Direct Nonradiative Energy Transfer Research Articles

Optically stimulated luminescence in state-of-the-art LYSO:Ce scintillators enables high spatial resolution 3D dose imaging

In this contribution, we study the optically stimulated luminescence (OSL) exhibited by commercial hbox {Lu}_{(2-x)}hbox {Y}_xhbox {SiO}_5:Ce crystals. This photon emission mechanism, complementary to scintillation, can trap a fraction of radiation energy deposited in the material and provides sufficient signal to develop a novel post-irradiation 3D dose readout. We characterize the OSL emission through spectrally and temporally resolved measurements and monitor the dose linearity response over a broad range. The measurements show that the hbox {Ce}^{3+} centers responsible for scintillation also function as recombination centers for the OSL mechanism. The capture to OSL-active traps competes with scintillation originating from the direct non-radiative energy transfer to the luminescent centers. An OSL response on the order of 100 ph/MeV is estimated. We demonstrate the imaging capabilities provided by such an OSL photon yield using a proof-of-concept optical readout method. A 0.1 hbox {mm}^3 spatial resolution for doses as low as 0.5 Gy is projected using a cubic crystal to image volumetric dose profiles. While OSL degrades the intrinsic scintillating performance by reducing the number of scintillation photons emitted following the passage of ionizing radiation, it can encode highly resolved spatial information of the interaction point of the particle. This feature combines ionizing radiation spectroscopy and 3D reusable dose imaging in a single material.

Photoluminescence Quenching in Quantum Emitter, Metallic Nanoparticle, and Graphene Hybrids

We have developed a theory for photoluminescence quenching and plasmonic properties in hybrid nanosystems made from three nanosystems such as a quantum emitters, metallic nanoparticles, and graphene. The metallic nanoparticles and graphene have surface plasmons which couple with probe photons and create surface plasmon polaritons. Therefore, the excitons in the quantum emitters interact with surface plasmon polaritons via the dipole-dipole interaction. Due to this interaction, energy is exchanged between the nanosystems. The second quantized formulation and the quantum density matrix method have been used to calculate photoluminescence and the radiative and non-radiative decay processes in the presence of dipole-dipole interaction. We have compared our theory with experiments of two and three nanosystems, and a good agreement between theory and experiments is achieved. It has been found that the photoluminescence quenching in hybrid systems not only occurs through the direct non-radiative energy transfer from the quantum emitter to the metal nanoparticle and to graphene but also occurs through the indirect non-radiative energy transfer from quantum emitter to the metal nanoparticle via graphene and from the quantum emitter to graphene via metal nanoparticle. These are interesting findings and they can be used to fabricate nanoswitches and nanosensors for medical applications.

Energy Transfer Study of Symmetric Polyisoprene−Poly(methyl methacrylate) Diblock Copolymers Bearing Dyes at the Junctions: Dye Orientation

Direct nonradiative energy transfer experiments are described for the lamellar structures formed in films of poly(isoprene-b-methyl methacrylate) in which individual polymers are labeled at the junction with a single dye molecule. The donor was 9-phenanthryl (9Phe), and we compare two acceptors (1-anthryl (1An) and 2-anthryl (2An)) which differ in the orientation of their transition moments with respect to the polymer backbone. Experiments on the lamellar structures formed in films of PI-9Phe-PMMA/PI-1An-PMMA and PI-9Phe-PMMA/PI-2An-PMMA provide identical values of the interface thickness (1.1 ± 0.2 nm) between the PI and PMMA domains. The fact that a common value is obtained for the two sets of experiments indicates that the orientation parameter κ2 for the energy transfer process in this rigid system makes an identical contribution to both sets of experiments. We take these results as justification for using an averaged value of the orientation parameter 〈|κ|〉2 in fitting experimental data to the underlying theory of energy transfer. We find that in our experiments the local correlation in orientation of the polymer chains in the region of the interface is too small to be detected.

