Long-wavelength Excitations Research Articles

Hemicyanine-linked pyrimidine mimics as solvatochromic fluorophores with visible excitation wavelengths

The design of solvatochromic fluorescent nucleosides with visible excitation wavelengths is a goal towards the generation of modified oligonucleotides for fluorimetry-based molecular imaging. Herein, two hemicyanine-linked C5-2′-deoxyuridine nucleosides (PyI-dU and APPy-dU) have been synthesized by first generating hemicyanine-alkyne precursors that were attached via the alkyne moiety to 5-iodo-2′-deoxyuridine (5-I-dU) by Sonogashira coupling. The photophysical properties of the hemicyanine-linked dU probes have been characterized and compared to the corresponding properties of the hemicyanine-alkyne precursors. The nucleoside probe PyI-dU exhibits optical features that mimic the properties of the free hemicyanine-alkyne precursor, while the APPy-dU probe displays more favorable optical properties (longer excitation wavelength, brighter emission in water) than its precursor that is ascribed to π-stacking interactions between the hemicyanine dye with the dU nucleobase. Overall, probe APPy-dU is a superior solvatochromic fluorophore than PyI-dU suggesting its greater utility for fluorescent imaging applications.

Fluorescence quantum yield of CDOM in coastal zones of the Arctic seas

ABSTRACTAlong with traditional optical indices, calculated from absorption and fluorescence spectra to describe chromophoric dissolved organic matter (CDOM) naturally occurring in water, the fluorescence quantum yield (FQY) becomes significant. Knowledge of CDOM optical properties is important for satellite remote sensing as well as for lidar ground-true measurements. The FQY as a function of excitation wavelength within 240–500 nm range for a variety of the Arctic shelf waters was determined for the first time in order to identify the characteristic chromophores peculiar to different regions of the Arctic basin affected by freshwater runoff. The surface water samples were collected during several cruises in 2015–2017 in the following sites: the mouth areas of the Khatanga and Lena Rivers (the Laptev Sea), the delta area of the Northern Dvina River (the White Sea), desalinated waters of the Kara Sea (influenced by freshwater of the Ob and Yenisei Rivers) and the East Siberian Sea (influenced by freshwaters of the Indigirka and Kolyma Rivers), as well the shelf areas of those seas not affected by terrigenous runoff. To characterize DOM, conventional optical indices SR, HIX, and BIX were calculated. In most cases, important humic character of DOM was established, while the contribution of autochthonous organic matter varied from low to intermediate level. For the samples with terrestrial impact, the FQY decreased from excitation at 240 nm to 270–280 nm and then increased, demonstrating two peaks at 340 and 380 nm, with constant decrease towards longer excitation wavelengths; at λex = 380 nm FQY varied from 1.4% to 3.1%. In some cases, additional maximum at 270 nm of FQY-excitation dependency was observed as an indicator of autochthonous nature of biological material. Minimal FQY was measured for the White Sea surface waters, the maximal for the Laptev and East Siberian seas.

Electron mass enhancement and magnetic phase separation near the Mott transition in double-layer ruthenates

We present a detailed investigation of the specific heat of Ca3(Ru1-xMx)2O7 (M = Ti, Fe, Mn) single crystals. Depending on the dopant and doping level, three distinct regions are present: a quasitwo-dimensional metallic state with antiferromagnetic (AFM) order formed by ferromagnetic bilayers (AFM-b), a Mott insulating state with G-type AFM order (G-AFM), and a localized state with a mixed AFM-b and G-AFM phase. Our specific heat data provide deep insights into the Mott transitions induced by Ti and Mn doping. We observed not only an anomalous large mass enhancement, but also an additional term in the specific heat, i.e., C ∝ T2, in the localized region. The C ∝ T2 term is most likely due to long-wavelength excitations with both FM and AFM components. A decrease in the Debye temperature is observed in the G-type AFM region, indicating lattice softening associated with the Mott transition.

