Visualization by means of coherent light sheets: application to different flows

Light-sheet generator systems using a sweeping, focused laser beam and spherocylindrical optical components are described and their respective performances discussed. They allow the visualization of supersonic air flows by means of light scattering. Tomographic recordings give a three-dimensional reconstruction of the flow (especially the stationary structures) and can be realized in real-time.     

The water/graphitic-carbon interaction energy

The water/graphitic-carbon interaction energy was obtained for a sample having a water surface site adsorption density of 13.3 μmol m⁻². The interaction energy was determined from the spreading pressure of water, its surface tension and the water contact angle and using a formula obtained by the combination of the Young equation with a general equation of pair interaction. The values obtained for contact angles 42° and 86° are 7.63 and 7.18 kJ mol⁻¹ of water are similar to the water binding energies obtained from molecular dynamic simulations of water droplets on a graphite surface: 6.7-8.33 kJ mol⁻¹.     

Surface molecular imprinting on g-C3N4 photooxidative nanozyme for improved colorimetric biosensing

Graphitic carbon nitride (g-C₃N₄), as a visible-light-active organic semiconductor, has attracted growing attentions in photocatalysis and photoluminescence-based biosensing. Here, we demonstrated the intrinsic photooxidase activity of g-C₃N₄ and then surface molecular imprinting on g-C₃N₄ nanozymes was achieved for improved biosensing. Upon blue LED irradiation, the g-C₃N₄ exhibited superior enzymatic activity for oxidation of chromogenic substrate like 3,3′,5,5′-tetramethylbenzidine (TMB) without destructive H₂O₂. The oxidation was mainly ascribed to O₂⁻ that was generated during light irradiation. The surface molecular imprinting on g-C₃N₄ can lead to an over 1000-fold alleviation in matrix-interference from serum samples, 4-fold improved enzymatic activity as well as enhanced substrate specificity comparing with bare g-C₃N₄ during colorimetric sensing. Also, the MIP-g-C₃N₄ possesses a high affinity to TMB with a K_m value of only 22 μmol/L, much lower than other comment nanozymes like AuNPs, Fe₃O₄ NPs, etc. It was successfully applied for detection of cysteine in serum sample with satisfactory recoveries.     

Microstructural and Dielectric Characterization of Sol-Gel Derived Silicon Oxycarbide Glass Sheets

Thin (50—1000 m) silicon oxycarbide glass sheets were synthesized by the pyrolysis of gel sheets obtained from a methyl-modified silica sol containing colloidal silica under inert atmosphere between 900 and 1450°C. The microstructure of these glass sheets was investigated with the help of high resolution scanning and transmission electron microscopy (HR-SEM and HR-TEM), X-ray diffraction and Raman spectroscopy and their dielectric properties were determined. The surface morphology as observed with HR-SEM did not exhibit a notable temperature dependence. HR-TEM studies showed that the glass sheets sintered up to 1200°C are amorphous, whereas those sintered at 1450°C contain uniformly dispersed crystallites of SiC and graphite. X-ray diffraction studies were found in agreement with the HR-TEM results. Raman spectroscopy showed that free carbon is present as an amorphous phase till a temperature of 1000°C, whereas at temperatures 1200°C, the presence of graphitic carbon was observed. Silicon oxycarbide glass sheets heat treated at temperatures up to 1200°C, showed a dielectric constant between 4.1 ± 0.11 and 4.6 ± 0.15 in the frequency range from 75 kHz to 5 MHz, with corresponding losses between 0.0008 and 0.1100. Such silicon oxycarbide glass sheets sintered at 1200°C could find an application as substrates for electronic packaging.     

Static and Dynamic Nanosheets from Selective Assembly of Geometric Macrocycle Isomers

In contrast to the significant advances that have been made in the construction of two‐dimensional (2D) nanostructures, the rational modification from static to dynamic 2D sheets remains a great challenge. Static and dynamic sheets formed from selective self‐assembly of geometric macrocycle isomers based on anthracene units are presented. The self‐assembly of the cis isomer generates static planar sheets, whereas the trans isomer forms dynamic rolled sheets which are reversibly unrolled upon stimulation by a thermal signal. Furthermore, the mixed solution of the two isomers exhibits self‐sorting behavior, generating the coexistence of the two independent self‐assembled structures, the planar sheets and the folded scrolls. The self‐sorted supramolecular objects with considerable shape and size differences are able to be readily separated, one isomer from the other.     

