Excitation Laser Line Research Articles

1,10-Phenanthroline Adsorption on Iron Electrode Monitored by Surface-Enhanced Raman Scattering (SERS). Comparison to SERS of Phen and Its Transition Metal Complex on Silver Electrode

The SER spectra of 1,10-phenanthroline (phen) adsorbed on Fe electrode in chloride medium as a function of applied potentials at 632.8 and 514.5 nm excitation wavelengths are presented. SERS spectra of phen on Co and Ni electrodes at 632.8 nm are also shown. The SER signatures of the surface complex indicate its similarity with the chemically prepared [M(phen)3]2+, where M = Fe2+, Co2+, or Ni2+. The same changes in relative intensities of the bands with applied potential are observed for phen on transition metals and for [M(phen)3]2+ on silver electrodes. These spectral changes at 632.8 nm excitation radiation resemble the spectral changes in the resonance Raman spectra of [M(phen)3]Cl2 at different excitation laser lines. At the potential of maximum intensity, the strongest band in phen on Ag SER spectrum is ca. 70 times higher than that in phen on Fe, while the phen on Fe SER excitation profile is ca. 5 times higher than that of phen on Ag. At 632.8 nm radiation, the band at 557 cm-1 for phen on Fe show...

Shuttling of G Protein Subunits between the Plasma Membrane and Intracellular Membranes

Heterotrimeric G proteins (alphabetagamma) mediate the majority of signaling pathways in mammalian cells. It is long held that G protein function is localized to the plasma membrane. Here we examined the spatiotemporal dynamics of G protein localization using fluorescence recovery after photobleaching, fluorescence loss in photobleaching, and a photoswitchable fluorescent protein, Dronpa. Unexpectedly, G protein subunits shuttle rapidly (t1/2 < 1 min) between the plasma membrane and intracellular membranes. We show that consistent with such shuttling, G proteins constitutively reside in endomembranes. Furthermore, we show that shuttling is inhibited by 2-bromopalmitate. Thus, contrary to present thought, G proteins do not reside permanently on the plasma membrane but are constantly testing the cytoplasmic surfaces of the plasma membrane and endomembranes to maintain G protein pools in intracellular membranes to establish direct communication between receptors and endomembranes.

Assessing the Role of the Active-site Cysteine Ligand in the Superoxide Reductase from Desulfoarculus baarsii

Superoxide reductase is a novel class of non-heme iron proteins that catalyzes the one-electron reduction of O(2)(.) to H(2)O(2), providing an antioxidant defense in some bacteria. Its active site consists of an unusual non-heme Fe(2+) center in a [His(4) Cys(1)] square pyramidal pentacoordination. In this class of enzyme, the cysteine axial ligand has been hypothesized to be an essential feature in the reactivity of the enzyme. Previous Fourier transform infrared spectroscopy studies on the enzyme from Desulfoarculus baarsii revealed that a protonated carboxylate group, proposed to be the side chain of Glu(114), is in interaction with the cysteine ligand. In this work, using pulse radiolysis, Fourier transform infrared, and resonance Raman spectroscopies, we have investigated to what extent the presence of this Glu(114) carboxylic lateral chain affects the strength of the S-Fe bond and the reaction of the iron active site with superoxide. The E114A mutant shows significantly modified pulse radiolysis kinetics for the protonation process of the first reaction intermediate. Resonance Raman spectroscopy demonstrates that the E114A mutation results in both a strengthening of the S-Fe bond and an increase in the extent of freeze-trapping of a Fe-peroxo species after treatment with H(2)O(2) by a specific strengthening of the Fe-O bond. A fine tuning of the strength of the S-Fe bond by the presence of Glu(114) appears to be an essential factor for both the strength of the Fe-O bond and the pK(a) value of the Fe(3+)-peroxo intermediate species to form the reaction product H(2)O(2).

Raman dispersion in polyaniline base forms

Raman spectra of polyaniline in its emeraldine (EB) and pernigraniline (PB) base forms were obtained using several excitation laser lines and Raman dispersion of the bands was measured. Only the νC N band value depends on excitation wavelength for both polymeric forms. These changes of vibrational frequencies with excitation wavelength were analyzed using amplitude mode model (AMM). ∏ n ( ω n R / ω n 0 ) 2 versus excitation photon energies plot was linear in the electronic absorption transition region, for EB at ca. 640 nm and for PB at ca. 620 nm. The dispersion values, D (slope of ∏ n ( ω n R / ω n 0 ) 2 versus excitation photon energies plot) were 0.058 and 0.038 eV −1 for EB and PB, respectively, which indicate that EB has the first excited state of even parity presenting lower energy when comparing to that of PB.