Long-wavelength phonons in the crystalline and pasta phases of neutron-star crusts

We study the long-wavelength excitations of the inner crust of neutron stars, considering three phases: cubic crystal at low densities, rods and plates near the core-crust transition. To describe the phonons, we write an effective Lagrangian density in terms of the coarse-grained phase of the neutron superfluid gap and of the average displacement field of the clusters. The kinetic energy, including the entrainment of the neutron gas by the clusters, is obtained within a superfluid hydrodynamics approach. The potential energy is determined from a model where clusters and neutron gas are considered in phase coexistence, augmented by the elasticity of the lattice due to Coulomb and surface effects. All three phases show strong anisotropy, i.e., angle dependence of the phonon velocities. Consequences for the specific heat at low temperature are discussed.

Excitation-Dependent Fluorescence Quantum Yield for Freshwater Chromophoric Dissolved Organic Matter from Northern Russian Lakes

Advanced fluorescence analysis within the wide range of excitation wavelengths from 230 to 510 nm accompanied with chromatography was used to study natural chromophoric dissolved organic matter (CDOM) from three freshwater Karelian lakes. The influence of excitation wavelength (λex) on fluorescence quantum yield and emission maximum position was determined. The CDOM fluorescence quantum yield has reached a minimum at λex∼270–280 nm and a maximum at λex∼340–360 nm. It was monotonously decreasing after 370 nm towards longer excitation wavelengths. Analytical reversed-phase high-performance liquid chromatography with multiwavelength fluorescence detector characterized distribution of fluorophores between hydrophilic/hydrophobic CDOM parts. This technique revealed “hidden” protein-like fluorophores for some CDOM fractions, in spite of the absence of protein-like fluorescence in the initial CDOM samples. The humic-like fluorescence was documented for all hydrophilic and hydrophobic CDOM chromatographic peaks, and its intensity was decreasing along with peaks’ hydrophobicity. On contrary, the protein-like fluorescence was found only in the hydrophobic peaks, and its intensity was increasing along with peaks’ hydrophobicity. This work provides new data on the CDOM optical properties consistent with the formation of supramolecular assemblies controlled by association of low-molecular size components. In addition, these data are very useful for understanding the CDOM function in the environment.

Dual-Excitation Nanocellulose Plasmonic Membranes for Molecular and Cellular SERS Detection.

We demonstrate that cellulose nanofiber (CNF) biomaterials with high transparency and mechanical robustness can be combined with gold nanorods to form a multifunctional porous membrane for dual-mode surface-enhanced Raman scattering (SERS) detection of both small molecules and cells. The nanoporous nature of the nanofiber membranes allows for effective molecular filtration and preconcentration of the analytes, further boosting the SERS performance. Specifically, because of the low fluorescence and Raman background of the CNF matrix, extremely low loading density of gold nanorods can be used. The nanorod assemblies within the CNF network can be resonantly driven by a 532 nm laser (transverse plasmonic mode) and near resonantly driven at by a 785 nm laser (longitudinal mode), facilitating dual operational modes at two excitation wavelengths. The shorter wavelength excitation mode yields better Raman scattering efficiency and has been demonstrated to be capable of detecting rhodamine 6G (R6G) dyes down to picomolar concentrations. On the other hand, the longer wavelength excitation mode provides autofluorescence suppression for the better detection of microorganisms such as Escherichia coli, shortening the required integration time from hours to minutes. Upon drastically lowering the spectral background noise and utilizing nanofiltration, the plasmonic CNF membranes reported here show significantly improved SERS sensitivity and detection fidelity as compared to traditional metal, metal oxide, synthetic polymer, and paper SERS substrates.

Multifunctional Dithiadiazolyl Radicals: Fluorescence, Electroluminescence, and Photoconducting Behavior in Pyren-1′-yl-dithiadiazolyl

