Rezwan, Kurosch
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Rezwan, Kurosch
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Rezwan, Kurosch
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Rezwan, K.
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Item-typ:Veröffentlichung, Hierarchically ordered micro/meso/macroporous polymer-derived ceramic monoliths fabricated by freeze-casting(Elsevier Science, 2015-10-17); ; ; A hierarchically-ordered macro/meso/microporous SiOC monolith was obtained via freeze-casting using commercial polysiloxane as a raw material and silica sol as a binder and template source. The pre-ceramic polymer polysiloxane was pyrolyzed at 600 °C to produce a hydrophilic surface; higher temperatures would fully decompose the organic groups. When silica sol and polysiloxane precursor were combined in freeze-casting method, after pyrolysis a polymer-derived SiOC ceramic monolith with a lamellar pore morphology and a hierarchically-ordered pore structure was obtained. Decomposition of the polysiloxane precursors results in the development of micropores, and particle packing is believed to be responsible for the mesopore formation. Macro/mesoporous hierarchically-ordered ceramics with a specific surface area of 74 m2/g are preserved at pyrolysis temperatures as high as 1000 °C. The influence of H44-derived filler amount (10 wt–40 wt%), freezing temperature (−20 °C, −80 °C, −150 °C), and pyrolysis temperature (600 °C, 700 °C, 1000 °C) on open porosity, pore size distribution, and surface characteristics were investigated.Wissenschaftlicher ArtikelBand:36Heft:1119 102 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Amorphous arsenic sulfide nanoparticles in a shallow water hydrothermal system(Elsevier Science, 2019-04-23); ; ; ; Hydrothermal fluids can contain trace elements such as arsenic (As), which are toxic to surrounding biota. In these kind of fluids, the bioavailability and biotransformation of As have been investigated but so far the ratio of total soluble As (<200 nm) versus the amount of As contained in a nanoparticulate phase has not been reported. Here, for the first time, the presence of As in the nanoparticulate fraction (between 200 and 20 nm) is described for arsenic-rich hydrothermal fluids in a marine shallow-water hydrothermal system. Samples of diffusively venting hydrothermal fluids, pore-water and seawater were collected in the hydrothermal system located in Paleochori Bay, Milos Island (Greece), and the fraction between 200 and 20 nm (As200-20) was studied. Up to 38% of the soluble arsenic was present within the As200-20 fraction in pore fluids, 10 to 20% in hydrothermal fluids and 5% in seawater. Identification and characterization of particles in hydrothermal fluid, pore-water and seawater was performed by scanning electron microscopy coupled to energy dispersive X-ray spectroscopy (SEM-EDX), transmission electron microscopy (TEM) with selected area electron diffraction (SAED) and dynamic light scattering (DLS). The particles are of spherical morphology with a polydisperse size distribution (PDI: 0.37) and diameters close to 100 nm. EDX studies confirmed a chemical composition rich in As and S. The SAED pattern revealed absence of a crystal phase indicating the presence of an amorphous arsenic sulfide material. These results bring into discussion the role of the nanoparticulate fraction for As dispersion, bioavailability, and potentially harmful effects in marine coastal ecosystems.Wissenschaftlicher ArtikelBand:211100 158 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Water-based freeze casting: Adjusting hydrophobic polymethylsiloxane for obtaining hierarchically ordered porous SiOC(Wiley, 2017-05-07); ; ; ; The hydrophobic properties of methyl poly siloxane (MK) were pushed into the“hydrophilic”range by cross-linking it with (3-aminopropyl)triethoxysilane(APTES) and subsequent pyrolysis to enable water-based freeze casting. Fillerproperties are investigated by varying the ratios of MK to APTES (1:1, 1:2, 1:3,1:4, 1:5), and pyrolysis temperatures (400°C, 500°C, 600°C) for the purpose ofdetermining an optimal set of characteristics for freeze casting. Additionally, fillerselection for this purpose is facilitated by analysis of zeta potential values andvapor adsorption. It was found that water-based freeze casting with hybrid fillers,followed by a pyrolysis step (600°C-700°C), leads to a SiOC ceramic monolithwith a lamellar pore morphology and a hierarchically ordered micro/meso/macrop-ore structure. Samples pyrolyzed at 1000°C contain mesopores, having a SSA ashigh as 51.6 m2/g. The hierarchically porous structure is very promising for appli-cations involving gas or liquid transportation.Wissenschaftlicher ArtikelBand:100Heft:5113 192 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Macro/mesoporous SiOC ceramics of anisotropic structure for cryogenic engineering(Elsevier, 2017-09-01); ; ; ; Macro/mesopore SiOC ceramic monoliths of anisotropic structure were prepared by freeze casting, using methy phenyl polysiloxane (H44) or methy polysiloxane (MK) and (3-aminopropyl)triethoxysilane (APTES) as precursors. Influence of pyrolysis and testing temperature on compressive strength was investigated. Monoliths pyrolyzed at 700 °C