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Poly(N,N-bis(2-methoxyethyl)acrylamide) (PbMOEAm) featuring two classical chemical motifs from non-ionic water-soluble polymers, namely, the amide and ethyleneglycolether moieties, was synthesized by reversible addition fragmentation transfer (RAFT) polymerization. This tertiary polyacrylamide is thermoresponsive exhibiting a lower critical solution temperature (LCST)-type phase transition. A series of homo- and block copolymers with varying molar masses but low dispersities and different end groups were prepared. Their thermoresponsive behavior in aqueous solution was analyzed via turbidimetry and dynamic light scattering (DLS). The cloud points (CP) increased with increasing molar masses, converging to 46 degrees C for 1 wt% solutions. This rise is attributed to the polymers' hydrophobic end groups incorporated via the RAFT agents. When a surfactant-like strongly hydrophobic end group was attached using a functional RAFT agent, CP was lowered to 42 degrees C, i.e., closer to human body temperature. Also, the effect of added salts, in particular, the role of the Hofmeister series, on the phase transition of PbMOEAm was investigated, exemplified for the kosmotropic fluoride, intermediate chloride, and chaotropic thiocyanate anions. A pronounced shift of the cloud point of about 10 degrees C to lower or higher temperatures was observed for 0.2 M fluoride and thiocyanate, respectively. When PbMOEAm was attached to a long hydrophilic block of poly(N,N-dimethylacrylamide) (PDMAm), the cloud points of these block copolymers were strongly shifted towards higher temperatures. While no phase transition was observed for PDMAm-b-pbMOEAm with short thermoresponsive blocks, block copolymers with about equally sized PbMOEAm and PDMAm blocks underwent the coil-to-globule transition around 60 degrees C.
The catalytic activity of metal nanoparticles (NPs) supported on porous supports can be controlled by various factors, such as NPs size, shape, or dispersivity, as well as their interaction with the support or the properties of the support material itself. However, these intrinsic properties are not solely responsible for the catalytic behavior of the overall reaction system, as the local environment and surface coverage of the catalyst with reactants, products, intermediates and other invloved species often play a crucial role in catalytic processes as well. Their contribution can be particularly critical in liquid-phase reactions with gaseous reactants that often suffer from low solubiltiy. One example is (D)-glucose oxidation with molecular oxygen over gold nanoparticles supported on porous carbons. The possibility to promote oxygen delivery in such aqueous phase oxidation reactions via the immobilization of heterogenous catalysts onto the interface of perfluorocarbon emulsion droplets is reported here. Gold-on-carbon catalyst particles can stabilize perfluorocarbon droplets in the aqueous phase and the local concentration of the oxidant in the surroundings of the gold nanoparticles accelerates the rate-limiting step of the reaction. Consequently, the reaction rate of a system with the optimal volume fraction of fluorocarbon is higher than a reference emulsion system without fluorocarbon, and the effect is observed even without additional oxygen supply.
The use of organic materials with reversible redox activity holds enormous potential for next-generation Li-ion energy storage devices. Yet, most candidates are not truly sustainable, i.e., not derived from renewable feedstock or made in benign reactions. Here an attempt is reported to resolve this issue by synthesizing an organic cathode material from tannic acid and microporous carbon derived from biomass. All constituents, including the redox-active material and conductive carbon additive, are made from renewable resources. Using a simple, sustainable fabrication method, a hybrid material is formed. The low cost and ecofriendly material shows outstanding performance with a capacity of 108 mAh g(-1) at 0.1 A g(-1) and low capacity fading, retaining approximately 80% of the maximum capacity after 90 cycles. With approximately 3.4 V versus Li+/Li, the cells also feature one of the highest reversible redox potentials reported for biomolecular cathodes. Finally, the quinone-catecholate redox mechanism responsible for the high capacity of tannic acid is confirmed by electrochemical characterization of a model compound similar to tannic acid but without catecholic groups.
Electrical actuation of coated and composite fibers based on poly[ethylene-co-(vinyl acetate)]
(2020)
Robots are typically controlled by electrical signals. Resistive heating is an option to electrically trigger actuation in thermosensitive polymer systems. In this study electrically triggerable poly[ethylene-co-(vinyl acetate)] (PEVA)-based fiber actuators are realized as composite fibers as well as polymer fibers with conductive coatings. In the coated fibers, the core consists of crosslinked PEVA (cPEVA), while the conductive coating shell is achieved via a dip coating procedure with a coating thickness between 10 and 140 mu m. The conductivity of coated fibers sigma = 300-550 S m(-1) is much higher than that of the composite fibers sigma = 5.5 S m(-1). A voltage (U) of 110 V is required to heat 30 cm of coated fiber to a targeted temperature of approximate to 65 degrees C for switching in less than a minute. Cyclic electrical actuation investigations reveal epsilon '(rev) = 5 +/- 1% reversible change in length for coated fibers. The fabrication of such electro-conductive polymeric actuators is suitable for upscaling so that their application potential as artificial muscles can be explored in future studies.
Polypropylene as one of the world's top commodity polymers is also widely used in the textile industry. However, its non-polar nature and partially crystalline structure significantly complicate the process of industrial coloring of polypropylene. Currently, textiles made of polypropylene or with a significant proportion of polypropylene are dyed under quite harsh conditions, including the use of high pressures and temperatures, which makes this process energy intensive. This research presents a three-step synthesis of coloring agents, capable of adhering onto synthetic polypropylene yarns without harsh energy-consuming conditions. This is possible by encapsulation of organic pigments using trimethoxyphenylsilane, introduction of surface double bonds via modification of the silica shell with trimethoxysilylpropylmethacrylate and final attachment of highly adhesive anchor peptides using thiol-ene chemistry. We demonstrate the applicability of this approach by dyeing polypropylene yarns in a simple process under ambient conditions after giving a step-by-step guide for the synthesis of these new dyeing agents. Finally, the successful dyeing of the yarns is visualized, and its practicability is discussed.
The replacement of oxygen by sulfur atoms of [1,3]-dioxolo[4.5-f]benzodioxole (DBD) fluorescent dyes is an efficient way to adjust the photophysical properties (sulfur tuning). While previously developed S-4-DBD dyes exhibit considerably red-shifted absorption and emission wavelength, the heavy atom effect of four sulfur atoms cause low fluorescence quantum yields and short fluorescence lifetimes. Herein, we demonstrate that the replacement of less than four sulfur atoms (S-1-DBD, 1,2-S-2-DBD, and 1,4-S-2-DBD dyes) permits a fine-tuning of the photophysical properties. In some cases, a similar influence on the wavelength without the detrimental effect on the quantum yields and lifetimes is observed. Furthermore, the synthetic accessibility of S-1- and S-2-DBD dyes is improved, compared with S-4-DBD dyes. For coupling with biomolecules a series of reactive derivatives of the new dyes were developed (azides, OSu esters, alkynes, maleimides).
Enzyme degradable polymersomes from chitosan-g-[poly-l-lysine-block-epsilon-caprolactone] copolymer
(2020)
The scope of this study includes the synthesis of chitosan-g-[peptide-poly-epsilon-caprolactone] and its self-assembly into polymeric vesicles employing the solvent shift method. In this way, well-defined core-shell structures suitable for encapsulation of drugs are generated. The hydrophobic polycaprolactone side-chain and the hydrophilic chitosan backbone are linked via an enzyme-cleavable peptide. The synthetic route involves the functionalization of chitosan with maleimide groups and the preparation of polycaprolactone with alkyne end-groups. A peptide functionalized with a thiol group on one side and an azide group on the other side is prepared. Thiol-ene click-chemistry and azide-alkyne Huisgen cycloaddition are then used to link the chitosan and poly-epsilon-caprolactone chains, respectively, with this peptide. For a preliminary study, poly-l-lysin is a readily available and cleavable peptide that is introduced to investigate the feasibility of the system. The size and shape of the polymersomes are studied by dynamic light scattering and cryo-scanning electron microscopy. Furthermore, degradability is studied by incubating the polymersomes with two enzymes, trypsin and chitosanase. A dispersion of polymersomes is used to coat titanium plates and to further test the stability against enzymatic degradation.