The pyren-1'-yl-functionalized dithiadiazolyl (DTDA) radical, C16H9CNSSN (1), is monomeric in solution and exhibits fluorescence in the deep-blue region of the visible spectrum (440 nm) upon excitation at 241 nm. The salt [1][GaCl4] exhibits similar emission, reflecting the largely spectator nature of the radical in the fluorescence process, although the presence of the radical leads to a modest quenching of emission (ΦF = 98% for 1+ and 50% for 1) through enhancement of non-radiative decay processes. Time-dependent density functional theory studies on 1 coupled with the similar emission profiles of both 1+ and 1 are consistent with the initial excitation being of predominantly pyrene π-π* character. Spectroscopic studies indicate stabilization of the excited state in polar media, with the fluorescence lifetime for 1 (τ = 5 ns) indicative of a short-lived excited state. Comparative studies between the energies of the frontier orbitals of pyren-1'-yl nitronyl nitroxide (2, which is not fluorescent) and 1 reveal that the energy mismatch and poor spatial overlap between the DTDA radical SOMO and the pyrene π manifold in 1 efficiently inhibit the non-radiative electron-electron exchange relaxation pathway previously described for 2. Solid-state films of both 1 and [1][GaCl4] exhibit broad emission bands at 509 and 545 nm, respectively. Incorporation of 1 within a host matrix for OLED fabrication revealed electroluminescence, with CIE coordinates of (0.205, 0.280) corresponding to a sky-blue emission. The brightness of the device reached 1934 cd/m2 at an applied voltage of 16 V. The crystal structure of 1 reveals a distorted π-stacked motif with almost regular distances between the pyrene rings but alternating long-short contacts between DTDA radicals. Solid state measurements on a thin film of 1 reveal emission occurs at shorter wavelengths (375 nm) whereas conductivity measurements on a single crystal of 1 show a photoconducting response at longer wavelength excitation (455 nm).

A rapidly expanding Bose-Einstein condensate: an expanding universe in the lab.

We study the dynamics of a supersonically expanding ring-shaped Bose-Einstein condensate both experimentally and theoretically. The expansion redshifts long-wavelength excitations, as in an expanding universe. After expansion, energy in the radial mode leads to the production of bulk topological excitations - solitons and vortices - driving the production of a large number of azimuthal phonons and, at late times, causing stochastic persistent currents. These complex nonlinear dynamics, fueled by the energy stored coherently in one mode, are reminiscent of a type of "preheating" that may have taken place at the end of inflation.

Non-Euclidean Geometry, Nontrivial Topology and Quantum Vacuum Effects

Space out of a topological defect of the Abrikosov–Nielsen–Olesen (ANO) vortex type is locally flat but non-Euclidean. If a spinor field is quantized in such a space, then a variety of quantum effects are induced in the vacuum. On the basis of the continuum model for long-wavelength electronic excitations originating in the tight-binding approximation for the nearest-neighbor interaction of atoms in the crystal lattice, we consider quantum ground-state effects in Dirac materials with two-dimensional monolayer structures warped into nanocones by a disclination; the nonzero size of the disclination is taken into account, and a boundary condition at the edge of the disclination is chosen to ensure self-adjointness of the Dirac–Weyl Hamiltonian operator. We show that the quantum ground-state effects are independent of the disclination size, and we find circumstances in which they are independent of parameters of the boundary condition.

Energy loss of surface plasmon polaritons on Ag nanowire waveguide

Metal nanostructures can support surface plasmon polaritons (SPPs) propagating beyond diffraction limit, which enables the miniaturizing of optical devices and the integrating of on-chip photonic and electronic circuits. Various surface plasmon based optical components have already been developed such as plasmonic routers, detectors, logic gates, etc. However, the high energy losses associated with SPPs' propagation have largely hampered their applications in nanophotonic devices and circuits. Developing the methods of effectively reducing energy loss is significant in this field. In this review, we mainly focus on the energy losses when SPPs propagate in Ag nanowires (NWs). Researches on energy loss mechanism, measurement approaches and methods of reducing energy loss have been reviewed. Owing to their good morphology and high crystallinity as well as low loss in visible spectrum, chemically synthesized Ag NWs are a promising candidate for plasmonic waveguides. The energy losses mainly arise from inherent Ohmic damping, scattering process, leaky radiation and absorption of substrate. These processes can be influenced by excitation wavelength, the geometry of NW and the dielectric environment, especially the effect of substrate, which is discussed in the review. Longer excitation wavelength and larger NW diameter can induce decreased mode confinements and smaller Ohmic loss. The experimental methods to measure the energy loss have been summarized. Researches on reducing energy loss have been reviewed including applying dielectric layer or graphene between NW and substrate, replacing commonly used substrate with a dielectric multilayer substrate, introducing gain materials, and forming hybrid waveguides by using the semiconductor or dielectric NW. Specifically, the leaky radiation can be prevented when an appropriate dielectric layer is placed between NW and substrate, and the mode confinement can be reduced which leads to decreased Ohmic loss. The gain materials can be used to compensate for the energy loss during propagation. Compared with metal waveguides, semiconductor or dielectric NWs suffer lower energy losses while decreased field confinement. Then the hybrid waveguides constructed by metal and dielectric NWs can combine their advantages, which possesses reduced propagation loss. In addition, the plasmon modes in NWs in a homogeneous medium and a substrate are briefly discussed respectively, followed by the introduction to fundamental properties of SPPs propagation. Finally, perspectives of the future development of reducing energy loss are given. The researches on reducing energy loss are crucial for designing and fabricating the nanophotonic devices and integrated optical circuits.

