Adsorptive Square‐Wave Voltammetry for the Analysis and Speciation of Complexes of Cd(II)‐Ferron

The theory of adsorptive stripping square‐wave voltammetry (SWV) for relatively low ligand concentrations is employed to determine the reduction mechanism of Cd(II)‐ferron complexes accumulated on a static mercury drop electrode at different pH values. The electrochemical behavior of ferron molecules indicated that the adsorptive concentration of Cd(II) is possible in solutions with 3.5<pH<11, providing a wide pH range where the interference of other ligands present in real samples would be not so critical. Cyclic voltammetry experiments were also performed for the purpose of comparison. Fitting between experimental and theoretical square‐wave voltammograms shows that the prevailing species at the reaction layer coincide with the equilibrium bulk distribution. The simulation procedure indicated that the electrochemical rate constants of Cd(II)‐ferron complexes varied from (6±1) s^?1 to (0.17±0.01) s^?1 for solutions analyzed at pH 3.9 and 10.8, respectively. Changes at the surface concentrations are discussed considering the ligand to complex ratios at the electrode surface and at the solution bulk. From this analysis it is possible to infer that the oxidized metal species are produced in the electrolytic solution instead of on the electrode surface.     

Carbohydrates electrocatalytic oxidation using CNT-NiCo-oxide modified electrodes

A new sensor for an amplified electrochemical detection of carbohydrates is proposed, where carbohydrates are oxidized by CNT-NiCo-oxide composite in basic solutions. Cyclic voltammograms of the modified electrode show a stable and well defined redox couple in alkaline media due to the synergy of Ni(II)/Ni(III) system with Co(II)/Co(III). The modified electrode shows excellent electrocatalytic activity towards monosaccharides oxidation at reduced overpotential in alkaline solutions. Six monosaccharides were determined amperometrically at the surface of this modified electrode with high sensitivity over a wide range of concentrations, from 0.02 up to 12.12 mM. Low detection limit of 5 μM for glucose could be obtained.     

Voltammetric determination of cadmium (II) based on a composite film of a thiol-functionalized mesoporous molecular sieve and an ionic liquid

A composite film made from a thiol-functionalized mesoporous molecular sieve and an ionic liquid is introduced for use in a voltammetric sensor for Cd(II). The electrode exhibits excellent sensitivity towards Cd(II) in showing a markedly increased stripping peak current. Following the optimization of the experimental parameters, a linear response is obtained in the concentration range from 29?nM to 0.87?mM of Cd(II). The detection limit is as low as 1.0?nM (at S/N?=?3) after an accumulation at ?1.1?V for 4?min. The method was successfully applied to determine Cd(II) in water samples. Features such as large electroactive area, fast electron transfer and low background current make this electrode a promising platform for fabricating reliable electrochemical sensors for various species, such as heavy metals and environmental pollutants.

Electrochemical behavior and antioxidant activity of tetradentate Schiff bases and their copper(II) complexes

This study describes the effects of the substituents on electrochemical behavior and antioxidant activity of the six tetradentate Schiff bases, containing ethane-1,2-diamine or propane-1,2-diamine as the amine part and pentane-2,4-dione and/or 1-phenylbutane-1,3-dione as ??-diketone, and corresponding copper(II) complexes. Cyclic voltammograms of these compounds were recorded in dimethylsulfoxide and 0.1?M sodium perchlorate as supporting electrolyte with glassy carbon as working electrode at different scan rates. The voltammograms of Schiff bases alone showed only one irreversible peak. Voltammograms recorded for complexes showed the presence of quasi-reversible processes taking place at the metal center and reversible process at the ligand part. Both steric and inductive effects of substituents and structure of imine bridge of Schiff base ligands as well as complexes were discussed. These effects appear relevant for the antioxidant activity. Antioxidant activity of the investigated compounds expressed as Trolox equivalent antioxidant capacity is also discussed. The electrochemical behavior showed a high correlation with the antioxidant activity for investigated compounds.     

