Eduardo Méndez

Toward decentralized analysis of mercury (II) in real samples. A critical review on nanotechnology-based methodologies.

In recent years, it has increased the number of works focused on the development of novel nanoparticle-based sensors for mercury detection, mainly motivated by the need of low cost portable devices capable of giving fast and reliable analytical response, thus contributing to the analytical decentralization. Methodologies employing colorimetric, fluorometric, magnetic, and electrochemical output signals allowed reaching detection limits within the pM and nM ranges. Most of these developments proved their suitability in detecting and quantifying mercury (II) ions in synthetic solutions or spiked water samples. However, the state of art in these technologies is still behind the standard methods of mercury quantification, such as cold vapor atomic absorption spectrometry and inductively coupled plasma techniques, in terms of reliability and sensitivity. This is mainly because the response of nanoparticle-based sensors is highly affected by the sample matrix. The developed analytical nanosystems may fail in real samples because of the negative incidence of the ionic strength and the presence of exchangeable ligands. The aim of this review is to critically consider the recently published innovations in this area, and highlight the needs to include more realistic assays in future research in order to make these advances suitable for on-site analysis.

Electrochemical and optical study of rhenium layers formed on gold electrodes

Films obtained after cathodic polarization at Ec, −0.2 V≤Ec≤0.1 V (vs. RHE), in aqueous acid perrhenate have been investigated using cyclic voltammetry and ellipsometry. The thickness and optical indices corresponding to the electrodeposited rhenium layer were calculated. The results indicate the formation of a composite layer, with a volumetric fraction of about 30% of metallic rhenium and hydrogen occlusion.

Silver nanoparticle aggregation not triggered by an ionic strength mechanism

The synthesis of stable colloidal solutions of silver nanoparticles is a major goal in the industry to control their fate in aqueous solutions. The present work studies 10–20-nm silver nanoparticle aggregation triggered by the presence of chloride ions. The aggregation process was followed by UV–Vis–NIR spectroscopy and transmission electron microscopy. We found that the mechanism involved differs from the classic explanation of nanoparticle aggregation triggered by an increase in the ionic strength. Moreover, our results give evidence that even when nanoparticles are resistant to an increment of the total amount of ions, the formation of insoluble salts in the vicinity of the nanoparticle is enough to induce the aggregation. The presence of silver chloride around the silver nanoparticles was documented by an X-ray diffraction pattern and electrochemical methods because chloride anions are ubiquitous in real media; this alternative process jeopardized the development of many applications with silver nanoparticles that depend on the use of stable colloids.

Redox stability of Re(V)–amine complexes

The complete electrochemical characterization of three amine-related complexes containing the ½ReO2� þ core was achieved by the use of three different electrode materials. The ½Re V O2� þ core, the amine group and iodide were characterized based on the voltammetric behavior on glassy carbon, nickel and gold, respectively. � 2002 Elsevier Science B.V. All rights reserved.

A comparative study of electrochemical and optical properties of rhenium deposited on gold and platinum

Rhenium-containing films were grown on gold and platinum after different potentiostatic and potentiodynamic polarizations in the - 0.20 V to 0.70 V range (vs rhe) in aqueous acid perrhenate. Experimental data were obtained using cyclic voltammetry and ellipsometry, from which the thickness and optical indices of the electrodeposited rhenium layer were calculated. Metallic rhenium deposition on gold takes place at potentials within the hydrogen evolution reaction. Rhenium oxide on platinum is formed in the hydrogen adatom potential domain, whereas metallic rhenium is deposited concurrently with the hydrogen adsorption and evolution reactions on the same metal.

Kinetics of rhenium dioxide deposition on columnar structured platinum electrodes

The kinetics concerning the early stage of the electrodeposition of rhenium dioxide on columnar-structured platinum electrodes from acid aqueous containing perrhenate ions was studied. The results demonstrate a zero-order kinetics obtained by holding the potential at two different values.

