Voltammetric Behaviour of 7‐Methylguanine Using Screen‐printed Graphite Electrodes: towards a Guanine Methylation Electrochemical Sensor

The determination of 7‐methylguanine (7mG) is an interesting analytical target since its quantification can be correlated with the methylation of cytosine which is one of the most relevant epigenetic modification of DNA. In this work, screen‐printed graphite electrodes were used to study the electrochemical characterization of 7mG by cyclic and square wave voltammetry. 7mG concentration and pH were examined, as well as the repeatability of the measurements. The electrochemical response of 7mG in the presence of guanine and adenine was also investigated. Results aim to the future development of an electrochemical sensor for the methylation degree in DNA.     

Electrochemical Oxidation of 8‐Oxo‐2′‐Deoxyguanosine on Glassy Carbon,Gold, Platinum and Tin(IV) Oxide Electrodes

Cyclic voltammetry was used to investigate the oxidation of 8‐oxo‐2′‐deoxyguanosine (8‐oxo‐dG) on the glassy carbon (GC), platinum, gold and SnO₂ electrodes over a range of the sweep rate, 8‐oxo‐dG concentration and the solution pH. Reaction mechanism that is common to all these electrodes involves the two‐electron two‐proton charge transfer step followed by the irreversible chemical reaction(s). Rate of the charge transfer reaction decreases with the increasing solution pH (GC, Pt, Au), and depends on the nature of the electrode material following the sequence GC>Pt, Au>>SnO₂. These effects can be related to the degree of oxidation of the electrode surface (Pt, Au, SnO₂), or to the density of the active surface sites (GC). Any of these electrodes can be used for the fabrication of an amperometric detector for 8‐oxo‐dG .     

Electrochemical Oxidation of Desipramine Drug in the Presence of 4,6‐Dimethylpyrimidine‐2‐thiol Nucleophile in Aqueous Acidic Medium

Electrochemical oxidation of desipramine (DES) has been studied in the presence of 4,6‐dimethylpyrimidine‐2‐thiol (DMPT) as nucleophile in acidic medium by means of cyclic voltammetry, controlled‐potential electrolysis and spectroscopic data, as diagnostic techniques. Voltammetric studies of electro‐oxidation of desipramine were realized in a range of pH 1.0 to 8.0 in the absence and presence of DMPT. The results indicate the participation of the product of electrochemical oxidation of desipramine in the reaction with DMPT with ECEC electrochemical mechanism. However, the voltammetry and coulometry results imply existence of a catalytic (EC′) electrochemical mechanism in parallel with ECEC electrochemical mechanism. The product has been characterized by IR, ¹H NMR, ¹³C NMR and MS methods.     

Electrochemical Oxidation Mechanism of Ethidium Bromide at a Glassy Carbon Electrode

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.     

Electrochemical Oxidation of Rutin

An electrochemical investigation of rutin oxidation on a glassy carbon electrode was carried out using cyclic voltammetry, differential pulse voltammetry and square‐wave voltammetry over a wide pH interval. The electrochemical oxidation is a complex process, which proceeds in a cascade mechanism, related with the 4‐hydroxyl groups of the rutin molecule. The catechol 3′,4′‐dihydroxyl group is the first to be oxidized by a two‐electron – two‐proton reversible oxidation reaction, followed by an irreversible oxidation reaction due to the 5,7‐dihydroxyl group. Both mechanisms are pH dependent. An adsorption process is also observed and the oxidation products block the electrode surface.     

Human Cytochrome P450 (CYP1A2)‐dsDNA Interaction in situ Evaluation Using a dsDNA‐electrochemical Biosensor

Human cytochrome CYP1A2 is one of the major hepatic cytochrome P450s involved in many drugs metabolism, and chemical carcinogens activation. The CYP1A2‐dsDNA interaction in situ evaluation using a DNA‐electrochemical biosensor and differential pulse voltammetry was investigated. A dsDNA‐electrochemical biosensor showed that CYP1A2 interacted with dsDNA causing conformational changes in the double helix chain and DNA oxidative damage. A preferential interaction between the dsDNA guanosine residues and CYP1A2 was found, as free guanine and 8‐oxoguanine, a DNA oxidative damage biomarker, oxidation peaks were detected. This was confirmed using guanine and adenine homopolynucleotides‐electrochemical biosensors. The CYP1A2‐dsDNA interaction and dsDNA conformation changes was also confirmed by UV‐Vis spectrophotometry.     