This opinion article describes recent approaches to use the "biorefinery" concept to lower the carbon footprint of typical mass polymers, by replacing parts of the fossil monomers with similar or even the same monomer made from regrowing dendritic biomass. Herein, the new and green catalytic synthetic routes are for lactic acid (LA), isosorbide (IS), 2,5-furandicarboxylic acid (FDCA), and p-xylene (pXL). Furthermore, the synthesis of two unconventional lignocellulosic biomass derivable monomers, i.e., alpha-methylene-gamma-valerolactone (MeGVL) and levoglucosenol (LG), are presented. All those have the potential to enter in a cost-effective way, also the mass market and thereby recover lost areas for polymer materials. The differences of catalytic unit operations of the biorefinery are also discussed and the challenges that must be addressed along the synthesis path of each monomers.
A method for the fabrication of well-defined metallic nanostructures is presented here in a simple and straightforward fashion. As an alternative to lithographic techniques, this routine employs microcontact printing utilizing wrinkled stamps, which are prepared from polydimethylsiloxane (PDMS), and includes the formation of hydrophobic stripe patterns on a substrate via the transfer of oligomeric PDMS. Subsequent backfilling of the interspaces between these stripes with a hydroxyl-functional poly(2-vinyl pyridine) then provides the basic pattern for the deposition of citrate-stabilized gold nanoparticles promoted by electrostatic interaction. The resulting metallic nanostripes can be further customized by peeling off particles in a second microcontact printing step, which employs poly(ethylene imine) surface-decorated wrinkled stamps, to form nanolattices. Due to the independent adjustability of the period dimensions of the wrinkled stamps and stamp orientation with respect to the substrate, particle arrays on the (sub)micro-scale with various kinds of geometries are accessible in a straightforward fashion. This work provides an alternative, cost-effective, and scalable surface-patterning technique to fabricate nanolattice structures applicable to multiple types of functional nanoparticles. Being a top-down method, this process could be readily implemented into, e.g., the fabrication of optical and sensing devices on a large scale.
We search for homovalent alternatives for A, B, and X-ions in ABX(3) type inorganic halide perovskites suitable for tandem solar cell applications. We replace the conventional A-site organic cation CH3NH3, by 3 inorganic cations, Cs, K, and Rb, and the B site consists of metals; Cd, Hg, Ge, Pb, and Sn This work is built on our previous high throughput screening of hybrid perovskite materials (Kar et al 2018 J. Chem. Phys. 149, 214701). By performing a systematic screening study using Density Functional Theory (DFT) methods, we found 11 suitable candidates; 2 Cs-based, 3 K-based and 6 Rb-based that are suitable for tandem solar cell applications.
Stem cells are capable of sensing and processing environmental inputs, converting this information to output a specific cell lineage through signaling cascades. Despite the combinatorial nature of mechanical, thermal, and biochemical signals, these stimuli have typically been decoupled and applied independently, requiring continuous regulation by controlling units. We employ a programmable polymer actuator sheet to autonomously synchronize thermal and mechanical signals applied to mesenchymal stem cells (MSC5). Using a grid on its underside, the shape change of polymer sheet, as well as cell morphology, calcium (Ca2+) influx, and focal adhesion assembly, could be visualized and quantified. This paper gives compelling evidence that the temperature sensing and mechanosensing of MSC5 are interconnected via intracellular Ca2+. Up-regulated Ca2+ levels lead to a remarkable alteration of histone H3K9 acetylation and activation of osteogenic related genes. The interplay of physical, thermal, and biochemical signaling was utilized to accelerate the cell differentiation toward osteogenic lineage. The approach of programmable bioinstructivity provides a fundamental principle for functional biomaterials exhibiting multifaceted stimuli on differentiation programs. Technological impact is expected in the tissue engineering of periosteum for treating bone defects.
The article describes a systematic investigation of the effects of an aqueous NaOH treatment of 3D printed poly(lactic acid) (PLA) scaffolds for surface activation. The PLA surface undergoes several morphology changes and after an initial surface roughening, the surface becomes smoother again before the material dissolves. Erosion rates and surface morphologies can be controlled by the treatment. At the same time, the bulk mechanical properties of the treated materials remain unaltered. This indicates that NaOH treatment of 3D printed PLA scaffolds is a simple, yet viable strategy for surface activation without compromising the mechanical stability of PLA scaffolds.
Nanoporous carbon materials (NCMs) provide the "function" of high specific surface area and thus have large interface area for interactions with surrounding species, which is of particular importance in applications related to adsorption processes. The strength and mechanism of adsorption depend on the pore architecture of the NCMs. In addition, chemical functionalization can be used to induce changes of electron density and/or electron density distribution in the pore walls, thus further modifying the interactions between carbons and guest species. Typical approaches for functionalization of nanoporous materials with regular atomic construction like porous silica, metal-organic frameworks, or zeolites, cannot be applied to NCMs due to their less defined local atomic construction and abundant defects. Therefore, synthetic strategies that offer a higher degree of control over the process of functionalization are needed. Synthetic approaches for covalent functionalization of NCMs, that is, for the incorporation of heteroatoms into the carbon backbone, are critically reviewed with a special focus on strategies following the concept "from molecules to materials." Approaches for coordinative functionalization with metallic species, and the functionalization by nanocomposite formation between pristine carbon materials and heteroatom-containing carbons, are introduced as well. Particular focus is given to the influences of these functionalizations in adsorption-related applications.
A thermodynamic study of the adsorption of Human Serum Albumin (HSA) onto spherical polyelectrolyte brushes (SPBs) by isothermal titration calorimetry (ITC) is presented. The SPBs are composed of a solid polystyrene core bearing long chains of poly(acrylic acid). ITC measurements done at different temperatures and ionic strengths lead to a full set of thermodynamicbinding constants together with the enthalpies and entropies of binding. The adsorption of HSA onto SPBs is described with a two-step model. The free energy of binding Delta Gb depends only weakly on temperature because of a marked compensation of enthalpy by entropy. Studies of the adsorbed HSA by Fourier transform infrared spectroscopy (FT-IR) demonstrate no significant disturbance in the secondary structure of the protein. The quantitative analysis demonstrates that counterion release is the major driving force for adsorption in a process where proteins become multivalent counterions of the polyelectrolyte chains upon adsorption. A comparison with the analysis of other sets of data related to the binding of HSA to polyelectrolytes demonstrates that the cancellation of enthalpy and entropy is a general phenomenon that always accompanies the binding of proteins to polyelectrolytes dominated by counterion release.
Luminescent Ionogels with Excellent Transparency, High Mechanical Strength, and High Conductivity
(2020)
The paper describes a new kind of ionogel with both good mechanical strength and high conductivity synthesized by confining the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([Bmim][NTf₂]) within an organic–inorganic hybrid host. The organic–inorganic host network was synthesized by the reaction of methyltrimethoxysilane (MTMS), tetraethoxysilane (TEOS), and methyl methacrylate (MMA) in the presence of a coupling agent, offering the good mechanical strength and rapid shape recovery of the final products. The silane coupling agent 3-methacryloxypropyltrimethoxysilane (KH-570) plays an important role in improving the mechanical strength of the inorganic–organic hybrid, because it covalently connected the organic component MMA and the inorganic component SiO₂. Both the thermal stability and mechanical strength of the ionogel significantly increased by the addition of IL. The immobilization of [Bmim][NTf₂] within the ionogel provided the final ionogel with an ionic conductivity as high as ca. 0.04 S cm⁻¹ at 50 °C. Moreover, the hybrid ionogel can be modified with organosilica-modified carbon dots within the network to yield a transparent and flexible ionogel with strong excitation-dependent emission between 400 and 800 nm. The approach is, therefore, a blueprint for the construction of next-generation multifunctional ionogels.
