1,679 research outputs found

    Mechanisms of light energy harvesting in dendrimers and hyperbranched polymers

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    Since their earliest synthesis, much interest has arisen in the use of dendritic and structurally allied forms of polymer for light energy harvesting, especially as organic adjuncts for solar energy devices. With the facility to accommodate a proliferation of antenna chromophores, such materials can capture and channel light energy with a high degree of efficiency, each polymer unit potentially delivering the energy of one photon-or more, when optical nonlinearity is involved. To ensure the highest efficiency of operation, it is essential to understand the processes responsible for photon capture and channelling of the resulting electronic excitation. Highlighting the latest theoretical advances, this paper reviews the principal mechanisms, which prove to involve a complex interplay of structural, spectroscopic and electrodynamic properties. Designing materials with the capacity to capture and control light energy facilitates applications that now extend from solar energy to medical photonics. © 2011 by the authors; licensee MDPI, Basel, Switzerland

    Energy harvesting: a review of the interplay between structure and mechanism

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    The science of energy harvesting has recently undergone radical change, with the advent of new materials exploiting mechanisms fundamentally different from those of traditional solar cells. Utilizing principles that are in many cases acquired from breakthroughs in molecular photobiology, the introduction of a range of new synthetic polymers, multichromophore arrays and nanoparticle-based materials heralds a marked resurgence of interest, a shift of focus and heightened expectations in the science of light-harvesting. The interplay between structure and mechanism significantly impinges upon issues extending from fundamental theory to the principles of energy-harvesting materials design. Understanding and exploiting the principles allows materials to be engineered that can harness absorbed energy with heightened efficiency. Two of the key areas of application are dendrimers and rare-earth doped solids

    Electronic coupling mechanisms and characteristics for optically nonlinear photoactive nanomaterials

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    In a range of nanophotonic energy harvesting materials, resonance energy transfer (RET) is the mechanism for the intermolecular and intramolecular transfer of electronic excitation following the absorption of ultraviolet/visible radiation. In the nonlinear intensity regime, suitably designed materials can exhibit two quite different types of mechanism for channeling the excitation energy to an acceptor that is optically transparent at the input frequency. Both mechanisms are associated with two-photon optical excitation - of either a single donor, or a pair of donor chromophores, located close to the acceptor. In the former case the mechanism is two-photon resonance energy transfer, initiated by two-photon absorption at a donor, and followed by RET directly to the acceptor. The probability for fulfilling the initial conditions for this mechanism (for the donors to exhibit two-photon absorption) is enhanced at high levels of optical input. In the latter twin-donor mechanism, following initial one-photon excitations of two electronically distinct donors, energy pooling results in a collective channeling of their energy to an acceptor chromophore. This mechanism also becomes effective under high intensity conditions due to the enhanced probability of exciting donor chromophores within close proximity of each other and the acceptor. In this paper we describe the detailed balance of factors that determines the favored mechanism for these forms of optical nonlinearity, especially electronic factors. Attention is focused on dendrimeric nanostar materials with a propensity for optical nonlinearity

    Energy transfer in macromolecular arrays

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    Design and Synthesis of Fluorescent Silica Nanoparticle Conjugates for Metal Ion Sensing Applications

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    A novel series of fluorescent nanoparticle conjugates were designed and synthesized for the selective turn-off sensing of low concentrations of Cu2+ ion (nM-μM) in 2:1 ethanol:10 mM HEPES (pH 7). Silica nanoparticles (~250 nm) were modified with heterobifunctional polyethylene glycol (PEG) linkers and a third generation PAMAM dendron to function as chelator for Cu2+ ions. The organic dye fluorescein isothiocyanate (FITC) was subsequently conjugated to the dendron to act as a fluorescent sensitizer for the nanoparticle that is quenched upon the binding of Cu2+. Fluorescent nanoparticle conjugates using third and fourth generation PAMAM dendrimers (SNP-G3-FITC and SNP-G4-FITC) were synthesized for comparison. The dendron conjugate (SNP-PEG8-G3S-FITC) was determined to have a higher dynamic range (0.10-1.99 μM Cu2+) than the dendrimer counterparts (0.02 -0.30 μM Cu2+). A follow- up series of sensors were designed and synthesized using FITC-conjugated silica nanoparticles for use in characterization of the surface using Fluorescence Resonance Energy Transfer (FRET). These conjugates provide examples of an attractive modular framework for other fluorescent nanoparticles to be synthesized and tailored to analytes of interest. Silica nanoparticle conjugates can be readily synthesized using known reactions, easily isolated from reaction solutions, and they remain undetectable in fluorescent experiments

    Impact of PAMAM dendrimers on the photophysics of linked fluorophores: a spectroscopy and microscopy approach.