had the highest compressive strength both at 77 K (14.0 ± 4.3 MPa) and 293 K (7.7 ± 1.8 MPa), regardless of precursors. Compressive strength of monolith in parallel direction is around twice as much as perpendicular direction. Compressive strength of both monoliths in liquid nitrogen (77 K) was around twice of that in air (293 K) probably due to low temperature and liquid resistance. Anisotropic expansion was observed, and the shrinkage in parallel direction was almost twice of that in perpendicular direction, which can be verified by the Schapery equation. Monolith made from H44 showed a much higher coefficient of thermal expansion at 77 K than monolith made from MK and APTES, probably due to difference in composition and measurement condition. Thermal conductivities and specific heat capacities displayed an upward trend from low to warmer temperature. The minimum and maximum values for thermal conductivity are 0.2 and 1.2 W m− 1 K− 1. The maximum heat conductivities might be determined mainly by the macroporosity and the thermal conductivity of the hybrid material.Wissenschaftlicher ArtikelBand:13497 122 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Antibacterial active open-porous hydroxyapatite/lysozyme scaffolds suitable as bone graft and depot for localised drug deliveryAn engineered synthetic scaffold for bone regeneration should provide temporary structural support and a medium for controlled and localised release of bioavailable medical drugs. In this work, a method is proposed to incorporate biologically active agents without impairing agent activity into open-porous resorbable hydroxyapatite scaffolds. Scaffolds are obtained by a one-pot freeze gelation process and loaded with different amounts of lysozyme, a model macromolecular drug with antibacterial activity. The antibacterial activity is tested by submerging hydroxyapatite scaffolds with 0.5 to 2.5 wt.% lysozyme into two different bacteria stock solutions. A complete dieback of M. luteus bacteria when in contact with the scaffolds is observed. Higher lysozyme amount in the scaffold leads to faster dieback. In contact with scaffolds containing 2.5 wt.% lysozyme after 30 min, no viable bacteria can be observed. An amount of 0.5 wt.% lysozyme in the scaffolds is sufficient to kill all bacteria after a contact time of 24 h. For L. innocua, a bacteriostatic effect is observed. The scaffolds have spongiosa-like stability and are suitable bone implant substitutes. As agents are released from the scaffolds by degrees over a time period of at least 9 days, they are particularly attractive as depot for localised drug delivery of bioactive macromolecular drugs.Wissenschaftlicher ArtikelBand:31Heft:8131 127 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, An evaluation of colloidal and crystalline properties of CaCO3 nanoparticles for biological applicationsBiodegradable calcium carbonate carriers are a promising and safe nanoparticle platform which might enable various applications as an engineered nanomaterial in health care, food and cosmetics. However, engineered nanoparticles can exhibit new forms of toxicity that must be carefully evaluated before being widely adopted in consumer products or novel drug delivery systems. To this end, we studied four common calcium carbonate particle systems (calcite nanoparticles, amorphous sub-micrometer and vaterite sub-micrometer and micrometer particles) and compared their behavior in biological medium and in cell culture experiments. The thermodynamically stable calcite phase is shown to maintain its morphological features as no phase transformation occurs. Size- and time-dependent phase transformation of the less stable vaterite particles are observed within 96h in cell medium. The protein serum albumin can be an effective inhibitor of phase-transition and it is shown to improve colloidal stability. The impact of the biological environment goes beyond protein-corona formation, as we observed rapid dissolution of amorphous particles in high ionic strength cell medium, but not in Millipore water. Cellular responses of human osteoblasts against CaCO3 particles indicate that increased intracellular calcium ions improve viability and that particle internalization is not size-dependent. Useful insights for designing CaCO3-based delivery systems are provided and also corroborate to the idea that intrinsic material properties as well as environmental conditions are of relevance for the successful implementation of dispersed CaCO3 particles in drug delivery systems and in other applications.Wissenschaftlicher ArtikelBand:78103 208 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Proteolytic ceramic capillary membranes for the production of peptides under flow(Elsevier, 2019-04-15); ; ; ; In this study, we investigate the effect of membrane surface functionalization on the immobilization of the protease subtilisin A and its performance in the production of peptides from the model protein casein under flow. The surface of tubular ceramic membranes was silanized to yield carboxylated and aminated supports for enzyme immobilization via non-covalent and carbodiimide activated binding. The protease density correlated with electrostatic interactions between the positively charged enzyme and the