Ilanna C. Lopes

Electroanalytical determination of carbendazim by square wave adsorptive stripping voltammetry with a multiwalled carbon nanotubes modified electrode

A preconcentrating/voltammetric multiwalled carbon nanotube modified glassy carbon electrode (MWCNT–GCE) has been developed for stripping analysis of carbendazim (Methyl Benzimidazol-2-yl Carbamate—MBC), based on dispersing MWCNT in water. The effect of experimental variables, such as the dispersion and loading of MWCNT, was assessed. A quasi-reversible behavior for MBC in acetic acid/acetate buffer 0.1 mol L−1 (pH 4.7) was verified and its high effective pre-concentration was attributed to the high adsorption capability and enormous surface area of the MWCNT. No evidence of carry-over effect, combined with the easiness of electrode preparation, led to the development of a highly sensitive and reliable method with an experimental work range from 0.256 to 3.11 µmol L−1 with a detection limit of 10.5 ppb for a short (60 s) accumulation period. Measurement of MBC in a river water sample was demonstrated. The accuracy of the method for real sample analysis was assessed by estimating the apparent recovery (93 ± 2.9% and 86 ± 4.1% for 4.3 × 10−7 mol L−1) for a MBC spiked river water sample.

In situ electrochemical evaluation of anticancer drug temozolomide and its metabolites–DNA interaction

Temozolomide (TMZ) is an antineoplastic alkylating agent with activity against serious and aggressive types of brain tumours. It has been postulated that TMZ exerts its antitumor activity via its spontaneous degradation at physiological pH. The in vitro evaluation of the interaction of TMZ and its final metabolites, 5-aminoimidazole-4-carboxamide (AIC) and methyldiazonium ion, with double-stranded DNA (dsDNA) was studied using differential pulse voltammetry at a glassy carbon electrode. The DNA damage was electrochemically detected following the changes in the oxidation peaks of guanosine and adenosine residues. The results obtained revealed the decrease of the dsDNA oxidation peaks with incubation time, showing that TMZ and AIC/methyldiazonium ion interact with dsDNA causing its condensation. Furthermore, the experiments of the in situ TMZ and AIC/methyldiazonium ion–dsDNA interaction using the multilayer dsDNA-electrochemical biosensor confirmed the condensation of dsDNA caused by these species and showed evidence for a specific interaction between the guanosine residues and TMZ metabolites, since free guanine oxidation peak was detected. The oxidative damage caused to DNA bases by TMZ metabolites was also detected electrochemically by monitoring the appearance of the 8-oxoguanine/2,8-dyhydroxyadenine oxidation peaks. Nondenaturing agarose gel electrophoresis of AIC/methyldiazonium ion–dsDNA samples confirmed the occurrence of dsDNA condensation and oxidative damage observed in the electrochemical results. The importance of the dsDNA-electrochemical biosensor in the in situ evaluation of TMZ–dsDNA interactions is clearly demonstrated.

In situ evaluation of gemcitabine-DNA interaction using a DNA-electrochemical biosensor.

The electrochemical behaviour of the cytosine nucleoside analogue and anti-cancer drug gemcitabine (GEM) was investigated at glassy carbon electrode, using cyclic, differential pulse and square wave voltammetry, in different pH supporting electrolytes, and no electrochemical redox process was observed. The evaluation of the interaction between GEM and DNA in incubated solutions and using the DNA-electrochemical biosensor was studied. The DNA structural modifications and damage were electrochemically detected following the changes in the oxidation peaks of guanosine and adenosine residues and the occurrence of the free guanine residues electrochemical signal. The DNA-GEM interaction mechanism occurred in two sequential steps. The initial process was independent of the DNA sequence and led to the condensation/aggregation of the DNA strands, producing rigid structures, which favoured a second step, in which the guanine hydrogen atoms, participating in the C-G base pair, interacted with the GEM ribose moiety fluorine atoms.

Microcystin-LR and chemically degraded microcystin-LR electrochemical oxidation

Microcystins (MCs) are cyclic hepatotoxic heptapeptides produced by certain strains of freshwater cyanobacteria toxic for humans and animals. The electrochemical behaviour of microcystin-LR (MC-LR) at a glassy carbon electrode (GCE) was investigated using cyclic voltammetry (CV), square wave voltammetry (SWV) and differential pulse voltammetry (DPV). The oxidation of MC-LR is a diffusion-controlled irreversible and pH-independent process that occurs with the transfer of only one electron and does not involve the formation of any electroactive oxidation product. Upon incubation in different pH electrolytes, homogeneous degradation of MC-LR in solution was electrochemically detected by the appearance of a new oxidation peak at a lower potential. The electrochemical behaviour of chemically degraded MC-LR is an irreversible, pH-dependent process, and involves the formation of two redox products that undergo reversible oxidation. The formation of degradation products of MC-LR was confirmed by HPLC with UV detection at room temperature. Experiments were also carried out in solutions containing constituent MC-LR amino acids, which enabled the understanding of the MC-LR electron transfer reaction and degradation. An oxidation mechanism for MC-LR is proposed.

Sorbic Acid and Its Degradation Products: Electrochemical Characterization

The electrochemical redox behavior of sorbic acid (SA), an important food preservative, was investigated at a glassy carbon electrode using cyclic, differential pulse, and squarewave voltammetry over a wide pH range. The oxidation of SA is an irreversible, diffusion-controlled, and pH-independent process that occurs with the transfer of only one electron and does not involve the formation of any electroactive oxidation product. Adsorption of SA at GCE electrodes was also observed. Following incubation in different pH electrolytes, the degradation of SA was electrochemically detected by the appearance of a new oxidation peak at lower potential value. The degradation products, formed homogenously in solution, undergo irreversible oxidation and lead to the formation of two oxidation products that strongly adsorb on the electrode surface and are reversibly oxidized. SA degradation was also confirmed using HPLC and UV-Vis spectrophotometry. A mechanism for oxidation of SA and its degradation products in aqueous solutions was proposed.

Cyanobacterial Hepatotoxins Oxidation Mechanisms and Interaction with DNA

Cyanobacteria (blue-green algae) produce a variety of harmful xadcompounds known as cyanotoxins. Microcystin-LR (MC-LR) and nodularin (NOD) are two potent cyanotoxins with strong hepatotoxic activity which pose a major risk to animals and humans, causing illness and death. Several methods for the detection and degradation of these toxins have been developed over the last years due to MC-LR and NOD strong toxic effects. An electrochemical investigation of MC-LR and NOD electrochemical behaviour, as well as their chemical degradation, is described and emerges as an innovation in the cyanotoxins detection field. The MC-LR and the NOD-DNA interactions were investigated using a DNA-electrochemical xadbiosensor, adding more data to the studies of genotoxicity and carcinogenic potential associated to these toxins.