- Bayreuther Graduiertenschule für Mathematik und Naturwissenschaften (BayNAT) (20)
Development of an artificial silk protein on the basis of a lacewing egg stalk protein
- Silks are widely used in textile industry as clothing and furnishings due to their tensile strength, smoothness, soft texture, lustre, and drape. Most commonly silk of the mulberry silkworm Bombyx mori (B. mori) is used in such applications, however, silks evolved independently in many different arthropods for various purposes.1 During evolution the different silks were optimised for their task-specific uses over millions of years, e.g. adopting different mechanical properties. The mechanical properties mainly derive from the protein secondary structure and its higher order arrangement in silk fibres. Spider silk, for example, is known for its tensile properties surpassing nylon, Kevlar®, silkworm silk, and high-tensile steel.2-5 Beyond their mechanical properties, some silks are also reported to be biocompatible and non-immunogenic.6 One beneficial feature of silk proteins is the possibility to process them into various morphologies.7, 8
Several of these silk features make them interesting for material scientists, intending to produce silks with tuneable properties depending on the desired application, ranging from technical ones such as high performance fibres to medical ones such as drug delivery.
This thesis deals with the characterisation and reproduction of a less explored silk, the lacewing egg stalk silk. Mechanical testing revealed a strong dependence on the relative humidity. In the dry state at 30% relative humidity, the stalks are quite rigid and break at an elongation of 2% whereas at 70% and 100% relative humidity they elongate up to 434%. This extension is accompanied by a secondary structure change from cross-ß to parallel-ß. The cross-ß structure in unstretched stalks provides bending stiffness and rigidity to the stalk, and this bending stiffness gets lost when the stalks are stretched. In this thesis a model is proposed which explains these differences at various relative humidity on the molecular level, wherein changes in the strength of hydrogen bonds upon exposure to water (a hydrogen bond donor/acceptor) in combination with multiple disulphide cross-links (which are not affected by water) act together and are responsible for this behaviour.
Based on consensus sequences of published sequence data (derived from MalXB2 an egg stalk protein of Mallada signata (M. signata)),9 an engineered egg stalk protein named N[AS]8C was recombinantly produced.
To produce an artificial stalk, a droplet of a solution of purified N[AS]8C was placed on a substrate, and tweezers were used to pull out a fibre. After drying, and post treatment, the properties of the artificial stalks were investigated in comparison to the natural ones. Mechanical testing revealed similar behaviour at 30% relative humidity, but at 70% and 100% relative humidity the artificial stalks were not as extensible as the natural ones. This corresponds to the fact, that no cross-ß structure was formed, and, therefore, no rearrangement into parallel-ß structure was possible.
Subsequently, N[AS]8C was processed into non-fibrous morphologies. It was possible to produce capsules, hydrogels, foams, and films. The foams show an interesting micro and nano structure which differs from that of recombinant spider silk. The cavities are filled with a mesh of nano fibres building a 3D scaffold.
Films are a morphology with potential for application in cell culture. Fibroblast attachment on N[AS]8C films is quite poor. Therefore, we tried to induce guided fibroblast growth on patterned protein films. A first layer of the films was cast from ntagCysC16-c(RGDfK), an engineered spider silk protein coupled with the integrin recognition motif RGD to provide a protein layer to which fibroblasts attached well. The second protein layer was produced using a PDMS (polydimethylsiloxane) template and N[AS]8C. Fibroblasts grown on these films adhere only to the RGD modified spider silk and not to the N[AS]8C areas. A second feature of such films is to orient the fibroblasts on films with alternating lines of the two proteins. Such films might be useful for tissue engineering to control cell adhesion and get a structured cell pattern. This is essential for many tissues such as bones, muscles, and epithelia tissue. The low cell adhesion properties of N[AS]8C films might be interesting for coatings for applications where cell adhesion is not desired such as stents or catheters.
