Voltammetric Behavior of Semicarbazide at Graphene Modified Electrode and Application to Detection

Herein voltammetric behaviors of semicarbazide (SEM) are investigated by employing a graphene modified working electrode, which displays attractive electroanalytical properties. In the acetate buffer solution of pH 4.00, there is a well‐defined oxidative peak of SEM, attributing to the irreversible and adsorption‐controlled electrode reaction with 2 electrons participation. The values of apparent heterogeneous electron transfer rate constant k′_s with the redox are 0.0061 s^?1 and 0.0009 s^?1, respectively, for two scan rate section. After the experimental parameters, which influence the voltammetric responses of SEM, including supporting electrolyte, pH, accumulation time and accumulation potential, etc., are optimized, it is found that the anodic peak current of SEM is proportional to its concentration in the range of 4 ‐ 40 μmol/L with a detection limit of 1 μmol/L (S/N = 3). Then, an electrochemical method for detecting SEM quantitatively is developed successfully. The concentrations of SEM in fortified tap water samples are tested with satisfactory recovery, indicating that the novel method is strongly promising in the environmental monitoring application.     

Significance of redistribution reactions detected by in situ atomic force microscopy during early stages of fast scan rate redox cycling experiments at a solid 7,7,8,8-tetracyanoquinodimethane-glassy carbon electrode-aqueous (electrolyte) interface

The reduction of solid 7,7,8,8-tetracyanoquinodimethane (TCNQ) at an electrode-TCNQ-aqueous (electrolyte) is complex, irrespective of whether the solid on the electrode surface is attached by direct adherence or formed by electrochemical deposition. In order to understand the origin of reaction pathways that accompany the [TCNQ]^0/− process, fast scan rate (0.1 V s⁻¹) redox cycling and potential step experiments on TCNQ mechanically attached to a glassy carbon electrode placed in aqueous solution containing 0.1 M electrolyte (KCl, CsCl, or Et₄NCl) have been monitored by the technique of in situ atomic force microscopy (AFM). The shapes of cycling voltammograms are consistent with the presence of a mixture of diffusion and surface processes in the initial cycles. AFM results show that, during the early stage of the redox cycling experiments, electrochemical reduction of TCNQ to sparingly soluble TCNQ⁻ is accompanied by a redistribution process. This rearrangement results in the transformation of arrays of almost amorphous solid to a lower energy microcrystalline state which has a more thin film-type appearance. When CsCl is the electrolyte, long needle-type crystals are detected by the AFM method after long periods of redox cycling. The identity of the cation in the supporting electrolyte and the solubility of the reduced salt formed by reduction of TCNQ affect the nature of the voltammetry observed during early stages of redox cycling. When the redistribution process is completed and the stable crystalline phase is formed, the voltammetry of the [TCNQ]^0/− couple is predominantly controlled by a nucleation-growth mechanism. Received: 8 March 1999 / Accepted: 12 April 1999     

The influence of alkali metal cations on the rate of the Fe(CN)64−/Fe(CN)63− electrode process

The rate of the hexacyanoferrate redox system shows a first order dependence on the concentration of the cationic component of the supporting electrolyte. The catalytic influence of the alkali metal cations on the electrode process increases in the order Li⁺<Na⁺<K⁺～Cs⁺. The temperature dependence of the rate constant of the electrode process in KF and LiNO₃ has been measured and the results show that the activated complex is formed by the collision or association of a cation of the supporting electrolyte with the reactant anion, which may already be paired with one cation. It is suggested that this mechanism may be applicable to other electrode reactions involving highly charged species.     

