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The development and optimization of carbonaceous materials is of great interest for several applications including gas sorption, electrochemical storage and conversion, or heterogeneous catalysis. In this thesis, the exploration and optimization of nitrogen containing carbonaceous materials by direct condensation of smart chosen, molecular precursors will be presented. As suggested with the concept of noble carbons, the choice of a stable, nitrogen-containing precursor will lead to an even more stable, nitrogen doped carbonaceous material with a controlled structure and electronic properties. Molecules fulfilling this requirement are for example nucleobases. The direct condensation of nucleobases leads to highly nitrogen containing carbonaceous materials without any further post or pretreatment. By using salt melt templating, pore structure adjustment is possible without the use of hazardous or toxic reagents and the template can be reused.
Using these simple tools, the synergetic effect of the pore structure and nitrogen content of the materials can be explored. Within this thesis, the influence of the condensation parameters will be correlated to the structure and performance of the materials. First, the influence of the condensation temperature to the porosity and nitrogen content of guanine will be discussed and the exploration of highly CO2 selective structural pores in C1N1 materials will be shown. Further tuning the pore structure of the materials by salt melt templating will be then explored, the potential of the prepared materials as heterogeneous catalysts and their basic catalytic strength will be correlated to their nitrogen content and pore morphology. A similar approach is used to explore the water sorption behavior of uric acid derived carbonaceous materials as potential sorbents for heat transformation applications. Changes in maximum water uptake and hydrophilicity of the prepared materials will be correlated to the nitrogen content and pore architecture. Due to the high thermal stability, porosity, and nitrogen content of ionic liquid derived nitrogen doped carbonaceous materials, a simple impregnation and calcination route can be conducted to obtain copper nano cluster decorated nitrogen-doped carbonaceous materials. The activity as catalyst for the oxygen reduction reaction of the obtained materials will be shown and structure performance relations are discussed.
In conclusion, the versatility of nitrogen doped carbonaceous materials with a nitrogen to carbon ratio of up to one will be shown. The possibility to tune the pore structure as well as the nitrogen content by using a simple procedure including salt melt templating as well as the use of molecular precursors and their effect on the performance will be discussed.
The negative impact of crude oil on the environment has led to a necessary transition toward alternative, renewable, and sustainable resources. In this regard, lignocellulosic biomass (LCB) is a promising renewable and sustainable alternative to crude oil for the production of fine chemicals and fuels in a so-called biorefinery process. LCB is composed of polysaccharides (cellulose and hemicellulose), as well as aromatics (lignin). The development of a sustainable and economically advantageous biorefinery depends on the complete and efficient valorization of all components. Therefore, in the new generation of biorefinery, the so-called biorefinery of type III, the LCB feedstocks are selectively deconstructed and catalytically transformed into platform chemicals. For this purpose, the development of highly stable and efficient catalysts is crucial for progress toward viability in biorefinery. Furthermore, a modern and integrated biorefinery relies on process and reactor design, toward more efficient and cost-effective methodologies that minimize waste. In this context, the usage of continuous flow systems has the potential to provide safe, sustainable, and innovative transformations with simple process integration and scalability for biorefinery schemes.
This thesis addresses three main challenges for future biorefinery: catalyst synthesis, waste feedstock valorization, and usage of continuous flow technology. Firstly, a cheap, scalable, and sustainable approach is presented for the synthesis of an efficient and stable 35 wt.-% Ni catalyst on highly porous nitrogen-doped carbon support (35Ni/NDC) in pellet shape. Initially, the performance of this catalyst was evaluated for the aqueous phase hydrogenation of LCB-derived compounds such as glucose, xylose, and vanillin in continuous flow systems. The 35Ni/NDC catalyst exhibited high catalytic performances in three tested hydrogenation reactions, i.e., sorbitol, xylitol, and 2-methoxy-4-methylphenol with yields of 82 mol%, 62 mol%, and 100 mol% respectively. In addition, the 35Ni/NDC catalyst exhibited remarkable stability over a long time on stream in continuous flow (40 h). Furthermore, the 35Ni/NDC catalyst was combined with commercially available Beta zeolite in a dual–column integrated process for isosorbide production from glucose (yield 83 mol%).