Monsoonal affected dynamics of nitrate and dissolved organic carbon in a mountainous catchment under intensive land-use
- In recent decades, complex mountainous landscapes have been increasingly under pressure by deforestation and intensified highland agriculture. This trend not only compromises the quality of water in the highlands, but also impacts the availability of water resources downstream. Hence, an effective and sustainable management of these mountainous regions is essential and needed to mitigate this risk. Developing sustainable management principles to ensure the freshwater supply, however, requires a profound understanding of decisive factors and processes that control the water quality in mountainous landscapes. Amongst other substances, nitrate and dissolved organic carbon (DOC) play a critical role in the ecosystem health of water bodies. The major focus of this dissertation is on determining the nitrate and DOC mobilization processes and dynamics in the Haean Catchment, a mountainous watershed located in South Korea, which is agriculturally productive and strongly influenced by the prevailing East-Asian monsoon. The primary objective was to identify decisive factors that control the nitrate and DOC mobilization processes and dynamics in such catchments.
To these ends, we conducted stream water quality and discharge measurements during a range of conditions, from monsoonal rainfall events to dry, baseflow conditions. Assessments were completed along the topographic elevation gradient of the catchment, from an upland deciduous forest, over areas intensively used for agriculture, down to the catchment outlet. In order to gain a better understanding of nutrient fate within the Haean Catchment, we investigated river-aquifer exchange fluxes. In addition to hydraulic gradient monitoring, we used heat as tracer. To resolve the river-aquifer exchange fluxes, we set up a two-dimensional flow and heat transport model using the numerical code HydroGeoSphere (HGS). Potential effects of river-aquifer exchange dynamics on local water quality were investigated by collecting both, river and groundwater samples. Furthermore, we determined the impact of the ridge and furrow cultivation that is commonly applied in South Korean dryland agriculture, on nitrate leaching and evaluated fertilizer best management practices (FBMPs) using a three-dimensional flow and solute transport model (HGS).
Our results show that DOC and nitrate sources as well as their mobilization differ between the forest and agricultural river sites. In the forest, elevated in-stream DOC concentrations during rainfalls were due to hydrologic flushing of soluble organic matter in upper soil horizons with a strong dependency on pre-storm wetness conditions. Nitrate contributions to the stream occurred predominantly along interflow transport pathways.
At the agricultural sites, in-stream DOC concentrations were considerably higher and supplied from adjacent rice paddies. The highest in-stream nitrate concentrations occurred in the lower agricultural part of the catchment. Through hydraulic gradient monitoring, we identified in this part of the catchment a distinct connection between the river and aquifer, and nitrate-rich groundwater inputs to the river elevated the in-stream nitrate concentration. In the mid-elevation portion of the catchment, however, a limited connectivity between the river and aquifer was identified and river water quality was consequently unaffected in these areas.
The results of our HGS modeling study show a high temporal and spatial variability in river-aquifer exchange fluxes with frequent flow reversals during the monsoon season. Our results also suggest that these flow reversals may enhance the natural attenuation of nitrate in the shallow groundwater below the stream. The simulation results on evaluating FBMPs demonstrate that applying a combination of several FBMPs such as an adapted placement and timing is recommended to minimize the risk of groundwater nitrate contamination.
Overall, this dissertation demonstrates that the hydrologic pathways resulting from the monsoon climate drive the in-stream DOC dynamics in the forested catchment, whereas sources and mobilization of DOC in downstream agricultural areas are mainly controlled by the specific land-use type. In particular, the widely used rice paddy “plot-to-plot” irrigation system in Korean highlands was shown to control in-stream DOC concentrations. Nitrate dynamics in streams and aquifers in the agricultural areas of the catchment reflect the combined effects of land-use type and monsoonal hydrology. Particularly, the highly variable river-aquifer exchange fluxes with frequent flow reversals, which may enhance the nitrate attenuation, are driven by monsoonal extreme precipitation events. Since it has been forecasted that global warming will increase the frequency and the intensity of extreme precipitation events also in other regions of the world, our results will become increasingly important in future water quality assessments and research.
