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- Institut für Chemie (194) (remove)
Well-defined dihydroxy telechelic oligodepsipeptides (oDPs), which have a high application potential as building blocks for scaffold materials for tissue engineering applications or particulate carrier systems for drug delivery applications are synthesized by ring-opening polymerization (ROP) of morpholine-2,5-diones (MDs) catalyzed by 1,1,6,6-tetra-n-butyl-1,6-distanna-2,5,7,10-tetraoxacyclodecane (Sn(IV) alkoxide). In contrast to ROP catalyzed by Sn(Oct)(2), the usage of Sn(IV) alkoxide leads to oDPs, with less side products and well-defined end groups, which is crucial for potential pharmaceutical applications. A slightly faster reaction of the ROP catalyzed by Sn(IV) alkoxide compared to the ROP initiated by Sn(Oct)(2)/EG is found. Copolymerization of different MDs resulted in amorphous copolymers with T(g)s between 44 and 54 degrees C depending on the molar comonomer ratios in the range from 25% to 75%. Based on the well-defined telechelic character of the Sn(IV) alkoxide synthesized oDPs as determined by matrix-assisted laser desorption/ionization time of flight measurements, they resemble interesting building blocks for subsequent postfunctionalization or multifunctional materials based on multiblock copolymer systems whereas the amorphous oDP-based copolymers are interesting building blocks for matrices of drug delivery systems.
Stepped supporting tools were developed and used in the university seminar Organic Chemistry taken by nonmajor chemistry students, which supported self-regulated learning. These supporting tools were also used for accompanying homework, which included a QR code that led to additional supporting tools. The application of stepped supporting tools in the seminars was evaluated by a four-item Likert scale. The students assessed the tools as a helpful instrument for solving tasks in chemistry.
The aqueous self-assembly behavior of a series of poly(ethylene glycol)-poly(l-/d-lactide) block copolymers and corresponding stereocomplexes is examined by differential scanning calorimetry, dynamic light scattering, and transmission electron microscopy. Block copolymers assemble into spherical micelles and worm-like aggregates at room temperature, whereby the fraction of the latter seemingly increases with decreasing lactide weight fraction or hydrophobicity. The formation of the worm-like aggregates arises from the crystallization of the polylactide by which the spherical micelles become colloidally unstable and fuse epitaxically with other micelles. The self-assembly behavior of the stereocomplex aggregates is found to be different from that of the block copolymers, resulting in rather irregular-shaped clusters of spherical micelles and pearl-necklace-like structures.
Analytical epigenetics
(2018)
The field of epigenetics describes the relationship between genotype and phenotype, by regulating gene expression without changing the canonical base sequence of DNA. It deals with molecular genomic information that is encoded by a rich repertoire of chemical modifications and molecular interactions. This regulation involves DNA, RNA and proteins that are enzymatically tagged with small molecular groups that alter their physical and chemical properties. It is now clear that epigenetic alterations are involved in development and disease, and thus, are the focus of intensive research. The ability to record epigenetic changes and quantify them in rare medical samples is critical for next generation diagnostics. Optical detection offers the ultimate single-molecule sensitivity and the potential for spectral multiplexing. Here we review recent progress in ultrasensitive optical detection of DNA and histone modifications.
Material surfaces with tailored aerophobicity are crucial for applications where gas bubble wettability has to be controlled, e.g., gas storage and transport, electrodes, bioreactors or medical devices. Here, we present switchable underwater aerophobicity of hydrophobic polymeric substrates, which respond to heat with multilevel micro-and nanotopographical changes. The cross-linked poly[ethylene-co-(vinyl acetate)] substrates possess arrays of microcylinders with a nanorough top surface. It is hypothesized that the specific micro-/nanotopography of the surface allows trapping of a water film at the micro interspace and in this way generates the aerophobic behavior. The structured substrates were programmed to a temporarily stable, nanoscale flat substrate showing aerophilic behavior. Upon heating, the topographical changes caused a switch in contact angle from aerophilic to aerophobic for approaching air bubbles. In this way, the initial adhesion of air bubbles to the programmed flat substrate could be turned into repellence for the recovered substrate surface. The temperature at which the repellence of air bubbles starts can be adjusted from 58 +/- 3 degrees C to 73 +/- 3 degrees C by varying the deformation temperature applied during the temperature-memory programming procedure. The presented actively switching polymeric substrates are attractive candidates for applications, where an on-demand gas bubble repellence is advantageous. (c) 2018 Helmholtz-Zentrum Geesthacht, Zentrum fur Material- und Kustenforschung. Published by Elsevier Ltd.
We discuss recent investigations of the interaction of polyelectrolytes with proteins. In particular, we review our recent studies on the interaction of simple proteins such as human serum albumin (HSA) and lysozyme with linear polyelectrolytes, charged dendrimers, charged networks, and polyelectrolyte brushes. In all cases discussed here, we combined experimental work with molecular dynamics (MD) simulations and mean-field theories. In particular, isothermal titration calorimetry (ITC) has been employed to obtain the respective binding constants K-b and the Gibbs free energy of binding. MD simulations with explicit counterions but implicit water demonstrate that counterion release is the main driving force for the binding of proteins to strongly charged polyelectrolytes: patches of positive charges located on the surface of the protein become multivalent counterions of the polyelectrolyte, thereby releasing a number of counterions condensed on the polyelectrolyte. The binding Gibbs free energy due to counterion release is predicted to scale with the logarithm of the salt concentration in the system, which is verified by both simulations and experiment. In several cases, namely, for the interaction of proteins with linear polyelectrolytes and highly charged hydrophilic dendrimers, the binding constant could be calculated from simulations to very good approximation. This finding demonstrated that in these cases explicit hydration effects do not contribute to the Gibbs free energy of binding. The Gibbs free energy can also be used to predict the kinetics of protein uptake by microgels for a given system by applying dynamic density functional theory. The entire discussion demonstrates that the direct comparison of theory with experiments can lead to a full understanding of the interaction of proteins with charged polymers. Possible implications for applications, such as drug design, are discussed.
Strategies to surface-functionalize scaffolds by covalent binding of biologically active compounds are of fundamental interest to control the interactions between scaffolds and biomolecules or cells. Poly(para-dioxanone) (PPDO) is a clinically established polymer that has shown potential as temporary implant, eg, for the reconstruction of the inferior vena cava, as a nonwoven fiber mesh. However, PPDO lacks suitable chemical groups for covalent functionalization. Furthermore, PPDO is highly sensitive to hydrolysis, reflected by short in vivo half-life times and degradation during storage. Establishing a method for covalent functionalization without degradation of this hydrolyzable polymer is therefore important to enable the surface tailoring for tissue engineering applications. It was hypothesized that treatment of PPDO with an N-hydroxysuccinimide ester group bearing perfluorophenyl azide (PFPA) under UV irradiation would allow efficient surface functionalization of the scaffold. X-ray photoelectron spectroscopy and attenuated total reflectance Fourier-transformed infrared spectroscopy investigation revealed the successful binding, while a gel permeation chromatography study showed that degradation did not occur under these conditions. Coupling of a rhodamine dye to the N-hydroxysuccinimide esters on the surface of a PFPA-functionalized scaffold via its amine linker showed a homogenous staining of the PPDO in laser confocal microscopy. The PFPA method is therefore applicable even to the surface functionalization of hydrolytically labile polymers, and it was demonstrated that PFPA chemistry may serve as a versatile tool for the (bio-)functionalization of PPDO scaffolds.
The hydrolytic stability of polymers to be used for coatings in aqueous environments, for example, to confer anti-fouling properties, is crucial. However, long-term exposure studies on such polymers are virtually missing. In this context, we synthesized a set of nine polymers that are typically used for low-fouling coatings, comprising the well-established poly(oligoethylene glycol methylether methacrylate), poly(3-(N-2-methacryloylethyl-N,N-dimethyl) ammoniopropanesulfonate) (“sulfobetaine methacrylate”), and poly(3-(N-3-methacryamidopropyl-N,N-dimethyl)ammoniopropanesulfonate) (“sulfobetaine methacrylamide”) as well as a series of hitherto rarely studied polysulfabetaines, which had been suggested to be particularly hydrolysis-stable. Hydrolysis resistance upon extended storage in aqueous solution is followed by ¹H NMR at ambient temperature in various pH regimes. Whereas the monomers suffered slow (in PBS) to very fast hydrolysis (in 1 M NaOH), the polymers, including the polymethacrylates, proved to be highly stable. No degradation of the carboxyl ester or amide was observed after one year in PBS, 1 M HCl, or in sodium carbonate buffer of pH 10. This demonstrates their basic suitability for anti-fouling applications. Poly(sulfobetaine methacrylamide) proved even to be stable for one year in 1 M NaOH without any signs of degradation. The stability is ascribed to a steric shielding effect. The hemisulfate group in the polysulfabetaines, however, was found to be partially labile.
Here we present a self-made annular continuous-flow reactor that can be used in the UV/vis range in an internal numbering-up manner. As a model reaction, we chose a powerful batch-scale-limited benzoannelation method, namely, an intramolecular photodehydro-Diels-Alder (IMPDDA) reaction. The scale-up potential of this particular photochemical benchmark reaction toward the preparation of macrocylic (1,7)naphthalenophanes by variation of selected flow parameters is presented.
Different signal amplification strategies to improve the detection sensitivity of immunoassays have been applied which utilize enzymatic reactions, nanomaterials, or liposomes. The latter are very attractive materials for signal amplification because liposomes can be loaded with a large amount of signaling molecules, leading to a high sensitivity. In addition, liposomes can be used as a cell-like "bioscaffold" to directly test recognition schemes aiming at cell-related processes. This study demonstrates an easy and fast approach to link the novel hydrophobic optical probe based on [1,3]dioxolo[4,5-f]-[1,3]benzodioxole (DBD dye mm239) with tunable optical properties to hydrophilic recognition elements (e.g., antibodies) using liposomes for signal amplification and as carrier of the hydrophobic dye. The fluorescence properties of mm239 (e.g., long fluorescence lifetime, large Stokes shift, high photostability, and high quantum yield), its high hydrophobicity for efficient anchoring in liposomes, and a maleimide bioreactive group were applied in a unique combination to build a concept for the coupling of antibodies or other protein markers to liposomes (coupling to membranes can be envisaged). The concept further allowed us to avoid multiple dye labeling of the antibody. Here, anti-TAMRA-antibody (DC7-Ab) was attached to the liposomes. In proof-of-concept, steady-state as well as time-resolved fluorescence measurements (e.g., fluorescence depolarization) in combination with single molecule detection (fluorescence correlation spectroscopy, FCS) were used to analyze the binding interaction between DC7-Ab and liposomes as well as the binding of the antigen rhodamine 6G (R6G) to the antibody. Here, the Forster resonance energy transfer (FRET) between mm239 and R6G was monitored. In addition to ensemble FRET data, single-molecule FRET (PIE-FRET) experiments using pulsed interleaved excitation were used to characterize in detail the binding on a single-molecule level to avoid averaging out effects.
Femtosecond-pulsed laser written and etched fiber bragg gratings for fiber-optical biosensing
(2018)
We present the development of a label-free, highly sensitive fiber-optical biosensor for online detection and quantification of biomolecules. Here, the advantages of etched fiber Bragg gratings (eFBG) were used, since they induce a narrowband Bragg wavelength peak in the reflection operation mode. The gratings were fabricated point-by-point via a nonlinear absorption process of a highly focused femtosecond-pulsed laser, without the need of prior coating removal or specific fiber doping. The sensitivity of the Bragg wavelength peak to the surrounding refractive index (SRI), as needed for biochemical sensing, was realized by fiber cladding removal using hydrofluoric acid etching. For evaluation of biosensing capabilities, eFBG fibers were biofunctionalized with a single-stranded DNA aptamer specific for binding the C-reactive protein (CRP). Thus, the CRP-sensitive eFBG fiber-optical biosensor showed a very low limit of detection of 0.82 pg/L, with a dynamic range of CRP detection from approximately 0.8 pg/L to 1.2 µg/L. The biosensor showed a high specificity to CRP even in the presence of interfering substances. These results suggest that the proposed biosensor is capable for quantification of CRP from trace amounts of clinical samples. In addition, the adaption of this eFBG fiber-optical biosensor for detection of other relevant analytes can be easily realized.
The article describes the surface modification of 3D printed poly(lactic acid) (PLA) scaffolds with calcium phosphate (CP)/gelatin and CP/chitosan hybrid coating layers. The presence of gelatin or chitosan significantly enhances CP co-deposition and adhesion of the mineral layer on the PLA scaffolds. The hydrogel/CP coating layers are fairly thick and the mineral is a mixture of brushite, octacalcium phosphate, and hydroxyapatite. Mineral formation is uniform throughout the printed architectures and all steps (printing, hydrogel deposition, and mineralization) are in principle amenable to automatization. Overall, the process reported here therefore has a high application potential for the controlled synthesis of biomimetic coatings on polymeric biomaterials.
Dynamic and direct visualization of interfacial evolution is helpful in gaining fundamental knowledge of all-solid-state-lithium battery working/degradation mechanisms and clarifying future research directions for constructing next-generation batteries. Herein, in situ and in operando synchrotron X-ray tomography and energy dispersive diffraction were simultaneously employed to record the morphological and compositional evolution of the interface of InLi-anode|sulfide-solid-electrolyte during battery cycling. Compelling morphological evidence of interfacial degradation during all-solid-state-lithium battery operation has been directly visualized by tomographic measurement. The accompanying energy dispersive diffraction results agree well with the observed morphological deterioration and the recorded electrochemical performance. It is concluded from the current investigation that a fundamental understanding of the phenomena occurring at the solid-solid electrode|electrolyte interface during all-solid-state-lithium battery cycling is critical for future progress in cell performance improvement and may determine its final commercial viability.
The combination of gold nanoparticles with liposomes is important for nano- and biotechnology. Here, we present direct, label-free characterization of liposome structure and composition at the site of its interaction with citrate-stabilized gold nanoparticles by surface-enhanced Raman scattering (SERS) and cryogenic electron microscopy (cryo-EM). Evidenced by the vibrational spectra and cryo-EM, the gold nanoparticles destroy the bilayer structure of interacting liposomes in the presence of a high amount of citrate, while at lower citrate concentration the nanoparticles interact with the surface of the intact liposomes. The spectra of phosphatidylcholine and phosphatidylcholine/sphingomyelin liposomes show that at the site of interaction the lipid chains are in the gel phase. The SERS spectra indicate that cholesterol has strong effects on the contacts of the vesicles with the nanoparticles. By combining cryo-EM and SERS, the structure and properties of lipid nanoparticle composites could be tailored for the development of drug delivery systems.
