Spectroscopic investigations of OH- influence on near-infrared fluorescence quenching of Yb3+/Tm3+ co-doped sodium-metaphosphate glasses

Energy transfer processes were studied in two sets of Yb3+ and Tm3+ co-doped sodium-metaphosphate glasses, prepared in air and nitrogen atmospheres. Using Forster, Dexter, and Miyakawa theoretical models, the energy transfer parameters were calculated. The main ion-ion energy transfer processes analyzed were energy migration among Yb3+ ions, cross-relaxations between Yb3+ and Tm3+ ions, and interactions with OH- radicals. The results indicated that Yb -> Tm energy transfer favors 1.8 mu m emissions, and there is no evidence of concentration quenching up to 2% Tm2O3 doping. As expected, samples prepared in nitrogen atmosphere present higher fluorescence quantum efficiency than those prepared in air, and this feature is specially noted in the near-infrared region, where the interaction with the OH- radicals is more pronounced. (c) 2007 Published by Elsevier B.V.

Low-energy muon-transfer reaction from hydrogen isotopes to helium isotopes

Direct muon transfer in low-energy collisions of the muonic hydrogen H-mu and helium (He++) is considered in a three-body quantum-mechanical framework of coordinate-space integro-differential Faddeev-Hahn-type equations within two- and six-state close coupling approximations. The final-state Coulomb interaction is treated without any approximation employing appropriate Coulomb waves in the final state. This procedure of treating Coulomb interaction leads to much improved results for low-energy transfer rates. The present results agree reasonably well with previous semiclassical calculations. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Optical properties and frequency upconversion fluorescence in a Tm3+-doped alkali niobium tellurite glass

Optical spectroscopic properties of Tm3+-doped 60TeO(2)-10GeO(2)-10K(2)O-10Li(2)O-10Nb(2)O(5) glass are reported. The absorption spectra were obtained and radiative parameters were determined using the Judd-Ofelt theory. Characteristics of excited states were studied in two sets of experiments. Excitation at 360 nm originates a relatively narrow band emission at 450 nm attributed to transition D-1(2)-->F-3(4) of the Tm3+ ion with photon energy larger than the band-gap energy of the glass matrix. Excitation at 655 nm originates a frequency upconverted emission at 450 nm (D-1(2)-->F-3(4)) and emission at 790 nm (H-3(4)-->H-3(6)). The radiative lifetimes of levels D-1(2) and H-3(4) were measured and the differences between their experimental values and the theoretical predictions are understood as due to the contribution of energy transfer among Tm3+ ions. (C) 2003 American Institute of Physics.

Efficient energy upconversion emission in Tm3+/Yb3+-codoped TeO2-based optical glasses excited at 1.064 mu m

Efficient energy upconversion of cw radiation at 1.064 mum into blue, red, and near infrared emission in Tm3+-doped Yb3+-sensitized 60TeO(2)-10GeO(2)-10K(2)O-10Li(2)O-10Nb(2)O(5) glasses is reported. Intense blue upconversion luminescence at 485 nm corresponding to the Tm3+ (1)G(4)--> H-3(6) transition with a measured absolute power of 0.1 muW for 800 mW excitation power at room temperature is observed. The experimental results also revealed a sevenfold enhancement in the upconversion efficiency when the sample was heated from room temperature to 235 degreesC yielding 0.7 muW of blue absolute fluorescence power for 800 mW pump power. High brightness emission around 800 nm (F-3(4)--> H-3(6)) in addition to a less intense 655 nm ((1)G(4)--> H-3(4) and F-3(2,3)--> H-3(6)) fluorescence is also recorded. The energy upconversion excitation mechanism for thulium emitting levels is assigned to multiphonon-assisted anti-Stokes excitation of the ytterbium-sensitizer followed by multiphonon-assisted sequential energy-transfer processes. (C) 2001 American Institute of Physics.

