Nonisothermal crystallization behavior of the poly(ethylene glycol) block in poly(L-lactide)-poly(ethylene glycol) diblock copolymers: Effect of the poly(L-lactide) block length

The confined crystallization behavior, melting behavior, and nonisothermal crystallization kinetics of the poly(ethylene glycol) block (PEG) in poly(L-lactide)poly(ethylene glycol) (PLLA-PEG) diblock copolymers were investigated with wideangle X-ray diffraction and differential scanning calorimetry. The analysis showed that the nonisothermal crystallization behavior changed from fitting the Ozawa equation and the Avrami equation modified by Jeziorny to deviating from them with the molecular weight of the poly(L-lactide) (PLLA) block increasing. This resulted from the gradual strengthening of the confined effect, which was imposed by the crystallization of the PLLA block. The nucleation mechanism of the PEG block of PLLA15000-PEG5000 at a larger degree of supercooling was different from that of PLLA2500-PEG5000, PLLA5000-PEG5000, and PEG5000 (the numbers after PEG and PLLA denote the molecular weights of the PEG and PLLA blocks, respectively). They were homogeneous nucleation and heterogeneous nucleation, respectively.

Isothermal crystallization behavior of the poly(L-lactide) block in poly(L-lactide)-poly(ethylene glycol) diblock copolymers: Influence of the PEG block as a diluted solvent

Isothermal crystallization kinetics and morphology of the poly(L-lactide) block in poly(L-lactide)poly(ethylene glycol) diblock copolymers were studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. The results were compared with that of the PLLA homopolymer. The introduction of the PEG block accelerated the crystallization rate of the PLLA block and promoted to form ring-banded spherulites. The analysis of isothermal crystallization kinetics has shown that the PLLA homopolymer accorded with the Avrami equation. But the PLLA block of the diblock copolymers deviated from the Avrami equation, which resulted from increasing of the crystallization rate and occurring of the second crystallization process. The equilibrium melting temperature (T,,) of the PLLA block fell with its molecular weight decreasing. The conditions to obtain more regular ring-banded spherulites were below: the sample was the PLLA block of LA(5) EG(5); the crystallization temperature was about from 95 degrees C to 100 degrees C, which almost corresponded to regime II.

Optimization Studies to Improve MSC-based Cardiac Cell Therapy : Cytokine Preconditioning and Nanoparticle Coupling

Contexte: La cardiopathie ischémique (IHD) reste une cause majeure de mortalité en Amérique du Nord. La thérapie cellulaire cardiaque (CCT) a émergé comme une thérapie prometteuse pour aider à guérir certaines malades cardiaques. Parmi les cellulaires avec propriétés pluripotentes, les cellules stromales mésenchymateuses (MSC) sont prometteuses. Cependant, plusieurs questions demeurent non résolues et certaines défis empêchent l'application clinique de la CCT se dans l'IHD, tels que le faible taux de rétention cellulaire in situ, le suivi des cellules in vivo post-implantation et post-acheminements et l`apoptose. Ici, le traitement préliminaire des MSC avec des facteurs de croissance et leur couplage avec des nanoparticules (NP) seront étudiés comme des méthodes pour optimiser MSC. Méthodes: Des MSCs provenant du rat (rMSC) et du cochon (pMSC) ont été isolés à partir de moelle osseuse. Les rMSC ont été préconditionnées avec SDF-1a, TSG-6 et PDGF-BB, et ensuite soumises à une hypoxie, une privation de sérum et a un stress oxydatif. Des études de cicatrisation ont également été effectués avec rMSCs préconditionnées. En parallèle, de nouvelles NP ferromagnétiques liées aux silicones ont été synthétisées. Les NPs ont été couplées aux pMSCs suivant leur fonctionnalisation avec l`anticorps, CD44, un antigène de surface du MSC bien connu. Par la suite, les études de biocompatibilité ont été réalisées sur pMSC-NP et en incluant des tests des processus cellulaires tels que la migration, l'adhésion, la prolifération et les propriétés de la différenciation. Résultats: Parmi toutes les cytokines testées, PDGF-BB a démontré la plus grande capacité à améliorer la survie de MSC dans des conditions d'hypoxie, de privation de sérum et en reponse au stress oxydatif. La conjugaison de NP a atténué la migration et la prolifération des pMSCs, mais n`a pas changé leur capacité de différenciation. Enfin, la complexe du MSC-NP est détectable par IRM. Conclusion: Nos données suggèrent que de nouvelles stratégies, telles que traitement préliminaire de PDGF-BB et le couplage des nanoparticules ferromagnétiques, peuvent être considérés comme des avenues prometteuse pour optimiser les MSCs pour la CCT.