DNA-binding Activity of the ERp57 C-terminal Domain Is Related to a Redox-dependent Conformational Change

ERp57, a member of the protein-disulfide isomerase family, although mainly localized in the endoplasmic reticulum is here shown to have a nuclear distribution. We previously showed the DNA-binding properties of ERp57, its association with the internal nuclear matrix, and identified the C-terminal region, containing the a' domain, as being directly involved in the DNA-binding activity. In this work, we demonstrate that its DNA-binding properties are strongly dependent on the redox state of the a' domain active site. Site-directed mutagenesis experiments on the first cysteine residue of the -CGHC-thioredoxin-like active site lead to a mutant domain (C406S) lacking DNA-binding activity. Biochemical studies on the recombinant domain revealed a conformational change associated with the redox-dependent formation of a homodimer, having two disulfide bridges between the cysteine residues of two a' domain active sites. The formation of intermolecular disulfide bridges rather than intramolecular oxidation of active site cysteines is important to generate species with DNA-binding properties. Thus, in the absence of any dedicated motif within the protein sequence, this structural rearrangement might be responsible for the DNA-binding properties of the C-terminal domain. Moreover, NADH-dependent thioredoxin reductase is active on intermolecular disulfides of the a' domain, allowing the control of dimeric protein content as well as its DNA-binding activity. A similar behavior was also observed for whole ERp57.

Multiphoton Microscopy for the Investigation of Dermal Penetration of Nanoparticle-Borne Drugs

Multiphoton microscopy (MPM) of a dually fluorescence-labeled model system in excised human skin is employed for high-resolution three-dimensional (3D) visualization in order to study the release, accumulation, and penetration properties of drugs released from nanoscale carrier particles in dermal administration. Polymer particles were covalently labeled with fluorescein, whereas Texas Red as a drug-model was dissolved in the particles to be released to the formulation matrix. Single nanoparticles on skin could easily be localized and imaged with diffraction-limited resolution. The temporal evolution of the fluorescent drug-model concentration in various skin compartments over more than 5 hours was investigated by multiphoton spectral imaging of the same area of the specimen. The 3D penetration profile of the drug model in correlation with skin morphology and particle localization information is obtained by multiple laser line excitation experiments. MPM combined with spectral imaging was found to allow noninvasive long-term studies of particle-borne drug-model penetration into skin with subcellular resolution. By dual color labeling, a clear discrimination between particle-bound and released drug model was possible. The introduced technique was shown to be a powerful tool in revealing the dermal penetration properties and pathways of drugs and nanoscale drug vehicles on microscopic level.

Raman spectroscopic investigation of carbon nanowalls

Two-dimensional carbon nanowalls (CNWs) were prepared by microwave plasma-enhanced chemical-vapor deposition and scanning electron microscopy was used to observe their morphologies. The Raman observations of different sample orientations and polarizations show that CNWs are well crystallized. Micro-Raman scattering measurements were also carried out with different excitation laser lines (325, 488, 514, 532, and 633 nm). The D band shows a very strong shift of 46.19 cm(-1)eV with excitation laser energy and this has been explained by the double resonance effect. The decreasing intensity ratios IDIG and ID'/IG with increasing laser excitation energy were detected and discussed.

Carbon Nanotube Bundles as Molecular Assemblies for the Detection of Polycyclic Aromatic Hydrocarbons: Surface-Enhanced Resonance Raman Spectroscopy and Theoretical Studies

In this work surface-enhanced resonance Raman spectroscopic experiments have demonstrated that metallic single-walled carbon nanotubes can be used as chemical assemblies between the pyrene analyte and the silver colloidal surface. Pyrene has been detected at concentrations lower than 10(-9) M by use of the 514.5 nm excitation laser line. A charge transfer from the surface to the nanotube characterizes the nanotube-silver surface interaction. The pyrene-nanotube interaction occurs through a pi-pi electronic stacking. Extended Hückel calculations based on a simplified molecular model for the analyte/nanotube/surface system support the experimental conclusions. The nanotube-pyrene distance is 3.4 A, and the most probable orientation for pyrene is confirmed to be plane parallel to the nanotube surface. An energy transfer from the silver surface to the nanotube/analyte system is verified.