Exploring the binding mechanism of Guaijaverin to human serum albumin: Fluorescence spectroscopy and computational approach

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Systematic studies of protein immobilization by surface plasmon field-enhanced fluorescence spectroscopy

The research interest of this study is to investigate surface immobilization strategies for proteins and other biomolecules by the surface plasmon field-enhanced fluorescence spectroscopy (SPFS) technique. The recrystallization features of the S-layer proteins and the possibility of combining the S-layer lattice arrays with other functional molecules make this protein a prime candidate for supramolecular architectures. The recrystallization behavior on gold or on the secondary cell wall polymer (SCWP) was recorded by SPR. The optical thicknesses and surface densities for different protein layers were calculated. In DNA hybridization tests performed in order to discriminate different mismatches, recombinant S-layer-streptavidin fusion protein matrices showed their potential for new microarrays. Moreover, SCWPs coated gold chips, covered with a controlled and oriented assembly of S-layer fusion proteins, represent an even more sensitive fluorescence testing platform. Additionally, S-layer fusion proteins as the matrix for LHCII immobilization strongly demonstrate superiority over routine approaches, proving the possibility of utilizing them as a new strategy for biomolecular coupling. In the study of the SPFS hCG immunoassay, the biophysical and immunological characteristics of this glycoprotein hormone were presented first. After the investigation of the effect of the biotin thiol dilution on the coupling efficiently, the interfacial binding model including the appropriate binary SAM structure and the versatile streptavidin-biotin interaction was chosen as the basic supramolecular architecture for the fabrication of a SPFS-based immunoassay. Next, the affinity characteristics between different antibodies and hCG were measured via an equilibrium binding analysis, which is the first example for the titration of such a high affinity interaction by SPFS. The results agree very well with the constants derived from the literature. Finally, a sandwich assay and a competitive assay were selected as templates for SPFS-based hCG detection, and an excellent LOD of 0.15 mIU/ml was attained via the “one step” sandwich method. Such high sensitivity not only fulfills clinical requirements, but is also better than most other biosensors. Fully understanding how LHCII complexes transfer the sunlight energy directionally and efficiently to the reaction center is potentially useful for constructing biomimetic devices as solar cells. After the introduction of the structural and the spectroscopic features of LHCII, different surface immobilization strategies of LHCII were summarized next. Among them the strategy based on the His-tag and the immobilized metal (ion) affinity chromatography (IMAC) technique were of great interest and resulted in different kinds of home-fabricated His-tag chelating chips. Their substantial protein coupling capacity, maintenance of high biological activity and a remarkably repeatable binding ability on the same chip after regeneration was demonstrated. Moreover, different parameters related to the stability of surface coupled reconstituted complexes, including sucrose, detergent, lipid, oligomerization, temperature and circulation rate, were evaluated in order to standardize the most effective immobilization conditions. In addition, partial lipid bilayers obtained from LHCII contained proteo-liposomes fusion on the surface were observed by the QCM technique. Finally, the inter-complex energy transfer between neighboring LHCIIs on a gold protected silver surface by excitation with a blue laser (λ = 473nm) was recorded for the first time, and the factors influencing the energy transfer efficiency were evaluated.

Fluorescence and photobleaching dynamics of single light-harvesting complexes

Single light-harvesting complexes LH-2 from Rhodopseudomonas acidophila were immobilized on various charged surfaces under physiological conditions. Polarized light experiments showed that the complexes were situated on the surface as nearly upright cylinders. Their fluorescence lifetimes and photobleaching properties were obtained by using a confocal fluorescence microscope with picosecond time resolution. Initially all molecules fluoresced with a lifetime of 1 ± 0.2 ns, similar to the bulk value. The photobleaching of one bacteriochlorophyll molecule from the 18-member assembly caused the fluorescence to switch off completely, because of trapping of the mobile excitations by energy transfer. This process was linear in light intensity. On continued irradiation the fluorescence often reappeared, but all molecules did not show the same behavior. Some LH-2 complexes displayed a variation of their quantum yields that was attributed to photoinduced confinement of the excited states and thereby a diminution of the superradiance. Others showed much shorter lifetimes caused by excitation energy traps that are only ≈3% efficient. On repeated excitation some molecules entered a noisy state where the fluorescence switched on and off with a correlation time of ≈0.1 s. About 490 molecules were examined.

