A novel method for fast determination of vitamin B6 by fast continuous cyclic voltammetry

In this work a novel method for the determination of Vitamin B6 in flow-injection systems has been developed. The fast Fourier transform continuous cyclic voltammetry (FFTCV) at gold microelectrode in flowing solution system was used for determination of Vitamin B6. This method is rapid, simple and highly sensitive procedures allowing the determination of Vitamin B6 in pharmaceutical analysis. The effects of various parameters on the sensitivity of the method were investigated. The best performance was obtained with the pH value of 2, scan rate value of 30 V/s, accumulation potential of 200 mV and accumulation time of 0.3 s. The proposed method has some advantages over other reported methods such as, no need for the removal of oxygen from the test solution, a sub-nanomolar detection limit, and finally the method is fast enough for the determination of any such compound, in a wide variety of chromatographic methods. To obtain a sensitive determination, the integration range of currents was set for all the potential scan ranges, including oxidation and reduction of the Au surface electrode, while performing the measurements. The potential waveform, consisting of the potential steps for cleaning, accumulation and potential ramp of analyte, was applied on an Au disk microelectrode (12.5 μm in radius) in a continuous way. The detection limit of the method for Vitamin B6 was 2.8 pg/ml. The relative standard deviation of the method at 2.1% was 8 runs. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 2, pp. 173–181. The text was submitted by the authors in English.     

Data-aquisition system for cyclic voltammetry at a microelectrode and application to fast electrode kinetics

A data-aquisition system for fast micro-electrode voltammetry is described. The computer-controlled system can be used at scan rates up to 10 000 V s^?1. The response of the system was examined by monitoring fast electrochemical reactions. A heterogeneous rate constant of 1.1 cm s^?1 was obtained for oxidation of ferrocene in acetonitrile. Oxidation of anthracene in acetonitrile was also studied; the enthalpy of activation was evaluated as 13 kJ mol^?1 for the reaction of the anthracene radical cation with acetonitrile.     

Theory and application of cyclic voltammetry at a hemispherical microelectrode for a quasi-reversible reaction

The theory of cyclic voltammetry for a quasi-reversible reaction at a hemispherical microelectrode has been deduced. From the results calculated according to the theory, the heterogeneous rate constant and charge transfer coefficient can be measured with the cyclic voltammetric data obtained at an intermediate scan rate on a hemispherical microelectrode. To verify the theory, the kinetic constant for reduction of benzoquinone in acetonitrile has been determined. The result is coincident with that measured by a fast scan rate at a platinum microdisk electrode. The kinetic constant and charge transfer coefficient for reduction of 2,3,5,6-tetrachlorobenzoquinone have also been determined with the method described in this paper.     

钡离子在玫红酸钠修饰碳糊电极上电化学固相微萃取研究

本文以电化学活性的玫红酸钠修饰碳糊电极,利用玫红酸盐与钡离子的络合效应,实现钡离子的电化学固相微萃取。以循环伏安法研究了钡离子的固相微萃取及其最佳实验条件。固相微萃取的动力学符合S形曲线模型,获得表观一级反应速率常数为2.183 min-1。固相微萃取的热力学遵循Freundlich等温吸附模型,吸附常数为n=11.4,k=1.025。     

Functioning of antimony film electrode in acid media under cyclic and anodic stripping voltammetry conditions

New insights into the functioning, i.e. electrochemical behaviour and analytical performance, of in situ prepared antimony film electrodes (SbFEs) under square-wave anodic stripping (SW-ASV) and cyclic (CV) voltammetry conditions are presented by studying several key operational parameters using Pb(II), Cd(II) and Zn(II) as model analyte ions. Five different carbon- and metal-based substrate transducer electrodes revealed a clear advantage of the former ones while the concentration of the precursor Sb(III) ion exhibited a distinct influence on the ASV functioning of the SbFE. Among six acids examined as potential supporting electrolytes the HNO₃ was demonstrated to yield nearly identical results in conducting ASV experiments with SbFE as so far almost exclusively used HCl. This is extremely important as HNO₃ is commonly employed acidifying agent in trace metal analysis, especially in elemental mass spectrometry measurements. By carrying out a systematic CV and ASV investigation using a medium exchange protocol, we confirmed the formation of poorly soluble oxidized Sb species at the substrate electrode surface at the end of each stripping step, i.e. at the potentials beyond the anodic dissolution of the antimony film. Hence, the significance of the cleaning and initializing the surface of a substrate electrode after accomplishing a stripping step was thoroughly studied in order to find conditions for a complete removal of the adhered Sb-oxides and thus to assure a memory-free functioning of the in situ prepared SbFE. Finally, the practical analytical application of the proposed ASV method was successfully tested and evaluated by measuring the three metal analytes in ground (tap) and surface (river) water samples acidified with HNO₃. Our results approved the appropriateness of the SbFE and the proposed method for measuring low μg L⁻¹ levels of some toxic metals, particularly taking into account the possibility of on-field testing and the use of low cost instrumentation.     