Dynamics of Chain Exchange Between Self-Assembled Diblock Copolymer Micelles of Poly(ethylene oxide)-block-Polylactide Studied by Direct Nonradiative Excitation Energy Transfer

A series of diblock poly(ethylene oxide)-block-polylactide copolymers with fairly narrow distribution of molar masses and compositions was prepared and characterized. The copolymers form multimolecular spherical micelles in 1,4-dioxane-water mixtures. The chain exchange between micelles formed by fluorescence-labeled copolymers was studied by direct nonradiative excitation energy transfer (NRET) fluorescence measurements in water-rich media containing 10 vol.% of 1,4-dioxane. The equilibration rate, i.e., the rate of unimer chain exchange between micelles obeys basically the theoretically predicted scaling relations. It slows down with the length of soluble blocks (a quadratic decrease) and considerably (an exponential decrease) with the length of insoluble chains. Scaling exponents were found lower than those predicted. The study shows that nanoparticle systems based on poly(ethylene oxide)-block-polylactide copolymers with required properties for various biomedical applications can be designed, prepared and their properties can be optimized.

Energy Transfer Study of the Interface Thickness in Symmetrical Isoprene−Methyl Methacrylate Diblock Copolymers

We describe direct nonradiative energy transfer (DET) experiments on mixtures of two symmetrical poly(isoprene-b-methyl methacrylate) block copolymers of similar molar mass and composition, bearing different dyes at the junction. One polymer has a donor dye (9-phenanthrene) at the junction, and the other has an acceptor dye (2-anthracene). Small-angle X-ray scattering experiments show that films of the mixed diblock copolymers have a lamellar morphology with a spacing that varies with composition from 24 to 26 nm. Fluorescence decay profiles from these films were analyzed in terms of an energy transfer model that takes account of the distribution of junctions across the interface. On the basis of the value R0 = 2.3 nm for the characteristic ET distance and an assumed value of 〈κ2〉 = 0.476 for the preaveraged orientation parameter for randomly oriented immobile dipoles, we calculate an interface thickness of 1.6 ± 0.1 nm. This value is independent of the acceptor/donor ratio (i.e., the acceptor concentration) in the films.

Thermodynamics and Morphology of Latex Blend Films

We propose a model to describe direct nonradiative energy transfer (DET) between donors and acceptors chemically attached to the two different components of a latex blend. The model describes the case of one polymer dispersed as spheres of identical diameter in a continuous matrix of the second polymer. The segment distribution of the two polymers in the interface region is calculated taking into account the binodal compositions of the polymers in the different blend domains, the amount of each component, and the theory of the interfacial region between two polymers. The model was used to characterize the interface between poly(butyl methacrylate) (PBMA) and poly(2-ethylhexyl methacrylate) (PEHMA) domains in a binary blend, that we have examined previously (Macromolecules 2000, 33, 5863). We found a drastic dependence of the donor fluorescence survival probability curves on the Flory−Huggins χ parameter and on the PBMA:PEHMA weight ratio. Since the latter ratio is known from the experimental conditions, we were able to calculate χ = 0.021 from the measurements with very low uncertainty, and infer the interface thickness between the domains in the blend as approximately 5.5 nm.

Review of polystyrene diffusion studies in latex particles by small-angle neutron scattering