8-Allylcoumarins are conveniently accessible through a microwave-promoted tandem Claisen rearrangement/Wittig olefination/cyclization sequence. They serve as a versatile platform for the annellation of five- to seven-membered rings using ring-closing olefin metathesis (RCM). Furano-, pyrano-, oxepino- and azepinocoumarins were synthesized from the same set of precursors using Ru-catalyzed double bond isomerizations and RCM in a defined order. One class of products, pyrano[2,3-f]chromene-2,8-diones, were inaccessible through direct RCM of an acrylate, but became available from the analogous allyl ether via an assisted tandem catalytic RCM/allylic oxidation sequence.
The series of novel 3,3′-bis(trisarylsilyl)- and 3,3′-bis(arylalkylsilyl)-substituted binaphtholate rare-earth-metal complexes 2a–i (SiR3 = Si(o-biphenylene)Ph (a), SiCyPh2 (b), Si-t-BuPh2 (c), Si(i-Pr)3 (d), SiCy2Ph (e), Si(2-tolyl)Ph2 (f), Si(4-t-Bu-C6H4)3 (g), Si(4-MeO-C6H4)Ph2 (h), SiBnPh2 (i)) have been prepared via arene elimination from [Ln(o-C6H4CH2NMe2)3] (Ln = Y, Lu) and the corresponding 3,3′-bis(silyl)-substituted binaphthol. The complexes exhibit high catalytic activity in the hydroamination/cyclization of aminoalkenes, with activities exceeding 1000 h–1 for (R)-2f-Ln, (R)-2g-Ln, and (R)-2h-Ln in the cyclization of 2,2-diphenylpent-4-enylamine (3a) at 25 °C, while the rigid dibenzosilole-substituted complexes (R)-2a-Ln and the triisopropylsilyl-substituted complexes (R)-2d-Ln exhibited the lowest activity in the range of 150–270 h–1. Catalysts (R)-2b-Lu, (R)-2c-Lu, (R)-2f-Lu, and (R)-2i-Lu provide the highest selectivities for the majority of the substrates, while the yttrium congeners are usually less selective. The highest enantioselectivities of 96% ee were observed using (R)-2a-Lu and (R)-2c-Lu in the cyclization of (4E)-2,2,5-triphenylpent-4-enylamine (9). The reactions show apparently zero-order rate dependence on substrate concentration and first-order rate dependence on catalyst concentration, with some reactions exhibiting a slightly accelerated rate at high conversion due to a shift in the equilibrium between a less active, higher coordinate catalyst species in favor of a more active, lower coordinate species as a result of weaker binding of the hydroamination product in comparison to the aminoalkene substrate. The shift in equilibrium from the higher to the lower coordinate species is also entropically favored at elevated temperatures, which results in an unusual increase in selectivity in the cyclization of 2,2-dimethylpent-4-enylamine (3d), presumably due to a higher selectivity of the lower coordinate catalyst species. All binaphtholate yttrium complexes, except (R)-2a-Y, are catalytically active in the intermolecular hydroamination of benzylamines with terminal alkenes. The highest selectivity of 66% ee was observed for the reaction of benzylamine with 4-phenyl-1-butene using (R)-2h-Y at 110 °C.
Non-Born-Oppenheimer quantum dynamics of H-2(+) and HD+ excited by single one-cycle laser pulses linearly polarized along the molecular (z) axis have been studied within a three-dimensional model, including the internuclear distance R and electron coordinates z and rho, by means of the numerical solution of the time-dependent Schrodinger equation on the timescale of about 200 fs. Laser carrier frequencies corresponding to the wavelengths of lambda(l) = 400 and 50 nm have been used and the amplitudes of the pulses have been chosen such that the energies of H-2(+) and HD+ are above the dissociation threshold after the ends of the laser pulses. It is shown that excitation of H-2(+) and HD+ above the dissociation threshold is accompanied by formation of vibrationally "hot" and "cold" ensembles of molecules. Dissociation of vibrationally "hot" molecules does not prevent the appearance of post-laser-pulse electronic oscillations, parallel z oscillations, and transversal rho oscillations. Moreover, dissociation of "hot" molecules does not influence characteristic frequencies of electronic z and rho oscillations. The main difference between the laser-induced quantum dynamics of homonuclear H-2(+) and its heteronuclear isotope HD+ is that fast post-laser-pulse electronic z oscillations in H-2(+) are regularly shaped with the period of tau(shp) approximate to 30 fs corresponding to nuclear oscillations in H-2(+), while electronic z oscillations in HD+ arise as "echo pulses" of its initial excitation and appear with the period of tau(echo) approximate to 80 fs corresponding to nuclear motion in HD+. Accordingly, corresponding power spectra of nuclear motion contain strong low-frequency harmonics at omega(shp) = 2 pi/tau(shp) in H2(+) and omega(echo) = 2 pi/tau(echo) in HD+. Power spectra related to both electronic and nuclear motion have been calculated in the acceleration form. Both higher- and lower-order harmonics are generated at the laser wavelength lambda(l) = 400 nm, while only lower-order harmonics are well pronounced at lambda(l) = 50 nm. It is also shown that a rationalized harmonic order, defined in terms of the frequency of the laser-induced electronic z oscillations, agrees with the concept of inversion symmetry for electronic motion in diatomic molecules.
Fabrication of well-ordered porous silicon tubular structures using colloidal lithography and metal assisted chemical etching is reported. A continuous hexagonal hole/particle gold pattern was designed over monocrystalline silicon through deposition of polyNIPAM microspheres, followed by the surface decoration with gold nanoparticles and thermal treatment. An etching reaction with HF, ethanol and H2O2 dissolved the silicon in contact with the metal nanoparticles (NP), creating a porous tubular array in the "off-metal area". The morphological characterization revealed the formation of a cylindrical hollow porous tubular shape with external and internal diameter of approx. 900 nm and 400 nm respectively, though it can be tuned to other desired sizes by choosing an appropriate dimension for the microspheres. The porous morphology and optical properties were studied as a function of resistivity of silicon substrates. Compared to two different gold templates on cSi and nontubular porous pillar structures, porous silicon tubular framework revealed a maximum surface enhanced Raman scattering enhancement factor of 10(6) for the detection of 6-mercaptopurine (6-MP). Due to the large surface area available for any surface modification, open nanostructured platforms such as those studied here have potential applications in the field of reflection/photoluminescene and SERS based optical bio-/chemical sensors.
This research addressed the question, if it is possible to simplify current microcontact printing systems for the production of anisotropic building blocks or patchy particles, by using common chemicals while still maintaining reproducibility, high precision and tunability of the Janus-balance
Chapter 2 introduced the microcontact printing materials as well as their defined electrostatic interactions. In particular polydimethylsiloxane stamps, silica particles and high molecular weight polyethylenimine ink were mainly used in this research. All of these components are commercially available in large quantities and affordable, which gives this approach a huge potential for further up-scaling developments. The benefits of polymeric over molecular inks was described including its flexible influence on the printing pressure. With this alteration of the µCP concept, a new method of solvent assisted particle release mechanism enabled the switch from two-dimensional surface modification to three-dimensional structure printing on colloidal silica particles, without changing printing parameters or starting materials. This effect opened the way to use the internal volume of the achieved patches for incorporation of nano additives, introducing additional physical properties into the patches without alteration of the surface chemistry.
The success of this system and its achievable range was further investigated in chapter 3 by giving detailed information about patch geometry parameters including diameter, thickness and yield. For this purpose, silica particles in a size range between 1µm and 5µm were printed with different ink concentrations to change the Janus-balance of these single patched particles. A necessary intermediate step, consisting of air-plasma treatment, for the production of trivalent particles using "sandwich" printing was discovered and comparative studies concerning the patch geometry of single and double patched particles were conducted. Additionally, the usage of structured PDMS stamps during printing was described. These results demonstrate the excellent precision of this approach and opens the pathway for even greater accuracy as further parameters can be finely tuned and investigated, e.g. humidity and temperature during stamp loading.
The performance of these synthesized anisotropic colloids was further investigated in chapter 4, starting with behaviour studies in alcoholic and aqueous dispersions. Here, the stability of the applied patches was studied in a broad pH range, discovering a release mechanism by disabling the electrostatic bonding between particle surface and polyelectrolyte ink. Furthermore, the absence of strong attractive forces between divalent particles in water was investigated using XPS measurements. These results lead to the conclusion that the transfer of small PDMS oligomers onto the patch surface is shielding charges, preventing colloidal agglomeration. However, based on this knowledge, further patch modifications for particle self-assembly were introduced including physical approaches using magnetic nano additives, chemical patch functionalization with avidin-biotin or the light responsive cyclodextrin-arylazopyrazoles coupling as well as particle surface modification for the synthesis of highly amphiphilic colloids. The successful coupling, its efficiency, stability and behaviour in different solvents were evaluated to find a suitable coupling system for future assembly experiments. Based on these results the possibility of more sophisticated structures by colloidal self-assembly is given.
Certain findings needed further analysis to understand their underlying mechanics, including the relatively broad patch diameter distribution and the decreasing patch thickness for smaller silica particles. Mathematical assumptions for both effects are introduced in chapter 5. First, they demonstrate the connection between the naturally occurring particle size distribution and the broadening of the patch diameter, indicating an even higher precision for this µCP approach. Second, explaining the increase of contact area between particle and ink surface due to higher particle packaging, leading to a decrease in printing pressure for smaller particles.
These calculations ultimately lead to the development of a new mechanical microcontact printing approach, using centrifugal forces for high pressure control and excellent parallel alignment of printing substrates. First results with this device and the comparison with previously conducted by-hand experiments conclude this research. It furthermore displays the advantages of such a device for future applications using a mechanical printing approach, especially for accessing even smaller nano particles with great precision and excellent yield.
In conclusion, this work demonstrates the successful adjustment of the µCP approach using commercially available and affordable silica particles and polyelectrolytes for high flexibility, reduced costs and higher scale-up value. Furthermore, its was possible to increase the modification potential by introducing three-dimensional patches for additional functionalization volume. While keeping a high colloidal stability, different coupling systems showed the self-assembly capabilities of this toolbox for anisotropic particles.
Hepcidin-25 was identified as themain iron regulator in the human body, and it by binds to the sole iron-exporter ferroportin. Studies showed that the N-terminus of hepcidin is responsible for this interaction, the same N-terminus that encompasses a small copper(II) binding site known as the ATCUN (amino-terminal Cu(II)- and Ni(II)-binding) motif. Interestingly, this copper-binding property is largely ignored in most papers dealing with hepcidin-25. In this context, detailed investigations of the complex formed between hepcidin-25 and copper could reveal insight into its biological role. The present work focuses on metal-bound hepcidin-25 that can be considered the biologically active form. The first part is devoted to the reversed-phase chromatographic separation of copper-bound and copper-free hepcidin-25 achieved by applying basic mobile phases containing 0.1% ammonia. Further, mass spectrometry (tandemmass spectrometry (MS/MS), high-resolutionmass spectrometry (HRMS)) and nuclear magnetic resonance (NMR) spectroscopy were employed to characterize the copper-peptide. Lastly, a three-dimensional (3D)model of hepcidin-25with bound copper(II) is presented. The identification of metal complexes and potential isoforms and isomers, from which the latter usually are left undetected by mass spectrometry, led to the conclusion that complementary analytical methods are needed to characterize a peptide calibrant or referencematerial comprehensively. Quantitative nuclear magnetic resonance (qNMR), inductively-coupled plasma mass spectrometry (ICP-MS), ion-mobility spectrometry (IMS) and chiral amino acid analysis (AAA) should be considered among others.
Utilization of sunlight for energy harvesting has been foreseen as sustainable replacement for fossil fuels, which would also eliminate side effects arising from fossil fuel consumption such as drastic increase of CO2 in Earth atmosphere. Semiconductor materials can be implemented for energy harvesting, and design of ideal energy harvesting devices relies on effective semiconductor with low recombination rate, ease of processing, stability over long period, non-toxicity and synthesis from abundant sources. Aforementioned criteria have attracted broad interest for graphitic carbon nitride (g-CN) materials, metal-free semiconductor which can be synthesized from low cost and abundant precursors. Furthermore, physical properties such as band gap, surface area and absorption can be tuned. g-CN was investigated as heterogeneous catalyst, with diversified applications from water splitting to CO2 reduction and organic coupling reactions. However, low dispersibility of g-CN in water and organic solvents was an obstacle for future improvements.
Tissue engineering aims to mimic natural tissues mechanically and biologically, so that synthetic materials can replace natural ones in future. Hydrogels are crosslinked networks with high water content, therefore are prime candidates for tissue engineering. However, the first requirement is synthesis of hydrogels with mechanical properties that are matching to natural tissues. Among different approaches for reinforcement, nanocomposite reinforcement is highly promising.
This thesis aims to investigate aqueous and organic dispersions of g-CN materials. Aqueous g-CN dispersions were utilized for visible light induced hydrogel synthesis, where g-CN acts as reinforcer and photoinitiator. Varieties of methodologies were presented for enhancing g-CN dispersibility, from co-solvent method to prepolymer formation, and it was shown that hydrogels with diversified mechanical properties (from skin-like to cartilage-like) are accessible via g-CN utilization. One pot photografting method was introduced for functionalization of g-CN surface which provides functional groups towards enhanced dispersibility in aqueous and organic media. Grafting vinyl thiazole groups yields stable additive-free organodispersions of g-CN which are electrostatically stabilized with increased photophysical properties. Colloidal stability of organic systems provides transparent g-CN coatings and printing g-CN from commercial inkjet printers.
Overall, application of g-CN in dispersed media is highly promising, and variety of materials can be accessible via utilization of g-CN and visible light with simple chemicals and synthetic conditions. g-CN in dispersed media will bridge emerging research areas from tissue engineering to energy harvesting in near future.
Water at α-alumina surfaces
(2018)
The (0001) surface of α-Al₂O₃ is the most stable surface cut under UHV conditions and was studied by many groups both theoretically and experimentally. Reaction barriers computed with GGA functionals are known to be underestimated. Based on an example reaction at the (0001) surface, this work seeks to improve this rate by applying a hybrid functional method and perturbation theory (LMP2) with an atomic orbital basis, rather than a plane wave basis. In addition to activation barriers, we calculate the stability and vibrational frequencies of water on the surface. Adsorption energies were compared to PW calculations and confirmed PBE+D2/PW stability results. Especially the vibrational frequencies with the B3LYP hybrid functional that have been calculated for the (0001) surface are in good agreement with experimental findings. Concerning the barriers and the reaction rate constant, the expectations are fully met. It could be shown that recalculation of the transition state leads to an increased barrier, and a decreased rate constant when hybrid functionals or LMP2 are applied.