Direct electrochemistry of myoglobin in a room temperature ionic liquid aqueous solution and its catalysis to H<Subscript>2</Subscript>O<Subscript>2</Subscript>

Using 1-ethyl-2-methylimidazolium trifluoroacetate (EMImTfa) as the supporting electrolyte, a couple of well-defined and reversible redox peaks of Myb could be observed at the basal plane graphite (BPG) electrode through direct electron transfer between the protein and the BPG electrode, whose anodic and cathodic peak potentials were at −0.098 V and −0.144 V vs. Ag | AgCl, respectively. Both anodic and cathodic peak currents increased linearly with the potential scan rates. Compared with the supporting electrolyte of phosphate buffer solution, EMImTfa played an important role for the direct electron transfer between Myb and the BPG electrode. Further investigation suggested that Myb was adsorbed tightly on the surface of the BPG electrode in the presence of EMImTfa to form a stable, approximate monolayer Myb film. Myb adsorbed on the BPG electrode surface could retain its biological activity and showed a remarkable electrocatalytic activity for the reduction of H₂O₂ in an EMImTfa aqueous solution. Based on these, a third-generation biosensor could be constructed to directly detect the concentration of H₂O₂ in EMImTfa aqueous solution with a limit of detection of 3.24 × 10⁻⁸ M. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 3, pp. 363–368. The text was submitted by the authors in English.     

Electrochemical and Spectroelectrochemical Studies on Pyridoxine Hydrochloride Using a Poly(methylene blue) Modified Electrode

The electrochemical redox processes of pyridoxine hydrochloride (VB₆) at a poly(methylene blue) film modified glass carbon electrode (PMBE) in a phosphate buffer solution (PBS, pH 8.0) were studied by cyclic voltammetry. The VB₆ electrode reaction with quasi‐reversible characteristics was diffusion‐controlled at low scan rates and adsorption‐controlled at high scan rates. The anodic peak current positive to 0.6 V (vs. SCE) was found to be proportional to the concentration of VB₆ in the range of 0.010 to 1.03 mg?mL^?1 with a detection limit of 1.34 μg mL^?1. Fluorescence and UV‐vis absorption spectroelectrochemical measurements suggest that the pyridine ring was not destroyed over the potential range from ?0.8 to 1 V (vs. SCE), and the electrocatalytic generation of pyridoxal was anodically started at 0.57 V.     

The solid state electrochemistry of samarium (III) hexacyanoferrate (II)

A new electroactive polynuclear inorganic compound of a rare earth metal hexacyanoferrate, samarium hexacyanoferrate (SmHCF), was prepared chemically and characterized using techniques of FTIR spectroscopy, thermogravimetric analysis (TGA), X-ray powder diffraction, UV–Vis spectrometry and X-ray photoelectron spectroscopy (XPS) etc. The cyclic voltammetric behavior of SmHCF mechanically attached to the surface of graphite electrode was well defined and exhibited a pair of redox peaks with the formal potential of 180.5 mV (versus SCE) at a scan rate of 100 mV/s in 0.2-M NaCl solution and the redox peak currents increased linearly with the square root of the scan rates up to as high as 1,000 mV/s. The effects of the concentration of supporting electrolyte on the electrochemical characteristics of SmHCF and the transport behavior of K⁺, Na⁺ and Li⁺ counter-ions through the ion channel of SmHCF were studied by voltammetry.     

A Study of Nickel Electrodeposition on Paraffin‐Impregnated Graphite Electrode

The aim of the present work was to elucidate the mechanism of electrolytic deposition of Ni on paraffin‐impregnated graphite electrode (PIGE). This process is influenced by H₂ evolution, which occurs in the same potential region. On the basis of the results obtained by linear and cyclic voltammetry, elimination voltammetry with linear scan (EVLS) was used to evaluate both processes. H₂ Evolution alone was studied in sulfate supporting electrolyte, and the previously suggested mechanism for this process according to Volmer–Heyrovsky was confirmed by EVLS. It was found that both the Ni²⁺ concentration and pH affect the polarization behavior of PIGE significantly. Two separated cathodic peaks were observed at low Ni²⁺ and high H⁺ concentrations, and the separation was better at higher scan rates. EVLS confirmed the most‐probable mechanism of Ni deposition as being controlled by slow transfer of the first electron under formation of [NiOH]⁺ as an intermediate. EVLS also indicated slow reduction of H⁺ preceding the reduction of Ni²⁺. The same was confirmed by studying the anodic dissolution at different switching potentials. The results were complemented by scanning electron microscopy (SEM).     

