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The electrocatalytic oxidation of tartaric acid on a carbon paste electrode modified with cobalt (II)-phthalocyanine was demonstrated and applied to the development of a highly sensitive, simple, fast and inexpensive voltammetric sensor to determine tartaric acid. The electrochemical behavior of the modified electrode was investigated by cyclic and square wave voltammetry, and the effect of experimental variables, such as dispersion and loading of cobalt (II)-phthalocyanine, together with optimum conditions for sensing the analyte by square wave voltammetry were assessed. In addition, the absence of a significant memory effect combined with the ease of electrode preparation led to the development of a sensitive and direct method to determine tartaric acid in wines. Interferences from other low molecular weight organic acids commonly present in wines were circumvented by using a multiway calibration technique, successfully obtaining the second order advantage by modeling voltammetric data with unfolded partial least square with residual bilinearization (U-PLS/RBL). A linear response range between 10 and 100 μmol L−1 (r = 0.9991), a relative prediction error of 4.55% and a recovery range from 96.41 to 102.43% were obtained. The proposed method is non-laborious, since it does not use sample pretreatment such as filtration, extraction, pre-concentration or cleanup procedures.

Raltitrexede (RTX) is a folate analogue that belongs to the antimetabolites family and has antineoplastic activity correlated to inhibition of the thymidylate synthase (TS) enzyme. This study addresses the electrochemical characterization of RTX at a glassy carbon electrode, on a wide interval of pH, using voltammetric techniques. The interaction between RTX and double helix DNA (dsDNA) in physiological medium is also addressed, using a DNA-electrochemical biosensor. A mechanism for the electrochemical oxidation of RTX is proposed. RTX is irreversibly electroxidized under a predominantly diffusion controlled and pH-dependent process. The oxidation process in acid and physiological media occurs in two consecutive steps. The first oxidation step is pH-independent and associated with the transfer of one electron of the nitro group at N10 position by releasing a methyl cation. The second oxidation step is pH-dependent and occurs with the transfer of one electron and one proton from the carbon at C9 position, followed by a direct attack by a water molecule leading to an irreversible dissociation of an oxidation product of RTX. A strong interaction between RTX and dsDNA was revealed by using a multilayer dsDNA electrochemical biosensor. The experiments demonstrated that RTX interacts and binds to the dsDNA helix by different interaction modes, suggesting an intercalation in between the DNA base pairs leading to defects in its secondary structure.

Isatin methyl derivatives have several pharmacotherapeutic applications, such as antibacterial, antifungal, antiviral, anticonvulsants and anticancer activities. The electrochemical behaviours of thirteen isatin-substituted derivatives with different functional groups, such as isatin 1-morpholinomethyl, isatin 1-hydroxymethyl, and isatin 1-, 5- or 7-methyl, and their halogenated derivatives, in phosphate buffer pH = 7.2, at a glassy carbon electrode, by cyclic, differential pulse and square wave voltammetry, were investigated. The oxidation mechanism of all isatin derivatives occurred in two consecutive irreversible charge transfer pH-dependent reactions. The first anodic reaction was on the benzene ring, with the generation of one hydroxyl group attached to the ring, which was oxidized in the second step to para- and/or ortho-quinone derivatives, and/or polymeric products. The reduction mechanism of all isatin derivatives, was an irreversible cathodic process, dependent on pH and the presence/absence of attached halogenated groups. The isatin derivatives reduction, in the absence of halogens, caused the irreversible cleavage of the carbonyl group, at the position C3, in the heterocyclic ring, and, in the presence of halogens, the cathodic peak corresponded to the carbon-halogen bond reduction. The functional groups attached to the isatin ring at the 1-position, morpholinomethyl-, hydroxymethyl- and methyl- are not electroactive, and do not affect considerably the isatin derivatives redox mechanism, since the oxidation occurs at the benzene ring, and the reduction at the carbonyl C3-position.

