Reduction of cytochrome c at a lipid bilayer modified electrode

The reduction of horse heart cytochrome c has been investigated at a platinum electrode modified with a lipid bilayer membrane (BLM) which immobilized vinyl ferrocene as an electron mediator. The current—voltage curves show that the direct electrochemistry of cytochrome c at the metal electrode occurs quite efficiently. An adsorption equilibrium constant for cytochrome at the BLM surface, as well as an electron transfer rate constant between the protein and the modified electrode have been estimated from these results. The values of both parameters are much higher than those reported with other types of electrode modifications, indicating that a lipid bilayer-modified platinum electrode system using vinyl ferrocene as a mediator provides substantial improvements in electrochemical activity of cytochrome c at metal electrodes. The potential for modifying and utilizing this new class of “biomembrane-like” electrode surface for metalloprotein electrochemistry is briefly discussed.     

Reduction of cytochrome c at a lipid bilayer modified electrode

The reduction of horse heart cytochrome c has been investigated at a platinum electrode modified with a lipid bilayer membrane (BLM) which immobilized vinyl ferrocene as an electron mediator. The current-voltage curves show that the direct electrochemistry of cytochrome c at the metal electrode occurs quite efficiently. An adsorption equilibrium constant for cytochrome at the BLM surface, as well as an electron transfer rate constant between the protein and the modified electrode have been estimated from these results. The values of both parameters are much higher than those reported with other types of electrode modifications, indicating that a lipid bilayer-modified platinum electrode system using vinyl ferrocene as a mediator provides substantial improvements in electrochemical activity of cytochrome c at metal electrodes. The potential for modifying and utilizing this new class of “biomembrane-like” electrode surface for metalloprotein electrochemistry is briefly discussed.     

The mechanism of the platinum indicator electrode in argentometry

A study has been made of the behaviour of platinum and some other inert electrodes in silver nitrate titrations. Where the metal surface has been subjected to a reducing treatment, such as cathodic polarization, before use, the electrode will often function as a silver indicator electrode throughout the titration. It has been shown experimentally that this is due to the formation of a layer of metallic silver upon the electrode by interaction of the surface with silver ions in solution. If the metal surface undergoes such pretreatment that it is oxidized, then its potential normally remains at the oxide value during a silver nitrate titration until the silver ion concentration is sufficiently great for this value to be exceeded by the silver silver ion potential; formation of metallic silver then again takes place and from this point onwards the electrode behaves as a silver electrode. A detailed interpretation of the behaviour of platinum after various pretreatments has been made along these lines.     

Microscopic modelling of the reduction of a Zn(II) aqua-complex on metal electrodes

The mechanism of the Zn(II) reduction from acid aqueous solution at a metal electrode surface has been elucidated at a microscopic level. The Anderson–Newns model was employed in order to construct the adiabatic potential energy surfaces along the solvent coordinate for several reactions as a function of the electrode–reactant distance and the overpotential. A quantum chemical approach was employed to treat the coupling of the reactant to the metal electrode within a cluster model. The reduction of a [Zn(H₂O)₆]²⁺ complex was found to proceed in the adiabatic regime, the transfer of the first electron being rate-determining. Main attention is focused on effects of a qualitative nature, which are discussed in the light of available experimental data. The electrode charge excess and distance of maximal approach were found to affect significantly the Frank–Condon barriers of the reaction. An experimentally observed dependence of the rate constant of Zn(II) reduction upon the electrode material has been interpreted.     

Specular reflectance studies of bromide adsorption on gold

Specular reflectance changes have been used to examine the specific adsorption of bromide on gold in the presence of a large excess of supporting electrolyte (NaF) which is not specifically adsorbed. A linear relation has been demonstrated between the reflectance changes and the surface excess of bromide through the examination of the time dependence of the reflectance under conditions where the rate of adsorption of the bromide is diffusion controlled and hence known. The adsorption isotherms have been found to follow Temkin behavior. The electrosorption valency has been evaluated from the charge and surface excess at constant potential and found to be ?0.49 to ?0.59, depending on the potential. Various mechanisms for the subtantial changes in reflectance attending the specific adsorption of anions are discussed. The observed effects cannot be explained on the basis of changes in the charge on the electrode and corresponding changes in the contribution of the conduction band to the surface optical properties. The principal mechanism is proposed to be modifications in the surface electronic states of the metal electrode through direct orbital interactions between the adsorbed anions and the metal.     