The impact that catalysis has on global economy and environment is substantial, since 85% of all chemical industrial processes are catalytic. Among those, 80% of the processes are heterogeneously catalyzed, 17% make use of homogeneous catalysts, and 3% are biocatalytic processes. Especially in the pharmaceutical and agrochemical industry, a significant part of these processes involves chiral compounds. Obtaining enantiomerically pure compounds is necessary and it is usually accomplished by asymmetric synthesis and catalysis, as well as chiral separation. The efficiency of these processes may be vastly improved if the chiral selectors are positioned on a porous solid support, thereby increasing the available surface area for chiral recognition. Similarly, the majority of commercial catalysts are also supported, usually comprising of metal nanoparticles (NPs) dispersed on highly porous oxide or nanoporous carbon material.
Materials that have exceptional thermal and chemical stability, and are electrically conductive are porous carbons. Their stability in extreme pH regions and temperatures, the possibility to tailor their pore architecture and chemical functionalization, and their electric conductivity have already established these materials in the fields of separation and catalysis. However, their heterogeneous chemical structure with abundant defects make it challenging to develop reliable models for the investigation of structure-performance relationships. Therefore, there is a necessity for expanding the fundamental understanding of these robust materials under experimental conditions to allow for their further optimization for particular applications. This thesis gives a contribution to our knowledge about carbons, through different aspects, and in different applications.
On the one hand, a rather exotic novel application was investigated by attempts in synthesizing porous carbon materials with an enantioselective surface. Chapter 4.1 described an approach for obtaining mesoporous carbons with an enantioselective surface by direct carbonization of a chiral precursor. Two enantiomers of chiral ionic liquids (CIL) based on amino acid tyrosine were used as carbon precursors and ordered mesoporous silica SBA-15 served as a hard template for obtaining porosity. The chiral recognition of the prepared carbons has been tested in the solution by isothermal titration calorimetry with enantiomers of Phenylalanine as probes, as well as chiral vapor adsorption with 2-butanol enantiomers. Measurements in both solution and the gas phase revealed the differences in the affinity of carbons towards two enantiomers.
The atomic efficiency of the CIL precursors was increased in Chapter 4.2, and the porosity was developed independently from the development of chiral carbons, through the formation of stable composites of pristine carbon and CIL-derived coating. After the same set of experiments for the investigation of chirality, the enantiomeric ratios of the composites reported herein were even higher than in the previous chapter.
On the other hand, the structure‒activity relationship of carbons as supports for gold nanoparticles in a rather traditional catalytic model reaction, on the interface between gas, liquid, and solid, was studied. In Chapter 5.1 it was shown on the series of catalysts with different porosities that the kinetics of ᴅ-glucose oxidation reaction can be enhanced by increasing the local concentration of the reactants around the active phase of the catalyst. A large amount of uniform narrow mesopores connected to the surface of the Au catalyst supported on ordered mesoporous carbon led to the water confinement, which increased the solubility of the oxygen in the proximity of the catalyst and thereby increased the apparent catalytic activity of this catalyst.
After increasing the oxygen concentration in the internal area of the catalyst, in Chapter 5.2 the concentration of oxygen was increased in the external environment of the catalyst, by the introduction of less cohesive liquids that serve as efficient solvent for oxygen, perfluorinated compounds, near the active phase of the catalyst. This was achieved by a formation of catalyst particle-stabilized emulsions of perfluorocarbon in aqueous ᴅ-glucose solution, that further promoted the catalytic activity of gold-on-carbon catalyst.
The findings reported within this thesis are an important step in the understanding of the structure-related properties of carbon materials.
In the present thesis, self-assembly of hydrophilic polymers, reinforced hydrogels and inorganic/polymer hybrids were examined. The thesis describes an avenue from polymer synthesis via various methods over polymer self-assembly to the formation of polymer materials that have promising properties for future applications.
Hydrophilic polymers were utilized to form multi-phase systems, water-in-water emulsions and self-assembled structures, e.g. particles/aggregates or hollow structures from completely water-soluble building blocks. The structuring of aqueous environments by hydrophilic homo and block copolymers was further utilized in the formation of supramolecular hydrogels with compartments or specific thermal behavior. Furthermore, inorganic graphitic carbon nitride (g-CN) was utilized as photoinitiator for hydrogel formation and as reinforcer for hydrogels. As such, hydrogels with remarkable mechanical properties were synthesized, e.g. high compressibility, high storage modulus or lubricity. In addition, g-CN was combined with polymers for a broad range of materials, e.g. coatings, films or latex, that could be utilized in photocatalytic applications. Another inorganic material class was combined with polymers in the present thesis as well, namely metal-organic frameworks (MOFs). It was shown that the pore structure of MOFs enables improved control over tacticity and achievement of high molar masses. Furthermore, MOF-based polymerization catalysis was introduced with improved control for coordinating monomers, catalyst recyclability and decreased metal contamination in the product. Finally, the effect of external influence on MOF morphology was studied, e.g. via solvent or polymer additives, which allowed the formation of various MOF structures.
Overall, advances in several areas of polymer science are presented in here. A major topic of the thesis was hydrophilic polymers and hydrogels that currently constitute significant materials in the polymer field due to promising future applications in biomedicine. Moreover, the combination of polymers with materials from other areas of research, i.e. g-CN and MOFs, provided various new materials with remarkable properties also of interest for applications in the future, e.g. coatings, particle structures and catalysis.
Highly luminescent indium phosphide zinc sulfide (InPZnS) quantum dots (QDs), with zinc selenide/zinc sulfide (ZnSe/ZnS) shells, were synthesized. The QDs were modified via a post-synthetic ligand exchange reaction with 3-mercaptopropionic acid (MPA) and 11-mercaptoundecanoic acid (MUA) in different MPA:MUA ratios, making this study the first investigation into the effects of mixed ligand shells on InPZnS QDs. Moreover, this article also describes an optimized method for the correlation of the QD size vs. optical absorption of the QDs. Upon ligand exchange, the QDs can be dispersed in water. Longer ligands (MUA) provide more stable dispersions than short-chain ligands. Thicker ZnSe/ZnS shells provide a better photoluminescence quantum yield (PLQY) and higher emission stability upon ligand exchange. Both the ligand exchange and the optical properties are highly reproducible between different QD batches. Before dialysis, QDs with a ZnS shell thickness of ~4.9 monolayers (ML), stabilized with a mixed MPA:MUA (mixing ratio of 1:10), showed the highest PLQY, at ~45%. After dialysis, QDs with a ZnS shell thickness of ~4.9 ML, stabilized with a mixed MPA:MUA and a ratio of 1:10 and 1:100, showed the highest PLQYs, of ~41%. The dispersions were stable up to 44 days at ambient conditions and in the dark. After 44 days, QDs with a ZnS shell thickness of ~4.9 ML, stabilized with only MUA, showed the highest PLQY, of ~34%.
