Publications

Abstract The adsorption behaviour of a series of phosphatidylcholines (PCs) with saturated carbon chains of different length (DLPC, DPPC, DSPC, DAPC, and DBPC) at the electrified 1,2-dichloroethane/water interface was studied by measuring electrochemical impedance spectroscopy at the polarized interface. Two different trends in the interfacial capacitance were observed for any of the PCs the capacity dependence on the applied potential: strong adsorption occurs at negative potential with a marked decrease of C(E); increase of capacity is observed at positive potentials. It is demonstrated that the interfacial lipid adsorption was dependent on phospholipid concentration, applied potential, and phospholipid chain length. The potential and concentration dependence of the interfacial capacitance in the presence of these phospholipids were successfully described by a model based on the solution of the Poisson-Boltzmann equation in the interfacial region.

Abstract An electrochemical approach for the improved electrochemical sensing of DNA was developed in this study based on the oxidation signals of silver and DNA base, guanine by using disposable pencil graphite electrode (PGE) electrodes. The easy surface modification of disposable electrodes PGEs with nucleic acids was performed by passive adsorption using amino linked DNA oligonucleotide attached onto the surface of silver nanoparticles (Ag-NPs). Firstly, the microscopic characterization of silver nanoparticles was investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and the electrochemical behaviour of these NPs was studied by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. Then, the overall performance of this novel electrochemical DNA sensing method based on these nanoparticles is studied and discussed in terms of optimum analytical conditions, such as; the effect of DNA concentration, NPs concentration and different buffer solutions, etc. in order to obtain silver and guanine oxidation signals in higher sensitivity and selectivity. The main features related with this electrochemical assay based on silver nanoparticles are discussed and compared with other assays reported in the literatures.

Abstract We present a proposal for research, development and construction of international communities in Portuguese using digital platforms. The idea was born following an international conference on distance teaching and learning, in which a meeting on 'E-learning and Portuguese-language culture' was promoted. The established contacts gave place to a new project to stimulate learning communities via the internet. Currently, the countries involved are Portugal, Brazil and Mozambique, but in a near future other Portuguese-speaking communities will be able to participate. The theoretical framework is provided by Vigotsky's ideas on learning contexts. Copyright © 2007 Inderscience Enterprises Ltd.

Abstract Multilayer films composed of poly(L-arginine) (pArg) and mercaptoundecanoic acid (MUA) stabilized gold nanoparticles (Au-MUA NPs) have been fabricated based on the electrostatic layer-by-layer self-assembly technique upon a gold electrode modified with a first layer of mercaptosuccinic acid (MSA). The formation of the pArg/Au-MUA NP self-assemblies as alternative multilayers was confirmed by UV-vis absorption spectroscopy and atomic force microscopy while their electrochemical properties were studied using cyclic voltammetry, square wave voltammetry, and electrochemical impedance spectroscopy. Charge transport through the multilayer was studied experimentally by using the redox pair [Fe(CN)(6)](3-/4-). It was found that these new assemblies have a high permeability to the probe ions. The presence of the Au-MUA NPs greatly improves the conductivity and the electron-transfer ability of the film which exhibited new electrical properties characterized by a low impedance response and enhanced electric current as more layers were added for both Au-MUA NP and pArg terminated multilayers. It is concluded that the behavior observed is based on two cumulative contributions: electron transfer mediated by the Au-MUA NPs layers and ionic diffusion favored by the poly(L-arginine) layers due to the Donnan inclusion. The films obtained showed high conductive properties which represent very promising features for the construction of electrochemical sensors or nanoelectronic devices.

Abstract The standard molar enthalpies of formation of the 3-methyl-N-R-2-quinoxalinecarboxamide-1,4-dioxides (R = H, phenyl, 2-tolyl) in the gas phase were derived using the values for the enthalpies of combustion of the crystalline compounds, measured by static bomb combustion calorimetry, and for the enthalpies of sublimation, measured by Knudsen effusion, at T = 298.15 K. These values have also been used to calibrate a computational procedure that has been employed to estimate the gas-phase enthalpies of formation of the corresponding 3-methyl-N-R-2-quinoxalinecarboxamides and also to compute the first, second, and mean N-O bond dissociation enthalpies in the gas phase. It is found that the size of the substituent almost does not influence the computed N-O bond dissociation enthalpies; the maximum enthalpic difference is similar to 5 kJ center dot mol(-1).

