Radiationless Energy Transfer Research Articles

Metal-Enhanced Fluorescence from Gold Surfaces: Angular Dependent Emission

The first observation of Metal-Enhanced Fluorescence (MEF) from large gold colloids is presented. Gold colloids, 40 and 200 nm diameter, were deposited onto glass substrates in a homogeneous fashion. The angular-dependent fluorescence emission of FITC-HSA, adsorbed onto gold colloids, was measured on a rotating stage which was used to evaluate MEF at all spatial angles. The emission intensity of FITC-HSA was found to be up to 2.5-fold brighter than the emission on bare glass substrates at an angle of 270 degrees. This is explained by the Radiating Plasmon Model, whereby the combined system, composed of the fluorophore and the metal colloids, emits with the photophysical characteristics of the fluorophore, after the excitation and the partial radiationless energy transfer between the excited states of the fluorophore and the surface plasmons of the gold colloids. The fluorescence enhancement was found to be higher with 200 nm gold colloids as compared to 40 nm colloids due to the increased contribution of the scattering portion of the 200 nm gold colloid extinction spectrum. These observations suggest that gold colloids could be used in MEF applications, offering more stable surfaces than the commonly used silvered surfaces, for applications requiring longer term storage and use.

MAPK-activated Protein Kinase-2 (MK2)-mediated Formation and Phosphorylation-regulated Dissociation of the Signal Complex Consisting of p38, MK2, Akt, and Hsp27

The p38 MAPK and heat shock protein 27 (hsp27) form a signaling complex with serine/threonine kinase Akt and MAPK-activated protein kinase-2 (MK2), which plays an important role in controlling stress-induced apoptosis and reorganizing actin cytoskeleton. However, regulation of the complex is poorly understood. In this study, the interaction between p38 and hsp27 was visualized in single living L929 cells using fluorescence resonance energy transfer technology, while their association with Akt was examined by immunoprecipitation analysis. Under normal growth conditions, p38 kinase constitutively interacts with hsp27. When cells were exposed to H(2)O(2) or stimulated by arachidonic acid, this interaction was disrupted. However, inhibition of the activation of p38 and Akt by selective inhibitors or overexpression of the kinase-dead mutant of p38 diminished such effects. Furthermore, mutation of phosphorylation sites of hsp27 renders the interaction resistant to H(2)O(2) and arachidonic acid. It was interesting to find that the interaction disappeared in the cells from MK2-knock-out mice or the cells treated with lemptomycin B that blocks export of MK2 from nucleus to cytosol. However, MK2 is not required for the association of hsp27 with Akt. This study suggests that MK2 mediates the incorporation of p38 into the pre-existing complex of hsp27 with Akt. Phosphorylation of hsp27 finally breaks the signaling complex.

Photonic and Electrochemical Properties of Adsorbed [Ru(dpp)2(Qbpy)]2+ Luminophores

Dense monolayers of [Ru(dpp)2Qbpy]2+, where dpp is 4,4'-diphenylphenanthroline and Qbpy is 2,2':4,4' ':4'4' '-quarterpyridyl, have been formed by spontaneous adsorption onto clean platinum microelectrodes. The cyclic voltammetry of these monolayers is nearly ideal, and three redox states are accessible over the potential range of +/-1.3 V. Chronoamperometry conducted on the microsecond time scale has been used to probe the dynamics of heterogeneous electron transfer and indicates that the standard heterogeneous electron-transfer rate constant, k degrees , is approximately 106 s-1. The metal complex emits at approximately 600 nm in fluid and solid solution as well as when bound to a platinum electrode surface within a dense monolayer. In the case of the monolayers, it appears that the excited states are not completely deactivated by radiationless energy transfer to the metal because electronic coupling between the adsorbates and the electrode is weak. The dynamics of lateral electron transfer between the electronically excited Ru2+* and ground-state Ru3+ species has been explored by measuring the luminescence intensity after defined quantities of Ru3+ have been produced electrochemically within the monolayer. The rate of lateral electron transfer is between 8 x 106 and 3 x 108 M-1 s-1, indicating efficient electron transfer between adsorbates in close-packed assemblies. Voltammetry conducted at megavolt per second scan rates has been used to directly probe the redox properties of the electronically excited species.