DNA damage has been associated with cancer. Alkylating agents interact with DNA to yield a variety of products, such as 7-methylguanosine (7-mGuo), a predominant adduct, 5-methylcytosine, 3-methyladenosine and o-6-methylguanosine. This work revealed, for the first time, the oxidation mechanism of 7-mGuo on a glassy carbon electrode (GCE) using cyclic and differential pulse voltammetry, as well as impedance spectroscopy. 7-mGuo is irreversibly oxidized in a pH dependent single step reaction, predominantly under diffusion control. The anodic behaviour of 7-mGuo was compared with the anodic behaviour of other guanine derivatives. An electroanalytical method was proposed for the determination of 7-mGuo using a GCE by differential pulse voltammetry at physiological pH, with a limit of detection of 3.26 μmol L−1 and a limit of quantification of 10.88 μmol L−1. The electrochemical detection of 7-mGuo was also investigated in the presence of the potential interferents guanine, guanosine and 7-methylguanine (7-mGua). The study demonstrated that the anodic peak of 7-mGuo did not suffer from any interference from these species, since its oxidation happens at a very distinct potential. A differential pulse voltammetry study of 7-mGua and 7-mGuo in the presence of double-helix DNA (dsDNA) or DNA bases was carried out aiming to investigate the possibility of using a DNA-electrochemical biosensor for detection of DNA methylation. The electrochemical data revealed that the DNA methylation damage can be detected by a distinct signal and quantified using a DNA biosensor.

An amine-functionalized porous sol–gel silica film was shown to be an effective platform to immobilize small anionic redox mediators of high solubility on solid electrodes by electrostatic interact...

The electrochemical oxidation of 5-methylcytosine (5-mCyt) was studied at glassy carbon electrode (GCE) in aqueous solution, over a wide pH range, using cyclic, square wave and differential pulse voltammetry, as well as compared with the anodic behaviour of other similar bases, Cyt, thymine (Thy) and uracil (Ura). The results revealed that the electrode reaction of 5-mCyt occurs in a single, one-electron one-proton, irreversible step controlled by diffusion, generating intermediate radicals that quickly react with water and/or dimerize. Moreover, in general, it was observed that the C5 methylated pyrimidines are more easily oxidized and thus are more reactive in relation to the unmethylated bases, Cyt and Ura. A mechanism for the electro-oxidation of 5-mCyt is proposed. Using differential pulse voltammetry (DPV) experimental conditions, such as the electrode size, supporting electrolyte composition, pH and influence of possible interferents (guanine, 7-methylguanine, adenine and Cyt), were also investigated for determination of 5-mCyt with low detection limit. Under the best conditions, the DPV method proposed, with a pre-treated 3.0 mm diameter GCE, provided a linear analytical curve to 5-mCyt in phosphate buffer solution (pH 7.0) in the concentration range from 3 up to 15 µmol L−1, with a detection limit of 0.11 µmol L−1 and a high correlation coefficient (r = 0.997). A new simple and sensitive electroanalytical method using an unmodified GCE for the quantification of DNA methylation is then presented.

This study investigates redox properties of fluorescein (FLSC), a fluorescent tracer with many applications in several areas, markedly in biochemical research and health care diagnosis, on glassy carbon electrode (GCE) at a wide interval of pH by using voltammetric techniques. Three peaks were observed at different potentials. The investigation revealed that FLSC is irreversibly electroxidized under a diffusion-controled and pH–dependent process. The oxidation process in acid and physiological media occurs in two consecutive steps with formation of a main electroactive oxidation product in acid medium. Both oxidation steps involve the transfer of one electron and one proton, corresponding to the oxidation of phenolic groups with formation of ortho-quinone derivatives, which are reversibly reduced to form catechol derivatives, and/or polymeric products. One electron and one proton are removed from the phenolic group at the position C6’ at the first step and at position C3’ at the second step. The diffusion coefficient of FLSC was assessed in pH=7.0 phosphate buffer (9.77×10−5 cm2 s−1). A differential pulse voltammetric method for determination of FLSC in physiological medium was also proposed.

A voltammetric study of the redox behavior of procarbazine (PCZ), one of the most used chemotherapy agents to face Hodgkin’s lymphoma, was carried out to investigate its oxidation at glassy carbon electrode (GCE). Essays of cyclic, differential pulse and square-wave voltammetries, on a large pH interval, revealed the electrochemical oxidation behavior of PCZ as an irreversible adsorption-controlled oxidation process, occurring in four consecutive charge transfer steps. The first, second and third steps are pH-dependent and occur with the transfer of one electron and one proton, leading to the formation of an electroactive product in acid media, while the last one is pH-independent, involving the transfer of one electron. A reaction mechanism for the electrochemical oxidation of PCZ is proposed, where, in the first and second step the oxidation occurs on the hydrazine group, followed by isomerization and, after hydrolysis, conversion to benzaldehyde-procarbazine. The third and fourth steps are associated with the oxidation of the benzaldehyde-procarbazine with the production of the N-isopropylterephthalic acid.

