Electrochemical properties of sulfur adsorbed on gold electrodes

The electrochemical properties of sulfur adsorbed on gold electrodes were studied in 10^?5M solutions of S^2? in 1 M NaOH. In general, ∵_S is less than a monolayer. At E=0.05 V only, a monolayer will be formed after long times. The sulfur layer is stable in the potential range between ?0.6 and +0.4 V. At lower potentials, sulfur can be desorbed cathodically (charge Q_red), but at higher potentials, where layers of gold oxide are formed, the sulfur is oxidized anodically (charge Q_ox). From the ratio Q_red·6/Q_ox=γ, the electrosorption valency γ=?2 is obtained. This means, that the sulfide ions are almost completely discharged during adsorption. The same layer can be formed by adsorption from polysulfide solutions, which can be explained by a break of the sulfur bond and adsorption of single sulfur atoms. The double layer capacity decreases during adsorption of sulfur indicating the formation of an insulating sulfur layer with a dielectric constant of about 2. The anodic adsorption of sulfide ions is limited by diffusion only. For longer polarisation times, the coverage is independent of time, i.e. place exchange reactions between Au and S can be excluded. The cathodic desorption as well as the anodic oxidation of the adsorbed sulfur are potential dependent charge transfer processes, as can be concluded from potentiodynamic measurements with various sweep rates.     

Surfactant-sensitive polymeric membrane electrodes

The development and testing is described of surfactant-sensitive electrodes in which the active element is a plasticised polymeric membrane containing a dissolved complex of a cationic and anionic surfactant. Electrodes made with poly(vinyl chloride) membranes plasticised with 40–60% tricresyl phosphate and containing cetyl trimethylammonium dodecyl sulphate are reasonably satisfactory for determining sodium dodecyl sulphate activities below and above the critical micelle concentration. A new class of membrane electrodes has been introduced in which ion-exchange groups are chemically bound to the ends of the poly(vinyl chloride) chains either through the use of an amine as chain transfer agent during polymerisation or by using the SO₃^? radical-anion as polymerisation initiator. The resulting electrodes are specific to anionic or to cationic surfactants but selectivity between different surfactants of the same charge sign is not high. The electrode lifetimes in micellar solutions are much higher than those of previously described electrodes because the electroactive material cannot be lost by solubilisation.     

Electrochemical properties of nanoporous carbon electrodes in various nonaqueous electrolytes

Electrical double layer and electrochemical characteristics at the nanoporous carbon|acetonitrile interface with additions of Et₄NBF₄, Et₃MeNBF₄, EtMe₃NBF₄, LiClO₄, and LiBF₄ have been studied by cyclic voltammetry and impedance spectroscopy methods. A value of zero charge potential, dependent on the structure of the cations as well as on the composition of the anions, the region of ideal polarizability, and other characteristics has been established. Analysis of the complex plane plots shows that the nanoporous carbon|acetonitrile+0.1 M electrolyte (Et₄NBF₄, Et₃MeNBF₄, or EtMe₃NBF₄) interface can be simulated by the equivalent circuit, in which the two parallel conduction parts in the solid and liquid phases are interconnected by the double layer capacitance in parallel with the complex admittance of the hindered reaction of the charge transfer process or of the partial charge transfer (i.e. adsorption stage limited) process. The values of the characteristic frequency depend on the electrolyte composition and on the electrode potential, i.e. on the nature of the ions adsorbed at the surface of the nanoporous carbon electrode. In the region of moderate a.c. frequencies, the modified Randles-like equivalent circuit has been used for simulation of the complex plane plots. In the region of negative surface charge densities, the intercalation process of Li⁺ ions from LiClO₄ and LiBF₄ solutions into the surface film is possible and these data can be simulated using the modified Ho et al. model or Meyer et al. model. Electronic Publication     

Solvent polymeric membrane ion-selective electrodes

The significant role of poly(vinyl chloride) as a support polymer in solvent polymeric ion-selective electrodes is briefly reviewed. This lays emphasis on the way PVC has contributed to the relatively high state of development of conventional ion-selective electrodes, microelectrodes, coated-wire electrodes and ion-sensitive field effect transistors based on liquid ion-exchanger and neutral carrier ionophore sensors. Also discussed are possibilities of using alternative polymer matrices for overcoming some of the shortfalls of the PVC electrodes.     

