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Ionic rectifier membranes or devices generate uni-directional ion transport to convert an alternating current (AC) ion current input into stored energy or direct current (DC) in the form of ion/salt gradients. Electrochemical experiments 80 years ago were conducted on biological membrane rectifier systems, but today a plethora of artificial ionic rectifier types has been developed and electroanalytical tools are employed to explore mechanisms and performance. This overview focuses on microscale ionic rectifiers with a comparison to nano- and macroscale ionic rectifiers. The potential is surveyed for applications in electrochemical analysis, desalination, energy harvesting, electrochemical synthesis, and in selective ion extraction.

Since the research direction of supercapacitors (SC) has been deviated from the principle of causing capacitance, the revisit may be a future task in exploiting electric power storage. SC have been developed by applying ionic properties in order to enhance heterogeneity of ionic distributions. However, SC belong to double-layer capacitances (DLC), which are caused by orientation of solvent dipoles without any direct relation to ionic properties. DLC are always complicated with ion-independent characteristics such as frequency dispersion, voltage dependence, and participation in redox reactions, whereas these complications have not yet been considered in development of SC. Especially, pseudo capacitances should be examined through isolating the contribution of DLC from the redox properties.

The design of powerful, more biocompatible microrobots calls for faster catalytic reactions. Here we demonstrate a two-fold increase in the speed of photocatalytic TiO2-metal Janus micromotors via a Au/Ag bi-layered coating. Electrochemical measurements show that such a bimetallic coating is a better photocatalyst than either metal alone. Similarly, an additional sputtered Ag layer could also significantly increase the speed of Pt-PS or TiO2-Pt micromotors, suggesting that applying bimetallic coatings is a generalizable strategy in the design of faster catalytic micromotors.

A facile and efficient synthesis of hemin containing metal–organic framework (MOF) nanocomposite was reported. Two dimensional (2D) Ni–MOF nanosheets have been used as a new type of matrix material to immobilize hemin. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS)–mapping, thermal gravity analysis (TGA) suggest the hemin is uniformly distributed in the Ni–MOF nanosheets. Ultraviolet–visible spectroscopy (UV–vis) analysis shows that hemin molecules are combined with MOFs nanosheets through π–π interaction. Electrochemical study indicates the Ni–MOFs/Hemin nanocomposite exhibits high electro–catalytic activities for H2O2 reduction and the biosensor offers high performance with a low detection limit of 0.2 μM and a wide linear range of 1 μM–0.4 mM. Furthermore, the Ni–MOFs/Hemin nanocomposite based biosensor displays high selectivity and stability owing to the protective effect of the 2D MOF nanosheets. More importantly, the biosensor was used to determine the H2O2 levels in human serum and disinfectant samples with satisfactory results.

Cation transport through nanochannels in graphene oxide can be rectified to give ionic diode devices for future applications, for example, in desalination. A film of graphene oxide is applied to a 6 μm thick poly(ethylene terephthalate) substrate with a 20 μm diameter microhole and immersed in aqueous HCl solution. Strong diode effects are observed even at high ionic strength (0.5 M). Switching between open and closed states, microhole size effects, and time-dependent phenomena are explained on the basis of a simplified theoretical model focusing on the field-driven transport within the microhole region. In aqueous NaCl, competition between Na + transport and field-driven heterolytic water splitting is observed but shown to be significant only at low ionic strength. Therefore, nanostructured graphene oxide is demonstrated to exhibit close to ideal behavior for future application in ionic diode desalination of seawater.

The voltammetry of hydrogen ion is revisited quantitatively at a thin wire platinum electrode at scan rates less than 8 V s−1 in the context of the amount of adsorption and the negative capacitance. The first reduction wave was attributed to the reduction of adsorbed hydrogen ion, which is inconsistent with the Volmer step. The second one was composed of the partially-diffusion control current and the negatively capacitive current associated with the redox reaction. The capacitance causes the potential shift at high scan rates owing to the relaxation of the electric field for the faradaic reactions.

Electrode reactions without deliberately adding supporting electrolyte are complicated with electric migrations associated with redox reactions, formation of double layers and ion–ion interactions relevant to electrode reactions. The complications can be realized quantitatively through voltammetry at various sized microelectrodes, partly because of negligibly small solution resistance effects and partly because of steady-state voltammograms, which can be analyzed theoretically without capacitive time-dependent current. The present review deals with (i) techniques of fabrication of many sized microelectrodes, (ii) conditions of keeping low electrolyte concentrations, (iii) ionic contribution of forming double layers, (iv) participation in counter ions for electrode reactions and (v) change in ionic environment by redox reactions.