Voltammetric determination of cadmium(II) using a chemically modified electrode

An 1-(pyridylazo)-2-naphthol modified glassy carbon electrode has been investigated as sensor for the measurement of trace levels of Cd2+. Cd2+ is deposited on the surface of a PAN modified glassy carbon electrode at -1.10 V (vs. SCE) via forming Cd2+-PAN and subsequent reduction at the electrode. In the following step, Cd-PAN is oxidized, and voltammograms are recorded by scanning the potential in a positive direction. Calibration plots were found to be linear in the range 2 x 10(-8) mol/L to 8 x 10(-7) mol/L. The detection limit was 5 x 10(-10) mol/L, and the coefficient of variation, determined on one single electrode at a concentration of 5 x 10(-7) mol/L, was calculated to be 3.2% (n = 5). Using this new kind of modified electrode, trace levels of Cd(II) in water samples were determined; the average recovery was calculated to be 98.78%.     

Determination of electron transfer kinetic parameters by fourier transform electrochemical impedance spectroscopic analysis

A new attempt to obtain electron transfer kinetic parameters at an electrified electrode/electrolyte interface using Fourier transform electrochemical impedance spectroscopic (FTEIS) analyses of small potential step chronoamperometric currents is presented. The kinetic parameters thus obtained allowed mass transport free voltammograms to be constructed in an overpotential region, where the diffusion limits the electron transfer reaction, using the Butler-Volmer (B-V) relation. The B-V voltammograms clearly distinguish electrode reactions that are not much different in their electron transfer kinetic parameters, thus showing very similar normal linear sweep voltammetric (SCV) behaviors. Electrochemical reduction of p-benzoquinone, which displays nearly the same SCV responses at a gold electrode regardless whether the electrode is covered by a thiolated beta-cyclodextrin self-assembled monolayer, was taken as an example for the demonstration. The results show that the two voltametrically similar systems display very different electron transfer characteristics.     

Copper Complexation with Soluble and Surface Freshwaters Ligands

Speciation of copper has been done using samples collected at different times of the year (December 92 and October 93) and in three sites of a polluted river (Este River, Northern Portugal). Filtered samples and the suspended particulate matter were titrated with metal ion and the labile metal concentrations measured by anodic stripping voltammetry (DPASV). An extra peak in the Cu voltammograms has been noticed when titrating filtered samples and its origin was investigated. Results have suggested that the extra peak is due to copper(I) stabilized by ligands adsorbed on the mercury electrode and a model for the electrochemical mechanism is proposed. From titrations of the same samples with zinc, cadmium and lead it has been concluded that there are two types of organics in the river water: macromolecules and small molecules with D_ML≈D_M with higher affinity for soft cations such as Cd(II) and Cu(I), that can be adsorbed on mercury electrode as anions.     

Study of electrochemical properties of pyrrolidinofullerenes by microelectrode voltammetry

The electrochemical behavior of nine pyrrolidinofullerenes has been investigated by cyclic voltammetry on a gold microdisk electrode. Four reversible reduction peaks and two irreversible reduction peaks are observed for each fullerene derivative. The half-wave potentials of all pyrrolidinofullerenes are more negative than those of C₆₀ itself. The diffusion coefficient of these compounds is measured by their steady-state voltammograms.     

Electrochemical Response of Polyaniline (PANI)/Thiokol Rubber (TR) Films in Cysteine Solution

The electrochemical character of polyaniline/thiokol rubber (PANI/TR) composite film on a Pt electrode was investigated in a cysteine sulphuric acid solution by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The significant change of the peak current and peak potential in the cyclic voltammograms was observed. The irreversible peaks of cyclic voltammetry for cysteine/cystine redox couple at a platinum electrode have turned into one pair of nearly reversible peaks at the above electrode; the reversibility of the redox couple has been greatly improved. We found that thiokol rubber could improve the electrocatalysis of the PANI film to the electrochemical reaction with S‐S bonds, as well as the adhesion of the PANI film to the Pt substrate. Several effective factors, such as the concentration of thiokol rubber in the nonaqueous electrodeposition solution, different potential scan rate, immersing time of the film electrode in the studied electrolyte were all taken into account.     

