A New Cyclic Bisdesmoside from Tubers of Bolbostemma paniculatum

The structure of tubeimoside V (1), a new cyclic bisdesmoside, isolated from tubers of Bolbostemma paniculatum (Tu Bei Mu), was established by means of 2D NMR spectral and chemical methods. Compound 1 has inter-saccharide chain bridging by a dicrotalic acid to form aunique macrocyclic structure.     

Process Parameters Optimization,Characterization, and Application of KOH-Activated Norway Spruce Bark Graphitic Biochars for Efficient Azo Dye Adsorption

In this work, Norway spruce bark was used as a precursor to prepare activated biochars (BCs) via chemical activation with potassium hydroxide (KOH) as a chemical activator. A Box–Behnken design (BBD) was conducted to evaluate and identify the optimal conditions to reach high specific surface area and high mass yield of BC samples. The studied BC preparation parameters and their levels were as follows: pyrolysis temperature (700, 800, and 900 °C), holding time (1, 2, and 3 h), and ratio of the biomass: chemical activator of 1: 1, 1.5, and 2. The planned BBD yielded BC with extremely high SSA values, up to 2209 m²·g⁻¹. In addition, the BCs were physiochemically characterized, and the results indicated that the BCs exhibited disordered carbon structures and presented a high quantity of O-bearing functional groups on their surfaces, which might improve their adsorption performance towards organic pollutant removal. The BC with the highest SSA value was then employed as an adsorbent to remove Evans blue dye (EB) and colorful effluents. The kinetic study followed a general-order (GO) model, as the most suitable model to describe the experimental data, while the Redlich–Peterson model fitted the equilibrium data better. The EB adsorption capacity was 396.1 mg·g⁻¹. The employment of the BC in the treatment of synthetic effluents, with several dyes and other organic and inorganic compounds, returned a high percentage of removal degree up to 87.7%. Desorption and cyclability tests showed that the biochar can be efficiently regenerated, maintaining an adsorption capacity of 75% after 4 adsorption–desorption cycles. The results of this work pointed out that Norway spruce bark indeed is a promising precursor for producing biochars with very promising properties.     

Dioxygenase-catalysed cis-dihydroxylation of meta-substituted phenols to yield cyclohexenone cis-diol and derived enantiopure cis-triol metabolites

cis-Dihydroxylation of meta-substituted phenol (m-phenol) substrates, to yield the corresponding cyclohexenone cis-diol metabolites, was catalysed by arene dioxygenases present in mutant and recombinant bacterial strains. The presence of cyclohexenone cis-diol metabolites and several of their cyclohexene and cyclohexane cis-triol derivatives was detected by LC-TOFMS analysis and confirmed by NMR spectroscopy. Structural and stereochemical analyses of chiral ketodiol bioproducts, was carried out using NMR and CD spectroscopy and stereochemical correlation methods. The formation of enantiopure cyclohexenone cis-diol metabolites is discussed in the context of postulated binding interactions of the m-phenol substrates at the active site of toluene dioxygenase (TDO).     

Development and trends in synchrotron studies of ancient and historical materials

Synchrotron photon-based methods are increasingly being used for the physico-chemical study of ancient and historical materials (archaeology, palaeontology, conservation sciences, palaeo-environments). In particular, parameters such as the high photon flux, the small source size and the low divergence attained at the synchrotron make it a very efficient source for a range of advanced spectroscopy and imaging techniques, adapted to the heterogeneity and great complexity of the materials under study. The continuous tunability of the source — its very extended energy distribution over wide energy domains (meV to keV) with a high intensity — is an essential parameter for techniques based on a very fine tuning of the probing energy to reach high chemical sensitivity such as XANES, EXAFS, STXM, UV/VIS spectrometry, etc. The small source size attained (a few micrometres) at least in the vertical plane leads to spatial coherence of the photon beams, giving rise in turn to a series of imaging methods already crucial to the field. This review of the existing literature shows that microfocused hard X-ray spectroscopy (absorption, fluorescence, diffraction), full-field X-ray tomography and infrared spectroscopy are the leading synchrotron techniques in the field, and presents illustrative examples of the study of ancient and historical materials for the various methods. Fast developing analytical modalities in scanning spectroscopy (STXM, macro-XRF scanning) and novel analytical strategies regarding optics, detectors and other instrumental developments are expected to provide major contributions in the years to come. Other energy domains are increasingly being used or considered such as far-infrared and ultraviolet/visible for spectroscopy and imaging. We discuss the main instrumental developments and perspectives, and their impact for the science being made on ancient materials using synchrotron techniques.     