Luminescence, electronic absorption and vibrational IR and Raman studies of binary and ternary cerium ortho-, pyro- and meta-phosphates doped with Pr 3+ ions

The broad class of polycrystalline ortho-[Ba 3Ce(PO 4) 3, Ca 3Ce(PO 4) 3, Ba 6Ce(PO 4) 5, K 3Ce(PO 4) 2. Na 3Ce(PO 4) 2, Na 3− x K x Ce(PO 4) 2 ( x = 0.5, 1.5 and 2.5)], pyro-[NaCeP 2O 7] and meta-[NaCe(PO 3) 4, KCe(PO 3) 4, K 2Ce(PO 3) 5] phosphates was prepared in the solid state reaction. The Pr 3+ ions have been used as active probe for studies of the spectroscopic properties of these materials. Optical absorption, emission as well as infrared and Raman spectroscopic methods have been applied to characterise the properties and structure of the compounds studied. Their electronic spectra were discussed in terms of the Ce 3+ ion spectroscopic characteristics. The 2F 5/2 → 2F 7/2 transition appears in the typical for this ion region, i.e., at about 2000 cm −1. The absorption bands in the range 25,000–50,000 cm −1 have been assigned to the 4f 1 → 5d 1 transitions of the cerium ion. The role of these transitions in the radiative and radiation-less energy transfer mechanism was discussed. This paper discusses also the emission of Ce 3+ and Pr 3+ and Stokes shift.

Competition between inter- and intra- molecular energy exchanges in a simple quantum model of a dimer

We propose a fully quantum model to describe the dynamics of a possible radiationless energy transfer process between identical and nearly localized molecules or monomers coupled through a dipole–dipole term. The system is studied as an environmentally isolated dimeric pair and we find that its dynamics exhibits a competition between the process ruling out the transfer of energy among different degrees of freedom of a given monomer and the one steering the intermolecular passage of excitations from a monomer to the other one. Such a competition is quantitatively characterized investigating on the temporal behaviour of quantum covariances of some couples of appropriate observables having a clear physical meaning.

Studies of quenching and enhancement of fluorescence of methyl orange adsorbed on silver colloid

The surface fluorescence of methyl orange (MO) adsorbed onto colloidal silver was investigated. It was found that the 420 nm fluorescence band of MO aqueous solution quenched sharply. While the enhancement factor of the other fluorescence band at 530 nm was up to 20, and at the same time, the band had a red shift of 30 nm. These notable phenomena are explained in the main aspects of the influence of fluorescence quenching and fluorescence enhancing by the radiationless energy transfer channel.

Fluorescence resonance energy transfer disposable sensor for copper(II)

A disposable sensor has been developed for the measurement of copper(II) concentration in aqueous solution based on a change in the fluorescence of porphyrazine 2,7,12,17-tetra- tert-butyl-5,10,15,20-tetraaza-21H,23H-porphine (TP). The sensor was constructed by spin-coating a polyester support with a PVC solution containing TP, a plasticizer, the chelating agent Zincon and the ion-pairing benzetonium chloride. The measurement principle is based on the radiationless resonance energy transfer (RET) from TP immobilized in membrane, and acting as fluorescence donor, to Zincon acting as an acceptor induced by copper(II). The absorption spectrum of the Zincon–Cu(II) complex presents adequate overlapping with the emission spectrum of TP, producing a useful analytical signal by the RET process. The disposable sensor responds to copper(II) irreversibly over a dynamic range from 0.039 to 14 μmol L −1 (2.5–890 μg L −1) with a sensor-to-sensor reproducibility (relative standard deviation RSD) of 1.9%, as log a C u 2 + , at the medium level of the range and a response time of 10 min. The performance of the optical disposable sensor was tested for the analysis of copper in different types of natural waters (river, well, spring and swimming pool), validating results against a reference procedure.