Ethanol oxidation reactions using SnO(2)@Pt/C as an electrocatalyst

This paper presents a study on the ethanol oxidation reaction using SnO(2)@Pt/C core-shell structures as electrocatalysts. All the materials used, including Pt/C and PtSn/C E-tek, were 20% (w/w) metal on carbon. The formation of core-shell nanoparticles (SnO(2)@Pt/C) was measured by UV-vis spectrophotometry. X-ray diffraction measurements showed Pt (shell) diffraction patterns without influence from the SnO(2) core and without any shift in 2 theta values for Pt. The diameters of the core-shell particle structures, measured using high-resolution transmission electron microscopy images, were in the range of 3-16 nm. The electrochemical profile for SnO(2)@Pt/C in an acidic medium (H(2)SO(4) at a concentration of 0.5 mol L(-1)) was almost the same as the typical electrochemical behavior for Pt in an acidic medium. Furthermore, the onset potential for the ethanol oxidation reaction using SnO(2)@Pt/C was almost the same as that for PtSn/C E-tek (0.23 V versus the reversible hydrogen electrode). However, the mass current peak densities for ethanol oxidation were 50% higher on SnO(2)@Pt/C than on PtSn/C E-tek. In the polarization curve, the mass current density for ethanol oxidation was higher at all potentials for SnO(2)@Pt/C when compared to Pt/C and PtSn/C E-tek. At 0.5 V, the current mass density for ethanol oxidation on SnO(2)@Pt was 2.3 times of that for the same process on the commercial material. The electrocatalytic activity of SnO(2)@Pt/C for ethanol oxidation was associated with an increase in the electrochemically active surface area. However, an electronic effect should also be considered because the Pt shell changes its electronic structure in the presence of the foreign core. (C) 2010 Elsevier B.V. All rights reserved.

Análise de nanoestruturas por espectroscopia de impedância para células fotoeletroquímicas

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Novos materiais multifuncionais para aplicação em fotocatálise

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Design and Characterization of New Luminescent Sensors and Labels

