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The goal of this PhD thesis is to apply fluorescence microscopy to investigate the interplay between local catalytic performance and catalyst porosity and to derive structure-activity and selectivity relationships for heterogeneous catalysts at the level of individual catalytic turnovers. To achieve this goal, I employed both diffraction-limited and super-resolution fluorescence microscopy with support from other techniques.A first objective was to study the effects of dealumination on the distribution of acid sites inside individual mordenite crystals and the impact hereof on the catalytic activity. Using super-resolution fluorescence microscopy, Raman microspectroscopy, and focused-ion-beam-assisted scanning electron microscopy I identified significant variations in catalytic properties inside individual dealuminated mordenites as well as strong variations between individual catalyst crystals. The origin of this suboptimal catalytic performance could be linked to variabilities that exist during commercial, large-scale dealumination.Secondly I studied the effect of solvents on the catalytic performance of acid H-ZSM-5. Using fluorescence microscopy with the acid-catalyzed furfuryl alcohol oligomerization reaction I discovered that the reaction preferentially occurs in a subset of the ZSM-5 pores. Using solvents of different polarity this pore selectivity could be altered. This result can be used to selectively perform catalytic reactions in either of the micropore subsystems.Later, this fluorescence based approach was extended to study the catalytic activity of metal-organic frameworks. For ZIF-8 I could prove that the reactivity is limited to the outer surface and bulk crystal defects. This inefficient use of the MOF material can be abated by the introduction of larger mesopores. In this project, I used oleic acid etching to increase the molecular penetration of the whole crystal volume.In conclusion, in this thesis I have applied fluorescence microscopy to resolve the structure-activity relationships in zeolites and metal-organic frameworks, and suggested strategies to optimize the catalytic activity. The results and the wealth of inferences therefrom demonstrate how fluorescence microscopy can enrich catalysis research as a characterization method. Such studies can be used to advance the field of catalyst design and development.
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The rational improvement of solid catalysts requires a thorough understanding of the structure-activity relationship down to the smallest possible length scales. Recently developed approaches that correlate fluorescence microscopy (FM) and scanning electron microscopy (SEM) allow such insights by direct linking of nanoscale catalytic activity maps, as recorded using fluorescence, to the local structural context. The goal of this PhD was to investigate structure-activity relationships at the nanoscale, by developing an integrated light and electron microscope (ILEM) that combines the power of single molecule sensitive FM to resolve reactivity at the nanoscale with a high-end SEM. Initially, the ILEM was applied to visualize silver nanoparticle photodeposition from an aqueous silver(I) solution on individual ZnO crystals, in real time. This was enabled by the ability to simultaneously perform local UV irradiation using the integrated light microscope and SEM imaging at the same region of interest (ROI) while the sample was contained inside a specialized liquid cell. This research revealed that silver nanoparticle formation predominantly occurs at crystal edges. However, the contribution of the electron beam during silver nanoparticle deposition was found to be non-negligible. Follow-up research was performed by applying the ILEM in a correlative fashion; i.e., by performing structural imaging before and after, and not during, UV induced silver photodeposition. As such, the facet dependent photocatalytic reactivity could be explored at the single particle level and, at the sub-particle level, variations were related to crystallographic structural features and defects. The first correlative super-resolution fluorescence and electron microscopic investigation of zeolite catalysts was made possible after resolving several technical challenges encountered during the ZnO research and by further optimizing the fluorescence microscope to enable the detection of individual catalytic fluorogenic conversions. This improved setup made it possible to directly observe the effect of intercrystalline intergrowths on the overall catalytic performance of acid mordenite zeolites. By determining the orientation of the individual reaction products compared to the underlying zeolitic framework, it was found that shape-selectivity was maintained at the defect-rich intergrowth boundary. Hence, the intergrowth was identified as a void space that facilitates mass transport into these pores. Acid leaching did not dramatically change this, as activity increased on previously active regions while the molecular orientation was maintained. A second correlative zeolite structure-activity investigation targeted individual ZSM-22 catalyst particles. The typical needle-shaped morphology of these particles results from a lateral fusion of elementary nanorods and indirect experimentation already suggested that during this lateral fusion, external, catalytically inactive, aluminum is converted into catalytically active internal aluminum. This was confirmed by the visualization of catalyst activity and shape-selectivity at the sub-particle level on the ILEM. In summary, over the course of this PhD, a performant ILEM was developed that combined super-resolution fluorescence microscopy based on the localization of individual fluorescent molecules with high-end field emission SEM. Also the necessary experimental routines and software tools were developed, enabling the determination of the structure-activity relationship of heterogeneous catalysts at the nanoscale. This powerful research tool allows a direct correlation of catalyst structure and activity beyond the single particle level. It is anticipated that the further application and development of this ILEM will ultimately lead to a more rationalized catalyst optimization.
