Glassy carbon electrode modified with a film of poly(Toluidine Blue O) and carbon nanotubes for nitrite detection

This work describes the modification of a glassy carbon electrode with poly(Toluidine Blue O) (GC/poly-TBO) and single-walled carbon nanotubes (SWCNT) for the electrocatalytic oxidation of nitrite. GC/poly-TBO was prepared by electropolymerization and used as such or after immobilizing SWCNT on the polymeric film to give a composite GC/poly-TBO-SWCNT electrode. The electrochemical and catalytic behavior of both electrodes was studied comparatively. It was observed that the presence of SWCNT contributed to enhance the electrocatalytic response for nitrite oxidation, as measured by amperometry at +0.92 V vs. Ag/AgCl/KCl_sat and pH 7. The response was linear with respect to the nitrite concentration in the 0.001–4 mM range, with a detection limit of 0.37 μM (based on signal to noise ratio of 3) for GC/poly-TBO-SWCNT. The proposed method was also applied to the determination of nitrite in a wastewater sample and compared to the spectrophotometric method.     

Kinetic 12C/13C isotope fractionation by invertase: evidence for a small in vitro isotope effect and comparison of two techniques for the isotopic analysis of carbohydrates

The natural ¹³C/¹²C isotope composition (δ¹³C) of plants and organic compounds within plant organs is a powerful tool to understand carbon allocation patterns and the regulation of photosynthetic or respiratory metabolism. However, many enzymatic fractionations are currently unknown, thus impeding our understanding of carbon trafficking pathways within plant cells. One of them is the ¹²C/¹³C isotope effect associated with invertases (EC 3.2.1.26) that are cornerstone enzymes for Suc metabolism and translocation in plants. Another conundrum of isotopic plant biology is the need to measure accurately the specific δ¹³C of individual carbohydrates. Here, we examined two complementary methods for measuring the δ¹³C value of sucrose, glucose and fructose, that is, off‐line high‐performance liquid chromatography (HPLC) purification followed by elemental analysis and isotope ratio mass spectrometry (EA‐IRMS) analysis, and gas chromatography‐combustion (GC‐C)‐IRMS. We also used these methods to determine the in vitro ¹²C/¹³C isotope effect associated with the yeast invertase. Our results show that, although providing more variable values than HPLC～EA‐IRMS, and being sensitive to derivatization conditions, the GC‐C‐IRMS method gives reliable results. When applied to the invertase reaction, both methods indicate that the ¹²C/¹³C isotope effect is rather small and it is not affected by the use of heavy water (D₂O). Copyright © 2009 John Wiley & Sons, Ltd.     

Azidation of β-carbonyl lactones and lactams

The direct azidation of various heterocyclic β-ketoesters, lactones, and lactams is reported. By using tosylazide and an organic base such as l-proline or TBD, the direct α-insertion of azide into these substrates was achieved in moderate to good yields, without competitive deacylating diazo transfer. This procedure represents an interesting alternative to the usual two-step approach of α-halogenation and subsequent displacement with azide ion.     

Criteria to define the standard deviation for proficiency assessment for the determination of essential trace elements in serum: comparison of Z-scores based on the Horwitz function or on biological variability

A critical issue in the organisation of Proficiency Testing/External Quality Assessment Schemes is the definition of the criteria against which the performance of individual laboratories should be evaluated. Organisers of EQAS in Occupational and Environmental Laboratory Medicine (http://www.occupational-environmental-laboratory.com) collaborate to define common acceptable levels of performance. The aim of this study was to compare the Horwitz function to the Fraser’s approach. Sets of results obtained from the distribution of test materials in the Network schemes (for the measurands: copper, selenium or zinc in serum) were used to calculate Z-scores according to both approaches. Quality specifications derived from both approaches were also compared to the standard deviations obtained. Except for selenium, Horwitz criteria suggests a more stringent evaluation than Fraser criteria, the latter being very stringent as regard the participant analytical variability.     