The aim of this Ph.D. project has been the design and characterization of new and more efficient luminescent tools, in particular sensors and labels, for analytical chemistry, medical diagnostics and imaging. Actually both the increasing temporal and spatial resolutions that are demanded by those branches, coupled to a sensitivity that is required to reach the single molecule resolution, can be provided by the wide range of techniques based on luminescence spectroscopy. As far as the development of new chemical sensors is concerned, as chemists we were interested in the preparation of new, efficient, sensing materials. In this context, we kept developing new molecular chemosensors, by exploiting the supramolecular approach, for different classes of analytes. In particular we studied a family of luminescent tetrapodal-hosts based on aminopyridinium units with pyrenyl groups for the detection of anions. These systems exhibited noticeable changes in the photophysical properties, depending on the nature of the anion; in particular, addition of chloride resulted in a conformational change, giving an initial increase in excimeric emission. A good selectivity for dicarboxylic acid was also found. In the search for higher sensitivities, we moved our attention also to systems able to perform amplification effects. In this context we described the metal ion binding properties of three photoactive poly-(arylene ethynylene) co-polymers with different complexing units and we highlighted, for one of them, a ten-fold amplification of the response in case of addition of Zn2+, Cu2+ and Hg2+ ions. In addition, we were able to demonstrate the formation of complexes with Yb3+ an Er3+ and an efficient sensitization of their typical metal centered NIR emission upon excitation of the polymer structure, this feature being of particular interest for their possible applications in optical imaging and in optical amplification for telecommunication purposes. An amplification effect was also observed during this research in silica nanoparticles derivatized with a suitable zinc probe. In this case we were able to prove, for the first time, that nanoparticles can work as “off-on” chemosensors with signal amplification. Fluorescent silica nanoparticles can be thus seen as innovative multicomponent systems in which the organization of photophysically active units gives rise to fruitful collective effects. These precious effects can be exploited for biological imaging, medical diagnostic and therapeutics, as evidenced also by some results reported in this thesis. In particular, the observed amplification effect has been obtained thanks to a suitable organization of molecular probe units onto the surface of the nanoparticles. In the effort of reaching a deeper inside in the mechanisms which lead to the final amplification effects, we also attempted to find a correlation between the synthetic route and the final organization of the active molecules in the silica network, and thus with those mutual interactions between one another which result in the emerging, collective behavior, responsible for the desired signal amplification. In this context, we firstly investigated the process of formation of silica nanoparticles doped with pyrene derivative and we showed that the dyes are not uniformly dispersed inside the silica matrix; thus, core-shell structures can be formed spontaneously in a one step synthesis. Moreover, as far as the design of new labels is concerned, we reported a new synthetic approach to obtain a class of robust, biocompatible silica core-shell nanoparticles able to show a long-term stability. Taking advantage of this new approach we also showed the synthesis and photophysical properties of core-shell NIR absorbing and emitting materials that proved to be very valuable for in-vivo imaging. In general, the dye doped silica nanoparticles prepared in the framework of this project can conjugate unique properties, such as a very high brightness, due to the possibility to include many fluorophores per nanoparticle, high stability, because of the shielding effect of the silica matrix, and, to date, no toxicity, with a simple and low-cost preparation. All these features make these nanostructures suitable to reach the low detection limits that are nowadays required for effective clinical and environmental applications, fulfilling in this way the initial expectations of this research project.

Microfluidic device and interfacial transport: application to biomolecules and nanostructures

The aim of my dissertation is to provide new knowledge and applications of microfluidics in a variety of problems, from materials science, devices, and biomedicine, where the control on the fluid dynamics and the local concentration of the solutions containing the relevant molecules (either materials, precursors, or biomolecules) is crucial. The control of interfacial phenomena occurring in solutions at dierent length scales is compelling in nanotechnology for devising new sensors, molecular electronics devices, memories. Microfluidic devices were fabricated and integrated with organic electronics devices. The transduction involves the species in the solution which infills the transistor channel and confined by the microfluidic device. This device measures what happens on the surface, at few nanometers from the semiconductor channel. Soft-lithography was adopted to fabricate platinum electrodes, starting from platinum carbonyl precursor. I proposed a simple method to assemble these nanostructures in periodic arrays of microstripes, and form conductive electrodes with characteristic dimension of 600 nm. The conductivity of these sub-microwires is compared with the values reported in literature and bulk platinum. The process is suitable for fabricating thin conductive patterns for electronic devices or electrochemical cells, where the periodicity of the conductive pattern is comparable with the diusion length of the molecules in solution. The ordering induced among artificial nanostructures is of particular interest in science. I show that large building blocks, like carbon nanotubes or core-shell nanoparticles, can be ordered and self-organised on a surface in patterns due to capillary forces. The eective probability of inducing order with microfluidic flow is modeled with finite element calculation on the real geometry of the microcapillaries, in soft-lithographic process. The oligomerization of A40 peptide in microconfined environment represents a new investigation of the extensively studied peptide aggregation. The added value of the approach I devised is the precise control on the local concentration of peptides together with the possibility to mimick cellular crowding. Four populations of oligomers where distinguished, with diameters ranging from 15 to 200 nm. These aggregates could not be addresses separately in fluorescence. The statistical analysis on the atomic force microscopy images together with a model of growth reveal new insights on the kinetics of amyloidogenesis as well as allows me to identify the minimum stable nucleus size. This is an important result owing to its implications in the understanding and early diagnosis and therapy of the Alzheimer’s disease

Supramolecular approaches to organized luminescent nanostructures for sensing, labeling and imaging applications