Electrochemical Oxidation of Berberine and of Its Oxidation Products at a Glassy Carbon Electrode

The electrochemical behavior of berberine, an isoquinoline plant alkaloid with a wide spectrum of physiological effects, was studied at a glassy carbon electrode using cyclic, differential pulse and square‐wave voltammetry. The oxidation of berberine is a quasireversible, diffusion‐controlled process and occurred in a cascade mechanism with the formation of several oxidation products. The diffusion coefficient of berberine was calculated from cyclic voltammetry studies to be D=1.69×10^?6 cm² s^?1. The oxidation process of berberine is also pH dependent and the number of electrons and protons transferred was determined using differential pulse voltammetry. The formation of several oxidation products that adsorbed at the glassy carbon electrode surface was observed and their electrochemical behavior characterized. A mechanism for the oxidation of berberine at a glassy carbon electrode was proposed.     

Antidiabetic Drug Metformin Oxidation and in situ Interaction with dsDNA Using a dsDNA‐electrochemical Biosensor

The antidiabetic drug metformin (MET) is one of a group of emerging pharmaceutical drug contaminants in the wastewater treatment plants. The electrochemical behaviour of MET?Cu(II) complex by differential pulse and square wave voltammetry, in a wide pH range, at a glassy carbon electrode modified with a carbon black dihexadecylphosphate film (CB?DHP/GCE), was investigated. The MET?Cu(II) complex oxidation showed one pH‐dependent process, which leads to the formation of an oxidation product, being oxidized at a lower potential. The electroanalytical MET?Cu(II) complex detection limit, LOD=0.63 μM, and quantification limit, LOQ=2.09 μM, were obtained, and the MET?Cu(II) complex determination in wastewater samples collected from a senior residence effluent, using the CB?DHP/GCE, was achieved. Considering MET toxicity, the electrochemical evaluation of MET?dsDNA interaction, in incubated solutions and using dsDNA‐electrochemical biosensors, following the changes in the oxidation peaks of guanosine and adenosine residues electrochemical currents, was also investigated. The MET?dsDNA interaction mechanism, for shorter times, occurs by the binding of MET molecules in the minor grooves of the dsDNA, and for long times, the stabilization of the MET?dsDNA complex, causing a local distortion and/or unwinding of dsDNA morphology, is described. However, MET did not promote DNA oxidative damage.     

Electrochemical Oxidation of Sanguinarine and of Its Oxidation Products at a Glassy Carbon Electrode – Relevance to Intracellular Effects

The electrochemical behavior of sanguinarine, a quaternary benzophenanthridine glycoside alkaloid with antimicrobial, anti‐inflammatory, antioxidant and/or immune‐stimulatory activities, was studied at a glassy carbon electrode using cyclic, differential pulse, and square wave voltammetry. The oxidation of sanguinarine is a quasireversible, diffusion‐controlled process and occurred in a cascade mechanism with the formation of several oxidation products which adsorbed at the electrode surface. The oxidation of sanguinarine is pH dependent and involves the transfer of the same number of electrons and protons. The adsorbed sanguinarine oxidation products are reversibly oxidized at the glassy carbon electrode surface and their oxidation for a wide range of pHs was also studied by differential pulse and square wave voltammetry. A mechanism for the oxidation of sanguinarine at glassy carbon electrode is proposed.     

An Electrochemical Sensor Based on Gold Nanoparticles Incorporated in Mesoporous MFI Zeolite for Determination of Purine Bases in DNA

An electrochemical sensor was developed based on gold nanoparticles incorporated in mesoporous MFI zeolite for the determination of purine bases. Au nanoparticles (AuNPs) were incorporated into the mesoporous MFI zeolite (AuNPs/m‐MFI) by post‐grafting reaction. The composite materials were characterized by transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS) and electrochemical methods. Au nanoparticles with a size of 5‐20 nm are uniformly dispersed in the pores of mesoporous MFI zeolite. And the morphology of MFI zeolite can be perfectly kept after pore expansion and Au nanoparticles incorporation. The electrocatalytic oxidation of purine bases (guanine and adenine in DNA) is investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The surface‐confined Au nanoparticles provide the good catalytic activity for oxidation of purine bases. The simultaneous detection of guanine and adenine can be achieved at AuNPs/m‐MFI composites modified glassy carbon electrode (GCE). The electrochemical sensor based on AuNPs/m‐MFI exhibits wide linear range of 0.5–500 μM and 0.8–500 μM with detection limit of 0.25 and 0.29 μM for guanine and adenine, respectively. Moreover, the electrochemical sensor is applied to evaluation of guanine and adenine in herring sperm DNA samples with satisfactory results.     

