Electrochemical Behavior of Thalidomide at a Glassy Carbon Electrode

Thalidomide is an oral drug marketed in the 1950s as a sedative and an anti‐emetic during pregnancy that was removed from the market when its teratogenic side effects appeared in new born children due to inadequate tests to assess the drug's safety. Recent studies evaluating the use of thalidomide in cancer and HIV diseases have sparked renewed interest. The electrochemical behavior of thalidomide on a glassy carbon electrode has been investigated using cyclic, differential and square‐wave voltammetry in aqueous media at different pHs. The oxidation mechanism of thalidomide is an irreversible, adsorption‐controlled process, pH dependent up to values close to the pK_a and occurs in two consecutive charge transfer reactions. A mechanism of oxidation of thalidomide involving one electron and one proton to produce a cation radical, which reacts with water and yields a final hydroxylated product is proposed. The reduction of thalidomide is also a pH dependent, irreversible process and occurs in a single step, with the same number of electrons and protons transferred. The reduction mechanism involves the protonation of the nitrogen that bridges the two cyclic groups, and the product of the protonation reaction causes irreversible dissociation. Both thalidomide and the non electroactive oxidation and reduction products are strongly adsorbed on the glassy carbon electrode surface.     

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.     

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 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.     

Electrochemical Oxidation of Pentoxifylline and its Analysis in Pure and Pharmaceutical Formulations at a Glassy Carbon Electrode

Abstract

The oxidative behavior of pentoxifylline was studied at a glassy carbon electrode in phosphate buffer solutions using cyclic and differential-pulse voltammetry. The oxidation process was shown to be irreversible over the pH range (3.0–9.0) and was diffusion controlled. The possible mechanism of the oxidation of pentoxifylline was investigated by means of cyclic voltammetry and UV-Vis spectroscopy. An analytical method was developed for the determination of pentoxifylline in phosphate buffer solution at pH 3.0 as a supporting electrolyte. The anodic peak current varied linearly with pentoxifylline concentration in the range 2.0 × 10^?8 M to 6.0 × 10^?7 M of pentoxifylline with a limit of detection (LOD) of 4.42 × 10^?10 M. The proposed method was applied to the determination of pentoxifylline in pure and pharmaceutical formulations.     

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.     

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.     

Oxidation Mechanism of Fluorescein at Glassy Carbon Electrode

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.     

Electrochemical Redox Behaviour of Temozolomide Using a Glassy Carbon Electrode

The electrochemical behaviour of temozolomide on a glassy carbon electrode has been investigated. The reduction of temozolomide is an irreversible process, pH dependent, and the mechanism involves the addition of one electron and one proton to C5 to form an anion radical, causing the irreversible breakdown of the tetrazinone ring. The oxidation mechanism of temozolomide is an irreversible, adsorption‐controlled process, pH dependent up to value close to the pK_a and occurs in two consecutive charge transfer reactions, with the formation of the hydroxylated product. The electroanalytical determination of TMZ led to a detection limit of 1.1 µM.     

Electrochemical Behavior and Voltammetric Determination of Curcumin at Electrochemically Reduced Graphene Oxide Modified Glassy Carbon Electrode

This work presents a sensitive voltammetric method for determination of curcumin by using a electrochemically reduced graphene oxide (ERGO) modified glass carbon electrode (GCE) in 100 mM KCl‐10 mM sodium phosphate buffer solution (pH 7.40). The electrochemical behaviors of curcumin at ERGO/GCE were investigated by cyclic voltammetry, suggesting that the ERGO/GCE exhibits excellent electrocatalytic activity towards curcumin, compared with bare GCE and GO/GCE electrodes. The electrochemical reaction mechanisms of curcumin, demethoxycurcumin and bisdemethoxycurcumin at the ERGO/GCE were also investigated and discussed systematically. Under physiological condition, the modified electrode showed linear voltammetric response from 0.2 μM to 60.0 μM for curcumin, with the detection limit of 0.1 μm. This work demonstrates that the graphene‐modified electrode is a promising strategy for electrochemical determination of biological important phenolic compounds.     

Sensitive Voltammetric Determination of Baicalein at Thermally Reduced Graphene Oxide Modified Glassy Carbon Electrode

This work presents a sensitive voltammetric method for determination of the flavonoid baicalein by using a thermally reduced graphene oxide (TRGO) modified glassy carbon electrode (GCE) in 100 mM KCl‐10 mM sodium phosphate buffer solution (pH 7.40). The surface morphology and structure of TRGO investigated by atomic force microscopy, FT‐IR spectroscopy and Raman spectroscopy reveal that the TRGO prepared maintained as single or bilayer sheets and with significant edge‐plane‐like defect sites. The TRGO/GCE modified electrode shows more favorable electron transfer kinetics for potassium ferricyanide and potassium ferrocyanide probe molecules, which are important electroactive compounds, compared with bare GCE and GO/GCE electrodes. The electrochemical behaviors of baicalein at the TRGO/GCE were investigated by cyclic voltammetry, suggesting that the TRGO/GCE exhibits excellent electrocatalytic activity to baicalein. Under physiological conditions, the modified electrode showed linear voltammetric response from 10 nM to 10 µM for baicalein, with a detection limit of 6.0 nM. This work demonstrates that the graphene‐modified electrode is a promising tool for electrochemical determination of flavonoid drugs.     