Finally, 35Ni/NDC was applied for the valorization of industrial waste products, namely sodium lignosulfonate (LS) and beech wood sawdust (BWS) in continuous flow systems. The LS depolymerization was conducted combining solvothermal fragmentation of water/alcohol mixtures (i.e.,methanol/water and ethanol/water) with catalytic hydrogenolysis/hydrogenation (SHF). The depolymerization was found to occur thermally in absence of catalyst with a tunable molecular weight according to temperature. Furthermore, the SHF generated an optimized cumulative yield of lignin-derived phenolic monomers of 42 mg gLS-1. Similarly, a solvothermal and reductive catalytic fragmentation (SF-RCF) of BWS was conducted using MeOH and MeTHF as a solvent. In this case, the optimized total lignin-derived phenolic monomers yield was found of 247 mg gKL-1.
The optical properties of chromophores, especially organic dyes and optically active inorganic molecules, are determined by their chemical structures, surrounding media, and excited state behaviors. The classical optical go-to techniques for spectroscopic investigations are absorption and luminescence spectroscopy. While both techniques are powerful and easy to apply spectroscopic methods, the limited time resolution of luminescence spectroscopy and its reliance on luminescent properties can make its application, in certain cases, complex, or even impossible. This can be the case when the investigated molecules do not luminesce anymore due to quenching effects, or when they were never luminescent in the first place. In those cases, transient absorption spectroscopy is an excellent and much more sophisticated technique to investigate such systems. This pump-probe laser-spectroscopic method is excellent for mechanistic investigations of luminescence quenching phenomena and photoreactions. This is due to its extremely high time resolution in the femto- and picosecond ranges, where many intermediate or transient species of a reaction can be identified and their kinetic evolution can be observed. Furthermore, it does not rely on the samples being luminescent, due to the active sample probing after excitation. In this work it is shown, that with transient absorption spectroscopy it was possible to identify the luminescence quenching mechanisms and thus luminescence quantum yield losses of the organic dye classes O4-DBD, S4-DBD, and pyridylanthracenes. Hence, the population of their triplet states could be identified as the competitive mechanism to their luminescence. While the good luminophores O4-DBD showed minor losses, the S4-DBD dye luminescence was almost entirely quenched by this process. However, for pyridylanthracenes, this phenomenon is present in both the protonated and unprotonated forms and moderately effects the luminescence quantum yield. Also, the majority of the quenching losses in the protonated forms are caused by additional non-radiative processes introduced by the protonation of the pyridyl rings. Furthermore, transient absorption spectroscopy can be applied to investigate the quenching mechanisms of uranyl(VI) luminescence by chloride and bromide. The reduction of the halides by excited uranyl(VI) leads to the formation of dihalide radicals X^(·−2). This excited state redox process is thus identified as the quenching mechanism for both halides, and this process, being diffusion-limited, can be suppressed by cryogenically freezing the samples or by observing these interactions in media with a lower dielectric constant, such as ACN and acetone.
Eukaryotic cells can be regarded as complex microreactors capable of performing various biochemical reactions in parallel which are necessary to sustain life. An essential prerequisite for these complex metabolic reactions to occur is the evolution of lipid membrane-bound organelles enabling compartmental- ization of reactions and biomolecules. This allows for a spatiotemporal control over the metabolic reactions within the cellular system. Intracellular organi- zation arising due to compartmentalization is a key feature of all living cells and has inspired synthetic biologists to engineer such systems with bottom-up approaches.
Artificial cells provide an ideal platform to isolate and study specific re- actions without the interference from the complex network of biomolecules present in biological cells. To mimic the hierarchical architecture of eukaryotic cells, multi-compartment assemblies with nested liposomal structures also re- ferred to as multi-vesicular vesicles (MVVs) have been widely adopted. Most of the previously reported multi-compartment systems adopt bulk method- ologies which suffer from low yield and poor control over size. Microfluidic strategies help circumvent these issues and facilitate a high-throughput and robust technique to assemble MVVs of uniform size distribution.
In this thesis, firstly, the bulk methodologies are explored to build MVVs and implement a synthetic signalling cascade. Next, a polydimethylsiloxane (PDMS)-based microfluidic platform is introduced to build MVVs and the significance of PEGylated lipids for the successful encapsulation of inner com- partments to generate stable multi-compartment systems is highlighted.
Next, a novel two-inlet channel PDMS-based microfluidic device to create MVVs encompassing a three-step enzymatic reaction cascade is presented. A directed reaction pathway comprising of the enzymes α-glucosidase (α-Glc), glucose oxidase (GOx), and horseradish peroxidase (HRP) spanning across three compartments via reconstitution of size-selective membrane proteins is described. Furthermore, owing to the monodispersity of our MVVs due to microfluidic strategies, this platform is employed to study the effect of com- partmentalization on reaction kinetics.
