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‘Smart’ Janus emulsions
(2021)
Emulsions constitute one of the most prominent and continuously evolving research areas in Colloid Chemistry, which involves the preparation of mixtures or dispersions of immiscible components in a continuous medium. Besides conventional oil-in-water or water-in-oil emulsions, other emulsions of complex droplet morphologies have recently attracted significant research interests. Especially Janus emulsions, in which each droplet is comprised of two distinct sub-regions, have shown versatile potential applications. One of their advantages is the possibility of compartmentalization, which enables to play with two different chemistries in a single droplet. Though microfluidic methods are conventionally used to prepare Janus emulsions, their industrial applications are largely hindered by low throughput and extensive instrumentations. Recently, it has been discovered that simply one-pot moderate/high energy emulsification is also capable of developing Janus morphology, although their preparation and stabilization remain rather substantially challenging. This cumulative doctoral thesis focuses on the preparation and characterization of ‘smart’ Janus emulsions, i.e. Janus emulsions with special stimuli-responsive features. One-step moderate/high energy emulsification of olive and silicone oil in an aqueous medium was carried out. Special consideration was devoted to the interfacial tensions among the components to maintain the criteria of forming characteristic droplet architectures, in addition to avoiding multiple emulsion destabilization phenomena like imminent phase separation or even separated droplet formation. A series of investigations were conducted related to the formation of complexes of charged macromolecules and role of them as stabilizers to achieve stable Janus emulsions for a realistic timeframe (more than 3 months). The correlation between the size of the stabilizer particles and the droplet size of emulsion was established. Furthermore, it was observed that Janus emulsion gels with interesting rheological properties can be fabricated in the presence of suitable polyelectrolyte complexes. Janus emulsions that could be influenced by pH, temperature or magnetic field were successfully produced in presence of characteristic stimuli-responsive stabilizers. Afterwards, the effect of these changes was studied by different characterization techniques. The size and morphology could be tuned easily by changing the pH. The incorporation of iron oxide magnetic nanoparticles (synthesized separately by a co-precipitation method) to one component of the Janus emulsion was carried out so that the movement and orientation of the complex droplets in aqueous media could be controlled by an external magnetic field. Additionally, temperature-triggered instantaneous reversible breakdown of Janus droplets was also accomplished. The responses of the Janus droplets by the stimuli were well-documented and explained. Another goal of the present contribution was to exploit this special morphological feature of emulsions as a template for producing porous materials. This was demonstrated by the preparation of ultralight magnetic responsive aerogels, utilizing Janus emulsion gels. The produced aerogels also showed the capacity to separate toxic dye from water. To the best of our knowledge, this is the first example of investigation towards batch scale production of Janus emulsion with such special stimuli-responsive properties by a simple bulk emulsification method.
Zur Charakterisierung von amphiphilen Blockcopolymeren aus N-Vinylpryrrolidon und Vinylacetat
(2009)
Untersuchungen zur Entwicklung und Synthese neuartiger Gelenkstäbe basierend auf Oligospiroketalen
(2019)
Totalsynthese benzoannellierter Sauerstoffheterocyclen durch Mikrowellen induzierte Tandem-Sequenzen
(2019)
Thermodynamics, kinetics and rheology of surfactant adsorption layers at water/oil interfaces
(2012)
Nanoparticles of magnetite (Fe3O4) are envisioned to find used in diverse applications, ranging from magnetic data storage, inks, ferrofluids as well as in magnetic resonance imaging, drug delivery, and hyperthermia cancer treatment. Their magnetic properties strongly depend on their size and morphology, two properties that can be synthetically controlled. Achieving appropriate control under soft chemical conditions has so far remained a challenging endeavor. One proven way of exerting this desired control has been using a biomimetic approach that emulates the proteome of magnetotactic bacteria by adding poly-L-arginine in the co- precipitation of ferrous and ferric chloride. The objective of the work presented here is to understand the impact of this polycation on the formation mechanism of magnetite and, through rational design, to enhance the control we can exert on magnetite nanoparticle size and morphology. We developed a SAXS setup to temporally and structurally resolve the formation of magnetite in the presence of poly-L-arginine in situ. Using analytical scattering models, we were able to separate the scattering contribution of a low-density 5 nm iron structure from the contribution of the growing nanoparticles. We identified that the low-density iron structure is a metastable precursor to the magnetite particles and that it is electrostatically stabilized by poly-L-arginine. In a process analogous to biomineralization, the presence of the charged macromolecule thus shifts the reaction mechanism from a thermodynamically controlled one to a kinetically controlled one. We identify this shift in reactions mechanism as the cornerstone of the proposed mechanism and as the crucial step in the paradigm of this extraordinary nanoparticle morphology and size control. Based on SAXS data, theoretical considerations suggest that an observed morphological transition between spherical, solid, and sub-structured mesocrystalline magnetite nanoparticles is induced through a pH-driven change in the wettability of the nanoparticle surface. With these results, we further demonstrate that SAXS can be an invaluable tool for investigating nanoparticle formation. We were able to change particle morphology from spherically solid particles to sub-structured mesocrystals merely by changing the precipitation pH. Improving the synthesis sustainability by substituting poly-L-arginine with renewable, polysaccharide-based polycations produced at the metric ton scale, we demonstrated that the ability to alter the reaction mechanism of magnetite can be generically attributed to the presence of polycations. Through meticulous analysis and the understanding of the formation mechanism, we were able to exert precise control over particle size and morphology, by adapting crucial synthesis parameters. We were thus able to grow mesocrystals up to 200 nm and solid nanocrystals of 100 nm by adding virtually any strong polycation. We further found a way to produce stable single domain magnetite at only slightly increased alkalinity, as magnetotactic bacteria do it. Thus through the understanding of the biological system, the consecutive biomimetic synthesis of magnetite and the following understanding of the mechanism involved in the in vitro synthesis, we managed to improve the synthetic control over the co-precipitation of magnetite, coming close biomineralization of magnetite in magnetotactic bacteria. Polyanions, in both natural as well as in synthetic systems, have been in the spotlight of recent research, yet our work shows the pivotal influence polycations have on the nucleation of magnetite. This work will contribute significantly to our ability to tailor magnetite nanoparticle size and morphology; in addition, we presume it will provide us with a model system for studying biomineralization of magnetite in vitro, putting the spotlight on the important influence of polycations, which have not had the scientific attention they deserve.