Almost optimal convergence of the point vortex method for vortex sheets using numerical filtering

Standard numerical methods for the Birkhoff-Rott equation for a vortex sheet are unstable due to the amplification of roundoff error by the Kelvin-Helmholtz instability. A nonlinear filtering method was used by Krasny to eliminate this spurious growth of round-off error and accurately compute the Birkhoff-Rott solution essentially up to the time it becomes singular. In this paper convergence is proved for the discretized Birkhoff-Rott equation with Krasny filtering and simulated roundoff error. The convergence is proved for a time almost up to the singularity time of the continuous solution. The proof is in an analytic function class and uses a discrete form of the abstract Cauchy-Kowalewski theorem. In order for the proof to work almost up to the singularity time, the linear and nonlinear parts of the equation, as well as the effects of Krasny filtering, are precisely estimated. The technique of proof applies directly to other ill-posed problems such as Rayleigh-Taylor unstable interfaces in incompressible, inviscid, and irrotational fluids, as well as to Saffman-Taylor unstable interfaces in Hele-Shaw cells.

     

Physical properties of silicone foams filled with carbon nanotubes and functionalized graphene sheets

Free-rising silicone foams were made with loading fractions of up to 0.25 wt.-% functionalized graphene sheets (FGS) and up to 1.0 wt.-% carbon nanotubes (CNTs) using hydrogen as blowing agent. Scanning electron microscopy of the samples revealed an open cellular structure and a homogeneous dispersion of both types of nanofillers. The incorporation of nanofiller affected the foaming process and thus the final foam density and cellular structure. Transmission electron microscopy revealed the formation of a CNT network throughout the sample, while FGS presented an exfoliated and intercalated dispersion. The thermal stability of the samples was drastically affected by the presence of both nanofillers. Both nanofillers showed a positive effect on the compressive response of the foams. However, the nanocomposite foams were found to decrease the acoustic absorption with nanofiller content probably due to the variable foam structure and improved stiffness.     

Supramolecular bidentate ligands by metal-directed in situ formation of antiparallel beta-sheet structures and application in asymmetric catalysis

The principles of protein structure design, molecular recognition, and supramolecular and combinatorial chemistry have been applied to develop a convergent metal-ion-assisted self-assembly approach that is a very simple and effective method for the de novo design and the construction of topologically predetermined antiparallel beta-sheet structures and self-assembled catalysts. A new concept of in situ generation of bidentate P-ligands for transition-metal catalysis, in which two complementary, monodentate, peptide-based ligands are brought together by employing peptide secondary structure motif as constructing tool to direct the self-assembly process, is achieved through formation of stable beta-sheet motifs and subsequent control of selectivity. The supramolecular structures were studied by (1)H, (31)P, and (13)C NMR spectroscopy, ESI mass spectrometry, X-ray structure analysis, and theoretical calculations. Our initial catalysis results confirm the close relationship between the self-assembled sheet conformations and the catalytic activity of these metallopeptides in the asymmetric rhodium-catalyzed hydroformylation. Good catalyst activity and moderate enantioselectivity were observed for the selected combination of catalyst and substrate, but most importantly the concept of this new methodology was successfully proven. This work presents a perspective interface between protein design and supramolecular catalysis for the design of beta-sheet mimetics and screening of libraries of self-organizing supramolecular catalysts.     

Folding control in cyclic peptides through N-methylation pattern selection: formation of antiparallel beta-sheet dimers, double reverse turns and supramolecular helices by 3alpha,gamma cyclic peptides

Peptide foldamers constitute a growing class of nanomaterials with potential applications in a wide variety of chemical, medical and technological fields. Here we describe the preparation and structural characteristics of a new class of cyclic peptide foldamers (3alpha,gamma-CPs) that, depending on their backbone N-methylation patterns and the medium, can either remain as flat rings that dimerize through arrays of hydrogen bonds of antiparallel beta-sheet type, or can fold into twisted double reverse turns that, in the case of double gamma-turns, associate in nonpolar solvents to form helical supramolecular structures. A 3alpha,gamma-CP consists of a number of multiples of a repeat unit made up of four amino acid residues of alternating chirality: three corresponding to alpha-amino acids and one to a gamma-amino acid (a cis-3-aminocycloalkanecarboxylic acid).