Migration-assisted energy transfer at conjugated polymer/metal interfaces

The dynamics of exciton quenching in a conjugated polymer due to the presence of metal films is analyzed using time-resolved photoluminescence. The quenching is governed by direct radiationless energy transfer to the metal and is further enhanced by diffusion of excitons into the depletion area of the exciton population at the polymer/metal interface. The time-resolved luminescence is described by a numerical exciton diffusion model with the energy transfer incorporated via long-range dipole-dipole interaction at the metallic mirror. This allows us to disentangle the contributions from direct energy transfer to the metal and exciton migration, to the exciton quenching process. For an aluminum electrode strong exciton quenching occurs in a region of typically $15\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, which can be decomposed in a characteristic energy-transfer range of $7.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and an exciton diffusion length of $6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.

Strong luminescence intensity modulation near a metal-organic interface

The dependence of the luminescence intensity on the distance between an ultra thin PPV probe layer and a metal surface is investigated. We have employed a new Layer-by-Layer method, which allows control of layer deposition at the monolayer level. A precise variation of the metal/PPV separation over the entire substrate surface was achieved by the deposition of an inert spacer layer. A strong dependence of the PPV emission on the spacer layer and metal thickness is observed. The effect of the morphology of the metal surface is also discussed. These results are explained in terms of a competing short range radiationless energy transfer and intensity modulation due to interference effects produced by the optical cavity formed by metal and polymer films.

Effects of Chlorpromazine·HCl on the Structural Parameters of Bovine Brain Membranes

Fluorescence probes located in different membrane regions were used to evaluate the effects of chlorpromazine .HCl on structural parameters (transbilayer lateral mobility, annular lipid fluidity, protein distribution, and lipid bilayer thickness) of synaptosomal plasma membrane vesicles (SPMVs) isolated from bovine cerebral cortex. The experimental procedure was based on the selective quenching of 1,3-di(1-pyrenyl)propane (Py-3-Py) by trinitrophenyl groups, radiationless energy transfer from the tryptophan of membrane proteins to Py-3-Py, and energy transfer from Py-3-Py monomers to 1-anilinonaphthalene-8-sulfonic acid (ANS). In this study, chlorpromazine .HCl decreased the lateral mobility of Py-3-Py in a concentration dependent-manner, showed a greater ordering effect on the inner monolayer than on the outer monolayer, decreased annular lipid fluidity in a dose dependent-manner, and contracted the membrane lipid bilayer. Furthermore, the drug was found to have a clustering effect on membrane proteins.

A miniaturized heterogeneous fluorescence immunoassay on gold-coated nano-titer plates.

A miniaturized heterogeneous fluorescence immunoassay utilizing radiationless energy transfer to gold surfaces for fluorescence quenching is described. The phase-separation fluorescence immunoassay (PSFIA), a competitive heterogeneous assay, is carried out in the nanowells of a gold-coated nano-titer plate (NTP). Small analytes such as atrazine and histamine can be detected in nanoliter volumes with low limits of detection, maintaining a high degree of parallelization and miniaturization. The assay is characterized and optimized with respect to gold-layer thickness, surface concentration of immobilized analyte derivative, and antibody concentration. Various immobilization strategies are discussed.

Effects of dopamine HCl on structural parameters of bovine brain membranes.

Fluorescence probes located in different membrane regions were used to evaluate the effect of dopamine.HCl on the structural parameters (transbilayer lateral mobility, annular lipid fluidity, protein distribution, and thickness of the lipid bilayer) of synaptosomal plasma membrane vesicles (SPMV), which were obtained from the bovine cerebral cortex. An experimental procedure was used based on selective quenching of 1,3-di(1-pyrenyl)propane (Py-3-Py) by trinitrophenyl groups, and radiationless energy transfer from the tryptophan of membrane proteins to Py-3-Py and energy transfer from Py-3-Py monomers to 1-anilinonaphthalene-8-sulfonic acid (ANS) was also utilized. Dopamine.HCl increased both the bulk lateral mobility and annular lipid fluidity, and it had a greater fluidizing effect on the inner monolayer than on the outer monolayer. Furthermore, the drug had a clustering effect on membrane proteins.