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Microplastic pollution is one of the most critical environmental issues in the world right now. Many studies about human exposure to these micro-and nanoplastics (MNP) and the toxic effects they can cause are being conducted at this moment. Quantification and identification of these MNP lie at the bottom of solving this global problem. The most popular identification methods are Fourier-transform infrared spectroscopy and Raman spectroscopy. These techniques are lacking in non-destructive analysis with high sensitivity and low background noise. This thesis aims to develop an optical spectroscopy method to detect and identify microplastics after staining with Nile Red. The latter is a lipophilic solvatochromic dye. Staining will cause the particle to fluoresce, with its emission spectrum depending on its polarity. More polar plastics will have a more red-shifted emission maximum. Via hyperspectral analysis, the MNP can be identified based on this spectrum. Fluorescence lifetime imaging microscopy is also implemented. The fluorescence lifetime is the time that a fluorophore spends in the excited state, after absorbing a photon, before returning to the ground state. The lifetime depends on the fluorophore's micro-environment and therefore is also different for each MNP type. Differentiation is possible based on this lifetime value, but a graphical, fit-free phasor approach is also used. This allows for the mapping of the lifetime distribution in an image. When two different MNP types are present in one sample, they will each have a separate distribution cloud on the phasor plot, allowing for the graphical distinguishment of MNP. The plastics used are PP, PVC, HDPE, PET, and PS, these samples contain a range of shapes and sizes. A bought suspension of pure PS polymer particles of a fixed shape and size of 0,6 μm was also examined. The spectral data were processed in two ways. By calculating a ‘weighted average’ with the intensity values at each wavelength. This resulted in each MNP being attributed a weighted average value. After recording spectra of different particles, a statistical analysis, ANOVA and t-test, was done. This showed that the weighted averages of PP & HDPE and PS (self-made) & PVC do not differ significantly. The second option is through comparing the spectra themselves, using a relative similarity percentage. Because of the low similarity MNP gave when compared with themselves, only MNP with a significantly large similarity percentage could be distinguished. The same statistical tests were done on lifetime data. Measurements done on different particles of the same plastic showed that only HDPE & PET lifetimes did not differ significantly. Multiple measurements done on the same particle showed slightly different lifetimes. It could be concluded that HDPE & PET and PS (self-made and 0,6 μm) & PET cannot be differentiated. Finally, phasor plot analysis of samples containing two different plastics showed a possibility of recognizing MNP types based on the location of their phasor cloud on a phasor plot. When combining these two techniques, making use of the ’weighted average’ value and lifetime analysis, it could be possible to identify each plastic. The combination with similarly weighted averages, PP & HDPE give significantly different fluorescent lifetimes values and are possible to separate on a phasor plot. Still, both techniques have some flaws and improvement is possible.