The thiol of human serum albumin: Acidity, microenvironment and mechanistic insights on its oxidation to sulfenic acid

Abstract Human serum albumin (HSA) has a single reduced cysteine residue, Cys34, whose acidity has been controversial. Three experimental approaches (pH‐dependence of reactivity towards hydrogen peroxide, ultraviolet titration and infrared spectroscopy) are used to determine that the pKa value in delipidated HSA is 8.1±0.2 at 37 °C and 0.1 M ionic strength. Molecular dynamics simulations of HSA in the sub‐microsecond timescale show that while sulfur exposure to solvent is limited and fluctuating in the thiol form, it increases in the thiolate, stabilized by a persistent hydrogen‐bond (HB) network involving Tyr84 and bridging waters to Asp38 and Gln33 backbone. Insight into the mechanism of Cys34 oxidation by H2O2 is provided by ONIOM(QM:MM) modeling including quantum water molecules. The reaction proceeds through a slightly asynchronous SN2 transition state (TS) with calculated &Dgr;‡G and &Dgr;‡H barriers at 298 K of respectively 59 and 54 kJ mol−1 (the latter within chemical accuracy from the experimental value). A post‐TS proton transfer leads to HSA–SO− and water as products. The structured reaction site cages H2O2, which donates a strong HB to the thiolate. Loss of this HB before reaching the TS modulates Cys34 nucleophilicity and contributes to destabilize H2O2. The lack of reaction‐site features required for differential stabilization of the TS (positive charges, H2O2 HB strengthening) explains the striking difference in kinetic efficiency for the same reaction in other proteins (e.g. peroxiredoxins). The structured HB network surrounding HSA–SH with sequestered waters carries an entropic penalty on the barrier height. These studies contribute to deepen the understanding of the reactivity of HSA–SH, the most abundant thiol in human plasma, and in a wider perspective, provide clues on the key aspects that modulate thiol reactivity against H2O2. Graphical abstract Figure. No Caption available. HighlightsThe pKa for delipidated HSA thiol is 8.1 at 37 °C and 0.1 M ionic strength.HSA thiolate is stabilized by H‐bonding with Tyr84 and waters bridging to Asp38.Calculated &Dgr;H‡ and &Dgr;G‡ to oxidize Cys34 by H2O2 respectively are 54 and 59 kJ mol−1.A post‐transition state proton transfer leads to HSA sulfenate and water.Theoretical data is in excellent agreement with temperature dependency of kinetics.

Nanotubos de orientación vertical autoensamblados por método electroquímico: síntesis y caracterización

Nanotube arrays of TiO2 have received great attention in various areas, such as gas- sensing, photocatalysis, solar energy conversion, and biomaterials. Therefore, particular attempts are carried out to fabricate the vertically oriented TiO2 nanotube arrays by electrochemical anodization in order to improve its properties to meet the practical application requirements. In the present work, vertically oriented TiO2 nanotube arrays have been fabricated by a selforganized electrochemical anodization process of titanium foil by use of glycerol based electrolyte (containing 0.27M NH4F and 50 vol% H2O). The effect of anodizing potential and growing time in the final TiO2 resulting nanostructure has been studied and discussed. XRD and SEM analysis were employed to characterize crystallographic phase, the overall crystalline quality, the possible texture of the nanotubes, and the morphological nanotube properties. In the present work the wetting properties of the nanoporous TiO2 nanotube structure have also been studied to determine the hidrophobic and hidrophylic grade. Ordered high-aspect-ratio TiO2 nanotube arrays with an inner-tube diameter of ca. 35 nm and a tube height of several tens of microns have been obtained. As confirmed by X-ray diffraction studies, the as-prepared TiO2 nanotubes are amorphous but crystallize with annealing in an oxygen atmosphere at 550 oC, exhibiting then, the anatase phase with a minor quantity of the rutile one.

Voltammetric Characterization of (ReO) 3+ Containing Complexes

In the present work, the voltammetric profiles of Au-pc in 1 mM solutions of K[Re V OCl2(L)] (L = dianions of H2hida or H2eida; hida, n-(2,6-dimethylphenyl carbamoylmethyl)imino diacetic acid, and eida, 2,6-diethyl ida) complexes were evaluated. The obtained electrochemical response was the result of the contributions coming from the [ReO] 3+ core (Epa at ca. 0.2 V vs. Ag/AgNO3) and the carboxylic group of the ligand. There where also observed redox contributions ascribed to the presence of perrhenate coming from the decomposition of the complex with the lost of ligands, in agreement with the low stability of the studied complexes.