The common thread of this thesis is the will of investigating properties and behavior of assemblies. Groups of objects display peculiar properties, which can be very far from the simple sum of respective components’ properties. This is truer, the smaller is inter-objects distance, i.e. the higher is their density, and the smaller is the container size. “Confinement” is in fact a key concept in many topics explored and here reported. It can be conceived as a spatial limitation, that yet gives origin to unexpected processes and phenomena based on inter-objects communication. Such phenomena eventually result in “non-linear properties”, responsible for the low predictability of large assemblies. Chapter 1 provides two insights on surface chemistry, namely (i) on a supramolecular assembly based on orthogonal forces, and (ii) on selective and sensitive fluorescent sensing in thin polymeric film. In chapters 2 to 4 confinement of molecules plays a major role. Most of the work focuses on FRET within core-shell nanoparticles, investigated both through a simulation model and through experiments. Exciting results of great applicative interest are drawn, such as a method of tuning emission wavelength at constant excitation, and a way of overcoming self-quenching processes by setting up a competitive deactivation channel. We envisage applications of these materials as labels for multiplexing analysis, and in all fields of fluorescence imaging, where brightness coupled with biocompatibility and water solubility is required. Adducts of nanoparticles and molecular photoswitches are investigated in the context of superresolution techniques for fluorescence microscopy. In chapter 5 a method is proposed to prepare a library of functionalized Pluronic F127, which gives access to a twofold “smart” nanomaterial, namely both (i)luminescent and (ii)surface-functionalized SCSSNPs. Focus shifts in chapter 6 to confinement effects in an upper size scale. Moving from nanometers to micrometers, we investigate the interplay between microparticles flowing in microchannels where a constriction affects at very long ranges structure and dynamics of the colloidal paste.

Nanocapsules for self-healing materials

Im Rahmen dieser Arbeit wurden neue Ansätze für das Konzept der kapselbasierten Selbstheilungsmaterialien untersucht. Die Verkapselung von Selbstheilungsreagenzien in funktionellen Nanokapseln wurde dabei mittels drei verschiedener Herstellungsmethoden in Miniemulsion durchgeführt. Zunächst wurde die Synthese von Kern-Schale-Partikeln mit verkapselten Monomeren für die Ringöffnungs-Metathese-Polymerisation über freie radikalische Polymerisation in Miniemulsionstropfen beschrieben. Durch orthogonale Reaktionen wurden dabei verschiedene chemische Funktionalisierungen in die Schale eingebracht. Die Rolle des Tensides, das Verhältnis von Kernmaterial zu Monomer sowie die Variation der Lösungsmittelqualität hatte dabei einen Einfluss auf die Struktur der Kolloide. Die Heilungsreagenzien blieben auch nach der Verkapselung aktiv, was durch erfolgreich durchgeführte Selbstheilungsexperimente gezeigt werden konnte. Im zweiten Abschnitt wurde die Synthese von Silica-Nanocontainern für Selbstheilungsmaterialien über Hydrolyse und Polykondensation von Alkoxysilanen an der Grenzfläche der Miniemulsionstropfen beschrieben. Dieser Ansatz ermöglichte die effiziente Verkapselung sowohl von Monomeren als auch von Lösungen der Katalysatoren für die Metathese-Polymerisation in einem Einstufenprozess. Die Größe der Kapseln, die Dicke der Schale und der Feststoffgehalt der Dispersionen konnte dabei in einem weiten Bereich variiert werden. Anhand von erfolgreich durchgeführten Selbstheilungsreaktionen, die über Thermogravimetrie und 13C-NMR-Spektroskopie verfolgt wurden, konnte gezeigt werden, dass die Selbstheilungsreagenzien nach der Verkapselung aktiv blieben. Das dritte Konzept behandelte die Herstellung von polymeren Nanokapseln mittels Emulsions-Lösungsmittelverdampfungstechnik, welche eine milde Methode zur Verkapselung darstellt. Es wurde eine allgemeine und einfache Vorgehensweise beschrieben, in der Selbstheilungsreagenzien in polymeren Nanokapseln unter Verwendung von kommerziell erhältlichen Polymeren als Schalenmaterial verkapselt wurden. Zudem wurden Copolymere aus Styrol und verschiedenen hydrophilen Monomeren über freie radikalische Polymerisation sowie über polymeranaloge Reaktionen hergestellt. Diese statistischen Copolymere waren ebenso wie Blockcopolymere zur Herstellung von wohldefinierten Kern-Schale-Nanopartikeln mittels Emulsions-Lösungsmittelverdampfungsprozess geeignet. rnrnDes Weiteren wurde ein neues Konzept für die Synthese von pH-responsiven Nanokapseln aus tensidfreien Emulsionen unter Verwendung von Copolymeren aus Styrol und Trimethylsilylmethacrylat beschrieben. Der vorgeschlagene synthetische Ansatz ermöglicht dabei die erste Synthese von Nanokapseln über den Emulsions-Lösungsmittelverdampfungsprozess in Abwesenheit eines Tensides. Eine vollständig reversible Aggregation ermöglichte eine leichte Trennung der Nanokapseln von der kontinuierlichen Phase sowie eine Erhöhung der Konzentration der Nanokapseldispersionen auf das bis zu fünffache. Darüber hinaus war es möglich, Selbstheilungsreagenzien in stabilem Zustand zu verkapseln. Abschließend wurde die elektrochemische Abscheidung von mit Monomer gefüllten Nanokapseln in eine Zinkschicht zur Anwendung im Korrosionsschutz behandelt.

