http://hdl.handle.net/10900/42133 Sat, 22 Aug 2026 04:06:35 GMT 2026-08-22T04:06:35Z http://hdl.handle.net/10900/182418 Synthesis and Characterization of Molecular Sensors Responsive to Amino Acid Neurotransmitters for Magnetic Resonance Imaging Toljić, Đorđe This thesis explores the development of responsive MRI contrast agents (CA) targeting glutamate (Glu) and γ-aminobutyric acid (GABA), the major excitatory and inhibitory neurotransmitters (NT). Chapter 1 outlines key concepts in MRI physics and discusses both non-specific and stimulus-responsive MRI CA, including small-molecule and nanoscale dendrimer-based systems. It also covers fundamental aspects of chemical neurotransmission and highlights challenges in designing CA that respond to zwitterionic amino acid neurotransmitters. Additionally, it reviews current progress in MR-based imaging of Glu and GABA. The examined bismacrocyclic CA ligate amino acids (AA) via predominant Gd-Ocarboxylate interaction and multiple H-bonds between ammonium cation and the 18C6 moiety. Thus, the focus of Chapter 2 is on chemical modification of binding sites with emphasis on the 18-crown-6 (18C6) fragment to selectively improve the affinity towards zwitterionic NT. This approach implied the synthesis of a family of Gd-complexes with varying natures of both binding sites and revealing structure-reactivity relationships using various analytical techniques, including relaxometric profiling and DFT calculations. In Chapter 2 it is revealed that recognition of AA transmitters by cyclen-18C6 chemosensors is governed by the nature of the pending arm on the 18C6 motif. Based on these findings, Chapter 3 reports a novel design of a ditopic molecular host with improved affinity to Glu and GABA transmitters suitable for T2/T1w ratiometric MR imaging. The application of a bSSFP pulse sequence on phantoms demonstrated signal enhancement for cooperatively bound Glu and GABA over hydrogen carbonate, wherein an MR signal of hydrogen carbonate remained unchanged. After successful preparation of a synthetic host with higher selectivity for AA over hydrogen carbonate, the focus has turned on biocompatibility improvements. Thus, Chapter 4 describes a synthetic approach for the preparation of the first dendrimer-GBCA conjugate responsive to AA. The previously characterized small-molecule Gd-chelate in Chapter 2 was grafted onto the end amines of the G4 PAMAM dendrimer in order to assess the impact of conjugation on binding AA. To tune solubility at physiological pH, the conjugate was PEGylated with hydrophilic polyethylene glycol. Efficacy of the probe was evaluated in relaxometric titration with AA and competitive metabolites, and the potential for ratiometric T2/T1w imaging was estimated. Chapter 5 describes the development of a novel approach intended for the 13C chemical shift imaging (CSI) of Glu induced by the lanthanide chelate. To test the effects of lanthanide ions, a single bismacrocyclic chelator with the highest affinity to Glu and GABA (Chapter 2) was metalated with various Ln3+ ions, other than Gd3+, to produce a series of ditopic paramagnetic receptors. The 13C NMR titration of Ln-chelates performed with Gly-13C2 showed a stronger impact than that observed for Glu-13C5. The resonances of carboxylic carbons of ternary host-guest adducts exhibited the greatest signal change, presumably due to the shortest Ln-13Ccarboxylic distance. Thu, 20 Aug 2026 00:00:00 GMT http://hdl.handle.net/10900/182418 2026-08-20T00:00:00Z http://hdl.handle.net/10900/182361 Analysis of the assembly of the virulence-associated Salmonella type III secretion system Daouda Agbanrin, Maradona Salmonella enterica serovar Typhimurium utilizes the type III secretion system (T3SS) to directly translocate bacterial effector proteins into the host cytosol, thereby establishing infection and promoting intracellular survival within host cells. Although the overall architecture and function of the T3SS have been extensively characterized, the precise order of assembly of its substructures, particularly the base components, remains incompletely understood. Conflicting models propose either an outside-in assembly pathway initiated at the outer