Ernesto J. Calvo

Study of atmospheric particulate matter in Buenos Aires city

Abstract The data reported here constitute the results of the first long-term monitoring campaign of daily PM10 and PM2.5 concentration levels in Buenos Aires city. Twenty-four hour averages atmospheric concentrations of PM10 and PM2.5 were measured (not simultaneously) at a site near downtown Buenos Aires, since December 1998–September 1999. The values of PM2.5 concentrations correlate well with the concentrations of carbon monoxide (CO) during the winter period, indicating that direct traffic emissions have an important contribution to PM2.5. The data are less correlated for the case of PM10, indicating that the sources of the coarse fraction are not only traffic emissions, with an important contribution of other sources, for example re-suspended material. Scanning electron microscopy and atomic force microscopy images of atmospheric particles on nucleopore filters are shown. Information about the ion and metal content of these particles is reported.

Localised electrochemical impedance spectroscopy with high lateral resolution by means of alternating current scanning electrochemical microscopy

A new method for measuring local interfacial impedance properties with high lateral resolution was developed by combination of electrochemical impedance spectroscopy (EIS) with scanning electrochemical microscopy (SECM). Alternating current scanning electrochemical microscopy (AC-SECM) allowed to identify and visualise microscopic domains of different conductivity/electrochemical activities on solid/liquid interfaces immersed into an electrolyte. The performance of the method was illustrated by imaging an array of Pt-band microelectrodes in solutions of low conductivity in the absence of any redox mediator.

The electrochemical reduction of hydrogen peroxide on polycrystalline copper in borax buffer

Abstract The electrochemical reduction of hydrogen peroxide on copper electrodes in a borax buffer has been studied. The effects of surface oxidation and chloride ions in the electrolyte have been investigated. Hydrogen peroxide can be electrochemically reduced on Cu(I) oxidized surfaces. In the absence of chloride, the reduction is catalysed by a redox cycle involving the Cu(I)/Cu(II) couple. In chloride-containing solutions, the surface oxides are dissolved and the reduction becomes more irreversible. There appears to be a change in the reduction mechanism at potentials more negative than −0.45 V.

Layer-by-Layer Self-Assembled Osmium Polymer-Mediated Laccase Oxygen Cathodes for Biofuel Cells: The Role of Hydrogen Peroxide

High potential purified Trametes trogii laccase has been studied as a biocatalyst for oxygen cathodes composed of layer-by-layer self-assembled thin films by sequential immersion of mercaptopropane sulfonate-modified Au electrode surfaces in solutions containing laccase and osmium-complex bound to poly(allylamine), (PAH-Os). The polycation backbone carries the Os redox relay, and the polyanion is the enzyme adsorbed from a solution of a suitable pH so that the protein carries a net negative charge. Enzyme thin films were characterized by quartz crystal microbalance, ellipsometry, cyclic voltammetry, and oxygen reduction electrocatalysis under variable oxygen partial pressures with a rotating disk electrode. New kinetic evidence relevant to biofuel cells is presented on the detection of traces of H(2)O(2), intermediate in the O(2) reduction, with scanning electrochemical microscopy (SECM). Furthermore the inhibitory effect of peroxide on the biocatalytic current resulted in abnormal current dependence on the O(2) partial pressure and peak shape with hysteresis in the polarization curves under stagnant conditions, which is offset upon stirring with the RDE. The new kinetic evidence reported in the present work is very relevant for the operation of biofuel cells under stagnant conditions of O(2) mass transport.

Electrical Communication between Electrodes and Enzymes Mediated by Redox Hydrogels.

Different redox polymers based on poly(allylamine) with covalently attached ferrocene and pyridine groups that coordinate iron and ruthenium complexes were prepared, and hydrogels were obtained by cross-linking them with epichlorohydrin. Charge propagation from the underlying electrode, through the redox polymer and electrical communication with the enzyme FADH(2) of glucose oxidase, was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The effects of electrolyte composition, concentration of enzyme and substrate, and electrode potential are reported. The role of different redox mediators covalently attached to the polymer backbone is discussed in terms of driving force and electrostatic barriers.