This paper reviews small-angle neutron scattering (SANS) and some results from direct nonradiative energy transfer (DET), for the observation of the diffusion coefficients of polystyrene chains at latex interfaces. To compare SANS with DET, doubly labeled polystyrene with deuterium and fluorescence groups were synthesized, showing that while SANS and DET produce comparable data in terms of diffusion coefficients, both results differ in detail, each having their own advantages. Chain confinement, ionic end groups, and short branch effects on interdiffusion were studied. Large polymer chains confined in small particles have non-Gaussian shapes that store rubber elastic energy. Rapid, non-diffusion relaxation is inhibited because the density would be required to become less than normal. Hence confinement effects on the diffusion rate are not significant. Using the DET method, ionic end-groups were found to increase the early-time apparent interdiffusion coefficients during film formation. The early-time apparent diffusion coefficients of polystyrene with varying end-groups were found to increase as follows: The higher apparent diffusion coefficients of the chains with ionic groups are presumably due to a surface segregation of the end-groups caused by the polar, aqueous environment during latex synthesis. The interdiffusion behavior of sulfite-ended polystyrene (Mn ≅ 300 000 g/mol) with H-ends, one sulfite end, and two sulfite ends were compared via SANS and DET. The diffusion coefficients of polystyrene with one or two sulfite end groups were five times and ten times lower than that of polystyrene, respectively. The ionic end group effects on the reduced diffusion coefficients are interpreted as the competition between enhancement by the surface segregation of end groups and reduction by end group aggregation. Noting that sulfate end groups diffused faster, while sulfite end groups diffused slower, the effect is complex, and not yet fully resolved. Diffusion coefficients of polystyrene with branches were studied by DET. Short branches work to decrease the Tg and hence increase the diffusion coefficients. However, after the experimental temperature, T, is converted to a normalized temperature, T-Tg, the diffusion coefficients are found to be almost independent upon the number of branches and the length of branches. The branch length ranged from one-carbon to 40 carbons. Side chains of entanglement molecular weight or longer may be required to significantly reduce the diffusion coefficient. Copyright © 2002 John Wiley & Sons, Ltd.

Hydrophobically Modified Amphiphilic Block Copolymer Micelles in Non-Aqueous Polar Solvents. Fluorometric, Light Scattering and Computer-Based Monte Carlo Study

The micellization behavior of a hydrophobically modified polystyrene-block-poly(methacrylic acid) diblock copolymer, PS-N-PMA-A, tagged with naphthalene between blocks and with anthracene at the end of the PMA block, was studied in 1,4-dioxane-methanol mixtures by light scattering and fluorescence techniques. The behavior of a single-tagged sample, PS-N-PMA, and low-molar-mass analogues was studied for comparison. Methanol-rich mixtures with 1,4-dioxane are strong selective precipitants for PS. Multimolecular micelles with compact PS cores and PMA shells may be prepared indirectly by dialysis from 1,4-dioxane-rich mixtures, or by a slow titration of copolymer solutions in 1,4-dioxane-rich solvents with methanol under vigorous stirring. In tagged micelles, the naphthalene tag is trapped in nonpolar and fairly viscous core/shell interfacial region. In hydrophobically modified PS-N-PMA-A micelles, the hydrophobic anthracene at the ends of PMA blocks tends to avoid the bulk polar solvent and buries in the shell. The distribution of anthracene tags in the shell is a result of the enthalpy-to-entropy interplay. The measurements of direct nonradiative excitation energy transfer were performed to estimate the distribution of anthracene-tagged PMA ends in the shell. The experimental fluorometric data show that anthracene tags penetrate into the inner shell in methanol-rich solvents. Monte Carlo simulations were performed on model systems to get reference data for analysis of time-resolved fluorescence decay curves. A comparison of experimental and simulated decays indicates that hydrophobic traps return significantly deep into the shell (although not as deep as in aqueous media). The combined light scattering, fluorometric and computer simulation study shows that the conformational behavior of shell-forming PMA blocks in non-aqueous media is less affected by the presence of nonpolar traps than that in aqueous media.

Interface Thickness of a Styrene−Methyl Methacrylate Block Copolymer in the Lamella Phase by Direct Nonradiative Energy Transfer

This paper examines the use of direct nonradiative energy transfer (DET) experiments to determine the interface thickness in films of symmetric poly(styrene-b-methyl methacrylate) (PS−PMMA) block c...