Spectroscopic Characterization of Polyaniline Formed by Using Copper(II) in Homogeneous and MCM-41 Molecular Sieve Media

This work emphasizes the important role of the synthetic parameters in the structure of the polymeric material obtained in the aniline polymerization. The polymers formed by the oxidative polymerization of aniline by copper(II) ions in acidic aqueous solution, acetonitrile/water medium, and also copper(II) acetate complex encapsulated into MCM-41 molecular sieve were characterized by resonance Raman spectroscopy using three exciting laser lines and other techniques such as UV-vis, FTIR, and XANES (Nitrogen K edge). Additionally the products were investigated by thermogravimetric analysis and powder X-ray diffraction. When Cu(II) ions in acidic aqueous medium are used, emeraldine salt (ES-PANI) is formed through the usual head-to-tail polymerization mechanism, while in acetonitrile/water medium a polymer is observed having mainly phenazine-like rings, quinonediimine, and/or phenylenediamine segments in the chains, suggesting that a distinct mechanism is operating. The average molecular weights of the free polymers synthesized in water and in acetronile/water were, respectively, ca. 37 300 and 16 900 Da. The encapsulated polymer synthesized in Cu(II)-MCM-41 is a polymeric mixture of (i) ES-PANI and (ii) the polymer obtained when this metal cation was used as oxidant in acetonitrile/water medium. All the characterization data were compared to those ones obtained for standard free polyaniline and also for the encapsulated polymer into mesoporous MCM-41 formed by using persulfate in acidic aqueous medium as oxidant.

Pressure dependence of the optical and vibrational properties of hexagonal and cubic CdS films grown on GaAs

AbstractThe pressure dependence of the excitonic photoluminescence and Raman scattering in both hexagonal and cubic CdS films grown on GaAs substrates is reported. We found that the pressure coefficients of the cubic CdS films are equal to those of the hexagonal films within experimental uncertainties. The main difference between their optical and vibrational properties seems to lie in their exciton energies, the hexagonal phase having a slightly higher energy than the cubic phase. Strong enhancement in multi‐longitudinal optical (LO) phonon scattering has been observed by “pressure‐tuning” the exciton energy to resonate with the excitation laser line. As a result we have been able to determine the pressure coefficients of the higher order LO phonons in CdS for modes as high as 5LO. We found that the pressure coefficients of these higher order LO modes when normalized by the phonon frequency exhibit a systematic decrease with the order of the mode. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Resonant Raman study of confined Se single helix and Se8 rings

Electron-phonon coupling in Se species confined in the nanoporous matrix has been investigated by using resonant Raman spectra performed with several different laser lines from deep blue to near infrared. The spectra strongly depend on the energies of the excitation laser lines. The one-phonon symmetric A1 modes for Se single helix and Se8 rings are enhanced in the vicinity of their absorption bands. Detailed analysis shows that the Raman band in the high-frequency range of 450–550cm−1 is composed of three individual second-order Raman bands for the confined Se species. These two-phonon Raman shifts occur at twice the frequency shift of the first-order Raman lines, and their intensities are also enhanced when the excitation laser energy matches an electronic transition in Se nanospecies.

Photochemical Kinetics of Maleic to Fumaric Acid on Silver Nanoparticle Surfaces

A visible photochemistry of maleic to fumaric acid adsorbed on silver nanoparticle surfaces was investigated as probed by SERS using a simple flow method. Photoisomerization of maleic to fumaric acid was consecutively observed in the condition of various flow rates, which varied the exposure time of laser beam. The sequential SERS spectra of maleic acid indicated that the photochemical isomerization and desorption took place simultaneously on silver nanoparticle surfaces as a function of laser fluency and wavelength. For 530.9nm laser line excitation, the rate constant coefficients were obtained with a = 5.9 <TEX>$sec^{-1}$</TEX> mW for isomerization and b = 13.9 <TEX>$sec^{-1}$</TEX> mW for desorption, which <TEX>$k_1\;=\;aI^n\;and\;k_2\;=\;bI^m$</TEX>. Both reactions were one photon process (n = 1, m = 1) of a visible light and relatively fast process whose decay time was in the range of milli-second for 50 mW laser power. The rate of photochemical reaction increased on going toward the blue and photodesorption was a dominant process. A simple flow method used in this study was very useful to study a relatively fast photochemical reaction of molecules adsorbed on silver nanoparticle surfaces.

