality Al‐rich AlGaN/AlN multiple quantum wells (MQWs) under selective excitation condition in order to investigate carrier recombination dynamics around the Mott transition. Fast and slow lifetime components (12 ps and 210 ps) were definitely observed under high excitation conditions at 5.5 K and were attributed to electron‐hole plasma and the exciton many‐body effect, respectively. It was found that the remarkable difference in the radiative lifetimes caused strongly excitation‐power dependent internal quantum efficiencies. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Quenched-in vacancies in Fe-Al alloys

Quenched-in vacancies in Fe-Al alloys with Al content ranging from 24 to 49 at.% were investigated employing two complementary techniques of positron annihilation: slow positron implantation spectroscopy and positron lifetime measurements. It was found that quenched alloys exhibit a very high concentration of vacancies. Although the free positron component cannot be resolved in positron lifetime spectrum in majority of samples, the concentration of quenched-in vacancies can be still determined from the positron diffusion length measured by a variable energy positron beam. The lowest concentration of vacancies was found in a stoichiometric (SM) Fe3Al alloy. The concentration of defects increases with increasing degree of non-stoichiometry with respect to Fe3Al, i.e. in alloys with under-SM and over-SM Al concentration. However, the increase in concentration of quenched-in defects is more pronounced in Al-rich alloys, i.e. alloys containing more than 25 at.% of Al.

Excess carrier recombination lifetime of bulk n-type 3C-SiC

Transient absorption technique was used to determine carrier lifetimes in 3C-SiC grown on Si and 6H-SiC substrates. A slow lifetime component originated from minority carrier traps and pointed out to the trap saturation with increasing injection. Recombination lifetime in different samples varied between 0.5–120 ns. Its value decreased with excess carrier density in the transition range between minority-carrier-lifetime and high-injection lifetime but abnormally increased above the carrier density of 2×1017 cm−3. Negligible contribution of surface and Auger recombination to recombination lifetime peculiarities was observed. Possible mechanisms of the observed lifetime variation are discussed.

Photoresponse of electrical conductivity in an organic superconductor investigated by time-resolved resistance measurements

Photoirradiation effect on the electrical conductivity has been examined for an organic superconductor κ-(BEDT-TTF) 2Cu[N(CN) 2]Br, where BEDT-TTF represents bis(ethylenedithio)tetrathiafulvalene, based on the time-resolved resistance measurements. Visible laser pulses with a wavelength of 532 nm were irradiated on the crystal surface, and the transient voltage signals induced by photoirradiation in the current-biased sample were monitored to obtain the transient time profile of the resistance. The increase of the resistance upon photoirradiation is observed with a slow decay lifetime of the order of millisecond both in the metallic phase and in the superconducting phase. The time profiles are remarkably different from each other, and a slow rise was observed only in the time profiles of the superconducting phase.

Highly fluorescing solid DNA-cationic polyelectrolyte complexes prepared from a natural DNA and a poly(fluorenevinylene-alt-phenylene) bearing quaternary ammonium pendants

A fluorescing, copolymer (Q)-bearing, quaternary ammonium pendant was mixed with excess natural salmon sperm DNA with a molecular weight of 1.3×106 (2,000 base pairs) to afford highly fluorescing, complex mixtures. The fluorescence life-time of the polymer Q was greatly increased when mixed with DNA: for the mixture of Q:DNA=1:750 the fast and slow decay lifetimes increased from ca. 10 to 100 ps and from 20 ps to ca. 1 ns, respectively. The enhanced fluorescence of the mixtures was ascribed to efficient compartmentalization and reduced conformational relaxation of the polymer Q by complexation with excess DNA.

One-dimensional Silicon−Cadmium Selenide Heterostructures

We report the synthesis and characterization of 1D Si-CdSe heteronanostructures with different morphologies such as coaxial, biaxial, sandwiched, pattern wrapping, coiling, structures etc., via a one-step metal catalyzed thermal evaporation method. Both Si and CdSe exhibit single crystalline characteristics in the heterostructures, as revealed by scanning transmission electron microscopy. The Si nanowires formed directly from the Si substrate via the solid-liquid-solid process acts as the absorption site for CdSe deposition as well as the template for the formation of 1D Si-CdSe heterostructures. Time-resolved X-ray excited optical luminescence from the 1D Si-CdSe heteronanostructures reveals two main emission features at 530 and 637 nm with slow and fast decay lifetime, respectively. The 530 and 637 nm emission is associated with the Si and CdSe component of the heterostructures, respectively.