Iron spin crossovers at high pressures and temperatures and their effects on materials relevant tot he Earth’s lower mantle and core
- Iron is the most abundant element by mass in the Earth. The iron content and its spin or oxidation state have a major influence on the physical properties of the main phases in the Earth’s interior. Therefore it is of vast importance to understand the behavior of iron in mineral phases at the temperature and pressure conditions of the Earth’s interior. This cumulative thesis investigates Fe spin crossovers in iron-containing magnesium aluminum silicates, iron-bearing silicate glasses, the iron carbide Fe3C and the effect of Fe spin crossovers on the Fe/Mg partitioning between perovskite and ferropericlase in pyrolitic model system of the Earth’s lower mantle. The goal is first to understand the nature of the Fe spin crossover in respect to its oxidation state and second to estimate the consequences of their occurrence to processes and the structure in the Earth. Central tools in these studies are laser heated diamond anvil cells, to reach the pressure and temperature conditions of the Earth’s interior, Mössbauer spectroscopy, which is a sensitive probe for detecting structural and spin changes in Fe-bearing materials, and analytical transmission electron microscopy, as a probe of chemistry and oxidation state on the nm-scale. In this cumulative thesis I present the results of five research articles. For the analysis of conventional and recently developed synchrotron energy domain Mössbauer spectra the computer program MossA is introduced, which builds the basis for the analysis and interpretation of the results for the other studies. Based on synchrotron Mössbauer spectroscopy and electrical conductivity measurements of Fe-bearing silicate aluminum perovskite it is shown that Fe3+ occupies the dodecahedral A-site of the perovskite structure and remains in the high-spin state throughout the pressure and temperature conditions of the Earth’s lower mantle. Furthermore, a study on the electronic behavior of Fe in a Fe2+-rich aluminous silicate glass and a Fe3+-rich sodium silicate glass infers that no sharp high spin to low spin crossover occurs in silicate melts in the Earth’s lower mantle. This result excludes the possibility of negatively buoyant melts in the lower mantle in an early magma ocean solely due to strong preferential partitioning of iron into the melt phase, which would be induced by a Fe low-spin bearing melt. New insights into to decoupled partitioning behavior of Fe2+ and Fe3+ between the two dominant phases of the Earth’s lower mantle, perovskite and ferropericlase, are presented. The intermediate spin to low spin crossover of Fe2+ in perovskite at about 110 GPa seems to have a strong effect on partitioning and oxidation state of Fe. It leads to a change of the partitioning behavior of Fe between perovskite and ferropericlase and induces a reduction of Fe3+ to Fe2+ in perovskite. Finally, a Mössbauer spectroscopic and single-crystal x-ray diffraction study of Fe3C reveals a two-stage loss of magnetism in Fe3C at high pressures at room temperature: a ferro- to paramagnetic transition around 8-10 GPa and a para- to nonmagnetic transition at about 22 GPa.
Describing Charge Transfer in Extended Donor-Acceptor Systems with Density Functional Theory
- It is a long-standing problem of (time-dependent) density functional theory ((TD)DFT) that traditional functionals severely underestimate charge transfer (CT) excitations. In particular, the theoretical description of donor-acceptor (DA) systems is plagued by this shortcoming. DA systems are frequently used as light absorbing components in organic photovoltaic devices. The lowest electronic excitation in these molecules is usually influenced by CT.
In order to support the systematic development of new DA systems that are needed to improve the efficiency of organic solar cells it is a prerequisite for theory to reliably predict the electronic properties of this system class.
We demonstrate that the tuned range-separated hybrid (RSH) approach predicts these excitations in accordance with experiment. The approach can be regarded as an implicitly defined density functional within the generalized Kohn-Sham (GKS) scheme of DFT. Its main ingredient is the range-separation parameter that determines the splitting between long- and short-range exchange. It is obtained from first principles by enforcing the ionization potential theorem of GKS theory.
We consider DA systems of various sizes that are composed of thiophene as donor and benzothiadiazole or naphthalene diimide as acceptor. We show how the optical and electronic properties can be tailored by changing the conjugation length and the arrangement of the donor and acceptor components. We also address the downsides that accompany the use of tuned RSH functionals. Due to the way the approach is implicitly defined anew for each system it is not size consistent. By calculating ground state properties of atoms and diatomic molecules we report size consistency errors and demonstrate consequences of the size consistency violation, e.g., the incorrect prediction of binding energies.
In order to reliably predict CT excitations within the Kohn Sham scheme of DFT the exchange correlation potential approximation has to incorporate particle number discontinuities. A candidate potential with the necessary features is the Becke-Johnson potential that is based on semi-local ingredients and is therefore computationally attractive for the treatment of very large systems. We show, however, that the potential cannot be applied in TDDFT because it is not a functional derivative and violates the zero-force theorem. We discuss a procedure on the basis of density path integrals that transforms the BJ potential into a functional derivative of a corresponding energy expression.