Abstract The gaseous standard molar enthalpies of formation of two 2-R-3-methylquinoxaline-1,4-dioxides (R = benzoyl or tert-butoxycarbonyl), at T = 298.15 K, were derived using the values for the enthalpies of formation of the compounds in the condensed phase, measured by static bomb combustion calorimetry, and for the enthalpies of sublimation, measured by Knudsen effusion, using a quartz crystal oscillator. The three dimensional structure of 2-tert-butoxycarbonyl-3-methylquinoxaline-1,4-dioxide has been obtained by X-ray crystallography showing that the two N-O bond lengths in this compound are identical. The experimentally determined geometry in the crystal is similar to that obtained in the gas-phase after computations performed at the B3LYP/6-311 + G(2d,2p) level of theory. The experimental and computational results reported allow to extend the discussion about the influence of the molecular structure on the dissociation enthalpy of the N-O bonds for quinoxaline 1,4-dioxide derivatives. As found previously, similar N-O bond lengths in quinoxaline-1,4-dioxide compounds are not linked with N-O bonds having the same strength. Copyright (c) 2007 John Wiley & Sons, Ltd.

Abstract Silver thin films were deposited with different preferential orientations and special attention was paid to the bioreactivity of the surfaces. The study was essentially focused on the evaluation of the films by x-ray diffraction (XRD), atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), electron probe microanalysis (EPMA) and contact angle measurements. The deposited thin films were characterized before and after immersion in S-enriched simulated human plasma in order to estimate the influence of the preferential crystallographic orientation on the in vitro behaviour. Silver thin films with and without ( 111) preferential crystallographic orientation were deposited by r.f. magnetron sputtering to yield nanocrystalline coatings, high compact structures, very hydrophobic surfaces and low roughness. These properties reduce the chemisorption of reactive species onto the film surface. The in vitro tests indicate that silver thin films can be used as coatings for biomaterials applications.

Abstract The latest results of voltammetric research on the ionic transfer process of ionisable drugs across bare and lipid-modified liquid-liquid interfaces are reviewed. In recent years, two voltammetric methods have been extensively applied to this purpose, i.e. the classical four electrode voltammetry at the interface between two immiscible electrolyte solutions, and the "three-phase electrode." Thus, a brief background of the methodologies and some successful examples of their application are highlighted in this work. Particular attention is given to the ionic transfer kinetics and to the electro-chemical characterization of the drug-membrane interactions between the ionized drugs and lipid-modified interfaces. Future trends in this area are also mentioned in connection with high-throughput assessment of ADMET properties of drugs.

Abstract The quenching of the fluorescence of excitation emission matrices (EEM) of two samples of dissolved organic matter (DOM) [fulvic acid from a dam water (FA) and a commercial humic acid (HA)], provoked by the metal ions Cu(II), UO22+, Fe(III) and Hg(II), was studied by principal component analysis (PCA) and multivariate curve resolution with alternating least squares (MCR-ALS). PCA of the individual EEM, sets of EEM and the sequential analysis of EEM allows the determination of the number of components that provoke linearly independent variations in the EEM and to assess the trend and stability of the quenching process. Four and three components were detected in the EEM of FA and HA, respectively. Some of these components show quenching in the presence of the studied metal ions and other are not affected. Also, the occurrence of scattering due to hydrolysis of metal ions is detected in the PCA sequential analysis of EEM as function of the metal ion concentration. Excitation and emission spectra and quenching profiles were extracted from EEM using MCR-ALS with non-negativity constraints. Stability constants between FA and HA with the studied metal ions were estimated by a modified Stern-Volmer equation.

Abstract The factors that affect trihalomethane (THM) (chloroform, bromodichloromethane, chlorodibromomethane and bromoform) formation from the chlorination of aqueous solutions of hydrophobic fulvic acids (FA) were investigated in a prototype laboratorial simulation using factorial analysis. This strategy involved a fractional factorial design (16 plus 5 center experiments) of five factors (fulvic acids concentration, chlorine dose, temperature, pH and bromide concentration) and a Box Behnken design (12 plus 3 center experiments) for the detailed analysis of three factors (FA concentration, chlorine dose and temperature). The concentration of THM was determined by headspace analysis by GC-ECD. The most significant factors that affect the four THM productions were the following: chloroform-FA concentration and temperature; bromodichloromethane-FA concentration and chlorine dose; chlorodibromomethane-chlorine dose; and, bromofonn-chlorine dose and bromide concentration. Moreover, linear models were obtained for the four THM concentrations in the disinfection solution as function of the FA concentration, chlorine dose and temperature, and it was observed that the complexity of the models (number of significant factors and interactions) increased with increasing bromine atoms in the THM. Also, this study shows that reducing the FA concentration the relative amount of bromated THM increases.