supports, with the highest enzyme density reached on negatively charged, carboxylated membranes (0.019 molecules/nm², noncovalent approach). Enzyme leaching was reduced by covalent binding of protease to carboxylated supports (5% leached) and slightly improved by binding to aminated membranes (46%) over non-covalent binding to unfunctionalized reference capillaries (66%). Regarding carbodiimide activated immobilization, protease on unfunctionalized and aminated supports exhibited a significantly larger specific activity (0.99 μmol/min/mg) than enzymes on carboxylated surfaces (0.15 μmol/min/mg), which suggests preferred enzyme orientation. In protein hydrolysis, these differences in surface-enzyme interactions were reflected by variations in peptide composition and degree of hydrolysis. Accordingly, we demonstrate that surface functionalization critically determines the surface properties of protease support materials for the production of peptides under flow and allows tailoring the performance of proteolytic capillary membranes.Wissenschaftlicher ArtikelBand:147119 97 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Arsenic and sulfur nanoparticle synthesis mimicking environmental conditions of submarine shallow-water hydrothermal vents(2022-01); ; ; Arsenic and sulfur mineralization is a natural phenomenon occurring in hydrothermal systems where parameters like temperature and organic matter (OM) can influence the mobilization of the toxic metalloid in marine environments. In the present study we analyze the influence of temperature and OM (particularly sulfur-containing additives) on As and S precipitation based on the recent discovery of As-rich nanoparticles in the hydrothermal system near the coast of the Greek island Milos. To this end, we experimentally recreate the formation of amorphous colloidal particles rich in As and S via acidification (pH 3-4) of aqueous precursors at various temperatures. At higher temperatures, we observe the formation of monodisperse particles within the first 24 h of the experiment, generating colloidal particles with diameters close to 160 nm. The S:As ratio and particle size of the synthetized particles closely correlates with values for AsxSy particles detected in the hydrothermal system off Milos. Furthermore, organic sulfur containing additives (cysteine and glutathione, GSH) are a key factor in the process of nucleation and growth of amorphous colloidal AsxSy particles and, together with the temperature gradient present in shallow hydrothermal vents, dictate the stabilization of As-bearing nanomaterials in the environment. Based on these findings, we present a simple model that summarizes our new insights into the formation and mobility of colloidal As in aquatic ecosystems. In this context, amorphous AsxSy particles can present harmful effects to micro- and macro-biota not foreseen in bulk As material.Buch136 111 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Fatigue behavior and damage analysis of PIP C/SiC composite(Elsevier Science, 2022-10-01); ; ; ; In this work, we study the fatigue behavior of a C/SiC composite produced by several cycles of polymer infil- tration and pyrolysis (PIP). Fatigue tests were performed with maximum stresses corresponding to 60–90% of the tensile strength of the composite. During the fatigue tests, acoustic emission (AE) monitoring was performed and the measured AE energy was utilized to quantify the damage and distinguish possible damage mechanisms. Most of the fatigue damage in the form of matrix cracking, interface damage and fiber breakage occurs in the first cycle. As loading cycles proceeded, damage in form of matrix crack re-opening and interfacial friction constantly accumulates. Nevertheless, all samples survived the run-out of 1,000,000 cycles. After the fatigue tests, an in- crease of the tensile strength is observed. This phenomenon is associated with the relief of process-induced in- ternal thermal stresses and the weakening of the fiber-matrix interface. In general, the studied material shows very high relative fatigue limit of 90% of its tensile strengthWissenschaftlicher ArtikelBand:42Heft:1371 75 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Distribution of water in ceramic green bodies during drying(Elsevier, 2019-04-30); ; ; ; In order to investigate drying mechanisms at different stages, the distribution of water within the ceramic green bodies at different scales has been examined. The experimental measurements, using a simple weighing technique and Magnetic Resonance Imaging (MRI), show that during the first stage of drying involving shrinkage the material is constituted of uniquely solid and water with no gradient in water content within the sample. Then, during the second stage of drying, significant differences of water content as a function of position appear. As a complement, at the grain scale, observations using environmental scanning electron microscopy were made giving useful information on the solid–liquid–gas interfaces in the near surface part of the green body. Finally, the gradients in the water distribution were exploited to make a simple estimate of the diffusion coefficient of water with its dependence on the moisture content.Wissenschaftlicher ArtikelBand:39Heft:10157 103