Electrochemical properties of CdSe and CdTe quantum dots

Semiconductor nanocrystal quantum dots (QDs), owing to their unique opto-electronic properties determined by quantum confinement effects, have been the subject of extensive investigations in different areas of science and technology in the past two decades. The electrochemical behaviour of QDs, particularly for CdSe and CdTe nanocrystals, has also been explored, although to a lesser extent compared to the optical properties. Voltammetric measurements can be used to probe the redox levels available for the nanocrystals, which is an invaluable piece of information if these systems are involved in electron transfer processes. Electrochemical data can also foster the interpretation of the spectroscopic properties of QDs, and give insightful information on their chemical composition, dimension, and surface properties. Hence, electrochemical methods constitute in principle an effective tool to probe the quality of QD samples in terms of purity, size dispersion, and surface defects. The scope of this critical review is to discuss the results of electrochemical studies carried out on CdSe and CdTe core and core-shell semiconductor nanocrystals of spherical shape. Examples of emerging or potential applications that exploit electroactive quantum dot-based systems will also be illustrated.     

Imino‐ and Azasugar Protonation Inside Human Acid β‐Glucosidase,the Enzyme that is Defective in Gaucher Disease

Gaucher disease is caused by mutations in human acid β‐glucosidase or glucocerebrosidase (GCase), the enzyme responsible for hydrolysis of glucosyl ceramide in the lysosomes. Imino‐ and azasugars such as 1‐deoxynojirimycin and isofagomine are strong inhibitors of the enzyme and are of interest in pharmacological chaperone therapy of the disease. Despite several crystal structures of the enzyme with the imino‐ and azasugars bound in the active site having been resolved, the actual acid–base chemistry of the binding is not known. In this study we show, using photoinduced electron transfer (PET), that 1‐deoxynojirimycin and isofagomine derivatives are protonated by human acid β‐glucosidase when bound, even if they are completely unprotonated outside the enzyme. While isofagomine derivative protonation to some degree was foreshadowed by earlier crystal structures, 1‐deoxynojirimycin derivatives were not believed to act as basic amines in the enzyme.     

ANCUT2, a Thermo-alkaline Cutinase from <Emphasis Type="Italic">Aspergillus nidulans</Emphasis> and Its Potential Applications

Biochemical characterization of purified ANCUT2 cutinase from Aspergillus nidulans is described. The identified amino acid sequence differs from that predicted in Aspergillus genomic databases in amino acids not relevant for catalysis. The enzyme is thermo-alkaline, showing its maximum activity at pH 9 and 60 °C, and it retains more than 60% of its initial activity after incubation for 1 h at 60 °C for pH values between 6 and 10. ANCUT2 is more active towards long-chain esters and it hydrolyzes cutin; however, it also hydrolyzes short-chain esters. Cutinase is inhibited by metal ions, PMSF, SDS, and EDTA (10 mM). It retains 50% of its activity in most of the solvents tested, although it is more stable in hydrophobic solvents. According to its found biochemical properties, preliminary assays demonstrate its ability to synthesize methyl esters from sesame oil and the most likely application of this enzyme remains in detergent formulations.     

Extraction and characterization of cellulose nanowhiskers from <Emphasis Type="Italic">Mandacaru</Emphasis> (<Emphasis Type="Italic">Cereus jamacaru</Emphasis> DC.) spines

Cellulose nanowhiskers were extracted from the spines of Mandacaru (Cereus jamacaru DC.), a cactus native to the Caatinga biome of northeastern Brazil, using sulfuric acid hydrolysis preceded by alkaline treatment and bleaching. Results showed that three bleaching steps were required to remove most of the non-cellulosic constituents of the spines that yielded 77.4% cellulose. Nanowhiskers size decreased from about 400 to 260 nm when extraction time varied from 60 to 120 min, this was also evidenced by X-ray diffraction. Alkaline treated and bleached samples had lower thermal stability as compared to untreated spines due to removal of lignin and increased surface area. The amount of time samples were treated with sulfuric acid influenced the thermal stability and consequently the degree of crystallinity of the nanowhiskers. Cellulose nanowhiskers were obtained from Mandacaru spines providing a new renewable source of reinforcement with potential applications in nanocomposites.     

