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Proteins are natural polypeptides produced by cells; they can be found in both animals and plants, and possess a variety of functions. One of these functions is to provide structural support to the surrounding cells and tissues. For example, collagen (which is found in skin, cartilage, tendons and bones) and keratin (which is found in hair and nails) are structural proteins. When a tissue is damaged, however, the supporting matrix formed by structural proteins cannot always spontaneously regenerate. Tailor-made synthetic polypeptides can be used to help heal and restore tissue formation.
Synthetic polypeptides are typically synthesized by the so-called ring opening polymerization (ROP) of α-amino acid N-carboxyanhydrides (NCA). Such synthetic polypeptides are generally non-sequence-controlled and thus less complex than proteins. As such, synthetic polypeptides are rarely as efficient as proteins in their ability to self-assemble and form hierarchical or structural supramolecular assemblies in water, and thus, often require rational designing. In this doctoral work, two types of amino acids, γ-benzyl-L/D-glutamate (BLG / BDG) and allylglycine (AG), were selected to synthesize a series of (co)polypeptides of different compositions and molar masses.
A new and versatile synthetic route to prepare polypeptides was developed, and its mechanism and kinetics were investigated. The polypeptide properties were thoroughly studied and new materials were developed from them. In particular, these polypeptides were able to aggregate (or self-assemble) in solution into microscopic fibres, very similar to those formed by collagen. By doing so, they formed robust physical networks and organogels which could be processed into high water-content, pH-responsive hydrogels. Particles with highly regular and chiral spiral morphologies were also obtained by emulsifying these polypeptides. Such polypeptides and the materials derived from them are, therefore, promising candidates for biomedical applications.
Surface-enhanced Raman scattering (SERS) is a promising tool to obtain rich chemical information about analytes at trace levels. However, in order to perform selective experiments on individual molecules, two fundamental requirements have to be fulfilled. On the one hand, areas with high local field enhancement, so-called “hot spots”, have to be created by positioning the supporting metal surfaces in close proximity to each other. In most cases hot spots are formed in the gap between adjacent metal nanoparticles (NPs). On the other hand, the analyte has to be positioned directly in the hot spot in order to profit from the highest signal amplification. The use of DNA origami substrates provides both, the arrangement of AuNPs with nm precision as well as the ability to bind analyte molecules at predefined positions. Consequently, the present cumulative doctoral thesis aims at the development of a novel SERS substrate based on a DNA origami template. To this end, two DNA-functionalized gold nanoparticles (AuNPs) are attached to one DNA origami substrate resulting in the formation of a AuNP dimer and thus in a hot spot within the corresponding gap. The obtained structures are characterized by correlated atomic force microscopy (AFM) and SERS imaging which allows for the combination of structural and chemical information.
Initially, the proof-of principle is presented which demonstrates the potential of the novel approach. It is shown that the Raman signal of 15 nm AuNPs coated with dye-modified DNA
(dye: carboxytetramethylrhodamine (TAMRA)) is significantly higher for AuNP dimers arranged on a DNA origami platform in comparison to single AuNPs. Furthermore, by attaching single TAMRA molecules in the hot spot between two 5 nm AuNPs and optimizing the size of the AuNPs by electroless gold deposition, SERS experiments at the few-molecule level are presented. The initially used DNA origami-AuNPs design is further optimized in many respects. On the one hand, larger AuNPs up to a diameter of 60 nm are used which are additionally treated with a silver enhancement solution to obtain Au-Ag-core-shell NPs. On the other hand, the arrangement of both AuNPs is altered to improve the position of the dye molecule within the hot spot as well as to decrease the gap size between the two particles. With the optimized design the detection of single dye molecules (TAMRA and cyanine 3 (Cy3)) by means of SERS is demonstrated. Quantitatively, enhancement factors up to 10^10 are estimated which is sufficiently high to detect single dye molecules.
In the second part, the influence of graphene as an additional component of the SERS substrate is investigated. Graphene is a two-dimensional material with an outstanding combination of electronical, mechanical and optical properties. Here, it is demonstrated that
single layer graphene (SLG) replicates the shape of underlying non-modified DNA origami
substrates very well, which enables the monitoring of structural alterations by AFM imaging.