Fluorescence lifetime imaging microscopy of Chlamydomonas reinhardtii: non‐photochemical quenching mutants and the effect of photosynthetic inhibitors on the slow chlorophyll fluorescence transient

Fluorescence lifetime-resolved images of chlorophyll fluorescence were acquired at the maximum P-level and during the slower transient (up to 250 s, including P-S-M-T) in the green photosynthetic alga Chlamydomonas reinhardtii. At the P-level, wild type and the violaxanthin-accumulating mutant npq1 show similar fluorescence intensity and fluorescence lifetime-resolved images. The zeaxanthin-accumulating mutant npq2 displays reduced fluorescence intensity at the P-level (about 25-35% less) and corresponding lifetime-resolved frequency domain phase and modulation values compared to wild type/npq1. A two-component analysis of possible lifetime compositions shows that the reduction of the fluorescence intensity can be interpreted as an increase in the fraction of a short lifetime component. This supports the important photoprotection function of zeaxanthin in photosynthetic samples, and is consistent with the notion of a 'dimmer switch'. Similar, but quantitatively different, behaviour was observed in the intensity and fluorescence lifetime-resolved imaging measurements for cells that were treated with the electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, the efficient PSI electron acceptor methyl viologen and the protonophore nigericin and. Lower fluorescence intensities and lifetimes were observed for all npq2 mutant samples at the P-level and during the slow fluorescence transient, compared to wild type and the npq1 mutant. The fluorescence lifetime-resolved measurements during the slow fluorescence changes after the P level up to 250 s for the wild type and the two mutants, in the presence and absence of the above inhibitors, were analyzed with a graphical procedure (polar plots) to determine lifetime compositions. At higher illumination intensity, wild type and npq1 cells show a rise in fluorescence intensity and corresponding rise in the species concentration of the slow lifetime component after the initial decrease following the P level. This reversal is absent in the npq2 mutant, and for all samples in the presence of the inhibitors. Lifetime heterogeneities were observed in experiments averaged over multiple cells as well as within single cells, and these were followed over time. Cells in the resting state (induced by several hours of darkness), instead of the normal swimming state, show shortened lifetimes. The above results are discussed in terms of a superposition of effects on electron transfer and protonation rates, on the so-called 'State Transitions', and on non-photochemical quenching. Our data indicate two major populations of chlorophyll a molecules, defined by two 'lifetime pools' centred on slower and faster fluorescence lifetimes.

Slow luminescence from Er3+ centers in Er-Doped GeGaSe chalcogenide glasses observed by wideband quadrature frequency-resolved spectroscopy

Wideband QFRS from 2 ns to 160 s of photoluminescence (PL) from Er-doped GeGaSe glasses reveals a PL component peaked at a slow lifetime τH ≈ 16 s at temperatures 3.7 and 294 K besides the prominent Er3+ emission peak at τEr ≈ 3.3 ms. The τH component has sharp PL peaks at photon energies ∼0.8 and ∼1.3 eV. On the other hand, the QFRS spectrum of the undoped sample has three peaks at 3.7 K; the first two peaks at ∼20 ns and ∼90 μs are associated with singlet and triplet excitons, and the slowest peak has a lifetime τH ≈ 16 s similar to that observed in the Er-doped sample. IR-induced quenching of the slow components of both the undoped and doped samples has been observed, suggesting a type of slow emission related to tunneling between tail-trapped photocarriers and D0 centers. The experimental results suggest also that the defect-related Auger effect probably mediates the energy transfer from precursors to Er3+ centers.

Functional Heterogeneity of Photosystem II in Domain Specific Regions of the Thylakoid Membrane of Spinach (Spinacia oleracea L.)