Further integration of cell-free expression module into the MVVs would allow for gene-mediated signal transduction within artificial eukaryotic cells. Therefore, the chemically inducible cell-free expression of a membrane protein alpha-hemolysin and its further reconstitution into liposomes is carried out.
In conclusion, the present thesis aims to build artificial eukaryotic cells to achieve size-selective chemical communication that also show potential for applications as micro reactors and as vehicles for drug delivery.
The controlled dosage of substances from a device to its environment, such as a tissue or an organ in medical applications or a reactor, room, machinery or ecosystem in technical, should ideally match the requirements of the applications, e.g. in terms of the time point at which the cargo is released. On-demand dosage systems may enable such a desired release pattern, if the device contain suitable features that can translate external signals into a release function. This study is motivated by the opportunities arising from microsystems capable of an on-demand release and the contributions that geometrical design may have in realizing such features. The goals of this work included the design, fabrication, characterization and experimental proof-of-concept of geometry-assisted triggerable dosing effect (a) with a sequential dosing release and (b) in a self-sufficient dosage system. Structure-function relationships were addressed on the molecular, morphological and, with a particular attention, the device design level, which is on the micrometer scale. Models and/or computational tools were used to screen the parameter space and provide guidance for experiments.
Membrane contact sites are of particular interest in the field of synthetic biology and biophysics. They are involved in a great variety of cellular functions. They form in between two cellular organelles or an organelle and the plasma membrane in order to establish a communication path for molecule transport or signal transmission.
The development of an artificial membrane system which can mimic membrane contact sites using bottom up synthetic biology was the goal of this research study. For this, a multi - compartmentalised giant unilamellar vesicle (GUV) system was created with the membrane of the outer vesicle mimicking the plasma membrane and the inner GUVs posing as cellular organelles.
In the following steps, three different strategies were used to achieve an internal membrane - membrane adhesion.
The goal of regenerative medicine is to guide biological systems towards natural healing outcomes using a combination of niche-specific cells, bioactive molecules and biomaterials. In this regard, mimicking the extracellular matrix (ECM) surrounding cells and tissues in vivo is an effective strategy to modulate cell behaviors. Cellular function and phenotype is directed by the biochemical and biophysical signals present in the complex 3D network of ECMs composed mainly of glycoproteins and hydrophilic proteoglycans. While cellular modulation in response to biophysical cues emulating ECM features has been investigated widely, the influence of biochemical display of ECM glycoproteins mimicking their presentation in vivo is not well characterized. It remains a significant challenge to build artificial biointerfaces using ECM glycoproteins that precisely match their presentation in nature in terms of morphology, orientation and conformation. This challenge becomes clear, when one understands how ECM glycoproteins self-assemble in the body. Glycoproteins produced inside the cell are secreted in the extra-cellular space, where they are bound to the cell membrane or other glycoproteins by specific interactions. This leads to elevated local concentration and 2Dspatial confinement, resulting in self-assembly by the reciprocal interactions arising from the molecular complementarity encoded in the glycoprotein domains. In this thesis, air-water (A-W) interface is presented as a suitable platform, where self-assembly parameters of ECM glycoproteins such as pH, temperature and ionic strength can be controlled to simulate in vivo conditions (Langmuir technique), resulting in the formation of glycoprotein layers with defined characteristics. The layer can be further compressed with surface barriers to enhance glycoprotein-glycoprotein contacts and defined layers of glycoproteins can be immobilized on substrates by horizontal lift and touch method, called Langmuir-Schäfer (LS) method. Here, the benefit of Langmuir and LS methods in achieving ECM glycoprotein biointerfaces with controlled network morphology and ligand density on substrates is highlighted and contrasted with the commonly used (glyco)protein solution deposition (SO) method on substrates. In general, the (glyco)protein layer formation by SO is rather uncontrolled, influenced strongly by (glyco)protein-substrate interactions and it results in multilayers and aggregations on substrates, while the LS method results in (glyco)proteins