membrane via SctC (InvG) formation, followed by recruitment of the remaining basal body components SctD (PrgH) and SctJ (PrgK), or an inside-out process originating at the inner membrane through assembly of the export apparatus, followed by recruitment of SctJ and SctD and finally SctC. The aim of this study was to clarify this conflict using a combination of in vivo photocrosslinking and luciferase-based secretion assays. Photocrosslinking experiments revealed that the interaction between SctD and SctJ remained detectable in the absence of SctC, although with reduced signal intensity compared to the wild type, indicating that SctD-SctJ assembly depends on prior assembly of SctC. Similarly, the SctD-SctC interaction was still detected in the absence of SctJ, albeit with reduced intensity, suggesting that stable SctD-SctC association depends on prior SctJ assembly. Interactions involving sorting platform components, including SctD-SctK and SctK-SctQ, also remained detectable in ΔsctC and ΔsctJ strains, indicating that early recruitment of cytoplasmic sorting platform components can occur before completion of the basal body. However, the reduced crosslinking signals observed in mutant backgrounds suggest that the fully assembled basal body contributes to stabilization of these interactions. Functional analyses using NanoLuc-based secretion assays further demonstrated that no increased accumulation of the early substrate InvJ occurred in the periplasm of ΔsctC strains, indicating that secretion initiation requires the presence of SctC. Together, our findings reconcile the previously proposed inside-out and outside-in assembly models by supporting a refined hybrid mechanism in which SctJ and SctC assemble independently at the inner and outer membranes, respectively, followed by insertion of SctD between both structures to complete basal body assembly. Fri, 31 Dec 2027 00:00:00 GMT http://hdl.handle.net/10900/182361 2027-12-31T00:00:00Z http://hdl.handle.net/10900/182333 Pre-Ignition Fire Detection Using Semiconducting Metal Oxide Gas Sensors – Experimental and Chemometric Investigations Schynowski, Leah Katharina Aufgrund der unzureichenden Möglichkeiten, die derzeit zur Branddetektion in bemannten Habitaten im All zur Verfügung stehen, beschäftigt sich diese Arbeit mit der Entwicklung einer Alternative zu der gebräuchlichen Partikeldetektion. Durch fehlende Sedimentation in Mikrogravitation, sowie die Agglomeration der Partikel in der erzwungenen Konvektion an Bord, kommt es zu veränderten Anforderungen an die Detektoren, im Vergleich zu terrestrischen Anwendungen. Beispielsweise konnte in einer Experimentreihe die unzureichende Detektion, des auf der Internationalen Raumstation verwendeten optischen Detektors, gegenüber kleiner Partikel, emittiert von erhitzten Raumfahrtwerkstoffen, festgestellt werden. Angesichts des enormen Risikos, das in dem abgeschlossenen Volumen eines Weltraumhabitats von den bei einem Brand entstehenden Gase ausgeht, ist eine schnelle Reaktion, möglicherweise sogar Evakuierung, entscheidend für das Überleben der Besatzung. Zudem führt wiederholte Detektion von Staub und anderen Partikeln zu Fehlalarmen, welche die Alarmbereitschaft der Crew erheblich beeinflussen und kurzzeitige Deaktivierung der Lüftung sowie sorgfältige Protokollierung der Situation nach sich ziehen. Die Problematik könnte sich möglicherweise nach dem Ende der Internationalen Raumstation weiter zuspitzen, da durch die geplanten Missionen zum Mond und Mars planetare Stäube in die Habitate gebracht werden könnten. Die derzeitige Maßnahme, um während der regelmäßigen Reinigungen keine Fehlalarme zu provozieren, ist die simple Abschaltung der Detektoren. Diese Arbeit beschreibt die ersten Schritte in der Entwicklung eines Systems zur Detektion während der Entstehungsphase eines Brandes, vor Entzündung und dem Entstehen einer Flamme. Definiert durch die Zeit als entscheidende Ressource, sowie die Anforderungen an Energieverbrauch, Masse und Volumen, überzeugen miniaturisierte Metalloxid-Halbleitersensoren (SMOX) als Hauptkomponenten des zu entwickelnden Systems. Durch ihre hohe Sensitivität gegenüber flüchtigen organischen Stoffen, welche abhängig