Design, Synthesis and Application of Cylindrical Polymer Brushes: From Nanostructures to Advanced Materials
- This thesis focuses on the design, synthesis and application of cylindrical polymer brushes (CPBs). Herein, we investigated the scission behavior of polyelectrolyte CPBs on different surfaces, developed novel synthetic pathways for well-defined CPBs via reversible addition-fragmentation chain transfer (RAFT) polymerization, designed and prepared complex functional CPBs for light-harvesting and energy transfer, and utilized CPBs as templates for the synthesis of novel one-dimensional (1D) organic/inorganic hybrid nanostructures.
The ‘grafting-from’ approach was chosen as the general method to synthesize well-defined CPBs with various chemical and structural compositions. The linear polymer backbones (polyinitiators) were obtained by anionic polymerization or RAFT polymerization, whereas the side chains were grafted by atom transfer radical polymerization (ATRP) or RAFT polymerization. The obtained CPBs possess a narrow molecular weight distribution in both the backbone and the side chains.
The polymer backbone of core-shell CPBs consisting of a poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) core block and a poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) shell block was ruptured upon drying on solid surfaces, when suf-ficient Coulombic interactions between the shell block and the surface were formed. We controlled this scission behavior by tuning the surface interactions through switching the surface nature, shell quaternization, varying pH, or adding multivalent counterions. This study demonstrates that core-shell CPBs serve as a tool to directly compare the weak intermolecular forces with the strong carbon-carbon covalent bonds.
A novel ‘grafting-from’ approach was developed to overcome the challenges of synthesizing well-defined CPBs from a linear polymer backbone with a high density of RAFT functionalities. In this so-called “CTA-shuttled” R-group approach, a certain amount of low-molecular-weight chain transfer agent (CTA) was added to the polymerization system, serving as shuttles to transfer active radicals among the individual growing CPBs. Well-defined CPBs with polystyrene or poly(tert-butyl acrylate) branches and core-shell CPBs with polystyrene-block-poly(N-isopropylacrylamide) branches were synthesized, with the molecular weight distribution much narrower than that from the conventional R-group approach. Monte Carlo simulations confirmed that the advantage of the “CTA-shuttled” R-group approach consists in the release of the active radicals from the trapping CPB systems.
Imitating the natural “energy cascade” architecture, we developed single-molecular, rod-like nano-light harvesters (NLHs) on the basis of CPBs. Herein, a number of block copolymer side chains carrying light absorbing antennae groups (9,9-diethylfluorene, energy donors) were tethered to a linear polymer backbone containing emitting groups (anthracene, energy acceptors). These NLHs provide very efficient energy absorption and energy transfer from antennae to energy acceptors. Furthermore, we were able to manipulate the efficiency of energy transfer by tuning the distance between energy donors and energy acceptors in physical and/or chemical ways. This CPB-based NLH architecture presents a novel concept to design light harvesting materials and can readily be transplanted to any other applications in photoelectronic devices.
Core-shell CPBs with a poly(acrylic acid) (PAA) core block and a PDMAEMA shell block were employed as templates for the preparation of various rare-earth metal cations (Ln3+) incorporated silica hybrid nanoparticles (NPs). A tight chelation of Ln3+ ions in the PAA core and a crosslinked silica layer deposited on the shell provide a very stable encapsulation of Ln3+ ions within the hybrid NPs and thus a high biocompatibility. The silica hybrid NPs obtain unique and diverse properties from the incorporated Ln3+ ions, such as visible photoluminescence, paramagnetic behavior, and a longitudinal relaxation time (T1) shortening effect. This novel template-directed approach succeeds in combining different functional centers via loading in-situ mixed Ln3+ ions into individual CPBs resulting in multicomponent hybrid NPs, which possess both visible photoluminescence and T1 contrast enhancement and can thus be applied as multimodal bioimaging probes.