Enhancement of electron transfer kinetics on a polyaniline-modified electrode in the presence of anionic dopants

The electrocatalytic redox behavior of 1,4-naphthoquinone (NQ) has been studied on a polyaniline-modified platinum electrode (PANI) using cyclic voltammetry and rotating disc electrode (RDE) as diagnostic techniques. The modified electrode was prepared by electropolymerization of aniline in different acidic solutions. The PANI showed electrocatalytic activity toward the redox behavior of NQ. This process includes the participation of PANI to the redox reaction of NQ via the surface catalysis phenomena. The cyclic voltammograms of NQ in HCl on the PANI-Cl-modified electrode showed an overlapped oxidation peak, the peak potential of which did not change with increasing scan rate. The influence of other anions including and as dopant was also studied and compared with Cl⁻. The use of HClO₄ as a supporting electrolyte resulted in well-separated redox peaks. The RDE voltammogram was used to obtain a quantitative assessment of reaction rate at the PANI-modified electrode. It was found that PANI acts as an electrocatalyst for NQ reduction with decreasing ΔE _p and increasing k°.     

Direct electrochemical response of Myoglobin using a room temperature ionic liquid, 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate,as supporting electrolyte

Direct electrochemical response of Myoglobin (Myb) at the basal plane graphite (BPG) electrode was observed when a room temperature ionic liquid (RTIL), 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate ([HEMIm][BF₄]), was used as the supporting electrolyte. In a 0.17 M [HEMIm]BF₄ aqueous solution, a couple of well-defined redox peaks of Myb could be obtained, whose anodic and cathodic peak potentials were at −0.158 and −0.224 V (vs. Ag/AgCl), respectively. Both anodic and cathodic peak currents increased linearly with the potential scan rate. Compared with the supporting electrolyte of phosphate buffer, [HEMIm][BF₄] played an obvious promotion for the direct electron transfer between Myb and the BPG electrode. Further investigation suggested that Myb was adsorbed tightly on the surface of the BPG electrode in the presence of [HEMIm][BF₄] to form a stable, approximate monolayer Myb film. Myb adsorbed on the BPG electrode surface showed a remarkable electrocatalytic activity for the reduction of oxygen in a [HEMIm][BF₄] aqueous solution. Based on these, a third-generation biosensor could be constructed to directly detect the concentration of oxygen in aqueous solution with a limit of detection of 2.3 × 10⁻⁸ M.     

Interfering influence of silver on the voltammetric behaviour of Cd2+ and Zn2+ ions on hmde in supporting electrolytes containing nitrates

A new kind of secondarily formed peaks was found in cyclic and stripping voltammetry in neutral sulphate, perchlorate and nitrate supporting electrolytes containing some divalent cations and a substance (for example O₂), the reduction of which gives as a by-product OH^? ions. The hydroxides deposited in the vicinity of the mercury electrode, in the course of a cathodic scan, react during the anodic scan according to the reaction Hg+Me(OH)₂=Hg(OH)₂+Me²⁺+2e forming a new, separate anodic peak.It was found that silver exerts a catalytic effect on the reduction of NO₃^? ions on the mercury electrode. In neutral nitrate supporting electrolyte containing Ag⁺ ions the hydroxides of some cations (Cd²⁺, Zn²⁺, Mn²⁺, Co²⁺ and Ni²⁺) were deposited during the cathodic scan or during the preelectrolysis. Afterwards, in the course of the anodic scan, a new peak, of the kind described above, was observed. The same effect was formerly interpreted, for Zn²⁺ and Cd²⁺, as evidence for the formation of intermetallic compounds, AgZn and AgCd.     