A multilayer dsDNA-electrochemical biosensor for the in situ electrochemical evaluation of anticancer antibody rituximab (RTX) interaction with double-stranded DNA (dsDNA), with differential pulse voltammetry was used. The decrease of the dsDNA oxidation peaks with incubation time was observed and was confirmed that RTX binds causing dsDNA condensation, and no evidence for DNA base oxidative damage was detected electrochemically. Nondenaturing agarose gel-electrophoresis of RTX-dsDNA samples confirmed the occurrence of condensation in the dsDNA structure.

The electrochemical behaviour of ortho-tyrosine (o-Tyr) and 3-nitro-tyrosine (3-NO2-Tyr) in aqueous media, over a wide pH interval, at glassy carbon electrode (GCE), was investigated and compared with para-tyrosine (p-Tyr), using cyclic, differential pulse and square wave voltammetry, as well as electrochemical impedance spectroscopy. In general, the p- and o-Tyr undergo oxidation in a single irreversible step pH-dependent, with the transfer of one electron and one proton, from the phenolic group to formation of Tyr phenoxy radical (Tyr ). However, while the p-Tyr radical preferably polymerizes, forming a resistive film on the GCE surface, the o-Tyr reacts preferentially with water with formation of o- and p-quinone derivatives, that are adsorbed and reversibly reduced on the GCE surface. In relation the 3-NO2-Tyr its oxidation occurs, in general, in two irreversible steps. The first step is pH-dependent, while the second is pH-independent, indicating the absence of protons in the process. The first process correspond to the oxidation of the phenolic group to form 3-NO2-Tyr , which reacts in different ways, polymerizing, forming a resistive film on the GCE surface and/or being directly electro-oxidized to a cationic product (second step). The voltammetric data also showed that the 3-NO2-Tyr phenol group is more difficult to oxidize when compared to p- and o-Tyr molecules. Moreover, unlike p-Tyr and o-Tyr that present no cathodic peak, 3-NO2-Tyr suffers in acid medium electro-reduction in a single irreversible step with formation of two electroactive products. Such processes were assigned to the reduction of the nitro group to form hydroxylamine and amine. Thus, is clearly demonstrated that the nitro group attached, as well as the phenolic group position at the Tyr molecule, strongly influence its redox properties. The redox mechanism of o-Tyr and 3-NO2-Tyr are presented and discussed.

The electrochemical behavior of ethidium bromide (EtBr) on a glassy carbon electrode (GCE) was studied by cyclic, square wave and differential pulse voltammetry over a wide pH interval. The results revealed that the oxidation mechanism of EtBr is an irreversible and adsorption-controlled electrode process that occurs in two consecutives steps. The first step is pH-dependent and occurs at the amino group in the C8 position with the formation of ortho- and para-quinone derivatives, while the second step is pH-independent and occurs at the amino group in the C3 position. A square wave method for quantitative determination of EtBr is also proposed.

Rituximab (RTX) is a human/murine chimeric monoclonal antibody (mAb) that specifically targets the transmembrane protein CD20 of B-cells. The oxidation mechanism of native and denatured RTX was investigated on glassy carbon electrode. The denaturing agent sodium dodecyl sulfate and the redutancts tris(2-carboxyethyl)phosphine and dithiothreitol were used. Significant differences were observed for native and denatured RTX oxidation due to morphological changes and unfolding of the RTX native structure. Native RTX presented only one oxidation peak of tyrosine and tryptophan residues, whereas in denatured RTX were detected three peaks corresponding to the oxidation of tyrosine, tryptophan and histidine residues.

Isatin derivatives possess a broad pharmacological profile and clinical applications, and the electrochemical behaviour of isatin-3-hydrazone, 7-methylisatin-3-hydrazone, isatin-3-semicarbazone, isatin-3-thiosemicarbazone, 5-bromoisatin-3-thiosemicarbazone and 5-nitroisatin, at a glassy carbon electrode, using cyclic, square-wave and differential pulse voltammetry over a wide pH range, was investigated and compared with isatin. The oxidation mechanism of isatin-hydrazone and isatin-semicarbazone derivatives is an irreversible, pH-dependent, adsorption-controlled process, and occurs in two consecutive charge transfer reactions with the formation of electroactive oxidation products, and their reduction mechanism is similar to the reduction of isatin, one single irreversible, pH-dependent cathodic process. The redox processes of isatin-hydrazone derivatives are associated with the hydrazone group, and for isatin-semicarbazone derivatives with the semicarbazone group. The oxidation mechanism of nitroisatin is also similar to isatin but the reduction is an irreversible, pH-dependent process that occurs in two consecutive charge transfer reactions with the formation of one electroactive reduction product. The results showed that all functional groups attached to the isatin ring structure give rise to different redox mechanisms, and the isatin derivatives investigated present new redox properties.