Simultaneous and Sensitive Detection of Cd(II) and Pb(II) Using a Novel Bismuth Film/Ordered Mesoporous Carbon‐molecular Wire Modified Graphite Carbon Paste Electrode

An electrochemical sensor for the simultaneous and sensitive detection of Cd(II) and Pb(II) is proposed on the basis of square‐wave anodic stripping voltammetry (SWASV) experiments using a novel bismuth film/ordered mesoporous carbon‐molecular wire modified graphite carbon paste electrode (Bi/OMC‐MW/GCPE). Ordered mesoporous carbon (OMC) and molecular wire (MW) (diphenylacetylene) were used as the modifier and binder, respectively. The Bi/OMC‐MW/GCPE was prepared with the addition of graphite powder, OMC and DPA at the ratio of 2 : 1 : 1. The electrochemical properties and morphology of the electrode were characterized by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), SWASV and scanning electron microscopy (SEM). The parameters affecting the stripping current response were investigated and optimized. The experimental results show that the prepared electrode exhibited excellent electrochemical performance, good electrical conductivity and a high stripping voltammetric response. Under optimized conditions, a linear range was achieved over a concentration range from 1.0 to 70.0 μg/L for both Cd(II) and Pb(II) metal ions, with detection limits of 0.07 μg/L for Cd(II) and 0.08 μg/L for Pb(II) (S/N=3) with the deposition time 150 s. Moreover, the sensor exhibited improved sensitivity and reproducibility compared to traditional CPEs. The fabricated electrode was then successfully used to satisfactorily detect Cd(II) and Pb(II) in real soil samples.     

Mass transfer to tubular electrodes. Part 2: CE process

The kinetic equations for an electrochemical process consisting of a homogeneous first‐order chemical reaction followed by electron transfer at the electrode surface are solved numerically, for linear sweep voltammetry under hydrodynamical conditions in a tubular electrode. Models for both the cases involving reversible as well as irreversible electrode charge transfer reaction are investigated. The influence on current–potential voltammograms of the experimentally measurable parameters like the potential scan rate, axial flow rate and chemical equilibrium parameter is examined and depicted graphically. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis,characterization and electrochemical behavior of a new diimine-dioxime compound and its application in developing Hg(II)-selective membrane electrode

In this work, a new diimine-dioxime compound (N,N′-bis[1-biphenyl-2-hydroxyimino-2-(4-chloroanilino)-1-ethylidene]-1,4-phenylenediamine) was synthesized and characterized by a combination of elemental analyses, FT-IR, ¹H- and ¹³C-NMR spectra. The extraction ability of the new compound has been examined in chloroform by using several transition metal picrates such as Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Pb(II), Cd(II) and Hg(II). It has been observed that diimine-dioxime compound shows a high affinity to Hg(II) ion. The electrochemical measurements of the compound were performed by cyclic voltammetry in acetonitrile solution at room temperature, and two irreversible oxidation waves were observed. A Hg(II)-selective electrode based on the diimine-dioxime compound has been developed. The electrode showed linear responses with Nernstian slopes of 33 ± 1 mV per decade over a wide concentration range (1.0 × 10^?2–8.0 × 10^?6 M). The limit of detection was 2.4 × 10^?6 M. The electrode has a response time about 10–15 s and it did not show a considerable divergence in its potential response over a period of 1 month. The proposed electrode revealed selectivity towards Hg(II) ion in the presence of various cations. The electrode could be used over a wide pH range of 4.0–9.0. The electrode can be successfully used as an indicator electrode for potentiometric titration of Hg(II) with EDTA.     