Electrochemical characterization of insulating silica-modified electrodes: Transport properties and physicochemical features

The effect of depositing different numbers of insulating layers from a silica sol onto an ITO support was investigated to elucidate the changes occurring to diffusion and transfer mechanisms compared with bare electrodes. The electrochemical studies highlighted unexpected trends, which were discussed with respect to literature models and interpreted in the light of the physicochemical characterization (by FE-SEM, AFM, UV–vis transmittance) and particularly the hydrophilicity of the layers.     

Electrochemical properties of mixed self-assembled monolayers on gold electrodes containing mercaptooctylhydroquinone and alkylthiols

Mixed self-assembled monolayers of 2-(mercaptooctyl)hydroquinone (QH2) and alkylthiols were formed on gold electrodes in EtOH and the redox process of the hydroquinone moiety of QH2 was characterized by cyclic voltammetry (CV) in 0.1 M H(2)SO(4). The monolayers were formed at a series of QH2:alkylthiol ratios and the QH2:alkylthiol ratio in solution was compared to the electrochemical response from QH2 in the obtained monolayer. Mixed monolayers of QH2 with hexylthiol, dodecylthiol, and octadecylthiol were studied. The length of the alkylthiol is crucial for the electrochemical response from QH2 in the monolayer. The total concentration of thiols during monolayer formation and incubation times were also studied and low concentrations of < 2.5 mM and long incubation times gave rise to lower peak separation, lower peak half widths in the CVs of the mixed monolayers, and lower background current. The stability of a pure QH2 monolayer and a 1:4 QH2:hexylthiol monolayer toward high potentials of up to 1.5 V versus Ag/AgCl was also studied and it was observed that the mixed monolayer is significantly more stable than the pure QH2 monolayer.     

Electrochemical properties of Li symmetric solid-state cell with NASICON-type solid electrolyte and electrodes

All-solid-state phosphate symmetric cells using Li₃V₂(PO₄)₃ for both the positive and negative electrodes with the phosphate Li_1.5Al_0.5Ge_1.5(PO₄)₃ as the solid electrolyte were proposed. Amorphous Li_1.5Al_0.5Ge_1.5(PO₄)₃ was added into the electrode to increase the interface area between the active materials and the electrolyte. Any other phases were not formed at the electrode/electrolyte interface even after hot pressing at 600 °C. The discharge capacity was 92 mAh g^? 1 at 22 µA cm^? 2 at 80 °C, and 38 mAh g^? 1 at 25 °C, respectively. Symmetric cell configuration leads to simplify the fabrication process for all-solid-state batteries and will reduce manufacturing costs.     

Electrochemical properties of diamond-like carbon electrodes prepared by the pulsed laser deposition method

Diamond-like carbon electrodes (DLCEs) have been synthesized by the pulsed laser deposition method. The surface structure of the DLCEs has been studied by atomic force microscopy and the root-mean-square roughness has been established as R _ms≥81 ?. Electrochemical impedance spectroscopy and cyclic voltammetry data show that DLCEs are nearly ideally polarizable in the potential region –0.4<E<1.1 V (vs. Ag|AgCl|sat. KCl in H₂O) in 0.1 M NaF+H₂O solution. Various equivalent circuits have been used for fitting the complex plane and Bode plots. A very good agreement between experimental and calculated Nyquist curves has been established if the charge transfer and double layer charging at the surface, intercalation of the H⁺ and (or) Na⁺ ions and solid phase diffusion inside the nanoparticle, as well as the effect of an insulating film at the surface (i.e. surrounding the nanoparticles), are taken into account.     