Intra- vs intermolecular photoinduced electron transfer reactions of a macrocyclic donor-acceptor dyad

The synthesis, structural characterization, and photophysical behavior of a 14-membered tetraazamacrocycle with pendant 4-dimethylaminobenzyl (DMAB) and 9-anthracenylmethyl groups is reported (L-3, 6-((9-anthracenylmethyl)amino)-trans-6,13-dimethyl-13-((4-dimethylaminobenzyl)amino)-1,4,8,11-tetraaza-cyclotetradecane). In its free base form, this compound displays rapid intramolecular photoinduced electron transfer (PET) quenching of the anthracene emission, with both the secondary amines and the DMAB group capable of acting as electron donors. When complexed with Zn(II), the characteristic fluorescence of the anthracene chromophore is restored as the former of these pathways is deactivated by coordination. Importantly, it is shown that the DMAB group, which remains uncoordinated and PET active, acts only very weakly to quench emission, by comparison to the behavior of a model Zn complex lacking the pendant DMAB group, [ZnL2](2+) (Chart 1). By contrast, Stern-Volmer analysis of intermolecular quenching of [ZnL2](2+) by N,N-dimethylaniline (DMA) has shown that this reaction is diffusion limited. Hence, the pivotal role of the bridge in influencing intramolecular PET is highlighted.

Determination of the orientational distribution and orientation factor for transfer between membrane-bound fluorophores using a confocal microscope

Orientational fluorophores have been a useful tool in physical chemistry, biochemistry, and more recently structural biology due to the polarized nature of the light they emit and that fact that energy can be transferred between them. We present a practical scheme in which measurements of the intensity of emitted fluorescence can be used to determine limits on the mean and distribution of orientation of the absorption transition moment of membrane-bound. uorophores. We demonstrate how information about the orientation of. uorophores can be used to calculate the orientation factor k(2) required for use in FRET spectroscopy. We illustrate the method using images of AlexaFluor probes bound to MscL mechanosensitive transmembrane channel proteins in spherical liposomes.

Energy level decay and excited state absorption processes in erbium-doped tellurium glass

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

New bodipy dyads for two-photons fluorescence imaging in cells

Fluorescent probes are essential tools for studying biological systems. The last decade has witnessed particular interest in the development of two-photon excitable probes, due to their advantageous features in tissue imaging compared to the corresponding one-photon probes [1]. Recently, we have designed and synthetized an aminonaphthalimide–BODIPY derivative as energy transfer cassettes and were found to show very fast and efficient BODIPY fluorescence sensitization [2]. This was observed upon one- and two-photon excitation, which extends the application range of the investigated bichromophoric dyads in terms of accessible excitation wavelengths. In order to increase the two-photon absorption of the system aminonaphthalimide fluorophore was replace with a Prodan analog (BODIPY dyad 1), which presents found a variety of applications as probes and labels in biology [3]. The two-photon absorption cross-section  of the dyads is significantly incremented by the presence of the 6-acetyl-2-naphthylamine donor group. The emission maximum of a BODIPY fluorophore can significantly be red-shifted in comparison to their precursors by conjugation with aromatic aldehydes. [4] We use a synthetic strategy to obtain BODIPY dyad 2 that incorporates an imidazole ring. This molecule can be used in biological media as a near-neutral pH indicator based on one- and two-photon excitable BODIPY acceptor.

Quantum Dots As Biophotonics Tools.