Furthermore, the molecular beam scattering of water on (0001) surface was studied. In a previous work by Hass the dissociation was studied by AIMD of molecularly adsorbed water, referring to an equilibrium situation. The experimental method to obtaining this is pinhole dosing. In contrast to this earlier work, the dissociation process of heavy water that is brought onto the surface from a molecular beam source was modeled in this work by periodic ab initio molecular dynamics simulations. This experimental method results in a non-equilibrium situation. The calculations with different surface and beam models allow us to understand the results of the non-equilibrium situation better. In contrast to a more equilibrium situation with pinhole dosing, this gives an increase in the dissociation probability, which could be explained and also understood mechanistically by those calculations.
In this work good progress was made in understanding the (1120) surface of α-Al₂O₃ in contact with water in the low-coverage regime. This surface cut is the third most stable one under UHV conditions and has not been studied to a great extent yet. After optimization of the clean, defect free surface, the stability of different adsorbed species could be classified. One molecular minimum and several dissociated species could be detected. Starting from these, reaction rates for various surface reactions were evaluated. A dissociation reaction was shown to be very fast because the molecular minimum is relatively unstable, whereas diffusion reactions cover a wider range from fast to slow. In general, the (112‾0) surface appears to be much more reactive against water than the (0001) surface. In addition to reactivity, harmonic vibrational frequencies were determined for comparison with the findings of the experimental “Interfacial Molecular Spectroscopy” group from Fritz-Haber institute in Berlin. Especially the vibrational frequencies of OD species could be assigned to vibrations from experimental SFG spectra with very good agreement. Also, lattice vibrations were studied in close collaboration with the experimental partners. They perform SFG spectra at very low frequencies to get deep into the lattice vibration region. Correspondingly, a bigger slab model with greater expansion perpendicular to the surface was applied, considering more layers in the bulk. Also with the lattice vibrations we could obtain reasonably good agreement in terms of energy differences between the peaks.
This project was focused on exploring the phase behavior of poly(styrene)187000-block-poly(2-vinylpyridine)203000 (SV390) with high molecular weight (390 kg/mol) in thin films, in which the self-assembly of block copolymers (BCPs) was realized via thermo-solvent annealing. The advanced processing technique of solvent vapor treatment provides controlled and stable conditions.
In Chapter 3, the factors to influence the annealing process and the swelling behavior of homopolymers are presented and discussed. The swelling behavior of BCP in films is controlled by the temperature of the vapor and of the substrate, on one hand, and variation of the saturation of the solvent vapor atmosphere (different solvents), on the other hand. Additional factors like the geometry and material of the chamber, the type of flow inside the chamber etc. also influence the reproducibility and stability of the processing. The slightly selective solvent vapor of chloroform gives 10% more swelling of P2VP than PS in films with thickness of ~40 nm.
The tunable morphology in ultrathin films of high molecular weight BCP (SV390) was investigated in Chapter 4. First, the swelling behavior can be precisely tuned by temperature and/or vapor flow separately, which provided information for exploring the multiple-parameter-influenced segmental chain mobility of polymer films. The equilibrium state of SV390 in thin films influenced by temperature was realized at various temperatures with the same degree of swelling. Various methods including characterization with SFM, metallization and RIE were used to identify the morphology of films as porous half-layer with PS dots and P2VP matrix. The kinetic investigations demonstrate that on substrates with either weak or strong interaction the original morphology of the BCP with high molecular weight is changed very fast within 5 min, and the further annealing serves for annihilation of defects.
The morphological development of symmetric BCP in films with thickness increasing from half-layer to one-layer influenced by confinement factors of gradient film thicknesses and various surface properties of substrates was studied in Chapter 5. SV390 and SV99 films show bulk lamella-forming morphology after slightly selective solvent vapor (chloroform) treatment. SV99 films show cylinder-forming morphology under strongly selective solvent vapor (toluene) treatment since the asymmetric structure (caused by toluene uptake in PS blocks only) of SV99 block copolymer during annealing. Both kinds of morphology (lamella and cylinder) are influenced by the film thickness. The annealed morphology of SV390 and SV99 influenced by the combination of confined film and substrate property is similar to the morphology on flat silicon wafers. In this chapter the gradients in the film thickness and surface properties of the substrates with regard to their influence on the morphological development in thin BCP films are presented. Directed self-assembly (graphoepitaxy) of this SV390 was also investigated to compare with systematically reported SV99.
In Chapter 6 an approach to induced oriented microphase separation in thick block copolymer films via treatment with the oriented vapor flow using mini-extruder is envisaged to be an alternative to existing methodologies, e.g. via non-solvent-induced phase separation. The preliminary tests performed in this study confirm potential perspective of this method, which alters the structure through the bulk of the film (as revealed by SAXS measurements), but more detailed studies have to be conducted in order to optimize the preparation.
Die intrazelluläre Markierung mit geeigneten Reagenzien ermöglicht ihre bildgebende Darstellung in lebenden Organismen. Dieses Verfahren (auch „Zell-Tracking“ genannt) wird in der Grundlagenforschung zur Entwicklung zellulärer Therapien, für die Erforschung pathologischer Prozesse, wie der Metastasierung, sowie für Therapiekontrollen eingesetzt. Besondere Bedeutung haben in den letzten Jahren zelluläre Therapien mit Stammzellen erlangt, da sie großes Potential bei der Regeneration von Geweben bei Krankheiten wie Morbus Parkinson oder Typ-1-Diabetes versprechen. Für die Entwicklung einer zellulären Therapie sind Informationen über den Verbleib der applizierten Zellen in vivo (Homing-Potential), über ihre Zellphysiologie sowie über die Entstehung möglicher Entzündungen notwendig. Das Ziel der vorliegenden Arbeit war daher die Synthese von Markierungsreagenzien, die nicht nur eine effiziente Zellmarkierung ermöglichen, sondern einen synergistischen Effekt hinsichtlich des modalitätsübergreifenden Einsatzes in den bildgebenden Verfahren MRT und Laser-Ablation(LA)-ICP-MS erlauben. Die MRT-Bildgebung ermöglicht die nicht invasive Nachverfolgung markierter Zellen in vivo und die LA-ICP-MS die anschließende ex vivo Analytik zur Darstellung der Elementverteilung (Bioimaging) in einer Biopsieprobe oder in einem Gewebeschnitt. Für diese Zwecke wurden zwei verschiedene Markierungsreagenzien mit dem kontrastgebenden Element Gadolinium synthetisiert. Gadolinium eignet sich aufgrund seines hohen magnetischen Moments hervorragend für die MRT-Bildgebung und da es in Biomolekülen nicht natürlich vorkommt, konnten die Reagenzien gleichermaßen für die Zellmarkierung und das Bioimaging mit der LA-ICP-MS untersucht werden. Für die Synthese eines makromolekularen Reagenzes wurde das kommerziell verfügbare Dendrimer G5-PAMAM über bifunktionelle Linker mit dem Chelator DOTA funktionalisiert, um anschließend Gadolinium zu komplexieren. Ein zweites, nanopartikuläres Reagenz wurde über eine Solvothermal-Synthese erhalten, bei der Ln:GdVO4-Nanokristalle mit einer funktionellen Polyacrylsäure(PAA)-Hülle dargestellt wurden. Die Dotierung der Ln:GdVO4-PAA Nanokristalle mit verschiedenen Lanthanoiden (Ln=Eu, Tb) zeigte ihre prinzipielle Multiplexfähigkeit in der LA-ICP-MS. Beide Markierungsreagenzien zeichneten sich durch gute Bioverträglichkeiten und r1-Relaxivitäten aus, was zudem ihr Potential für Anwendungen als präklinische „blood-pool“ MRT-Kontrastmittel belegte. Die Untersuchung der Zellmarkierung erfolgte anhand einer Tumorzelllinie und einer Stammzelllinie, wobei beide Zellarten erfolgreich intrazellulär mit beiden Reagenzien markiert wurden. Nach der Zellmarkierung veranschaulichte die in vitro MRT-Bildgebung von Zell-Phantomen eine deutlichere Kontrastverstärkung der Zellen nach der Markierung mit den Nanokristallen im Vergleich zum kommerziellen Kontrastmittel Magnevist®. Die hohe Effizienz der Zellmarkierung mit den Nanokristallen und die damit verbundenen hohen Signalintensitäten in einer einzelnen Zelle erlaubten beim Bioimaging mit der LA-ICP-MS, Messungen bis zu einer Auflösung von 4 µm Laser Spot Size. Nach der Zellmarkierung mit den DOTA(Gd3+)-funktionalisierten G5-PAMAM Dendrimeren waren hingegen Aufnahmen mit der LA-ICP-MS nur bis zu einer Auflösung von 12 µm Laser Spot Size möglich. Insgesamt waren die Ln:GdVO4-PAA Nanokristalle mit größerer Ausbeute und kostengünstiger herstellbar als die DOTA(Gd3+)-funktionalisierten G5-PAMAM Dendrimere und zeigten zudem eine effizientere Zellmarkierung. Die Ln:GdVO4-PAA Nanokristalle erscheinen somit für das Zell-Tracking als besonders vielversprechend. Darauf aufbauend wurden die Nanokristalle zur Etablierung der Antikörper-Konjugation ausgewählt, was sie für die molekulare in vivo Bildgebung sowie für die Immuno-Bildgebung von Gewebeschnitten oder Biopsie-Proben mit der LA-ICP-MS anwendbar macht.
Deoxyribonucleic acid (DNA) is the carrier of human genetic information and is exposed to environmental influences such as the ultraviolet (UV) fraction of sunlight every day. The photostability of the DNA against UV light is astonishing. Even if the DNA bases have a strong absorption maximum at around 260 nm/4.77 eV, their quantum yield of photoproducts remains very low 1. If the photon energies exceed the ionization energy (IE) of the nucleobases ( ̴ 8-9 eV) 2, the DNA can be severely damaged. Photoexcitation and -ionization reactions occur, which can induce strand breaks in the DNA. The efficiency of the excitation and ionization induced strand breaks in the target DNA sequences are represented by cross sections. If Si as a substrate material is used in the VUV irradiation experiments, secondary electrons with an energy below 3.6 eV are generated from the substrate. This low energy electrons (LEE) are known to induce dissociative electron attachment (DEA) in DNA and with it DNA strand breakage very efficiently. LEEs play an important role in cancer radiation therapy, since they are generated secondarily along the radiation track of ionizing radiation.
In the framework of this thesis, different single stranded DNA sequences were irradiated with 8.44 eV vacuum UV (VUV) light and cross sections for single strand breaks (SSB) were determined. Several sequences were also exposed to secondary LEEs, which additionally contributed to the SSBs. First, the cross sections for SSBs depending on the type of nucleobases were determined. Both types of DNA sequences, mono-nucleobase and mixed sequences showed very similar results upon VUV radiation. The additional influence of secondarily generated LEEs resulted in contrast in a clear trend for the SSB cross sections. In this, the polythymine sequence had the highest cross section for SSBs, which can be explained by strong anionic resonances in this energy range. Furthermore, SSB cross sections were determined as a function of sequence length. This resulted in an increase in the strand breaks to the same extent as the increase in the geometrical cross section. The longest DNA sequence (20 nucleotides) investigated in this series, however, showed smaller cross section values for SSBs, which can be explained by conformational changes in the DNA. Moreover, several DNA sequences that included the radiosensitizers 5-Bromouracil (5BrU) and 8-Bromoadenine (8BrA) were investigated and the corresponding SSB cross sections were determined. It was shown that 5BrU reacts very strongly to VUV radiation leading to high strand break yields, which showed in turn a strong sequence-dependency. 8BrA, on the other hand, showed no sensitization to the applied VUV radiation, since almost no increase in strand breakage yield was observed in comparison to non-modified DNA sequences.
In order to be able to identify the mechanisms of radiation damage by photons, the IEs of certain DNA sequences were further explored using photoionization tandem mass spectrometry. By varying the DNA sequence, both the IEs depending on the type of nucleobase as well as on the DNA strand length could be identified and correlated to the SSB cross sections. The influence of the IE on the photoinduced reaction in the brominated DNA sequences could be excluded.
Nanophotonics is the field of science and engineering aimed at studying the light-matter interactions on the nanoscale. One of the key aspects in studying such optics at the nanoscale is the ability to assemble the material components in a spatially controlled manner. In this work, DNA origami nanostructures were used to self-assemble dye molecules and DNA coated plasmonic nanoparticles. Optical properties of dye nanoarrays, where the dyes were arranged at distances where they can interact by Förster resonance energy transfer (FRET), were systematically studied according to the size and arrangement of the dyes using fluorescein (FAM) as the donor and cyanine 3 (Cy 3) as the acceptor. The optimized design, based on steady-state and time-resolved fluorometry, was utilized in developing a ratiometric pH sensor with pH-inert coumarin 343 (C343) as the donor and pH-sensitive FAM as the acceptor. This design was further applied in developing a ratiometric toxin sensor, where the donor C343 is unresponsive and FAM is responsive to thioacetamide (TAA) which is a well-known hepatotoxin. The results indicate that the sensitivity of the ratiometric sensor can be improved by simply arranging the dyes into a well-defined array. The ability to assemble multiple fluorophores without dye-dye aggregation also provides a strategy to amplify the signal measured from a fluorescent reporter, and was utilized here to develop a reporter for sensing oligonucleotides. By incorporating target capturing sequences and multiple fluorophores (ATTO 647N dye molecules), a reporter for microbead-based assay for non-amplified target oligonucleotide sensing was developed. Analysis of the assay using VideoScan, a fluorescence microscope-based technology capable of conducting multiplex analysis, showed the DNA origami nanostructure based reporter to have a lower limit of detection than a single stranded DNA reporter. Lastly, plasmonic nanostructures were assembled on DNA origami nanostructures as substrates to study interesting optical behaviors of molecules in the near-field. Specifically, DNA coated gold nanoparticles, silver nanoparticles, and gold nanorods, were placed on the DNA origami nanostructure aiming to study surface-enhanced fluorescence (SEF) and surface-enhanced Raman scattering (SERS) of molecules placed in the hotspot of coupled plasmonic structures.
In dieser Arbeit steht die Entwicklung einer Sensorplattform für biochemische Anwendungen, welche auf einem optischen Detektionsprinzips beruht, im Vordergrund. Während der Entwicklung wurden zwei komplementäre Konzeptideen behandelt, zum einen ein Sensor, der auf photonischen Kristallen und Wellenleiterstrukturen basiert und zum anderen einen faserbasierten Sensor, der chemisch modifizierte Faser-Bragg-Gitter enthält. Das optische Detektionsprinzip in beiden Sensorideen ist die resultierende Brechungsindexänderung als messbare physikochemische Kenngröße.
