Silicon Oxycarbide (SiOC)-Supported Ionic Liquids: Heterogeneous Catalysts for Cyclic Carbonate Formation.

Silicon oxycarbides (SiOCs) impregnated with tetrabutylammonium halides (TBAX) were investigated as an alternative to silica-based supported ionic liquid phases for the production of bio-based cyclic carbonates derived from limonene and linseed oil. The support materials and the supported ionic liquid phases (SILPs) were characterized via Fourier transform infrared spectroscopy, thermogravimetric analysis, nitrogen adsorption, X-ray photoelectron spectroscopy, microscopy, and solvent adsorption. The silicon oxycarbide supports were pyrolyzed at 300-900 °C prior to being coated with different tetrabutylammonium halides and further used as heterogeneous catalysts for the formation of cyclic carbonates in batch mode. Excellent selectivities of 97-100% and yields of 53-62% were obtained with tetrabutylammonium chloride supported on the silicon oxycarbides. For comparison, the catalytic performance of commonly employed silica-supported ionic liquids was investigated under the same conditions. The silica-supported species triggered the formation of a diol as a byproduct, leading to a lower selectivity of 87% and a lower yield of 48%. Ultimately, macroporous monolithic SiOC-SILPs with suitable permeability characteristics (k1 = 10-11 m2) were produced via photopolymerization-assisted solidification templating and applied for the selective and continuous production of limonene carbonate with supercritical carbon dioxide as the reagent and sole solvent. Constant product output over 48 h without concurrent catalyst leaching was achieved.

A Covalent Organic Framework/Graphene Dual-Region Hydrogel for Enhanced Solar-Driven Water Generation.

Solar-driven water generation is a sustainable water treatment technology, helping to relieve global water scarcity issues. However, this technology faces great challenges due to the high energy consumption of water evaporation yielding low evaporation rates. Here, a covalent organic framework (COF)/graphene dual-region hydrogel, containing hydrophilic and hydrophobic regions in one material, is developed through a facile in situ growth strategy. The hydrophilic COF is covering parts of the hydrophobic graphene regions. Through accurate control of both wetting regions, the hybrid hydrogel shows effective light-harvesting, tunable wettability, optimized water content, and lowered energy demand for water vaporization. Acting as solar absorber, the dual-region hydrogel exhibits a steam generation rate as high as 3.69 kg m-2 h-1 under 1 sun irradiation (1 kW m-2), which competes well with other state-of-the-art materials. Furthermore, this hydrogel evaporator can be used to produce drinkable water from seawater and sewage, demonstrating the potential for water treatment.

Hydrothermal polymerization of porous aromatic polyimide networks and machine learning-assisted computational morphology evolution interpretation.

We report on the hydrothermal polymerization (HTP) of polyimide (PI) networks using the medium H2O and the comonomers 1,3,5-tris(4-aminophenyl)benzene (TAPB) and pyromellitic acid (PMA). Full condensation is obtained at minimal reaction times of only 2 h at 200 °C. The PI networks are obtained as monoliths and feature thermal stabilities of >500 °C, and in several cases even up to 595 °C. The monoliths are built up by networks of densely packed, near-monodisperse spherical particles and annealed microfibers, and show three types of porosity: (i) intrinsic inter-segment ultramicroporosity (<0.8 nm) of the PI networks composing the particles (∼3-5 µm), (ii) interstitial voids between the particles (0.1-2 µm), and (iii) monolith cell porosity (∽10-100 µm), as studied via low pressure gas physisorption and Hg intrusion porosimetry analyses. This unique hierarchical porosity generates an outstandingly high specific pore volume of 7250 mm3 g-1. A large-scale micromorphological study screening the reaction parameters time, temperature, and the absence/presence of the additive acetic acid was performed. Through expert interpretation of hundreds of scanning electron microscopy (SEM) images of the products of these experiments, we devise a hypothesis for morphology formation and evolution: a monomer salt is initially formed and subsequently transformed to overall eight different fiber, pearl chain, and spherical morphologies, composed of PI and, at long reaction times (>48 h), also PI/SiO2 hybrids that form through reaction with the reaction vessel. Moreover, we have developed a computational image analysis pipeline that deciphers the complex morphologies of these SEM images automatically and also allows for formulating a hypothesis of morphology development in HTP that is in good agreement with the manual morphology analysis. Finally, we upscaled the HTP of PI(TAPB-PMA) and processed the resulting powder into dense cylindrical specimen by green solvent-free warm-pressing, showing that one can follow the full route from the synthesis of these PI networks to a final material without employing harmful solvents.

Hyperspectral optical coherence tomography for in vivo visualization of melanin in the retinal pigment epithelium.

Previous studies for melanin visualization in the retinal pigment epithelium (RPE) have exploited either its absorption properties (using photoacoustic tomography or photothermal optical coherence tomography [OCT]) or its depolarization properties (using polarization sensitive OCT). However, these methods are only suitable when the melanin concentration is sufficiently high. In this work, we present the concept of hyperspectral OCT for melanin visualization in the RPE when the concentration is low. Based on white light OCT, a hyperspectral stack of 27 wavelengths (440-700 nm) was created in post-processing for each depth-resolved image. Owing to the size and shape of the melanin granules in the RPE, the variations in backscattering coefficient as a function of wavelength could be identified-a result which is to be expected from Mie theory. This effect was successfully identified both in eumelanin-containing phantoms and in vivo in the low-concentration Brown Norway rat RPE.