This chapter provides a short review of quantum dots (QDs) physics, applications, and perspectives. The main advantage of QDs over bulk semiconductors is the fact that the size became a control parameter to tailor the optical properties of new materials. Size changes the confinement energy which alters the optical properties of the material, such as absorption, refractive index, and emission bands. Therefore, by using QDs one can make several kinds of optical devices. One of these devices transforms electrons into photons to apply them as active optical components in illumination and displays. Other devices enable the transformation of photons into electrons to produce QDs solar cells or photodetectors. At the biomedical interface, the application of QDs, which is the most important aspect in this book, is based on fluorescence, which essentially transforms photons into photons of different wavelengths. This chapter introduces important parameters for QDs' biophotonic applications such as photostability, excitation and emission profiles, and quantum efficiency. We also present the perspectives for the use of QDs in fluorescence lifetime imaging (FLIM) and Förster resonance energy transfer (FRET), so useful in modern microscopy, and how to take advantage of the usually unwanted blinking effect to perform super-resolution microscopy.

G-quadruplex Formation Enhances Splicing Efficiency Of Pax9 Intron 1.

G-quadruplexes are secondary structures present in DNA and RNA molecules, which are formed by stacking of G-quartets (i.e., interaction of four guanines (G-tracts) bounded by Hoogsteen hydrogen bonding). Human PAX9 intron 1 has a putative G-quadruplex-forming region located near exon 1, which is present in all known sequenced placental mammals. Using circular dichroism (CD) analysis and CD melting, we showed that these sequences are able to form highly stable quadruplex structures. Due to the proximity of the quadruplex structure to exon-intron boundary, we used a validated double-reporter splicing assay and qPCR to analyze its role on splicing efficiency. The human quadruplex was shown to have a key role on splicing efficiency of PAX9 intron 1, as a mutation that abolished quadruplex formation decreased dramatically the splicing efficiency of human PAX9 intron 1. The less stable, rat quadruplex had a less efficient splicing when compared to human sequences. Additionally, the treatment with 360A, a strong ligand that stabilizes quadruplex structures, further increased splicing efficiency of human PAX9 intron 1. Altogether, these results provide evidences that G-quadruplex structures are involved in splicing efficiency of PAX9 intron 1.

Photophysical properties and quantum chemical studies of poly(2,7-9,9'-dihexylfluorene-dyil)

This work reports the photophysical properties (excitation and fluorescence spectra, fluorescence quantum yield, fluorescence lifetimes) of the poly(2,7-9,9'-dihexylfluorene-dyil) in dilute solutions of four solvents (toluene, tetrahydrofuran, chloroform and ethyl acetate) as well as the properties in solid state. Photoluminescence showed spectra characteristic of disordered α-backbone chain conformation. Simulation of the electronic absorption spectra of oligomers containing 1 to 11 mers showed that the critical conjugation length is between 6 and 7 mers. We also estimated the theoretical dipole moments which indicated that a coil conformation is formed with 8 repeating units per turn. We also showed that some energy transfer process appears in solid state which decreases the emission lifetime. Furthermore, based on luminescent response of the systems herein studied and electroluminescent behavior reported on literature, both photo and electroluminescence emissions arise from the same emissive units.

Frequency upconversion luminescence from Yb(+3)-Tm(+3) codoped PbO-GeO(2) glasses containing silver nanoparticles

Infrared-to-visible and infrared-to-infrared frequency upconversion processes in Yb(3+)-Tm(3+) doped PbO-GeO(2) glasses containing silver nanoparticles (NPs) were investigated. The experiments were performed by exciting the samples with a diode laser operating at 980 nm (in resonance with the Yb(3+) transition (2)F(7/2)->(2)F(5/2)) and observing the photoluminescence (PL) in the visible and infrared regions due to energy transfer from Yb(3+) to Tm(3+) ions followed by excited state absorption in the Tm3+ ions. The intensified local field in the vicinity of the metallic NPs contributes for enhancement in the PL intensity at 480 nm (Tm(3+) :(1)G(4)->(3)H(6)) and at 800 nm (Tm(3+) : (3)H(4) -> (3)H(6)). (C) 2009 American Institute of Physics. [doi:10.1063/1.3211300]