生物功能电极I: GOD修饰电极的电化学活性

以二环己基碳化二亚胺为活化剂将葡萄糖氧化酶(GOD)共价键接在玻碳电极上, 伏安实验观察到酶与电极基体的直接电子传递, 有观电子传递速度常数约为1s^-^1, 过程归因于全酶中辅基FAD的氧化还原转变。Ag^+离子的存在强烈地阻碍酶辅基的还原, 这与该离子抑制酶活性的机理可能有联系。Ag^+的抑制作用可由EDTA处理或电化学处理而解除, GOD电极对氧和苯醌的电还原有催化作用。测定了苯醌同还原态GOD的化学反应速度常数, 并讨论用苯醌代替氧作为生物电催化中的电子传递体的优点。     

Electrochemical regeneration of sulfur loaded electrodes

Electrochemical oxidation of sulfide is a promising technique for its removal from wastewaters. Generally, the main product of this oxidation reaction is elemental sulfur. The latter deposits as a solid on the electrode and deactivates it. Therefore, an efficient and effective regeneration technique of sulfur loaded electrodes is required for the practical implementation of this technology. Here we demonstrate a method for in situ reduction of electrodeposited sulfur on carbon fibre electrodes to sulfide/polysulfides, at low energy input. The intermediary coulombic efficiency (CE) values strongly depend on pH and buffer capacity of the solution. These values were recorded up to 435 ± 5% due to simultaneous chemical dissolution of sulfur to polysulfides under alkaline conditions. This process demonstrates the potential for continuously removing dissolved sulfide from wastewaters at an anode as sulfur and recovering the deposited sulfur based on regular switching between anodic sulfide oxidation and cathodic sulfur reduction.     

Electrochemical properties of modified carbon paste electrodes containing some amino derivatives of 9,10-anthraquinone

The preparation and aqueous electrochemistry of carbon paste electrodes modified by some amino-substituted 9,10-anthraquinones are reported. In all cases, the reduction processes studied by cyclic voltammetry reveal a quasi-reversible behavior. The half-wave potentials were calculated as a function of the solution pH and from the resulting potential-pH plots the formal potentials and pK _a ^′ values of different redox and acid-base couples involved at various pH ranges were evaluated. The diffusion coefficients of different anthraquinone derivatives used in paraffin oil were calculated by chronoamperometry. Received: 3 May 1999 / Accepted: 15 November 1999     

Development of polymeric membranes for zinc ion-selective electrodes

Several polymeric membranes for zinc ion-selective electrodes have been investigated. By optimizing the choice of solvent mediator and ligand, selectivity for zinc ions can be obtained. The applicability of a theory proposing membrane selectivities as a result of both solvent and site properties is demonstrated. The concept of solubility parameters is used in discussing the detection limits obtained. The best electrode found is based on a PVC membrane containing the zinc salt of di(2-ethylhexyl)phosphoric acid dissolved in tri(2-ethylhexyl)phosphate. It is the first ion-selective electrode which responds primarily to zinc. The sensor, which has a lifetime of at least two months, is characterized by a rapid response, potential stability and good sensitivity caused by a super-Nernstian slope (43.8mV/p Zn); the detection limit is 4.5 ± 0.1 pZn.     

Electrochemical properties of Nile Blue covalently immobilized on self-assembled thiol-monolayer modified gold electrodes.

A monolayer of Nile Blue (NB) has been covalently immobilized on the self-assembled thiol-monolayer modified gold electrode. Cyclic voltammograms indicated a stable and reverse redox process of NB bonded on the electrode surface. The mechanisms of redox process coupling with proton transfer were proposed. The NB-modified electrode showed excellent electrocatalytic activity toward Nicotinamide adenine dinucleotide (NADH) oxidation and horseradish peroxidase (HRP) reduction. A hydrogen peroxide biosensor based on NB as a mediator has been demonstrated.     