The anodic oxidation of the hydroxyanthraquinones carminic acid (CA) and chrysazin (CR) was investigated. The oxidation of CA proceeds in a pH-dependent cascade mechanism, concerning the hydroquinone, the catechol and the 3-OH groups in the anthraquinone moiety. The oxidation of the hydroquinone following the catechol electron-donating groups occurs first at low positive potentials, the 3-OH group is oxidized irreversibly at a higher potential. The oxidation of CR is pH-dependent and occurs in successive steps. Oxidation of the hydroquinone tautomer in the CR-ring occurs first, and the symmetrical 1-OH and 8-OH groups are irreversibly oxidized at the same higher potential.

Protein reducing agents dithiothreitol (DTT) or tris(2-carboxyethyl)phosphine (TCEP) are crucial to disrupt disulfide bonds for qualitative and quantitative analysis in proteomics. Protein bond disruption is very important for analyzing proteins as single subunits. The electrochemical oxidation mechanism of DTT and TCEP was investigated using cyclic and differential pulse voltammetry over a wide pH range on a glassy carbon electrode. The oxidation mechanism of both compounds is irreversible, pH-dependent and proceeds in a single step. The anodic oxidation of DTT is associated with the oxidation of the sulfhydryl group with lowest pKa, while the anodic oxidation of TCEP is associated with the oxidation of the phosphine group in the molecule. The diffusion coefficients of DTT and TCEP were calculated, and the redox mechanisms are proposed. Keywords: Dithiothreitol, Tris(2-carboxyethyl)phosphine, Proteins, Oxidation, Glassy carbon electrode

The electrochemical behavior of antioxidants daidzein (DZ) and 7-hydroxy-4-chromone (7-OH-4-CHM) was investigated. The oxidation of DZ is irreversible, pH-dependent, and occurs in two steps, the first of 4’-OH on the B-ring and the second on the 7-OH on the A-ring. The oxidation of 7-OH-4-CHM occurs on the 7-OH on the A-ring, is irreversible, pH-dependent and proceeds in a single step. The DZ diffusion coefficient was calculated in pH 7.0 phosphate buffer to be DDZ = 8.1 � 10 � 5 cm 2 s � 1 . The detection limits of DZ and 7-OH-4-CHM was calculated as LODDZ = 0.08 mM and LOD7-OH-4-CHM = 0.13 mM.

Clioquinol is an antifungal, antiprotozoal and an Alzheimer's disease drug with cytotoxic activity toward human cancer cells. The electrochemical behavior of clioquinol and its oxidation product was studied using cyclic, differential pulse and square-wave voltammetry over a wide pH range on a glassy carbon electrode. The results revealed that the oxidation of clioquinol is an irreversible pH-dependent process that proceeds with the transfer of one electron and one proton in an adsorption-controlled mechanism and results in the formation of a main oxidation product, which adsorbs very strongly on the glassy carbon surface. The charge transfer coefficient was calculated as 0.64. The adsorbed oxidation product presented reversible redox behavior, with two electron and two proton transfer. The electrochemical oxidation of clioquinol as a phenolic compound involves the formation of a phenoxy radical which reacts in at least two ways: in one pathway the radical initiates polymerization, the products remaining at the electrode surface, and in the other the radical is oxidized to a quinone-like structure. A mechanism for the oxidation of clioquinol is proposed.

The evaluation of the interaction of hazard compounds, divalent cations Pb, Cd, Ni and Pd, with double-stranded DNA (dsDNA), forming a metal–DNA complex, was studied by atomic force microscopy (AFM) and differential pulse voltammetry (DPV) on two carbon electrode surfaces, highly oriented pyrolytic graphite (HOPG) and glassy carbon (GC). The electrochemical behaviour of these metal–DNA complexes was related to the different adsorption patterns and conformational changes obtained by the AFM images on the HOPG surface. The dsDNA interaction was specific with each metal cation, inducing structural changes in the B-DNA structure, local denaturation of the double helix and oxidative damage. AFM images showed an increase of the electrode surface coverage for lead, cadmium and nickel DNA complexes. For cadmium and nickel–DNA complexes oxidative damage to DNA was electrochemically detected for the concentrations studied. Palladium interaction with dsDNA induced condensation of the dsDNA secondary structure, which led to the aggregation of helixes forming very compact and thick filaments in the network film. The voltammetric data for the palladium–DNA complex showed a sharp decrease of the guanosine and adenosine oxidation peak currents, consistent with the AFM results of DNA condensation in the presence of palladium, but no DNA oxidative damage was detected for the range of concentrations used.