Evaluation of Strategies to Improve the Production of Alkaline Protease PrtA from Aspergillus nidulans

Aspergillus nidulans produces several proteases. The prtA gene encodes a major protease, and two approaches were explored to achieve the overproduction of this enzyme. Molecular cloning of the mature form of this enzyme in Pichia pastoris resulted in the production of an inactive form. In addition, the presence of this enzyme was toxic for the host and resulted in cell lysis. The modification of the culture medium constituents resulted in a 6.4-fold increase in enzyme production. The main effect was achieved through the use of organic nitrogen sources. Although it was previously shown that the PrtA protease shows promiscuous esterase activity, the production of this enzyme was not induced by lipidic sources.     

A characterization of linearly semisimple groups

Let G = SpecA be an affine K-group scheme and à = {w ∈ A*: dim _K A*·w < ∞, dim _K w· A* < ∞}. Let 〈?,?〉: A* × Ã → K, 〈w, \(\tilde w\)〉:=tr(w~w), be the trace form. We prove that G is linearly reductive if and only if the trace form is non-degenerate on A*.     

Estimation of the Defect Status on Visual Field Longitudinal Data

In this paper, we show the use of Multivariate Time Series models, Markov Random Fields and Bayesian methodologies to solve an applied ophthalmological problem related to the study of glaucoma. Glaucoma is a very serious and widely extended eye disease characterized by a gradual decrease in the intensity of the patient’s sight. It is not, however, homogeneous over all the visual field, and starts at one or several sites and gradually spreads to nearby sites. Measurement of the patient’s “seeing threshold” at different points in the visual field is an important diagnostic tool for glaucoma and other diseases. It results in a map with 52 numerical values, each of which represents the level of intensity perceived by the patient at that site, and ranges from 0 (complete blindness) to 35 (exceptional vision). Additionally a “defect status” variable can be attached at each site in the visual field. This variable would indicate whether the site is normal or defective. Using Bayesian methodologies, the “defect status” process can be regarded as a parameter of the probability distribution of the thresholds and can be estimated as the maximum of its posterior distribution. The stochastic model assumed for the observed “threshold”, given the “defect status”, is a first order autoregressive integrated model (VARI(1,1)) in time, with first order homogeneous spatial correlation. The defect status is modeled by using a Spatiotemporal Autologistic Model with non-homogeneous spatial dependence. This dependence assumes that the propagation of the lesions follows the directions taken by the nerve fibers. MCMC methods are used to jointly estimate the defect status, and parameters and hyperparameters of the model.     

Ending Aging in Super Glassy Polymer Membranes

Aging in super glassy polymers such as poly(trimethylsilylpropyne) (PTMSP), poly(4‐methyl‐2‐pentyne) (PMP), and polymers with intrinsic microporosity (PIM‐1) reduces gas permeabilities and limits their application as gas‐separation membranes. While super glassy polymers are initially very porous, and ultra‐permeable, they quickly pack into a denser phase becoming less porous and permeable. This age‐old problem has been solved by adding an ultraporous additive that maintains the low density, porous, initial stage of super glassy polymers through absorbing a portion of the polymer chains within its pores thereby holding the chains in their open position. This result is the first time that aging in super glassy polymers is inhibited whilst maintaining enhanced CO₂ permeability for one year and improving CO₂/N₂ selectivity. This approach could allow super glassy polymers to be revisited for commercial application in gas separations.     

On the electrical conductivity of PVDF composites with different carbon-based nanoadditives

Composites based on poly(vinylidene fluoride) (PVDF) and different carbon additives, such as carbon nanofibers (CNF), graphite (G), expanded graphite (EG), and single-walled carbon nanotubes (SWCNT) have been prepared by nonsolvent precipitation, from solution, and subsequent melt processing. From a structural point of view, the α-crystal phase is the predominant crystal form in all the nanocomposites. However, those containing CNF, G, and EG at high nanoadditive content present also β-crystal phase. Even though the intrinsic thermal properties of PVDF are hardly affected, the nanoadditives act as nucleating agents for the crystallization. In regard to the electrical properties, all nanocomposites exhibit a percolating behavior. Moreover, the fact that the nanocomposites present both high dc conductivity and high dielectric constant, in a certain nanoadditive concentration range below the percolation threshold, suggests that a tunneling conduction mechanism for charge transport is present. With regard to the ac electrical properties, depending on the morphology of the different additives, the charge transport above percolation threshold can be explained taking into account the anomalous diffusion effect for high nanoadditive content or an intercluster polarization mechanism when the nanoadditive concentration decreases.