Copper‐based Metal‐organic Framework for Non‐enzymatic Electrochemical Detection of Glucose

A non‐enzymatic electrochemical glucose sensor based on a Cu‐based metal‐organic framework (Cu‐MOF) modified electrode was developed. The Cu‐MOF was prepared by a simple ionothermal synthesis, and the characterizations of the Cu‐MOF were studied by Fourier transform infrared spectroscopy (FT‐IR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), single‐crystal X‐ray powder diffraction (SCXRD), and X‐ray powder diffraction (XRD). Electrochemical behaviors of the Cu‐MOF modified electrode to glucose were measured by differential pulse voltammetry (DPV). The electrochemical results showed that the Cu‐MOF modified electrode exhibited an excellent electro‐catalytic oxidation towards glucose in the range of 0.06 μM to 5 mM with a sensitivity of 89 μA/mM cm² and a detection limit of 10.5 nM. Moreover, the fabricated sensor showed a high selectivity to the oxidation of glucose in coexistence with other interferences. The sensor was satisfactorily applied to the determination of glucose in urine samples. With the significant electrochemical performances, MOFs may provide a suitable platform in the construction of kinds of electrochemical sensors and/or biosensors and hold a great promise for sensing applications.     

A cyclodextrin host-guest recognition approach to a label-free electrochemical DNA hybridization biosensor

A novel label-free electrochemical DNA hybridization biosensor using a β-cyclodextrin/poly(N-acetylaniline)/carbon nanotube composite modified screen printed electrode (CD/PNAANI/CNT/SPE) has been developed. The proposed DNA hybridization biosensor relies on the intrinsic oxidation signals of guanine (G) and adenine (A) from single-stranded DNA entered into the cyclodextrin (CD) cavity. Due to the binding of G and A bases to complementary cytosine and thymine bases in dsDNA, the signals obtained for ssDNA were much higher than that of dsDNA. The synergistic effect of the multi-walled carbon nanotubes provides a significantly enhanced voltammetric signal, and the CD encapsulation effect makes anodic peaks of G and A shift to less positive potentials than that at the bare SPE. The peak heights of G and A signals are dependent on both the number of the respective bases in oligonucleotides and the concentration of the target DNA sequences. Hybridization of complementary strands was monitored through the measurements of oxidation signal of purine bases, which enabled the detection of target sequences from 0.01 to 1.02 nmol μl(-1) with the detection limit of target DNA as low as 5.0 pmol μl(-1) (S/N = 3). Implementation of label-free and homogeneous electrochemical hybridization detection constitutes an important step toward low-cost, simple, highly sensitive and accurate DNA assay. Discrimination between complementary, noncomplementary, and two-base mismatch targets was easily accomplished using the proposed electrode.     

Pathways of Electrochemical Oxidation of Indolic Compounds

The electrochemical behaviour of indole and a group of indole‐containing compounds with a substituent at the C3 position, indol‐3‐acetamide (IAM), tryptamine, gramine, indole acetic acid (IAA), indole propionic acid (IPA), indole butyric acid (IBA) and tryptophan, was investigated at a glassy carbon electrode, in order to determine their oxidation pathways. Indole undergoes one irreversible pH dependent oxidation, whereas the oxidation process of indole derivatives was more complex, a two step, the oxidation at C2 position on the pyrrole ring followed by the hydroxylation at the C7 position of the benzene moiety of indoles, irreversible pH dependent oxidation.     