In this way, it is shown that graphene encapsulation significantly increases the structural
stability of bare DNA origami substrates towards mechanical force and prolonged exposure
to deionized water.
Furthermore, SLG is used to cover DNA origami substrates which are functionalized with a
40 nm AuNP dimer. In this way, a novel kind of hybrid material is created which exhibits
several advantages compared to the analogue non-covered SERS substrates. First, the fluorescence background of dye molecules that are located in between the AuNP surface and SLG is efficiently reduced. Second, the photobleaching rate of the incorporated dye molecules is decreased up to one order of magnitude. Third, due to the increased photostability of the investigated dye molecules, the performance of polarization-dependent series measurements on individual structures is enabled. This in turn reveals extensive information about the dye molecules in the hot spot as well as about the strain induced within the graphene lattice.
Although SLG can significantly influence the SERS substrate in the aforementioned ways, all
those effects are strongly related to the extent of contact with the underlying AuNP dimer.
In complement to the well-established zwitterionic monomers 3-((2-(methacryloyloxy)ethyl)dimethylammonio)propane-1-sulfonate (“SPE”) and 3-((3-methacrylamidopropyl)dimethylammonio)propane-1-sulfonate (“SPP”), the closely related sulfobetaine monomers were synthesized and polymerized by reversible addition-fragmentation chain transfer (RAFT) polymerization, using a fluorophore labeled RAFT agent. The polyzwitterions of systematically varied molar mass were characterized with respect to their solubility in water, deuterated water, and aqueous salt solutions. These poly(sulfobetaine)s show thermoresponsive behavior in water, exhibiting upper critical solution temperatures (UCST). Phase transition temperatures depend notably on the molar mass and polymer concentration, and are much higher in D2O than in H2O. Also, the phase transition temperatures are effectively modulated by the addition of salts. The individual effects can be in parts correlated to the Hofmeister series for the anions studied. Still, they depend in a complex way on the concentration and the nature of the added electrolytes, on the one hand, and on the detailed structure of the zwitterionic side chain, on the other hand. For the polymers with the same zwitterionic side chain, it is found that methacrylamide-based poly(sulfobetaine)s exhibit higher UCST-type transition temperatures than their methacrylate analogs. The extension of the distance between polymerizable unit and zwitterionic groups from 2 to 3 methylene units decreases the UCST-type transition temperatures. Poly(sulfobetaine)s derived from aliphatic esters show higher UCST-type transition temperatures than their analogs featuring cyclic ammonium cations. The UCST-type transition temperatures increase markedly with spacer length separating the cationic and anionic moieties from 3 to 4 methylene units. Thus, apparently small variations of their chemical structure strongly affect the phase behavior of the polyzwitterions in specific aqueous environments.
Water-soluble block copolymers were prepared from the zwitterionic monomers and the non-ionic monomer N-isopropylmethacrylamide (“NIPMAM”) by the RAFT polymerization. Such block copolymers with two hydrophilic blocks exhibit twofold thermoresponsive behavior in water. The poly(sulfobetaine) block shows an UCST, whereas the poly(NIPMAM) block exhibits a lower critical solution temperature (LCST). This constellation induces a structure inversion of the solvophobic aggregate, called “schizophrenic micelle”. Depending on the relative positions of the two different phase transitions, the block copolymer passes through a molecularly dissolved or an insoluble intermediate regime, which can be modulated by the polymer concentration or by the addition of salt. Whereas, at low temperature, the poly(sulfobetaine) block forms polar aggregates that are kept in solution by the poly(NIPMAM) block, at high temperature, the poly(NIPMAM) block forms hydrophobic aggregates that are kept in solution by the poly(sulfobetaine) block. Thus, aggregates can be prepared in water, which switch reversibly their “inside” to the “outside”, and vice versa.