High-Temperature Nonequilibrium Bose Condensation Induced by a Hot Needle.

We investigate theoretically a one-dimensional ideal Bose gas that is driven into a steady state far from equilibrium via the coupling to two heat baths: a global bath of temperature T and a "hot needle," a bath of temperature T_{h}≫T with localized coupling to the system. Remarkably, this system features a crossover to finite-size Bose condensation at temperatures T that are orders of magnitude larger than the equilibrium condensation temperature. This counterintuitive effect is explained by a suppression of long-wavelength excitations resulting from the competition between both baths. Moreover, for sufficiently large needle temperatures ground-state condensation is superseded by condensation into an excited state, which is favored by its weaker coupling to the hot needle. Our results suggest a general strategy for the preparation of quantum degenerate nonequilibrium steady states with unconventional properties and at large temperatures.

Photon-Induced Light Emission from Foamed Gold with Micro/Nanohollow Sphere Structures.

We present a study on photon-induced light emission at room temperature from macroscale foamed gold with micro/nanoscale hollow spheres synthesized by seed-mediated growth method. Samples with a fixed sphere diameter but different Au densities are examined. It is demonstrated that strong and characteristic light emission from these samples can be achieved under optical excitation. In a short excitation wavelength regime (280–470 nm), the peak position in the photoemission spectrum increases almost linearly with excitation wavelength. In a relatively long-wavelength excitation regime (478–520 nm), photoluminescence (PL) can be observed where the peak position in the PL spectrum depends very little on excitation wavelength and two peaks can be seen in the PL emission spectrum. These effects do not change significantly with varying sample density, although it is found that the intensity of the light emission increases with sample density. We find that the features of the PL emission from foamed gold with micro/nanoscale hollow spheres differ significantly from those observed for Au nanoparticles. This study is relevant to the application of Au micro/nanostructures as advanced optoelectronic materials and devices.

Novel Fluorescent Microemulsion: Probing Properties, Investigating Mechanism, and Unveiling Potential Application.

Nanoscale microemulsions have been utilized as delivery carriers for nutraceuticals and active biological drugs. Herein, we designed and synthesized a novel oil in water (O/W) fluorescent microemulsion based on isoamyl acetate, polyoxyethylene castor oil EL (CrEL), and water. The microemulsion emitted bright blue fluorescence, thus exhibiting its potential for active drug detection with label-free strategy. The microemulsion exhibited excitation-dependent emission and distinct red shift with longer excitation wavelengths. Lifetime and quantum yield of fluorescent microemulsion were 2.831 ns and 5.0%, respectively. An excellent fluorescent stability of the microemulsion was confirmed by altering pH, ionic strength, temperature, and time. Moreover, we proposed a probable mechanism of fluorochromic phenomenon, in connection with the aromatic ring structure of polyoxyethylene ether substituent in CrEL. Based on our findings, we concluded that this new fluorescent microemulsion is a promising drug carrier that can facilitate active drug detection with a label-free strategy. Although further research is required to understand the exact mechanism behind its fluorescence property, this work provided valuable guidance to develop new biosensors based on fluorescent microemulsion.

A fluorescence-based imaging method to measure in vitro and in vivo mitophagy using mt-Keima.