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    Biophysics is a relatively new approach to the Life Sciences, founded on a strongly multidisciplinary background of Biology, Chemistry and Physics. It focuses on the biochemical processes that characterize the carbon-based life, and aims to explain them as fully as possible trying to frame them in a theoretical model. In order to reach this goal, fluorescence microscopy and spectroscopy, paticularly in time-resolved variants, are establishing as primary research tools. Some of the most recent and promising applications of Biophysics are in the biomedical field. One of the scientists’ ultimate purpose would be realizing nano-devices capable of diagnosing and healing the single diseased cells, without involving the healthy ones. It is evident how nano-medicine would be much less invasive and much more accurate than its macroscopic counterpart. To achieve this goal, it is desirable to develop and create a multivalent dispositive with the following characteristics: a scaffold that should be biocompatible and metabolizable by the body once its job is finished, cell-penetrating and able to carry targeting agents (peptides, antibodies, small ligands), sensing and/or imaging moieties (fluorophores or probes of other kind) and actuators (drugs or similar). In this view, many macromolecules have been proposed as building blocks for this nanotool, like nanoparticles, nanotubes and dendrimers. Dendrimers are highly branched synthetic polymeric molecules, with all bonds emanating radially from a central core with highly reproducible composition. They have revealed a considerable potential in several biological and biomedical applications; one of the interesting features of these macromolecules is that they have surface groups that can be successfully functionalized in a controlled way, e.g. with drugs, fluorophores, or other contrasting or sensing agents, in order to serve as biosensors in the cellular environment or as drug carriers once they are internalized in living cells or organisms ([1], [2]). This emerging scenario motivates the present thesis work. Indeed, while there exist many papers and works regarding (or even exploiting) fluorophore-functionalized dendrimers ([3], [4]), a careful analysis of the impact of dendrimers on the photophysics of those fluorophores is still missing. In the work described in this thesis, several of the fluorescence techniques most widely used in Biophysics have been employed to study the physical-chemical properties of Polyamidoamine (PAMAM) dendrimers functionalized with Carboxyfluorescein N-hydroxy succinimide ester (5(6)-FAM SE or NHS-carboxyfluorescein) fluorescent dyes. In particular, the dendrimers used are of Generation 4 (G4) and have an ethylendiamine core. Beyond the classical spectroscopic techniques aimed at investigating absorption and fluorescence properties of the samples, three specific fluorescence microscopy techniques have been exploited: Fluorescence Lifetime Imaging Microscopy (FLIM), Fluorescence Correlation Spectroscopy (FCS) and Single Molecule Detection (SMD) ([5]). The ultimate goal of this project is to study how the optical and mechanical properties of the samples may vary with respect to the dye alone by tuning relevant parameters, such as the charge on the surface of the dendrimer (i.e. by acetylating the usually positively charged amino end groups), the number of fluorophores on the surface, the pH, and other significant quantities that may affect the internalization, diffusion and the general behavior of dendrimers in cells. In order to reach this goal, molar extinction coefficients of the charged samples and quantum yields of either charged and acetylated samples were determined; these values were compared with the known ones of the dye alone (also confirmed by measurements I performed firsthand). We found that the direct linkage to the dendrimer surface affects the optical properties of the dye by a similar extent for charged and neutral samples: in particular, we observed a strong decrease in its average quantum yield in a range of about 80 − 90% for samples with different numbers of linked fluorophores close to physiological pH (7.4). Secondly, absorption and emission spectra at various pH were recorded for all of the charged samples and for two of the neutral ones: we noted differences in the shape and peak wavelength of the spectra with respect to those of the non-conjugated fluorophore, which we interpreted as caused by interactions between the dye(s) and the dendrimer local environment. Finally, measurements performed by means of fluorescence microscopy (FLIM, FCS) and Single-Molecule techniques allowed us to better elucidate the results obtained with spectroscopy methods, reaching the following conclusions: there are interactions between NHS-carboxyfluorescein fluorophores bound directly to the surface of PAMAM dendrimers with the dendrimers themselves, which affect significantly the photophysical properties of the dyes; there are different configurations which cause different brightnesses for the dyes; these configurations seem to evolve dynamically in the single dendrimer-dye(s) systems, probably according to the ever-changing local conformation of these complexes. This thesis is organized as follows: • Chapter 1 focuses on dendrimers, with particular attention to their chemical structure, the unique features of the dendritic architecture with respect to the linear one, and their biological applications. • Chapter 2 starts with a brief overview of the general theory that underlies the interactions between light and matter. In particular, I describe the processes of absorption and emission of radiation, the mechanism of fluorescence and its typical characteristics, such as lifetime, quantum yield and quenching. Then, the main spectroscopic and fluorescence microscopy techniques exploited during the experimental part of this project are illustrated, with particular emphasis on the three mentioned above. • In Chapter 3, after a brief outline of the chemical and optical properties of fluorescein and its derivative dyes, the materials and methods are listed, and the obtained results reported, for each technique employed. • Chapter 4 reports the main experimental results of this thesis work. In this chapter (and in the conclusions) a qualitative model of interpretation for the peculiar behavior of the system dendrimer-fluorophores is proposed and the evidences observed in the experiments are enclosed in a global descriptive frame. Moreover, as an example of future perspectives of this thesis work, I report a comparison with an improved sample developed at NEST Laboratories, with a spacer inserted between the dendrimer surface and the fluorophore in order to stabilize its optical properties by reducing the interactions with the dendrimer. References [1] C.C. Lee , J.A. MacKay, J.M. Frechet, F.C. Szoka Designing Dendrimers for Biological Applications. Nat. Biotechnol. 2005, 23 (12) 1517-26. [2] J.M. Oliveira, A.J. Salgadoc, J.F. Manoa and R.L. Reisa Dendrimers and Derivatives as a Potential Therapeutic Tool in Regenerative Medicine Strategies - A Review. Progress in Polymer Science, 35, 2010, 1163-1194. [3] L. Albertazzi, M. Serresi, A. Albanese and F. Beltram Dendrimer Internalization and Intracellular Trafficking in Living Cells. Mol. Pharmaceutics, 2010, 7 (3), pp 680-688. [4] A. Saovapackhiran, A. D’Emanuele, D. Attwood and J. Penny Surface Modification of PAMAM Dendrimers Modulates the Mechanism of Cellular Internalization. Bioconjugate Chem., 2009, 20, 693-701. [5] J. R. Lakowicz Principles of Fluorescence Spectroscopy, Third Edition, Chapters 22, 23, 24. Springer, 2006