Exciton photoluminescence from ZnO layers produced by laser-induced gas breakdown processing

The plasma of optically-excited gas breakdown has been used to treat a Zn target in atmospheric pressure gases (air, O₂, N₂, Ar). The breakdown is produced near the target by a pulsed CO₂ laser radiation, yielding to a local erosion of the target under the irradiation spot and the formation of a porous nanostructured layer, consisting of ZnO nanoscale spheres. We show that the produced nanostructured layers exhibit an intense exciton emission band in the ultraviolet range (380–385 nm), while defect-related photoluminescent bands were weak and could be completely removed by varying the fabrication parameters. Properties of the produced layers were found to be very promising for the development of optoelectronic devices. PACS 81.16.Mk; 81.05.-t     

Investigation of perceptual constancy in the temporal-envelope domain

The ability to discriminate complex temporal envelope patterns submitted to temporal compression or expansion was assessed in normal-hearing listeners. An XAB, matching-to-sample-procedure was used. X, the reference stimulus, is obtained by applying the sum of two, inharmonically related, sinusoids to a broadband noise carrier. A and B are obtained by multiplying the frequency of each modulation component of X by the same time expansion/compression factor, alpha (alphain[0.35-2.83]). For each trial, A or B is a time-reversed rendering of X, and the listeners' task is to choose which of the two is matched by X. Overall, the results indicate that discrimination performance degrades for increasing amounts of time expansion/compression (i.e., when alpha departs from 1), regardless of the frequency spacing of modulation components and the peak-to-trough ratio of the complex envelopes. An auditory model based on envelope extraction followed by a memory-limited, template-matching process accounted for results obtained without time scaling of stimuli, but generally underestimated discrimination ability with either time expansion or compression, especially with the longer stimulus durations. This result is consistent with partial or incomplete perceptual normalization of envelope patterns.     

The nature of the predominant acceptors in high quality zinc telluride

We report very sharp bound exciton luminescence spectra in high quality melt-grown very lightly compensated ZnTe, p-type with N_A-N_D in the low 10⁺¹⁵ cm^-3. Bound exciton localisation energies at seven shallow neutral acceptors with E_A between ~55 and ~150 meV are very insensitive to E_A. Optical absorption and dye laser luminescence excitation spectroscopy were necessary to obtain a full separation of the transitions due to different acceptors, together with a study of certain ‘two-hole’ luminescence satellites in which the acceptor is left in a series of orbital states after bound exciton decay. Two shallow acceptors are P_Te and As_Te, a third possibly Li_Zn while a fourth, relatively prominent in our best undoped crystals, may be a complex. A deeper, 150 meV acceptor, frequently reported in the ZnTe literature and electrically dominant in most of our undoped crystals has the Zeeman character of a point defect. We present clear evidence from our spectra that this energy does not represent the binding of a single hole at a doubly ionized cation vacancy, a popular attribution since 1963. This acceptor may be covered by another impurity, possibly Cu_Zn. We also report bound phonon effects, lifetime broadening of excited bound exciton states and observe a single unidentified donor with E_D ~18.5 meV. This energy is determined using selective dye laser excitation at the weak neutral donor bound exciton line and from the onset of valence band to ionized donor photo-absorption.     

Size dependent photoluminescence from Si nanoclusters produced by laser ablation

We report a photoluminescence study of silicon nanoclusters deposited by laser ablation. This technique allows to obtain clusters with a reduced size dispersion. Taking advantage of that as well as of the possibility to tune selectively the cluster size relatively to the preparation parameters, we are able to make a fine correlation between the emission band and the corresponding emitting size. The photoluminescence band can be tuned in a wide spectral region spreading from the near ultraviolet to the near infrared depending on preparation conditions. The correlation with atomic force microscopy measurements provides a size dependence of the luminescence which is fully consistent with the quantum confinement interpretation. To our knowledge, we present here the first observation of a wide spectral range tuning of the emission bands of nanocrystalline silicon by using a dry technique compatible with clean vacuum processing.     

Improving impossible-differential attacks against Rijndael-160 and Rijndael-224

Impossible-differential attacks are a very efficient form of cryptanalysis against block ciphers. In this paper, we improve the existing impossible-differential attacks against Rijndael-160 and Rijndael-224.