含硝基咔唑类有机电催化剂的合成及其对醇的电催化氧化

有机小分子的电催化氧化是催化领域的一个重要研究内容.通过醇的选择性氧化合成相应醛或酮类化合物在精细化学品和有机化学中间体的合成领域均具有十分重要的意义.有机电催化合成用电子代替强氧化还原剂,可以使反应在比较温和的条件下进行.但在直接电氧化合成反应中,电极表面容易生成有机聚合物膜,使电极钝化,电流效率急剧下降.而在电子转移媒质作用下的有机电催化反应不仅可以避免电极表面钝化,还可以控制目标产物的过度氧化.三芳胺类化合物是一类新型的电氧化还原媒质,由于其具有较宽的电化学氧化还原电位已引起研究者的广泛关注.咔唑类化合物相比于三芳胺类具有更好的平面性,使得取代基效应更为显著.咔唑类化合物被广泛作为荧光材料,但用于电化学方面的研究很少.本文通过在咔唑类化合物中引入具有强吸电子性的硝基以提高该类化合物的氧化电位,并将其作为有机电催化媒质,采用间接电化学氧化的方式,在室温下研究醇的电化学催化氧化反应,合成相应醛类化合物.
　　我们合成了三种含硝基咔唑类有机电催化剂,通过1H NMR对其结构进行了鉴定.采用循环伏安法测试了该类有机电催化剂的电化学氧化还原性能.发现取代基的电子效应对有机电催化剂的氧化还原电位及电化学氧化还原可逆性有很大的影响,供电子基(–OCH3)的引入使氧化电位负移(0.717 V),吸电子基(–Br)的引入使氧化电位明显正移(1.282 V).同时,取代基的引入有效改善了有机电催化剂的电化学可逆性,从而可以作为有效的电氧化还原媒质应用于电化学氧化反应中.而当把化合物中的NO2还原为NH2后,咔唑类化合物的电化学氧化还原可逆性完全消失,表明硝基的引入对咔唑类有机电催化剂的电化学性能有很大的影响.
　　循环伏安结果发现,在咔唑类硝基化合物的作用下,对甲氧基苯甲醇(p-MBzOH)的电化学氧化峰电位从1.350 V降至1.286 V,表明可以在较低电位下进行电解,有效降低了电氧化反应的能耗,同时氧化峰电流明显增加,说明该类有机电催化剂对p-MBzOH具有良好的电催化性能.随着p-MBzOH浓度的增加,氧化峰电流也明显增大,说明在咔唑类有机电催化剂的作用下, p-MBzOH可以在比较高的浓度下进行电化学氧化电解.通过对不同对位取代基的苯甲醇类化合物进行循环伏安研究,发现含硝基咔唑类化合物对具有较高氧化电位的反应底物均表现出良好的电催化氧化性能.
　　在含硝基咔唑类有机电催化剂的氧化电位(1.28 V)和室温下,对不同浓度的p-MBzOH进行恒电位电解6 h,发现当催化剂的用量为底物的2.5 mol%时, p-MBzOH可以完全转化为相应的醛类目标产物.而且恒电位电解后分离回收的含硝基咔唑类有机电催化剂仍具有良好的电化学氧化还原可逆性.     

Electrochemical Redox Behavior of Omeprazole Using a Glassy Carbon Electrode

The electrochemical redox behavior of omeprazole (OMZ), a gastric acid pump inhibitor, was investigated at a glassy carbon electrode using cyclic, differential pulse and square‐wave voltammetry over a wide pH range. The pH‐dependent oxidation occurs in two irreversible consecutive charge transfer reactions. Adsorption of the nonelectroactive product was also observed. The first oxidation involves removal of one electron, followed by deprotonation and leads to the formation of a hydroxylated species. The second oxidation process is related to the hydroxyl and amino groups in the benzimidazole moiety. The reduction is irreversible, also pH‐dependent, and occurs in a single step at the sulfoxide group in a diffusion‐controlled mechanism. The diffusion coefficient of omeprazole was calculated to be D_OMZ=2.31×10^?6 cm² s^?1.     

Electrochemical Behavior of Some Thiotriazoles in Aqueous‐Alcoholic Media at GCE

An electrochemical study related to the electrooxidation of 4‐amino‐3‐thio‐5‐methyl‐1,2,4‐triazole (I), 4‐amino‐3‐thio‐5‐phenyl‐1,2,4‐triazole (II) and 3‐thio‐5‐phenyl‐1,2,4‐triazole (III), in 10% v/v methanol‐acetate buffer pH 4.6 has been performed. A variety of electrochemical techniques such as differential pulse voltammetry, cyclic voltammetry, double‐potential step chronoamperometry, rotating‐disk electrode voltammetry and coulometry, were employed to clarify that the mechanism of the electrode process follows the oxidation of thiol compounds. All the compounds exhibit similar redox behavior under the given conditions. They display one irreversible oxidation peak, which is diffusion controlled. From the plot of current function in cyclic voltammetry and the ratio of i_c/i_a less than one in double‐potential step chronoamperometry, it was established that these compounds undergo an one electron oxidation followed by a dimerization process involving the formation of disulfide derivative (EC mechanism). The pK_a values were obtained by the dependence of limiting current and potential with in the wide pH interval. The transfer coefficients, the diffusion coefficients and rate constant of coupled chemical reaction were also reported. The substituent effects were also investigated.     

