Electrochemical and ESR studies of <Emphasis Type="Italic">tert</Emphasis>-butanol oxidation mechanism in the presence of radical cations pyrazine-di-N-oxide and its substituted derivatives as mediators

The methods of cyclic voltammetry, ESR electrolysis, and quantum chemical simulation were used to study the tert-butanol (tert-BuOH) oxidation mechanism in the presence of mediator cation radicals of pyrazine-di-N-oxide, 2,5-di-Me- and 2,3,5,6-tetra-Me-pyrazine-di-N-oxdides. This study was carried out on carbon glass (CG) and Pt electrodes in 0.1 M LiClO₄ solution in acetonitrile and on Au electrode in tert-butanol containing 0.05 M LiClO₄. The ESR spectra of cation and anion radicals of aromatic di-N-oxides were recorded in tert-BuOH. The quantum chemical simulation of the reaction between pyrazine-di-N-oxide radical cation and C-H bond in tert-BuOH was performed. The results were explained in the terms of the general two-electron oxidation mechanism of tert-BuOH in the complex with aromatic di-N-oxide cation radical as mediator.     

Electrochemical oxidation of luteolin at a glassy carbon electrode and its application in pharmaceutical analysis

Luteolin is a flavonoid reported to occur widely in many medicinal plants. The electrochemical behavior of luteolin was studied in phosphate buffer solution (PBS) of pH 4.0 at a glassy carbon electrode (GCE) by cyclic voltammetry (CV) and differential pulse voltammetric method (DPV). The results indicated the well-defined redox peak of luteolin which was involving two electrons and two protons was observed and the electrode process is adsorption-controlled. The charge transfer coefficient (alpha) was calculated as 0.66. The relationships between oxidation peak current and the concentration of luteolin are linear in the range of 1.0 x 10(-8) - 1.0 x 10(-6) M by DPV method. The detection limit had been estimated as 5.0 x 10(-9) M. The facile and rapid method has been successfully applied to the detection of luteolin in tablets.     

Anodic Stripping Voltammetric Detection of Arsenic(III) at Gold Nanoparticle‐Modified Glassy Carbon Electrodes Prepared by Electrodeposition in the Presence of Various Additives

The simple, fast and highly sensitive anodic stripping voltammetric detection of As(III) at a gold (Au) nanoparticle‐modified glassy carbon (GC) (nano‐Au/GC) electrode in HCl solution was extensively studied. The Au nanoparticles were electrodeposited onto GC electrode using chronocoulometric technique via a potential step from 1.1 to 0 V vs. Ag|AgCl|NaCl (sat.) in 0.5 M H₂SO₄ containing Na[AuCl₄] in the presence of KI, KBr, Na₂S and cysteine additives. Surfaces of the resulting nano‐Au/GC electrodes were characterized with cyclic voltammetry. The performances of the nano‐Au/GC electrodes, which were prepared using different concentrations of Na[AuCl₄] (0.05–0.5 mM) and KI additive (0.01–1.0 mM) at various deposition times (10–30 s), for the voltammetric detection of As(III) were examined. After the optimization, a high sensitivity of 0.32 mA cm^?2 μM^?1 and detection limit of 0.024 μM (1.8 ppb) were obtained using linear sweep voltammetry.     

Voltammetric determination of trace amounts of gold(III) with a carbon paste electrode modified with chelating resin

A carbon paste electrode modified with chelating resin (ammino-isopropylmercaptan-type cross-linked chelating resins) for the voltammetric determination of gold(III) was characterized by cyclic voltammetry. The gold(III) ion is accumulated on the surface of the modified electrode only by the chelating effect of the modifier in the carbon paste, without application of a potential. After exchange of the medium the accumulated amount of gold(III) is determined by voltammetry in a blank electrolyte solution. The response depends on both the concentration of gold and the accumulation time. For a 5-min preconcentration time, a linear calibration graph was obtained in the range 3 × 10^?8-1 × 10^?6 M and the detection limit was about 1 × 10^?8 M. A combination of chemical and electrochemical renewal allows the use of a single modified electrode in multiple analytical determinations over several days. For ten preconcentration—determination—renewal cycles [2 × 10^?7 M Au(III)], the response could be reproduced with 4.7% relative standard deviation. Many parameters such as the composition of the paste and pH influence the response of the measurement. Many other metal ions have no or little effect on the determination of gold. The procedure was applied to the determination of gold in minerals, copper and anode mud, with good results.     