layers with a more homogenous presentation. To achieve the goal of realizing defined ECM layers on substrates, ECM glycoproteins having the ability to self-assemble were selected: Collagen-IV (Col-IV) and fibronectin (FN). Highly packed FN layer with uniform presentation of ligands was deposited on polydimethysiloxane VIII (PDMS) by LS method, while a heterogeneous layer was formed on PDMS by SO with prominent aggregations visible. Mesenchymal stem cells (MSC) on PDMS equipped with FN by LS exhibited more homogeneous and elevated vinculin expression and weaker stress fiber formation than on PDMS equipped with FN by SO and these divergent responses could be attributed to the differences in glycoprotein presentation at the interface. Col-IV are scaffolding components of specialized ECM called basement membranes (BM), and have the propensity to form 2D networks by self-polymerization associated with cells. Col- IV behaves as a thin-disordered network at the A-W interface. As the Col-IV layer was compressed at the A-W interface using trough barriers, there was negligible change in thickness (layer thickness ~ 50 nm) or orientation of molecules. The pre-formed organization of Col-IV was transferred by LS method in a controlled fashion onto substrates meeting the wettability criterion (CA ≤ 80°). MSC adhesion (24h) on PET substrates deposited with Col-IV LS films at 10, 15 and 20 mN·m-1 surface pressures was (12269.0 ± 5856.4) cells for LS10, (16744.2 ± 1280.1) cells for LS15 and (19688.3 ± 1934.0) cells for LS20 respectively. Remarkably, by selecting the surface areal density of Col-IV on the Langmuir trough on PET, there is a linear increase between the number of adherent MSCs and the Col-IV ligand density. Further, FN has the ability to self-stabilize and form 2D networks (even without compression) while preserving native β-sheet structure at the A-W interface on a defined subphase (pH = 2). This provides the possibility to form such layers on any vessel (even on standard six-well culture plates) and the cohesive FN layers can be deposited by LS transfer, without the need for expensive LB instrumentation. Multilayers of FN can be immobilized on substrates by this approach, as easily as Layer-by-Layer method, even without the need for secondary adlayer or activated bare substrate. Thus, this facile glycoprotein coating strategy approach is accessible to many researchers to realize defined FN films on substrates for cell culture. In conclusion, Langmuir and LS methods can create biomimetic glycoprotein biointerfaces on substrates controlling aspects of presentation such as network morphology and ligand density. These methods will be utilized to produce artificial BM mimics and interstitial ECM mimics composed of more than one ECM glycoprotein layer on substrates, serving as artificial niches instructing stem cells for cell-replacement therapies in the future.
The present work focuses on minimising the usage of toxic chemicals by integration of the biobased monomers, derived from fatty acid esters, to photopolymerization processes, which are known to be nature friendly. Internal double bond present in the oleic acid was converted to more reactive (meth)acrylate or epoxy group. Biobased starting materials, functionalized by different pendant groups, were used for photopolymerizing formulations to design of new polymeric structures by using ultraviolet light emitting diode (UV-LED) (395 nm) via free radical polymerization or cationic polymerization.
New (meth)acrylates (2,3 and 4) consisting of two isomers, methyl 9-((meth)acryloyloxy)-10-hydroxyoctadecanoate / methyl 9-hydroxy-10-((meth)acryloyloxy)octadecanoate (2 and 3) and methyl 9-(1H-imidazol-1-yl)-10-(methacryloyloxy)octadecanoate / methyl 9-(methacryloyloxy)-10-(1H-imidazol-1-yl)octadecanoate (4), modified from oleic acid mix, and ionic liquid monomers (1a and 1b) bearing long alkyl chain were polymerized photochemically. New (meth)acrylates are based on vegetable oil, and ionic liquids (ILs) have nonvolatile behaviour. Therefore, both monomer types have green approach. Photoinitiated polymerization of new (meth)acrylates and ionic liquids was investigated in the presence of ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate (Irgacure® TPO−L) or di(4-methoxybenzoyl)diethylgermane (Ivocerin®) as photoinitiator (PI). Additionally, the results were discussed in comparison with those obtained from commercial 1,6-hexanediol di(meth)acrylate (5 and 6) for deeper investigation of biobased monomer’s potential to substitute petroleum derived materials with renewable resources for possible coating applications. Kinetic study shows that methyl 9-(1H-imidazol-1-yl)-10-(methacryloyloxy)octadecanoate / methyl 9-(methacryloyloxy)-10-(1H-imidazol-1-yl)octadecanoate (4) and ionic liquids (1a and 1b) have quantitative conversion after irradiation process which is important for practical applications. On the other hand, heat generation occurs in a longer time during the polymerization of biobased systems or ILs.