Mitophagy is a cellular process that selectively removes damaged, old or dysfunctional mitochondria. Defective mitophagy is thought to contribute to normal aging and to various neurodegenerative and cardiovascular diseases. Previous methods used to detect mitophagy in vivo were cumbersome, insensitive and difficult to quantify. We created a transgenic mouse model that expresses the pH-dependent fluorescent protein mt-Keima in order to more readily assess mitophagy. Keima is a pH-sensitive, dual-excitation ratiometric fluorescent protein that also exhibits resistance to lysosomal proteases. At the physiological pH of the mitochondria (pH 8.0), the shorter-wavelength excitation predominates. Within the acidic lysosome (pH 4.5) after mitophagy, mt-Keima undergoes a gradual shift to longer-wavelength excitation. In this protocol, we describe how to monitor mitophagic flux in living cells over an 18-h time frame, as well as how to quantify mitophagy using the mt-Keima probe. This protocol also describes how to use confocal microscopy to visualize mitophagy in living tissues obtained from mt-Keima transgenic mice. With this protocol, the mt-Keima probe can reliably be imaged within the first 60 min after tissue collection. We also describe how to apply mt-Keima with stimulated emission depletion (STED) microscopy, which can potentially provide substantially higher-resolution images. Typically, the approximate time frame for time-lapse fluorescence imaging of mt-Keima is 20 h for living cells. For confocal analysis of tissue from an mt-Keima mouse, the whole procedure generally takes no longer than 60 min, and the STED imaging usually takes <2 h.

Graphene quantum dots derived from hollow carbon nano-onions

Herein, carbon nano-onions (CNOs) with different structures have been investigated as precursors for the synthesis of graphene quantum dots (GQDs). It was found that hollow CNOs yield GQDs with a uniform size distribution, whereas metal encapsulation in the CNO structure is disadvantageous for the same. Furthermore, the hollow CNOs are also advantageous for the synthesis of GQDs with a yellow-green hybrid luminescence and long-ranged excitation wavelength (λ ex)-independent photoluminescent (PL) behavior, in which the λ ex upper limit was 480 nm. These features enable safe sensing and cell tracking applications with a longer excitation wavelength in the visible light region. The potential applications of the synthesized GQDs as fluorescent sensing probes for detecting Cu(II) ions and non-toxic cell imaging under visible light excitation have been demonstrated. This means that sensing and bioimaging can be accomplished in the natural environment with no need for UV excitation. This work provides a reference to researchers in tailoring CNO structures in terms of their inner space to synthesize GQDs with the desired luminescence behavior.

Spin dynamics of antiferromagnets in the presence of a homogeneous magnetization

We use general hydrodynamic equations to determine the long-wavelength spin excitations in isotropic antiferromagnets in the presence of a homogeneous magnetization. The latter may be induced, such as in antiferromagnets in an external magnetic field, or spontaneous, such as in ferrimagnetic or canted phases that are characterized by the coexistence of antiferromagnetic and ferromagnetic order. Depending on the physical situation, we find propagating spin waves that are gapped in some cases and gapless in others, diffusive modes, or relaxational modes. The excitation spectra turn out to be qualitatively different depending on whether or not the homogeneous magnetization is a conserved quantity. The results lay the foundation for a description of a variety of quantum phase transitions, including the transition from a ferromagnetic metal to an antiferromagnetic one, and the spin-flop transitions that are observed in some antiferromagnets. They also are crucial for incorporating weak-localization and Altshuler-Aronov effects into the descriptions of quantum phases in both clean and disordered magnetic metals.

Observation of three-component fermions in the topological semimetal molybdenum phosphide.

In quantum field theory, Lorentz invariance leads to three types of fermion-Dirac, Weyl and Majorana. Although the existence of Weyl and Majorana fermions as elementary particles in high-energy physics is debated, all three types of fermion have been proposed to exist as low-energy, long-wavelength quasiparticle excitations in condensed-matter systems. The existence of Dirac and Weyl fermions in condensed-matter systems has been confirmed experimentally, and that of Majorana fermions is supported by various experiments. However, in condensed-matter systems, fermions in crystals are constrained by the symmetries of the 230 crystal space groups rather than by Lorentz invariance, giving rise to the possibility of finding other types of fermionic excitation that have no counterparts in high-energy physics. Here we use angle-resolved photoemission spectroscopy to demonstrate the existence of a triply degenerate point in the electronic structure of crystalline molybdenum phosphide. Quasiparticle excitations near a triply degenerate point are three-component fermions, beyond the conventional Dirac-Weyl-Majorana classification, which attributes Dirac and Weyl fermions to four- and two-fold degenerate points, respectively. We also observe pairs of Weyl points in the bulk electronic structure of the crystal that coexist with the three-component fermions. This material thus represents a platform for studying the interplay between different types of fermions. Our experimental discovery opens up a way of exploring the new physics of unconventional fermions in condensed-matter systems.