Dual-Responsive Polymer and Hybrid Systems: Applications for Gene Delivery and Hydrogels
- This thesis focused on the synthesis of functional materials based on water-soluble and responsive polymers, in particular poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA). The dual-responsive behavior and polycationic character at physiological pH of PDMAEMA lead to outstanding properties and thus, to a versatile component for water-based applications. The main concept of the thesis was to combine the ability for gene delivery of PDMAEMA with the magnetic properties of iron oxide nanoparticles to enable an activity of the gene vector in an applied external magnetic field. Another point was to apply the dual-responsive behavior of PDMAEMA (temperature and pH) for physically cross-linked hydrogels.
Initial studies on magnetic dual-responsive gene vectors revealed a facile synthesis of PDMAEMA-grafted iron oxide nanoparticles utilizing dopamine as physically binding anchor group for the polymer chains. Here, a dopamine-based ATRP initiator was applied for the surface modification of the nanoparticles, which enabled a controlled polymerization technique via the “grafting-from” approach. Gene transfection experiments with CHO-K1 cells show that the transfection efficiency is significantly higher than for poly(ethyleneimine) (PEI), which is regarded as the “gold standard” among the polycationic gene vectors. Although the hybrid particles show a considerably high molecular weight (4.3 MDa), which should lead to a significant increase of the cytotoxicity as observed for linear PDMAEMA their cytotoxicity is remarkably low, lower than that of PEI. Thus, the excellent performance in gene delivery experiments can be attributed to the star-like architecture of the PDMAEMA. Moreover, the uptake of our superparamagnetic gene vector into the cells enables a magnetic cell separation by applying an external magnetic field.
However, due to the non-covalent bonds of dopamine to the iron oxide nanoparticles, the PDMAEMA chains undergo a detachment with time from the nanoparticle surface. This led to the synthesis of PDMAEMA-based magnetic core-shell-corona nanoparticles. Here, the iron oxide nanoparticles were covered with a thin silica shell in order to link the PDMAEMA chains covalently to the inorganic core via silane chemistry. This approach revealed stable dual-responsive hybrid nanoparticles with irreversible binding of the polymer chains and a high long-term stability in aqueous media. These hybrid star-like particles also show excellent gene delivery. The inter-polyelectrolyte complex formation between the PDMAEMA corona of the core-shell-corona particles and pDNA showed that the pDNA molecules are individually complexed with single nanoparticles at N/P ratios (polymer nitrogen / pDNA phosphorous) where the best transfection results are obtained. The magnetic cell separation was further improved by using a Magnetic Activated Cell Sorting system (MACSTM). The magnetically separated cells maintain a high transfection efficiency as well as viability and could even be further cultivated.
Another aspect of this thesis was to include PDMAEMA as stimuli-responsive block in a double switchable block copolymer-based hydrogel. For this purpose, we chose a physically cross-linked ABCBA pentablock terpolymer system, which was polymerized via sequential ATRP and consist of a water-soluble PEO middle block, two dual-responsive (temperature/pH) PDMAEMA B-blocks as well as two thermo-responsive poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA) A-blocks (PDEGMA-b-PDMAEMA-b-PEO-b-PDMAEMA-b-PDEGMA). The aggregation behavior in dilute solution was investigated via temperature-dependent Dynamic Light Scattering (DLS) revealing that both stimuli-responsive blocks can be triggered separately and the coil-to-globule transition temperatures of the stimuli-responsive blocks were found to be strongly dependent on the block lengths for low molecular weights. In concentrated solutions, however, rheology studies did not show a further change in the mechanical properties after gelation for the investigated ABCBA pentablock terpolymer compositions. As a result, the principle of our complex system points towards a successful synthesis of a dual-responsive ABCBA pentablock terpolymer hydrogel system, which may show two distinct phase transition even for the gel state, if longer block lengths of the outer A- and B-blocks would be applied.
Janus Particles at Interfaces
- This thesis describes the synthesis and the characterization of both polymeric and hybrid Janus particles of well-defined size, shape and functionality and their high potential for applications in colloidal and material science.
Soft Janus particles, based on polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (SBM) triblock terpolymers, represent a fascinating group of polymeric materials because their size, shape and functionality directly influences their adsorption behavior at liquid-liquid interfaces.
