Lucia H. Mascaro

Direct electrochemical determination of carbaryl using a multi-walled carbon nanotube/cobalt phthalocyanine modified electrode.

The electrochemical detection of carbaryl at low potentials, in order to avoid matrix interferences, is an important challenge. This study describes the development, electrochemical characterization and utilization of a glassy carbon (GC) electrode modified with multi-wall carbon nanotubes (MWCNT) plus cobalt phthalocyanine (CoPc) for the quantitative determination of carbaryl in natural waters. The surface morphology was examined by scanning electron microscopy, enhanced sensitivity was observed with respect to bare glassy carbon and electrocatalytic effects reduced the oxidation potential to +0.80 V vs. SCE in acetate buffer solution at pH 4.0. Electrochemical impedance spectroscopy was used to estimate the rate constant of the oxidation process and square-wave voltammetry to investigate the effect of electrolyte pH. Square-wave voltammetry in acetate buffer solution at pH 4.0, allowed the development of a method to determine carbaryl, without any previous step of extraction, clean-up, or derivatization, in the range of 0.33-6.61 micromol L(-1), with a detection limit of 5.46+/-0.02 nmol L(-1) (1.09+/-0.02 microg L(-1)) in water. Natural water samples spiked with carbaryl and without any purification step were successfully analyzed by the standard addition method using the GC/MWCNT/CoPc film electrode.

Electrodeposition of Ni-Mo and Fe-Mo alloys from sulfate-citrate acid solutions

This paper reports on a study of the electrodeposition of Ni-Mo and Fe-Mo alloys on Pt electrodes in a sodium citrate solution at pH 4.0. The alloys were subjected to cyclic voltammetry at different ion concentration ratios to identify their behavior. The electrodeposited coatings were analyzed by SEM, EDX and XRD. The results of the cyclic voltammetry tests indicated the presence of dissolution peaks only with high Ni or Fe ion concentrations. The SEM analysis revealed that the two alloys had dissimilar morphologies and the Ni-Mo alloys were homogeneous and thick. The Mo and O contents were higher in the Ni-Mo than in the Fe-Mo alloys. Anomalous deposition was found to occur in the co-deposition of Ni and Mo.

Underpotential deposition of silver on polycrystalline platinum studied by cyclic voltammetry and rotating ring-disc techniques

The underpotential deposition of Ag (UPD Ag) on polycrystalline Pt surfaces has been studied combining cyclic voltammetry and rotating ring-disc measurements. A new voltammetric methodology is proposed for the proper calculation of the anodic charge density in UPD Ag desorption measurements. It involves an evaluation of the surface oxidation contribution by measurement of the charge related to PtO reduction in the following scan of the voltammetric experiment. This methodology allows one to obtain the individual contributions to the total anodic charge density. Collecting experiments carried out with the rotating ring-disc system fully confirmed the validity of the procedure. In this way, a complete monolayer of Ag deposited on polycrystalline Pt from a 0.5 M H2SO4–5.0 × 10−6M AgNO3 solution showed an experimental maximum coverage of 90%. The remaining surface was covered by residual H atoms. This phenomenon was evident for either pure UPD Ag or for depositions involving also bulk Ag. Therefore, apart from the misfit between ad-atoms and substrate, UPD Ag occurs by a straightforward mechanism involving one-electron transfer and resulting in a complete monolayer of Agads. The reappearance of adsorbed hydrogen was observed for depositions beyond two monolayers. This was associated with a surface rearrangement followed by three-dimensional nucleation of Ag that generates free-sites for H-adsorption. No evidence of Ag insertion in the Pt phase was detected in this work.

Influence of the different carbon nanotubes on the development of electrochemical sensors for bisphenol A

Different methods of functionalisation and the influence of the multi-walled carbon nanotube sizes were investigated on the bisphenol A electrochemical determination. Samples with diameters of 20 to 170 nmwere functionalized in HNO3 5.0 mol L(-1) and a concentrated sulphonitric solution. The morphological characterisations before and after acid treatment were carried out by scanning electron microscopy and cyclic voltammetry. The size and acid treatment affected the oxidation of bisphenol A. The multi-walled carbon nanotubes with a 20-40 nm diameter improved the method sensitivity and achieved a detection limit for determination of bisphenol A at 84.0 nmol L(-1).

A Comparative Electrochemical Behaviour Study and Analytical Detection of the p-Nitrophenol Using Silver Solid Amalgam, Mercury, and Silver Electrodes

This work reports a comparative electrochemical behaviour study and p-nitrophenol analytical detection using silver solid amalgam, hanging dropping mercury, and silver electrodes. For this, square wave voltammetry was employed, where the analytical responses and the redox mechanisms could be compared for reduction processes of 4-nitrophenol by analysis of the voltammetric responses. The analytical performance of the electrode was evaluated and detection and quantification limits, recovery percentages, repeatability, and reproducibility for the silver solid amalgam and hanging dropping mercury electrodes presented similar values; the results presented for the silver electrode indicated worse analytical parameters than the other electrodes. The results indicate that the silver solid amalgam electrode can be considered a suitable tool and an interesting alternative for the analytical determination of 4-nitrophenol, as well as for the determination of other biological and environmentally interesting compounds that present analytical responses on mercury surfaces.