The poly(meth)acrylates modified from (meth)acrylated fatty acid methyl ester monomers generally show a low glass transition temperature because of the presence of long aliphatic chain in the polymer structure. However, poly(meth)acrylates containing aromatic group have higher glass transition temperature. Therefore, new 4-(4-methacryloyloxyphenyl)-butan-2-one (7) was synthesized which can be a promising candidate for the green techniques, such as light induced polymerization. Photokinetic investigation of the new monomer, 4-(4-methacryloyloxyphenyl)-butan-2-one (7), was discussed using Irgacure® TPO−L or Ivocerin® as photoinitiator. The reactivity of that monomer was compared to commercial 2-phenoxyethyl methacrylate (8) and phenyl methacrylate (9) basis of the differences on monomer structures. The photopolymer of 4-(4-methacryloyloxyphenyl)-butan-2-one (7) might be an interesting candidate for the coating application with the properties of quantitative conversion and high molecular weight. It also shows higher glass transition temperature.
In addition to the linear systems based on renewable materials, new crosslinked polymers were also designed in this thesis. Therefore, isomer mixture consisting of ethane-1,2-diyl bis(9-methacryloyloxy-10-hydroxy octadecanoate), ethane-1,2-diyl 9-hydroxy-10-methacryloyloxy-9’-methacryloyloxy10’-hydroxy octadecanoate and ethane-1,2-diyl bis(9-hydroxy-10-methacryloyloxy octadecanoate) (10) was synthesized by derivation of the oleic acid which has not been previously described in the literature. Crosslinked material based on this biobased monomer was produced by photoinitiated free radical polymerization using Irgacure® TPO−L or Ivocerin® as photoinitiator. Furthermore, material properties were diversified by copolymerization of 10 with 4-(4-methacryloyloxyphenyl)-butan-2-one (7) or methyl 9-(1H-imidazol-1-yl)-10-(methacryloyloxy)octadecanoate / methyl 9-(methacryloyloxy)-10-(1H-imidazol-1-yl)octadecanoate (4). In addition to this, influence of comonomer with different chemical structure on the network system was investigated by analysis of thermo-mechanical properties, crosslink density and molecular weight between two crosslink junctions. An increase in the glass transition temperature caused by copolymerization of biobased monomer 10 with the excess amount of 4-(4-methacryloyloxyphenyl)-butan-2-one (7) was confirmed by both techniques, differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). On the other hand, crosslink density decreased as a result of copolymerization reactions due to the reduction in the mean functionality of the system. Furthermore, surface characterization has been tested by contact angle measurements using solvents with different polarity.
This work also contributes to the limited data reported about cationic photopolymerization of the epoxidized vegetable oils in the literature in contrast to the widely investigation of thermal curing of the biorenewable epoxy monomers. In addition to the 9,10-epoxystearic acid methyl ester (11), a new monomer of bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (12) has been synthesized from oleic acid. These two biobased epoxies have been polymerized via cationic photoinitiated polymerization in the presence of bis(t-butyl)-iodonium-tetrakis(perfluoro-t-butoxy)aluminate ([Al(O-t-C4F9)4]-) and isopropylthioxanthone (ITX) as photinitiating system. Polymerization kinetic of 9,10-epoxystearic acid methyl ester (11) and bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (12) was investigated and compared with the kinetic of commercial monomers being 3,4-epoxycyclohexylmethyl-3’,4’-epoxycyclohexane carboxylate (13), 1,4-butanediol diglycidyl ether (14), and diglycidylether of bisphenol-A (15). Both biobased epoxies (11 and 12) showed higher conversion than cycloaliphatic epoxy (13), and lower reactivity than 1,4-butanediol diglycidyl ether (14). Additional network systems were designed by copolymerization of bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (12) and diglycidylether of bisphenol-A (15) in different molar ratios (1:1; 1:5; 1:9). It addresses that, final conversion is dependent on polymerization rate as well as physical processes such as vitrification during polymerization. Moreover, low glass transition temperature of homopolymer derived from bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (12) was successfully increased by copolymerization with diglycidylether bisphenol-A (15). On the other hand, the surface produced from bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (12) shows hydrophobic character. Higher concentration of biobased diepoxy (12) in the copolymerizing mixture decreases surface free energy. Network systems were also investigated according to the rubber elasticity theory. Crosslinked polymer derived from the mixture of bis-(9,10-epoxystearic acid) 1,2-ethanediyl ester (12) and diglycidylether of bisphenol-A (15) (molar ratio=1:5) exhibits almost ideal polymer network.