Picosecond Fluorescence Studies of the Surface Morphology of Charged Polystyrene Latex Particles

Two negatively charged polystyrene latex dispersions with particles of similar diameter (around 100 nm) but different surface charge density (1.7 μC/cm2 for CG18 and 2.3 μC/cm2 for CG5) were compared in terms of their surface morphology. Since they were synthesized in the presence of a redox initiator (potassium persulfate/sodium metabisulfite), particles bore sulfate and sulfonate charged groups at the surface. Fluorescence decay measurements were performed in situ on these latex samples, using a pair of fluorescent cationic dyes adsorbed onto the surface: Rhodamine 6G (R6G) as the energy transfer donor and malachite green carbinol hydrochloride (MG) as the acceptor. Since the kinetics of the direct nonradiative energy transfer process between the dyes is sensitive to the donor and acceptor distributions, fluorescence decay measurements provide information about the binding distribution of the cationic dyes, which reflects the distribution of the negative charges at the latex particle surface. For the h...

In-line monitoring of immiscible polymer blends in a rheometer with a fiber-optics-assisted fluorescence detection system

In-line studies of the initial stages of shear-induced coalescence in two-phase polymer blends were carried out with a home-built device combining a cone and plate rheometer and a fiber-optic-assisted fluorescence detection system. A blend of 90 wt % poly(2-ethylhexyl methacrylate) (PEHMA) and 10 wt % poly(butyl methacrylate) (PBMA) was prepared by the casting of films onto a solid substrate from mixed aqueous latex dispersions of the two polymers. The dispersions were prepared via emulsion polymerization under conditions in which both components were formed as spherical particles with a very narrow size distribution. By using a 14:1 particle ratio of PEHMA to PBMA, we obtained films in which 120-nm PBMA particles were surrounded by a PEHMA matrix. The blend contained phenanthrene-labeled PBMA particles and anthracene-labeled PBMA particles in a ratio of 4:1, whereas the PEHMA matrix polymer was unlabeled. We monitored the anthracene-to-phenanthrene fluorescence intensity ratio (I470/I360) as a measure of direct nonradiative energy transfer from phenanthrene to anthracene, whereas the blend was sheared at different shear rates and temperatures. Under no-shear conditions, the results of in-line experiments were in good agreement with the results of off-line measurements of energy transfer by conventional techniques. In blends under shear, the two sets of experiments, in-line and off-line, did not agree with each other. The cause of this disagreement was associated with normal forces in the blend under shear that affected the optical path length and the relative intensities of the fluorescence signals of the phenanthrene and anthracene groups in the blend. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2302–2316, 2001

SANS Study of Sulfonate End Group Effect on Polystyrene Self-Diffusion

Film formation from a latex involves interdiffusion of polymer chains. The interdiffusion behavior of polystyrene with H ends, one sulfonate end, and two sulfonate ends are compared via small-angle neutron scattering (SANS) and a direct nonradiative energy transfer technique (DET) at short times. High molecular weight (Mn ≅ 300 000) anionically synthesized polystyrenes were confined in latex particles utilizing an artificial miniemulsification technique. Interdiffusion of the polystyrenes in a latex film was carried out at temperatures of 125−145 °C. The diffusion coefficients of polystyrene with H ends were five times and 10 times higher than that of polystyrene with one sulfonate end and two sulfonate ends, respectively. The probable cause is end-to-end aggregation of the chains, supported by the ratio Rg/M1/2 remaining substantially constant.

Cross-Linking, Miscibility, and Interface Structure in Blends of Poly(2-ethylhexyl methacrylate) Copolymers. An Energy Transfer Study

