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Sigal A.,National University of Cordoba | Rojas M.I.,INFIQC | Leiva E.P.M.,INFIQC
Physical Review Letters | Year: 2011

Density functional theory (DFT) calculations are performed for the adsorption energy of hydrogen and oxygen on graphene decorated with a wide set of metals (Li, Na, K, Al, Ti, V, Ni, Cu, Pd, Pt). It is found that oxygen interferes with hydrogen adsorption by either blocking the adsorption site or by the irreversible oxidation of the metal decoration. The most promising decorations are Ni, Pd, and Pt due to a reasonable relationship of adsorption energies which minimize the oxygen interference. The DFT results are used to parametrize a statistical mechanical model which allows evaluation of the effect of partial pressures in the gas phase during storage. According to this model, even in the most promising case, it is necessary to reduce the oxygen partial pressure close to ultrahigh vacuum conditions to allow hydrogen storage. © 2011 American Physical Society.

Sigal A.,National University of Cordoba | Rojas M.I.,INFIQC | Leiva E.P.M.,INFIQC
International Journal of Hydrogen Energy | Year: 2011

In the present work, the decoration of a graphene sheet with nickel is considered as a hydrogen storage material by means of density functional theory calculations. A number of factors relevant for the handling and operation of this material were analyzed. This includes the interaction with potentially interfering chemicals, hydride formation and the hydrogen storage capacity. The present results show that unless the access of oxygen to the surface is restricted, its strong bond to the decorated systems will preclude the practical use for hydrogen storage. In the best case, the energy required to replace an adsorbed oxygen molecule by hydrogen is of the order of 1.7 eV, something that indicates the severe problem that the presence of oxygen represents for this type of systems. © 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Soria-Castro S.M.,INFIQC | Caminos D.A.,INFIQC | Penenory A.B.,INFIQC
RSC Advances | Year: 2014

Microwave irradiation promotes a quick aromatic nucleophilic substitution by a thermally induced electron transfer process to form new C-C bonds by the coupling of aryl radicals and enolate nucleophiles. Diverse 2-aryl-1- phenylethanones can be prepared by the direct α-arylation of acetophenone with different haloarenes. The ketone enolate anion is generated by deprotonation with tBuOK in DMSO and the reaction is carried out in a closed microwave vessel at 70-100°C for 10 min. This simple procedure also allows the synthesis of deoxybenzoin and indole heterocycle derivatives by inter- or intra-molecular ring closure reactions, with moderate to excellent substitution yields. This journal is © the Partner Organisations 2014.

(Figure presented) The synthesis of 6-substituted 2,4-dimethoxypyrimidines with different nucleophiles was accomplished with good to excellent yields (50-95%) through S RN1 reactions, starting from commercially available 6-chloro-2,4-dimethoxypyrimidine (1). Hydrolysis of these derivatives gave access to 6-substituted uracils with good yields and short times by the use of microwave irradiation. The preparation of uracils from 1 without the isolation of 2,4-dimethoxypyrimidine derivatives affords a rapid access to these compounds in good yields and excellent purity by avoiding an unnecessary step of purification. © 2010 American Chemical Society.

Heredia A.A.,INFIQC | Penenory A.B.,INFIQC
European Journal of Organic Chemistry | Year: 2013

A new protocol for the one-pot synthesis of styryl alkyl sulfides was developed. This methodology involves the in situ generation of thiolate anions by nucleophilic substitution between potassium thioacetate and alkyl halides followed by fragmentation. Further reactions of these thiolate anions with substituted (E,Z)-β-styryl halides gave the corresponding sulfides with retention of stereochemistry in good to excellent yields. This procedure does not require a metal catalyst, it proceeds under mild conditions and in short times, and it is free from malodorous and air-sensitive alkyl thiols. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Vazquez C.I.,INFIQC | Lacconi G.I.,INFIQC
Journal of Electroanalytical Chemistry | Year: 2013

Silver electrodeposition onto HOPG electrodes in the presence of picolinic acid (PA) has been studied by cyclic voltammetry and chronoamperometry. Changes in the nucleation and growth mechanisms, which are dependent on the PA concentration and solution pH, have been observed. Formation of complexes with Ag+ ions and adsorption of the additive molecules on both, the substrate and the growing silver crystallites can be correlated with the potential dependence of the kinetic parameters, N0 and A. The amount, distribution (random or localized on the defects sites) and size of the crystallites on the substrate are influenced by the composition of the solution (PA concentration and pH) when the control of the nucleation and growth processes is regulated by application of a double potential program. © 2013 Elsevier B.V. All rights reserved.

