Electrocatalytic Behavior of Glassy Carbon Electrodes Modified with Multiwalled Carbon Nanotubes and Cobalt Phthalocyanine for Selective Analysis of Dopamine in Presence of Ascorbic Acid

This work describes the development, electrochemical characterization and utilization of a cobalt phthalocyanine modified carbon nanotube electrode for the quantitative determination of dopamine in 0.2 mol L^?1 phosphate buffer contaminated with high concentration of ascorbic acid. The electrode surface was analyzed by cyclic voltammetry and electrochemical impedance spectroscopy which showed a modified surface presenting a charge transfer resistance of 500 Ω, against the 16.46 kΩ value found for the bare glassy carbon surface. A pseudo rate constant value of 5.4×10^?4 cm s^?1 for dopamine oxidation was calculated. Voltammetric experiments showed a shift of the peak potential of DA oxidation to less positive value at 390 mV as compared with that of a bare GC electrode at 570 mV. The electrochemical determination of dopamine, in presence of ascorbic acid in concentrations up to 0.1 mol L^?1 by differential pulse voltammetry, yielded a detection limit as low as 2.56×10^?7 mol L^?1.     

Investigation of 13CH3OH methanol: new laser lines and assignments

We have reinvestigated ¹³CH₃OH as a source of far-infrared (FIR) laser emission using a CO₂ laser as a pumping source. Thirty new FIR laser lines in the range 36.5 μm to 202.6 μm were observed and characterized. Five of them have wavelengths between 36.5 and 75 μm and have sufficient intensity to be used in LMR spectroscopy. Using Fourier-transform spectroscopic data in the infrared (IR) and FIR regions we have determined the assignment for 10 FIR laser transitions and predict nine frequencies for laser lines which have yet to be observed. Received: 17 July 2000 / Published online: 6 December 2000     

Determination of yttrium and lanthanum in zirconium dioxide by HPLC, X-ray fluorescence and neutron activation analyses

Summary Lanthania-and yttria-stabilized zirconium oxide ceramics have been examined using High Performance Liquid Chromatography (HPLC), thermal neutron activation (NA) and X-ray Fluorescence (XRF) analyses and thus determine the stabilizer content. Ceramic powders with the composition ZrO₂:x mol % La₂O₃ (x=5, 10, 15, 20 and 33) and ZrO₂:x mol % Y₂O₃ (x=10, 15, 20 and 25) were prepared by the citrate and the co-precipitation techniques, respectively. The lanthanum content was determined by HPLC (x=5.09, 9.78, 14.98, 19.81 and 25.94) and NA (x=5.15, 10.32, 17.25, 21.08 and 27.97) analyses, the yttrium content by HPLC (x=8.5, 13.5, 17.9 and 22.1) and XRF (x=9.9, 15.8, 20.1 and 24.9) analyses. An experimental sequence, based on continuous dilution of ceramic powder solutions, is proposed for preparing samples for HPLC measurements. A swimming pool nuclear reactor is used for NA analysis. The quantitative determinations of yttrium and lanthanum doping levels obtained using those techniques are described.     

Modeling cellobiose hydrolysis with integrated kinetic models

The enzyme cellobiase Novozym 188, which is used for improving hydrolysis of bagasse with cellulase, was characterized in its commercial available form and integrated kinetic models were applied to the hydrolysis of cellobiose. The specific activity of this enzyme was determined for pH values from 3.0–7.0, and temperatures from 40–75°C, with cellobiose at 2 g/L. Thermal stability was measured at pH 4.8 and temperatures from 40–70°C. Substrate inhibition was studied at the same pH, 50°C, and cellobiose concentrations from 0.4–20 g/L. Product inhibition was determined at 50°C, pH 4.8, cellobiose concentrations of 2 and 20 g/L, and initial glucose concentration nearly zero or 1.8 g/L. The enzyme has shown the greatest specific activity, 17.8 U/mg, at pH 4.5 and 65°C. Thermal activation of the enzyme followed Arrhenius equation with the Energy of Activation being equal to 11 kcal/mol for pH values 4 and 5. Thermal deactivation was adequately modeled by the exponential decay model with Energy of Deactivation giving 81.6 kcal/mol. Kinetics parameters for substrate uncompetitive inhibition were: Km=2.42 mM, V _max=16.31 U/mg, Ks=54.2 mM. Substrate inhibition was clearly observed above 10 mM cellobiose. Product inhibition at the concentration studied has usually doubled the time necessary to reach the same conversion at the lower temperature tested.     