碳糊电极阳极吸附伏安法测定洛美沙星

报道了碳糊电极阳极吸附伏安法测定洛美沙星的新方法。在0.096mol L的KHP NaOH(pH5.4)缓冲液中,用碳糊电极为工作电极,在0.3V(vs.SCE)富集一定时间,然后从0.3～1 3V以300mV s扫速线性扫描,记录其在1 02V的二次导数阳极溶出峰。溶出峰电流与洛美沙星浓度在8.0×10-9～8.0×10-8mol L(富集90s)和8.0×10-8～8.0×10-7mol L(富集30s)范围内呈良好的线性关系,相关系数分别为0.9844和0.9967,检出限为9.0×10-10mol L(S N=3)。探讨了洛美沙星在碳糊电极上的伏安性质和电极反应机理,并且成功地应用于人体尿液中洛美沙星的测定,结果与紫外光度法基本吻合。     

聚L-色氨酸修饰电极的制备及对多巴胺的测定

研究了聚L 色氨酸修饰玻碳电极的制备及其多巴胺在该修饰电极上的循环伏安特性,建立了循环伏安法测定多巴胺的电化学分析新方法。在pH7.0的磷酸盐缓冲溶液中,用该电极测定多巴胺的线性范围为:2.0×10-6～5.0×10-4mol L,检测限为1.5×10-7mol L。已用于药剂中多巴胺的测定。     

Ac voltammetry studies of electron transfer kinetics for a redox couple attached via short alkanethiols to a gold electrode

Ac voltammetry (ACV) is used to determine the chain length dependence of electron transfer kinetics between gold bead electrodes and ruthenium redox centers attached to the electrode surface via short alkanethiols. The pentaamminepyridine ruthenium redox centers are attached to pre-assembled monolayers of mercaptoalkanecarboxylic acids (HS(CH₂)_mCOOH, m=5, 7, 10). Equations for faradaic admittance of strongly adsorbed, non-interacting electroactive species are fitted to the experimental faradaic admittance data. Kinetic heterogeneity is suggested to be a probable reason for the apparent increase in the standard rate constant (k_s) and decrease in the total coverage of redox centers (θ_total) as the perturbation frequency increases. Because of the frequency dependence of k_s and because of limitations in the correction for uncompensated resistance, faradaic admittance at a fixed phase angle of 70° is chosen to compare k_s for different chain lengths. The plot of ln(k_s) versus m (the number of methylene units) is linear with a slope of −1.2±0.1 per CH₂.     

Study on the adsorptive catalytic voltammetry of aloe-emodin at a carbon paste electrode

Recently, the studies and applications for the pola-rographic catalytic waves of organic compounds at dropping mercury electrode made considerable head-way. Tovopova et al.[1―4] reported the polarographic catalytic waves of the organic compounds containi…     

Voltammetry and determination of metronidazole at a carbon fiber microdisk electrode

The electrochemistry of metronidazole, 1-(hydroxyethyl)-2-methyl-5-nitroimidazole, was investigated at a carbon fiber microdisk electrode in pH 9 Britton Robinson buffer. Under these conditions, the reduction of metronidazole is controlled by both mass transport to the microdisk and adsorption with an equilibrium constant of 4 × 10³ mol⁻¹ dm³ and a saturation coverage of 0.88 × 10⁻⁸ mol cm⁻². The adsorption and accumulation of metronidazole on the surface of the carbon fiber allows its determination at low concentrations by square wave adsorptive stripping voltammetry. A detection limit for metronidazole of 5 × 10⁻⁷ mol dm⁻³ and a R.S.D. of 3.7% at 1 × 10⁻⁶ mol dm⁻³ (n = 4) were obtained with a two electrode system with no stirring during the accumulation step. Based on this method, a simple procedure for the determination of metronidazole in urine is described which requires no pre-treatment of the sample before analysis.     