von der Temperatur aus Polymeren entweichen, sind die kommerziellen Sensoren SGP40 (Sensirion AG, Schweiz) geeignete Kandidaten. Als Testmaterialien wurden die in der Raumfahrt verwendeten Kunststoffe Polymethylmethacrylat (PMMA), Polydimethylsiloxan (PDMS), Nomex® IIIA und Kapton® gewählt. Erkenntnisse, über aus den Materialien ausgasende Spezies sowie Zersetzungstemperaturen, wurden mittels Gaschromatographie-Massenspektrometrie und thermogravimetrischer Analyse gewonnen. Ein Großteil der Arbeit beschreibt die Entwicklung einer geeigneten Testanlage zur Durchführung der Detektionsexperimente. Um die Anlage auch in zukünftigen Arbeiten möglichst vielseitig verwenden zu können, wurden einige Funktionalitäten (wie beispielsweise automatisierte Lüftung und die Injektion von Analyten) eingerichtet und deren Entwicklung sowie nachfolgende Validierung dokumentiert. In diesem Aufbau, unter Verwendung eines Rohrofens, wurden die Testmaterialien und die Sensorantworten der SGP40 ausgewertet. Durch Modellierung der Betriebstemperatur der Sensoren, unter Verwendung chemometrischer Methoden, konnten gewonnene Modellvorhersagen präzisiert werden. So gelang es mittlere relative Fehler zwischen realen Temperaturen und Vorhersage von 4.74 % für PMMA, 5.52 % für Nomex® IIIA, 14.20 % für PDMS und 14.94 % für Kapton® zu erreichen. Die mittleren absoluten Fehler der Random-Forest-Regression liegen zwischen etwa 3 K und 10 K. Zur Evaluierung des Potenzials des SGP40 eine Gasquelle zu lokalisieren, wurde in der genannten Testanlage eine Machbarkeitsstudie mit PMMA als repräsentativen Werkstoff durchgeführt. Dabei wurde der Rohrofen zwischen drei möglichen Positionen verschoben, und die entstehenden Gase wurden mit 32 im 1 m^3 großen Testvolumen verteilten Sensoren gemessen. Nicht nur konnten mittels dynamischer Zeitnormierung und k-Nächste-Nachbarn-Algorithmus unter Leave-One-Out-Kreuzvalidierung 100 % der Positionen richtig zugeordnet werden, das Experiment lieferte auch wichtige Einblicke in die Bewegungen der Gaswolke und die hierdurch mehr oder weniger geeigneten Sensorpositionen. Durch die in dieser Arbeit gesammelten Informationen wurde die Basis für die Entwicklung eines Systems zur Detektion während der Entstehungsphase eines Brandes geschaffen und entsprechend weitere Schritte konkretisiert. Neben den gewonnenen Erkenntnissen, wurden Hard- und Software für die Testanlage entwickelt und dokumentiert, welche weiterhin für das Projekt der Branddetektion sowie darüber hinaus Anwendung in verschiedenen Bereichen der Gassensorik finden werden.; The issue of insufficient technology for smoke or fire detection in habitable spacecraft was the main motivator for this work. Due to complex shapes, sizes, and movements of particles and agglomerates in microgravity and forced convection, the requirements for detectors differ from terrestrial application. As a consequence, frequent false alarms decrease the alertness of the crew on missions. Regular deactivation of the detection system, as currently practiced during cleaning activities on the International Space Station (ISS), is an unsatisfactory solution. The current detector’s lack of sensitivity toward small particles generated from overheated spacecraft materials, as revealed by experiments, makes innovation all the more necessary. This work documents the first steps in the overall project to develop a suited pre-ignition fire detection system. With the key limitation of time, pre-ignition fire detection is targeted using highly sensitive, small, power-efficient, and fast semiconducting metal oxide (SMOX)-based sensors. As the main component of the detection system, the commercially available sensor SGP40 (Sensirion AG, Switzerland) was chosen and investigated, to address the current issues on the ISS and more crucially, the arising obstacles, caused by planetary dust in future missions to the Moon and Mars. Poly(methyl methacrylate) (PMMA), poly(dimethylsiloxane) (PDMS), Nomex® IIIA, and Kapton® were selected as materials used in space applications and their thermal behavior and outgassing analyzed. A test-apparatus was built to facilitate the detection experiment, while doubling as a multi-purpose