Gold(III) reduction in a tris-HCl buffer: Effect of riboflavin,rutin, 1,1-dipyridyl,and 1-naphthol

Reduction of chloroauric acid on platinum and gold electrodes in a 0.1 M tris-HCl buffer of pH 8 containing riboflavin, rutin, 1,1-dipyridyl, or 1-naphthol is studied by cyclic voltammetry and in situ ESR methods. On the basis of the obtained data it is assumed that in the buffer there occurs the reduction of Au(III) to Au(I). In the presence of 1,1-dipyridyl, there occurs the reduction of complex [Au(III)-1,1-dipyridyl]. The reduction of Au(III) in the presence of 1-naphthol is realized in the composition of complex [Au(III)-tris-1-naphthol]. The hampering of the electrode process of the Au(III) reduction in the presence of 1-naphthol is caused by the adsorption of the [tris-1-naphthol] associates at the electrode surface. The presence of Au(III) does not exert any influence on the process of electroreduction of riboflavin. The obtained results make it possible to presume that the resistance of gold-accumulating cells Micrococcus luteus toward toxic compounds that are inhibitors of the respiratory chain, such as 1,1-dipyridyl and 1-naphthol, is caused by their binding in gold-containing complexes in the composition of Au-protein.     

Sensitive determination of dopamine in the presence of uric acid and ascorbic acid using TiO2 nanotubes modified with Pd, Pt and Au nanoparticles

A simple modified TiO(2) nanotubes electrode was fabricated by electrodeposition of Pd, Pt and Au nanoparticles. The TiO(2) nanotubes electrode was prepared using the anodizing method, followed by modifying Pd nanoparticles onto the tubes surface, offering a uniform conductive surface for electrodeposition of Pt and Au. The performance of the modified electrode was characterized by cyclic voltammetry and differential pulse voltammetry methods. The Au/Pt/Pd/TiO(2) NTs modified electrode represented a high sensitivity towards individual detection of dopamine as well as simultaneous detection of dopamine and uric acid using 0.1 M phosphate buffer solution (pH 7.00) as the base solution. In both case, electro-oxidation peak currents of dopamine were linearly related to accumulated concentration over a wide concentration range of 5.0 × 10(-8) to 3.0 × 10(-5) M. However in the same range of dopamine concentration, the sensitivity had a significant loss at Pt/Pd/TiO(2) NTs electrode, suggesting the necessity for Au nanoparticles in modified electrode. The limit of the detection was determined as 3 × 10(-8) M for dopamine at signal-to-noise ratio equal to 3. Furthermore, the Au/Pt/Pd/TiO(2) NTs modified electrode was able to distinguish the oxidation response of dopamine, uric acid and ascorbic acid in mixture solution of different acidity. It was shown that the modified electrode possessed a very good reproducibility and long-term stability. The method was also successfully applied for determination of DA in human urine samples with satisfactory results.     

核固红及稀土核固红极谱行为的研究

在由酸性至碱性的水溶液中均获得了核固红的极谱吸附波, 证实在酸性底液中核固红在汞电极上的吸附符合Frumkin等温式. 对pH由7至11的碱性底液中核固红电还原的ECE机制作了研究, 还获得并研究了在pH11附近, 乙二胺-氯化钾底液中的稀土-核固红体系的极谱络合吸附波。     

Poly(Methyl Red) Modified Glassy Carbon Electrodes: Electrosynthesis,Characterization, and Sensor Behavior

Poly(methyl red), PMR, was electropolymerized on glassy carbon electrodes by potential cycling in 50 mM phosphate buffer solution at pH 7.0 and 8.0 and Britton Robinson buffer solution in the pH range 7.0‐11.0. The electrochemical behavior of PMR modified electrodes was investigated by cyclic voltammetry in Britton Robinson buffer solution at different pHs from 5.0 to 10.0 and found that the best PMR film formation was obtained at pH 9.0. Uric acid was quantitatively determined at PMR modified electrodes by cyclic voltammetry and differential pulse voltammetry in Britton Robinson buffer at pH 5.0. Both methods presented a linear dependence between the anodic peak current and the concentration of uric acid in the range of 0.4 to 60 μM and 0.08 to 100 μM with the limits of detection of 0.038 and 0.009 μM for cyclic voltammetry and differential pulse voltammetry, respectively. Poly(methyl red) as redox mediator allowed the determination of uric acid without any interferences from the substances in serum samples.     