Beyond backscattering: optical neuroimaging by BRAD.

Optical coherence tomography (OCT) is a powerful technology for rapid volumetric imaging in biomedicine. The bright field imaging approach of conventional OCT systems is based on the detection of directly backscattered light, thereby waiving the wealth of information contained in the angular scattering distribution. Here we demonstrate that the unique features of few-mode fibers (FMF) enable simultaneous bright and dark field (BRAD) imaging for OCT. As backscattered light is picked up by the different modes of a FMF depending upon the angular scattering pattern, we obtain access to the directional scattering signatures of different tissues by decoupling illumination and detection paths. We exploit the distinct modal propagation properties of the FMF in concert with the long coherence lengths provided by modern wavelength-swept lasers to achieve multiplexing of the different modal responses into a combined OCT tomogram. We demonstrate BRAD sensing for distinguishing differently sized microparticles and showcase the performance of BRAD-OCT imaging with enhanced contrast for ex vivo tumorous tissue in glioblastoma and neuritic plaques in Alzheimer's disease.

Image-Based Chemical Structure Determination.

Chemical imaging is a powerful tool for understanding the chemical composition and nature of heterogeneous samples. Recent developments in elemental, vibrational, and mass-spectrometric chemical imaging with high spatial resolution (50-200 nm) and reasonable timescale (a few hours) are capable of providing complementary chemical information about various samples. However, a single technique is insufficient to provide a comprehensive understanding of chemically complex materials. For bulk samples, the combination of different analytical methods and the application of statistical methods for extracting correlated information across different techniques is a well-established and powerful concept. However, combined multivariate analytics of chemical images obtained via different imaging techniques is still in its infancy, hampered by a lack of analytical methodologies for data fusion and analysis. This study demonstrates the application of multivariate statistics to chemical images taken from the same sample via various methods to assist in chemical structure determination.

Planar, Polysilazane-Derived Porous Ceramic Supports for Membrane and Catalysis Applications.

Porous, silicon carbonitride-based ceramic support structures for potential membrane and catalysis applications were generated from a preceramic polysilazane precursor in combination with spherical, ultrahigh-molecular weight polyethylene microparticles through a sacrificial filler approach. A screening evaluation was used for the determination of the impact of both porogen content and porogen size on pore structure, strength, and permeability characteristics of planar specimens. By optimizing both the composition as well as cross-linking parameters, maximum characteristic biaxial flexural strengths of 65 MPa and porosities of 42% were achieved. The evolution of an interconnected, open-pore network during thermal porogen removal and conversion of the preceramic polymer led to air permeabilities in the order of 10-14 m2. The materials were further exposed to long-term heat treatments to demonstrate the stability of properties after 100 h at 800°C in oxidizing, inert, and reducing environments. The determined performance, in combination with the versatile preparation method, illustrates the feasibility of this processing approach for the generation of porous ceramic support structures for applications at elevated temperatures in a variety of fields, including membrane and catalysis science.

A novel processing approach for free-standing porous non-oxide ceramic supports from polycarbosilane and polysilazane precursors.

In this contribution, a low-pressure/low-temperature casting technique for the preparation of novel free-standing macrocellular polymer-derived ceramic support structures is presented. Preceramic polymers (polycarbosilane and poly(vinyl)silazane) are combined with sacrificial porogens (ultra-high molecular weight polyethylene microbeads) to yield porous ceramic materials in the Si-C or Si-C-N systems, exhibiting well-defined pore structures after thermal conversion. The planar-disc-type specimens were found to exhibit biaxial flexural strengths of up to 60 MPa. In combination with their observed permeability characteristics, the prepared structures were found to be suitable for potential applications in filtration, catalysis, or membrane science.

Polarization sensitive optical coherence tomography of melanin provides intrinsic contrast based on depolarization.

Polarization sensitive optical coherence tomography (PS-OCT) is a functional extension of OCT. In addition to imaging based on tissue reflectivity, PS-OCT also enables depth-resolved mapping of sample polarization properties such as phase-retardation, birefringent axis orientation, Stokes vectors, and degree of polarization uniformity (DOPU). In this study, PS-OCT was used to investigate the polarization properties of melanin. In-vitro measurements in samples with varying melanin concentrations revealed polarization scrambling, i.e. depolarization of backscattered light. Polarization scrambling in the PS-OCT images was more pronounced for higher melanin concentrations and correlated with the concentration of the melanin granules in the phantoms. Moreover, in-vivo PS-OCT was performed in the retinas of normal subjects and individuals with albinism. Unlike in the normal eye, polarization scrambling in the retinal pigment epithelium (RPE) was less pronounced or even not observable in PS-OCT images of albinos. These results indicate that the depolarizing appearance of pigmented structures like, for instance, the RPE is likely to be caused by the melanin granules contained in these cells.