Das aus der Natur bekannte Phänomen der photonischen Kristalle, das u. a. bei Opalen und bei Schmetterlingen zu finden ist, wurde bereits 1887 von Lord Rayleigh beschrieben. Er beschrieb die optischen Eigenschaften von periodischen mehrschichtigen Filmen, welche als vereinfachtes Modell eines eindimensionalen photonischen Kristalls verstanden werden können. Die Periodizität der Brechungsindexänderung resultiert in einem optischen Filter für Frequenzen in einem bestimmten spektralen Bereich, weshalb dann dort keine Lichtausbreitung mehr möglich ist. Wird dieses System aber durch eine Defektstelle in der Brechungsindexperiodizität gestört, sodass daraus zwei perfekt periodische Systeme entstehen, ist die Lichtausbreitung für eine bestimmte Frequenz dennoch möglich. In der Folge resultiert daraus ein schmalbandiges Signal im Transmissionsspektrum. Die erlaubte Frequenz ist dabei u. a. abhängig vom Brechungsindexunterschied des periodischen Systems, d.h. Veränderung des Brechungsindexes einer Schicht führt zu einer spektralen Verschiebung der erlaubten Frequenz, dadurch kann dieses Sensorkonzept für biochemische Sensorik ausgenutzt werden [1]. Diese Entwicklung des auf photonischen Kristallen basierenden Sensors war eine Kooperation mit dem Industriepartner „Nanoplus GmbH“. In der Doktorarbeit wurden Simulationen und praktischen Arbeiten zur Designentwicklung des Sensors und die Arbeiten an einem ersten Modellaufbau für die biochemischen Anwendungen durchgeführt.
Für den faserbasierten Sensor wurden Faser-Bragg-Gitter in den Faserkern hineingeschrieben. Hill et al. entdeckten 1978, dass solche Gitterstrukturen genau wie photonische Kristalle als optische Filter fungieren [2]. Die Gitter bestehen dabei aus Änderungen des Brechungsindexes im Faserkern. Im Laufe der nächsten vierzig Jahren wurden verschiedene Einschreibetechniken und Gitterstrukturen entwickelt, weshalb die Eigenschaften der jeweiligen Gitterstrukturen variieren. Eine solche Gitterstruktur sind u. a. die Faser-Bragg-Gitter, deren Gitterperiode, d. h. die Abstände der Brechungsindexmodifikationen, sich im Nanometer- bis Mikrometerbereich befinden. Aufgrund der kleinen Gitterperiode wird eine rückwärtsführende Welle im Kern für eine bestimmte Frequenz bzw. Wellenlänge, der Bragg-Wellenlänge, erzeugt. Im Endeffekt resultiert daraus ein schmalbandiges Signal sowohl im Transmissionsspektrum, als auch im Reflexionsspektrum. Die Resonanzwellenlänge ist dabei proportional zu der Gitterperiode und dem effektiven Brechungsindex, welcher vom Brechungsindex des Kerns und des kernumgebenen Materials abhängig ist. Letztlich eignet sich diese Technik für physikochemische Sensorik. Im Rahmen dieser Arbeit wurden die Gitter mit Hilfe einer relativen neuen Herstellungsmethode in die Fasern geschrieben [3]. Anschließend stand die Entwicklung eines Biosensors im Vordergrund, wobei zunächst ein Protokoll zum Ätzen der Faser mit Flusssäure entwickelt worden ist, dass das System sensitiv zum umgebenen Brechungsindex macht. Am Ende wurde ein Modellaufbau realisiert, indem ein Modellsystem, hier die Detektion vom C-reaktiven Protein mittels spezifischen einzelsträngigen DNS-Aptameren, erfolgreich getestet und quantifiziert worden ist.
1 Mandal, S.; Erickson, D. Nanoscale Optofluidic Sensor Arrays. Opt. Express 2008, 16 (3), 1623–1631.
2 Hill, K. O.; Fujii, Y.; Johnson, D. C.; Kawasaki, B. S. Photosensitivity in Optical Fiber Waveguides: Application to Reflection Filter Fabrication. Appl. Phys. Lett. 1978, 32 (10), 647–649.
3 Martínez, A.; Dubov, M.; Khrushchev, I.; Bennion, I. Direct Writing of Fibre Bragg Gratings by Femtosecond Laser. Electron. Lett. 2004, 40 (19), 1170.
Amorphous calcium carbonate(ACC) is a wide spread biological material found in many organisms, such as sea Urchins and mollusks, where it serves as either a precursor phase for the crystalline biominerals or is stabilized and used in the amorphous state. As ACC readily crystallizes, stabilizers such as anions, cations or macromolecules are often present to avoid or delay unwanted crystallization. Furthermore, additives often control the properties of the materials to suit the specific function needed for the organism. E.g. cystoliths in leaves that scatter light to optimize energy uptake from the sun or calcite/aragonite crystals used in protective shells in mussels and gastropods. Lifetime of the amorphous phase is controlled by the kinetic stability against crystallization. This has often been linked to water which plays a role in the mobility of ions and hence the probability of forming crystalline nuclei to initiate crystallization. However, it is unclear how the water molecules are incorporated within the amorphous phase, either as liquid confined in pores, as structural water binding to the ions or as a mixture of both. It is also unclear how this is perturbed when additives are added, especially Mg2+, one the most common additives found in biogenic samples. Mg2+ are expected to have a strong influence on the water incorporated into ACC, given the high energy barrier to dehydration of magnesium ions compared to calcium ions in solution.
During the last 10-15 years, there has been a large effort to understand the local environment of the ions/molecules and how this affects the properties of the amorphous phase. But only a few aspects of the structure have so far been well-described in literature. The reason for this is partly caused by the low stability of ACC if exposed to air, where it tends to crystallize within minutes and by the limited quantities of ACC produced in traditional synthesis routes. A further obstacle has been the difficulty in modeling the local structure based on experimental data. To solve the problem of stability and sample size, a few studies have used stabilizers such as Mg2+ or OH- and severely dehydrated samples so as to stabilize the amorphous state, allowing for combined neutron and x-ray analysis to be performed. However, so far, a clear description of the local environments of water present in the structure has not been reported.
In this study we show that ACC can be synthesized without any stabilizing additives in quantities necessary for neutron measurements and that accurate models can be derived with the help of empirical-potential structural refinement. These analyses have shown that there is a wide range of local environments for all of the components in the system suggesting that the amorphous phase is highly inhomogeneous, without any phase separation between ions and water. We also showed that the water in ACC is mainly structural and that there is no confined or liquid-like water present in the system. Analysis of amorphous magnesium carbonate also showed that there is a large difference in the local structure of the two cations and that Mg2+ surprisingly interacts with significantly less water molecules then Ca2+ despite the higher dehydration energy. All in all, this shows that the role of water molecules as a structural component of ACC, with a strong binding to cat- and anions probably retard or prevents the crystallization of the amorphous phase.
Die vorliegende Arbeit behandelt die Synthese und Charakterisierung von funktionalisierten Alkydharzen und die photoinduzierte Polymerisation dieser unter Einsatz einer Quecksilberdampflampe oder einer UV LED mit unterschiedlicher Lichtintensität. Der Fokus dieser Arbeit bestand in der gezielten Substitution der internalen Doppelbindungen der Fettsäureester durch reaktivere Gruppen, wie Acrylate oder Methacrylate, welche für Alkydharze in dieser Form so in der Literatur nicht beschrieben sind. Untersuchungen des Polymerisationsverhaltens dieser funktionalisierten Harze wurden mit der Photo DSC durchgeführt, wobei Bis – (4 – methoxybenzoyl) diethylgermanium als Photoinitiator diente. Die Ergebnisse haben gezeigt, dass die Harze radikalisch polymerisiert werden können und eine geringere Abhängigkeit von der Umgebungsatmosphäre (Luftsauerstoff bzw. Stickstoff) vorliegt. Dies ist so in der Literatur für funktionalisierte Alkydharze nicht bekannt. Abmischungen von unterschiedlichen Monomeren und funktionalisierten Harzen bewirkten eine Steigerung der Viskosität sowie eine Verringerung der Sauerstoffinhibierung im Zuge der photoinduzierten Polymerisation unter Luftsauerstoff für die Quecksilberdampflampe und der UV LED.
Zur Untersuchung der sauerstoffinhibierenden Wirkung der Harze sind Synthesen unterschiedlicher, funktionalisierter Ölsäuremethylester als Modellsubstanzen durchgeführt worden. Ein verbessertes Polymerisationsverhalten und eine geringe Abhängigkeit von der Umgebungsatmosphäre konnte für die Modelle nachgewiesen werden. Zur Aufklärung des verbesserten Polymerisationsverhaltens sind gezielt Substituenten (Imidazol, Brom, Alkohol, Acetat) in den funktionalisierten Ölsäuremethylester eingebaut worden, um den Einfluss dieser aufzuzeigen. Im Rahmen dieser Synthesen sind neuartige Strukturen synthetisiert worden, welche so in der Literatur nicht beschrieben sind. Die Gegenüberstellung der Polymerisationszeit, der Umsatz der (Meth-)Acrylatgruppen sowie die Zeit zum Erreichen der maximalen Polymerisationsgeschwindigkeit unter Verwendung von unterschiedlichen UV Lichtquellen hat einen Einfluss der Substituenten auf das Polymerisationsverhalten gezeigt.
Various ways of preparing enantiomerically pure 2-amino[6]helicene derivatives were explored. Ni(0) mediated cyclotrimerization of enantiopure triynes provided (M)- and (P)-7,8-bis(p-tolyl)hexahelicene-2-amine in >99% ee as well as its benzoderivative in >99% ee. The stereocontrol was found to be inefficient for a 2- aminobenzo[6]helicene congener with an embedded five-membered ring. Helically chiral imidazolium salts bearing one or two helicene moieties have been synthesized and applied in enantioselective [2+2+2] cyclotrimerization catalyzed by an in situ formed Ni(0)-NHC complex. The synthesis of the first helically chiral Pd- and Ru-NHC complexes and their application in enantioselective catalysis was demonstrated. The latter shows promising results in enantioselective olefin metathesis reactions. A mechanistic proposal for asymmetric ring closing metathesis is provided.
The aim of this doctoral thesis was to establish a technique for the analysis of biomolecules with infrared matrix-assisted laser dispersion (IR-MALDI) ion mobility (IM) spectrometry. The main components of the work were the characterization of the IR-MALDI process, the development and characterization of different ion mobility spectrometers, the use of IR-MALDI-IM spectrometry as a robust, standalone spectrometer and the development of a collision cross-section estimation approach for peptides based on molecular dynamics and thermodynamic reweighting.
First, the IR-MALDI source was studied with atmospheric pressure ion mobility spectrometry and shadowgraphy. It consisted of a metal capillary, at the tip of which a self-renewing droplet of analyte solution was met by an IR laser beam. A relationship between peak shape, ion desolvation, diffusion and extraction pulse delay time (pulse delay) was established. First order desolvation kinetics were observed and related to peak broadening by diffusion, both influenced by the pulse delay. The transport mechanisms in IR-MALDI were then studied by relating different laser impact positions on the droplet surface to the corresponding ion mobility spectra. Two different transport mechanisms were determined: phase explosion due to the laser pulse and electrical transport due to delayed ion extraction. The velocity of the ions stemming from the phase explosion was then measured by ion mobility and shadowgraphy at different time scales and distances from the source capillary, showing an initially very high but rapidly decaying velocity. Finally, the anatomy of the dispersion plume was observed in detail with shadowgraphy and general conclusions over the process were drawn.
Understanding the IR-MALDI process enabled the optimization of the different IM spectrometers at atmospheric and reduced pressure (AP and RP, respectively). At reduced pressure, both an AP and an RP IR-MALDI source were used. The influence of the pulsed ion extraction parameters (pulse delay, width and amplitude) on peak shape, resolution and area was systematically studied in both AP and RP IM spectrometers and discussed in the context of the IR-MALDI process. Under RP conditions, the influence of the closing field and of the pressure was also examined for both AP and RP sources. For the AP ionization RP IM spectrometer, the influence of the inlet field (IF) in the source region was also examined. All of these studies led to the determination of the optimal analytical parameters as well as to a better understanding of the initial ion cloud anatomy.
The analytical performance of the spectrometer was then studied. Limits of detection (LOD) and linear ranges were determined under static and pulsed ion injection conditions and interpreted in the context of the IR-MALDI mechanism. Applications in the separation of simple mixtures were also illustrated, demonstrating good isomer separation capabilities and the advantages of singly charged peaks. The possibility to couple high performance liquid chromatography (HPLC) to IR-MALDI-IM spectrometry was also demonstrated. Finally, the reduced pressure spectrometer was used to study the effect of high reduced field strength on the mobility of polyatomic ions in polyatomic gases.
The last focus point was on the study of peptide ions. A dataset obtained with electrospray IM spectrometry was characterized and used for the calibration of a collision cross-section (CCS) determination method based on molecular dynamics (MD) simulations at high temperature. Instead of producing candidate structures which are evaluated one by one, this semi-automated method uses the simulation as a whole to determine a single average collision cross-section value by reweighting the CCS of a few representative structures. The method was compared to the intrinsic size parameter (ISP) method and to experimental results. Additional MD data obtained from the simulations was also used to further analyze the peptides and understand the experimental results, an advantage with regard to the ISP method. Finally, the CCS of peptide ions analyzed by IR-MALDI were also evaluated with both ISP and MD methods and the results compared to experiment, resulting in a first validation of the MD method. Thus, this thesis brings together the soft ionization technique that is IR-MALDI, which produces mostly singly charged peaks, with ion mobility spectrometry, which can distinguish between isomers, and a collision cross-section determination method which also provides structural information on the analyte at hand.
Synthesis of artificial building blocks for sortase-mediated ligation and their enzymatic linkage
(2018)
The enzyme Sortase A catalyzes the formation of a peptide bond between the recognition sequence LPXTG and an oligoglycine. While manifold ligations between proteins and various biomolecules, proteins and small synthetic molecules as well as proteins and surfaces have been reported, the aim of this thesis was to investigate the sortase-catalyzed linkage between artificial building blocks. Hence, this could pave the way for the use of sortase A for tasks from a chemical point of view and maybe even materials science.