The surface catalytic mechanism: a comparative study with square-wave and staircase cyclic voltammetry

A pseudo-first-order catalytic mechanism in which both reactant and product of a redox reaction are strongly immobilized on an electrode surface is theoretically analysed under conditions of square-wave (SWV) and staircase cyclic voltammetry (SCV). A mathematical procedure is developed under diffusionless conditions. The relationships between the properties of the voltammetric response and both the kinetic parameters of the redox reaction and the parameters of the excitation signal are studied. The phenomenon of the quasi-reversible maximum is discussed. A comparative study between SWV and SCV is presented and the limitations and advantages of both techniques, from analytical and kinetic points of view, are discussed. The theoretical predictions are experimentally confirmed by the redox reaction of azobenzene in the presence of hydrogen peroxide as an oxidizing agent. Electronic Publication     

Chemically modified pencil electrodes for application in reagentless chronopotentiometric antioxidant activity measurement

Recently a novel chronopotentiometric method has been introduced that was found useful for assessing antioxidant activity. In our experiments, the performance of chemically modified working electrodes made of different pencil materials were tested in the novel method. Redox mediator film of Meldola Blue was formed on electrode surfaces. The stability of the film, the number of molecules structuring it has been studied. In antioxidant measurements, controlled potential oxidation of the mediator was made followed by recording the open cell potential in time. The initial slope of this was used as analytical signal. Results proved applicability of the pencil probes.     

Fabrication and electrochemical behaviors of thin film three electrode-measurement unit with microdisk array as working electrode

A three electrode-measurement unit with 33 microdisk electrode-array (MDA) as working electrode was constructed by lithography.Its electrochemical behavior was observed.The results of the oxidation of ferrocene in CH3CN-H2O mixed media demonstrate that high quality electrochemical data for the system without supporting electrolyte can be achieved with the MDA unit constructed in the present work.     

Study of the adsorption of cytochrome c on a gold nanoparticle – modified gold electrode by using cyclic voltammetry, electrochemical impedance spectroscopy and chronopotentiometry

Adsorption of cytochrome c (Cyt c) on a gold nanoparticle – modified gold electrode was studied by using cyclic voltammetry, electrochemical impedance spectroscopy and chronopotentiometry in a phosphate buffer solution. It is shown that the charge transfer resistance is directly proportional to the amount of adsorbed Cyt c. The effects of temperature and time on the course of adsorption were also studied. The trends obtained in ΔG_ADS showed that Cyt c was found to have a smaller affinity for the modified electrode as indicated by their smaller negative ΔG_ADS values.     

多壁碳纳米管修饰电极检测盐酸氯丙嗪的研究

制备了多壁碳纳米管修饰玻碳电极,采用循环伏安法(CV)研究了盐酸氯丙嗪在修饰电极上的电化学特性,发展了一种新的检测盐酸氯丙嗪的电化学分析方法。在最佳实验条件下,用循环伏安法检测盐酸氯丙嗪,其响应电流与盐酸氯丙嗪的浓度在8.0×10-5~1.0×10-3mol/L范围内有很好的线性关系,线性方程为Ip(A)=0.0106c(mol/L)-8×10-8(R2=0.999,n=6),检出限为6.2×10-6mol/L(S/N=3)。方法可用于盐酸氯丙嗪片的测定。     

嵌入式碳纳米管石墨电极测定灯盏花素的研究

制备嵌入式多壁碳纳米管修饰石墨电极,利用循环伏安法(CV)研究灯盏花素在嵌入式多壁碳纳米管修饰石墨电极(ESCFE)上的电化学氧化行为,结果表明,灯盏花素在修饰电极上出现一对明显的准可逆氧化还原峰,峰电位分别为Epa=0.17 V和Epc=0.05 V(△E=0.12 V),峰电流分别为ipa=42.79μA,ipc=...     