Tetrahalidocuprat(II)-komplexe : Untersuchungen zur Relation von Struktur- und EPR-Parametern
(2013)
Synthesis of stimuli-responsive and switchable inorganic nanoparticles for biomedical applications
(2010)
This thesis focuses on the synthesis of novel functional materials based on plasmonic nanoparticles. Three systems with targeted surface modification and functionalization have been designed and synthesized, involving modified perylenediimide doped silica-coated silver nanowires, polydopamine or TiO2 coated gold-palladium nanorods and thiolated poly(ethylene glycol) (PEG-SH)/dodecanethiol (DDT) modified silver nanospheres. Their possible applications as plasmonic resonators, chiral sensors as well as photo-catalysts have been studied. In addition, the interaction between silver nanospheres and 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) molecules has also been investigated in detail.
In the first part of the thesis, surface modification on Ag nanowires (NWs) with optimized silica coating through a modified Stöber method has been firstly conducted, employing sodium hydroxide (NaOH) to replace ammonia solution (NH4OH). The coated silver nanowires with a smooth silica shell have been investigated by single-particle dark-field scattering spectroscopy, transmission electron microscopy and electron-energy loss spectroscopy to characterize the morphologies and structural components. The silica-coated silver nanowires can be further functionalized with fluorescent molecules in the silica shell via a facile one-step coating method. The as-synthesized nanowire is further coupled with a gold nanosphere by spin-coating for the application of the sub-diffractional chiral sensor for the first time. The exciton-plasmon-photon interconversion in the system eases the signal detection in the perfectly matched 1D nanostructure and contributes to the high contrast of the subwavelength chiral sensing for the polarized light.
In the second part of the thesis, dumbbell-shaped Au-Pd nanorods coated with a layer of polydopamine (PDA) or titanium dioxide (TiO2) have been constructed. The PDA- and TiO2- coated Au-Pd nanorods show a strong photothermal conversion performance under NIR illumination. Moreover, the catalytic performance of the particles has been investigated using the reduction of 4-nitrophenol (4-NP) as the model reaction. Under light irradiation, the PDA-coated Au-Pd nanorods exhibit a superior catalytic activity by increasing the reaction rate constant of 3 times. The Arrhenius-like behavior of the reaction with similar activation energies in the presence and absence of light irradiation indicates the photoheating effect to be the dominant mechanism of the reaction acceleration. Thus, we attribute the enhanced performance of the catalysis to the strong photothermal effect that is driven by the optical excitation of the gold surface plasmon as well as the synergy with the PDA layer.
In the third part, the kinetic study on the adsorption of 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquino-dimethane (F4TCNQ) on the surface of Ag nanoparticles (Ag NPs) in chloroform has been reported in detail. Based on the results obtained from the UV-vis-NIR absorption spectroscopy, cryogenic transmission electron microscopy (cryo-TEM), scanning nano-beam electron diffraction (NBED) and electron energy loss spectroscopy (EELS), a two-step interaction kinetics has been proposed for the Ag NPs and F4TCNQ molecules. It includes the first step of electron transfer from Ag NPs to F4TCNQ indicated by the ionization of F4TCNQ, and the second step of the formation of Ag-F4TCNQ complex. The whole process has been followed via UV-vis-NIR absorption spectroscopy, which reveals distinct kinetics at two stages: the instantaneous ionization and the long-term complex formation. The kinetics and the influence of the molar ratio of Ag NPs/F4TCNQ molecules on the interaction between Ag NPs and F4TCNQ molecules in the organic solution are reported herein for the first time. Furthermore, the control experiment with silica-coated Ag NPs indicates that the charge transfer at the surface between Ag NPs and F4TCNQ molecules has been prohibited by a silica layer of 18 nm.