    Ultrafast Non-Forster Intramolecular Donor Acceptor Excitation Energy Transfer

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    Ultrafast intramolecular electronic energy transfer in a conjugated donor-acceptor system is simulated using nonadiabatic excited-state molecular dynamics. After initial site-selective photoexcitation of the donor, transition density localization is monitored throughout the S-2 -> S-1 internal conversion process, revealing an efficient unidirectional donor acceptor energy-transfer process. Detailed analysis of the excited state trajectories uncovers several salient features of the energy-transfer dynamics. While a weak temperature dependence is observed during the entire electronic energy relaxation, an ultrafast initially temperature-independent process allows the molecular system to approach the S-2-S-1 potential energy crossing seam within the first ten femtoseconds. Efficient energy transfer occurs in the absence of spectral overlap between the donor and acceptor units and is assisted by a transient delocalization phenomenon of the excited-state wave function acquiring Frenkel-exciton character at the moment of quantum transition.This project has received funding from the Universidad Carlos III de Madrid, the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement n° 600371, el Ministerio de Economia y Competitividad (COFUND2014-51509), el Ministerio de Educación, cultura y Deporte (CEI-15-17) and Banco Santander. This work was partially supported by CONICET, UNQ, ANPCyT (PICT-2014-2662). We also acknowledge support of the Center for Integrated Nano-technology (CINT), a U.S. Department of Energy, Office of Basic Energy Sciences user facility, as well as additional funding from the Bavarian University Centre for Latin America (BAYLAT). The work in Mons is supported by BELSPO through the PAI P6/27 Functional Supramolecular Systems project and by the Belgian National Fund for Scientific Research FNRS/F.R.S. DB is a Research Director of FNRS
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