and future relevant gas mixing system. The design and development process as well as information on the construction and software are documented as part of this work to support maintenance and promote adaptations to the needs of scientific tasks. The polymers were heated in a controlled manner, in a specialized tube furnace, and the thermally-driven outgassing measured the SGP40 sensors, in constant and temperature modulated operation. In combination with suited chemometric methods, predictions on material temperatures resulted in mean absolute errors of 4.74 % for PMMA, 5.52 % for Nomex® IIIA, 14.20 % for PDMS, and 14.94 % for Kapton® . Mean relative errors ranged from 3 K to 10 K, using random forest regression. Additionally, the materials’ relevant characteristics of degradation temperature and outgassing species were assessed using thermogravimetric analysis-mass spectrometry (TGA-MS) and gas chromatography-mass spectrometry (GC-MS). Further investigations on the ability of the commercial sensor were conducted, to deliver spatial information on a source. For this purpose, 32 sensors were distributed in a chemical sensor grid, in the 1 m^3 test volume of the test-apparatus, in an attempt to localize the origin of heated PMMA in a three-class classification. The patterns of the sensor responses led to a classification accuracy of 100 % using dynamic time warping distances with the k-nearest neighbors algorithm. Furthermore, information on the plume movement was gained, leading to the identification of optimal sensor placement in the present configuration. The results of this work form the basis for further development of pre-ignition fire detection, while the specialized test system will remain in use beyond the scope of the project. Fri, 14 Aug 2026 00:00:00 GMT http://hdl.handle.net/10900/182333 2026-08-14T00:00:00Z http://hdl.handle.net/10900/182332 Spatial Network Design of the eLTER Research Infrastructure Ohnemus, Thomas Research Infrastructures (RIs) are key to face the grand environmental challenges humanity encounters by providing standardized, accessible, large-scale data. Yet, observation locations in RIs tend to be biased. These biases cannot simply be judged by spatial distance. Rather, meaningful RI-specific indicators describing a status quo and future conditions should be the basis to uncover biases and inform an RI’s spatial network design. This thesis studies the spatial network design of the Integrated European Long-Term Ecosystem, critical zone and socio-ecological Research Infrastructure (eLTER RI). Therefore, eLTER RI’s target region was defined and a survey was conducted to describe eLTER RI’s network of in-situ facilities. As relevant indicators, six indicators collectively describing current ecosystem functioning were chosen and four indicators describing Global Change Pressure on ecosystems were developed. Based on these indicators, using a representativity analysis sampling bias was identified. Lastly, it was investigated whether in-situ facilities of other RIs can resolve sampling bias within eLTER RI. eLTER RI currently consists of 204 in-situ facilities, which underrepresent agricultural lands, dry bioclimates, areas with low economic density, areas projected to experience a strong increase and a strong decrease in precipitation, little change in biotemperature, both strong and hardly any land use change, and areas where water availability during a year becomes more uniform. Spatially, these conditions and therefore underrepresented regions cluster in the Iberian Peninsula, Fennoscandia and Eastern Europe. Facilities of other RIs can convincingly counteract bias in Eastern Europe, but not in the other regions. As long as biases within an RI persist, they must be addressed in data analyses, e.g. by removing data or explicitly mention biases uncovered. Only an unbiased in-situ facility network would allow confident upscaling of all data. The procedure to design the spatial network of an RI outlined in this thesis can function as a blueprint for continental or global scale RI development towards minimizing sampling bias. Fri, 14 Aug 2026 00:00:00 GMT http://hdl.handle.net/10900/182332 2026-08-14T00:00:00Z