A mild sonication and phase fractionation method has been used to isolate five regions of the thylakoid membrane in order to characterize the functional lateral heterogeneity of photosynthetic reaction centers and light harvesting complexes. Low-temperature fluorescence and absorbance spectra, absorbance cross-section measurements, and picosecond time-resolved fluorescence decay kinetics were used to determine the relative amounts of photosystem II (PSII) and photosystem I (PSI), to determine the relative PSII antenna size, and to characterize the excited-state dynamics of PSI and PSII in each fraction. Marked progressive increases in the proportion of PSI complexes were observed in the following sequence: grana core (BS), whole grana (B3), margins (MA), stroma lamellae (T3), and purified stromal fraction (Y100). PSII antenna size was drastically reduced in the margins of the grana stack and stroma lamellae fractions as compared to the grana. Picosecond time-resolved fluorescence decay kinetics of PSII were characterized by three exponential decay components in the grana fractions, and were found to have only two decay components with slower lifetimes in the stroma. Results are discussed in the framework of existing models of chloroplast thylakoid membrane lateral heterogeneity and the PSII repair cycle. Kinetic modeling of the PSII fluorescence decay kinetics revealed that PSII populations in the stroma and grana margin fractions possess much slower primary charge separation rates and decreased photosynthetic efficiency when compared to PSII populations in the grana stack.

Characterization of a SiC/SiC composite by X-ray diffraction, atomic force microscopy and positron spectroscopies

A SiC/SiC composite is characterized by X-ray diffraction, atomic force microscopy and various positron spectroscopies (slow positron implantation, positron lifetime and re-emission). It is found that besides its main constituent 3C–SiC the composite still must contain some graphite. In order to better interpret the experimental findings of the composite, a pyrolytic graphite sample was also investigated by slow positron implantation and positron lifetime spectroscopies. In addition, theoretical calculations of positron properties of graphite are presented.

Tracking Unfolding and Refolding of Single GFPmut2 Molecules

The unfolding and refolding kinetics of >600 single GFPmut2 molecules, entrapped in wet nanoporous silica gels, were followed by monitoring simultaneously the fluorescence emission of the anionic and neutral state of the chromophore, primed by two-photon excitation. The rate of unfolding, induced by guanidinium chloride, was determined by counting the number of single molecules that disappear in fluorescence images, under conditions that do not cause bleaching or photoinduced conversion between chromophore protonation states. The unfolding rate is of the order of 0.01 min −1, and its dependence on denaturant concentration is very similar to that previously reported for high protein load gels. Upon rinsing the gels with denaturant-free buffer, the GFPmut2 molecules refold with rates >10 min −1, with an apparently random distribution between neutral and anionic states, that can be very different from the preunfolding equilibrium. A subsequent very slow (lifetime of ∼70 min) relaxation leads to the equilibrium distribution of the protonation states. This mechanism, involving one or more native-like refolding intermediates, is likely rate limited by conformational rearrangements that are undetectable in circular dichroism experiments. Several unfolding/refolding cycles can be followed on the same molecules, indicating full reversibility of the process and, noticeably, a bias of denaturated molecules toward refolding in the original protonation state.

Enhanced septahedral ordering in cold Lennard-Jones fluids

We report molecular dynamics calculations on a two-component, cold, 15,625 particle, three-dimensional Lennard-Jones fluid. We calculated spherical harmonic components Q(LM) for the density of particles in the first coordination shell of each particle, as well as their spherical invariants <Q2L>, time-correlation functions, and wavelet density decompositions. The spherical invariants show that noncrystalline septahedral (<Q27>) ordering is important, especially at low temperature. While < Q210> could arise from icosahedral ordering, its behavior so closely tracks that of the nonicosahedral <Q211> that alternative origins for <Q210> need to be considered. Time correlation functions of spherical harmonic components are bimodal, with a faster temperature-independent mode and a slow, strongly temperature-dependent mode. Volume wavelet decompositions show that when T is reduced, the correlation length of <Q27> increases, especially below T=0.7 , but that the correlation length of <Q25> is independent of T .