Electrostatic interactions at self assembled molecular films of charged thiols on gold

Abstract Thiols of short alkyl chain length and different terminal group charge (COO − and NH 3 + ), and covalently attached ferrocene via terminal NH 2 and ferrocene carboxyaldehyde were self assembled on gold electrodes. Electron transfer to Fe(CN) 6 3− in solution and to covalently attached ferrocene centres were studied at different pH by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Changes in solution pH result in variation of the self assembled monolayer (SAM) terminal group charge. The disparate heterogeneous electron transfer apparent rates obtained from R ct have been described on the basis of the Boltzmann surface concentration factor and the potential distribution model of Smith and White and that of Fawcett for the Au–SAM-electrolyte interface.

The electrochemical reduction of oxygen on polycrystalline copper in borax buffer

Oxygen reduction proceeds on copper electrodes in boric + borate buffer by the four electron pathway. Difference reflectance spectroscope shows that surface oxide species are formed during oxygen reduction on pre-reduced copper. The redox couple Cu2O/CuO plays an important role in the reduction sequence and a CE mechanism is proposed, although direct electron transfer to the oxygen molecule from oxide-free surfaces appears possible. In this case, the behaviour of copper is similar to that of the isoelectronic metal gold.

Electrocatalysis of oxygen reduction at Fe3O4 oxide electrodes in alkaline solutions

The 0reduction of O2 at Fe3O4 cathodes in alkaline solutions takes place at electrode potentials where partial reduction of the hydrated Fe(III) surface layer occurs. Simultaneous electroreduction of the surface Fe(III) oxide and dioxygen suggests that octahedrally coordinated surface ferrous ions [Fe(II)OH−], are the active sites for oxygen electrocatalysis as proposed earlier for passive iron. The electroreduction of O2 on Fe3O4 cathodes is a two-electron process: O2 + H2O + 2e− → HO2− + HO− Further decomposition of the intermediate peroxide is catalysed by reduced magnetite with a high turnover: 2HO2− → O2 + 2HO− As a consequence of oxygen recycling the apparent number n* of electrons exchanged per oxygen molecule is strongly dependent on mass transport conditions. These results are relevant to understanding the mechanisms of reactions involved in the overall complex corrosion processes of ferrous alloys in aqueous aerated environments and in atmospheric corrosion.

Adsorption kinetics of charged thiols on gold nanoparticles

Colloidal Au nanoparticles have been functionalized with mercaptopropane sulfonate (MPS) and mercaptoethylamine (MEA) short alkanethiols in aqueous solutions. Adsorption kinetics of these charged thiols onto gold nanoparticles has been followed by the decrease in the surface plasmon band at 530 nm due to the red shift caused by particle-to-particle aggregation. Titration curves resulted from plotting the 530 and 800 nm-absorbance versus the added amount of thiol in solution, and yielded the mercaptane coverage on Au. While the adsorption of positively charged MEA on negatively charged Au colloid is very rapid, slower and more complex adsorption kinetics have been found for the negatively charged MPS.

Layer-by-layer electrostatic deposition of biomolecules on surfaces for molecular recognition, redox mediation and signal generation

Layer-by-layer supramolecular structures composed of alternate layers of negatively charged enzymes and cationic redox polyelectrolyte have been assembled. Glucose oxidase (GOx), lactate oxidase (LOx) and soybean peroxidase (SBP) have been electrically wired to the underlying electrode by means of poly(allylamine) with [Os(bpy)2ClPyCOH]+ covalently attached (PAA-Os) in organized structures with high spatial resolution. Biotinylated glucose oxidase has also been used to assemble step-by-step on antibiotin goat immunoglobulin (IgG) layers and the enzyme was electrically wired by PAA-Os. These spatially organized multilayers with mono- and bienzymatic schemes can work efficiently in molecular recognition, redox mediation and generation of an electrical signal. The concentration of redox mediator integrated into the multilayers, obtained from the voltammetric charge and an estimation of the layer thickness, exceeds by 100-fold the amount of deposited enzyme assessed by quartz crystal microbalance. Differences in GOx electrical wiring efficiency have been detected with the different assembling strategies. The surface concentration of electrically wired enzyme represents a small proportion of all the enzyme molecules present in the multilayers which can be oxidized by the soluble mediator [Os(bpy)2Cl PyCOOH]Cl. This proportion, as well as the rate of FADH2 oxidation by PAA-Os, increases with the number of electrically wired enzyme layers and with the spatial accessibility of the Os moiety to the enzyme active center.