Preparation of Molecularly Imprinted Electrochemical Sensor for Detection of Vincristine Based on Reduced Graphene Oxide/Gold Nanoparticle Composite Film

An innovative molecularly imprinted electrochemical sensor was fabricated based on reduced graphene oxide (RGO) and gold nanocomposite (Au) for rapid detection of vincristine (VCR). The RGO‐Au composite membrane was obtained via direct one‐step electrodeposition technique of graphene oxide (GO) and chloroauric acid (HAuCl₄) on the surface of a glassy carbon electrode (GCE) by means of cyclic voltammetry (CV) in the potential range between ?1.5 and 0.6 V in phosphate buffer solution (PBS) of pH 9.18, which is capable of effectively utilizing its superior electrical conductivity, larger specific surface area due to its synergistic effect between RGO and Au. The molecularly imprinted polymers (MIPs) were synthesized on the RGO‐Au modified glassy carbon electrode surface with VCR as the template molecular, methyl acrylic acid (MAA) as the functional monomer, and ethylene glycol maleic rosinate acrylate (EGMRA) as a cross‐linker. The performance of the sensor was investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) in detail. Under the optimum conditions, the fabricated sensor exhibited a linear relationship between oxidation peak current and VCR concentration over the range of 5.0×10^?8–5.0×10^?6 mol·L ^minus;1 with a correlation coefficient of 0.9952 and a detection limit (S/N=3) of 2.6×10^minus;8 mol·L^minus;1. The results indicated that the imprinted polymer films exhibited an excellent selectivity for VCR. The imprinted sensor was successfully used to determine VCR in real samples with recoveries of 90% –120% by using the standard addition method.     

木犀草素在玻碳电极上的直接电化学行为及其测定

应用循环伏安法研究木犀草素于玻碳电极的电化学行为.在磷酸盐缓冲液中(pH 4.0),-0.2~+0.8V电位区间内,木犀草素于玻碳电极表面发生的电极反应是吸附控制的准可逆2电子转移过程,电子转移系数α=0.66;建立了检测木犀草素含量的差示脉冲伏安法(DPV).在富集电位+0.4 V下,经富集240 s后,测得木犀草素氧化峰电流Ip与其浓度在1.0×10-8~1.0×10-6mol.L-1范围内呈良好的线性关系,最低检出限为5.0×10-9mol.L-1.本法操作简单、快速、灵敏、准确,可为木犀草素药物质量的控制和检测提供一种简便的新方法.     

Electrodeposited Cu‐Au Alloy Nanoparticles for Uric Acid Electrochemical Biosensor with Quick Response

A uric acid (UA) electrochemical biosensor based on the Cu‐Au alloy nanoparticles (NPs) and uricase was developed. The electrodeposition technique of Cu‐Au alloy NPs was selected to be a convenient potentiostatic method at –0.8 V in a single solution containing both Au(III) and Cu²⁺. Cyclic voltammetry and scanning electron microscopy proved the successful deposition of Cu‐Au alloy NPs. EIS demonstrated the good conductivity of Cu‐Au alloy NPs. The enzyme was immobilized on the surface of Cu‐Au alloy NPs modified electrode by casting with chitosan solution. The ultimate biosensor showed linear amperometric response towards UA in the concentration range of 3.0 to 26.0 μM with a detection limit of 0.8 μM. The main feature of the biosensor was its short response time, which was attributed to the good conductivity of Cu‐Au alloy NPs. Furthermore, the biosensor could avoid the interference of ascorbic acid and oxygen.     