A study of the differential capacitance of zinc in aqueous solution with potassium silicate additions

The differential capacitance of the polycrystalline zinc electrode has been studied in aqueous solutions of KCl, KNO₃ and KOH both with and without the addition of potassium silicate. Double layer capacitance measurements can be made in KCl and KNO₃ without the interaction of OH^? at low pH values < 3.0. The reduction of the nitrate ion takes place at the zinc electrode in aqueous potassium nitrate.The silicate ion is adsorbed on the zinc electrode in aqueous KOH solutions at a potential close to the dissolution potential. This results in inhibition of metal dissolution, due to limited interaction of OH^? with the metal surface. The electrode resistance is increased by this adsorbed layer of silicate ion. In alkaline solution the h.e.r. is stimulated by the addition of potassium silicate.     

Adsorption of insoluble surfactants at the Au(111)/solution interface

The adsorption of surfactants, which form insoluble monolayers on an aqueous substrate, onto a single crystal gold electrode have been described. Adsorption of this class of surfactants have been characterized using a combination of electrochemistry and Langmuir-Blodgett techniques. We have developed a technique to simultaneously measure the film pressure at the gas-solution (GS) interface and the film pressure of the surfactants that spread to the metal-solution (MS) interface. We have shown that surfactants such as octadecanol and stearic acid, which interact weakly with the metal surface, adsorb at an uncharged MS interface (at the potential of zero charge) and progressively desorb when the electrode surface is charged negatively. The electrode potential (charge density at the metal surface) influences the transfer of the surfactant from the GS interface to the MS interface. The transfer ratio is 1:1 at an uncharged MS interface, and is progressively reduced to zero when the MS interface is charged. We have employed 12-(9-anthroloxy) stearic acid, a surfactant dye molecule, to study the mechanism of potential induced desorption and adsorption of the film of insoluble molecules. With the help of electroreflectance spectroscopy and light scattering measurements, we have shown that if desorbed, the surfactant molecules form micelles (flakes or vesicles) that are trapped under the electrode surface. The micelles spontaneously spread back onto the electrode surface when the charge density at the metal approaches zero. The repeatable desorption and readsorption involve micellisation of the film at negative potentials and spontaneous spreading of the micelles to reform the monolayer at potentials close to pzc.     

Fabrication of metallic nanoparticles by electrochemical discharges

A method for the fabrication of metallic nanoparticles in large quantities by electrochemical discharges is presented. In an aqueous electrolyte, large current density (∼1 A/mm² at ∼20 V) leads to the formation of a ‘gas film’ around the electrode through which discharges occur. When metal ions are additionally present in the electrolyte and when the applied potential is cathodic, metal nanoparticles (typically 10–150 nm) are produced. The nanoparticles are formed in the solution and the gas film prevents them from depositing on the electrode. To control the size of the particles a method based on ‘rotating electrode’ is developed. Rotating the cathode rotates the fluid around it, which provides centrifugal force to the particles to move away from the electrode where they cannot grow. This method has been successfully used for fabrication of nanoparticles from several metal salts.     

Polarography in hexamethylphosphoramide: Effect of supporting electrolyte cations

Polarographic reductions of various metal ions such as the silver, cupric, zinc, cobaltous, nickel, ferric, ferrous ions and hydrogen ion in hexamethylphosphoramide (HMPA), have been investigated in the supporting electrolytes with various perchlorates. The reduction of most of these ions is strongly influenced by the cation of the supporting electrolyte. In the presence of the tetraethylammonium ion, when the size of the cation of the supporting electrolyte is small and easily adsorbed on the negatively charged electrode surface, the reductions of metal ions are controlled by some preceding processes and are naturally irreversible. The rate of reduction becomes more rapid with the increase of the size of the cation. Thus, in Hex₄NClO₄ or LiClO solutions, the reduction of these various metal ions takes place almost totally under diffusion control, although the waves of most of metal ions show a maximum. These effects of the cation of the supporting electrolytes on reduction can be explained as a phenomenon occurring on the electrode surface. This phenomenon has been reported in previous papers [1] on the reductions of the alkali and alkaline earth metal ions. The difference in the electrocapillary curves in these solutions is rarely shown at the potential around the electrocapillary maximum, but it is very obviously shown at more negative potential. The difference in the effect of the size of the cation of the supporting electrolyte on reduction of metal ion coincides well with the difference in the electrocapillary curves in these solutions: the effect of the size of the supporting electrolyte cation on the polarographic reduction is rarely shown at the potential around the electrocapillary maximum, but it is very obviously shown at more negative potential; therefore this effect is due to the electrode double-layer difference.     