A surface modification of ultraflat gold nanotriangles (AuNTs) with different shaped nanoparticles is of special relevance for surface-enhanced Raman scattering (SERS) and the photo-catalytic activity of plasmonic substrates. Therefore, different approaches are used to verify the flat platelet morphology of the AuNTs by oriented overgrowth with metal nanoparticles. The most important part for the morphological transformation of the AuNTs is the coating layer, containing surfactants or polymers. By using well established AuNTs stabilized by a dioctyl sodium sulfosuccinate (AOT) bilayer, different strategies of surface modification with noble metal nanoparticles are possible. On the one hand undulated superstructures were synthesized by in situ growth of hemispherical gold nanoparticles in the polyethyleneimine (PEI)-coated AOT bilayer of the AuNTs. On the other hand spiked AuNTs were obtained by a direct reduction of Au³⁺ ions in the AOT double layer in presence of silver ions and ascorbic acid as reducing agent. Additionally, crumble topping of the smooth AuNTs can be realized after an exchange of the AOT bilayer by hyaluronic acid, followed by a silver-ion mediated reduction with ascorbic acid. Furthermore, a decoration with silver nanoparticles after coating the AOT bilayer with the cationic surfactant benzylhexadecyldimethylammonium chloride (BDAC) can be realized. In that case the ultraviolet (UV)-absorption of the undulated Au@Ag nanoplatelets can be tuned depending on the degree of decoration with silver nanoparticles. Comparing the Raman scattering data for the plasmon driven dimerization of 4-nitrothiophenol (4-NTP) to 4,4′-dimercaptoazobenzene (DMAB) one can conclude that the most important effect of surface modification with a 75 times higher enhancement factor in SERS experiments becomes available by decoration with gold spikes.
Ammonia is a chemical of fundamental importance for nature`s vital nitrogen cycle. It is crucial for the growth of living organisms as well as food and energy source. Traditionally, industrial ammonia production is predominated by Haber- Bosch process (HBP) which is based on direct conversion of N2 and H2 gas under high temperature and high pressure (~500oC, 150-300 bar). However, it is not the favorite route because of its thermodynamic and kinetic limitations, and the need for the energy intense production of hydrogen gas by reforming processes. All these disfavors of HBP open a target to search for an alternative technique to perform efficient ammonia synthesis via electrochemical catalytic processes, in particular via water electrolysis, using water as the hydrogen source to save the process from gas reforming.
In this study, the investigation of the interface effects between imidazolium-based ionic liquids and the surface of porous carbon materials with a special interest in the nitrogen absorption capability. As the further step, the possibility to establish this interface as the catalytically active area for the electrochemical N2 reduction to NH3 has been evaluated. This particular combination has been chosen because the porous carbon materials and ionic liquids (IL) have a significant importance in many scientific fields including catalysis and electrocatalysis due to their special structural and physicochemical properties. Primarily, the effects of the confinement of ionic liquid (EmimOAc, 1-Ethyl-3-methylimidazolium acetate) into carbon pores have been investigated. The salt-templated porous carbons, which have different porosity (microporous and mesoporous) and nitrogen species, were used as model structures for the comparison of the IL confinement at different loadings. The nitrogen uptake of EmimOAc can be increased by about 10 times by the confinement in the pores of carbon materials compared to the bulk form. In addition, the most improved nitrogen absorption was observed by IL confinement in micropores and in nitrogen-doped carbon materials as a consequence of the maximized structural changes of IL. Furthermore, the possible use of such interfaces between EmimOAc and porous carbon for the catalytic activation of dinitrogen during the kinetically challenging NRR due to the limited gas absorption in the electrolyte, was examined. An electrocatalytic NRR system based on the conversion of water and nitrogen gas to ammonia at ambient operation conditions (1 bar, 25 °C) was performed in a setup under an applied electric potential with a single chamber electrochemical cell, which consists of the combination of EmimOAc electrolyte with the porous carbon-working electrode and without a traditional electrocatalyst. Under a potential of -3 V vs. SCE for 45 minutes, a NH3 production rate of 498.37 μg h-1 cm-2 and FE of 12.14% were achieved. The experimental observations show that an electric double-layer, which serves the catalytically active area, occurs between a microporous carbon material and ions of the EmimOAc electrolyte in the presence of sufficiently high provided electric potential. Comparing with the typical NRR systems which have been reported in the literature, the presented electrochemical ammonia synthesis approach provides a significantly higher ammonia production rate with a chance to avoid the possible kinetic limitations of NRR. In terms of operating conditions, ammonia production rate and the faradic efficiency without the need for any synthetic electrocatalyst can be resulted of electrocatalytic activation of nitrogen in the double-layer formed between carbon and IL ions.
Im Rahmen der vom Bundesministerium für Bildung und -forschung geförderten Forschungsinitiative „BonaRes – Boden als nachhaltige Ressource der Bioökonomie“ soll sich das Teilprojekt „I4S – integrated system for site-specific soil fertility management“ der Entwicklung eines integrierten Systems zum ortsspezifischen Management der Bodenfruchtbarkeit widmen. Hierfür ist eine Messplattform zur Bestimmung relevanter Bodeneigenschaften und der quantitativen Analyse ausgewählter Makro- und Mikronährstoffe geplant. In der ersten Phase dieses Projekts liegt das Hauptaugenmerk auf der Kalibrierung und Validierung der verschiedenen Sensoren auf die Matrix Boden, der Probennahme auf dem Acker und der Planung sowie dem Aufbau der Messplattform. Auf dieser Plattform sollen in der zweiten Phase des Projektes die verschiedenen Bodensensoren installiert, sowie Modelle und Entscheidungsalgorithmen zur Steuerung der Düngung und dementsprechend Verbesserung der Bodenfunktionen erstellt werden.
Ziel der vorliegenden Arbeit ist die Grundlagenuntersuchung und Entwicklung einer robusten Online-Analyse mittels Energie-dispersiver Röntgenfluoreszenzspektroskopie (EDRFA) zur Quantifizierung ausgewählter Makro- und Mikronährstoffe in Böden für eine kostengünstige und flächendeckende Kartierung von Ackerflächen. Für die Entwicklung eines Online-Verfahrens wurde ein dem Stand der Technik entsprechender Röntgenfluoreszenzmesskopf in Betrieb genommen und die dazugehörigen Geräteparameter auf die Matrix Boden optimiert. Die Bestimmung der analytischen Qualitäts-merkmale wie Präzision und Nachweisgrenzen fand für eine Auswahl an Nährelementen von Aluminium bis Zink statt. Um eine möglichst Matrix-angepasste Kalibrierung zu erhalten, wurde sowohl mit zertifizierten Referenzmaterialien (CRM), als auch mit Ackerböden kalibriert. Da einer der größten Nachteile der Röntgenfluoreszenzanalyse die Beeinflussung durch Matrixeffekte ist, wurde neben der klassischen univariaten Datenauswertung auch die chemometrische multivariate Methode der Partial Least Squares Regression (PLSR) eingesetzt. Die PLSR bietet dabei den Vorteil, Matrixeffekte auszugleichen, wodurch robustere Kalibriermodelle erhalten werden können. Zusätzlich wurde eine Hauptkomponentenanalyse (PCA) durchgeführt, um Gemeinsamkeiten und Ausreißer innerhalb des Probensets zu identifizieren. Es zeigte sich, dass eine Klassifizierung der Böden anhand ihrer Textur Sand, Schluff, Lehm und Ton möglich ist.
Aufbauend auf den Ergebnissen idealer Bodenproben (zu Tabletten gepresste luftgetrocknete Proben mit Korngrößen < 0,5 mm) wurde im Verlauf dieser Arbeit die Probenvorbereitung immer weiter reduziert und der Einfluss verschiedener Kenngrößen untersucht. Diese Einflussfaktoren können die Dichte und die Homogenität der Probe, sowie Korngrößeneffekte und die Feuchtigkeit sein. Anhand des RMSE (Wurzel der mittleren Fehlerquadratsumme) und unter Berücksichtigung der Residuen werden die jeweils erstellten Kalibriermodelle miteinander verglichen. Um die Güte der Modelle zu bewerten, wurden diese mit einem Testset validiert. Hierfür standen 662 Bodenproben von 15 verschiedenen Standorten in Deutschland zur Verfügung. Da die Ergebnisse an gepressten Tabletten für die Elemente Al, Si, K, Ca, Ti, Mn, Fe und Zn den Anforderungen für eine spätere Online-Analyse entsprechen, wurden im weiteren Verlauf dieser Arbeit Kalibriermodelle mit losen Bodenproben erstellt. Auch hier konnten gute Ergebnisse durch ausreichende Nachweisgrenzen und eine niedrige gemittelte Messabweichung bei der Vorhersage unbekannter Testproben erzielt werden. Es zeigte sich, dass die Vorhersagefähigkeit mit der multivariaten PLSR besser ist als mit der univariaten Datenauswertung, insbesondere für die Elemente Mn und Zn.