The adsorption behavior of Janus cylinders at liquid-liquid interfaces was studied using the pendant drop technique. The interfacial tension decreases with increasing Janus cylinder length and concentration. From the time evolution of the interfacial tension the characteristics of early and late stages of the Janus cylinder adsorption were specified. A series of TEM images of the liquid-liquid interface taken during the cylinder adsorption confirm these observations. Janus cylinders behave differently at the interfaces as compared to the block terpolymer precursor SBM and to cylinders of comparable sizes with a polybutadiene core and a homogeneous polystyrene shell.
Understanding the influence of particle size and architecture on the adsorption process is a very important criterion for an efficient industrial use of the Janus particles. To establish the effect of the Janus character together with the effect of particle shape on the interfacial activity and orientation of the Janus particles at an liquid-liquid interface, we present a combination of experimental and simulation data together with detailed studies elucidating the mechanisms governing the adsorption process of Janus spheres, Janus cylinders and Janus discs. These studies demonstrate that changes in the geometry of the particles strongly influence the stabilization of the liquid-liquid interface. As the shape changes from spheres to discs and cylinders, different adsorption kinetics, different packing behavior, different energy barriers and finally different equilibrium values for the interfacial tension can be found.
Another main point of this thesis was the synthesis of functional and/or stimuli-responsive hybrid core-shell-corona Janus particles based on inorganic colloids and the characterization of their unique properties and fascinating self-assembly behavior. The first step towards these Janus particles was to understand in detail the formation of core-shell-corona particles with a homogeneous corona, and then in a second step, to use our new knowledge to create hybrid core-shell-corona Janus particles.
We developed an easy and completely reproducible preparation and characterization of the solution behavior and functional properties of superparamagnetic and/or fluorescent, thermo-responsive inorganic/organic hybrid nanogels with an intermediate protective silica shell and an interactive polymer layer. These well-defined multifunctional nanogels were prepared via two consecutive encapsulation processes of superparamagnetic and/or fluorescent semiconductor nanocrystals with a silica layer and a crosslinked and responsive polymer poly(N-isopropylacrylamide) (PNIPAAm) corona. The precise adjustment of the conditions allows to achieve a reliable encapsulation and to either entrap several particles or single ones and to precisely tailor the thickness of the silica shell. Full functionality of the encapsulated nanocrystals is retained, but excellent wettability, biocompatibility, flexible surface chemistry, increased chemical stability are implemented together with a thermo-responsive polymer corona.
On the basis of our well-characterized core-shell particles we took advantage of the variable surface chemistry of the silica shell to combine the properties of the superparamagnetic core-shell nanoparticles with the catalytic character of nickel complexes in hybrid core-shell-corona nanoparticles forming heterogeneous nanocatalysts. In that way a heterogeneous catalyst was created for facile product separation in the catalytic conversion of olefins.
In the next level, an efficient strategy for the large-scale synthesis of well-defined hybrid Janus particles with a silica core (˂˂ 100 nm) and a stimuli-responsive PDMAEMA hemicorona was developed. The synthesis is based on a modified version of the Pickering emulsion technique in combination with surface-initiated atom transfer radical polymerization (ATRP) in a “grafting from” approach. First, 30 nm silica nanoparticles are immobilized at the interface of sub-micrometer sized droplets of poly(vinyl acetate). Since the nanoparticles are partially embedded, one hemisphere is protected. After the modification with an ATRP-initiator and the detachment of the modified silica particles, PDMAEMA was grafted from one hemisphere via ATRP. The obtained Janus nanoparticles are well-defined in size and shape and show stimuli-responsive structural changes depending on pH and temperature.
Structural and electronic properties of transition metal nanoalloys and magnetic compounds
- In transition metal clusters, potentially profitable technological applications and fascinating fundamental questions are closely connected. Bimetallic nanoalloys, e.g., have become increasingly popular as their performance in catalysis is often superior to their pure counterparts. Exemplary for this are gold-platinum (Au-Pt) nanoalloys that have been used as highly potent catalysts in electrocatalysis and in a variety of oxidation reactions. However, the mere existence of Au-Pt nanoalloys is astonishing, as Au and Pt cannot be mixed in bulk over a wide range of compositions. Furthermore, how a combination of Au and Pt in nanoalloys results in their special properties has not yet been determined conclusively.