Energy transfer—room temperature phosphorescence for the optosensing of transition metal ions

A room temperature phosphorescence (RTP) method for the optosensing of environmentally important transition metals (Hg(II), Cd(II), Pb(II), Zn(II), Cu(II), Ni(II), Co(II) and Fe(III)) in waters is described. The measurement principle is based on the radiationless energy transfer (ET) from the Al-ferron chelate immobilised on Dowex 1X2-200, and acting as the phosphorescent donor, to dithizone dye acting as the acceptor. Dithizone presents an absorption spectrum exhibiting adequate spectral overlap with the RTP emission spectrum of the donor and so giving rise to useful analytical RTP signals. The method has been developed by means of a flow injection system, where the immobilised Al-ferron chelate is packed into a conventional flow cell placed inside the cell holder of a phosphorimeter. After measurement, the sensing phase can be regenerated using ethanol. Working under optimized experimental conditions, including 0.1 M sodium acetate/acetic acid, pH 5.0, and an acceptor concentration of 5×10 −5 M, the selected metal ions form metal–dithizone complexes. The increase of the RTP signal observed can be related to the increase of the concentrations of the analytes (i.e. to the decrease of the free dithizone concentration). Both RTP intensity and triplet lifetime measurements can be used as analytical signals in the proposed method. Detection limits (3 σ) of 15–33 μg l −1 were found for the metal ions under study using RTP intensity measurements. The precision (R.S.D.) at 0.5 mg l −1 for all analytes was <1.8% ( n=5) and the calibration graphs were linear up to 1.5 mg l −1 (maximum concentration assayed).

Trapping energy from and injecting energy into dye-zeolite nanoantennae.

A photonic antenna is an organized multi-component arrangement in which several chromophores absorb the incident light and channel the excitation energy to a common acceptor component. Numerous attempts to make such systems have been presented in the literature. We have developed photonic antennae based on a host-guest system which shows exciting properties. The host material is zeolite L, a hexagonal crystal with one-dimensional channels along the crystal axis. As guests we have used a wide range of highly fluorescent dye molecules which can enter the channels but do not interact electronically because of geometrical constraints. These systems are able to collect and transport excitation energy by radiationless energy transfer. Light shining on the cylinder is first absorbed and the energy is then transported by the dye molecules inside the tubes to a desired region. The principle of two types of photonic antenna materials is illustrated below:

Time-Resolved Luminescence Energy Transfer Immunobinding Study Using a Ruthenium–Ligand Complex as a Donor Label

A novel immunosystem is described that exploits the effect of luminescence energy transfer from a luminescently labeled antigen to a fluorescent antibody. A luminescent ruthenium–ligand complex (D-455) with absorption/emission maxima at 456/639 nm, respectively, was employed as the donor label, and a squaraine-type cyanine label (636/655 nm), as the fluorescent acceptor label. Specifically, the system human serum albumin (HSA)/anti-HSA was studied. HSA was labeled with the donor dye D-455, and anti-HSA was labeled with the acceptor dye A-631. On formation of the antigen–antibody complex, energy transfer occurs. The radiationless energy transfer affects both the decay time of D-455 and the intensities of the emissions of both D-455 and A-631. The decay time of around 500 ns of D-455 allows frequency–domain measurements in the low kilohertz range and therefore can be based on the use of conventional optoelectronics. This also suggests gated measurements to be performed. The major difference from existing HSA immunosystems is the use of a slow decaying ruthenium–ligand complex as the donor and of a long-wave emitting cyanine acceptor dye having a high quantum yield and a decay kinetics that is governed by the rate of energy transfer from the slow decaying donor.

Fluorescence properties of tryptophan residues in the monomeric d-chain of Glossoscolex paulistus hemoglobin: an interpretation based on a comparative molecular model