Voltammetric studies of microcrystalline C60 adhered to a carbon electrode surface and placed in contact with aqueous electrolyte containing potassium ions

The reduction of microcrystalline C₆₀ fullerene, adhered at a carbon electrode and immersed in aqueous electrolyte, has been studied under various voltammetric conditions. This work reports mainly the voltammetric studies carried out principally in electrolyte containing potassium ions. Comparison of adherence techniques, such as solvent casting and mechanical transfer methods, are made to assess if the type of adhered techniques has any significant influence on the observed electrochemistry. The solvent casting method is found to produce three peaks in the potential for C₆₀^0/n- redox couple as compared to a single and large peak produced when a mechanical transfer technique is employed. When the reduction potential of microcrystalline C₆₀ in the presence of K⁺ is compared with other cations, such as Li, Na, Rb and Cs, it is observed that the shift of reduction potential follows the change in the hydration energy in the order Cs>Rb>K>Na>Li. In a mixed electrolyte study of CsCl/KCl, the reduction potential and peak shape of C₆₀^0/n- redox couple during cyclic voltammetry is observed to change with concentration of the cations and the observed electrochemistry can be attributed to a cation-exchange mechanism. The reduction of C₆₀ is irreversible in aqueous electrolyte containing alkaline cations as the re-oxidation process does not produce any observed electro-activity. Evidence of the formation of a passive coating of K _n C₆₀ fulleride, which does not appear to undergo dissolution is obtained under cyclic voltametric conditions. This coating remains electrochemically active in the presence of tetrabutylammonium ions in acetonitrile. Scan rate, chronocoulometric, and scanning electron microscopic studies provide evidence of the presence of a surface process involving solid–solid transformation.     

A voltammetric method for the quantitative determination of midecamycin compared to its simultaneous HPLC determination

The aim of this study was to present the quantitative determination of midecamycin at a gold electrode in 0.05 M NaHCO₃ using cyclic linear sweep voltammetry. It was found that the value of the oxidative peak of pure midecamycin at 0.85 V vs. SCE at a scan rate of 50 mV s⁻¹ is a linear function of the concentration in the range 0.1693–0.3289 mg cm⁻³. The value of its reductive peak at 0.3 V vs. SCE is a linear function of the concentration in the range 0.11396–0.3802 mg cm⁻³. HPLC analysis of the bulk of electrolyte confirmed the data obtained by analysis of the current peak values concerning the correlation with each investigated concentration of midecamycin. By MS spectrometry no degradation products were found in electrolyte during its quantitative determination.     

Electrooptical and electrochemical properties of an ionic conjugated polymer,poly(2-ethynylpyridinum-N-benzoylsulfonate)

The electrooptical and electrochemical properties of a self-dopable ionic conjugated polyacetylene, poly(2-ethynylpyridinium-N-benzoylsulfonate) (PEPBS), were studied. The photoluminescence spectra of the polymer showed that the photoluminescence peak was located at 545 nm corresponding to the photon energy of 2.27 eV. The cyclic voltammograms of the polymer exhibited reversible electrochemical behaviors between the doped and undoped peaks. It was found from the dependence of the oxidation current density of the polymer on scan rate that the kinetics of the redox process was diffusion-controlled. The electrical conductivity (σ) of undoped PEPBS was 5.7 × 10⁻⁹ S/cm. The text was submitted by the authors in English.     