木犀草素在离子液体修饰电极上的电催化氧化及其测定

用1-丁基-3-甲基咪唑六氟磷酸盐([BMIM]PF₆)疏水性离子液体修饰玻碳电极,在0.2 mol/L磷酸盐缓冲溶液(pH为4.0～8.0)中,运用循环伏安法（CV）和差示脉冲溶出伏安法(DPSV)研究了木犀草素在修饰电极上的电化学行为,建立了测定木犀草素含量的新方法。 实验结果表明,该修饰电极上木犀草素氧化、还原峰电位均负移,峰电流增大。 在-0.2～0.7 V电位区间,pH=7.0的磷酸盐缓冲溶液体系中,木犀草素在修饰电极表面发生的是受吸附控制的准可逆等电子等质子电极反应,电子转移系数α=0.5,吸附量为4.6×10-10 mol/cm²;木犀草素氧化峰电流与其浓度在1.0×10^-10～1.6×10^-8 mol/L范围内呈良好的线性关系,检出限达到3.2×10^-11 mol/L,回收率为98.7%～102.0%;该法操作简单、快速、灵敏、准确;可用于野菊花中类黄酮的测定。     

Electropolymerization of 1-naphthylamine and the structure of the polymer film

The electrochemical polymerization of 1-naphthylamine in acetonitrile solution was studied by cyclic voltammetry. Infrared spectroscopy, elemental analysis and quantum chemical calculation were used to investigate the polymer film formed. The results showed that in an acetonitrile solution containing sodium perchlorate and hydrochloric acid, the polymerization of 1-naphthylamine occurred at thep-position. A possible structure of poly(1-naphthylamine) is suggested.     

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.     

Electrochemically‐Induced Deposition of Amine‐Functionalized Silica Films on Gold Electrodes and Application to Cu(II) Detection in (Hydro)Alcoholic Medium

Well‐adherent amine‐functionalized porous silica films have been deposited on gold electrodes by combining the self‐assembly technology, the sol–gel process, and the electrochemical modulation of pH at the electrode/solution interface. A partial self‐assembled monolayer of mercaptopropyl‐trimethoxysilane (MPTMS) was first formed on disposable gold electrodes from recordable CDs (Au‐CDtrodes). The so pretreated MPTMS‐Au‐CDtrodes were immersed in a stable sol solution (pH 3) containing (3‐aminopropyl)‐triethoxysilane (APTES) and tetraethoxysilane (TEOS). Polycondensation of the APTES and TEOS precursors was then achieved by applying a negative potential for a given period of time to generate a local pH increase at the electrode/solution interface and promote the deposition of the amine functionalized silica film adhering well to the electrode surface owing to the MPTMS monolayer acting somewhat as a “molecular glue”. Various parameters affecting the electrodeposition process have been studied and the film permeability to redox probes in solution was characterized by cyclic voltammetry. The amine‐functionalized silica film electrodes were then applied to the preconcentration of copper(II) species prior to their electrochemical detection by anodic stripping differential pulse voltammetry. Getting high sensitivity has however required the application of an electrochemical pre‐activation step as the majority of the organo‐functional groups were in the form of ammonium moieties (because the film was prepared from an acidic sol). This was achieved by applying a sufficiently negative potential to the electrode surface to reduce protons and increase consequently the amine‐to‐ammonium ratio within the film and, thus, the efficiency of the precocentration process. The resulting device was then optimized for copper(II) determination in hydroalcoholic medium, giving rise to a linear response in the 0.1–10 μM concentration range.     

全固态酸度微电极组的设计与应用

以木犀草素为pH特性敏感物,碳纤维为导电基质,固体石蜡为粘合剂制成了木犀草素碳纤维酸度微电极,将该电极与自制导电凝胶型全固态微型Ag/AgCl电极和铂丝微电极联用,构成全固态酸度微电极组。 采用循环伏安法研究了该电极组对pH值为2.00~10.00的磷酸盐缓冲溶液有效酸度的响应,结果表明,循环伏安曲线上有一对可逆氧化还原峰,两峰的峰电势均随pH值呈线性变化,据此建立了方波伏安法测定溶液酸度的方法,氧化峰的峰电势(E_pa)与pH的线性回归方程为:E_pa(V)=-0.0567pH+0.603(r=0.999),体液中常见离子及蛋白质等不干扰测定。 该电极组微型、无毒、抗干扰能力强,应用于微量体表唾液和汗液酸度的测定,取得了令人满意的结果,为微量体液的实时、在体、现场检测提供了技术基础。     

Adsorptive and electrochemical behaviors of estradiol valerate at a mercury electrode