Resonance Raman scattering from phonon overtones in double-wall carbon nanotubes

The resonance behavior and the dispersion of the ${\mathrm{G}}^{\ensuremath{'}}$ line in double-wall carbon nanotubes were investigated by multifrequency Raman scattering. Using a large number of laser lines for excitation, the dispersion for the response from the inner tubes and from the outer tubes was found to be 85 and $99\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}∕\mathrm{eV}$, respectively. The reduction of the dispersion for the inner tubes is a consequence of their high curvature and suggests a flattening of the phonon dispersion at the $K$ point in the Brillouin zone. The frequency position for the ${\mathrm{G}}^{\ensuremath{'}}$ line of the inner tubes for a given laser energy was likewise strongly reduced as compared to values expected from tubes with standard diameter. This was again assigned to the strong curvature of the small-diameter inner tubes. Finally, the ${\mathrm{G}}^{\ensuremath{'}}$ line scattering cross sections for the outer tubes and for the inner tubes revealed resonances according to the transitions between the corresponding Van Hove singularities. The response from the inner tubes was particularly strong, in good agreement with the expected resonance enhancement resulting from their high curvature.

Gold nanostructured films deposited on polystyrene colloidal crystal templates for surface-enhanced Raman spectroscopy

Scanning electron microscopy in combination with transmission and reflectivity spectroscopy have been employed to characterize the structural, morphological and optical properties of thin gold film deposited on polystyrene colloidal crystal. Surface-enhanced Raman (SER) spectra of the probe Rhodamine 6G molecules adsorbed on the substrate have been also recorded by using the 514 and 633 nm laser lines for excitation in order to evaluate the performance of the gold nanostructured film as SERS-active substrate. Interesting correlations between the optical response, nanometer-scale morphology and SERS activity of the nanostructured film have been found.

Study of the spectroscopic properties and infrared-to-visible up-conversion fluorescence of Er3+-doped germanate glasses

Raman scattering, optical absorption and photoluminescence studies have been performed on Er3+-doped Na2O–Nb2O5–GeO2 germanate-based glasses. The main bands in the Raman spectra involve vibrational modes associated with the germanium oxide network. Judd–Ofelt intensity parameters were determined and then used to calculate the radiative transition rates and excited-state radiative lifetimes. Green and red up-conversion luminescence was observed at room temperature under 800 nm laser line excitation. The emission intensities have a quadratic dependence on the excitation pump power, indicating a two-step up-conversion mechanism. The intensity of the 660 nm red band shows an increase relative to the 554 nm green band with increasing Er3+ content. Combined with the time-resolved luminescence data of the emitting states, we suggest from these results that the up-conversion process is attributed to a competitive situation of excited-state absorption and energy transfer between excited ions accompanied by multiphonon relaxation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Origin of the 2450 cm −1 Raman bands in HOPG, single-wall and double-wall carbon nanotubes

The second order Raman signals around the G′-band region of graphite and carbon nanotubes have been investigated at more than 15 excitation laser lines. Two distinct Raman bands have been observed around 2700 cm −1; a prominent one is due to the so-called G′-band and the other is a weak band around 2450 cm −1. Both two bands can be from the double resonance process involving two phonons around the K-point in the phonon dispersion of a two-dimensional graphite. The 2450 cm −1-band has exhibited little power dependence, whereas the intensity of G′-band has shown large photon energy dependence as already reported. The 2450 cm −1-band and the G′-band correspond to non-dispersive q = 0 and fully-dispersive q = 2 k, respectively. From the phonon dispersion and the corresponding phonon frequency, the 2450 cm −1-band can be assigned as an overtone mode of LO phonon (i.e. 2LO). This is revealed by calculated Raman spectra of graphite with proper electron–phonon matrix elements. The present study is the first report on the origin and assignment of the 2450 cm −1-band, which is based on the double resonance Raman scattering.