For the proof of concept, the studied systems were kept as simple as possible at first by choosing easily accessible silica NPs and commercially available polymers. These building blocks were functionalized with peptide motifs for sortase-mediated ligation. Silica nanoparticles were synthesized with diameters of 60 and 200 nm and surface modified with C=C functionalities. Then, peptides bearing a terminal cysteine were covalently linked by means of a thiol-ene reaction. 60 nm SiO2 NPs were functionalized with pentaglycines, while peptides with LPETG motif were linked to 200 nm silica particles. Polyethyleneglycol (PEG) and poly(N isopropylacrylamide) (PNIPAM) were likewise functionalized with peptides by thiol-ene reaction between cysteine residues and C=C units in the polymer end groups. Hence, G5-PEG and PNIPAM-LPETG conjugates were obtained. With this set of building blocks, NP–polymer hybrids, NP–NP, and polymer–polymer structures were generated by sortase-mediated ligation and the product formation shown by transmission electron microscopy, MALDI-ToF mass spectrometry and dynamic light scatting, among others. Thus, the linkage of these artificial building blocks by the enzyme sortase A could be demonstrated.
However, when using commercially available polymers, the purification of the polymer–peptide conjugates was impossible and resulted in a mixture containing unmodified polymer. Therefore, strategies were developed for the own synthesis of pure peptide-polymer and polymer-peptide conjugates as building blocks for sortase-mediated ligation. The designed routes are based on preparing polymer blocks via RAFT polymerization from CTAs that are attached to N- or C-terminus, respectively, of a peptide. GG-PNIPAM was synthesized through attachment of a suitable RAFT CTA to Fmoc-GG in an esterification reaction, followed by polymerization of NIPAM and cleavage of the Fmoc protection group. Furthermore, several peptides were synthesized by solid-phase peptide synthesis. The linkage of a RAFT CTA (or
polymerization initiator) to the N-terminus of a peptide can be conducted in an automated fashion as last step in a peptide synthesizer. The synthesis of such a conjugate couldn’t be realized in the time frame of this thesis, but many promising strategies exist to continue this strategy using different coupling reagents. Such polymer building blocks can be used to synthesize protein-polymer conjugates catalyzed by sortase A and the approach can be carried on to the synthesis of block copolymers by using polymer blocks with peptide motifs on both ends.
Although the proof of concept demonstrated in this thesis only shows examples that can be also synthesized by exclusively chemical techniques, a toolbox of such building blocks will enable the future formation of new materials and pave the way for the application of enzymes in materials science. In addition to nanoparticle systems and block copolymers, this also includes combination with protein-based building blocks to form hybrid materials. Hence, sortase could become an enzymatic tool that complements established chemical linking technologies and provides specific peptide motifs that are orthogonal to all existing chemical functional groups.
Innerhalb dieser Doktorarbeit wurde eine neuartige Mikromanipulationstechnik für die lokale Flüssigkeitsabgabe am komplexen Drüsengewebe der Schabe P. americana charakterisiert und für die damit verbundene gezielte Manipulation von einzelnen Zellen in einem Zellkomplex (Gewebe) angewandt. Bei dieser Mikromanipulationstechnik handelt es sich um die seit 2009 bekannte nanofluidische Rasterkraftmikroskopie (FluidFM = fluidic force microscopy). Dabei werden sehr kleine mikrokanälige Rasterkraftspitzen bzw. Mikro-/Nanopipetten mit einer Öffnung zwischen 300 nm und 2 µm verwendet, mit denen es möglich ist, sehr kleine Volumina im Pikoliter- bis Femtoliter-Bereich (10-12 L – 10-15 L) gezielt und ortsgenau abzugeben. Das Ziel dieser Arbeit war die Analyse zellulärer Prozesse, wie z. B. Zell-Zell-Kommunikation oder Signalweiterleitung, zwischen benachbarten Zellen unter Zuhilfenahme der Fluoreszenzmikroskopie. Mit dieser Methode können die Zellen und ihre Bestandteile mittels vorheriger Farbstoffbeladung unter einem Mikroskop mit hohem Kontrast optisch dargestellt werden. Mit Hilfe der Fluoreszenzmikroskopie sollten schlussendlich die zellulären Reaktionen innerhalb des Gewebes nach der lokalen Manipulation visualisiert werden.
Zunächst wurde die Anwendung des Systems an Luft und wässriger Umgebung beschrieben. In diesem Zusammenhang wurde eine Reinigungs- und Beladungsmethode entwickelt, mit der es möglich war, die kostspieligen Mikro-/Nanopipetten zu reinigen und anschließend mehrmals wiederzuverwenden. Hierzu wurde eine alternative Methode getestet, mit der das Diffusionsverhalten von Farbstoffmolekülen in unterschiedlichen Medien untersucht werden kann. Des Weiteren wurden die Systemparameter optimiert, welche nötig sind, um zwischen der Probenoberfläche und der Pipette einen guten Pipettenöffnungs-abschluss zu erhalten. Dieser Abschluss ist essentiell, damit die abgegebene Flüssigkeit ausschließlich in der Abgaberegion mit der Probe wechselwirkt und die darauffolgenden Reaktionen nur innerhalb des Gewebes erfolgen, da ansonsten die Zell-Zell-Signalweiterleitung zwischen den Zellen nicht eindeutig nachvollzogen werden kann. Diese interzelluläre Kommunikation wurde anhand zweier sekundärer Botenstoffe (Ca2+ und NO) untersucht. Hierbei war es möglich einzelne lokale Reaktionen zu detektieren, welche sich über weitere Zellen ausbreiteten. Schlussendlich wurde die Fertigung einer speziellen Injektionspipette beschrieben, welche an zwei biologischen Systemen getestet wurde.
Health effects, attributed to the environmental pollution resulted from using solvents such as benzene, are relatively unexplored among petroleum workers, personal use, and laboratory researchers. Solvents can cause various health problems, such as neurotoxicity, immunotoxicity, and carcinogenicity. As such it can be absorbed via epidermal or respiratory into the human body resulting in interacting with molecules that are responsible for biochemical and physiological processes of the brain.
Owing to the ever-growing demand for finding a solution, an Ionic liquid can use as an alternative solvent. Ionic liquids are salts in a liquid state at low temperature (below 100 C), or even at room temperature. Ionic liquids impart a unique architectural platform, which has been interesting because of their unusual properties that can be tuned by simple ways such as mixing two ionic liquids.
Ionic liquids not only used as reaction solvents but they became a key developing for novel applications based on their thermal stability, electric conductivity with very low vapor pressure in contrast to the conventional solvents.
In this study, ionic liquids were used as a solvent and reactant at the same time for the novel nanomaterials synthesis for different applications including solar cells, gas sensors, and water splitting.
The field of ionic liquids continues to grow, and become one of the most important branches of science. It appears to be at a point where research and industry can work together in a new way of thinking for green chemistry and sustainable production.
New bio-based polymers
(2018)
Redox-responsive polymers, such as poly(disulfide)s, are a versatile class of polymers with potential applications including gene- and drug-carrier systems. Their degradability under reductive conditions allows for a controlled response to the different redox states that are present throughout the body. Poly(disulfide)s are typically synthesized by step growth polymerizations. Step growth polymerizations, however, may suffer from low conversions and therefore low molar masses, limiting potential applications. The purpose of this thesis was therefore to find and investigate new synthetic routes towards the synthesis of amino acid-based poly(disulfide)s.
The different routes in this thesis include entropy-driven ring opening polymerizations of novel macrocyclic monomers, derived from cystine derivatives. These monomers were obtained with overall yields of up to 77% and were analyzed by mass spectrometry as well as by 1D and 2D NMR spectroscopy. The kinetics of the entropy-driven ring-opening metathesis polymerization (ED-ROMP) were thoroughly investigated in dependence of temperature, monomer concentration, and catalyst concentration. The polymerization was optimized to yield poly(disulfide)s with weight average molar masses of up to 80 kDa and conversions of ~80%, at the thermodynamic equilibrium. Additionally, an alternative metal free polymerization, namely the entropy-driven ring-opening disulfide metathesis polymerization (ED-RODiMP) was established for the polymerization of the macrocyclic monomers. The effect of different solvents, concentrations and catalyst loadings on the polymerization process and its kinetics were studied. Polymers with very high weight average molar masses of up to 177 kDa were obtained. Moreover, various post-polymerization reactions were successfully performed.
This work provides the first example of the homopolymerization of endo-cyclic disulfides by ED-ROMP and the first substantial study into the kinetics of the ED-RODiMP process.
Time-dependent correlation function based methods to study optical spectroscopy involving electronic transitions can be traced back to the work of Heller and coworkers. This intuitive methodology can be expected to be computationally efficient and is applied in the current work to study the vibronic absorption, emission, and resonance Raman spectra of selected organic molecules. Besides, the "non-standard" application of this approach to photoionization processes is also explored. The application section consists of four chapters as described below.
In Chapter 4, the molar absorptivities and vibronic absorption/emission spectra of perylene and several of its N-substituted derivatives are investigated. By systematically varying the number and position of N atoms, it is shown that the presence of nitrogen heteroatoms has a negligible effect on the molecular structure and geometric distortions upon electronic transitions, while spectral properties are more sensitive: In particular the number of N atoms is important while their position is less decisive. Thus, N-substitution can be used to fine-tune the optical properties of perylene-based molecules.
In Chapter 5, the same methods are applied to study the vibronic absorption/emission and resonance Raman spectra of a newly synthesized donor-acceptor type molecule. The simulated absorption/emission spectra agree fairly well with experimental data, with discrepancies being attributed to solvent effects. Possible modes which may dominate the fine-structure in the vibronic spectra are proposed by analyzing the correlation function with the aid of Raman and resonance Raman spectra.
In the next two chapters, besides the above types of spectra, the methods are extended to study photoelectron spectra of several small diamondoid-related systems (molecules, radicals, and cations). Comparison of the photoelectron spectra with available experimental data suggests that the correlation function based approach can describe ionization processes reasonably well. Some of these systems, cationic species in particular, exhibit somewhat peculiar optical behavior, which presents them as possible candidates for functional devices.
Correlation function based methods in a more general sense can be very versatile. In fact, besides the above radiative processes, formulas for non-radiative processes such as internal conversion have been derived in literature. Further implementation of the available methods is among our next goals.
Synthesis, assembly and thermo-responsivity of polymer-functionalized magnetic cobalt nanoparticles
(2018)
This thesis mainly covers the synthesis, surface modification, magnetic-field-induced assembly and thermo-responsive functionalization of superparamagnetic Co NPs initially stabilized by hydrophobic small molecules oleic acid (OA) and trioctylphosphine oxide (TOPO), as well as the synthesis of both superparamagnetic and ferromagnetic Co NPs by using end-functionalized-polystyrene as stabilizer.
Co NPs, due to their excellent magnetic and catalytic properties, have great potential application in various fields, such as ferrofluids, catalysis, and magnetic resonance imaging (MRI). Superparamagnetic Co NPs are especially interesting, since they exhibit zero coercivity. They get magnetized in an external magnetic field and reach their saturation magnetization rapidly, but no magnetic moment remains after removal of the applied magnetic field. Therefore, they do not agglomerate in the body when they are used in biomedical applications. Normally, decomposition of metallic precursors at high temperature is one of the most important methods in preparation of monodisperse magnetic NPs, providing tunability in size and shape. Hydrophobic ligands like OA, TOPO and oleylamine are often used to both control the growth of NPs and protect them from agglomeration. The as-prepared magnetic NPs can be used in biological applications as long as they are transferred into water. Moreover, their supercrystal assemblies have the potential for high density data storage and electronic devices. In addition to small molecules, polymers can also be used as surfactants for the synthesis of ferromagnetic and superparamagnetic NPs by changing the reaction conditions. Therefore, chapter 2 gives an overview on the basic concept of synthesis, surface modification and self-assembly of magnetic nanoparticles. Various examples were used to illustrate the recent work.
The hydrophobic Co NPs synthesized with small molecules as surfactants limit their biological applications, which require a hydrophilic or aqueous environment. Surface modification (e.g., ligand exchange) is a general idea for either phase transition or surface-functionalization. Therefore, in chapter 3, a ligand exchange process was conducted to functionalize the surface of Co NPs. PNIPAM is one of the most popular smart polymers and its lower critical solution temperature (LCST) is around 32 °C, with a reversible change in the conformation structure between hydrophobic and hydrophilic. The novel nanocomposites of superparamagnetic Co NPs and thermo-responsive PNIPAM are of great interest. Thus, well-defined superparamagnetic Co NPs were firstly synthesized through the thermolysis of cobalt carbonyl by using OA and TOPO as surfactants. A functional ATRP initiator, containing an amine (as anchoring group) and a 2-bromopropionate group (SI-ATRP initiator), was used to replace the original ligands. This process is rapid and facial for efficient surface functionalization and afterwards the Co NPs can be dispersed into polar solvent DMF without aggregation. FT-IR spectroscopy showed that the TOPO was completely replaced, but a small amount of OA remained on the surface. A TGA measurement allowed the calculation of the grafting density of the initiator as around 3.2 initiator/nm2. Then, the surface-initiated ATRP was conducted for the polymerization of NIPAM on the surface of Co NPs and rendered the nanocomposites water-dispersible. A temperature-dependent dynamic light scattering study showed the aggregation behavior of PNIPAM-coated Co NPs upon heating and this process was proven to be reversible. The combination of superparamagnetic and thermo-responsive properties in these hybrid nanoparticles is promising for future applications e.g. in biomedicine.
In chapter 4, the magnetic-field-induced assembly of superparamagnetic cobalt nanoparticles both on solid substrates and at liquid-air interface was investigated. OA- and TOPO-coated Co NPs were synthesized via the thermolysis of cobalt carbonyl and dispersed into either hexane or toluene. The Co NP dispersion was dropped onto substrates (e.g., TEM grid, silicon wafer) and at liquid-air (water-air or ethylene glycol-air) interface. Due to the attractive dipolar interaction, 1-D chains formed in the presence of an external magnetic field. It is known that the concentration and the strength of the magnetic field can affect the assembly behavior of superparamagnetic Co NPs. Therefore, the influence of these two parameters on the morphology of the assemblies was studied. The formed 1-D chains were shorter and flexible at either lower concentration of the Co NP dispersion or lower strength of the external magnetic field due to thermal fluctuation. However, by increasing either the concentration of the NP dispersion or the strength of the applied magnetic field, these chains became longer, thicker and straighter. The reason could be that a high concentration led to a high fraction of short dipolar chains, and their interaction resulted in longer and thicker chains under applied magnetic field. On the other hand, when the magnetic field increased, the induced moments of the magnetic nanoparticles became larger, which dominated over the thermal fluctuation. Thus, the formed short chains connected to each other and grew in length. Thicker chains were also observed through chain-chain interaction. Furthermore, the induced moments of the NPs tended to direct into one direction with increased magnetic field, thus the chains were straighter. In comparison between the assembly on substrates, at water-air interface and at ethylene glycol-air interface, the assembly of Co NPs in hexane dispersion at ethylene glycol-air interface showed the most regular and homogeneous chain structures due to the better spreading of the dispersion on ethylene glycol subphase than on water subphase and substrates. The magnetic-field-induced assembly of superparamagnetic nanoparticles could provide a powerful approach for applications in data storage and electronic devices.