In-situ FT-IR studies on cyclic voltammetry of polythiophene

In-situ FT-IR spectra were taken during a cyclic voltammetric study of polythiophene in 1M LiClO₄ in propylene carbonate. The external reflection technique was used in the spectroscopic measurements. A broad band extending from 1500 cm^–1 to higher wave numbers appeared during oxidation of polythiophene with a potential scan from 2.8 V to 4.2 V. Bands due to doping of the polymer at 1330 and 1020 cm^–1 were also found to increase during the same potential scan. The appearance of spectral changes above 3.3 V supports the model of two types of doping processes occurring in the polymer. The observed changes almost disappeared during the reverse scan.     

Electrochemical behaviors of native and thermally denatured fish DNA in the presence of cytosine derivatives and porphyrin by cyclic voltammetry using boron-doped diamond electrode

The electrochemical behaviors of native and thermally denatured fish DNA was investigated using boron-doped diamond (BDD) film electrode by cyclic voltammetry. The BDD electrode afforded us to measure weak current less than muA for the DNA solution in 100 microl. The mixture of acetic acid and sodium acetate solution (0.2 M) was used as a supporting electrolyte. Two oxidation peaks were observed at about +1.1 V and +1.3 V at pH 4.6 for thermally denatured fish DNA. This is due to the oxidation of guanine and adenine in the denatured fish DNA, respectively. In contrast, the native fish DNA showed ill-defined peaks at +1.1 V. Furthermore, the electrochemical behaviors of thermally denatured fish DNA were studied in the presence of cytosine, cytidine, cytidine-5-monophosphate, tetrakis(1-methypyridinium-4-yl)porphyrin (H(2)(TMPyP)(4+)) and Ru(II)(TMPyP)(4+). The oxidation peak intensity at +1.1 V gradually decreased with the increase of the concentrations of the above compounds. Based on the above studies, electrochemical behaviors of the thermally denatured fish DNA at BDD electrode is discussed.     

Determination of nanomolar concentrations of lead and cadmium by anodic-stripping voltammetry at the silver electrode

Lead and cadmium have been determined by subtractive anodic-stripping voltammetry (SASV) in the square-wave mode at a silver electrode without removal of oxygen. The sensitivities and detection limits for the two metals differ considerably. Detection limits of 0.05 nM for lead and 1 nM for cadmium have been achieved following 90 s electrodeposition. The repeatability of consecutive SASV runs is good (for lead 0.5% at 20 nM for 30 s electrolysis, 5% at 0.3 nM for 60 s electrolysis; for cadmium 2.5% at 20 nM for 30 s electrolysis, 5% at 5 nM for 60 s). Hundreds of runs can be carried out without any pretreatment of the electrode. The high stability is attributed to renewal of the electrode surface that takes place during the electrodeposition step in a two-electrode cell: the silver counter/quasi-reference electrode generates silver ions that codeposit with lead and cadmium at the Ag-RDE, thus ensuring a continuity of the latter. Underpotential deposition (UPD) plays a central role in anodic-stripping voltammetry (ASV). During the deposition step, the adatom coverage of trace elements is in the range of 0.01-1% and no bulk deposition is invoked for metals that exhibit UPD. The UPD properties and, as a result, the ASV signals are strongly affected by the type and concentration of the supporting electrolyte. The effects of Cl⁻, Br⁻, SO₄²⁻ and NO₃⁻ are shown. The analysis of lead and cadmium in natural waters has been performed. Surfactants distort the SASV signal. In order to ensure surfactant-free solutions, the samples were pretreated by wet ashing.     

二苯并四硫代富瓦烯类衍生物的合成和电化学行为

合成了四个二苯并四硫代富瓦烯(DB-TTF)的衍生物(3a～b, 4a～b), 给出了它们的核磁共振氢谱和碳谱, 质谱,紫外吸收光谱及元素分析等数据. 用=循环伏安法测定了它们的氧化电位, 并将此与DB-TTF的氧化电位作了比较.发现: 两端取代的硫酮基对分子体系的吸电子效应较强,会明显增大分子的第一氧化电位 , 而且会降低分子内的Coulomb作用力, 相比之下, 在四周顶点上取代的烷氧基对分子体系的给电子效应较弱.