New frontiers in intravital microscopy of the kidney.

Intravital imaging with multiphoton microscopy enables the detailed study of dynamic cellular processes within functioning organs in living animals, in ways that would not otherwise be possible. It therefore represents a powerful tool in translational kidney research. In this article, we will discuss several new technical developments that have significantly increased the capabilities of kidney imaging. Important contemporary advances in biomedical imaging technology include longer wavelength excitation lasers, far-red emitting fluorescent reporters, highly sensitive detectors, fluorescence lifetime measurements, adaptive optics, microendoscopes, high-throughput automated analysis algorithms and tissue clearing techniques. Several recent studies have utilized intravital microscopy to gain valuable new insights into important physiological and pathophysiological processes in the kidney, such as renal handling of albumin and the cellular pathogenesis of acute kidney injury in sepsis. Major technological advances are rapidly expanding the frontiers of intravital microscopy, which is likely to play an increasingly important role in preclinical kidney research in the coming years.

Two red-emission single and double ‘arms’ fluorescent materials stemed from ‘one-pot’ reaction for hydrogen sulfide vivo imaging

The gasotransmitter hydrogen sulfide (H2S) has attracted attention in recent years for its contributions to human health and diseases. In this work, the design strategy relied on ‘one-pot’ by employing BODIPY 1 dye and P-hydroxybenzaldehyde afforded two key intermediates, then further reacted with NBD thus obtaining products ‘single’ and ‘double’ arms types molecules (BDP-N1 and BDP-N2). Firstly we concentrated on the comparative investigation their response abilities for hydrogen sulfide. The response mechanisms are all based on the selective thiolysis of 7-nitrobenzo[c-1,2,5] oxadiazole(NBD) moiety attached to the styryl-BODIPY scaffold by H2S. However, it is worth mentioned that double‘arms’showed longer wavelength excitation and emission resulting from extended conjugation. Furthermore, these successes of intracellular imaging experiments in A549 cells indicated that the two probes are suitable for imaging of exogenous or endogenous hydrogen sulfide in living cells or living zebrafish.

Niobium pentoxide: a promising surface-enhanced Raman scattering active semiconductor substrate

Surface-enhanced Raman scattering technique, as a powerful tool to identify the molecular species, has been severely restricted to the noble metals. The surface-enhanced Raman scattering substrates based on semiconductors would overcome the shortcomings of metal substrates and promote development of surface-enhanced Raman scattering technique in surface science, spectroscopy, and biomedicine studies. However, the detection sensitivity and enhancement effects of semiconductor substrates are suffering from their weak activities. In this work, a semiconductor based on Nb2O5 is reported as a new candidate for highly sensitive surface-enhanced Raman scattering detection of dye molecules. The largest enhancement factor value greater than 107 was observed with the laser excitation at 633 and 780 nm for methylene blue detection. As far as literature review shows, this is in the rank of the highest sensitivity among semiconductor materials; even comparable to the metal nanostructure substrates with “hot spots”. The impressive surface-enhanced Raman scattering activities can be attributed to the chemical enhancement dominated by the photo-induced charge transfer, as well as the electromagnetic enhancement, which have been supported by the density-functional-theory and finite element method calculation results. The chemisorption of dye on Nb2O5 creates a new highest occupied molecular orbital and lowest unoccupied molecular orbital contributed by both fragments in the molecule-Nb2O5 system, which makes the charge transfer more feasible with longer excitation wavelength. In addition, the electromagnetic enhancement mechanism also accounts for two orders of magnitude enhancement in the overall enhancement factor value. This work has revealed Nb2O5 nanoparticles as a new semiconductor surface-enhanced Raman scattering substrate that is able to replace noble metals and shows great potentials applied in the fields of biology related.