Au@Hg nanoalloy formation through direct amalgamation: Structural, spectroscopic, and computational evidence for slow nanoscale diffusion

Dynamic core-shell nanoparticles have received increasing attention in recent years. This paper presents a detailed study of Au-Hg nanoalloys, whose composing elements show a large difference in cohesive energy. A simple method to prepare Au@Hg particles with precise control over the composition up to 15 atom% mercury is introduced, based on reacting a citrate stabilized gold sol with elemental mercury. Transmission electron microscopy shows an increase of particle size with increasing mercury content and, together with X-ray powder diffraction, points towards the presence of a core-shell structure with a gold core surrounded by an Au-Hg solid solution layer. The amalgamation process is described by pseudo-zero-order reaction kinetics, which indicates slow dissolution of mercury in water as the rate determining step, followed by fast scavenging by nanoparticles in solution. Once adsorbed at the surface, slow diffusion of Hg into the particle lattice occurs, to a depth of ca. 3 nm, independent of Hg concentration. Discrete dipole approximation calculations relate the UV-vis spectra to the microscopic details of the nanoalloy structure. Segregation energies and metal distribution in the nanoalloys were modeled by density functional theory calculations. The results indicate slow metal interdiffusion at the nanoscale, which has important implications for synthetic methods aimed at core-shell particles.

Influência da interação dipolar nas fases magnéticas de nanopartículas esféricas com estrutura núcleo@camada

Bi-magnetic core@shell nanoparticle has attracted attention several researchers because great applicability that they offer. The possibility of combining different functionalities of magnetic materials make them a key piece in many areas as in data processing permanent magnets and biomagnetics sistems. These nanoparticles are controlled by intrinsic properties of the core and shell materials as well as the interactions between them, besides size and geometry effects. Thus, it was developed in this thesis a theoretical study about dipolar interaction contribution between materials different magnetic properties in bi-magnetic core@shell nanoparticles conventional spherical geometry. The materials were analyzed CoFe2O4, MnFe2O4 e CoFe2 in various combinations and sizes. The results show that the impact of the core dipole field in the shell cause reverse magnetization early its, before of the core, in nanoparticle of CoFe2O4(22nm)@CoFe2(2nm), thereby causing a decrease coercivity field of 65% in comparection with simple nanoparticle of CoFe2O4 (HC=13.6 KOe) of same diameter. The large core anisotropy in conventional nanoparticle makes it the a stable dipolar field source in the shell, that varies length scale of the order of the core radius. Furthermore, the impact of dipolar field is greatly enhanced by the geometrical constraints and by magnetics properties of both core@shell materials. In systems with core coated with a thin shell of thickness less than the exchange length, the interaction interface can hold reversal the shell occurring an uniform magnetization reversal, however this effect only is relevant on systems where the dipole field effects is weak compared with the exchange interaction.