Modeling electrochemical interfaces from ab initio molecular dynamics: water adsorption on metal surfaces at potential of zero charge

The origin of the potential difference between the potential of zero charge of a metal/water interface and the work function of the metal is a recurring issue because it is related to how water interacts with metal surface in the absence of surface charge. Recently ab initio molecular dynamics method has been used to model electrochemical interfaces to study interfacial potential and the structure of interface water. Here, we will first introduce the computational standard hydrogen electrode method, which allows for ab initio determination of electrode potentials that can be directly compared with experiment. Then, we will review the recent progress from ab initio molecular dynamics simulation in understanding the interaction between water and metal and its impact on interfacial potential. Finally, we will give our perspective for future development of ab initio computational electrochemistry.     

The electrocapillary effect at an electrode modified with an insoluble redox-active self-assembled monolayer

The interface between an electrolyte solution and a metal electrode coated with an oxidatively adsorbed, redox-active monolayer of long-chain thiols has been examined from a thermodynamic point of view. The electrode potential is assumed to vary within the region where no reductive desorption of the thiol occurs, so that the interface may formally be regarded as ideally polarizable. The analysis leads to an expression describing the potential dependence of interfacial tension in terms of the charge density on the metal, salt concentration, dielectric properties of the organic film, and the redox properties of the active terminal groups, which vary with the (average) distance from the electrode surface. This result generalizes the classical Lippmann equation to modified electrodes of the type considered.     

Determination of copper by electrothermal AAS after electrodeposition on a graphite disk electrode

The electrodeposition of copper on a graphite electrode at a constant potential with subsequent atomization in the graphite atomizer HGA-400 has been studied. A special graphite disk electrode is suitable for electrochemical enrichment at E = -0.7 V vs. SCE and the determination of copper by electrothermal-atomic absorption spectrometry (ET-AAS) if atomized at 2300 degrees C. In this way copper was determined in potable water and free Cu(2+) could be distinguished from that bound in chelate speciations after using a suitable deposition potential of the working electrode. This approach seems to be an alternative to the commonly used anodic stripping voltammetry (ASV) for the preconcentration and determination of free metal ions.     

Scanning electrochemical microscopy as a probe of Ag+ binding kinetics at Langmuir phospholipid monolayers

A new method has been developed for measuring local adsorption rates of metal ions at interfaces based on scanning electrochemical microscopy (SECM). The technique is illustrated with the example of Ag+ binding at Langmuir phospholipid monolayers formed at the water/air interface. Specifically, an inverted 25 microm diameter silver disc ultramicroelectrode (UME) was positioned in the subphase of a Langmuir trough, close to a dipalmitoyl phosphatidic acid (DPPA) monolayer, and used to generate Ag+ via Ag electro-oxidation. The method involved measuring the transient current-time response at the UME when the electrode was switched to a potential to electrogenerate Ag+. Since the Ag+/Ag couple is reversible, the response is highly sensitive to local mass transfer of Ag+ away from the electrode, which, in turn, is governed by the interaction of Ag+ with the monolayer. The methodology has been used to determine the influence of surface pressure on the adsorption of Ag+ ions at a phospholipid (dipalmitoyl phosphatidic acid) Langmuir monolayer. It is shown that the capacity for metal ion adsorption at the monolayer increased as the density of surface adsorption sites increased (by increasing the surface pressure). A model for mass transport and adsorption in this geometry has been developed to explain and characterise the adsorption process.     

Surface potential at the hematite-water interface

Construction of a metal oxide electrode enabling measurement of surface potential is described. The electrode was made using a hematite monocrystal, which avoids the problems arising from the possible porosity of the oxide layer. The potential of this electrode was measured as a function of pH. The hematite electrode provides reproducible results, especially in the acidic region. Surface potentials were calculated from electrode potentials using the electrokinetic isoelectric point. The slope of surface potential with respect to pH was found to be lower than the Nernstian, especially in the basic region. The effect was more pronounced at higher ionic strengths. It was shown how the measurement of surface potential can help to interpret the equilibrium data and evaluate the choice of a theoretical model describing the interfacial equilibrium at the metal oxide-water interface.     

Anodic stripping electrochemical analysis of metal nanoparticles

Traditional anodic stripping voltammetry (ASV) involves electrodeposition (reduction) of metal ions from solution over some time scale onto a working electrode followed by stripping (oxidation) of the deposited metal in a second step, where the stripping potential and quantity of charge passed provide information about the metal identity and solution concentration, respectively. ASV has recently been extended to the analysis of metal nanoparticles (NPs), which have grown popular because of their fascinating properties tunable by size, shape, and composition. There is a need for improved methods of NP analysis, and because metal NPs can be oxidized to metal ions, ASV is a logical choice. Early studies involved metal NPs as tags for the detection of biomolecules. More recently, anodic stripping has been used to directly analyze the physical, chemical, and structural properties of metal NPs. This review highlights the stripping analysis of NP assemblies on macroelectrodes, individual NPs in solution during collisions with a microelectrode, and a single NP attached to an electrode. A surprising amount of information can be learned from this very simple, low-cost technique.     