Der untersuchte Einfluss der Feuchtigkeit und der Korngrößen auf die Quantifizierung der Elementgehalte war vor allem bei leichteren Elementen deutlich zu sehen. Es konnte schließlich eine multivariate Kalibrierung unter Berücksichtigung dieser Faktoren für die Elemente Al bis Zn erstellt werden, so dass ein Einsatz an Böden auf dem Acker möglich sein sollte. Eine höhere Messunsicherheit muss dabei einkalkuliert werden. Für eine spätere Probennahme auf dem Feld wurde zudem der Unterschied zwischen statischen und dynamischen Messungen betrachtet, wobei sich zeigte, dass beide Varianten genutzt werden können. Zum Abschluss wurde der hier eingesetzte Sensor mit einem kommerziell erhältlichen Hand-Gerät auf sein Quantifizierungspotential hin verglichen. Der Sensor weist anhand seiner Ergebnisse ein großes Potential als Online-Sensor für die Messplattform auf. Die Ergebnisse unter Laborbedingungen zeigen, dass eine robuste Analyse Ackerböden unter Berücksichtigung der Einflussfaktoren möglich ist.
In recent years people have realised non-renewability of our modern society which relays on spending huge amounts of energy mostly produced from fosil fuels, such as oil and coal, and the shift towards more sustainable energy sources has started. However, sustainable sources of energy, such as wind-, solar- and hydro-energy, produce primarily electrical energy and can not just be poured in canister like many fosil fuels, creating necessity for rechragable batteries. However, modern Li-ion batteries are made from toxic heavy metals and sustainable alternatives are needed. Here we show that naturally abundant catecholic and guaiacyl groups can be utilised to replace heavy metals in Li-ion batteries.
Foremost vanillin, a naturally occurring food additive that can be sustainably synthesised from industrial biowaste, lignin, was utilised to synthesise materials that showed extraordinary performance as cathodes in Li-ion batteries. Furthermore, behaviour of catecholic and guiacyl groups in Li-ion system was compared, confirming usability of guiacayl containing biopolymers as cathodes in Li-ion batteries. Lastly, naturally occurring polyphenol, tannic acid, was incorporated in fully bioderived hybrid material that shows performance comparable to commercial Li-ion batteries and good stability.
This thesis presents an important advancement in understanding of biowaste derived cathode materials for Li-ion batteries. Further research should be conducted to better understand behaviour of guaiacyl groups during Li-ion battery cycling. Lastly, challenges of incorporation of lignin, an industrial biowaste, have to be addressed and lignin should be incorporated as a cathode material in Li-ion batteries.
Abstract. Catalysis is one of the most effective tools for the highly efficient assembly of complex molecular structures. Nevertheless, it is mainly represented by transition metal-based catalysts and typically is an energy consuming process. Therefore, photocatalysis utilizing solar energy is one of the appealing approaches to overcome these problems. A great alternative to classic transition metal-based photocatalysts, carbon nitrides, a group of organic polymeric semiconductors, have already shown their efficiency in water splitting, CO2 reduction, and organic pollutants degradation. However, these materials have also a great potential for the use in functionalization of complex organic molecules for synthetic needs as it was shown in recent years.
This work addresses the challenge to develop efficient system for heterogeneous organic photocatalysis, employing cheap and environmentally benign photocatalysts – carbon nitrides. Herein, fundamental properties of semiconductors are studied from the organic chemistry standpoint; the inherent properties of carbon nitrides, such as ability to accumulate electrons, are deeply investigated and their effect on the reaction outcome is established. Thus, understanding of the electron charging processes allowed for the synthesis of otherwise hardly-achieved diazetidines-1,3 by tetramerization of benzylamines. Furthermore, the high electron capacity of Potassium Poly(heptazine imide)s (K-PHI) made possible a multi-electron reduction of aromatic nitro compounds to bare or formylated anilines. Additionally, two deep eutectic solvents (DES) were designed as a sustainable reaction media and reducing reagent for this reaction. Eventually, the high oxidation ability of carbon nitride K-PHI is employed in a challenging reaction of halide anion oxidation (Cl―, Br―) to accomplish electrophilic substitution in aromatic ring. The possibility to utilize NaCl solution (seawater mimetic) for the chlorination of electron rich arenes was shown. Eventually, light itself is used as a tool in a chromoselective photocatalytic oxidation of aromatic thiols and thioacetatas to three different compounds, using UV, blue, and red LEDs.
All in all, the work enhances understanding the mechanism of heterogeneous photocatalysis in synthetic organic reactions and therefore, is a step forward to the sustainable methods of synthesis in organic chemistry.
In den letzten Jahrzehnten ist die Nachfrage nach kostengünstigen und flächendeckenden Kartierungsmöglichkeiten im Hinblick auf eine ertragssteigernde und umweltfreundlichere Bewirtschaftung von landwirtschaftlichen Nutzflächen stark gestiegen. Hierfür eignen sich spektroskopische Methoden wie die Röntgenfluoreszenzanalyse (RFA), Raman- und Gammaspektroskopie sowie die laserinduzierte Plasmaspektroskopie (LIBS). In Abhängigkeit von der Funktionsweise der jeweiligen Methoden werden Informationen zu verschiedensten Bodeneigenschaften wie Nährelementgehalt, Textur und pH-Wert erhalten.
Ziel dieser Arbeit ist die Entwicklung eines Online-LIBS-Verfahrens zur Nährelementbestimmmung und Kartierung von Ackerflächen. Die LIBS ist eine schnelle und simultane Multielementanalyse bei der durch das Fokussieren eines hochenergetischen Laserpulses Probenmaterial von der Probenoberfläche ablatiert wird und in ein Plasma überführt wird. Beim Abkühlen des Plasmas wird Strahlung emittiert, welche Rückschlüsse über die elementare Zusammensetzung der Probe gibt. Diese Arbeit ist im Teilprojekt I4S (Intelligenz für Böden) im Forschungsprogramm BonaRes (Boden als nachhaltige Ressource für die Bioökonomie) des Bundesministerium für Bildung und Forschung (BMBF) entstanden. Es wurden insgesamt 651 Bodenproben von verschiedenen Test-Agrarflächen unterschiedlichster Standorte Deutschlands gemessen, ausgewertet und zu Validierungszwecken mit entsprechender Referenzanalytik wie die Optische Emissionsspektroskopie mittels induktiv gekoppeltem Plasma (ICP-OES) und die wellenlängendispersive Röntgenfluoreszenzanalyse (WDRFA) charakterisiert.