Gutierrez F.,INFIQC | Rubianes M.D.,INFIQC | Rivas G.A.,INFIQC
Sensors and Actuators, B: Chemical | Year: 2012

In this work we report for the first time the use of the enzyme glucose oxidase (GOx) to efficiently disperse multiwall carbon nanotubes (CNT) and to confer biorecognition properties to the dispersed nanotubes. The optimum dispersion was obtained by sonicating for 15 min 1.0 mg/mL CNT in 1.0 mg/mL GOx solution prepared in 50:50 ethanol/water. The dispersion was evaluated by Scanning Electron Microscopy (SEM), Infrared (FT-IR) and Ultraviolet-visible (UV-vis) Spectroscopy. The electrochemical characterization of glassy carbon electrodes (GCE) modified with the dispersion (by dropping) was performed by Electrochemical Impedance Spectroscopy (EIS), Cyclic Voltammetry (CV), and Amperometry. The amount of electroactive GOx deposited on GCE (GCE/CNT-GOx) was 1.02 × 10 -10 mol cm -2 and the rate constant for the electron transfer between FAD center and the electrode was (2.9 ± 0.1) s -1 according to Laviron and (9.2 ± 1.3) s -1 considering the model proposed by Albery. The enzyme demonstrated to keep its biocatalytic activity even after dissolution in 50/50 v/v, ethanol-water solution and sonication for 15 min using either ferrocene methanol or oxygen as redox mediators. The sensitivity to glucose at 0.700 V obtained for seventeen electrodes prepared with 6 different dispersions was (3.2 ± 0.2) × 10 2 μA M -1, (r = 0.997), with an R.S.D. of 6.0%. The sensitivity remained highly constant after 30 days at room temperature (25 °C) and 4 °C, with average values of (3.21 ± 0.07) × 10 2 μA M -1, r = 0.9992 and (3.59 ± 0.08) × 10 2 μA M -1, r = 0.9990, respectively. The GCE/CNT-GOx can be used as platform to build supramolecular architectures for biosensing through the self-assembling of polyelectrolytes, opening the doors to new and exciting possibilities for the development of biosensors. © 2011 Published by Elsevier B.V.

This work reports on the analytical performance of glassy carbon electrodes (GCE) modified with a dispersion of multi-wall carbon nanotubes (MWCNT) in polyhistidine (Polyhis) (GCE/MWCNT-Polyhis) for the simultaneous determination of ascorbic acid (AA) and paracetamol (PA). The modified electrode exhibited enhanced current responses and lower oxidation overvoltages, demonstrating excellent catalytic activities towards AA and PA oxidation compared to bare GCE. The linear dependence between the anodic peak currents and the square root of scan rates over the range of 0.005-0.300 V s -1 demonstrate that the electrooxidation of AA and PA occurs under diffusional control. The MWCNT-Polyhis modified GCE displayed a sensitivity of (3.8 ± 0.1) × 10 4 μA M -1 (r = 0.998) and a detection limit of 0.76 μM for the selective determination of AA in the presence of 1.00 × 10 -4 M PA. Conversely, for the direct quantification of PA in the presence of 5.00 × 10 -4 M AA, the sensitivity and the detection limit were (6.3 ± 0.2) × 10 5 μA M -1 (r = 0.997) and 32 nM, respectively. The proposed electrochemical sensor was successfully applied for quantifying AA and PA in commercial pharmaceutical formulations without any sample pretreatment. © 2012 Elsevier B.V.

Romero M.R.,INFIQC | Ahumada F.,INFIQC | Garay F.,INFIQC | Baruzzi A.M.,INFIQC
Analytical Chemistry | Year: 2010

An amperometric sensor for lactate quantification is presented. The developed biosensor requires only 0.2 U of lactate oxidase, which is immobilized in a mucin/albumin hydrogel matrix. By protecting the platinum surface with a Nafion membrane, typical interference related to negatively charged species such as ascorbic acid has been minimized to practically undetectable levels. Electrochemical properties associated with the Nafion membrane are assessed as a function of Nafion concentration. In a phosphate buffer solution of pH 7.0, linear dependence of the catalytic current upon lactate bulk concentration was obtained between 2 and ∼1000 μM. A detection limit of 0.8 μM can be calculated considering 3 times the standard deviation of the blank signal divided by the sensitivity of the sensor. The lactate biosensor presents remarkable operational stability and sensitivity (0.537 ± 0.007) mA.M-1, where the error is the standard deviation of the slope calculated from the linear regression of the calibration curve of a fresh biosensor. In this regard, the sensor keeps practically the same sensitivity for 5 months, while the linear range decreases until an upper value of 0.8 mM is reached. Assays performed with whole blood samples spiked with 100 μM lactate gave (89 ± 6)% of recovery. © 2010 American Chemical Society.

Romero M.R.,INFIQC | Baruzzi A.M.,INFIQC | Garay F.,INFIQC
Sensors and Actuators, B: Chemical | Year: 2012

A comprehensive numerical treatment of the diffusion and reaction within a sandwich-type amperometric biosensor is presented. The model considers that the enzyme reacts according to a ping-pong mechanism and that it is entrapped into a so-called enzymatic matrix placed between two diffusion membranes. It is found that the concentrations of reagents and products within the sensor are difficult of being compared to those of the bulk. In this regard, the use of approximate analytical solutions would involve errors in the analysis of kinetic parameters corresponding to this kind of biosensors. Provided the mediator species are in high concentration or diffuse much faster than the substrate, the response time of a biosensor of this kind would be determined by the diffusion of the substrate though the external membrane. In this sense, those systems with immobilized mediators, in which diffusion of electrons or holes is assumed for the charge transport process, could be also described by this model. Thus, the thickness and the permeability to the analyte of the external membrane are critical parameters for improving the response time of a sandwich-type biosensor. The simulated curves are compared with experimental profiles corresponding to a lactate amperometric biosensor obtaining consistent results. In a future publication a non-linear fitting algorithm will be combined to the model for the extraction of kinetic and/or geometric parameters. © 2012 Elsevier B.V. All rights reserved.

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