We describe the miscibility of blends involving poly(ethylhexyl methacrylate) [PEHMA] latex copolymers using the direct nonradiative energy transfer (DET) technique. When the polymers in both components of a blend are PEHMA homopolymers, we obtain a fully mixed film. When one of the components in the blend is replaced with a PEHMA copolymer containing 5 mol % tert-butylcarbodiimidoethyl methacrylate (tBCEMA), we also obtain a fully mixed film. However, if a PEHMA copolymer containing 11 mol % methacrylic acid (MAA) is mixed with PEHMA homopolymer, the miscibility between the polymers is limited and is reduced further when the amount of MAA is increased to 20 mol %. Using a distribution model for energy transfer, we were able to determine the evolution of the interface thickness with annealing time. The maximum interface thickness attained in these blends decreases from δ = 15 nm to δ = 8 nm when the content of MAA in the blends increases from 11 mol % to 20 mol %. A freshly formed solvent-cast film prepared from a 1:1 blend of the PEHMA copolymer containing 11 mol % MAA and the PEHMA copolymer containing 5 mol % tBCEMA exhibits some polymer segregation. This persists in the solid film when the film is annealed for short times (20 min) at 60 °C. Over longer times, mixing of the copolymers occurs and reaches completion. We attribute this increase in miscibility to the formation of graft copolymer, which serves as a compatibilizing agent, through the reaction between the −COOH and −NCC− groups. When a film of the same composition is prepared from a blend of the two latex dispersions and annealed, a fully mixed film also results.

Energy Transfer Studies of Interpenetrating Polymer Networks: Characterizing the Interface in Polyacrylate−Polyurethane IPNs

Interpenetrating polymer networks (IPNs) of polyurethane (PU) and polyacrylate (PA) were prepared to give very small domains (ca. 20 nm) of PA dispersed in a PU matrix. In these samples the PU phase is labeled with phenanthrene, a donor for energy transfer experiments, and the PA phase is labeled with anthracene, the corresponding acceptor. Direct nonradiative energy transfer from phenanthrene to anthracene was used to study the IPN morphology. The interface and mixing between the microphases were analyzed in terms of a Helfand and Tagami (H−T) profile, which is commonly used to describe the interface in polymer blend systems. Exact fluorescence theoretical decay profiles were calculated according to the H−T profiles in a restricted geometry. Experimental fluorescence decays were compared to the theoretical decays, and the interface thickness was recovered. At the level of the model we used in our analysis, we found strong similarities between these IPNs and traditional polymer blends.

Polymer diffusion in latex films at ambient temperature

Polymer diffusion across interfaces at room temperature (21°C) was analyzed by direct nonradiative energy transfer (DET) in labeled latex films. Two modellatex polymers were examined: poly(butyl methacrylate) [PBMA, Mw = 3.5 × 104, Tg (dry) = 21°C] and a copolymer of 2-ethylhexyl methacrylate with 10 wt % (acetoacetoxy)-ethyl methacrylate [P(EHMA-co-AAEM), Mw = 4.8 × 104, Tg (dry) = −7°C]. Little energy transfer due to polymer diffusion was detected for the P(EHMA-co-AAEM) latex samples in the dispersed state or dried to solids content below ca. 90%, but above 90% solids, diffusion occurs among particles. For PBMA, diffusion occurs only after the film is dried (>97% solids) and aged. In the dry PBMA films, it requires 4–5 days at 21°C to reach a significant extent of mixing (fm = 0.3–0.4). This corresponds to an estimated penetration depth dapp of 30–40 nm and a mean apparent diffusion coefficient (Dapp) of 5 × 10−4 nm2/s. The corresponding Dapp value for the dry P(EHMA-co-AAEM) sample is 5 × 10−2 nm2/s, and it takes about 25–40 min for this polymer to reach fm of 0.3–0.4 with dapp of 20–30 nm. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1129–1139, 1998

Film formation of latex

Latex film formation, the process by which an aqueous dispersion of polymer particles is transformed into a continuous material, has a direct bearing on the final film morphology and properties. Each of the primary stages of film formation (evaporative drying and ordering; particle deformation; and polymer interdiffusion) have been studied experimentally. Recently-developed non-invasive techniques, including direct nonradiative energy transfer, atomic force microscopy, small angle neutron scattering, environmental scanning electron microscopy, and various optical techniques have greatly enhanced the study of these stages. Even so, the exact cause (or causes) of particle deformation is still a topic of considerable debate. Several factors are experimentally known to influence latex film formation: the ambient conditions; the presence of surfactants, plasticizers and pigments; and latex particle structure. An important aim in the study of latex film formation is to understand the mechanisms by which these and other factors affect the process. New challenges lie in the study of film formation of core-shell latices, latex blends and new compositions.