Electrochemical behaviour of isopropanol at platinum electrodes

The accessible potential range of isopropanol was found to be from +1.1 to -1.2 V vs. Ag/AgCl in non-aqueous 0.01 M LiCl, with acetone being formed at the anode and hydrogen at the cathode. Water was formed in a side-reaction, probably by ketal formation, the rate being proportional to the electrode area. Another side-reaction at the cathode produced an insoluble product after prolonged electrolysis. The coulometric current efficiency was nevertheless very high (99.6-99.97%) and almost independent of current density. The results indicate that the cathode reaction proceeded with somewhat less than theoretical current efficiency.     

Electrochemical study of carminomycin at solid electrodes

The electrochemical behaviour of carminomycin at carbon-paste, glassy-carbon and composite carbon—polymer electrodes has been compared. Three electroactive sites on the molecule have been identified. Two sites are involved in the oxidation processes, the first corresponding to the hydroquinone structure in the C-6/C-11 positions, the second attributed to the C-4 phenolic function. Applying a negative-going scan allows reduction of the quinone moiety in the C-5/C-12 positions. Both the quinone and hydroquinone functions exhibit quasi-reversible electrochemical reactions in acidic medium, becoming irreversible in alkaline solution. Adsorption phenomena accompany both the oxidation and reduction processes, primarily in neutral and alkaline media. The adsorption can be decreased by using the composite electrode, which is less sensitive to surface phenomena. This working electrode is proposed as the most suitable for the determination of carminomycin at pH 1. A linear calibration plot can be obtained over the range 1 × 10⁻⁶-1 × 10⁻⁴M, with a limit of detection of 6 × 10⁻⁸M.     

Chloroquine polymeric membrane electrodes: Development and applications

Three main types of PVC solvent polymeric membrane ion-selective electrodes for chloroquine are described. They are based on three ion-pairing agents namely dipicrylamine (DPA), tetraphenylborate (TPB) or tetrakis(4-chlorophenyl)borate (TCPB) with either dioctylphenyl phosphonate (DOPP) or trioctyl phosphate (TOP) solvent mediator. All electrodes exhibit Nernstian responses, fast dynamic response times and a wide useful pH range. The best all-round electrode is based on TPB and TOP plasticizing solvent mediators with a limit of detection of 7.1 × 10⁻⁶M and was utilized for the assay of chloroquine in tablets. Direct potentiometric determinations with either the analyte addition method or the normal calibration method gave results comparable to the official method.     

Electrochemical behavior of nitrogenated nanocrystalline diamond electrodes

Nitrogenated nanocrystalline diamond films with controlled conductivity are deposited from microwave plasma in CH₄-Ar-H₂-N₂ gas mixtures. They are characterized using atomic force microscopy, Raman spectroscopy, and electrophysical measurements. Their electrochemical properties are studied by cyclic voltammetry and electrochemical impedance spectroscopy. Kinetic parameters of reactions in [Fe(CN)₆]^3-/4- redox system are determined. The character of electrode behavior is controlled by the degree on nitrogenation. With the increasing of the nitrogen content in the reaction gas mixture (from 0 to 25%), the potential window somewhat narrows, the background current increases, the reversibility of reactions in the [Fe(CN)₆]^3-/4- redox system increases. By and large, the transition occurs from the electrochemical behavior of a “poor conductor” to that of a metal-like electrode.     

Perfluoro anion-exchange polymeric films on glassy carbon electrodes

Solutions of the perfluoro anion-exchange membrane TosHex® in a solvent mixture composed of methanol + isopropanol + water (1:1:1) were prepared and applied in coating glassy carbon electrodes. The evaporated films were used to accumulate the Fe(CN) ₆ redox couple on the electrode surface. The magnitude of the electrochemical response of the loaded films is comparable with that for Nafion® incorporated cationic redox species. The multicharged Fe(CN) ₆ couple accumulated in Tosflex® film causes an ion cross-linking of the polymeric backbone, thus decreasing ion transport in the film substantially.