S.A. Dassie

Collision as a way of forming bimetallic nanoclusters of various structures and chemical compositions

In the present work, a new way to obtain bimetallic nanoclusters of different structures and chemical compositions is proposed, which is based on computer simulations. Collision processes between two metal clusters of different natures are simulated through molecular-dynamics simulations using many-body potentials. Diverse diffusion mechanisms and structures can be observed, depending on the metals combined and the initial kinetic energies. The nanostructures we have found are core-shell (Pt-Au), alloyed (Pd-Au), and three-shell onionlike (Cu-Ag).

Electrochemical study of the interaction of alkali and alkaline-earth cations with a dibehenoyl phosphatidylcholine monolayer at the water/1,2-dichloroethane interface

Abstract The transfer of alkali and alkaline-earth cations across a dibehenoyl phosphatidyl choline (DBPC) monolayer is analysed by cyclic voltammetry and capacitance measurements. A cation adsorption process at the polar head of the phospholipid was evidenced. After the adsorption an enhancement of the current or a blocking of the transfer process was produced, depending on the nature and cation concentration. This blocking arises as a consequence of the packing induced by Li + or alkaline-earth cations adsorption at high concentrations which, in addition, is responsible for the destruction of the monolayer at high positive potentials. Capacitance measurements reinforce this statement. This paper also illustrates how electrochemical measurements at ITIES are a useful methodology for studying cation interactions with lipidic membranes and the transfer processes which occur across them.

Comparative analysis of the transfer of alkaline and alkaline-earth cations across the water/1,2-dichloroethane interface

The transfer of alkaline and alkaline-earth cations across the water/1,2-dichloroethane interface in presence of 1,10-phenanthroline (phen) in the organic phase was studied using cyclic voltammetry in a concentration range wide enough to cover the following experimental conditions: CM2+(w) ⪢ Cphen(o) and Cphen(o) ⪢ CM2+(w). The results obtained (when CM2+(w) ⪢ Cphen(o)) were compared with the theoretical voltammetric criteria reported by Homolka1l] for facilitated transfer. It appears that the transfer of all these cations occurs via a facilitated mechanism. The stoichiometry of all complexes formed in the organic phase is also reported. The stability constants for Ca2+-phen and Sr2+-phen complexes were estimated and compared with the tendency observed in the transfer potentials of these cations.

Electroreduction of methyl viologen in the presence of nitrite. Its influence on enzymatic electrodes

Abstract The electroreduction of methyl viologen (MV) in the presence of nitrite was studied by cyclic voltammetry. A catalytic wave for the reduction of MV 2+ was observed at −0.740 V for which an EC catalytic mechanism is proposed. The rate constant for this chemical reaction under pseudo-first-order conditions, evaluated using working curves, was employed in the simulation of the voltammetric response. The second-order rate constant was also evaluated. Influences of the reaction at −0.800 V on enzymatic electrodes employing nitrate reductase (NR) and MV + as mediator were also analysed by chronoamperometry.

Thermodynamic and structural analysis of homodimeric proteins: Model of β-lactoglobulin

The energetics of protein homo-oligomerization was analyzed in detail with the application of a general thermodynamic model. We have studied the thermodynamic aspects of protein-protein interaction employing β-lactoglobulin A from bovine milk at pH=6.7 where the protein is mainly in its dimeric form. We performed differential calorimetric scans at different total protein concentration and the resulting thermograms were analyzed with the thermodynamic model for oligomeric proteins previously developed. The thermodynamic model employed, allowed the prediction of the sign of the enthalpy of dimerization, the analysis of complex calorimetric profiles without transitions baselines subtraction and the obtainment of the thermodynamic parameters from the unfolding and the association processes and the compared with association parameters obtained with Isothermal Titration Calorimetry performed at different temperatures. The dissociation and unfolding reactions were also monitored by Fourier-transform infrared spectroscopy and the results indicated that the dimer of β-lactoglobulin (N(2)) reversibly dissociates into monomeric units (N) which are structurally distinguishable by changes in their infrared absorbance spectra upon heating. Hence, it is proposed that β-lactoglobulin follows the conformational path induced by temperature:N(2)⇌2N⇌2D. The general model was validated with these results indicating that it can be employed in the study of the thermodynamics of other homo-oligomeric protein systems.