Diclofenac on Boron-Doped Diamond Electrode: From Electroanalytical Determination to Prediction of the Electrooxidation Mechanism with HPLC-ESI/HRMS and Computational Simulations

Using square-wave voltammetry coupled to the boron-doped diamond electrode (BDDE), it was possible to develop an analytical methodology for identification and quantification of diclofenac (DCL) in tablets and synthetic urine. The electroanalytical procedure was validated, with results being statistically equal to those obtained by chromatographic standard method, showing linear range of 4.94 × 10(-7) to 4.43 × 10(-6) mol L(-1), detection limit of 1.15 × 10(-7) mol L(-1), quantification limit of 3.85 × 10(-7) mol L(-1), repeatability of 3.05% (n = 10), and reproducibility of 1.27% (n = 5). The association of electrochemical techniques with UV-vis spectroscopy, computational simulations and HPLC-ESI/HRMS led us to conclude that the electrooxidation of DCL on the BDDE involved two electrons and two protons, where the products are colorful and easily hydrolyzable dimers. Density functional theory calculations allowed to evaluate the stability of dimers A, B, and C, suggesting dimer C was more stable than the other two proposed structures, ca. 4 kcal mol(-1). The comparison of the dimers stabilities with the stabilities of the molecular ions observed in the MS, the compounds that showed retention time (RT) of 15.53, 21.44, and 22.39 min were identified as the dimers B, C, and A, respectively. Corroborating the observed chromatographic profile, dimer B had a dipole moment almost twice higher than that of dimers A and C. As expected, dimer B has really shorter RT than dimers A and C. The majority dimer was the A (71%) and the C (19.8%) should be the minority dimer. However, the minority was the dimer B, which was formed in the proportion of 9.2%. This inversion between the formation proportion of dimer B and dimer C can be explained by preferential conformation of the intermediaries (cation-radicals) on the surface.

Utilização de eletrodos sólidos de amálgama para a determinação analítica de compostos orgânicos e inorgânicos

This review reports the use of solid amalgam electrodes in the electroanalytical determination of organic and inorganic compounds. The different types of amalgam electrodes are presented, and attention is paid to solid amalgam electrode, and here is presented details about the pre-treatment for activation and renovation and also possible modifications in its surface. The wide potential range, higher signal-to-noise ratio, mechanical stability enabling their application in flowing systems, and principally their resistance toward passivation, indicate that the solid amalgam electrodes are environmentally friendly alternatives to mercury electrodes, without loss in the sensitivity and reproducibility in voltammetric responses.

Nucleation and growth of tin-zinc electrodeposits on a polycrystalline platinum electrode in tartaric acid

Current transients measurements and scanning electron microscopy were used to characterize the electrocrystallization process and morphology of tin-zinc alloys electrodeposited on Pt, in the absence and in the presence of tartaric acid. The model of Scharifker and Hills was used to analyze the current transients and it revealed that Sn-Zn electrocrystallization process in the presence of tartrate, under the studied conditions, is governed by three-dimensional progressive nucleation controlled by diffusion. In the absence of the complexant, the results indicated that nucleation process changes from instantaneous to progressive when the deposition potential becomes more negative, or when the incorporation of zinc occured to the deposit. The microscopic results showed deposits with two layers with different morphologies and is also influenced by deposition potential.

Eletrodegradação de Ponceau 2R utilizando ânodos dimensionalmente estáveis e Ti/Pt

This paper reports the electrochemical degradation of the azo dye Ponceau 2R under galvanostatic electrolysis in the 1 to 200 mA cm-2 range at room temperature using dimensionally-stable anodes of oxygen (DSA-O2), chlorine (DSA-Cl2) and a titanium electrode of platinum coated with platinum oxide (Ti/Pt). The methodology applied was efficient for removing the color of the Ponceau 2R and the highest percentage removal of total organic carbon was obtained at 200 mA cm-2. Despite not having been observed complete mineralization, approximately80% removal of aromatic rings was estimated, resulting in drastic reduction of toxicity of the sample.

Electrochemical Synthesis of Polyaniline/Poly-O-Aminophenol Copolymers in Chloride Medium

The copolymerization of o-aminophenol (OAP) and aniline (ANI) on Pt and ITO electrodes was studied using cyclic voltammetry in 0.1 M HCl/0.4 M NaCl solution. The films were characterized by SEM, cyclic voltammetry, and UV-Vis spectroscopy. The properties of the copolymer were compared with PANI and POAP films. The results strongly suggest that the growth of PANI-POAP films does not consist of the simple buildup of layers of homopolymers on the electrode surface as a result of OAP or ANI oxidation products in the monomer mixture, but that a new conducting polymer is formed by copolymerization.