Investigation of the Temperature Dependence of Free Volume in Polymethylpentene by Positron Annihilation Method

Positron lifetime measurements in polymethylpentene were performed in a wide temperature range. A conventional fast{slow coincidence lifetime spectrometer with plastic scintillators was used. All the measurements were made in vacuum. Additional measurements (X-ray difiraction, difierential scanning calorimetry, and mechanical spectroscopy) were also performed to establish properties of samples. Mean free volume radii in the investigated samples were estimated from the positron lifetimes measurements results. An increase in I3 with the elapsed time of measurements was observed.

Thermochromatium tepidumPhotoactive Yellow Protein/Bacteriophytochrome/Diguanylate Cyclase: Characterization of the PYP Domain

The purple phototrophic bacterium, Thermochromatium tepidum, contains a gene for a chimeric photoactive yellow protein/bacteriophytochrome/diguanylate cyclase (Ppd). We produced the Tc. tepidum PYP domain (Tt PYP) in Escherichia coli, and found that it has a wavelength maximum at 358 nm due to a Leu46 substitution of the color-tuning Glu46 found in the prototypic Halorhodospira halophila PYP (Hh PYP). However, the 358 nm dark-adapted state is in a pH-dependent equilibrium with a yellow species absorbing at 465 nm (pK(a) = 10.2). Following illumination at 358 nm, photocycle kinetics are characterized at pH 7.0 by a small bleach and red shift to what appears to be a long-lived cis intermediate (comparable to the I(2) intermediate in Hh PYP). The recovery to the dark-adapted state has a lifetime of approximately 4 min, which is approximately 1500 times slower than that for Hh PYP. However, when the Tt PYP is illuminated at pH values above 7.5, the light-induced difference spectrum indicates a pH-dependent equilibrium between the I(2) intermediate and a red-shifted 440 nm intermediate. This equilibrium could be responsible for the sigmoidal pH dependence of the recovery of the dark-adapted state (pK(a) = 8.8). In addition, the light-induced difference spectrum shows that, at pH values above 9.3, there is an apparent bleach near 490 nm superimposed on the 358 and 440 nm changes, which we ascribe to the equilibrium between the protonated and ionized dark-adapted forms. The L46E mutant of Tt PYP has a wavelength maximum at 446 nm, resembling wild-type Hh PYP. The kinetics of recovery of L46E following illumination with white light are slow (lifetime of 15 min at pH 7), but are comparable to those of wild-type Tt PYP. We conclude that Tt PYP is unique among the PYPs studied to date in that it has a photocycle initiated from a dark-adapted state with a protonated chromophore at physiological pH. However, it is kinetically most similar to Rhodocista centenaria PYP (Ppr) despite the very different absorption spectra due to the lack of E46.

Time-resolved photoluminescence in a-SiNx:H〈Nd〉 planar waveguides: evidence for stimulated emission

Abstract We performed lifetime measurements on the 1128 nmNd3+ emission from a neodymium-doped amorphous hydrogenated silicon sub-nitride (a SiNx:H〈Nd〉) planar waveguide. The 1.5 μm thick sample was prepared by reactive rf-sputtering. Lifetime measurements were performed exciting with a multiline Ar+ laser. The sample temperature was varied between 25 and 300 K, and the excitation power between 0.2 and 8 kW/cm2. In all measurement conditions the luminescence decay can be expressed by two exponentials. The fast decay has a lifetime between 40 and 60 μs and the slow decay has a lifetime between 1 and 3 ms. The excitation photon energy is not resonant with any of the Nd3+ transitions, consequently the excitation energy must be transferred from the host nitride to the Nd3+ ions. The fast lifetime is almost independent of the temperature, indicating that it is related to the excitation transfer process. As the temperature increases the probability of carrier recombination through processes that do not excite Nd3+ ions increases. The slow lifetime is associated with the intrinsic Nd3+ lifetime. It is shorter at low temperatures and high excitation rates. At 26 K, it decreases by a factor 2 when the excitation power goes from 2 to 8 kW/cm2. The lifetime decrease with the excitation power is associated with the onset of stimulated emission from the Nd3+ ions.