Selective detection of dopamine in the presence of ascorbic acid using carbon nanotube modified screen-printed electrodes

This work reports on the performance of carbon nanotube modified screen-printed electrodes (SPE-MWCNT) for the selective determination of dopamine (DA) in the presence of ascorbic acid (AA) by adsorptive stripping voltammetry (AdSV). Several operating conditions and parameters were examined including the electrochemical pre-treatment and the previous AA interaction and DA accumulation in the presence AA at physiological conditions. Under the chosen conditions, DA peak current of differential pulse voltammograms increases linearly with DA concentration in the range of 5.0 × 10⁻⁸ to 1.0 × 10⁻⁶ mol L⁻¹ with a limit of detection of 1.5 × 10⁻⁸ mol L⁻¹ in connection with 600 s accumulation time. The sensitivity obtained for DA was independent from the presence or absence of AA; therefore, the proposed method can be readily applied to detect DA in real samples. The proposed methodology was successfully used for the quantification of DA in urine samples.     

Amperometric Response of Hydrogen Peroxide at Carbon Nanotubes Paste Electrodes Modified with an Electrogenerated Poly(Fe(III)‐5‐amino‐phenantroline). Analytical Applications for Glucose Biosensing

This work reports the catalytic activity of a polymer electrogenerated from Fe(III)‐5‐amino‐1,10‐phenantroline solution at a carbon nanotubes paste electrode (CNTPE) towards the oxidation and mainly the reduction of hydrogen peroxide. The important role of carbon nanotubes on the generation of poly(Fe(III)‐5‐amino‐1,10‐phenantroline) is demonstrated through the comparison with the behavior of graphite paste electrode (CPE). The polymer electrogenerated at CNTPE largely improves the amperometric detection of hydrogen peroxide at ?0.100 V. The analytical application of the resulting electrode is demonstrated in connection with the design of a glucose biosensor based on the deposition of GOx and diluted Nafion on the top of the polymer‐modified CNTPE. The quantification of glucose in human serum samples showed a good correlation with the values obtained by the spectrophotometric technique.     

Exploiting automatic on-line renewable molecularly imprinted solid-phase extraction in lab-on-valve format as front end to liquid chromatography: application to the determination of riboflavin in foodstuffs

In the present work, it is proposed, for the first time, an on-line automatic renewable molecularly imprinted solid-phase extraction (MISPE) protocol for sample preparation prior to liquid chromatographic analysis. The automatic microscale procedure was based on the bead injection (BI) concept under the lab-on-valve (LOV) format, using a multisyringe burette as propulsion unit for handling solutions and suspensions. A high precision on handling the suspensions containing irregularly shaped molecularly imprinted polymer (MIP) particles was attained, enabling the use of commercial MIP as renewable sorbent. The features of the proposed BI-LOV manifold also allowed a strict control of the different steps within the extraction protocol, which are essential for promoting selective interactions in the cavities of the MIP. By using this on-line method, it was possible to extract and quantify riboflavin from different foodstuff samples in the range between 0.450 and 5.00 mg L⁻¹ after processing 1,000 μL of sample (infant milk, pig liver extract, and energy drink) without any prior treatment. For milk samples, LOD and LOQ values were 0.05 and 0.17 mg L⁻¹, respectively. The method was successfully applied to the analysis of two certified reference materials (NIST 1846 and BCR 487) with high precision (RSD < 5.5%). Considering the downscale and simplification of the sample preparation protocol and the simultaneous performance of extraction and chromatographic assays, a cost-effective and enhanced throughput (six determinations per hour) methodology for determination of riboflavin in foodstuff samples is deployed here.     

On the existence of bounded solutions for a nonlinear elliptic system

This work deals with the system (?Δ) ^m u?= a(x) v ^p , (?Δ) ^m v?=?b(x)?u ^q with Dirichlet boundary condition in a domain ${\Omega\subset\mathbb{R}^n}$ , where Ω is a ball if n ≥ 3 or a smooth perturbation of a ball when n?=?2. We prove that, under appropriate conditions on the parameters (a, b, p, q, m, n), any nonnegative solution (u, v) of the system is bounded by a constant independent of (u, v). Moreover, we prove that the conditions are sharp in the sense that, up to some border case, the relation on the parameters are also necessary. The case m?=?1 was considered by Souplet (Nonlinear Partial Differ Equ Appl 20:464–479, 2004). Our paper generalize to m ≥ 1 the results of that paper.     

Effect of oviductal secretion components on the fertilizing capacity of amphibian sperm: biological and ultrastructural studies

The present study was carried out to analyze the fertilization-supporting activity of Rhinella arenarum egg-jelly components. Spermatozoa were treated with diffusible factor (DF) constituted by the components released from the jelly coat into deionized water or with full jelly (FJ) containing all the components secreted by the oviductal pars convoluta (PC) during the transit of the oocytes through the duct, or with washed jelly (WJ) constituted only by structural components. Both jellies were solubilized by ultraviolet irradiation. These gametes were used for ultrastructural and biological studies in order to determine the acrosome state and the fertilizing capacity. Additional experiments were performed by using Ca(2+), a diffusible cation present in R. arenarum jelly envelopes. Results demonstrated a marked increase in the acrosome reaction (AR) of sperm treated with FJ or DF compared to the controls (Ringer's solution), no significant differences being observed between both treatments, while WJ showed low AR percentages similar to the ones obtained with the controls. The addition of Ca(2+) induced an increase in this parameter in a dose-dependent manner, although the values reached with FJ or DF were not attained. The results of the "in vitro" fertilization show a strong inverse association to the acrosome reaction (AR) rate. Treatment with Ca(2+) at the concentration present in the jelly (6.3 ± 0.9 mM) inhibited the fertilizing capacity as a function of incubation time, showing that at 2 min there was a decrease in the fertilization percentages compared to 10% Ringer's. Data indicated that Ca(2+) present in jelly is involved in the AR induction but suggests the possible participation of other diffusible and/or structural components of the oviductal secretion in this process.     