Using a facile and efficient method, two-dimensional (2D) ultra-thin MnO 2 nanosheets were successfully prepared. Due to the large specific surface area and mesoporous structure, MnO 2 nanosheet is a good candidate for use as an enhanced electrochemical sensing material. MnO 2 nanosheets were immobilized onto glassy carbon electrodes with Nafion film to construct a H 2 O 2 electrochemical nonenzymatic sensor. Amperometric study showed MnO 2 nanosheets exhibited very high electrocatalytic activity towards H 2 O 2 reduction. A very low detection limit (5 nM) was reached with a wide linear range (25 nM–2 μM and 10–454 μM) and a high sensitivity of 3261 mA M −1 cm −2 . The fabricated H 2 O 2 sensor also exhibited excellent selectivity, good reproducibility and long-time stability. Furthermore, the constructed high sensitive sensor was successfully applied to perform real-time monitoring trace concentration of H 2 O 2 released by SP2/0 cells, indicating that MnO 2 nanosheets provide a new platform for developing high performance electrochemical sensors in biological applications.

Rectified current-voltage curves were observed by micro-hole voltammetry in various concentrations of sodium ion electrolyte for transport through a micro-hole and a cationic exchange membrane (Nafion). Data suggest a linear current-concentration relation, implying electric migration-control. The rectification is observed as difference in current-potential slopes at positive and at negative applied voltages. The voltammograms are analyzed quantitatively in the context of Ohm's law as a function of both the geometry of the micro-hole and of the entire cell. The conductivity in the direction of the ion transport from the micro-hole to Nafion, corresponding to the lower conductance (closed diode), is proportional both to the cross-section area of the micro-hole and to the inverse micro-hole length. This is consistent with control by electro-migration within the micro-hole. In contrast, the conductivity in the opposite cation transport direction (open diode) is almost independent of the micro-hole geometry but varies with the length of the glass tube of the cell. The current-potential slope is consistent with the value calculated for the NaCl concentration and the geometry of the measurement cell. Under these conditions, the micro-hole is filled with a high concentration of NaCl as the flux of Na+ from the Nafion to the hole, and the high concentration causes the migration current larger that the current in the opposite direction. Consequently, the rectification ratio can be enhanced by careful design of both (i) the cell geometry including ion-exchange membranes and (ii) the micro-hole geometry rather than nano-scaled chemistry in holes.

Cyclic voltammograms of a ferrocenyl derivative in aqueous solution showed diffusion-controlled peak, of which currents deviated downward from the proportionality of the square-root of potential scan rates, v, and of which potentials were shifted with an increase in v. The dependence of the peak on v can be attributed conventionally to sluggish charge transfer rates and the solution resistance. This redox species has been proved to exhibit extremely fast transfer rates by microelectrode voltammetry. The solution resistance was suppressed by using small currents less than 3 μA at a thin cylindrical electrode even at scan rates of 8 V s−1. The dependence was inconsistent with the theory for the heterogeneous kinetics. It can be explained in terms of the negative capacitance associated with the redox reaction. Electrochemical reactions occur in the direction of relaxing the applied electric field, as for the double layer charging currents. However, the electrochemically generated charge enhances the electric field, so that the negatively charging current should flow. The negative capacitance was close to ca. −64 μF cm−2 for 1 mM redox concentration. The values varied linearly with the concentration.

Two-dimensional (2D) ultrathin nickel-cobalt phosphate nanosheets were synthesized using a simple one-step hydrothermal method. The morphology and structure of nanomaterials synthesized under different Ni/Co ratios were investigated by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Moreover, the influence of nanomaterials' structure on the electrochemical performance for glucose oxidation was investigated. It is found that the thinnest nickel-cobalt phosphate nanosheets synthesized with a Ni/Co ratio of 2:5 showed the best electrocatalytic activity for glucose oxidation. Also, the ultrathin nickel-cobalt phosphate nanosheet was used as an electrode material to construct a nonenzymatic electrochemical glucose sensor. The sensor showed a wide linear range (2-4470 μM) and a low detection limit (0.4 μM) with a high sensitivity of 302.99 μA·mM-1·cm-2. Furthermore, the application of the as-prepared sensor in detection of glucose in human serum was successfully demonstrated. These superior performances prove that ultrathin 2D nickel-cobalt phosphate nanosheets are promising materials in the field of electrochemical sensing.