Chapter 5 presented the synthesis of superparamagnetic and ferromagnetic cobalt nanoparticles through a dual-stage thermolysis of cobalt carbonyl (Co2(CO)8) by using polystyrene as surfactant. The amine end-functionalized polystyrene surfactants with different molecular weight were prepared via atom transfer radical polymerization technique. The molecular weight determination of polystyrene was conducted by gel permeation chromatography (GPC) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry techniques. The results showed that, when the molecular weight distribution is low (Mw/Mn < 1.2), the measurement by GPC and MALDI-ToF MS provided nearly similar results. For example, the molecular weight of 10600 Da was obtained by MALDI-ToF MS, while GPC gave 10500 g/mol (Mw/Mn = 1.17). However, if the polymer is poly distributed, MALDI-ToF MS cannot provide an accurate value. This was exemplified for a polymer with a molecular weight of 3130 Da measured by MALDI-TOF MS, while GPC showed 2300 g/mol (Mw/Mn = 1.38). The size, size distribution and magnetic properties of the hybrid particles were different by changing either the molecular weight or concentration of the polymer surfactants. The analysis from TEM characterization showed that the size of cobalt nanoparticles stabilized with polystyrene of lower molecular weight (Mn = 2300 g/mol) varied from 12–22 nm, while the size with middle (Mn = 4500 g/mol) and higher molecular weight (Mn = 10500 g/mol) of polystyrene-coated cobalt nanoparticles showed little change. Magnetic measurements exhibited that the small cobalt particles (12 nm) were superparamagnetic, while larger particles (21 nm) were ferromagnetic and assembled into 1-D chains. The grafting density calculated from thermogravimetric analysis showed that a higher grafting density of polystyrene was obtained with lower molecular weight (Mn = 2300 g/mol) than those with higher molecular weight (Mn = 10500 g/mol). Due to the larger steric hindrance, polystyrene with higher molecular weight cannot form a dense shell on the surface of the nanoparticles, which resulted in a lower grafting density. Wide angle X-ray scattering measurements revealed the epsilon cobalt crystalline phases of both superparamagnetic Co NPs coated with polystyrene (Mn = 2300 g/mol) and ferromagnetic Co NPs coated with polystyrene (Mn = 10500 g/mol). Furthermore, a stability study showed that PS-Co NPs prepared with higher polymer concentration and polymer molecular weight exhibited a better stability.
Due to a challenging population growth and environmental changes, a need for new routes to provide required chemicals for human necessities arises. An effective solution discussed in this thesis is industrial heterogeneous catalysis. The development of an advanced industrial heterogeneous catalyst is investigated herein by considering porous carbon nano-material as supports and modifying their surface chemistry structure with heteroatoms. Such modifications showed a significant influence on the performance of the catalyst and provided a deeper insight regarding the interaction between the surface structure of the catalyst and the surrounding phase. This thesis contributes to the few present studies about heteroatoms effect on the catalyst performance and emphasizes on the importance of understanding surface structure functionalization in a catalyst in different phases (liquid and gaseous) and for different reactions (hydrogenolysis, oxidation, and hydrogenation/ polymerization). Herein, the heteroatoms utilized for the modifications are hydrogen (H), oxygen (O), and nitrogen (N). The heteroatoms effect on the metal particle size, on the polarity of the support/ the catalyst, on the catalytic performance (activity, selectivity, and stability), and on the interaction with the surrounding phase has been explored. First hierarchical porous carbon nanomaterials functionalized with heteroatoms (N) is synthesized and applied as supports for nickel nanoparticles for hydrogenolysis process of kraft lignin in liquid phase. This reaction has been performed in batch and flow reactors for three different catalysts, two of comparable hierarchical porosity, yet one is modified with N and the other is not, and a third is a prepared catalyst from a commercial carbon support. The reaction production and analyses show that the catalysts with hierarchical porosity perform catalytically much better than in presence of a commercial carbon support with lower surface area. Moreover, the modification with N-heteroatoms enhanced the catalytic performance because the heteroatom modified porous carbon material with nickel nanoparticles catalyst (Ni-NDC) performed highest among the other catalysts. In the flow reactor, Ni-NDC selectively degraded the ether bonds (β-O-4) in kraft lignin with an activity of 2.2 x10^-4 mg lignin mg Ni-1 s-1 for 50 h at 350°C and 3.5 mL min-1 flow, providing ~99 % conversion to shorter chained chemicals (mainly guaiacol derivatives). Then, the functionalization of carbon surface was further studied in selective oxidation of glucose to gluconic acid using < 1 wt. % of gold (Au) deposited on the previously-mentioned synthesized carbon (C) supports with different functionalities (Au-CGlucose, Au-CGlucose-H, Au-CGlucose-O, Au-CGlucoseamine). Except for Au-CGlucose-O, the other catalysts achieved full glucose conversion within 40-120 min and 100% selectivity towards gluconic acid with a maximum activity of 1.5 molGlucose molAu-1 s-1 in an aqueous phase at 45 °C and pH 9. Each heteroatom influenced the polarity of the carbon differently, affecting by that the deposition of Au on the support and thus the activity of the catalyst and its selectivity. The heteroatom effect was further investigated in a gas phase. The Fischer-Tropsch reaction was applied to convert synthetic gas (CO and H2) to short olefins and paraffins using surface-functionalized carbon nanotubes (CNTs) with heteroatoms as supports for ion (Fe) deposition in presence and absence of promoters (Na and S). The results showed the promoted Fe-CNT doped with nitrogen catalyst to be stable up to 180 h and selective to the formation of olefins (~ 47 %) and paraffins (~6 %) with a conversion of CO ~ 92 % at a maximum activity of 94 *10^-5 mol CO g Fe-1 s-1. The more information given regarding this topic can open wide range of applications not only in catalysis, but in other approaches as well. In conclusion, incorporation of heteroatoms can be the next approach for an advanced industrial heterogeneous catalyst, but also for other applications (e.g. electrocatalysis, gas adsorption, or supercapacitors).
Reversible-deactivation radical polymerization (RDRP) is without any doubt one of the most prevalent and powerful strategies for polymer synthesis, by which well-defined living polymers with targeted molecular weight (MW), low molar dispersity (Ɖ) and diverse morphologies can be prepared in a controlled fashion. Atom transfer radical polymerization (ATRP) as one of the most extensive studied types of RDRP has been particularly emphasized due to the high accessibility to hybrid materials, multifunctional copolymers and diverse end group functionalities via commercially available precursors. However, due to catalyst-induced side reactions and chain-chain coupling termination in bulk environment, synthesis of high MW polymers with uniform chain length (low Ɖ) and highly-preserved chain-end fidelity is usually challenging. Besides, owing to the inherited radical nature, the control of microstructure, namely tacticity control, is another laborious task. Considering the applied catalysts, the utilization of large amounts of non-reusable transition metal ions which lead to cumbersome purification process, product contamination and complicated reaction procedures all delimit the scope ATRP techniques.
Metal-organic frameworks (MOFs) are an emerging type of porous materials combing the properties of both organic polymers and inorganic crystals, characterized with well-defined crystalline framework, high specific surface area, tunable porous structure and versatile nanochannel functionalities. These promising properties of MOFs have thoroughly revolutionized academic research and applications in tremendous aspects, including gas processing, sensing, photoluminescence, catalysis and compartmentalized polymerization. Through functionalization, the microenvironment of MOF nanochannel can be precisely devised and tailored with specified functional groups for individual host-guest interactions. Furthermore, properties of high transition metal density, accessible catalytic sites and crystalline particles all indicate MOFs as prominent heterogeneous catalysts which open a new avenue towards unprecedented catalytic performance. Although beneficial properties in catalysis, high agglomeration and poor dispersibility restrain the potential catalytic capacity to certain degree.
Due to thriving development of MOF sciences, fundamental polymer science is undergoing a significant transformation, and the advanced polymerization strategy can eventually refine the intrinsic drawbacks of MOF solids reversely. Therefore, in the present thesis, a combination of low-dimensional polymers with crystalline MOFs is demonstrated as a robust and comprehensive approach to gain the bilateral advantages from polymers (flexibility, dispersibility) and MOFs (stability, crystallinity). The utilization of MOFs for in-situ polymerizations and catalytic purposes can be realized to synthesize intriguing polymers in a facile and universal process to expand the applicability of conventional ATRP methodology. On the other hand, through the formation of MOF/polymer composites by surface functionalization, the MOF particles with environment-adjustable dispersibility and high catalytic property can be as-prepared.
In the present thesis, an approach via combination of confined porous textures from MOFs and controlled radical polymerization is proposed to advance synthetic polymer chemistry. Zn2(bdc)2(dabco) (Znbdc) and the initiator-functionalized Zn MOFs, ZnBrbdc, are utilized as a reaction environment for in-situ polymerization of various size-dependent methacrylate monomers (i.e. methyl, ethyl, benzyl and isobornyl methacrylate) through (surface-initiated) activators regenerated by electron transfer (ARGET/SI-ARGET) ATRP, resulting in polymers with control over dispersity, end functionalities and tacticity with respect to distinct molecular size. While the functionalized MOFs are applied, due to the strengthened compartmentalization effect, the accommodated polymers with molecular weight up to 392,000 can be achieved. Moreover, a significant improvement in end-group fidelity and stereocontrol can be observed. The results highlight a combination of MOFs and ATRP is a promising and universal methodology to synthesize versatile well-defined polymers with high molecular weight, increment in isotactic trial and the preserved chain-end functionality.
More than being a host only, MOFs can act as heterogeneous catalysts for metal-catalyzed polymerizations. A Cu(II)-based MOF, Cu2(bdc)2(dabco), is demonstrated as a heterogeneous, universal catalyst for both thermal or visible light-triggered ARGET ATRP with expanded monomer range. The accessible catalytic metal sites enable the Cu(II) MOF to polymerize various monomers, including benzyl methacrylate (BzMA), styrene, methyl methacrylate (MMA), 2-(dimethylamino)ethyl methacrylate (DMAEMA) in the fashion of ARGET ATRP. Furthermore, due to the robust frameworks, surpassing the conventional homogeneous catalyst, the Cu(II) MOF can tolerate strongly coordinating monomers and polymerize challenging monomers (i.e. 4-vinyl pyridine, 2-vinyl pyridine and isoprene), in a well-controlled fashion. Therefore, a synthetic procedure can be significantly simplified, and catalyst-resulted chelation can be avoided as well. Like other heterogeneous catalysts, the Cu(II) MOF catalytic complexes can be easily collected by centrifugation and recycled for an arbitrary amount of times.
The Cu(II) MOF, composed of photostimulable metal sites, is further used to catalyze controlled photopolymerization under visible light and requires no external photoinitiator, dye sensitizer or ligand. A simple light trigger allows the photoreduction of Cu(II) to the active Cu(I) state, enabling controlled polymerization in the form of ARGET ATRP. More than polymerization application, the synergic effect between MOF frameworks and incorporated nucleophilic monomers/molecules is also observed, where the formation of associating complexes is able to adjust the photochemical and electrochemical properties of the Cu(II) MOF, altering the band gap and light harvesting behavior. Owing to the tunable photoabsorption property resulting from the coordinating guests, photoinduced Reversible-deactivation radical polymerization (PRDRP) can be achieved to further simplify and fasten the polymerization.
More than the adjustable photoabsorption ability, the synergistic strategy via a combination of controlled/living polymerization technique and crystalline MOFs can be again evidenced as demonstrated in the MOF-based heterogeneous catalysts with enhanced dispersibility in solution. Through introducing hollow pollen pivots with surface immobilized environment-responsive polymer, PDMAEMA, highly dispersed MOF nanocrystals can be prepared after associating on polymer brushes via the intrinsic amine functionality in each DMAEMA monomer. Intriguingly, the pollen-PDMAEMA composite can serve as a “smart” anchor to trap nanoMOF particles with improved dispersibility, and thus to significantly enhance liquid-phase photocatalytic performance. Furthermore, the catalytic activity can be switched on and off via stimulable coil-to-globule transition of the PDMAEMA chains exposing or burying MOF catalytic sites, respectively.
Mit einer ausgewogenen Stoffauswahl aus den Teilgebieten Chemische Thermodynamik, Reaktionskinetik und Elektrochemie wird der Leser an das Studium der Physikalischen Chemie herangeführt. Das Verständnis der Theorie wird durch zahlreiche Aufgabenstellungen und die Angabe ihrer Lösungswege erleichtert. Das Buch gibt dem Studenten darüber hinaus Anregungen für ausgewählte Experimente zu den behandelten Teilgebieten, mit denen sich ein Grundverständnis physikalisch-chemischer Zusammenhänge entwickeln lässt.
Thermoresponsive block copolymers of presumably highly biocompatible character exhibiting upper critical solution temperature (UCST) type phase behavior were developed. In particular, these polymers were designed to exhibit UCST-type cloud points (Tcp) in physiological saline solution (9 g/L) within the physiologically interesting window of 30-50°C. Further, their use as carrier for controlled release purposes was explored. Polyzwitterion-based block copolymers were synthesized by atom transfer radical polymerization (ATRP) via a macroinitiator approach with varied molar masses and co-monomer contents. These block copolymers can self-assemble in the amphiphilic state to form micelles, when the thermoresponsive block experiences a coil-to-globule transition upon cooling. Poly(ethylene glycol) methyl ether (mPEG) was used as the permanently hydrophilic block to stabilize the colloids formed, and polyzwitterions as the thermoresponsive block to promote the temperature-triggered assembly-disassembly of the micellear aggregates at low temperature.