It was found that estradiol valerate could be adsorbed at a mercury electrode under open circuit. The adsorptive and electrochemical behaviors of estradiol valerate on a static mercury electrode were investigated by cyclic voltammetry, linear scan voltammetry and chronocoulometry. Based on this, a sensitive and selective adsorptive stripping square-wave voltammetric method was developed for the determination of estradiol valerate based on the optimization of solution conditions and electrochemical parameters. It was found that in a Britton-Robinson buffer solution containing 18% alcohol (pH 9.5), estradiol valerate gave a sensitive reductive peak at potential -1.29 V (vs. SCE) and the peak current was linear with the concentration of estradiol valerate in the range 2.0 x 10(-8)-2.5 x 10(-6) mol L-1. The detection limit was 1.1 x 10(-8) mol L-1. The interference of some common steroid estrogens was examined and it was found that they did not interfere in the determination of estradiol valerate in the present system.     

Voltammetric behavior and determination of bismuth on sodium humate modified carbon paste electrode

The preconcentration and voltammetric behavior of Bi^III on a sodium humate modified carbon paste electrode was studied by means of cyclic voltammetry (CV) and differential pulse stripping voltammetry (DPSV). The proposed measurement involves an initial nonelectrolytic preconcentration step in which Bi^III is complexed by the surface modifier in a solution of 0.05 M KNO₃-0.0106 M HNO₃ (pH 2.0) and a subsequent electrochemical scan step in which the preconcentrated Bi^III was reduced and then oxidized promptly in supporting electrolyte of 0.5 M HNO₃. The resulting DPSV anodic current was proportional to the concentration of Bi^III ion over the range of 4.78 × 10⁻⁸–1.44 × 10⁻⁵ M. The detection limit was 4.78 × 10⁻⁸ M. The proposed method was used to determine bismuth in various samples. Various factors affecting the electrode behavior were also investigated at the same time.     

Al Electrode Modified by Au Atoms as a Novel Electrode for Electrocatalytic Oxidation of Thiosulfate

An aluminum electrode modified with gold atoms was introduced as a novel electrode. Gold atoms were deposited both chemically and electrochemically onto the aluminum electrode to make an aluminum/gold (Al/Au) modified electrode (ME). The experimental results showed that the Al/Au modified electrode prepared by chemical deposition, exhibits much more current than the electrochemical deposition method. The electrochemical behavior of the Al/Au modified electrode was studied by cyclic voltammometry. This modified electrode showed two pairs of peaks, a₁c₁ and a₂c₂, with surface‐confined characteristics in a 0.5 M phosphate buffer. The dependence of E_pa of the second peak (a₂c₂) on pH shows a Nernestian behavior with a slope of 55 mV per unit pH. The effect of different supporting electrolytes, solution's pH and different scan rates on electrochemical behavior of Al/Au modified electrode was studied. Au deposited electrochemically on a Pt electrode (Pt/Au) was also used as another modified electrode. A comparative study of electrochemical behavior of bare Al, Pt/Au and Al/Au modified electrodes showed that both Pt/Au and Al/Au electrodes have the ability of electrocatalytic oxidation of S₂O₃^2?, but the electrocatalytic oxidation on the latter was better than the former. The kinetics of the catalytic reaction was investigated by using cyclic voltammetry and chronoamperometry techniques. The average value of the rate constant for the catalytic reaction and the diffusion coefficient were evaluated by means of chronoamperometry technique.     

Electrochemical behavior of dopamine at a quercetin-SAM-modified gold electrode and analytical application

A stable quercetin–thioglycolic acid-modified gold electrode (Qu–TCA/Au) was prepared as a self-assembled monolayer (SAM) and its electrochemical behavior was investigated by electrochemical methods. In 0.05-M phosphate buffer solution (pH 7.0) quercetin exhibits quasi-reversible signals at the Qu–TCA/Au electrode. The stability of the quercetin-modified gold electrode is very good. The quercetin self-assembled monolayer is an effective mediator for the oxidation of dopamine, which was investigated by cyclic voltammetry and differential pulse voltammetry. Ascorbic acid does not interfere with determination of dopamine at an electrode modified with a mixture of quercetin–thioglycolic acid and quercetin–11-mercaptoundecanoic acid. This modification allows dopamine to be determined in the presence of ascorbic acid in the range from 3×10^–5 to 3×10^–4 M. The detection limit is 1×10^–6 M. Scanning electrochemical microscopy (SECM) was employed to study the electrochemical performances of the modified gold electrode indicating different feedback modes at differently modified surfaces.