In the interest of producing functional catalysts from sustainable building-blocks, 1, 3-dicarboxylate imidazolium salts derived from amino acids were successfully modified to be suitable as N-Heterocyclic carbene (NHC) ligands within metal complexes. Complexes of Ag(I), Pd(II), and Ir(I) were successfully produced using known procedures using ligands derived from glycine, alanine, β-alanine and phenylalanine. The complexes were characterized in solid state using X-Ray crystallography, which allowed for the steric and electronic comparison of these ligands to well-known NHC ligands within analogous metal complexes.
The palladium complexes were tested as catalysts for aqueous-phase Suzuki-Miyaura cross-coupling. Water-solubility could be induced via ester hydrolysis of the N-bound groups in the presence of base. The mono-NHC–Pd complexes were seen to be highly active in the coupling of aryl bromides with phenylboronic acid; the active catalyst of which was determined to be mostly Pd(0) nanoparticles. Kinetic studies determined that reaction proceeds quickly in the coupling of bromoacetophenone, for both pre-hydrolyzed and in-situ hydrolysis catalyst dissolution. The catalyst could also be recycled for an extra run by simply re-using the aqueous layer.
The imidazolium salts were also used to produce organosilica hybrid materials. This was attempted via two methods: by post-grafting onto a commercial organosilica, and co-condensation of the corresponding organosilane. The co-condensation technique harbours potential for the production of solid-support catalysts.
Ionothermal carbon materials
(2016)
Alternative concepts for energy storage and conversion have to be developed, optimized and employed to fulfill the dream of a fossil-independent energy economy. Porous carbon materials play a major role in many energy-related devices. Among different characteristics, distinct porosity features, e.g., specific surface area (SSA), total pore volume (TPV), and the pore size distribution (PSD), are important to maximize the performance in the final device. In order to approach the aim to synthesize carbon materials with tailor-made porosity in a sustainable fashion, the present thesis focused on biomass-derived precursors employing and developing the ionothermal carbonization.
During the ionothermal carbonization, a salt melt simultaneously serves as solvent and porogen. Typically, eutectic mixtures containing zinc chloride are employed as salt phase. The first topic of the present thesis addressed the possibility to precisely tailor the porosity of ionothermal carbon materials by an experimentally simple variation of the molar composition of the binary salt mixture. The developed pore tuning tool allowed the synthesis of glucose derived carbon materials with predictable SSAs in the range of ~ 900 to ~ 2100 m2 g-1. Moreover, the nucleobase adenine was employed as precursor introducing nitrogen functionalities in the final material. Thereby, the chemical properties of the carbon materials are varied leading to new application fields. Nitrogen doped carbons (NDCs) are able to catalyze the oxygen reduction reaction (ORR) which takes place on the cathodic site of a fuel cell. The herein developed porosity tailoring allowed the synthesis of adenine derived NDCs with outstanding SSAs of up to 2900 m2 g-1 and very large TPV of 5.19 cm3 g-1. Furthermore, the influence of the porosity on the ORR could be directly investigated enabling the precise optimization of the porosity characteristics of NDCs for this application. The second topic addressed the development of a new method to investigate the not-yet unraveled mechanism of the oxygen reduction reaction using a rotating disc electrode setup. The focus was put on noble-metal free catalysts. The results showed that the reaction pathway of the investigated catalysts is pH-dependent indicating different active species at different pH-values. The third topic addressed the expansion of the used salts for the ionothermal approach towards hydrated calcium and magnesium chloride. It was shown that hydrated salt phases allowed the introduction of a secondary templating effect which was connected to the coexistence of liquid and solid salt phases. The method enabled the synthesis of fibrous NDCs with SSAs of up to 2780 m2 g-1 and very large TPV of 3.86 cm3 g-1. Moreover, the concept of active site implementation by a facile low-temperature metalation employing the obtained NDCs as solid ligands could be shown for the first time in the context of ORR.
Overall, the thesis may pave the way towards highly porous carbon with tailor-made porosity materials prepared by an inexpensive and sustainable pathway, which can be applied in energy related field thereby supporting the needed expansion of the renewable energy sector.