Poleable Nanoparticles as Fillers Towards Non-linear Optically Active Actuators

A new type of poleable dielectric elastomer is introduced herein. The elastomer contains polymer nanoparticles with frozen molecular dipoles, which can be oriented at elevated temperatures in an electric field via poling. The aim is to provide a soft material with high, tunable optical properties suitable for actuator and flexible electronics applications. To that end poleable polymeric nanoparticles with high dipole concentrations and glass transition temperatures well above room temperature will be needed to be used as filler in an elastomer matrix. The synthesis and characterization of such particles is presented in this manuscript. Polyhydroxyethyl methacrylate (PHEMA) nanoparticles were synthesized using miniemulsion polymerization. The particles were loaded with 4-[ ethyl (2-hydroxyethyl) amino]-4-nitrobenzene, usually called Disperse Red 1 (DR1), which has a large dipole moment (mu = 7.5 - 9.5 D). (1) The maximum dipole loadings is limited by the solubility of the dipole in the monomer solutions prior to polymerization. All samples show a glass transition temperature around 95 degrees C. Secondary electron microscopy (SEM) revealed spherical particles, the size of which was confirmed by dynamic light scattering (DLS) measurements. A composite was prepared by dispersing the particles in polydimethyl siloxane (PDMS).

Light trapping polymer-based coatings for cost effective flexible thin film photovoltaics

Marina Aymara González Lazo

Thin film silicon solar cells benefit from a lower fabrication cost than crystalline silicon cells, but they have a lower efficiency. The aim of this thesis was to explore the light-management capabilities of sub-micron polymer-based textures, both for the back reflector and for the front encapsulation of thin film silicon solar cells. The thin active layers of these devices require some form of photon path length enhancement to increase their efficiency. For this purpose, light trapping textures were replicated using transparent low shrinkage acrylate hyperbranched polymer nanocomposites. To enable low pressure replication of textures with enhanced mechanical performance, silica nanoparticles and silicon-based sol-gel precursors were combined to create novel low viscosity hybrid nanocomposite resins. Indeed, the viscosity of these hybrid suspensions was found to be one to two orders of magnitude lower than that of particulate suspensions with an equivalent silica loading. The hybrid resins were cured by combined UV polymerization and condensation, and the mechanical performance of the resulting nanocomposites was characterized in terms of their Vickers microhardness. The Vickers microhardness increased from 112 MPa for the unmodified polymer matrix to 190 MPa and 148 MPa for the hybrid nanocomposites and particulate nanocomposites, respectively, at 20 vol% silica, and reached as much as 287 MPa for the hybrid nanocomposites at 30 vol% silica. For the back reflector, light scattering (LS) textures in the form of random sub-micron pyramidal features were replicated in the nanocomposites using a nickel template and UV-nanoimprint lithography. The influence of nanoparticle content and pressure on the morphology and light scattering properties of the replicas was studied using scanning electron microscopy and optical analysis. The roughness and coherence length of the textures were similar to those of the template for all the compositions and process pressures investigated. After optimization of the dual-cure process, very high replication fidelity could be obtained in each case, leading to haze greater than 99 % over the whole of the visible light spectrum and very effective light scattering performance for a broad range of angles. A durable antireflective (AR) front encapsulation is also key to enhancing the photovoltaic performance of thin film silicon solar cells. The AR performance of hyperbranched polymer nanocomposite textures replicated from a moth eye pattern using UV-nanoimprint lithography was established both experimentally and through simulations. Optimum patterns were found on the basis of effective medium theory (EMT) to be arrays of paraboloids with a periodicity of 340 nm, which provided stable AR performance for aspect ratios in the range of 0.6 to 1.75. Good agreement was obtained between the simulated and measured optical behavior of such AR arrays, with a normal reflectance in the visible range of around 4 % when using a glass substrate. The simulations also predicted a reduction in the hemispherical reflectance from 15 % to 6 % for this texture. A hierarchical texture was replicated from a lotus leaf into the hyperbranched polymer modified with a perfluorinated acrylate, and the same material was also used to replicate the AR moth eye texture. In both cases, the replication fidelity was again found to be excellent and the textures exhibited improved hydrophobicity. [...]