The primary structure of the 142 residue Glossoscolex paulistus d-chain hemoglobin has been determined from Edman degradation data of 11 endo-Glu-C peptides and 11 endo-Lys-C peptides, plus the results of Edman degradation of the intact globin. Tryptophan occupies positions 15, 33 and 129. Homology modeling allowed us to assign the positions of these Trp residues relative to the heme and its environment. The reference coordinates of the indole rings (average coordinates of the C ε2 and C δ2 atoms) for W15 and W129 were 16.8 and 18.5 Å, respectively, from the geometric center of the heme, and W33 was located in close proximity to the heme group at a distance which was approximately half of that for W15 and W129. It was possible to identify three rotamers of W33 on the basis of electrostatic and Van der Waals energy criteria. The calculated distances from the center of the heme were 8.3, 8.4 and 9.1 Å for Rot1, Rot2 and Rot3, respectively. Radiationless energy transfer from the excited indole to the heme was calculated on the basis of Förster theory. For W33, the distance was more important than the orientation factor, κ 2, due to its proximity to the heme. However, based on κ 2, Rot2 (κ 2=0.945) was more favorable for the energy transfer than Rot1 (κ 2=0.433) or Rot3 (κ 2=0.125). In contrast, despite its greater distance from the heme, the κ 2 of W129 (2.903) established it as a candidate to be more efficiently quenched by the heme than W15 (κ 2=0.191). Although the Förster approach is powerful for the evaluation of the relative efficiency of quenching, it can only explain pico- and sub-nanosecond lifetimes. With the average lifetime, 〈τ〉=3 ns, measured for the apomonomer as the reference, the lifetimes calculated for each emitter were: W33-1 (1 ps), W33-2 (2 ps), W33-3 (18 ps), W129 (100 ps), and W15 (600 ps). Experimentally, there are four components for oxymonomers at pH 7: two long ones of 4.6 and 2.1 ns, which contribute approximately 90% of the total fluorescence, one of 300 ps (4%), and the last one of 33 ps (7.4%). It is clear that the equilibrium structure resulting from homology modeling explains the sub-nanosecond fluorescence lifetimes, while the nanosecond range lifetimes require more information about the protein in solution, since there is a significant contribution of lifetimes that resemble the apo molecule.

Playing with dye molecules at the inner and outer surface of zeolite L

Plants are masters of transforming sunlight into chemical energy. In the ingenious antenna system of the leaf, the energy of the sunlight is transported by chlorophyll molecules for the purpose of energy transformation. We have succeeded in reproducing a similar light transport in an artificial system on a nano scale. In this artificial system, zeolite L cylinders adopt the antenna function. The light transport is made possible by specifically organized dye molecules, which mimic the natural function of chlorophyll. Zeolites are crystalline materials with different cavity structures. Some of them occur in nature as a component of the soil. We are using zeolite L crystals of cylindrical morphology which consist of a continuous one-dimensional tube system and we have succeeded in filling each individual tube with chains of joined but noninteracting dye molecules. Light shining on the cylinder is first absorbed and the energy is then transported by the dye molecules inside the tubes to the cylinder ends. We expect that our system can contribute to a better understanding of the important light harvesting process which plants use for the photochemical transformation and storage of solar energy. We have synthesized nanocrystalline zeolite L cylinders ranging in length from 300 to 3000 nm. A cylinder of 800 nm diameter, e.g. consists of about 150 000 parallel tubes. Single red emitting dye molecules (oxonine) were put at each end of the tubes filled with a green emitting dye (pyronine). This arrangement made the experimental proof of efficient light transport possible. Light of appropriate wavelength shining on the cylinder is only absorbed by the pyronine and the energy moves along these molecules until it reaches the oxonine. The oxonine absorbs the energy by a radiationless energy transfer process, but it is not able to send it back to the pyronine. Instead it emits the energy in the form of red light. The artificial light harvesting system makes it possible to realize a device in which different dye molecules inside the tubes are arranged in such a way that the whole visible spectrum can be used by conducting light from blue to green to red without significant loss. Such a material could conceivably be used in a dye laser of extremely small size. The light harvesting nanocrystals are also investigated as probes in near-field microscopy, as materials for new imaging techniques and as luminescent probes in biological systems. The extremely fast energy migration, the pronounced anisotropy, the geometrical constraints and the high concentration of monomers which can be realized, have great potential in leading to new photophysical phenomena. Attempts are being made to use the efficient zeolite-based light harvesting system for the development of a new type of thin-layer solar cell in which the absorption of light and the creation of an electron-hole pair are spatially separated as in the natural antenna system of green plants. Synthesis, characterization and applications of an artificial antenna for light harvesting within a certain volume and transport of the electronic excitation energy to a specific place of molecular dimension has been the target of research in many laboratories in which different approaches have been followed. To our knowledge, the system developed by us is the first artificial antenna which works well enough to deserve this name. Many other highly organized dye–zeolite materials of this type can be prepared by similar methods and are expected to show a wide variety of remarkable properties. The largely improved chemical and photochemical stability of dye molecules inserted in an appropriate zeolite framework allows us to work with dyes which otherwise would be considered uninteresting because of their lack of stability. We have developed two methods for preparing well-defined dye–zeolite materials, one of them working at the solid–liquid and the other at the solid–gas interface. Different approaches for preparing similar materials are in situ synthesis (ship in a bottle) or different types of crystallization inclusion synthesis.