Morphological and luminescent studies on nanosized Er, Yb–Yttrium oxide up-converter prepared from different precursors

In this paper, we report luminescent and morphological studies with yttrium oxide samples doped with ytterbium and erbium. The samples were prepared by the combustion method and also from different precursors: oxalate, basic carbonate and polymeric resin. All powders were identified as being an yttrium oxide with a C-form structure, independent of the employed precursor. From mean crystallite size measurements, it was verified that oxides prepared through the polymeric precursor and combustion methods lead to the smallest crystallite size. Particle shape and size were investigated by SEM and TEM, and showed that both the oxalate precursor and the combustion methods do not provide oxide materials of suitable shape or size. On the other hand, the basic carbonate and polymeric precursors resulted in spherically shaped particles with an average diameter of 90 and 15 nm, respectively. Upon 980 nm diode laser excitation, green and red emission lines were detected for all samples and were assigned to the 2H 11/2, 4S 3/2→ 4I 15/2 and 4F 9/2→ 4I 15/2 transitions, respectively. Such transitions are characteristic for Er 3+ and result from energy transfer from Yb 3+ energy levels, 2F 7/2→ 2F 5/2. A relationship between the decrease in the mean crystallite size and the enhancement in red emission was also established as well as the influence of the presence of a high percentage of Yb 3+. Both factors promote ET from Yb 3+ ( 2F 5/2) to Er 3+ ( 4I 11/2).

Multiphonon resonant Raman scattering inZnOcrystals and nanostructured layers

Multiphonon resonant Raman scattering (RRS) was studied in unintentionally doped bulk $\mathrm{ZnO}$ crystals and layers, including nanostructured and highly conductive films, excited by 351.1 and $363.8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ laser lines in the temperature interval from 10 to $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The variation of resonant conditions with sample temperature and wavelength of the excitation laser line allowed us to discriminate between the incoming and outgoing exciton mediated RRS by LO phonons. The quenching of luminescence and enhancement of Raman scattering in nanostructured $\mathrm{ZnO}$ layers grown on single-layer opals enabled us to observe the Raman signal in resonant configuration thus making possible to study multiphonon RRS by Fr\ohlich-type vibrational modes related to nanocrystallites with sizes less than $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The emission spectra of highly conductive $\mathrm{ZnO}$ films grown on porous $\mathrm{InP}$ substrates were found to consist of multiphonon RRS lines superimposed on a broad asymmetric near band gap photoluminescence (PL) band. The occurrence of PL and RRS in highly conductive layers is attributed to tailing of the density of states caused by potential fluctuations and to the breakdown of the wave-vector conservation due to randomly distributed impurities (intrinsic defects).

Laser-Excited Luminescence of Trivalent Lanthanide Impurities and Local Structure in CeO2−ZrO2 Mixed Oxides

In this paper we report on the laser-excited Raman and luminescence spectroscopy of a series of ceria−zirconia mixed oxides, containing trivalent lanthanide ions as unintentional impurities, using 488.0-, 514.5-, and 1064.0-nm excitation wavelengths. We assigned the main bands located at Stokes shifts higher than 800 cm-1 from the laser lines to f−f emission transitions of Pr3+, Nd3+, Ho3+, and Er3+ ions. These ions can also be used as spectral probes of the cation sites in the solid solutions as their transition intensities are found to be dependent on the local symmetry of the sites in which they are accommodated. f−f emission transitions have also been found to overlap the vibrational Raman bands for Stokes shifts lower than 800 cm-1 from the laser lines. To overcome possible ambiguities in the structural information extracted from the Raman spectra, at least two laser excitation lines must be used.

Raman scattering of complex sodium aluminum hydride for hydrogen storage

Complex hydride materials are currently under investigation for potential use in hydrogen storage applications. In this report, the Raman spectra of sodium aluminum hydride (NaAlH 4) were collected using an excitation laser line at 633 nm. The interpretation and assignment of the observed spectrum of NaAlH 4 was aided by cluster-based density functional calculations, using hybrid density functional theory [B3LYP/6-31+G(d,p)], enabling the reliable interpretation of characteristic Raman bands. The assignment and identification of internal vibrational, torsional, and lattice vibrational modes are discussed, providing spectroscopic probes for future physicochemical studies following structural and systemic evolution during the hydriding/dehydriding processes of NaAlH 4.