XPS Studies of PtCl4 and H2PtCl6·6H20 doped polyacetylene

The PtCl₄ and H₂PtCl₆-6H₂O doped polyacetylene were studied by X-ray photoelection spectroscopy and transmission electron microscopy. We found that both Pt 4f and Cl 2p peaks could be resolved into two components both with a splitting of ca. 1.5 eV. The higher binding energy components of Pt 4f peak is attributed to Pt⁴⁺ and the lower binding energy one to Pt²⁺ species. From quantitative analysis of the results of decomposition of both Pt 4f and Cl 2p peaks it was found that an atomic ratio of chlorine to platinum for Pt²⁺ species is (Cl) / (Pt) = 2 and that for Pt⁴⁺ species is (Cl) / (Pt) = 6 for both PtCl₄ and H₂PtCl₆·6H₂O doped polyacetylene. The C 1s peaks could be decomposed into two components separated by ca. 1 eV. The intensity of the higher binding energy component increased with increasing dopant concentration. These indicate that the platinum salt doping proceeds through charge transfer from polyacetylene chain to platinum atom resulting in a partial reduction from Pt⁴⁺ to Pt²⁺ state. The existence of PtCl₂ cluster on the surface of the doped polyacetylene film was supported by transmission electron microscopy and electron diffraction observations. These results indicate that a random distribution of the dopant along the macromolecular chain, and the charge per carbon atom in the metallic region of doped polyacetylene has been estimated to be 0.2 |e|. From these results the mechanism of the PtCl₃ and H₂PtCl₆·6H₂O doping process in polyacetylene is clarified as follows: Thus the dopant anion in polyacetylene is PtCl,^2? for both PtCl₄ and H₂PtCl₆·6H₂O doping.     

Reduction of metol on mechanically renewed metal electrodes

The electroreduction of metol on mechanically renewed metallic electrodes is studied by direct voltammetry with linear potential sweep. Reduction peaks of metol are found in a neutral supporting electrolyte (0.02–0.5 M Na₂SO₄) on nickel, silver, and mercury electrodes before the potential of hydrogen liberation from the supporting electrolyte. The shape and parameters of the cathodic peak depend on an electrode material, and also on the composition and pH of the supporting solution. A probable mechanism of the electroreduction of metol is proposed. The regeneration of nickel and silver electrodes by in situ mechanical cutting of a 0.5-μm surface layer provides good reproducibility of the value of peak current; it is proportional to the concentration of metol in the range 2 × 10⁻³–1.8 × 10⁻² M.     

Adsorptive Stripping Square-Wave Voltammetry of Creatine

ABSTRACT

An adsorptive stripping square-wave voltammetric method for quantitative determination of creatine is developed. The basic redox properties of creatine are investigated by means of square-wave and cyclic staircase voltammetry. Creatine undergoes an irreversible reduction in neutral and acidic medium at a hanging mercury drop electrode. The square-wave voltammetric response of creatine depends on the parameters of the SW excitation signal as well as on the concentration and type of the supporting electrolyte, the accumulation time and the potential and pH of the medium. The optimal experimental conditions for quantitative determination of creatine are as follows: supporting electrolyte 0.1 mol/L KNO₃ buffered with 0.1 mol/L acetate buffer to pH = 4 and accumulation potential -1.2 V. The optimal SW parameters found are: frequency f = 120 Hz, amplitude E _sw = 30 mV, and scan increment dE = 4 mV. A detection limit of 6.6 x 10^?8 mol/L creatine was obtained after 30 s preconcentration period at accumulation potential -1.2 V. The correlation coefficients of the calibration curves at concentration levels of 10^?7 to 10^?5 mol/L creatine are greater than 0.99. The results of recovery tests range from 92.18% to 102.51%.     

AOT/Triton X-100混合反胶束体系的导电性能

采用聚乙二醇辛基苯基醚(TritonX-100)和二(2-乙基己基)琥珀酸磺酸钠(AOT)双表面活性剂,与正己烷、正己醇和水构成混合反胶束体系;研究了表面活性剂质量比、助表面活性剂含量、水油体积比和温度等因素对反胶束体系导电性能的影响,同时采用循环伏安法研究了K3Fe(CN)6/K4Fe(CN)6在该体系中的电化学行为.结果表明:由两种表面活性剂构成的反胶束体系电导率σ明显大于单一表面活性剂反胶束体系电导率;体系电导率随AOT与TritonX-100的质量比w(w=mAOT∶mTritonX-100)的变化而变化,w为0-0.4时,电导率随w增大而线性增大,之后增加趋势变缓;w=0.96时,σ达到稳定值576μS·cm-1.混合体系电导率随溶水量的增大及温度的上升而提高;而增加助表面活性剂可显著降低体系的电导率.在所研究体系中,Fe(CN)36-/Fe(CN)46-电化学反应对的氧化还原峰电位几乎不随扫描速率变化,峰电位差约为75mV,峰电流的比值约为1,氧化峰电流与扫描速率的平方根成正比,说明K3Fe(CN)6/K4Fe(CN)6在混合反胶束体系中显示出良好的氧化还原可逆性,反应由扩散步骤控制.     

