Mario V. Vázquez

Electrochemical methods as a tool for determining the antioxidant capacity of food and beverages: A review

The growing interest in functional foods had led to the use of analytical techniques to quantify some properties, among which is the antioxidant capacity (AC). In order to identify and quantify this capacity, some techniques are used, based on synthetic radicals capture; and they are monitored by UV-vis spectrophotometry. Electrochemical techniques are emerging as alternatives, given some of the disadvantages faced by spectrophotometric methods such as the use of expensive reagent not environmentally friendly, undefined reaction time, long sample pretreatment, and low precision and sensitivity. This review focuses on the four most commonly used electrochemical techniques (cyclic voltammetry, differential pulse voltammetry, square wave voltammetry and chronoamperometry). Some of the applications to determine AC in foods and beverages are presented, as well as the correlation between both spectrophotometric and electrochemical techniques that have been reported.

Enzymatic Functionalization of Cork Surface with Antimicrobial Hybrid Biopolymer/Silver Nanoparticles

Laccase-assisted assembling of hybrid biopolymer-silver nanoparticles and cork matrices into an antimicrobial material with potential for water remediation is herein described. Amino-functional biopolymers were first used as doping agents to stabilize concentrated colloidal dispersions of silver nanoparticles (AgNP), additionally providing the particles with functionalities for covalent immobilization onto cork to impart a durable antibacterial effect. The solvent-free AgNP synthesis by chemical reduction was carried out in the presence of chitosan (CS) or 6-deoxy-6-(ω-aminoethyl) aminocellulose (AC), leading to simultaneous AgNP biofunctionalization. This approach resulted in concentrated hybrid NP dispersion stable to aggregation and with hydrodynamic radius of particles of about 250 nm. Moreover, laccase enabled coupling between the phenolic groups in cork and amino moieties in the biopolymer-doped AgNP for permanent modification of the material. The antibacterial efficiency of the functionalized cork matrices, aimed as adsorbents for wastewater treatment, was evaluated against Escherichia coli and Staphylococcus aureus during 5 days in conditions mimicking those in constructed wetlands. Both intrinsically antimicrobial CS and AC contributed to the bactericidal effect of the enzymatically grafted on cork AgNP. In contrast, unmodified AgNP were easily washed off from the material, confirming that the biopolymers potentiated a durable antibacterial functionalization of the cork matrices.

NADH Oxidation onto Different Carbon-Based Sensors: Effect of Structure and Surface-Oxygenated Groups

Different carbon-based materials have been compared for the development of NADH sensors: glassy carbon electrodes (GCE), multiwalled carbon nanotubes (MWCNT), and carbon black (CB). The GCE and MWCNT has been subjected to oxidative pretreatment to study the influence of oxidative groups for NADH oxidation. The materials had been characterized by FT-IR to identify the surface composition. The response of bare (GC) and GC/modified electrodes toward potassium ferricyanide have been employed to obtain information about the electroactive area and electron transfer rate. Studies of NAD+/NADH redox behavior showed that MWCNT and GCE exhibit high degree of passivation while CB shows no fouling effects. Catalytic effect of surface-oxygenated groups was also proved for GCE and MWCNT, and both, O-GCE and O-MWCNT, exhibited a lower oxidation overpotential compared to the respective untreated materials. Chronoamperometric quantification showed a linear dependence between 2–18 μmol·L−1 and a detection limits of 6.2 μmol·L−1 (GCE), 5.4 μmol·L−1 (O-GCE), 3.2 μmol·L−1 (GCE/CB), 9.6 μmol·L−1 (GCE/MWCNT), and 4.9 μmol·L−1 (GCE/O-MWCNT) were obtained. The analytical performances suggest that a careful choice of the material for NADH sensing is necessary depending on the sensor application.

Effect of time and electrical potential gradient on the buffer capacity of soils, as measured using an electrokinetic method

The buffer capacity of soils is a property of interest in the field of environmental remediation, and is related to chemical properties such as the nature of soil humic material and the variable charged surfaces of clays and hydrated oxides. An electrokinetic technique has been proposed as a method for the determination of the buffer capacity of soils, making use of the electrolytic reactions of water that arise after the application of an electrical field. Such reactions produce hydronium and hydroxyl ions, which are continually entering the soil. A study was made of the effect of the duration of the electrokinetic process (24, 48 and 72 h), as well as of the electrical potential gradient applied to the soil (1.2 and 1.8 V/cm), in the determination of the buffer capacity of a soil of volcanic origin. It was observed that time had a significant effect on the value obtained for the buffer capacity in a basic medium (fβ–), and that both variables had a significant effect on that obtained in an acid medium (fβ+). Moreover, the buffer capacity of two soils of volcanic origin obtained using the electrokinetic technique was compared with that obtained using the traditional volumetric method. The results obtained using both techniques showed good agreement.

Antioxidant capacity of mango fruit (Mangifera indica). An electrochemical study as an approach to the spectrophotometric methods

The antioxidant capacity in mango (pulp, peel, and seed) was measured using spectrophotometric and electrochemical methods in order to make traditional methods comparable with electrochemical ones, using reference standards Gallic Acid and Trolox. ABTS, DPPH, Total polyphenols, and electrochemical index were evaluated. In order to present the electrochemical results in a more comparable way, the voltammetric charge (using Differential pulse voltammetry) was used. Spectrophotometric methods allowed to determine the difference in contents of metabolites with antioxidant capacity, except in peel and seed, while the electrochemical method separated the three extracts and is not affected by interferences. Spectrophotometric methods present a good correlation with electrochemical methods, using the same reference standards, therefore, a better comparison between methods is possible.

Electrokinetics as an Alternative for Soil and Compost Characterization

Considering the type of disturbance applied in an electrokinetic technique, a low intensity constant electric field, and the effects that this disturbance generates: motion of charged species; this technique can be considered as a characterization tool of porous materials that are in contact with ions in solution, as is the case of soil and compost.