Total Edge Irregularity Strength of Toroidal Fullerene

A toroidal fullerene (toroidal polyhex) is a cubic bipartite graph embedded on the torus such that each face is a hexagon. An edge irregular total k-labeling of a graph G is such a labeling of the vertices and edges with labels 1, 2, … , k that the weights of any two different edges are distinct, where the weight of an edge is the sum of the label of the edge itself and the labels of its two endvertices. The minimum k for which the graph G has an edge irregular total k-labeling is called the total edge irregularity strength, tes(G). In this paper we determine the exact value of the total edge irregularity strength of toroidal polyhexes.     

New synthesis method for M(II) chromites/silica nanocomposites by thermal decomposition of some precursors formed inside the silica gels

In this article, we present a new method for the obtaining of ZnCr₂O₄ and MgCr₂O₄ embedded in silica matrix. This method consists in the formation of Cr(III), Zn(II) and Cr(III), Mg(II) hydroxycarboxylate/carboxylate compounds, during the redox reaction between the nitrate ion and diol (1,3-propanediol), uniformly dispersed in the pores of hybrid gels. The thermal decomposition of these precursors leads to a mixture of corresponding metal oxides. The gels were synthesized starting from mixtures of Cr(NO₃)₃·9H₂O, Zn(NO₃)₂·6H₂O and Cr(NO₃)₃·9H₂O, Mg(NO₃)₂·6H₂O with tetraethyl orthosilicate and 1,3-propanediol for final compositions 50% ZnCr₂O₄/50% SiO₂ and 50% MgCr₂O₄/50% SiO₂. The obtained gels have been thermally treated at 140?°C, when the redox reaction nitrates-diol took place with formation of the precursors within the xerogels pores. The thermal decomposition of all precursors took place up to 300?°C, with formation of oxides mixtures (Cr₂O_3?+?x and ZnO) and (Cr₂O_3?+?x and MgO), respectively. At 400?°C, Cr₂O_3?+?x turn to Cr₂O₃ which reacts with ZnO forming ZnCr₂O₄/SiO₂. Starting with 400?°C, Cr₂O₃ reacts with MgO to an intermediary phase MgCrO₄, which decomposes with the formation of MgCr₂O₄/SiO₂. The formation of the precursors inside the silica matrix and the evolution of the crystalline phases were studied by thermal analysis, FT-IR spectrometry, XRD, and TEM.     

Synthesis and DPPH radical scavenging activity of prenylated phenol derivatives

The synthesis of twenty six prenylated phenols derivatives is reported. These compounds were obtained under mild conditions via Electrophilic Aromatic Substitution (EAS) coupling reactions between phenol derivatives containing electron-donor subtituents and 3-methyl-2-buten-1-ol using BF(3)×OEt(2). Dialkylations were also produced with this method. The formation of a chroman ring by intramolecular cyclization between a sp2 carbon from the prenyl group with the hydroxyl substituent in the ortho position occurred with some phenols. All the synthesized compounds were evaluated as antioxidants according to a DPPH radical scavenging activity assay. IC(50) values of five synthesized compounds indicated they were as good antioxidants as Trolox?.     

Electron states in a lattice of Au nanoparticles: the role of strain and functionalization

We make use of first-principles calculations to study the effects of functionalization and compression on the electronic properties of 2D lattices of Au nanoparticles. We consider Au38 particles capped by methylthiol molecules and possibly functionalized by the dithiolated conjugated molecules benzenedimethanethiol and benzenedicarbothialdehyde. We find that the nonfunctionalized lattices are insulating, with negligible band dispersions even for a compression of 20% of the lattice constant. Distinct behaviors of the dispersion of the lowest conduction band as a function of compression are predicted for functionalized lattices: The band dispersion of the benzenedimethanethiol-functionalized lattice increases considerably with compression, while that of the benzenedicarbothialdehyde-functionalized lattice decreases.     

Poly(ethylene oxide)s hydrophobically modified. Adsorption and spreading at the air-water interface

A comparative study of spread and adsorbed monolayer of poly(ethylene oxide)s of different molecular weight hydrophobically modified with alkyl isocyanates of different length chain is reported. The modification of the polymer was carried out according to reported procedures. The polymers obtained were studied at the air-water interface by Langmuir isotherms for spread monolayers and by Gibbs isotherms for the adsorption process. Isotherms obtained are interpreted in terms of the hydrophobic and hydrophilic balance of the polymers. Limiting area per repeating unit (A(0)) and collapse pressure (pi(c)) from spread monolayers were obtained. Spread monolayers of the hydrophobically modified polymers show larger collapse pressure values than unmodified polymer monolayers. In the adsorption process the excess surface concentration Gamma(infinity), area per repeat unit sigma, and efficiency of the adsorption were determined. The values of the area occupied per repeat unit in adsorbed monolayer (sigma) were larger than those of the spread monolayer. The efficiency of the adsorption of poly(ethylene oxide)s increases with the hydrophobic modification and with the alkyl chain length.