Heavy metal ions, lead, cadmium and nickel, are well known carcinogens with natural different origins and their direct mode of action is still not fully understood. A dsDNA-electrochemical biosensor, employing differential pulse voltammetry, was used for the in situ evaluation of Pb2+, Cd2+ and Ni2+ interaction with dsDNA. The results confirm that Pb2+, Cd2+ and Ni2+ bind to dsDNA, and that this interaction leads to different modifications in the dsDNA structure. These modifications were electrochemically recognized as changes in the oxidation peaks of guanosine and adenosine bases. Using homopolynucleotides of guanine and adenine it has been proved that the interaction between Pb2+ and DNA causes oxidative damage and preferentially takes place at adenine-containing segments, with the formation of 2,8-dihydroxyadenine, the oxidation product of adenine residues and a biomarker of DNA oxidative damage. The Pb2+ bound to dsDNA can still undergo oxidation. The interaction of Cd2+ and Ni2+ causes conformational changes, destabilizing the double helix, which can enable the action of other oxidative agents on DNA.

A universal and easily ON–OFF time operated coulometric flow cell for generation of chemicals is proposed. The cell can be used for generation of reagents, titrants or standard solutions, and can be coupled to analytical equipments in combination with flow systems for generation of micro-quantity amounts of chemicals in a wide concentration range. Platinum wire electrodes and other common laboratory available materials were employed in the cell assembly. The application of the cell for analytical purposes was illustrated by employing it for generating triiodide ions as a carrier in a flow injection analysis system applied to the analysis of dipyrone in pharmaceuticals, and as a titrant in a flow-batch analysis system applied for analysis of ascorbic acid in natural orange juice samples. Preliminary studies demonstrate that such a flow cell is suitable to allow the use of unstable reagents in analytical chemistry, since they can be generated in a closed system and promptly used in subsequent reactions. In addition, it offers advantages of saving chemicals, time and specially the laborious calibrating standard solutions preparation task of routine laboratories. Taking in account its analytical performance, the proposed cell promises to be an important tool in analytical chemistry with a great competition potential as compared to the other ones proposed in the literature.

Recently, it was demonstrated that colorectal cancer HT-29 cells can secrete epinephrine (adrenaline) in an autocrine manner to auto-stimulate cellular growth by adrenoreceptors activation, and that this secretion is enhanced by nicotine, showing an indirect relation between colorectal cancer and tobacco. The electrochemical behaviour of human colon adenocarcinoma HT-29 cells from a colorectal adenocarcinoma cell line, the hormone and neurotransmitter epinephrine, and nicotine, were investigated by cyclic voltammetry, using indium tin oxide (ITO), glassy carbon (GC) and screen printed carbon (SPC) electrodes. The oxidation of the HT-29 cells, previously grown onto ITO or SPC surfaces, followed an irreversible oxidation process that involved the formation of a main oxidation product that undergoes irreversible reduction, as in the epinephrine oxidation mechanism. The effect of nicotine stimulation of the HT-29 cells was also investigated. Nicotine, at different concentration levels 1, 2 and 15 mM, was introduced in the culture medium and an increase with incubation time, 0 to 3 h and 30 min, of the HT-29 cells oxidation and reduction peaks was observed. The interaction of nicotine with the HT-29 cells stimulated the epinephrine secretion causing an increase in epinephrine release concentration, and enabling the conclusion that epinephrine and nicotine play an important role in the colorectal tumour growth.

The interaction of thalidomide (TD) with double-stranded DNA (dsDNA) was studied using atomic force microscopy (AFM) at highly oriented pyrolytic graphite (HOPG), differential pulse voltammetry (DPV) at glassy carbon electrodes (GCE), UV-Vis and electrophoresis. After incubation of dsDNA with different concentrations of TD, the AFM images show the formation of thin and incomplete TD-DNA network films with a number of embedded molecular aggregates and regions of uncovered HOPG. Both the TD-dsDNA aggregates and network thickness directly depended on the TD concentration and incubation time. The voltammetric data also showed that the modifications caused by TD to the DNA double helical structure are time-dependent. In agreement with AFM, DPV, UV-Vis and electrophoresis results, a model is proposed for the TD-DNA interaction, considering that TD intercalates into the dsDNA, causing defects in the dsDNA secondary structure and DNA double helix unwinding. Moreover, both AFM and DPV show that condensation is caused to DNA by TD and occurs until 24 h of incubation, as well as DNA oxidative damage, detected electrochemically by the appearance of the 8-oxoGua and/or 2,8 oxoAde oxidation peak.