Advantages of a small-volume counter-current chromatography column

Counter-current chromatography (CCC) works with a support-free liquid stationary phase. This allows for preparative separations and purifications. However, there are serious technical constraints because of the need to keep a liquid stationary phase in a column. Centrifugal fields are used. A new commercial hydrodynamic 18 mL column made with a narrow-bore 0.8 mm Teflon tubing was evaluated by comparing it with older hydrodynamic CCC columns and a similar 19 mL column but made with 1.6 mm Teflon tubing. A small-volume CCC column allows for reliable and fast solute partition coefficient determination. When resolution is required, both high efficiency and liquid stationary phase retention are needed. Unfortunately, these two requirements bear technical contradictions. A column coiled with a narrow tubing bore will provide a high chromatographic efficiency while a column containing wider tubing bore will achieve higher stationary phase retention. In all cases, increasing the magnitude of the centrifugal field also increases the stationary phase retention. The solution is to build centrifuges able to produce high fields that will provide acceptable liquid phase retention with narrow-bore tubes. The new 18 mL 0.8 mm tubing bore column is able to rotate as fast as 2100 rpm generating a 240 × g field. The two older CCC columns cannot compete with the new one. However, the small 19 mL column with 1.6 mm bore tubing can be useful when fast results are desired without top resolution.     

Solubility of hydrocarbons in water: Experimental measurements and modeling using a group contribution with association equation of state (GCA-EoS)

In this communication, new experimental data on the solubility of n-hexane, cyclo-hexane and iso-octane in pure water are reported. The data have been measured using a static-analytic technique that takes advantage of a Rolsi™ sampling device in the temperature range of 298–353 K and at pressures up to 0.5 MPa. The experimental data measured in this work at 298 K have been compared with some selected data from the literature and good agreement is found. A group contribution plus association equation of state, namely the GCA-EoS, is used to model the phase equilibrium of water + hydrocarbon (C₂ to n-C₆, cy-C₆, i-C₄ and i-C₈) system. The predictions of the model are found in good agreement with the experimental data measured in this work and some selected data from the literature.     

Synthesis and biological evaluation of 3-aryltyramines as fragments binding to BACE-1 and BACE-2

3-Aryltyramines were prepared in one single step from tyramine and various arenediazonium salts by radical arylation. Binding as well as enzyme inhibition data of the 14 compounds do not prove true interaction with BACE-1. In contrast, with BACE-2 inhibition and binding could be confirmed indicating that 3-aryltyramines are potential starting points for a drug discovery effort.     

EDTA and DTPA modified ligands as sequestering agents for uranyl decorporation

Synthesis of modified EDTA and DTPA ligands and determination of their binding affinities for the uranyl cation are described.Thanks to a screening method, based on a chromophoric complex displacement procedure, chelating properties were studied in aqueous media under various pH conditions for evaluation of their in vivo uranyl-removal efficacy. Each ligand showed a more or less pronounced affinity for uranium. Specific ligands based on EDTA or DTPA analogues containing sulfocatecholamide (CAMS) were found to exhibit a significant affinity towards uranyl ion in acidic, neutral or basic conditions.     

Mechanical properties of natural rubber nanocomposites reinforced with cellulosic nanoparticles obtained from combined mechanical shearing,and enzymatic and acid hydrolysis of sisal fibers

In a previous work (Siqueira et al. 2010b) the preparation of cellulosic nanoparticles from sisal fibers using different processing routes, viz. a combination of mechanical shearing, acid and enzymatic hydrolysis was reported. It was shown that the pre-enzymatic hydrolysis treatment of bleached sisal pulp helps the preparation of well individualized rod-like nanocrystals. An amorphous polymer (natural rubber—NR) was chosen as model matrix to investigate the effect of these nanoparticles on the thermo-mechanical properties of nanocomposites. Both tensile tests and dynamic mechanical analyses showed improved stiffness for all nanocomposites. The enzymatic treatment allowed production of a huge range of cellulosic nanoparticles which provided completely different mechanical properties to NR matrix.     

Fluorescent nanocrystals grown in sol–gel thin films for generic stable and sensitive sensors

Through the sol–gel route, we have well-controlled the preparation of fluorescent organic nanocrystals grown in silicate thin films. This process is based on the confined nucleation and growth of dyes in the pores of wet gels. The resulting nanocomposite sol–gel thin films, coated onto low-cost substrates, exhibit coupled properties: transparency, stability, easy shaping of sol–gel thin films and high fluorescence intensity coming from organic nanocrystals. The sensitivity of the fluorescence intensity of nanocrystals to their environments can be exploited for the development of optical sensors. Indeed, Förster Resonance Energy Transfer can inhibit nanocrystal fluorescence when probe molecules are adsorbed or grafted on the nanocrystal surface after their diffusion through the pores of the sol–gel matrix. We investigated by time-resolved fluorescence spectroscopy the effect of nanocrystal size and probe concentration on the fluorescence quenching in presence of Methylene Blue used in this study as molecular probe. As strong fluorescence quenchings can be achieved, even for low probe concentrations, these hybrid organic–inorganic nanocoposites are promising for the development of sensor devices by increasing their fluorescence contrasts under specific chemical or biological environments.