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Fotokatalysatoren zoals zinkoxide zijn in staat om milieuverontreinigende stoffen af te breken na absorptie van voldoende energetisch licht. Een belangrijke beperking van dit fotokatalyse proces is de recombinatie van gegenereerde elektronen en gaten, wat resulteert in een verlies van fotokatalytische activiteit. Deze recombinatie wordt in dit onderzoek grotendeels voorkomen door afzetting van zilver nanopartikels op zinkoxide kristallen. De gegenereerde elektronen worden als het ware 'gevangen' in de zilver nanostructuren waardoor een verhoogde fotokatalytische activiteit wordt waargenomen. In een eerste fase worden twee reeds gepubliceerde procedures voor het genereren van ZnO microkristallen onder de loep genomen en verder verfijnd door verschillende parameters aan te passen. Op deze manier zijn drie zinkoxide microkristallen met een geschikte structuur voor dit onderzoek gesynthetiseerd: ZnO microrods [800 nm x 3,6 µm], ZnO microrods [1,5 µm x 2 µm] en ZnO staven [1 µm x 10 µm]. Deze ZnO kristallen verschillen in lengte, diameter en aanwezige kristalvlakken en hun structuur is uitvoerig door de scanning elektronenmicroscoop bestudeerd. Als vervolg worden zilver nanostructuren door middel van fotokatalytische reductie op de ZnO kristallen afgezet. Aan de hand van een fluorogene reactie wordt de fotokatalytische activiteit van de gesynthetiseerde halfgeleiders op bulk niveau bestudeerd. De reactie bestaat uit de fotoreductie van resazurine naar het fluorescente resorufine. Aan de hand van veranderingen in absorptie-eigenschappen en emissie-intensiteit van een resazurine oplossing in aanwezigheid van de katalysator, kan de efficiëntie van de fotokatalyse worden opgevolgd. Na afzetting van zilver nanostructuren op de zinkoxide kristallen wordt een meer efficiënte fotokatalysator verkregen gezien een twee- tot driemaal snellere katalytische omzetting kan worden waargenomen. In een laatste fase wordt de verhoogde fotokatalytische activiteit op 'single molecule' niveau onderzocht. Met een state-of-the-art geïntegreerde licht- en elektronenmicroscoop, worden de fotokatalytische omzettingen gecorreleerd met de oppervlakte-eigenschappen van de ZnO-zilver halfgeleiders. Zowel de 'in situ' zilverafzetting als de katalyse van de fluorogene reactie worden bij de drie gesynthetiseerde ZnO microkristallen geanalyseerd. Op het oppervlak van de ZnO microrods [800 nm x 3,6 µm] en ZnO staven [1 µm x 10 µm] worden dendriet- en plaatvormige zilverstructuren gevormd. Uit de overlap van de fotokatalytische activiteitsmappen met de structurele informatie via SEM blijkt dat de meeste activiteit wel degelijk op deze zilverstructuren is gelokaliseerd.
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The generation of clean, renewable energy sources has been a hot research topic for the past decades. Photocatalysis is a plausible way to store intermittently available solar energy in chemical bonds. So far, many metal oxides, metal sulfides, metal complexes, and their composites have been investigated for their efficiency as visible-light photocatalysts. It is still of great interest and practical importance to develop a cost-effective visible-light-driven catalyst for artificial photosynthesis.Recently, metal halide perovskites (MHPs) have been actively investigated to exploit their excellent optical properties, such as high visible-light absorption coefficients, long-range balanced electron-hole transport, etc., in combination with the potential of cheap and easy fabrication processes. However, the instability of MHPs in a polar environment limits the potential for their application as photocatalysts. To overcome this instability issue, reactions are usually performed in non-polar or low polarity solvents or in strongly concentrated metal halide solutions, which reverses the decomposition. As a prominent representative, cesium lead halides (CsPbX3, X = Cl, Br, I) have been seen as potential candidates for photocatalysis. Also, composite materials construction strategies have been actively utilized to optimize the activity and stability of MHPs in polar solvents. So far, CsPbBr3-based composites have been generated through the post-synthetic combination of preformed CsPbBr3 with other stabilizing materials. However, the in situ synthesis of composite materials offers enhanced surface contact with potentially improved activity and stability. Therefore, the in situ construction of CsPbBr3-based composites, comprising a type-II heterojunction, has been mainly investigated here.