Dipole−Dipole Electronic Energy Transfer. Fluorescence Decay Functions for Arbitrary Distributions of Donors and Acceptors. II. Systems with Spherical Symmetry

Many interesting systems of nanometer dimensions form spherically symmetric domains, either by design or through spontaneous self-assembly. The technique of direct nonradiative resonance energy transfer (DET) can be used to characterize the morphology of such structures on a nanometer scale. One needs to label the domain of interest with appropriate donor and acceptor dye moieties. One measures the fluorescence decay of the donors (ID(t)) in the presence of acceptors. The underlying geometry and physics of the system dictate how the dyes distribute themselves along the radial axis R of the system, according to concentration profiles (CD(R), and CA(R)) which need not be uniform. Because DET is sensitive to the distribution of dye interdistances, ID(t) contains information about the underlying morphology. In this work we obtain an analytic expression relating ID(t) to the donor/acceptor concentration profiles. This extends our previous contribution for systems with a plane of symmetry. The expression developed is general and capable of handling geometries in both extended and/or restricted spaces. It does not require a specific spatial locus for the donors nor does it require prior calculation of the pair-distribution function of the donor−acceptor distances. The development also provides one with an analytic expression for the evaluation of the interdistance distribution functions.

Direct non-radiative energy transfer across a sharp polymer interface

A simple interfacial model system is chosen in which fluorescent donors and acceptors attached to incompatible polymer chains are randomly distributed in respective phases separated by a sharp interface. Direct non-radiative energy transfer (DET) across the interface between the labeled polymer phases is observed by fluorescence decay measurements. The quantum efficiency of energy transfer ( Φ ET) is used to quantify DET. We show that Φ ET increases as the interfacial contact area between the phases increases and is inversely proportional to the diameter of the dispersed spherical donor-labeled particles. The data indicate that the degree of interpenetration at the interface is less than 2 nm.

Polyurethane−Polyacrylate Interpenetrating Networks. 2. Morphology Studies by Direct Nonradiative Energy Transfer Experiments

Two sequential urethane−acrylate interpenetrating network (IPN) systems were prepared in which the polyurethane (PU) phase is labeled with donor and acceptor dyes. Direct nonradiative energy transfer (DET) measurements on these systems indicate less efficient energy transfer in the IPN than in the corresponding pure PU matrix. This result is interpreted in terms of dilution of the dyes by mixing at the molecular level between the polyacrylate (PA) and PU components. Quantitative analysis of the changes in DET efficiency allows the extent of phase mixing to be calculated. The two SeqIPN's, and were labeled by incorporating phenanthrene and anthracene diols into the reaction mixture. Fluorescence decays of phenanthrene in these samples were measured by the single-photon-timing technique and analyzed in terms of both the Forster model for DET and the Perrin model for static quenching. Both analyses gave similar extents of phase mixing, and these values are in good accord with the results of electron microsco...

Effective Volume per Junction in Block Copolymer Interfaces Probed by Direct Energy Transfer

Direct nonradiative energy transfer (DET) experiments were carried out on a series of labeled polyisoprene-poly(methyl methacrylate) (PI-PMMA) and polystyrene-poly(methyl methacrylate) (PS-PMMA) diblock copolymers spanning a range of molecular weights and compositions. In all samples with spherical or cylindrical morphology, PMMA represents the major component, with PI or PS microdomains dispersed within the PMMA phase. These DET experiments yield the ratio of the microdomain size R to the interface thickness δ. We relate the R/δ ratio to the volume per junction υ j at the block copolymer interface and show that υ j does not depend upon the microdomain geometry or the overall block copolymer molecular weight, but only on the length N B of the shorter component. We derive an expression which accommodates the observed dependence of υ j on N B 1/3 .