Soft actuators have drawn significant attention due to their relevance for applications, such as artificial muscles in devices developed for medicine and robotics. Tuning their performance and expanding their functionality are frequently done by means of chemical modification. The introduction of structural elements rendering non-synthetic modification of the performance possible, as well as control over physical appearance and facilitating their recycling is a subject of a great interest in the field of smart materials. The primary aim of this thesis was to create a shape-memory polymeric actuator, where the capability for non-synthetic tuning of the actuation performance is combined with reprocessability. Physically cross-linked polymeric matrices provide a solid material platform, where the in situ processing methods can be employed for modification of the composition and morphology, resulting in the fine tuning of the related mechanical properties and shape-memory actuation capability.
The morphological features, required for shape-memory polymeric actuators, namely two crystallisable domains and anchoring points for physical cross-links, were embedded into a multiblock copolymer with poly(ε-caprolactone) and poly(L-lactide) segments (PLLA-PCL). Here, the melting transition of PCL was bisected into the actuating and skeleton-forming units, while the cross-linking was introduced via PLA stereocomplexation in blends with oligomeric poly(D-lactide) (ODLA). PLLA segment number average length of 12-15 repeating units was experimentally defined to be capable of the PLA stereocomplexes formation, but not sufficient for the isotactic crystallisation. Multiblock structure and phase dilution broaden the PCL melting transition, facilitating its separation into two conditionally independent crystalline domains. Low molar mass of the PLA stereocomplex components and a multiblock structure enables processing and reprocessing of the PLLA-PCL / ODLA blends with common non-destructive techniques. The modularity of the PLLA-PCL structure and synthetic approach allows for independent tuning of the properties of its components. The designed material establishes a solid platform for non-synthetic tuning of thermomechanical and structural properties of thermoplastic elastomers.
To evaluate the thermomechanical stability of the formed physical network, three criteria were appraised. As physical cross-links, PLA stereocomplexes have to be evenly distributed within the material matrix, their melting temperature shall not overlap with the thermal transitions of the PCL domains and they have to maintain the structural integrity within the strain ε ranges further applied in the shape-memory actuation experiments. Assigning PCL the function of the skeleton-forming and actuating units, and PLA stereocomplexes the role of physical netpoints, shape-memory actuation was realised in the PLLA-PCL / ODLA blends. Reversible strain of shape-memory actuation was found to be a function of PLA stereocomplex crystallinity, i.e. physical cross-linking density, with a maximum of 13.4 ± 1.5% at PLA stereocomplex content of 3.1 ± 0.3 wt%. In this way, shape-memory actuation can be tuned via adjusting the composition of the PLLA-PCL / ODLA blend. This makes the developed material a valuable asset in the production of cost-effective tunable soft polymeric actuators for the applications in medicine and soft robotics.
Synthesis and Characterization of Upconversion Nanaparticles for Applications in Life Sciences
(2021)
Ionizing radiation is used in cancer radiation therapy to effectively damage the DNA of tumors leading to cell death and reduction of the tumor tissue. The main damage is due to generation of highly reactive secondary species such as low-energy electrons (LEE) with the most probable energy around 10 eV through ionization of water molecules in the cells. A simulation of the dose distribution in the patient is required to optimize the irradiation modality in cancer radiation therapy, which must be based on the fundamental physical processes of high-energy radiation with the tissue. In the present work the accurate quantification of DNA radiation damage in the form of absolute cross sections for LEE-induced DNA strand breaks (SBs) between 5 and 20 eV is done by using the DNA origami technique. This method is based on the analysis of well-defined DNA target sequences attached to DNA origami triangles with atomic force microscopy (AFM) on the single molecule level. The present work focuses on poly-adenine sequences (5'-d(A4), 5'-d(A8), 5'-d(A12), 5'-d(A16), and 5'- 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. Additionally, DNA double strand breaks from a DNA hairpin 5'-d(CAC)4T(Bt-dT)T2(GTG)4 are examined for the first time and are compared with those of DNA single strands 5'-d(CAC)4 and 5'- d(GTG)4. The irradiation is made in the most likely energy range of 5 to 20 eV with an anionic resonance maximum around 10 eV independently of the DNA sequence. There is a clear difference between σSSB and σDSB of DNA single and double strands, where the strand break for ssDNA are always higher in all electron energies compared to dsDNA by the factor 3. A further part of this work deals with the characterization and analysis of new types of radiosensitizers used in chemoradiotherapy, which selectively increases the DNA damage upon radiation. Fluorinated DNA sequences with 2'-fluoro-2'-deoxycytidine (dFC) show an increased sensitivity at 7 and 10 eV compared to the unmodified DNA sequences by an enhancement factor between 2.1 and 2.5. In addition, light-induced oxidative damage of 5'-d(GTG)4 and 5'-d((CAC)4T(Bt-dT)T2(GTG)4) modified DNA origami triangles by singlet oxygen 1O2 generated from three photoexcited DNA groove binders [ANT994], [ANT1083] and [Cr(ddpd)2][BF4]3 illuminated in different experiments with UV-Vis light at 430, 435 and 530 nm wavelength is demonstrated. The singlet oxygen induced generation of DNA damage could be detected in both aqueous and dry environments for [ANT1083] and [Cr(ddpd)2][BF4]3.