Voltammetric Determination of Mercury(II) at Poly(3‐hexylthiophene) Film Electrode. Effect of Halide Ions

The well‐known method for the determination of mercury(II), which is based on the anodic stripping voltammetry of mercury(II), has been adapted for applications at the thin film poly(3‐hexylthiophene) polymer electrode. Halide ions have been found to increase the sensitivity of the mercury response and shift it more positive potentials. This behavior is explained by formation of mercuric halide which can be easily deposited and stripped from the polymer electrode surface. The procedure was optimized for mercury determination. For 120 s accumulation time, detection limit of 5 ng mL^?1 mercury(II) has been observed. The relative standard deviation is 1.3% at 40 ng mL^?1 mercury(II). The performance of the polymer film studied in this work was evaluated in the presence of surfactants and some potential interfering metal ions such as cadmium, lead, copper and nickel.     

Apparatus for mechanistic and analytical studies of the electrogenerated chemiluminescence of luminol

A new apparatus based on the rotating ring—disc electrode system is described. The symmetric double-step potential is connected to the ring electrode to oxidize luminol, while the disc electrode is maintained at a negative potential to reduce oxygen to hydrogen peroxide. Because of the electrode rotation, hydrogen peroxide is immediately transported to the ring electrode at which it reacts with luminol oxidation product to emit light. Preliminary electrogenerated chemiluminescence measurements indicate that the intensity of the chemiluminescence of luminol is highly dependent on the ring and disc electrode materials and that some metal ions have a catalytic or inhibitive effect on this luminescence reaction of luminol.     

New Analytical Applications for Sodium Diethyldithiocarbamate

Abstract

The electrochemical behavior of sodium diethyldithiocarbamate has been studied. A method has been developed to determine trace amounts of the reagent by preparatory electrolysis at a suitable potential which causes the sparingly soluble electrochemical product to precipitate on a small graphite electrode. The inverse polarogram gives a current which can be evaluated quantitatively, produced by the re-dissolving of the film on the electrode. Sodium diethyldithiocarbamate can also be used for electrochemically concentrating cobalt in the form of a sparingly soluble compound of Co(III) by measuring the current required to reduce (re-dissolve) the compound. The electrochemical oxidation reaction of sodium diethyldithiocarbamate to give insoluble films could be used for indirect determination of the metal ions which interact with the reagent.     

Probing electric fields at the ionic liquid-electrode interface using sum frequency generation spectroscopy and electrochemistry

The arrangement of ions at the platinum electrode in the room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate has been determined using sum frequency generation vibrational spectroscopy (SFG), electrochemical impedance spectroscopy (EIS), and the vibrational Stark effect. The results indicate that CO adsorbed on the Pt electrode has a Stark shift of 30-35 cm(-1)/V in the ionic liquid. The potential of zero charge (PZC) of the ionic liquid-Pt system is approximately -500 mV (vs Ag wire), with a capacitance of 0.12 F/m2. Further, polarization-dependent SFG experiments suggest the ions reorganize at the surface depending on the electrode charge. In combination, all these results indicate that the ions of a neat ionic liquid are organized in a Helmholtz layer at the electrified metal electrode interface.     

Electrochemical Impedance Characterization of Nafion‐Coated Carbon Film Resistor Electrodes for Electroanalysis

Carbon film disk electrodes with Nafion coatings have been characterized by electrochemical impedance spectroscopy (EIS) with a view to a better understanding of their advantages and limitations in electroanalysis, particularly in anodic stripping voltammetry of metal ions. After initial examination by cyclic voltammetry, spectra were recorded over the full potential range in acetate buffer solution at the bare electrodes, electrodes electrochemically pretreated in acid solution, and Nafion‐coated pretreated electrodes in the presence and absence of dissolved oxygen. EIS equivalent circuit analysis clearly demonstrated the changes between these electrode assemblies. In order to simulate anodic stripping voltammetry conditions, spectra were also obtained in the presence of cadmium and lead ions in solution at Nafion‐coated electrodes, both after metal ion deposition and following re‐oxidation. Permanent changes to the structure of the Nafion film occurred, which has implications for use of these electrode assemblies in anodic stripping voltammetry at relatively high trace metal ion concentrations.