Electrocatalytic Oxidation of Dopamine at a Nanocuprous Oxide‐Methylene Blue Composite Glassy Carbon Electrode

Cuprous oxide nanowhisker was prepared by using cetyltrimethyl ammonium bromide (CATB) as soft template, and was characterized by XRD and TEM methods. The electrochemical properties of nano‐Cu₂O and nano‐Cu₂O‐methylene blue (MB) modified electrode were studied. The experimental results indicate that nano‐Cu₂O shows a couple of redox peaks corresponding to the redox of Cu(II)/Cu(I), the peak currents are linear to the scan rates which demonstrate that the electrochemical response of Cu₂O is surface‐controlled. The composite nano‐Cu₂O‐Nafion‐MB modified electrode shows a trend of decrease of peak currents corresponding to the Cu (II)/Cu (I). However, the electrocatalytic ability of nano‐Cu₂O‐MB composite film to dopamine increases dramatically. At this composite electrode, dopamine shows a couple of quasireversible redox peaks with a peak separation of 106 mV, the peak current increases about 8 times and the oxidation peak potential decreases about 200 mV as compared to that at bare glassy carbon electrode. The peak currents change linearly with concentration of dopamine from 1×10^?7 to 3.2×10^?4 mol/L, the detection limit is 4.6×10^?8 mol/L. The composite electrode can effectively eliminate the interference of ascorbic acid and has better stability and excellent reproducibility.     

Facile cation electro-insertion into layer-by-layer assembled iron phytate films

Molecular layer-by-layer assembly from pre-saturated aqueous solutions of Fe³⁺ and phytate is employed to build up iron phytate deposits on tin-doped indium oxide (ITO) electrodes. Globular films with approximately 1 nm growth per layer are observed by AFM imaging and sectioning. In electrochemical experiments the iron phytate films show well-defined voltammetric responses consistent with an immobilised Fe(III/II) redox system in aqueous (LiClO₄, NaClO₄, KClO₄, phosphate buffer) and in ethanolic (LiClO₄, NaClO₄, NBu₄ClO₄) electrolyte solutions. The Fe(III/II) redox system is reversible and cation insertion/expulsion occurs fast on the timescale of voltammetric experiments even for more bulky NBu₄⁺ cations and in ethanolic solution. Peak shape analysis and scan rate dependent midpoint potentials suggest structural changes accompanying the redox process and limiting propagation. Iron phytate is proposed as a versatile and essentially colourless cation electro-insertion material and as a potential energy storage material.     

Remarkable Electrochemical Responses of Ferrocene/NaY Zeolite Composite modified Electrode Based on Hydrophobic Ionic Liquid

The ferrocene/NaY zeolite composites (Fc/NaY) are introduced on the surface of a glassy carbon electrode together with the hydrophobic ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIMPF₆). The modified electrode thus constructed exhibits a pair of reversible redox peaks corresponding to ferrocene. Additionally the peak separation remains almost constant (58–75 mV) and the value of the ratio i_pa/i_pc is close to 1 for scan rates in the range from 10 to 1000 mV s^?1. The effects of the scan rate, aqueous supporting electrolytes, hydrophobic ionic liquid and the contents of ferrocene encapsulated by electrochemistry are investigated. The extrazeolite electron transfer process is discussed. Furthermore, the Fc/NaY/IL‐modified electrode shows good mediation towards oxidation of ascorbic acid, dopamine, hydroquinone, and catechol.