Application of Novel Zn‐Ferrite Modified Glassy Carbon Paste Electrode as a Sensor for Determination of Cd(II) in Waste Water

This paper describes the preparation of a new sensor based on Zn‐ferrite modified glassy carbon paste electrode and its electrochemical application for the determination of trace Cd(II) ions in waste waters using differential pulse anodic stripping voltammetry (DPASV). Different Zn/Ni ferrite nanoparticles were synthesized and characterized using scanning electron microscopy (SEM) and X‐ray powder diffraction (XRPD). The prepared ferrite nanoparticles were used for the preparation of Zn‐ferrite‐modified glassy carbon paste electrode (ZnMGCPE) for determination of Cd(II) at nanomolar levels in waste water at pH 5. The different parameters such as conditions of preparation, Zn²⁺/Ni²⁺/Fe²⁺ ratio and electrochemical parameters, percentage of modifier, accumulation time, pH and accumulation potential were investigated. Besides, interference measurements were also evaluated under optimized parameters. The best voltammetric response was observed for ZnFe₂O₄ modifier, when the percentage of modifier was 3 %, accumulation time 9 min, pH of supporting electrolyte 5 and accumulation potential ?1.05 V. Thus prepared electrode displays excellent response to Cd(II) with a detection limit of 0.38 ppb, and selective detection toward Cd(II) was achieved.     

Electrochemical study of Azathioprine at thin carbon nanoparticle composite film electrode

Thin carbon nanoparticle/Nafion film (CNP/N), as a novel electrode material, is formed on the surface of the glassy carbon electrode in a simple solvent evaporation process. The electrochemical behavior of Azathioprine (Aza) at the CNP/N-modified electrode is investigated in detail by the means of cyclic voltammetry. During the electrochemical reduction of Aza, an irreversible cathodic peak is appeared. Cyclic voltammetric studies indicated that the reduction process has an irreversible and adsorption-like behavior. The observed reduction peak is attributed to a four-electron process referring to the reduction of nitro group to the corresponding hydroxylamine. The prepared electrode showed an excellent catalytic activity toward the electro-reduction of Aza leading to a significant improvement in sensitivity as compared to the bare glassy carbon electrode where the electrochemical activity for this compound is very weak.     

The i-E Equations of Steady-state Voltammograms at Microdisk Electrodes

Based on the theories of conventional electrodes, as well as the properties of microdisk electrode, the i-E equations for chronoamperometry at disk microelectrode for reversible, quasi-reversible and irreversible systems are derived. Steady-state voltammograms for the oxidation of [Fe(CN)6]4- , Fe2+ and ascorbic acid were measured at a series of microdisk electrodes with different radii. The conventional log-plot shows that oxidations of [Fe(CN)6]4- and ascorbic acid are reversible and totally irreversible, respectively, but the oxidation of Fe2+ is reversible at larger radius microdisk electrodes and quasi-reversible at smaller radius microdisk electrodes. The application of the log-plot to the voltammograms yielded a straight line, its slope allows us to evaluate the charge transfer coefficient and the intercept gives values of the electron transfer rate constant.     

Characterization and Applications of a Bismuth Bulk Electrode

A bismuth bulk electrode (BiBE), a new solid‐state electrode, is presented. The polycrystalline metal bismuth disk‐shaped electrode was examined for its anodic stripping voltammetry performance, which was found to be well comparable to that achieved with the bismuth or mercury film electrodes. Useful potential windows of the BiBE in aqueous solutions of pH 1 to 13 were found to range from approximately ?1.7 to ?0.1 V, depending on pH, where either hydrogen evolution or anodic dissolution of metallic bismuth limit the electrochemical inertness of the BiBE. Employing cyclic voltammetry (CV), the cathodic behavior of the BiBE was examined by testing inorganic (cadmium(II) ions) and organic (2‐nitrophenol) model compounds; a CV quasi‐reversible behavior was recorded in the case of the Cd(II)‐Cd(0) couple. The characteristics of the BiBE under anodic conditions, i.e., at bismuth surface coated with a thin conductive Bi₂O₃ film, was examined by testing two well‐established redox systems, potassium hexacyanoferate(III) and ruthenium(III) hexaaminechloride; a nearly reversible behavior was recorded in the latter case. Based on the presented preliminary results, BiBE can be considered as an interesting alternative to common solid and (toxic) mercury electrodes for possible use in electrochemical studies and electroanalytical applications.     