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In this thesis, an investigation on the influence of iron impurities on the optical properties of zeolites and the luminescence properties of silver-exchanged zeolites was performed. The main source of these impurities originates from the source materials used in the synthesis procedures, therefore, a comparative study on the optical purity between commercial zeolites and self-synthesized zeolites was performed. First, the influence of different synthesis procedures on the topology and crystal morphology of the self-made zeolites was determined. The followed procedures differed in silica source, organic template and overall batch composition. Each procedure contained multiple samples with a different crystallization time. Second, the presence of absorbing species, such as iron and organic components, was determined using DRS and ESR measurements. The DRS analysis gives direct information on the amount of absorbing species and the absorption region of the impurities. The relative concentration of iron in the zeolite was determined by ESR. Some of the self-synthesized samples were also subject to a post-synthesis treatment before the silver-exchange to extract the unavoidable trace amounts of iron impurities. After analyzing the physiochemical characteristics, the zeolites were exchanged with silver ions and a further thermal activation step was used to form the emissive silver clusters. These silver species are typically excited with UV light yielding a visible light emission and the corresponding quantum efficiency of emission (EQE) and the influence of the zeolite purity on it was studied in detail. In order to measure the EQE in a correct fashion, the exact position of the excitation and emission maxima of the silver-exchanged were determined using spectrofluorimeter. Based on the EQE and the 2D excitation-emission spectra obtained for the different samples the effect of several parameters, such as impurities, zeolite topology and counter-balancing ion on the l...
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De laatste decennia is er een toenemende belangstelling voor fotokatalyse. Eén van de voordelen is de mogelijkheid om zonlicht te gebruiken om bijvoorbeeld CO2 om te zetten in hernieuwbare chemische brandstoffen. TiO2 is een algemeen gebruikte fotokatalysator omdat het een grote band gap-energie, hoge stabiliteit en lage kosten heeft, maar de lichtabsorptie is beperkt tot het UV-gebied. Om de lichtabsorptie in het zichtbare lichtgebied te optimaliseren en de recombinatiesnelheid te verlagen, worden heterojunctie fotokatalysatoren gebruikt. De heterojunctievorming tussen de twee halfgeleiders in de fotokatalysator hangt af van de materiaaleigenschappen (bandgap, kristalstructuur, etc.) van de halfgeleiders. Het Type II- en Z-schema heterojuncties zijn de meest onderzochte typen heterojuncties, elk met zijn eigen voordelen en beperkingen. Het Z-schema vertoont een goed oxidatief en reducerend vermogen en ladingsscheiding, maar dit gaat ten koste van de helft van de ladingsdragers als gevolg van recombinatie aan het grensvlak tussen de twee halfgeleiders. In Type II vindt deze grensvlakrecombinatie niet plaats, waarbij de hoeveelheid beschikbare licht-gegenereerde ladingsdragers wordt gemaximaliseerd, maar ten koste van het oxidatieve en reductieve vermogen van respectievelijk de gaten en elektronen. In dit masterproefproject wordt de ladingsoverdracht in de heterojuncties met verschillende verhoudingen van CsPbBr3 nanokristallen en Bi2MoO6 nanosheets onderzocht. Eerst werd de synthesemethode bestudeerd om de vorming van de heterojunctie te verzekeren. De hot-injectie-methode is een gemakkelijke manier om perovskiet-nanokristallen met een kleine grootteverdeling op de Bi2MoO6-nanosheets te vormen. Om de gesynthetiseerde heterojunctie en de twee materialen in de heterojunctie te onderzoeken, werden de relevante materiaaleigenschappen geanalyseerd: (1) de bandgap van de halfgeleiders in de heterojuncties werd bestudeerd door Diffuse Reflectance Spectroscopie (DRS), (2) de kristalstructuur van de halfgeleiders geanalyseerd met röntgendiffractie (XRD), (3) de kristalgroottes gemeten met scanning elektronenmicroscopie (SEM) en berekend via de verandering in bandgap en de PL maxima en (4) de atomaire samenstelling van de heterojunctie werd bepaald door inductief gekoppelde plasma-optische emissiespectrometrie (ICP-OES). Om de licht-gegenereerde dynamiek van ladingsdragers in de composietmaterialen te analyseren, werden verschillende fotokatalytische tests met de gesynthetiseerde fotokatalysator uitgevoerd. De oxidatie van benzylalcohol tot benzaldehyde en de reductie van CO2 tot CO werden gebruikt voor de fotokatalyse met de heterojunctie. De redoxreacties zullen alleen plaatsvinden als de ladingsoverdracht de Z-schemaroute volgt, vanwege hun overeenkomstige redoxpotentialen. Er werd waargenomen dat de gecombineerde reactie, oxidatie van benzylalcohol en reductie van CO2, plaatsvond via de gewenste Z-schema-route, maar deze leidde tot een lage vorming van BAD en CO. Dit geeft aan dat de ladingsdragerroute veranderde van Z-schema naar Type II tijdens de fotokatalyse.