EPFL2015

Chemistry and Analysis of Nanoparticle-Modified Living Cells

Tanja Thomsen

Conventional therapeutics are often limited by their targeting ability, resulting in harmful and potential fatal side-effects for the patients. Recently, new strategies have been developed to improve target specificity of drugs in order to generate more efficient therapeutics. An innovative concept, examined in this thesis, is the use of cells as therapeutic carriers. This concept termed cell-mediated drug delivery, relies on the intrinsic targeting properties of cells to transport the therapeutic drug to the target location. The conjugation of nanoparticles, loaded with drugs or adjuvants, to the surface of cells enables the transport to the desired location and increases the local drug concentration, reducing the required therapeutic leading to further attenuation of side effects. Chapter 1 introduces the concept of cell-mediated drug delivery. It highlights the advantages of using cells decorated with nanoparticles as carriers for therapeutics as opposed to free (polymer) nanoparticles. Examples of various covalent and non-covalent nanoparticle conjugations strategies are discussed and an overview of all approaches, which have been used previously to attach nanoparticles to the cell surface, is presented. The importance of a thorough in vitro characterization of nanoparticle-modified cells is emphasized. The use of common characterization techniques such as flow cytometry, UV-Vis and fluorescence spectroscopy, fluorescence and electron microscopy, and radiolabeling is discussed in detail and examples of qualitative and quantitative in vitro analysis of nanoparticle- modified cells are reviewed. Chapter 2 contains an extensive in vitro analysis of cell-nanoparticle conjugates using fluorescent- and fluorescent label-free methods. The stability of nanoparticles loaded with cargo attached to cells was investigated by encapsulating three different green fluorescent dyes into nanoparticles and tracking the fluorescence signal over 24 h. Two of the dyes revealed a complete loss of fluorescence of a time period of 24 h, whereas a third entrapped dye and a covalently attached dye did not have a decrease in fluorescence over 24 h. Finally, we developed a fluorescent label-free method as an alternative to characterize nanoparticle-decorated cells without the need for any fluorescent label. Chapter 3 systematically explores conjugation chemistries to immobilize two platforms of functional biodegradable polymer nanoparticles (PEG-PLA (poly(ethylene glycol) - poly(lactide)) nanoparticles and carboxylic PLA nanoparticles) on T cells as potential carrier across the blood-brain barrier (BBB). We compared a series of covalent and non-covalent immobilization strategies by flow cytometry (FACS), confocal microscopy, and impact on cell proliferation and cellular viability. We found that T-cell surface coupling of nanoparticles is dependent on the ratio of nanoparticles to cells and that higher ratios led to more particles attached to cells. Furthermore, the most efficient strategy to attach nanoparticles was via ligand-receptor interactions (lectin - sialic acid and biotin - NeutrAvidin). In addition, nanoparticle-decorated T cells were investigated for their ability to cross an in vitro mouse BBB model under static conditions in a two-chamber assay. Decorating the cells with nanoparticles does not affect their ability to bind ICAM-1 (a protein involved in the transmigration process) or to cross the model biological barrier.

EPFL2020

UV-nanoimprint lithography and large area roll-to-roll texturization with hyperbranched polymer nanocomposites for light-trapping applications

Caroline Calderone, Philippe Couty, Diego Fischer, Marina Aymara González Lazo, Aïcha Hessler-Wyser, Yves Leterrier, Jan-Anders Månson, Rémy Teuscher

Light-trapping textures were produced in hyperbranched polymer (HBP) silica nanocomposites using a UV-nanoimprint lithography (UVNIL) replication method, either in batch or roll-to-roll processes. The hardness of the HBP was found to increase by a factor of 2.5 with the addition of 50 vol% of nanoparticles. A nickel master with random sub-micron pyramidal structures was used to imprint nanocomposites containing up to 20 vol% of silica on a polyethylene naphthalate (PEN) substrate. The influence of nanoparticle fraction and pressure on the texture morphology and light scattering properties of the replicas was studied using scanning electron microscopy and optical analysis. The roughness and coherence length of the textures were similar to those of the master for all investigated compositions and process pressures. Likewise, the light scattering performance of aluminum-coated texturized nanocomposites was identical to that of the metal template, with a haze of 90% over the 400–800 nm spectral range. Thin film amorphous silicon solar cells were deposited on the texturized substrates using a large-area roll-to-roll process. The photocurrent of these devices was found to be 23% higher than the reference value of a flat cell.