Cyclic voltammetry study of the peptide Lys-Cys-Thr-Cys-Cys-Ala [56-61] MT i in the presence of cadmium

The electrochemical study of the peptidic fragment Lys-Cys-Thr-Cys-Cys-Ala [56–61] MT I (FT) alone and in the presence of cadmium was performed, by cyclic voltammetry using a hanging mercury drop electrode, in order to enable comparisons between the behavior of Cd, Zn metallothioneins and that of this less complex molecule, intrinsic to the metallothionein structure. The influence of the equilibration time, of the scan rate, and of the solution pH was investigated. The influence of the addition of cadmium with different Cd–FT ratios was also studied. Samples containing both Cd and FT give complex voltammograms showing different peaks. Several electrochemical systems were distinguished: the mercury electrode itself is involved in the charge transfer step in the presence of thiol groups coming from the FT structure; another system can be attributed to the oxido-reduction of cadmium either as free ion or complexed by the peptide. The adsorption phenomena play an important role in the electrochemical process. Both dissolved and adsorbed species are electroactive.     

Cyclic voltammetry at microstructured electrodes

We present the theoretical treatment of cyclic voltammograms at microstructured electrodes. Calculations of voltammograms permit the determination of electrochemical parameters of redox systems in a single cell in parallel with the determinations of the spectroscopic parameters. The structural parameters of the electrode can be determined using the theoretical treatment presented if the electrochemical parameters of the redox system are known. Furthermore, lithographic-galvanic (LIGA) structures can be used as a model for microporous electrodes. Regression analysis was used to compare experimental and calculated cyclic voltammograms as well as to determine the electrochemical and spectroscopic parameters. A modified Randles-Sevčik equation has been derived to described the peak current dependence of cyclic voltammograms at micro-structured electrodes for both reversible and quasi-reversible charge transfer.     

电极/反相微乳液体系电沉积制备纳米金镀层

将含有氯化金的强酸性水溶液作为水相与Triton X-100、正己醇、正己烷组成反相微乳液体系, 并以该微乳液构成电极/反相微乳液电极系统, 利用电沉积方法成功地制备出纳米Au镀层. 循环伏安和交流阻抗对反相微乳液体系电沉积过程的研究发现, 微乳液中Au(III)的还原为完全不可逆过程, 其电化学反应的阻抗值约为具有相同表观浓度氯化金水溶液体系的5.5倍. SEM研究结果表明, 利用微乳液体系电沉积获得的金镀层由纳米Au颗粒组成, 直径为50 nm左右. 所制备的纳米Au修饰电极由于具有较大的比表面积, 其电化学性能优于纯Au电极, 该电极在酸性条件下有较好的析氢性能, 在碱性条件对丙三醇有较好的电催化氧化性能.     

Simultaneous Voltammetric Determination of Zn,Cd and Pb at Bismuth Nanopowder Electrodes with Various Particle Size Distributions

Effect of particle size distribution on sensor characteristics of nano‐Bi fixed electrode has been investigated using square wave anodic stripping voltammetry. Bi nanopowders with various particle size distributions were synthesized by gas condensation (GC) method with the change of chamber pressure. As the chamber pressure decreased, the size of Bi nanopowder became smaller with narrower distribution due to a shorter residence time of Bi vapor. The square wave anodic stripping voltammograms (SWASV) showed well‐defined and highly reproducible peaks at ?1.2 V, ?0.8 V and ?0.6 V (vs. SCE), which are closely related to the oxidation of Zn, Cd and Pb, respectively. The sensitivity and detection limit of the nano‐Bi fixed electrode were quantitatively estimated from the analyses of SWASV. From the results, it is concluded that as the size of Bi nanopowder becomes smaller with narrower particle size distribution, the sensitivity and detection limit of sensor electrode for Zn, Cd and Pb are improved, which is ascribed to the increase in electrochemical‐active surface area.