Cyclic Lipopeptide Antibiotic Daptomycin Electrochemical Oxidation at a Glassy Carbon Electrode

Daptomycin was the first approved drug from a new class of antimicrobials, the cyclic lipopeptides, and presents a broad spectrum of activity against a wide range of gram‐positive bacteria. The daptomycin redox behaviour, by cyclic, differential pulse and square wave voltammetry, in a wide pH range, at a glassy carbon electrode, was investigated. The daptomycin oxidation was a two‐step irreversible diffusion‐controlled process and the diffusion coefficient D_DPT = 2.32 × 10⁻⁵ cm² s⁻¹, was calculated. A detection limit LOD = 0.32 μM, was obtained. For the first time daptomycin, in fetal bovine serum biological fluid, using DP voltammetry, was determined.     

Electrochemical Biosensing Platform Using Carbon Nanotube Activated Glassy Carbon Electrode

Carbon nanotube enhanced electrochemically activated glassy carbon electrode (GCE) has been prepared and applied for sensitive electrochemical determination of DNA and DNA bases. The results indicate that the relative activation could efficiently enhance electron transfer at the pretreated GCE so that this carbon nanotube activated glassy carbon electrode could provide relatively low detection limit with good reproducibility for the respective biomolecular determination. Besides, greatly enhanced sensitivity could be obtained for the relevant electrochemical detection of the bio‐recognition process including DNA biosensing by using the carbon nanotube activated GCE. This approach provided a detection limit of 7.5 nM for guanine and 150 ng/mL for acid denatured DNA. These observations suggest that the carbon nanotube activated glassy carbon electrode could be utilized as a very sensitive and stable biosensor for some specific biological process.     

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.     

Determination of Theophylline at a Cysteic Acid Modified Glassy Carbon Electrode

A glassy carbon electrode modified with electrodeposited cysteic acid is proposed for the determination of theophylline. Estimation of the linear range, calibration function and limit of detection, as well as determination in real samples such as medication tablets were performed.     

Direct Electrochemical Oxidation of NADPH at a Low Potential on the Carbon Nanotube Modified Glassy Carbon Electrode

Introduction Because of its novel structural and electronic proper-ties, high chemical stability, and extremely high me-chanical strength and modulus,1 carbon nanotube (CNT), which has become a major subject of many experimen-tal and theoretical investigations, has a wide potential application from structural materials to nanoelectronic components2-12 since its initial discovery by Iijima13 in 1991 and the subsequent report about the synthesis of large quantities of CNT by Ebbesen and cowork…     

Proteasome Inhibitor Anticancer Drug Bortezomib Redox Behaviour at a Glassy Carbon Electrode

Bortezomib is the first therapeutic proteasome inhibitor used for cancer treatment. The redox behaviour of bortezomib was investigated over a wide pH range. Bortezomib undergoes electrochemical oxidation and reduction in independent mechanisms. The oxidation of bortezomib is pH‐dependent for pH<7.5 and occurs with the transfer of one electron and one proton involving the formation of two electroactive oxidation products. The reduction of bortezomib is quasi‐reversible, pH‐dependent, involving the transfer of two electrons and two protons and does not involve the formation of electroactive products. The value of pK_a≈7.5 was determined. Mechanisms for oxidation and reduction were proposed.     

甲氧苄啶在多壁碳纳米管-Nafion修饰电极上的电催化氧化及电分析方法

用循环伏安法(CV),计时库仑法(CC),计时电流法(CA),线性扫描伏安法(LSV)及电流-时间曲线研究了甲氧苄啶(trimethoprim, TMP)在碳纳米管-Nafion修饰电极(MWCNTs-Nafion/GCE)上的电化学行为,电化学动力学性质以及电分析方法.结果表明,TMP在GCE上有一个极弱的氧化峰,而在MWCNTs-Nafion/GCE上出现一个敏锐的氧化峰,表明MWCNTs-Nafion/GCE对TMP电化学氧化具有良好的催化作用.在扫描速度为10～800 mV/s时其氧化峰电流与扫描速度平方根(v1/2)呈良好线性关系,表明TMP在MWCNTs-Nafion/GCE上的伏安行为是受扩散控制的电化学过程.TMP在MWCNTs-Nafion/GCE上氧化峰电流与浓度在5.0×10-6～1.0×10-3 mol/L范围内呈良好线性关系;检出限为6.6×10-7 mol/L;RSD在0.75%～1 69%之间;加标回收率在98.1%～101.1%之间.本方法简便快捷,测定结果令人满意,可用于TMP的电化学定量测定.     

Firstly Electrochemical Examination of Myosmine at Glassy Carbon Electrode: Sensitive Determination in Tobacco Leaves by Differential Pulse Voltammetry

In this study, the electrochemical properties of myosmine, one of the tobacco alkaloids, were investigated for the first time using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques on glassy carbon electrode (GCE). Using GCE with CV technique, it gave an irreversible reduction peak with diffusion control at about −1.38 V in Britton-Robinson (BR, pH 12.0) medium. A good linear relationship between concentration and current in the range of 0.5 μM–3.5 μM in BR (pH 12.0) medium on GCE with DPV technique [ I p(μA)=0.847 C(μM)+0.114, r=0.995, n=7] has been observed. The proposed method has been successfully applied to tobacco leaves.