Label‐Free Bioelectronic Detection of Point Mutation by Using Peptide Nucleic Acid Probes

An electrochemical hybridization biosensor based on peptide nucleic acid (PNA) probes with a label‐free protocol is described. The detection of PNA‐DNA and DNA‐DNA hybridizations were accomplished based on the oxidation signal of guanine by using differential pulse voltammetry (DPV) at carbon paste electrode (CPE). It was observed that the oxidation signals of guanine obtained from the PNA and DNA probe modified CPEs were higher than those obtained from the PNA‐DNA and DNA‐DNA hybrid modified CPEs due to the accessible unbound guanine bases. The detection of hybridization between PNA probe and point mutation containing DNA target sequences was clearly observed due to the difference of the oxidation signals of guanine bases, because the point mutation was guanine nearly at the middle of the sequence. The effect of the DNA target concentration on the hybridization signal was also observed. The PNA probe was also challenged with excessive and equal amount of noncomplementary DNA and also mixtures of point mutation and target DNA.     

Antileishmanial Drug Miltefosine‐dsDNA Interaction in situ Evaluation with a DNA‐Electrochemical Biosensor

Leishmaniasis is one of the most important parasitic neglected disease. The electrochemical evaluation of the antileishmanial drug miltefosine‐dsDNA interaction was investigated in incubated solutions and using dsDNA‐electrochemical biosensors, following the changes in the oxidation peaks of guanosine and adenosine residues, and the occurrence of the free guanine residues, electrochemical signal. The electrochemical behaviour of miltefosine was also investigated, at a glassy carbon electrode, using cyclic, differential pulse and square wave voltammetry and no electrochemical redox processes were observed. The interaction mechanism of miltefosine‐dsDNA occurs in two ways: independent of the dsDNA sequence, and leading to the condensation/aggregation of DNA strands, producing a rigid miltefosine‐dsDNA complex structure, and a preferential interaction between the guanine hydrogen atoms in the C−G base pair and miltefosine, causing the release of guanine residues detected on the electrode surface. Miltefosine did not induce oxidative damage to DNA in the experimental conditions used.     

Oxidation Behavior of Insecticide Azoxystrobin and its Voltammetric Determination Using Boron‐doped Diamond Electrode

Electrochemical oxidation of azoxystrobin, a systemic fungicide commonly used in agriculture to protect a wide variety of crops, was investigated using cyclic voltammetry with a boron‐doped diamond electrode (BDDE) in aqueous buffer solutions. Two pH independent irreversible anodic current peaks controlled mostly by diffusion were observed in wide pH range (2 to 12) at potentials +1600 mV and +2150 mV vs. saturated silver‐silver chloride electrode. Mechanism of the electrochemical oxidation was proposed and supported with high performance liquid chromatography/mass spectrometry analysis of azoxystrobin solutions electrolyzed on carbon fiber brush electrode. The main product of the first two‐electron oxidation step was identified as methyl 2‐(2‐{[6‐(2‐cyanophenoxy)pyrimidin‐4‐yl]oxy}phenyl)‐2‐hydroxy‐3‐oxopropanoate. An analytical method for the determination of azoxystrobin in water samples and pesticide preparation by differential pulse voltammetry with BDDE was developed. The method provides a wide linear dynamic range (3.0×10^?7 to 2.0×10^?4 mol L^?1) with limit of detection 8×10^?8 mol L^?1. Accuracy of the method was evaluated by the addition and recovery method with recoveries ranging from 96.0 to 105.8 %. Interference study proved sufficient selectivity of the developed voltammetric method for the azoxystrobin determination in presence of azole fungicides as well as pesticides used to prevent the same crops.     

Electrochemical Oxidation of Sulfasalazine at a Glassy Carbon Electrode

Sulfasalazine (SSZ) is a pharmaceutical compound used for the treatment of rheumatoid arthritis. The electrochemical oxidation of SSZ at a glassy carbon electrode was studied by cyclic, differential pulse and square wave voltammetry in a wide pH range. For electrolytes with pH<11.0, the oxidation is an irreversible, diffusion‐control, pH‐dependent process that involves the transfer of one electron and one proton from the hydroxyl group of the salicylic moiety. For pH>11.0 the oxidation is pH‐independent, and a pK_a≈11 was determined. The formation of a quinone‐like oxidation product that undergoes two electrons and two protons reversible redox reaction was observed. Also, UV‐vis spectra of SSZ were recorded as a function of supporting electrolytes pH. An electrochemical oxidation mechanism was proposed.