It has been shown in empirical simulations and first-principles density functional theory (DFT) calculations that Au-Pt nanoalloys preferably arrange in a core-shell mixing pattern with Au forming a shell around a Pt core. This is in contradiction to many experimental studies that report the formation of solid solutions of Au and Pt. In the present work, this seeming discrepancy is addressed by simulating x-ray diffraction patterns that are experimentally used to characterize nanoalloys. It is shown that the interpretation of the diffraction patterns relies on questionable assumptions and therefore does not suffice as a definite characterization tool for Au-Pt nanoalloys.
To shed light on the special catalytic properties of Au-Pt nanoalloys under rather different experimental conditions, a thorough investigation of their electronic and structural properties has been carried out. It is found that features favorable for catalysis in Au-Pt nanoalloys emerge as a consequence of combining two fundamental properties: Pt contributes a high density of states close to the Fermi level, which promotes chemical activity. Au increases the structural flexibility of the Au-Pt system, which might be beneficial for the formation of active and element-specific binding sites as well as regeneration of the catalyst after the reaction.
Although DFT offers an attractive compromise between computational effort and accuracy for a theoretical description of Au-Pt nanoalloys, other transition metal compounds severely challenge existing DFT approximations. Manganese (Mn) doped silicon (Si) clusters represent an ideal model system to study the interaction of a single magnetic impurity with a semiconducting host both experimentally and theoretically. The transition from exohedral (lowly coordinated) to endohedral (highly coordinated) doping that occurs for Si clusters with more than ten atoms, is accompanied by complete quenching of the magnetic moment of Mn. We show that MnSi11+, the smallest endohedral cluster found in experiment, suffers strongly from a well-known general problem of most DFT approximations: the self-interaction error. Finally, a universal correlation between magnetic moment and the coordination of the Mn dopant is established that can be generalized to extended systems and suggests a route to stabilize the magnetic moment of bulk Mn-Si compounds.
Synthesis and Self-Assembly of Novel ABC Miktoarm Star Terpolymers
- A novel synthesis for ABC miktoarm star terpolymers and their self-assembly into complex superstructures in aqueous solution are described within this thesis. To this aim a modular route for such materials was developed, combining anionic polymerization and copper-catalyzed azide-alkyne cycloaddition. At the example of ABC miktoarm star terpolymers and an ABA’ miktoarm star copolymer containing a poly(N-methyl-2-vinypyridinium iodide) (P2VPq) segment, the counterion-mediated superstructure-formation of complex shaped aggregates was thoroughly investigated.
The key compound of the combinatorial synthesis is the newly synthesized 4-alkyne-substitued diphenylethylene derivative 1-[(4-(tert-butyldimethylsilyl)ethynyl)phenyl]-1-phenylethylene (“click-DPE”). This was applied in sequential anionic polymerization to prepare well-defined alkyne mid-functional diblock copolymers composed of polybutadiene (PB) as first and poly(tert-butyl methacrylate) (PtBMA), poly(2-vinylpyridine) (P2VP), or poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) as second block. The alkyne-midfunctional diblock copolymers were afterwards conjugated with azido-functional polystyrenes (PS), poly(ethylene oxide) (PEO), PtBMA and PDMAEMA to successfully obtain different novel ABC miktoarm star terpolymers with narrow molecular weight distributions.
For an ABC miktoarm star terpolymer consisting of arms of PB, PtBMA and P2VP it was demonstrated that after quaternization with methyl iodide (yielding BVqT) and dialysis to water the nature of the counterion allows for manipulation of the obtained structures. The miktoarm star architecture together with iodide as counterion is essential for this directed self-assembly. Transformation of iodide to triiodide, via the addition of iodine before dialysis to water, decreases the hydrophilicity of the P2VPq corona and therefore induces the directed self-assembly of spherical micelles with a PB/PtBMA core, into cylinders, superstructures thereof and finally barrel-shaped aggregates of up to 1 µm with an internal lamellar fine structure. Based on their appearance in transmission electron micrographs these were termed “woodlouse” aggregates. The compact particles consist of alternating lamellae of a partially demixed PB/PtBMA phase and a swollen P2VPq phase.