Für die Quantifizierung wurden zunächst die Messparameter des LIBS-Systems auf die Bodenmatrix optimiert und für die Elemente geeignete Linien ausgewählt sowie deren Nachweisgrenzen bestimmt. Es hat sich gezeigt, dass eine absolute Quantifizierung basierend auf einem univariaten Ansatz aufgrund der starken Matrixeffekte und der schlechten Reproduzierbarkeit des Plasmas nur eingeschränkt möglich ist. Bei Verwendung eines multivariaten Ansatz wie der Partial Least Squares Regression (PLSR) für die Kalibrierung konnten für die Nährelemente im Vergleich zur univariaten Variante Analyseergebnisse mit höherer Güte und geringeren Messunsicherheiten ermittelt werden. Die Untersuchungen haben gezeigt, dass das multivariate Modell weiter verbessert werden kann, indem mit einer Vielzahl von gut analysierten Böden verschiedener Standorte, Bodenarten und einem breiten Gehaltsbereich kalibriert wird. Mithilfe der Hauptkomponentenanalyse (PCA) wurde eine Klassifizierung der Böden nach der Textur realisiert. Weiterhin wurde auch eine Kalibrierung mit losem Bodenmaterial erstellt. Trotz der Signalabnahme konnten für die verschiedenen Nährelemente Kalibriergeraden mit ausreichender, analytischer Güte erstellt werden.
Für den Einsatz auf dem Acker wurde außerdem der Einfluss von Korngröße und Feuchtigkeit auf das LIBS-Signal untersucht. Die unterschiedlichen Korngrößen haben nur einen geringen Einfluss auf das LIBS-Signal und das Kalibriermodell lässt sich durch entsprechende Proben leicht anpassen. Dagegen ist der Einfluss der Feuchtigkeit deutlich stärker und hängt stark von der Bodenart ab, sodass für jede Bodenart ein separates Kalibriermodell für verschiedene Feuchtigkeitsgehalte erstellt werden muss. Mithilfe der PCA kann der Feuchtigkeitsgehalt im Boden grob abgeschätzt werden und die entsprechende Kalibrierung ausgewählt werden.
Diese Arbeit liefert essentielle Informationen für eine Echtzeit-Analyse von Nährelementen auf dem Acker mittels LIBS und leistet einen wichtigen Beitrag zu einer fortschrittlichen und zukunftsfähigen Nutzung von Ackerflächen.
Metal halide perovskites have merged as an attractive class of materials for photovoltaic applications due to their excellent optoelectronic properties. However, the long term stability is a roadblock for this class of material’s industrial pathway. Increasing evidence shows that intrinsic defects in perovskite promote material degradation. Consequently, understanding defect behaviours in perovskite materials is essential to further improve device stability and performance. This dissertation, hence, focuses on the topic of defect chemistry in halide perovskites.
The first part of the dissertation gives a brief overview of the defect properties in halide perovskite. Subsequently, the second part shows that doping methylammonium lead iodide with a small amount of alkaline earth metals (Sr and Mg) creates a higher quality, less defective material resulted in high open circuit voltages in both n-i-p and p-i-n architecture. It has been found that the mechanism of doping has two distinct regimes in which a low doping concentration enables the inclusion of the dopants into the lattice whereas higher doping concentrations lead to phase segregation. The material can be more n-doped in the low doping regime while being less n-doped in the high doping regime. The threshold of these two regimes is based on the atomic size of the dopants.
The next part of the dissertation examines the photo-induced degradation in methylammonium lead iodide. This degradation mechanism links closely to the formation and migration of ionic defects. After they are formed, these ionic defects can migrate, however, not freely depending on the defect concentration and their distribution. In fact, a highly concentrated defect region such as the grain boundaries can inhibit the migration of ionic defects. This has implications for material design as perovskite solar cells normally employ a polycrystalline thin-film which has a high density of grain boundary.
The final study presented in this PhD dissertation focuses on the stability of the state-of-the-art triple cation perovskite-based solar devices under external bias. Prolonged bias (more than three hours) is found to promote amorphization in halide perovskite. The amorphous phase is suspected to accumulate at the interfaces especially between the hole selective layer and perovskite. This amorphous phase inhibits the charge collection and severely affects the device performance. Nonetheless, the devices can recover after resting without bias in the dark. This amorphization is attributed to ionic defect migration most likely halides. This provides a new understanding of the potential degradation mechanisms in perovskite solar cells under operational conditions.
Lately, the integration of upconverting nanoparticles (UCNP) in industrial, biomedical and scientific applications has been increasingly accelerating, owing to the exceptional photophysical properties that UCNP offer. Some of the most promising applications lie in the field of medicine and bioimaging due to such advantages as, among others, deeper tissue penetration, reduced optical background, possibility for multicolor imaging, and lower toxicity, compared to many known luminophores. However, some questions regarding not only the fundamental photophysical processes, but also the interaction of the UCNP with other luminescent reporters frequently used for bioimaging and the interaction with biological media remain unanswered. These issues were the primary motivation for the presented work.
This PhD thesis investigated several aspects of various properties and possibilities for bioapplications of Yb3+,Tm3+-doped NaYF4 upconverting nanoparticles. First, the effect of Gd3+ doping on the structure and upconverting behaviour of the nanocrystals was assessed. The ageing process of the UCNP in cyclohexane was studied over 24 months on the samples with different Gd3+ doping concentrations. Structural information was gathered by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS), and discussed in relation to spectroscopic results, obtained through multiparameter upconversion luminescence studies at various temperatures (from 4 K to 295 K). Time-resolved and steady-state emission spectra recorded over this ample temperature range allowed for a deeper understanding of photophysical processes and their dependence on structural changes of UCNP.
A new protocol using a commercially available high boiling solvent allowed for faster and more controlled production of very small and homogeneous UCNP with better photophysical properties, and the advantages of a passivating NaYF4 shell were shown.
Förster resonance energy transfer (FRET) between four different species of NaYF4: Yb3+, Tm3+ UCNP (synthesized using the improved protocol) and a small organic dye was studied. The influence of UCNP composition and the proximity of Tm3+ ions (donors in the process of FRET) to acceptor dye molecules have been assessed. The brightest upconversion luminescence was observed in the UCNP with a protective inert shell. UCNP with Tm3+ ions only in the shell were the least bright, but showed the most efficient energy transfer.
In the final part, two surface modification strategies were applied to make UCNP soluble in water, which simultaneously allowed for linking them via a non-toxic copper-free click reaction to the liposomes, which served as models for further cell experiments. The results were assessed on a confocal microscope system, which was made possible by lesser known downshifting properties of Yb3+, Tm3+-doped UCNP. Preliminary antibody-staining tests using two primary and one dye-labelled secondary antibodies were performed on MDCK-II cells.
Hybrid organic-inorganic perovskites have attracted attention in recent years, caused by the incomparable increase in efficiency in energy convergence, which implies the application as an absorber material for solar cells. A disadvantage of these materials is, among others, the instability to moisture and UV-radiation. One possible solution for these problems is the reduction of the size towards the nano world. With that nanosized perovskites are showing superior stability in comparison to e.g. perovskite layers. Additionally to this the nanosize even enables stable perovskite structures, which could not be achieved otherwise at
room temperature.
This thesis is separated into two major parts. The separation is done by the composition and the band gap of the material and at the same time the shape and size of the nanoparticles. Here the division is made by the methylammonium lead tribromide nanoplatelets and the caesium lead triiodide nanocubes.
The first part is focusing on the hybrid organic-inorganic perovskite (methylammonium lead tribromide) nanoplatelets with a band gap of 2.35 eV and their thermal behaviour. Due to the challenging character of this material, several analysis methods are used to investigate the sub nano and nanostructures under the influence of temperature. As a result, a shift of phase-transition temperatures towards higher temperatures is observed. This unusual behaviour can be explained by the ligand, which is incorporated in the perovskite outer structure and adds phase-stability into the system.