Solar cells based on hybrid perovskites materials have become significantly important among the third generation photovoltaics over the last few years. The first solid state solar cell was reported in 2012. Over the years, the power conversion efficiencies of these devices have increased at a tremendous pace and this has made the perovskite solar cell devices a serious competitor in the well-established market of thin-film and wafer technologies. Over time, a large number of articles on this topic has been published in peer-reviewed journals. The presence of lead in the most efficient hybrid perovskite materials have raised questions about the possible toxicity of these devices and the extent of their environmental impact. Therefore, a lot of research has been devoted to finding alternative perovskite materials with similar or even better opto-electronic properties. An alternative strategy to improve the efficiency of thin film solar cells is to build efficient tandem cells by combining two or more perovskite materials with specifically tailored band gaps. The first step towards the development of perovskite-only tandem solar cells is to identify complementary hybrid perovskite materials with specific band gaps that maximize the efficiency of tandem solar cells. The optimal set of optical gaps for a tandem structure made of two materials is 1.9 eV and 1.0 eV. Since the electronic properties of hybrid perovskites are known to be strongly dependent on the composition and distortion of the crystal lattice, strong focus has been made towards the structure optimisation as well as the calculation of the energy band gaps of the materials using density functional theory (DFT). In an attempt to study the structure-property relationship of these perovskite materials and to find novel perovskite materials for future applications, researchers have employed computational screening procedures to study a large range of these materials by systematic replacement of the cations and anions from the prototypical perovskite. Density functional theory in particular is used as a theoretical tool, because of it’s precision to determine the properties of materials and also it’s computational viability in dealing with complex systems. In this thesis, the main focus is to do a systematic screening of the perovskite materials, of the composition ABX3 again by replacing the A-site, B-site and the X-site elements to find novel materials with band gaps suitable for application in tandem solar cells. As a first step towards contributing to this vibrant field of research, a high-throughput computational screening has been performed by replacing the metal and the halogen in the conventional CH3NH3PbI3 perovskites with homovalent metals and halogens to find materials in the desired range of band gaps that has already been mentioned earlier. This is achieved by performing a geometry optimisation on all the simulated structures followed by calculating their energy band gaps at the semilocal and the hybrid levels of theory. However, it is well known that the rotation of the organic cation CH3NH3 hinders the stability of these devices by the formation of hydrogen bonds between the hydrogen atoms of the cation and the halogens. This causes the materials to degrade under normal temperature and pressure conditions. As an attempt to prevent these devices from being unstable, a next step has been taken where the CH3NH3 cation has been replaced by inorganic cations of similar ionic radius. This is followed by another thorough screening, similar to the previous step. The stability of the materials has been determined by using the empirical Goldschmidt tolerance factor. As a last part of the thesis, a small proportion of the inorganic cation is mixed with CH3NH3 in order to form mixed-halide perovskites. These structures are optimised and their band gaps are calculated using density functional theory in order to predict materials suitable for single junction as well as tandem solar cell devices. It is expected that the contribution made through this thesis will be helpful for the progress of perovskite solar cells in terms of efficiencies and will also allow the community to explore the different properties these materials for further progress and development.