Three zwitterionic monomers were used for this studies, namely 3-((2-(methacryloyloxy)ethyl)dimethylammonio)propane-1-sulfonate (SPE), 4-((2-(methacryloyl- oxy)ethyl)dimethylammonio)butane-1-sulfonate (SBE), and 3-((2-(methacryloyloxy)ethyl)- dimethylammonio)propane-1-sulfate) (ZPE). Their (co)polymers were characterized with respect to their molecular structure by proton nuclear magnetic resonance (1H-NMR) and gel permeation chromatography (GPC). Their phase behaviors in pure water as well as in physiological saline were studied by turbidimetry and dynamic light scattering (DLS). These (co)polymers are thermoresponsive with UCST-type phase behavior in aqueous solution. Their phase transition temperatures depend strongly on the molar masses and the incorporation of co-monomers: phase transition temperatures increased with increasing molar masses and content of poorly water-soluble co-monomer. In addition, the presence of salt influenced the phase transition dramatically. The phase transition temperature decreased with increasing salt content in the solution. While the PSPE homopolymers show a phase transition only in pure water, the PZPE homopolymers are able to exhibit a phase transition only in high salinity, as in physiological saline. Although both polyzwitterions have similar chemical structures that differ only in the anionic group (sulfonate group in SPE and sulfate group in ZPE), the water solubility is very different. Therefore, the phase transition temperatures of targeted block copolymers were modulated by using statistical copolymer of SPE and ZPE as thermoresponsive block, and varying the ratio of SPE to ZPE. Indeed, the statistical copolymers of P(SPE-co-ZPE) show phase transitions both in pure water as well as in physiological saline. Surprisingly, it was found that mPEG-b-PSBE block copolymer can display “schizophrenic” behavior in pure water, with the UCST-type cloud point occurring at lower temperature than the LCST-type one.
The block copolymer, which satisfied best the boundary conditions, is block copolymer mPEG114-b-P(SPE43-co-ZPE39) with a cloud point of 45°C in physiological saline. Therefore, it was chosen for solubilization studies of several solvatochromic dyes as models of active agents, using the thermoresponsive block copolymer as “smart” carrier. The uptake and release of the dyes were explored by UV-Vis and fluorescence spectroscopy, following the shift of the wavelength of the absorbance or emission maxima at low and high temperature. These are representative for the loaded and released state, respectively. However, no UCST-transition triggered uptake and release of these dyes could be observed. Possibly, the poor affinity of the polybetaines to the dyes in aqueous environtments may be related to the widely reported antifouling properties of zwitterionic polymers.
The utilization of lignin as renewable electrode material for electrochemical energy storage is a sustainable approach for future batteries and supercapacitors. The composite electrode was fabricated from Kraft lignin and conductive carbon and the charge storage contribution was determined in terms of electrical double layer (EDL) and redox reactions. The important factors at play for achieving high faradaic charge storage capacity contribute to high surface area, accessibility of redox sites in lignin and their interaction with conductive additives. A thinner layer of lignin covering the high surface area of carbon facilitates the electron transfer process with a shorter pathway from the active sites of nonconductive lignin to the current collector leading to the improvement of faradaic charge storage capacity.
Composite electrodes from lignin and carbon would be even more sustainable if the fluorinated binder can be omitted. A new route to fabricate a binder-free composite electrode from Kraft lignin and high surface area carbon has been proposed by crosslinking lignin with glyoxal. A high molecular weight of lignin is obtained to enhance both electroactivity and binder capability in composite electrodes. The order of the processing step of crosslinking lignin on the composite electrode plays a crucial role in achieving a stable electrode and high charge storage capacity. The crosslinked lignin based electrodes are promising since they allow for more stable, sustainable, halogen-free and environmentally benign devices for energy storage applications. Furthermore, improvement of the amount of redox active groups (quinone groups) in lignin is useful to enhance the capacity in lithium battery applications. Direct oxidative demethylation by cerium ammonium nitrate has been carried out under mild conditions. This proves that an increase of quinone groups is able to enhance the performance of lithium battery. Thus, lignin is a promising material and could be a good candidate for application in sustainable energy storage devices.
Nanoparticles (NPs) are particles between 1 and 100 nanometers in size. They have attracted enormous research interests owing to their remarkable physicochemical properties and potential applications in the optics, catalysis, sensing, electronics, or optical devices. The thesis investigates systems of NPs attached to planar substrates.
In the first part of the results section of the thesis a new method is presented to immobilize NPs. In many NP applications a strong, persistent adhesion to substrates is a key requirement. Up to now this has been achieved with various methods, which are not always the optimum regarding adhesion strength or applicability. We propose a new method which uses capillarity to enhance the binding agents in the contact area between NP and substrate. The adhesion strength resulting from the new approach is investigated in detail and it is shown that the new approach is superior to older methods in several ways.
The following section presents the optical visualization of nano-sized objects through a combination of thin film surface distortion and interference enhanced optical reflection microscopy. It is a new, fast and non-destructive technique. It not only reveals the location of NPs as small as 20nm attached to planar surfaces and embedded in a molecularly thin liquid film. It also allows the measurement of the geometry of the surface distortion of the liquid film. Even for small NPs the meniscus reaches out for micrometers, which is the reason why the NPs produce such a pronounced optical footprint.
The nucleation and growth of individual bubbles is presented in chapter 5. Nucleation is a ubiquitous natural phenomenon and of great importance in numerous industrial processes. Typically it occurs on very small scales (nanometers) and it is of a random nature (thermodynamics of small systems). Up to now most experimental nucleation studies deal with a large number of individual nucleation processes to cope with its inherently statistical, spatio-temporal character. In contrast, in this thesis the individual O2-bubble formation from single localized platinum NP active site is studied experimentally. The bubble formation is initiated by the catalytic reaction of H2O2 on the Pt surface. It is studied how the bubble nucleation and growth depends on the NP size, the H2O2 concentration and the substrate surface properties. It is observed that in some cases the bubbles move laterally over the substrate surface, driven by the O2-production and the film ablation.
Photocatalysis is considered significant in this new energy era, because the inexhaustibly abundant, clean, and safe energy of the sun can be harnessed for sustainable, nonhazardous, and economically development of our society. In the research of photocatalysis, the current focus was held by the design and modification of photocatalyst.
As one of the most promising photocatalysts, g-C3N4 has gained considerable attention for its eye-catching properties. It has been extensively explored in photocatalysis applications, such as water splitting, organic pollutant degradation, and CO2 reduction. Even so, it also has its own drawbacks which inhibit its further application. Inspired by that, this thesis will mainly present and discuss the process and achievement on the preparation of some novel photocatalysts and their photocatalysis performance. These materials were all synthesized via the alteration of classic g-C3N4 preparation method, like using different pre-compositions for initial supramolecular complex and functional group post-modification. By taking place of cyanuric acid, 2,5-Dihydroxy-1,4-benzoquinone and chloranilic acid can form completely new supramolecular complex with melamine. After heating, the resulting products of the two complex shown 2D sheet-like and 1D fiber-like morphologies, respectively, which maintain at even up to high temperature of 800 °C. These materials cover crystals, polymers and N-doped carbons with the increase of synthesis temperature. Based on their different pre-compositions, they show different dye degradation performances. For CLA-M-250, it shows the highest photocatalytic activity and strong oxidation capacity. It shows not only great photo-performance in RhB degradation, but also oxygen production in water splitting. In the post-modification method, a novel photocatalysis solution was proposed to modify carbon nitride scaffold with cyano group, whose content can be well controlled by the input of sodium thiocyanate. The cyanation modification leads to narrowed band gap as well as improved photo-induced charges separation. Cyano group grafted carbon nitride thus shows dramatically enhanced performance in the photocatalytic coupling reaction between styrene and sodium benzenesulfinate under green light irradiation, which is in stark contrast with the inactivity of pristine g-C3N4.
Magnetische Eisenoxidnanopartikel werden bereits seit geraumer Zeit erfolgreich als MRT-Kontrastmittel in der klinischen Bildgebung eingesetzt. Durch Optimierung der magnetischen Eigenschaften der Nanopartikel kann die Aussagekraft von MR-Aufnahmen verbessert und somit der diagnostische Wert einer MR-Anwendung weiter erhöht werden. Neben der Verbesserung bestehender Verfahren wird die bildgebende Diagnostik ebenso durch die Entwicklung neuer Verfahren, wie dem Magnetic Particle Imaging, vorangetrieben. Da hierbei das Messsignal von den magnetischen Nanopartikeln selbst erzeugt wird, birgt das MPI einen enormen Vorteil hinsichtlich der Sensitivität bei gleichzeitig hoher zeitlicher und räumlicher Auflösung. Da es aktuell jedoch keinen kommerziell vertriebenen in vivo-tauglichen MPI-Tracer gibt, besteht ein dringender Bedarf an geeigneten innovativen Tracermaterialien. Daraus resultierte die Motivation dieser Arbeit biokompatible und superparamagnetische Eisenoxidnanopartikel für den Einsatz als in vivo-Diagnostikum insbesondere im Magnetic Particle Imaging zu entwickeln. Auch wenn der Fokus auf der Tracerentwicklung für das MPI lag, wurde ebenso die MR-Performance bewertet, da geeignete Partikel somit alternativ oder zusätzlich als MR-Kontrastmittel mit verbesserten Kontrasteigenschaften eingesetzt werden könnten.
Die Synthese der Eisenoxidnanopartikel erfolgte über die partielle Oxidation von gefälltem Eisen(II)-hydroxid und Green Rust sowie eine diffusionskontrollierte Kopräzipitation in einem Hydrogel.
Mit der partiellen Oxidation von Eisen(II)-hydroxid und Green Rust konnten erfolgreich biokompatible und über lange Zeit stabile Eisenoxidnanopartikel synthetisiert werden. Zudem wurden geeignete Methoden zur Formulierung und Sterilisierung etabliert, wodurch zahlreiche Voraussetzungen für eine Anwendung als in vivo-Diagnostikum geschaffen wurden. Weiterhin ist auf Grundlage der MPS-Performance eine hervorragende Eignung dieser Partikel als MPI-Tracer zu erwarten, wodurch die Weiterentwicklung der MPI-Technologie maßgeblich vorangetrieben werden könnte. Die Bestimmung der NMR-Relaxivitäten sowie ein initialer in vivo-Versuch zeigten zudem das große Potential der formulierten Nanopartikelsuspensionen als MRT-Kontrastmittel. Die Modifizierung der Partikeloberfläche ermöglicht ferner die Herstellung zielgerichteter Nanopartikel sowie die Markierung von Zellen, wodurch das mögliche Anwendungsspektrum maßgeblich erweitert wurde.
Im zweiten Teil wurden Partikel durch eine diffusionskontrollierte Kopräzipitation im Hydrogel, wobei es sich um eine bioinspirierte Modifikation der klassischen Kopräzipitation handelt, synthetisiert, wodurch Partikel mit einer durchschnittlichen Kristallitgröße von 24 nm generiert werden konnten. Die Bestimmung der MPS- und MR-Performance elektrostatisch stabilisierter Partikel ergab vielversprechende Resultate. In Vorbereitung auf die Entwicklung eines in vivo-Diagnostikums wurden die Partikel anschließend erfolgreich sterisch stabilisiert, wodurch der kolloidale Zustand in MilliQ-Wasser über lange Zeit aufrechterhalten werden konnte. Durch Zentrifugation konnten die Partikel zudem erfolgreich in verschiedene Größenfraktionen aufgetrennt werden. Dies ermöglichte die Bestimmung der idealen Aggregatgröße dieses Partikelsystems in Bezug auf die MPS-Performance.
In der vorliegenden Arbeit werden Wege zur Gewinnung verschiedener phenolischer Substanzen wie Lignin, Diarylheptanoide und 4-(3-Oxobutyl)phenol (Himbeerketon) aus dem Stamm der Hängebirke (Betula pendula) aufgezeigt. Durch Methacrylierung des 4-(3-Oxobutyl)phenols wurde ein Monomer erzeugt, welches mittels freier radikalischer Masse- und Lösungspolymerisation, sowie enzymatischer Polymerisation polymerisiert werden kann.
Eine erste Isolierung von Bestandteilen wurde durch Extraktion von Innenholz bzw. Rinde mit Methanol erzielt. Die in Methanol unlöslichen Bestandteile des Innenholzes und der Rinde wurden anschließend mit ausgewählten ionischen Flüssigkeiten extrahiert. Es wurde ein Verfahren zum selektiven Trennen der mit diesen ionischen Flüssigkeiten extrahierten Bestandteile in Cellulose, Hemicellulose, Lignin und mit Ethylacetat extrahierbare Bestandteile entwickelt. Hierdurch war es möglich, sowohl die verwendeten ionischen Flüssigkeiten als auch das Innenholz und die Rinde hinsichtlich ihres Extraktionsverhaltens miteinander zu vergleichen.
Ferner wurden verschiedene Strategien aufgezeigt, um insgesamt drei Spezies an Diarylheptanoiden aus dem methanolischen Extrakt der Rinde zu isolieren. Eines der gefundenen Diarylheptanoide (5 Hydroxy-1,7-bis(4-hydroxyphenyl)-3-heptanon) wurde via Retroaldolreaktion in 4 (3 Oxobutyl)phenol (Himbeerketon) und 3 (4 Hydroxyphenyl)propanal gespalten.
Es wurde die Verwendung des 4-(3-Oxobutyl)phenol als Monomerbestandteil untersucht. Hierfür wurde 4-(3-Oxobutyl)phenylmethacrylat synthetisiert und Wege zur Reinigung mittels Säulenchromatographie und Umkristallisation aufgezeigt. Anschließend wurde Poly(4-(3-oxobutyl)phenylmethacrylat) (PObMA) und Polybenzylmethacrylats (PBzMA) aus Massen- und Lösungspolymerisation hergestellt. Die Ausbeuten an PObpMA im Vergleich zum PBzMA liegen bei gleichen Reaktionsbedingungen auf gleichem Niveau. Im Kontrast hierzu ist der Polymerisationsgrad aus freier radikalischer Polymerisation in Masse des PObpMA im Vergleich zum PBzMA um den Faktor 3,7 größer. Die Glasübergangstemperaturen des PObpMA liegen bei gleichen Reaktionsbedingungen sowohl bei freier radikalischer Polymerisation in Masse, als auch bei Lösungspolymerisation über denen des PBzMA. Darüber hinaus wurde die Polymerisation von 4-(3-Oxobutyl)phenylmethacrylat und Benzylmethacrylat mit einem Initiatorsystem bestehend aus Meerrettichperoxidase, Acetylaceton und Wasserstoffperoxid bei Raumtemperatur beschrieben. Die mit enzymatischem Initiatorsystem erzeugten Produkte zeigten starke Übereinstimmung mit Produkten aus Lösungspolymerisationen, welche mit Azobis(isobutyronitril) initiiert wurden.
The ongoing trend of miniaturizing multifunctional devices, especially for minimally-invasive medical or sensor applications demands new strategies for designing the required functional polymeric micro-components or micro-devices. Here, polymers, which are capable of active movement, when an external stimulus is applied (e.g. shape-memory polymers), are intensively discussed as promising material candidates for realization of multifunctional micro-components. In this context further research activities are needed to gain a better knowledge about the underlying working principles for functionalization of polymeric micro-scale objects with a shape-memory effect. First reports about electrospun solid microfiber scaffolds, demonstrated a much more pronounced shape-memory effect than their bulk counterparts, indicating the high potential of electrospun micro-objects.