The second part of this thesis is focusing on the inorganic caesium lead triiodide nanocubes with a band gap of 1.83 eV. These nanocrystals are first investigated and compared by TEM, XRD and other optical methods. Within these methods, a cuboid and orthorhombic structure are revealed instead of the in literature described cubic shape and structure. Furthermore, these cuboids are investigated towards their self-assembly on a substrate. Here a high degree in self-assembly is shown. As a next step, the ligands of the nanocuboids are exchanged against other ligands to increase the charge carrier mobility. This is further investigated by the above-mentioned methods. The last section is dealing with the enhancement of the CsPbI3 structure, by incorporating potassium in the crystal structure. The results are suggesting here an increase in stability.
"How Wenzel and Cassie were wrong" – this was the eye-catching title of an article published by Lichao Gao and Thomas McCarthy in 2007, in which fundamental interpretations of wetting behavior were put into question. The authors initiated a discussion on a subject, which had been generally accepted a long time ago and they showed that wetting phenomena were not as fully understood as imagined. Similarly, this thesis tries to put a focus on certain aspects of liquid wetting, which so far have been widely neglected in terms of interpretation and experimental proof. While the effect of surface roughness on the macroscopically observed wetting behavior is commonly and reliably interpreted according to the well-known models of Wenzel and Cassie/Baxter, the size-scale of the structures responsible for the surface's rough texture has not been of further interest. Analogously, the limits of these models have not been described and exploited. Thus, the question arises, what will happen when the size of surface structures is reduced to the size of the contacting liquid molecules itself? Are common methods still valid or can deviations from macroscopic behavior be observed?
This thesis wants to create a starting point regarding these questions. In order to investigate the effect of smallest-scale surface structures on liquid wetting, a suitable model system is developed by means of self-assembled monolayer (SAM) formation from (fluoro)organic thiols of differing lengths of the alkyl chain. Surface topographies are created which rely on size differences of several Ångströms and exhibit surprising wetting behavior depending on the choice of the individual precursor system. Thus, contact angles are experimentally detected, which deviate considerably from theoretical calculations based on Wenzel and Cassie/Baxter models and confirm that sub-nm surface topographies affect wetting. Moreover, experimentally determined wetting properties are found to correlate well to an assumed scale-dependent surface tension of the contacting liquid. This behavior has already been described for scattering experiments taking into account capillary waves on the liquid surface induced by temperature and had been predicted earlier by theoretical calculations.
However, the investigation of model surfaces requires the provision of suitable precursor molecules, which are not commercially available and opens up a door to the exotic chemistry of fluoro-organic materials. During the course of this work, the synthesis of long-chain precursors is examined with a particular focus put on oligomerically pure semi-fluorinated n-alkyl thiols and n-alkyl trichlorosilanes. For this, general protocols for the syntheses of the desired compounds are developed and product mixtures are assayed to be separated into fractions of individual chain lengths by fluorous-phase high-performance liquid chromatography (F-HPLC).
The transition from model systems to technically more relevant surfaces and applications is initiated through the deposition of SAMs from long-chain fluorinated n-alkyl trichlorosilanes. Depositions are accomplished by a vapor-phase deposition process conducted on a pilot-scale set-up, which enables the exact control of relevant process parameters. Thus, the influence of varying deposition conditions on the properties of the final coating is examined and analyzed for the most important parameters. The strongest effect is observed for the partial pressure of reactive water vapor, which directly controls the extent of precursor hydrolysis during the deposition process. Experimental results propose that the formation of ordered monolayers rely on the amount of hydrolyzed silanol species present in the deposition system irrespective of the exact grade of hydrolysis. However, at increased amounts of species which are able to form cross-linked molecules due to condensation reactions, films deteriorate in quality. This effect is assumed to be caused by the introduction of defects within the film and the adsorption of cross linked agglomerates. Deposition conditions are also investigated for chain extended precursor species and reveal distinct differences caused by chain elongation.
Die Entwicklung nachhaltiger Bewirtschaftungs- und Produktionsmethoden ist eine der zentralen Fragestellungen der modernen Agrarwirtschaft. Die vorliegende Arbeit beschäftigt sich mit zwei Forschungsthemen, die das Konzept Nachhaltigkeit beinhalten. In beiden Fällen werden analytische Grundlagen für die Entwicklung entsprechender landwirtschaftlicher Arbeitsmethoden gelegt.
Das erste Thema ist an den sogenannten Präzisionsackerbau angelehnt. Bei diesem wird die Bearbeitung von Agrarflächen ortsabhängig ausgeführt. Das heißt, die Ausbringung von Saatgut, Dünger, Bewässerung usw. richtet sich nach den Eigenschaften des jeweiligen Standortes und wird nicht pauschal gleichmäßig über ein ganzes Feld verteilt. Voraussetzung hierfür ist eine genaue Kenntnis der Bodeneigenschaften. In der vorliegenden Arbeit sollten diese Parameter mittels der analytischen Technik der Laser-induzierten Breakdown Spektroskopie (LIBS), die eine Form der Elementaranalyse darstellt, bestimmt werden. Bei den hier gesuchten Bodeneigenschaften handelte es sich um die Gehalte von Nährstoffen sowie einige sekundäre Parameter wie den Humusanteil, den pH-Wert und den pflanzenverfügbaren Anteil einzelner Nährstoffe. Diese Eigenschaften wurden durch etablierte Referenzanalysen bestimmt. Darauf aufbauend wurden die Messergebnissen der LIBS-Untersuchungen durch verschiedene Methoden der sogenannten multivariaten Datenanalyse (MVA) ausgewertet. Daraus sollten Modelle zur Vorhersage der Bodenparameter in zukünftigen LIBS-Messungen erarbeitet werden. Die Ergebnisse dieser Arbeit zeigten, dass mit der Kombination von LIBS und MVA sämtliche Bodenparameter erfolgreich vorhergesagt werden konnten. Dies beinhaltete sowohl die tatsächlich messbaren Elemente als auch die sekundären Eigenschaften, welche durch die MVA mit den Elementgehalten in Zusammenhang gebracht wurden.
Das zweite Thema beschäftigt sich mit der Vermeidung von Verlusten durch Schädlingsbefall bei der Getreidelagerung. Hier sollten mittels der Ionenmobilitätsspektrometrie (IMS) Schimmelpilzkontaminationen detektiert werden. Dabei wurde nach den flüchtigen Stoffwechselprodukten der Pilze gesucht. Die durch Referenzmessungen mit Massenspektrometern identifizierten Substanzen konnten durch IMS im Gasvolumen über den Proben, dem sogenannten Headspace, nachgewiesen werden. Dabei wurde nicht nur die Anwesenheit einer Kontamination festgestellt, sondern diese auch charakterisiert. Die freigesetzten Substanzen bildeten spezifische Muster, anhand derer die Pilze identifiziert werden konnten. Hier wurden sowohl verschiedene Gattungen als auch einzelne Arten unterschieden. Die Messungen fanden auf verschiedenen Nährböden statt um den Einfluss dieser auf die Stoffwechselprodukte zu beobachten. Auch die sekundären Stoffwechselprodukte der Schimmelpilze, die Mykotoxine, konnten durch IMS detektiert werden.
Beide in dieser Arbeit vorgestellten Forschungsthemen konnten erfolgreich abgeschlossen werden. Sowohl LIBS als auch IMS erwiesen sich für den Nachweis der jeweiligen Analyten als geeignet, und der Einsatz moderner computergestützter Auswertemethoden ermöglichte die genaue Charakterisierung der gesuchten Parameter. Beide Techniken können in Form von mobilen Geräten verwendet werden und zeichnen sich durch eine schnelle und sichere Analyse aus. In Kombination mit entsprechenden Modellen der MVA sind damit alle Voraussetzungen für Vor-Ort-Untersuchungen und damit für den Einsatz in der Landwirtschaft erfüllt.
Seit Jahrzehnten stellen die molekularen Schalter ein wachsendes Forschungsgebiet dar. Im Rahmen dieser Dissertation stand die Verbesserung der thermischen Stabilität, der Auslesbarkeit und Schaltbarkeit dieser molekularen Schalter in komplexen Umgebungen mithilfe computergestützter Chemie im Vordergrund.