The general applicability of this counterion-mediated hierarchical self-assembly was furthermore demonstrated by using two other miktoarm star systems. For three ABC miktoarm star terpolymers of different composition, consisting of PB, PS and P2VPq segments (BVqS), a dependence of the morphology on the fraction of the hydrophilic block was determined, in analogy to diblock copolymers. For long P2VPq blocks stacked lamellar/disk-like structures evolve from micellar building units. In contrast, a short P2VPq segment yields multilamellar vesicles via fusion of vesicular primary building blocks. The vesicle walls are supposed to consist of a lamellar structure with the PB phase in the centre, shielded from the P2VPq corona by thin PS layers. At the example of one BVqS miktoarm star terpolymer the successful formation of nanohybrids containing gold nanoparticles within the P2VPq phase is demonstrated.
In the second system the low-Tg PB segment was replaced by a second PS block of different length (SVqS’). Even though vesicles serve as initial building units, the triiodide-induced superstructure formation leads to anisotropic aggregation of deformed vesicles, rather than to the fusion into multilamellar vesicles. This is attributed to the two glassy PS core blocks which minimize the dynamics during self-assembly and allow only minor rearrangement of the aggregated structures. Similar to the “woodlouse” aggregates from BVqT, lamellar structured particles of elongated shape were obtained from SVqS’, despite vesicles serving as primary building units. Consequently, the presented triiodide-directed self-assembly into complex superstructures is not restricted to miktoarm star polymers containing a low-Tg segment, as the rearrangement processes take place during the dialysis process, where the organic co-solvent enables sufficient mobility of the core-forming blocks.
Besides the introduction of a novel synthetic approach for the construction of miktoarm star terpolymers and the synthetic advance of the alkyne-functionalized DPE, the presented triiodide-mediated superstructure formation represents an interesting concept for directed self-assembly processes.
Impact of time and spatial averages on the energy balance closure
- Secondary circulations are large and relatively stationary eddies, which are caused by the surface heterogeneity and normally reside away from the ground. They are believed to be the cause of the energy balance closure problem at the earth's surface, because their contribution to the turbulent fluxes is missed by a fixed eddy-covariance tower measurement that has a typical averaging time of 30 minutes. In this thesis, data from the LITFASS-2003 experiment was used to investigate the impact of time and spatial averages on the energy balance closure. This data consisted of many observations over a large heterogeneous landscape that could generate secondary circulations; some of which might be still near the earth's surface.
For the time average analysis, the averaging time was extended to increase the possibility that secondary circulations were picked up by the sensor. Two approaches, which were the modified ogive analysis and the block ensemble average, were applied to analyze the data from ground-based measurements. The modified ogive analysis requiring a steady state condition, could extend the averaging time up to a few hours and suggested that an averaging time of 30 minutes was still overall sufficient for the eddy-covariance measurement over low vegetation. The block ensemble average, on the contrary, did not require a steady state condition, but could extend the averaging time to several days. However, this approach could only improve the energy balance closure for some sites during specific periods, when secondary circulations existed in the vicinity of the sensor. Based on this approach, it was found that the near-surface secondary circulations mainly transported sensible heat, which led to an alternative energy balance correction by the buoyancy flux ratio approach, in which the attribution of the residual depended on the relative contribution of the sensible heat flux to the buoyancy flux. The fraction of the residual attributed to the sensible heat flux by this energy balance correction was larger than in the energy balance correction that preserved the Bowen ratio.
In the spatial average analysis, two energy balance correction approaches, the buoyancy flux ratio and the Bowen ratio approaches, were applied to the area-averaged fluxes (composite fluxes) in order to include contribution from secondary circulations. These composite fluxes were aggregated from multiple ground-based measurements. The energy balance corrected fluxes were validated against the spatial average fluxes, which were measured by an aircraft and a large aperture scintillometer (LAS). In this validation, the backward Lagrangian footprint model was used to estimate the source area of the measurement. It was found that both energy balance correction approaches did improve the agreement between time and spatial averages fluxes. This suggested that the contribution from secondary circulations could be properly accounted by the energy balance correction.
All findings in this thesis, therefore, depict that secondary circulations significantly transport energy in the atmospheric surface layer. The energy balance correction, accomplished by using either the Bowen ratio approach or the buoyancy flux ratio approach, is necessary to estimate the actual vertical transport of energy at the earth's surface.