Atomistic computer simulations on the generation of bimetallic nanoparticles

Computer simulations on the generation of bimetallic nanoparticles are presented in this work. Two different generation mechanisms are simulated: (a) cluster-cluster collision by means of atom dynamics simulations; and (b) nanoparticle growth from a previous seed through grand canonical Monte Carlo (gcMC) calculations. When two metal nanoparticles collide, different structures are found: core/shell, alloyed and three-shell (A-B-A). On the other hand, the growth mechanism at different chemical potentials by means of gcMC reveals the same results as atom dynamics collisions do.

Thermodynamic model for the analysis of calorimetric data of oligomeric proteins.

The thermodynamic parameters for the process of protein unfolding can be obtained through differential scanning calorimetry. However, the unfolding process may not be a two-state one. Between the native and the unfolded state, there may be association or dissociation processes or the formation of an intermediate state. As a consequence of this, the precise interpretation of the calorimetric data should be done with a specific thermodynamic model. In this work, we present two general models for the unfolding process of an oligomeric protein: N n right harpoon over left harpoon nN right harpoon over left harpoon nD (model A) and N n right harpoon over left harpoon I n right harpoon over left harpoon nD (model B). In model A, the first step represents the dissociation of the oligomer into the monomeric native species, and the second step represents the denaturation process. In model B, the first step represents the conformational change of the oligomer, and the second step represents the dissociation of this species with the concomitant unfolding process. A canonical ensemble was employed to describe these systems, by considering that the total protein concentration remains constant. In the present work, we show and analyze the behavior of these systems in different conditions and how this analysis could help with the identification of the unfolding mechanism experimentally observed.

Adsorption of phenosafranin at the water ∣ 1,2 DCE interface: a voltammetric approach☆

Abstract In the present paper the interfacial behavior of a cationic dye, phenosafranin (PhS+), is voltammetrically analyzed. A sharp and narrow peak during the negative sweep was observed. The dependence of this peak on concentration, sweep rate and switch potential was a clear indication of an adsorption–desorption process similar to those observed on solid electrodes. It was determined that the adsorbed species results from the ion pairing between dicarbollylcobaltate (DCC−) and PhS+ at the interface as the process is observed only when the interface is polarized at the potential transfer of DCC−. The process fitted with a Frumkin isotherm and indicated a strong attractive interaction (g=−2.3) between the adsorbed species. This interaction was also analyzed spectrophotometrically.

Facilitated alkali ion transfer at the water 1,2-dichloroethane interphase: Ab-initio calculations concerning alkaline metal cation - 1,10-phenanthroline complexes.

Abstract A series of calculations on the energetics of complexation of alkaline metal cations with 1,10-phenanthroline are presented. It is an experimental fact that the ordering of the Gibbs energy of transfer across the water|1,2-dichloroethane interphase is governed by the charge/size ratio of the different cations; the larger cations showing the lower Gibbs energy of transfer. This ordering of the Gibbs energies of transfer is reverted in the presence of 1,10-phenanthroline in the organic phase. We have devised a thermodynamic cycle for the transfer process and, by means of ab initio calculations, have drawn the conclusion that in the presence of phenanthroline the Gibbs energy of transfer is governed by the stability of the phenanthroline/M+ complex, which explains the observed tendency from a theoretical point of view.

Computational Tools to Study and Predict the Long-Term Stability of Nanowires.

held in 2007, Gordon Moore recognized that by about 2020, his law would come up against the laws of physics. Furthermore, he recognized a change in a paradigm: the replacement of the top-down approach currently used for building circuits by a bottom-up procedure, where chips would be assembled using individual atoms or molecules. This is nothing but the realm of nanotechnology, while there is some consensus that the elementary switches of these circuits should be molecules with some feature allowing for the on/off status required for the components of logical devices, many questions remain concerning their stability. In the case of micrometric circuit components temperature may be an issue, but in the case of single molecules thermal effects may be overwhelming, since current flow occurs across a single bond. The lifetime of this bond, will determine the lifetime of the circuit component. Under these conditions, circuit engineering will be coming unexpectedly close to chemical kinetics. It still is far from clear which will be the technological procedure for the massive production of these molecular circuits. However, there are a number of experimental techniques for the study of their properties that are well established. These are shown schematically in Fig. 1. Fig. 1d shows a method devised to study the structure of monatomic nanowires (NWs). It has been developed by Kondo and Takayanagi (Kondo & Takayanagi, 1997) using High Resolution Transmission Electronic Microscopy (HRTEM) and allows the generation of suspended NWs. In this approach nanowires are generated in situ by focusing an electron beam on adjacent sites of a self-supported metal thin film (ca. 3 nm), making holes and allowing them to grow until a nanometric bridge is formed inside or between grains. The