A foldamer-based chiroptical molecular switch that displays complete inversion of the helical sense upon anion binding

Chiral indolocarbazole dimers fold into a helical conformation by virtue of intramolecular hydrogen bonds, as demonstrated by (1)H NMR and CD spectra and optical rotations. In particular, the optical properties of the dimers were found to be extremely sensitive to the nature of the solvent, depending on whether they are folded or not. The helical sense of the dimers can be reversibly switched by binding sulfate ion, which gives rise to complete inversion of the CD spectra. The binding mode and absolute stereochemistry of the sulfate complexes was unequivocally determined by single-crystal X-ray structures, which are all consistent with the CD and (1)H NMR spectra in solution.     

Abiotic reduction of uranium by mackinawite (FeS) biogenerated under sulfate-reducing condition

Sulfate-reducing bacteria and their by-products, such as iron sulfides, are widely distributed in groundwater and sediments, and can affect subsurface aqueous chemistry. Here we show the catalytic reduction of hexavalent uranium by FeS particles, which were largely generated by the activities of Desulfovibrio desulfuricans and D. vulgaris in anaerobic condition. Characterization of FeS particles by X-ray diffraction and high-resolution transmission electron microscopy revealed the presence of mackinawite having thin and flexible platy sheets with 0.5-nm lamellar spacing. This biogenic phase mediated abiotic reduction of U(VI) to U(IV) which was confirmed by UV–Vis absorption spectroscopy. The U conversion occurred through surface catalysis that involved adsorption of aqueous U(VI)–carbonate complexes (predominantly UO₂(CO₃) ₃ ^4? ) onto the mackinawite, but the transformed uranium was then released and remained in suspended form in the solution phase. This surface catalysis and subsequent U(IV) remobilization has not been reported as a pathway to occur under sulfate-reducing conditions. Our results suggest that the iron sulfide solid, which is characteristic of conductive property, is very sensitive and variable depending on the electron supplying and transferring environment, negatively affecting the surface uranium to be strongly stabilized and fixed on the FeS surface.     

Sorption of selenite onto chlorite considering mineral dissolution

In this study we investigated the sorption of selenite (SeO₃ ^2?) onto chlorite as a function of Se(IV) concentration, pH, and ionic strength. The sorption isotherm of Se(IV) onto chlorite was successfully presented by both the Langmuir isotherm and Tempkin equation although the Langmuir isotherm is somewhat better than the Tempkin equation. The sorption of Se(IV) onto chlorite was maintained to be constant at an acidic pH region, while the sorption decreased with an increasing pH at neutral and alkaline pH regions. However, the Se(IV) sorption onto chlorite was independent of the ionic strength of NaClO₄ solution. The amount of Se(IV) sorbed onto chlorite was significantly low compared to those of iron oxides such as apatite, goethite, hematite, and magnetite because of the lower content of Fe. We also investigated the effect of Fe(II) ions dissolved from chlorite on the Se(IV) sorption as a function of contact time. The chemical oxidation states of selenium sorbed onto chlorite surface were identified using X-ray absorption near edge structure (XANES) at the Pohang synchrotron light source. The amount of Fe(II) dissolved was increased by the contact time of 28 days but decreased after 28–56 days although the amount of dissolved Fe(II) ions was significantly small. This decrease of the dissolved Fe(II) may be due to the formation of Fe-oxyhydroxides such as ferrihydrite. The results of XANES measurements also showed that the Se(IV) sorbed onto chlorite was not reduced into Se(0) or Se(-II) even in the presence of Fe(II) ions in the solution because of the low Fe content of the chlorite although the mechanism was not clearly understood.     

Tilt angle change of nematic liquid crystal as a function of the number of graphite layers

The tilt angle of a nematic liquid crystal on a graphite flake was observed to change with increasing numbers of graphite layers. A portion of the substrate that induced homeotropic alignment was covered with graphite flakes, which induced a planar alignment. Nematic liquid crystals placed on the graphite deviated from vertical orientation to the polar angle. The angle of deviation appeared to be proportional to the number of layers and reached a limit, with almost planar alignment, at about 7–8 graphite layers. Although the main contributing factor to the tilt angle change was considered to be the result of van der Waals forces, it was seen that other long-range interaction forces needed to be considered to explain the experimental results obtained.     

Finely Formed,Kinetically Modulated Wrinkle Structures in UV‐Crosslinkable Liquid Prepolymers

Special characteristics of wrinkles such as a scattering source and a high surface area are finding use in high‐tech applications. UV‐crosslinkable prepolymers are occasionally used for fabricating wrinkled films. Wavelength of the wrinkles formed from the prepolymers is several tens and hundreds of micrometers. Here, a UV‐crosslinkable liquid prepolymer is synthesized to spontaneously form wrinkle structures in the order of several micrometers. Double layers with a very thin hard skin and a soft and contractible foundation are formed at the same time, by ensuring that all the absorbance wavelengths of the photoinitiator are shorter than the minimum wavelength at which the prepolymer is transparent. The rate of photo‐crosslinking reaction, R_p, is also found to affect the thickness of the skin and foundation layers at the early UV‐curing stage. The first‐order apparent rate constant, k_app, is between ≈0.20 and ≈0.69 s⁻¹ for the wrinkle formation. This wrinkle structures can be simply modulated by changing R_p.

     

Hydrothermal fabrication of well-ordered ZnO nanowire arrays on Zn foil: room temperature ultraviolet nanolasers

Well-ordered nanowires of the hexagonal wurtzite ZnO having an average diameter of 80 nm, a typical length of 12 μm, and a mean packing density of 7.5 nanowires μm⁻² have been directly grown on Zn foil in a preferred [0001] direction by a hydrothermal process and employed for room temperature ultraviolet nanolasers. The lasing action of arrayed ZnO nanowires has been observed from 370 to 400 nm with threshold irradiance of 25 kW cm⁻². Photoluminescence decays biexponentially: the fast component is attributed to free-exciton decay, and the slow one is to bound-exciton decay. The amplitude of the fast component increases whereas its lifetime decreases with the increment of threshold irradiance, suggesting that ZnO nanowire arrays undergo a change in the lasing mechanism from exciton–exciton scattering to electron–hole plasma recombination.     

Finite element analysis of quasistatic crack propagation in brittle media with voids or inclusions

A three-level finite element scheme is proposed for simulation of crack propagation in heterogeneous media including randomly distributed voids or inclusions. To reduce total degrees of freedom in the view of mesh gradation, the entire domain is categorized into three regions of different-level meshes: a region of coarse-level mesh, a region of intermediate-level mesh, and a region of fine-level mesh. The region of coarse-level mesh is chosen to be far from the crack to treat the material inhomogeneities in the sense of coarse-graining through homogenization, while the region near the crack is composed of the intermediate-level mesh to model the presence of inhomogeneities in detail. Furthermore, the region very near the crack tip is refined into the fine-level mesh to capture a steep gradient of elastic field due to the crack tip singularity. Variable-node finite elements are employed to satisfy the nodal connectivity and compatibility between the neighboring different-level meshes. Local remeshing is needed for readjustment of mesh near the crack tip in accordance with crack growth, and this is automatically made according to preset values of parameters determining the propagation step size of crack, and so the entire process is fully automatic. The effectiveness of the proposed scheme is demonstrated through several numerical examples. Meanwhile, the effect of voids and inclusions on the crack propagation is discussed in terms of T-stresses, with the aid of three-level adaptive scheme.