Based on these initial findings this thesis was aimed at exploring whether the alteration of the geometry of micro-scale electrospun polymeric objects can serve as suitable parameter to tailor their shape-memory properties. The central hypothesis was that different geometries should result in different degrees of macromolecular chain orientation in the polymeric micro-scale objects, which will influence their mechanical properties as well as thermally-induced shape-memory function. As electrospun micro-scale objects, microfiber scaffolds composed of hollow microfibers with different wall thickness and electrosprayed microparticles as well as their magneto-sensitive nanocomposites all prepared from the same polymer exhibiting pronounced bulk shape-memory properties were investigated. For this work a thermoplastic multiblock copolymer, named PDC, with excellent bulk shape-memory properties, associated with crystallizable oligo(ε-caprolactone) (OCL) switching domains, was chosen for the preparation of electrospun micro-scale objects, while crystallizable oligo(p-dioxanone) (OPDO) segments serve as hard domains in PDC.
In the first part of the thesis microfiber scaffolds with different microfiber geometries (solid or hollow with different wall thickness) were discussed. Hollow microfiber based PDC scaffolds were prepared by coaxial electrospinning from a 1, 1, 1, 3, 3, 3 hexafluoro-2-propanol (HFP) solution with a polymer concentration of 13% w·v-1. Here as a first step core-shell fiber scaffolds consisting of microfibers with a PDC shell and sacrificial poly(ethylene glycol) (PEG) core are generated. The hollow PDC microfibers were achieved after dissolving the PEG core with water. The utilization of a fixed electrospinning setup and the same polymer concentration of the PDC spinning solution could ensure the fabrication of microfibers with almost identical outer diameters of 1.4 ± 0.3 µm as determined by scanning electron microscopy (SEM). Different hollow microfiber wall thicknesses of 0.5 ± 0.2 and 0.3 ± 0.2 µm (analyzed by SEM) have been realized by variation of the mass flow rate, while solid microfibers were obtained by coaxial electrospinning without supplying any core solution. Differential scanning calorimetry experiments and tensile tests at ambient temperature revealed an increase in degree of OCL crystallinity form χc,OCL = 34 ± 1% to 43 ± 1% and a decrease in elongation of break from 800 ± 40% to 200 ± 50% associated with an increase in Young´s modulus and failture stress for PDC hollow microfiber scaffolds when compared with soild fibers. The observed effects were enhanced with decreasing wall thickness of the single hollow fibers. The shape-memory properties of the electrospun PDC scaffolds were quantified by cyclic, thermomechanical tensile tests. Here, scaffolds comprising hollow microfibers exhibited lower shape fixity ratios around Rf = 82 ± 1% and higher shape recovery ratios of Rr = 67 ± 1% associated to more pronounced relaxation at constant strain during the first test cycle and a lower switching temperature of Tsw = 33 ± 1 °C than the fibrous meshes consisting of solid microfibers. These findings strongly support the central hypothesis that different fiber geometries (solid or hollow with different wall thickness) in electrospun scaffolds result in different degrees of macromolecular chain orientation in the polymeric micro-scale objects, which can be applied as design parameter for tailoring their mechanical and shape-memory properties.
The second part of the thesis deals with electrosprayed particulate PDC micro-scale objects. Almost spherical PDC microparticles with diameters of 3.9 ± 0.9 μm (as determined by SEM) were achieved by electrospraying of HFP solution with a polymer concentration of 2% w·v-1. In contrast, smaller particles with sizes of 400 ± 100 nm or 1.2 ± 0.3 μm were obtained for the magneto-sensitive composite PDC microparticles containing 23 ± 0.5 wt% superparamagnetic magnetite nanoparticles (mNPs). All prepared PDC microparticles exhibited a similar overall crystallinity like the PDC bulk material as analyzed by DSC. AFM nanoindentation results revealed no influence of the nanofiller incorporation on the local mechanical properties represented by the reduced modulus determined for pure PDC microparticles and magneto-sensitive composite PDC microparticles with similar diameters around 1.3 µm. It was found that the reduced modulus of the nanocomposite microparticles increased substantially with decreasing particles size from 2.4 ± 0.9 GPa (1.2 µm) to 11.9 ± 3.1 GPa (0.4 µm), which can be related to a higher orientation of the macromolecules at the surface of smaller sized microparticles. The magneto-sensitivity of such nanocomposite microparticles could be demonstrated in two aspects. One was by attracting/collecting the composite micro-objects with an external permanent magnet. The other one was by a inductive heating to 44 ± 1 °C, which is well above the melting transition of the OCL switching domains, when compacted to a 10 x 10 mm2 film with a thickness of 10 µm and exposed to an alternating magnet field with an magnetic field strength of 30 kA·m-1. Both functions are of great relevance for designing next generation drug delivery systems combining targeting and on demand release.
By a compression approach shape-memory functionalization of individual microparticles could be realized. Here different programming pressures and compression temperatures were applied. The shape-recovery capability of the programmed PDC microparticles was quantified by online and off-line heating experiments analyzed via microscopy measurement. The obtained shape-memory properties were found to be strongly depending on the applied programming pressure and temperature. The best shape-memory performance with a high shape recovery rate of about Rr = 80±1% was obtained when a low pressure of 0.2 MPa was applied at 55 °C. Finally, it was demonstrated that PDC microparticles can be utilized as micro building parts for preparation of a macroscopic film with temporary stability by compression of a densely packed array of PDC microparticles at 60 °C followed by subsequent cooling to ambient temperature. This film disintegrates into individual microparticles upon heating to 60 °C. Based on this technology the design of stable macroscopic release systems can be envisioned, which can be easily fixed at the site of treatment (i.e. by suturing) and disintegrate on demand to microparticles facilitating the drug release.
In summary, the results of this thesis could confirm the central hypothesis that the variation of the geometry of polymeric micro-objects is a suitable parameter to adjust their shape-memory performance by changing the degree of macromolecular chain orientation in the specimens or by enabling new functions like on demand disintegration. These fundamental findings might be relevant for designing novel miniaturized multifunctional polymer-based devices.
Polymeric materials, which can perform reversible shape changes after programming, in response to a thermal or electrical stimulation, can serve as (soft) actuating components in devices like artificial muscles, photonics, robotics or sensors. Such polymeric actuators can be realized with hydrogels, liquid crystalline elastomers, electro-active polymers or shape-memory polymers by controlling with stumuli such as heat, light, electrostatic or magnetic field. If the application conditions do not allow the direct heating or electric stimulation of these smart devices, noncontact triggering will be required. Remotely controlled actuation have been reported for liquid crystalline elastomer composites or shape-memory polymer network composites, when a persistent external stress is applied during inductive heating in an alternating magnetic field. However such composites cannot meet the demands of applications requiring remotely controlled free-standing motions of the actuating components.
The current thesis investigates, whether a reprogrammable remotely controlled soft actuator can be realized by magneto-sensitive multiphase shape-memory copolymer network composites containing magnetite nanoparticles as magneto-sensitive multivalent netpoints. A central hypothesis was that a magnetically controlled two-way (reversible bidirectional) shape-memory effect in such nanocomposites can be achieved without application of external stress (freestanding), when the required orientation of the crystallizable actuation domains (ADs) can be ensured by an internal skeleton like structure formed by a second crystallizable phase determing the samples´s geometry, while magneto-sensitive iron oxide nanoparticles covalently integrated in the ADs allow remote temperature control. The polymer matrix of these composites should exhibit a phase-segregated morphology mainly composed of cyrstallizable ADs, whereby a second set of higher melting crystallites can take a skeleton like, geometry determining function (geometry determining domains, GDs) after programming of the composite and in this way the orientation of the ADs is established and maintained during actuation. The working principle for the reversible bidirectional movements in the multiphase shape-memory polymer network composite is related to a melting-induced contraction (MIC) during inductive heating and the crystallization induced elongation (CIE) of the oriented ADs during cooling. Finally, the amount of multivalent magnetosensitive netpoints in such a material should be as low as possible to ensure an adequate overall elasticity of the nanocomposite and at the same time a complete melting of both ADs and GDs via inductive heating, which is mandatory for enabling reprogrammability.
At first, surface decorated iron oxide nanoparticles were synthesized and investigated. The coprecipitation method was applied to synthesize magnetic nanoparticles (mNPs) based on magnetite with size of 12±3 nm and in a next step a ring-opening polymerization (ROP) was utilized for covalent surface modification of such mNPs with oligo(ϵ-caprolactone) (OCL) or oligo(ω-pentadecalactone) (OPDL) via the “grafting from” approach. A successful coating of mNPs with OCL and OPDL was confirmed by differential scanning calorimetry (DSC) experiments showing melting peaks at 52±1 °C for mNP-OCL and 89±1 °C for mNP-OPDL. It was further explored whether two-layered surface decorated mNPs, can be prepared via a second surface-initiated ROP of mNP-OCL or mNP-OPDL with ω-pentadecalactone or ϵ-caprolactone. The observation of two distinct melting transitions in DSC experiments as well as the increase in molecular weight of the detached coatings determined by GPC and 1H-NMR indicated a successful synthesis of the twolayered nanoparticles mNP-OCL-OPDL and mNP-OPDL-OCL. In contrast TEM micrographs revealed a reduction of the thickness of the polymeric coating on the nanoparticles after the second ROP, indicating that the applied synthesis and purification required further optimization.
For evaluating the impact of the dispersion of mNPs within a polymer matrix on the resulting inductive heating capability of composites, plain mNPs as well as OCL coated magnetite nanoparticles (mNP-OCLs) were physically incorporated into crosslinked poly(ε-caprolactone) (PCL) networks. Inductive heating experiments were performed with both networks cPCL/mNP and cPCL/mNP-OCL in an alternating magnetic field (AMF) with a magnetic field strength of H = 30 kA·m-1. Here a bulk temperature of Tbulk = 74±2 °C was achieved for cPCL/mNP-OCL, which was almost 20 °C higher than the melting transition of the PCL-based polymer matrix. In contrast, the composite with plain mNPs could only reach a Tbulk of 48±2 °C, which is not sufficient for a complete melting of all PCL crystallites as required for actuation.
The inductive heating capability of a multiphase copolymer nanocomposite network (designed as soft actuators) containing surface decorated mNPs as covalent netpoints was investigated. Such composite was synthesized from star-shaped OCL and OPDL precursors, as well as mNP-OCLs via reaction with HDI. The weight ratio of OPDL and OCL in the starting reaction mixture was 15/85 (wt%/wt%) and the amount of iron oxide in the nanocomposite was 4 wt%. DSC experiments revealed two well separated melting and crystallization peaks confirming the required phase-segregated morphology in the nanocomposite NC-mNP-OCL. TEM images could illustrate a phase-segregated morphology of the polymer matrix on the microlevel with droplet shaped regions attributed to the OPDL domains dispersed in an OCL matrix. The TEM images could further demonstrate that the nanoparticulate netpoints in NC-mNP-OCL were almost homogeneously dispersed within the OCL domains. The tests of the inductive heating capability of the nanocomposites at a magnetic field strength of Hhigh = 11.2 kA·m-1 revealed a achievable plateau surface temperature of Tsurf = 57±1 °C for NC-mNP-OCL recorded by an infrared video camera. An effective heat generation constant (̅P) can be derived from a multi-scale model for the heat generation, which is proportional to the rate of heat generation per unit volume of the sample. NC-mNP-OCL with homogeneously dispersed mNP-OCLs exhibited a ̅P value of 1.04±0.01 K·s- 1 at Hhigh, while at Hreset = 30.0 kA·m-1 a Tsurf of 88±1 °C (where all OPDL related crystallite are molten) and a ̅P value of 1.93±0.02 K·s-1 was obtained indicating a high magnetic heating capability of the composite.
The free-standing magnetically-controlled reversible shape-memory effect (mrSME) was explored with originally straight nanocomposite samples programmed by bending to an angle of 180°. By switching the magnetic field on and off the composite sample was allowed to repetitively heat to 60 °C and cool to the ambient temperature. A pronounced mrSME, characterized by changes in bending angle of Δϐrev = 20±3° could be obtained for a composite sample programmed by bending when a magnetic field strength of Hhigh = 11.2 kA·m-1 was applied in a multi-cyclic magnetic bending experiment with 600 heating-cooling cycles it could be shown that the actuation performance did not change with increasing number of test cycles, demonstrating the accuracy and reproducibility of this soft actuator. The degree of actuation as well as the kinetics of the shape changes during heating could be tuned by variation of the magnetic filed strength between Hlow and Hhigh or the magnetic field exposure time. When Hreset = 30.0 kA·m-1 was applied the programmed geometry was erased and the composite sample returned to it´s originally straight shape. The reprogrammability of the nanocomposite actuators was demonstrated by one and the same test specimen first exhibiting reversible angle changes when programmed by bending, secondly reprogrammed to a concertina, which expands upon inductive heating and contracts during cooling and finally reprogrammed to a clip like shape, which closes during cooling and opens when Hhigh was applied. In a next step the applicability of the presented remote controllable shape-memory polymer actuators was demonstrated by repetitive opening and closing of a multiring device prepared from NC-mNP-OCL, which repetitively opens and closes when a alternating magnetic field (Hhigh = 11.2 kA·m-1) was switched on and off.
For investigation of the micro- and nanostructural changes related to the actuation of the developed nanocomposite, AFM and WAXS experiments were conducted with programmed nanocomposite samples under cyclic heating and cooling between 25 °C and 60 °C. In AFM experiments the change in the distance (D) between representative droplet-like structures related to the OPDL geometry determining domains was used to calculate the reversible change in D. Here Drev = 3.5±1% was found for NC-mNP-OCL which was in good agreement with the results of the magneto-mechanical actuation experiments. Finally, the analysis of azimuthal (radial) WAXS scattering profiles could support the oriented crystallization of the OCL actuation domains at 25 °C.
In conclusion, the results of this work successfully demonstrated that shape-memory polymer nanocomposites, containing mNPs as magneto-sensitive multifunctional netpoints in a covalently crosslinked multiphase polymer matrix, exhibit magnetically (remotely) controlled actuations upon repetitive exposure to an alternating magnetic field. Furthermore, the (shape) memory of such a nanocomposite can be erased by exposing it to temperatures above the melting temperature of the geometry forming domains, which allows a reprogramming of the actuator. These findings would be relevant for designing novel reprogrammable remotely controllable soft polymeric actuators.
Direkte Arylierung
(2018)