Im ersten Projekt wurde die Kinetik der thermischen E → Z-Isomerisierung und die damit verbundene thermische Stabilität eines Azobenzol-Derivats untersucht. Dafür wurde Dichtefunktionaltheorie (DFT) in Verbindung mit der Eyring-Theorie des Übergangszustandes (TST) angewendet. Das Azobenzol-Derivat diente als vereinfachtes Modell für das Schalten in einer komplexen Umgebung (hier in metallorganischen Gerüsten). Es wurden thermodynamische und kinetische Größen unter verschiedenen Einflüssen berechnet, wobei gute Übereinstimmungen mit dem Experiment gefunden wurden. Die hier verwendete Methode stellte einen geeigneten Ansatz dar, um diese Größen mit angemessener Genauigkeit vorherzusagen.
Im zweiten Projekt wurde die Auslesbarkeit der Schaltzustände in Form des nichtlinearen optischen (NLO) Kontrastes für die Molekülklasse der Fulgimide untersucht. Die dafür benötigten dynamischen Hyperpolarisierbarkeiten unter Berücksichtigung der Elektronenkorrelation wurden mittels einer etablierten Skalierungsmethode berechnet. Es wurden verschiedene Fulgimide analysiert, wobei viele experimentelle Befunde bestätigt werden konnten. Darüber hinaus legte die theoretische Vorhersage für ein weiteres System nahe, dass insbesondere die Erweiterung des π-Elektronensystems ein vielversprechender Ansatz zur Verbesserung von NLO-Kontrasten darstellt. Die Fulgimide verfügen somit über nützliche Eigenschaften, sodass diese in Zukunft als Bauelemente in photonischen und optoelektronischen Bereichen Anwendungen finden könnten.
Im dritten Projekt wurde die E → Z-Isomerisierung auf ein quantenmechanisch (QM) behandeltes Dimer mit molekularmechanischer (MM) Umgebung und zwei Fluorazobenzol-Monomeren durch Moleküldynamik simuliert. Dadurch wurde die Schaltbarkeit in komplexer Umgebung (hier selbstorgansierte Einzelschichten = SAMs) bzw. von Azobenzolderivaten analysiert. Mit dem QM/MM Modell wurden sowohl Van-der-Waals-Interaktionen mit der Umgebung als auch elektronische Kopplung (nur zwischen QM-Molekülen) berücksichtigt. Dabei wurden systematische Untersuchungen zur Packungsdichte durchgeführt. Es zeigte sich, dass bereits bei einem Molekülabstand von 4.5 Å die Quantenausbeute (prozentuale Anzahl erfolgreicher Schaltprozesse) des Monomers erreicht wird. Die größten Quantenausbeuten wurden für die beiden untersuchten Fluorazobenzole erzielt. Es wurden die Effekte des Molekülabstandes und der Einfluss von Fluorsubstituenten auf die Dynamik eingehend untersucht, sodass der Weg für darauf aufbauende Studien geebnet ist.
Radical reactions have found many applications in carbohydrate chemistry, especially in the construction of carbon–carbon bonds. The formation of carbon–heteroatom bonds has been less intensively studied. This mini-review will summarize the efforts to add heteroatom radicals to unsaturated carbohydrates like endo-glycals. Starting from early examples, developed more than 50 years ago, the importance of such reactions for carbohydrate chemistry and recent applications will be discussed. After a short introduction, the mini-review is divided in sub-chapters according to the heteroatoms halogen, nitrogen, phosphorus, and sulfur. The mechanisms of radical generation by chemical or photochemical processes and the subsequent reactions of the radicals at the 1-position will be discussed. This mini-review cannot cover all aspects of heteroatom-centered radicals in carbohydrate chemistry, but should provide an overview of the various strategies and future perspectives
Interactions involving biological interfaces such as lipid-based membranes are of paramount importance for all life processes. The same also applies to artificial interfaces to which biological matter is exposed, for example the surfaces of drug delivery systems or implants. This thesis deals with the two main types of interface interactions, namely (i) interactions between a single interface and the molecular components of the surrounding aqueous medium and (ii) interactions between two interfaces. Each type is investigated with regard to an important scientific problem in the fields of biotechnology and biology:
1.) The adsorption of proteins to surfaces functionalized with hydrophilic polymer brushes; a process of great biomedical relevance in context with harmful foreign-body-response to implants and drug delivery systems.
2.) The influence of glycolipids on the interaction between lipid membranes; a hitherto largely unexplored phenomenon with potentially great biological relevance.
Both problems are addressed with the help of (quasi-)planar, lipid-based model surfaces in combination with x-ray and neutron scattering techniques which yield detailed structural insights into the interaction processes. Regarding the adsorption of proteins to brush-functionalized surfaces, the first scenario considered is the exposure of the surfaces to human blood serum containing a multitude of protein species. Significant blood protein adsorption was observed despite the functionalization, which is commonly believed to act as a protein repellent. The adsorption consists of two distinct modes, namely strong adsorption to the brush grafting surface and weak adsorption to the brush itself. The second aspect investigated was the fate of the brush-functionalized surfaces when exposed to aqueous media containing immune proteins (antibodies) against the brush polymer, an emerging problem in current biomedical applications. To this end, it was found that antibody binding cannot be prevented by variation of the brush grafting density or the polymer length. This result motivates the search for alternative, strictly non-antigenic brush chemistries. With respect to the influence of glycolipids on the interaction between lipid membranes, this thesis focused on the glycolipids’ ability to crosslink and thereby to tightly attract adjacent membranes. This adherence is due to preferential saccharide-saccharide interactions occurring among the glycolipid headgroups. This phenomenon had previously been described for lipids with special oligo-saccharide motifs. Here, it was investigated how common this phenomenon is among glycolipids with a variety of more abundant saccharide-headgroups. It was found that glycolipid-induced membrane crosslinking is equally observed for some of these abundant glycolipid types, strongly suggesting that this under-explored phenomenon is potentially of great biological relevance.
The DNA in living cells can be effectively damaged by high-energy radiation, which can lead to cell death. Through the ionization of water molecules, highly reactive secondary species such as low-energy electrons (LEEs) with the most probable energy around 10 eV are generated, which are able to induce DNA strand breaks via dissociative electron attachment. Absolute DNA strand break cross sections of specific DNA sequences can be efficiently determined using DNA origami nanostructures as platforms exposing the target sequences towards LEEs. In this paper, we systematically study the effect of the oligonucleotide length on the strand break cross section at various irradiation energies. The present work focuses on poly-adenine sequences (d(A₄), d(A₈), d(A₁₂), d(A₁₆), and d(A₂₀)) irradiated with 5.0, 7.0, 8.4, and 10 eV electrons. Independent of the DNA length, the strand break cross section shows a maximum around 7.0 eV electron energy for all investigated oligonucleotides confirming that strand breakage occurs through the initial formation of negative ion resonances. When going from d(A₄) to d(A₁₆), the strand break cross section increases with oligonucleotide length, but only at 7.0 and 8.4 eV, i.e., close to the maximum of the negative ion resonance, the increase in the strand break cross section with the length is similar to the increase of an estimated geometrical cross section. For d(A₂₀), a markedly lower DNA strand break cross section is observed for all electron energies, which is tentatively ascribed to a conformational change of the dA₂₀ sequence. The results indicate that, although there is a general length dependence of strand break cross sections, individual nucleotides do not contribute independently of the absolute strand break cross section of the whole DNA strand. The absolute quantification of sequence specific strand breaks will help develop a more accurate molecular level understanding of radiation induced DNA damage, which can then be used for optimized risk estimates in cancer radiation therapy.