Influência da interação dipolar nas fases magnéticas de nanopartículas esféricas com estrutura núcleo@camada

Bi-magnetic core@shell nanoparticle has attracted attention several researchers because great applicability that they offer. The possibility of combining different functionalities of magnetic materials make them a key piece in many areas as in data processing permanent magnets and biomagnetics sistems. These nanoparticles are controlled by intrinsic properties of the core and shell materials as well as the interactions between them, besides size and geometry effects. Thus, it was developed in this thesis a theoretical study about dipolar interaction contribution between materials different magnetic properties in bi-magnetic core@shell nanoparticles conventional spherical geometry. The materials were analyzed CoFe2O4, MnFe2O4 e CoFe2 in various combinations and sizes. The results show that the impact of the core dipole field in the shell cause reverse magnetization early its, before of the core, in nanoparticle of CoFe2O4(22nm)@CoFe2(2nm), thereby causing a decrease coercivity field of 65% in comparection with simple nanoparticle of CoFe2O4 (HC=13.6 KOe) of same diameter. The large core anisotropy in conventional nanoparticle makes it the a stable dipolar field source in the shell, that varies length scale of the order of the core radius. Furthermore, the impact of dipolar field is greatly enhanced by the geometrical constraints and by magnetics properties of both core@shell materials. In systems with core coated with a thin shell of thickness less than the exchange length, the interaction interface can hold reversal the shell occurring an uniform magnetization reversal, however this effect only is relevant on systems where the dipole field effects is weak compared with the exchange interaction.

POLYMER-NANOPARTICLE COMPOSITES FOR APPLICATIONS IN FLEXIBLE ELECTRONICS

The aim of this dissertation was to investigate flexible polymer-nanoparticle composites with unique magnetic and electrical properties. Toward this goal, two distinct projects were carried out. The first project explored the magneto-dielectric properties and morphology of flexible polymer-nanoparticle composites that possess high permeability (µ), high permittivity (ε) and minimal dielectric, and magnetic loss (tan δε, tan δµ). The main materials challenges were the synthesis of magnetic nanoparticle fillers displaying high saturation magnetization (Ms), limited coercivity, and their homogeneous dispersion in a polymeric matrix. Nanostructured magnetic fillers including polycrystalline iron core-shell nanoparticles, and constructively assembled superparamagnetic iron oxide nanoparticles were synthesized, and dispersed uniformly in an elastomer matrix to minimize conductive losses. The resulting composites have demonstrated promising permittivity (22.3), permeability (3), and sustained low dielectric (0.1), magnetic (0.4) loss for frequencies below 2 GHz. This study demonstrated nanocomposites with tunable magnetic resonance frequency, which can be used to develop compact and flexible radio frequency devices with high efficiency. The second project focused on fundamental research regarding methods for the design of highly conductive polymer-nanoparticle composites that can maintain high electrical conductivity under tensile strain exceeding 100%. We investigated a simple solution spraying method to fabricate stretchable conductors based on elastomeric block copolymer fibers and silver nanoparticles. Silver nanoparticles were assembled both in and around block copolymer fibers forming interconnected dual nanoparticle networks, resulting in both in-fiber conductive pathways and additional conductive pathways on the outer surface of the fibers. Stretchable composites with conductivity values reaching 9000 S/cm maintained 56% of their initial conductivity after 500 cycles at 100% strain. The developed manufacturing method in this research could pave the way towards direct deposition of flexible electronic devices on any shaped substrate. The electrical and electromechanical properties of these dual silver nanoparticle network composites make them promising materials for the future construction of stretchable circuitry for displays, solar cells, antennas, and strain and tactility sensors.

Synteza, modyfikacja powierzchni i charakterystyka fizykochemiczna wielofunkcyjnych nanomateriałów luminescencyjnych zawierających jony pierwiastków ziem rzadkich

Wydział Chemii: Zakład Ziem Rzadkich