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Ultrahigh molecular weight polyethylene (UHMWPE) is an aliphatic polymer made from ethylene monomers with an extraordinary high mechanical strength. With its excellent properties, this kind of material is widely used in mechanics, medical applications, military etc. Even though the final fiber product is extremely strong with a modulus as high as 7 GPa, there is still a lot potential to further improve this mechanical strength based on theoretical calculation. The strength of the fiber is currently mainly determined by the microstructure which is influenced by the fiber manufacturing process rather than by the intrinsic properties of the polymer chains. Plenty of studies so far have focused on the influence of various process parameters such as temperature, fiber draw ratio etc. Far less insights are available on the detailed UHMWPE polymer growth mechanism into the final powder particles and the molecular scale organization of the polymer chains into the final fiber. The main reason for this lack of insight is the absence of techniques that allow to directly observe the polymer distribution in the growing polymer particles and the processed fiber at the nanoscale in three dimensions. This research focuses on building new analytical tools for polyethylene to observe how the polymer is organized in the polymer particle and fiber in 3D dimensions separately. Both the UHMWPE powders and fibers are provided by DSM company. For the first part of the study, UHMWPE powders are labelled by fluorescent dye molecules. These fluorescent dyes are preloaded on the accessible active sites. Next the typical PE polymerization in heptane. The dye molecules hence allow tracing the distribution of the PE chains formed at the initially active sites throughout the final UHMWPE polymer product. The polymerization process was interrupted at different degrees of polymerization. By mapping out the distribution of these fluorescent dyes throughout the UHMWPE powder particles by using Stimulated Emission Depletion microscopy and Stimulated Raman Scattering microscope, will shed light on the catalytic process and yield a better understanding of the particle growth mechanism. For the second part, we will focus on the processed UHMWPE fibers. Here, unlabeled the typical UHMWPE is physically mixed with trace amounts of fluorescent dye molecules during the extrusion process. During gel spinning and further drawing, the individual UHMWPE chains will increasingly organize into crystalline domains. As a result, the fluorescent dye will remain in the non-crystalline domains of the imaged undrawn yarn fiber. By using the same microscopy techniques as for the UHMWPE powders, we mapped out the non-crystalline fiber domains both at the cross-section of the fiber as well as along the fiber length. The applied optical microscopic imaging approach combining different high-resolution molecular imaging techniques is novel in this field. For the UHMWPE powders, the catalyst fragmentation is the combination of shrinking core model and continuous bisection model and the polymer growth is via the multigrain model. For the UDY UHMWPE fiber, the contour of the non-crystalline domains was observed at the nanoscale. The images of the cross-section for fiber show nonhomogeneous distribution domains from the center to the edge. The images of the longitudinal section for fiber show the decisive location of domains and voids layer by layer, which can be used to guide industrial processing.
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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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