Crystal Engineering as a Tool for Directed Radiationless Energy Transfer in Layered {Λ-[Ru(bpy)3]Δ-[Os(bpy)3]}(PF6)4

New types of crystal structures with new physical properties such as energy harvesting can be engineered by exploiting the potentiality of chiral recognition. In the proposed strategy one takes advantage of the possibility that in true racemates one enantiomeric component in the crystal packing may be replaced by a different molecule of the same chirality and similar shape. This opens the path to a number of new properties. In the present investigation, we demonstrate that a system can be engineered, in which homochiral layers of Λ-[Ru(bpy)3]2+ rigorously alternate with homochiral layers of Δ-[Os(bpy)3]2+. This arrangement is realized in {Λ-[Ru(bpy)3]Δ-[Os(bpy)3]}(PF6)4. Due to the deliberately introduced lower dimensionality, the new crystalline system exhibits fascinating properties, in particular with respect to an interplay of processes of interlayer and intralayer radiationless energy transfer. Interestingly, in this system it is possible to achieve a controlled accumulation of excitation energy on a single crystallographic Δ-[Os(bpy)3]2+ site. Moreover, the excitation energy is absorbed in a wide range from the UV to the red side of the visible by both Λ-[Ru(bpy)3]2+ and Δ-[Os(bpy)3]2+ units, and one observes an intense red/infrared and highly resolved emission only of the low-energy Δ-[Os(bpy)3]2+ site, irrespective of the excitation wavelength used. The crystal structure of this newly engineered compound is determined for both the room-temperature phase (P32, a = 10.7012(5) Å, c = 16.3490(10) Å) as well as for the low-temperature phase (P3, a = 18.4189(10) Å, c = 16.2309(9) Å).

Exciton Migration and Cathode Quenching in Organic Light Emitting Diodes

The quantum efficiency of organic light emitting devices depends on the exciton emission efficiency. Exciton quenching at the metal cathode can be responsible for lowering the device performance. We consider exciton quenching by the metal cathode, taking into account both exciton diffusion and radiationless energy transfer to the metal. The quenching effect in tris(8-hydroxyquinoline)aluminum is analyzed, and ways to improve the device efficiency by suppressing the interaction with the electrode are discussed.

Phosphorescence and fluorescence characterization of fluorescein derivatives immobilized in various polymer matrices

The phosphorescence quantum yield, zero-time anisotropy, and lifetime are the three factors determining the suitability of a phosphorescent dye for rotational diffusion experiments. Furthermore, knowledge of the absolute orientation of the phosphorescence dipole moment in the molecular frame is a prerequisite for a quantitative analysis of time-resolved phosphorescence anisotropy (TPA) measurements in orientationally anisotropic systems. These factors have been characterized for four dye molecules, consisting of a fluorescein core with two or four substituted heavy-atoms (iodine and bromine). The characterization was performed by a combination of spectral and time-resolved phosphorescence and fluorescence measurements on dye molecules embedded in immobilizing polymer matrices. A quantitative criterion was found for the time-region where to use which dye, as well as for the dye concentrations to be used. For the latter criterion the effect of Forster radiationless energy transfer on the phosphorescence anisotropy was treated explicitly. The phosphorescence dipole moment was found to be tilted somewhat out of the molecular plane, and aligned more towards the in-plane fluorescence dipole moment than to the in-plane S1←S0 absorption dipole moment. The results for the four fluorescein derivatives indicate that the effect of the spin-orbital coupling is to pull the in-plane component of the phosphorescence dipole moment towards the fluorescence dipole moment, causing an increase of the phosphorescence quantum yield. This effect is primarily influenced by the Z-number of the substituted heavy atoms. The number of substituted heavy atoms plays a less important role.