‘Smart’ Janus emulsions
(2021)
Emulsions constitute one of the most prominent and continuously evolving research areas in Colloid Chemistry, which involves the preparation of mixtures or dispersions of immiscible components in a continuous medium. Besides conventional oil-in-water or water-in-oil emulsions, other emulsions of complex droplet morphologies have recently attracted significant research interests. Especially Janus emulsions, in which each droplet is comprised of two distinct sub-regions, have shown versatile potential applications. One of their advantages is the possibility of compartmentalization, which enables to play with two different chemistries in a single droplet. Though microfluidic methods are conventionally used to prepare Janus emulsions, their industrial applications are largely hindered by low throughput and extensive instrumentations. Recently, it has been discovered that simply one-pot moderate/high energy emulsification is also capable of developing Janus morphology, although their preparation and stabilization remain rather substantially challenging. This cumulative doctoral thesis focuses on the preparation and characterization of ‘smart’ Janus emulsions, i.e. Janus emulsions with special stimuli-responsive features. One-step moderate/high energy emulsification of olive and silicone oil in an aqueous medium was carried out. Special consideration was devoted to the interfacial tensions among the components to maintain the criteria of forming characteristic droplet architectures, in addition to avoiding multiple emulsion destabilization phenomena like imminent phase separation or even separated droplet formation. A series of investigations were conducted related to the formation of complexes of charged macromolecules and role of them as stabilizers to achieve stable Janus emulsions for a realistic timeframe (more than 3 months). The correlation between the size of the stabilizer particles and the droplet size of emulsion was established. Furthermore, it was observed that Janus emulsion gels with interesting rheological properties can be fabricated in the presence of suitable polyelectrolyte complexes. Janus emulsions that could be influenced by pH, temperature or magnetic field were successfully produced in presence of characteristic stimuli-responsive stabilizers. Afterwards, the effect of these changes was studied by different characterization techniques. The size and morphology could be tuned easily by changing the pH. The incorporation of iron oxide magnetic nanoparticles (synthesized separately by a co-precipitation method) to one component of the Janus emulsion was carried out so that the movement and orientation of the complex droplets in aqueous media could be controlled by an external magnetic field. Additionally, temperature-triggered instantaneous reversible breakdown of Janus droplets was also accomplished. The responses of the Janus droplets by the stimuli were well-documented and explained. Another goal of the present contribution was to exploit this special morphological feature of emulsions as a template for producing porous materials. This was demonstrated by the preparation of ultralight magnetic responsive aerogels, utilizing Janus emulsion gels. The produced aerogels also showed the capacity to separate toxic dye from water. To the best of our knowledge, this is the first example of investigation towards batch scale production of Janus emulsion with such special stimuli-responsive properties by a simple bulk emulsification method.
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.
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.
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.
Advanced hybrid materials are recognized as one of the most significant enablers for new technologies, which holds true especially on the quest for sustainable energy sources and energy production schemes (e.g., semiconductor based photocatalytic materials). Usually, a single component is far from meeting all the demands needed for these advanced applications. Hybrid materials are composed of at least two components commonly an inorganic and an organic material on the molecular level, which feature novel properties exceeding the sum of the individual parts and might be the milestones of next-generation applications. This dissertation aims to provide novel combinations of the metal-free semiconductor graphitic carbon nitride (g-C3N4) with polymers to obtain materials with advanced properties and applications. Visible light constitutes the core of the present work as it is the only energy source utilized either in synthesis or in the application process. In the area of applications by combination of g-C3N4 and polymers, two different hybrids were thoroughly elucidated, i.e.. their design and construction as well as potential application in photocatalysis. Novel soft 3D liquid objects were formed via charge-interaction driven interfacial jamming between polyelectrolytes in aqueous environment and colloidal dispersions of g-C3N4 in edible sunflower oil. As such, stable liquid objects could be molded into specific shapes and utilized for photodegradation of organic dyes in water. Furthermore, the grafting of polymers onto g-C3N4 was investigated. Allyl-end functionalized polymers were grafted onto g-C3N4 by a photoinitiated process to yield g-C3N4 with versatile and improved properties, e.g. advanced dispersibility enabling processing via spin coating. As g-C3N4 produces radicals under visible light irradiation, which is of significant interest for polymer science, g-C3N4 containing polymer latex and macrogel beads (MGB) were synthesized by emulsion photopolymerization and inverse suspension photopolymerization, respectively. A well-controlled emulsion photopolymerization process via g-C3N4 initiation was designed, which features synthesis of well-defined and cross-linked polymer particles. Furthermore, the polymerization process was investigated thoroughly, indicating an ad-layer polymerization in early stages of the process. The utilization of functionalized g-C3N4 allowed the polymerization of various monomer types. Moreover, g-C3N4 was utilized as photoinitiator in hydrogel MGB formation. The formed MGB properties could be tailored via process design, e.g. stirring rate, cross-linker content and g-C3N4 content. Finally, MGBs were introduced as photocatalyst for waste water remediation, i.e. the degradation of Rhodamine B in aqueous solution was studied. The present thesis therefore builds a bridge between g-C3N4 and polymers and provides strategies for hybrid material formation. Furthermore, several potential applications are revealed with significant implications for photocatalysis, polymerization processes and polymer materials.
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.
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.