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Developments in medical diagnostics, environmental monitoring, food safety and therapeutics require systems that make it possible to detect a wide range of molecules in a fast and easy, yet specific and sensitive manner. In light of these applications and driven by this demand, the field of biosensing has demonstrated huge potential. In spite of this potential, continuous improvements beyond the state of the art are necessary to help overcome remaining challenges and create new opportunities. Among others, the challenges can be correlated with three pivotal aspects of a biosensor: (i) the performance of the bioreceptor molecules, (ii) the nanoarchitecture of the biorecognition layer and (iii) the adequate signal generation.Although protein elements are frequently adopted as bioreceptors or signal amplifiers in biosensing applications, they often suffer from certain drawbacks related to their limited design flexibility and stability, the latter both in time and in varying assay conditions. Moreover, although highly relevant, the nanoarchitecture of the biorecognition layer (i.e. the bioreceptor positioning at the biosensing interface) is often neglected. In this context, the field of DNA-nanotechnology offers a number of solutions: (1) functional DNA nanotechnology forms an alternative for protein bioreceptors, (2) structural DNA nanotechnology can be applied to control the nanoarchitecture of the biorecognition layer and (3) dynamic DNA nanotechnology is known to enable extensive signal amplification. In this context, the goal of this dissertation was to exploit DNA nanotechnology (DNA probes and aptamers, DNA origami, DNA cascades) as a toolbox to design and develop novel DNA-based strategies for improved biosensing, with the final aim of moving towards DNA-only biosensors.

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Angiogenesis - the formation of new blood vessels from the pre-existing vasculature - occurs in both physiological and pathological conditions and is a hallmark of a wide range of diseases. During sprouting angiogenesis, invading endothelial cells exert cellular traction forces at cell-cell and cell-matrix interaction sites. While leader cells pulling on follower cells, follower cells pushing on leader cells, or both, have previously been postulated as mechanical forces that underlie the sprout outgrowth, forces during in vitro sprouting have never been measured directly. Deciphering the forces during this angiogenic sprouting - a process that is highly actomyosin-dependent - is at the heart of understanding sprouting angiogenesis.Within the scope of this study, first an in vitro model of angiogenesis is selected. With this basic model, the respective roles of actin and myosin for sprouting angiogenesis are investigated by targeting the actomyosin force generation of endothelial sprouts with small molecule inhibitors. Next, the in vitro model is extended to relate actomyosin-dependent cellular tractions to deformations of the surrounding matrix by means of 4D Displacement Microscopy. 4D Displacement Microscopy combines fluorescence microscopy with image registration algorithms to express cell-matrix mechanical interactions in terms of matrix deformation-based metrics. The model extensions include, amongst others: (1) Microscopically mapping matrix displacements around in vitro sprouts in 4D (in x, y, z and time), (2) inducing a stress-free reference state of the matrix by chemical relaxation of the cellular traction forces, and (3) calculating absolute displacements from the microscopy data with image registration algorithms. Microscopically mapped matrix displacements are here irrefutably coupled to the cellular traction forces of endothelial sprouts growing within the matrix.Tackling the core questions of the dissertation, 4D Displacement Microscopy is then applied to spatio-temporally analyse displacement field patterns around in vitro sprouts. Patterns are examined in relation to sprout morphology and dynamics; which are possible predictors of matrix displacement magnitudes. Recurrent patterns unravel how in vitro endothelial sprouts are mechanically interacting with the matrix, and shed light on the pulling versus pushing nature of forces underlying sprout protrusion dynamics of endothelial sprouts. Further analysis of local displacement fields around retracting and extending sprout protrusions confirms the expected predominant role of sprout pulling instead of sprout pushing forces. Ultimately, mathematically calculating the magnitudes of these sprout forces - for which matrix mechanical properties are needed - will lead to an even deeper understanding of sprouting angiogenesis by allowing comparative studies of hydrogels with different mechanical properties. Therefore, the 4D Displacement Microscopy analysis is taken a step further and as a proof of concept, cellular traction forces around in vitro angiogenic sprouts are estimated for the first time.Finally, modifications of the sample design allow conducting 4D Displacement Microscopy with Selective Plane Illumination microscopy (SPIM) instead of with Confocal Laser Scanning Microscopy (CLSM). SPIM-based displacement microscopy is promising for investigating fast processes of in vitro angiogenesis - such as protrusion dynamics - as it allows imaging multiple sprouts simultaneously at high temporal resolution, while maintaining the subcellular resolution required for the registration algorithms.In summary, this research highlights the key role of actomyosin-based traction forces for in vitro sprouting angiogenesis, it couples microscopically mapped matrix displacements to the cellular traction forces of endothelial sprouts, and it contributes to deciphering how in vitro endothelial sprouts mechanically and reciprocally interact with their micro-environment. 4D Displacement Microscopy (both with CLSM and SPIM) is shown to provide a quantitative and mechanically sound approach to advance the knowledge in the field, and forces around in vitro sprouts are reported for the first time.

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The goal of this thesis was to study photocatalytic N2 reduction to NH3 over metal halide perovskite-based photocatalysts. Apart from the main goal, focus was also laid on diverse characterization techniques such as UV-Vis and photoluminescence spectroscopy, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and ion chromatography. Various cesium lead bromide nanocrystal synthesis methodologies were explored, qualitative discussion on the nucleation and growth mechanism of cesium lead bromide quantum dots, and the role of ligands are also mentioned. In essence, the need for clean processes to produce NH3 and the potential of photocatalysis in achieving that goal has been highlighted. We have conducted a fundamental study on the N2 photofixation ability of cesium lead bromide quantum dots (CBR-QD) photocatalyst to NH3 using visible light ( > 420 nm), water and N2 as the inputs at ambient conditions. The effect of various sacrificial agents has also been tested. It is seen that our CBR-QD photocatalyst displays an ammonia production rate of 200 μmolh-1g-1 which is to our knowledge the highest reported value for all-inorganic perovskite-based photocatalysts. A bromine vacancy defect mediated N2 activation is hypothesized which however has to be verified for its validity and might be an interesting exercise to perform in the future. It is understood that photocatalytic N2 reduction to NH3 most probably will not replace the Bosch-Haber process due to the poor efficiencies of current photocatalysts, however, I believe that the motivation to work in this research line is not to replace the existing Bosch-Haber process but to reduce the dependency on it, which by itself is a great achievement, hence keeping the hunt for new materials and architectures wide open. This thesis also provides a fundamental understanding of N2 photofixation by semiconductor photocatalysis and marks the start of N2 photofixation ability of all-inorganic perovskites which may serve as a good foundation for future work.