The simplest non-minimal matter–geometry coupling in the <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>, <Emphasis Type="Italic">T</Emphasis>) cosmology

f(R, T) gravity is an extended theory of gravity in which the gravitational action contains general terms of both the Ricci scalar R and the trace of the energy-momentum tensor T. In this way, f(R, T) models are capable of describing a non-minimal coupling between geometry (through terms in R) and matter (through terms in T). In this article we construct a cosmological model from the simplest non-minimal matter–geometry coupling within the f(R, T) gravity formalism, by means of an effective energy-momentum tensor, given by the sum of the usual matter energy-momentum tensor with a dark energy contribution, with the latter coming from the matter–geometry coupling terms. We apply the energy conditions to our solutions in order to obtain a range of values for the free parameters of the model which yield a healthy and well-behaved scenario. For some values of the free parameters which are submissive to the energy conditions application, it is possible to predict a transition from a decelerated period of the expansion of the universe to a period of acceleration (dark energy era). We also propose further applications of this particular case of the f(R, T) formalism in order to check its reliability in other fields, rather than cosmology.     

Torsion effects on condensed matter: like a magnetic field but not so much

In this work, we study the effects of torsion due to a uniform distribution of topological defects (screw dislocations) on free spin/carrier dynamics in elastic solids. When a particle moves in such a medium, the effect of the torsion associated to the defect distribution is analogous to that of an applied magnetic field but with subtle differences. Analogue Landau levels are present in this system but they cannot be confined to two dimensions. In the case of spinless carriers, zero modes, which do not appear in the magnetic Landau levels, show up for quantized values of the linear momentum projected on the defects axis. Particles with spin are subjected to a Zeeman-like coupling between spin and torsion, which is insensitive to charge. This suggests the possibility of spin resonance experiments without a magnetic field for charged carriers or quasiparticles without electrical charge, like triplet excitons, for instance.     

Brazilian bioethanol program

Brazil is the largest producer of bioethanol, and sugarcane is the main raw material. Bioethanol, is produced by both batch and continuous processes, and in some cases, flocculating yeast is use. This article analyzes the Bracilian Ethanol Program. for the 1996–1997 havest, Brazil produced 14.16 billion L of ethanol and 13.8 million metrict of sugar, from 286 million metrict of sugarcane. These products were produced by 328 industries inactivity, with 101 autonomousethanol plants producing only ethanol, and 227 sugar mills producing sugar and ethanol. The sugar-ethanol market reaches about 7.5 billion US$/yr, accounting for direct and indirect revenues.     

Enhancement of selectivity for producing γ-cyclodextrin

The production of cyclodextrins (CDs) by cyclodextrin-glycosyl-transferase (CGTase) from Bacillus firmus was studied, with respect to the effect of the source of starch upon CD yield and on the selectivity for producing γ-CD. Cyclodextrin production tests were run for 24 h at 50°C, pH 8.0, and 1 mg/L of CGTase, and substrates were maltodextrin or the starches of rice, potato, cassava, and corn hydrolyzed up to D. E. 10. Cornstarch was the best substrate for producing γ-CD. Later, glycyrrhizin (2.5% [w/v]), which forms a stable complex with γ-CD, was added to the cornstarch reaction medium and increased the yield of γ-CD to about four times that produced with only maltodextrin, but the total yield of CDs remained practically unchanged. Therefore, the results showed that the studied CGTase is capable of giving relatively high yield of γ-CD in the presence of glycyrrhizin as complexant and cornstarch as substrate.     

Thin film deposition from plasmas of tetramethylsilane–helium–argon mixtures with oxygen and with nitrogen

Films were produced by plasma enhanced chemical vapor deposition (PECVD) of tetramethylsilane (TMS)–helium–argon mixtures with either oxygen or nitrogen in a vacuum system fed with radiofrequency power. Actinometric optical emission spectroscopy was used to determine trends in the concentrations of plasma species of interest (H, CH, O, CO, and CN) as a function of the ratio of the inorganic reactive gas (oxygen or nitrogen) to the monomer (TMS) in the system feed. As the ratio of oxygen to TMS in the feed is increased, the degree of oxygenation of the deposited material, as revealed by transmission infrared spectroscopy, is also increased. Similarly, the degree of nitrogenation of the films increases with increasing nitrogen to monomer ratio in the feed. Strong correlations exist between the plasma concentrations of the above-mentioned plasma species and film structure and composition. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1873–1879, 1998     

Photochemical Properties of trans‐[Ru(NH3)4(bpa)(L)]2+ (L = py,isn, 4‐acpy or 4‐pic)

Photochemical reactions of ruthenium (II) complexes of type trans‐[Ru(NH₃)₄LL']²⁺, where L is a nitrogenous heterocyclic ligand, pyridine (py), isonicotinamide (isn), 4‐acetylpyridine (4‐acpy) or 4‐picoline (4‐pic), and L´ is a 1,2‐bis(4‐pyridyl)ethane (bpa) ligand, were studied with the purpose of evaluating the ligand exchange when, in solution, the complexes are irradiated at the wavelengths of 365, 436, 480 and 519 nm. The study revealed that at lower wavelengths, a labilization process is observed for py and 4‐pic ligands, even at low quantum yields, indicating the dependence of the photolabeling process on the wavelength. The study also reveals that for the filters of greater wavelength, the processes of photolabilization do not occur for any of the studied complexes. The study also shows that there are no photolization processes for the complexes obtained with the isn and 4‐acpy ligands, and it is therefore possible to classify them as nonreactive.