Hemin, an iron porphyrin, functions as a carriage of dioxygen in mammalian respiration in the ferrous form. It works also as an electrochemical catalyst of the reduction of dioxygen by reproducing the ferrous form from the ferric one. Since the carriage requires stabilization of the dioxygen adduct, the catalytic reduction of dioxygen with the ferric form might hinder the stabilization. This work aims at finding the discrimination between the functions of the carriage and of the catalysis. A hint of the finding lies in the difference between the remarkable catalytic currents in high concentrations of hemin and negligibly small ones in very low concentrations. The catalytic current at the hemin-coated electrode is confirmed to occur at the stoichiometry of two hemin molecules and one dioxygen molecule. When this stoichiometry is applied to the dioxygen adduct of hemin as an intermediate species of the catalysis, high concentrations of dissolved hemin should provide the catalytic current. Spectro-electrochemistry demonstrates that ferrous hemin is not oxidized simply to the ferric form but is converted to other species, e.g. the dioxygen adduct.

The double layer capacitance per weight of graphene (GN) flakes in aqueous solution at the polarized potential increases with a decrease in the ac-frequency, obeying the inverse of the power law of the frequency. The power law is demonstrated to be equivalent to the constant phase element. The frequency-dependence increases with the thickness of the GN films. Thus, the lower the frequency and the thinner the film is, the larger is the capacitance density per weight. This is confirmed by cyclic voltammetry for several thickness of GN films and several scan rates. Resistance-like voltammograms at thick films are caused by frequency-dispersion. The overall capacitive properties of the GN films are as follows: the thickness of one GN flake is estimated to be 2 nm in average from the comparison of the thickness with the capacitances at the HOPG, which is consistent with values by the STM and AFM images.

A nonenzymatic glucose electrochemical sensor was constructed based on Au nanoparticles (AuNPs) decorated Ni metal-organic-framework (MOF)/Ni/NiO nanocomposite. Ni-MOF/Ni/NiO nanocomposite was synthesized by one-step calcination of Ni-MOF. Then AuNPs were loaded onto the Ni-based nanocomposites’ surface through electrostatic adsorption. Through characterization by transmission electron microscopy (TEM), high resolution TEM (HRTEM) and energy disperse spectroscopy (EDS) mapping, it is found that the AuNPs were well distributed on the surface of Ni-based nanocomposite. Cyclic voltammetric (CV) study showed the electrocatalytic activity of Au-Ni nanocomposite was highly improved after loading AuNPs onto it. Amperometric study demonstrated that the Au-Ni nanocomposites modified glassy carbon electrode (GCE) exhibited a high sensitivity of 2133.5 mA M−1 cm−2 and a wide linear range (0.4–900 μM) toward the oxidation of glucose with a detection limit as low as 0.1 μM. Moreover, the reproducibility, selectivity and stability of the sensor all exhibited outstanding performance. We applied the as-fabricated high performance sensor to measure the glucose levels in human serum and obtained satisfactory results. It is believed that AuNPs decorated Ni MOF/Ni/NiO nanocomposite provides a new platform for developing highly performance electrochemical sensors in practical applications.

Ni-MOF (metal–organic framework)/Ni/NiO/carbon frame nanocomposite was formed by combing Ni and NiO nanoparticles and a C frame with Ni-MOF using an efficient one-step calcination method. The morphology and structure of Ni-MOF/Ni/NiO/C nanocomposite were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and energy disperse spectroscopy (EDS) mapping. Ni-MOF/Ni/NiO/C nanocomposites were immobilized onto glassy carbon electrodes (GCEs) with Nafion film to construct high-performance nonenzymatic glucose and H2O2 electrochemical sensors. Cyclic voltammetric (CV) study showed Ni-MOF/Ni/NiO/C nanocomposite displayed better electrocatalytic activity toward glucose oxidation as compared to Ni-MOF. Amperometric study indicated the glucose sensor displayed high performance, offering a low detection limit (0.8 μM), a high sensitivity of 367.45 mA M–1 cm–2, and a wide linear range (from 4 to 5664 μM). Importantly, good reproducibility, long-time ...

When HOPG (highly oriented pyrolytic graphite) was electrochemically oxidized in alkali solution, STM observation showed that graphite oxide with homo-sized disks 15 nm in diameter and 0.5 nm in thickness was formed dispersively on the HOPG surface. With an increase in the anodic charge, the number of the disks enhanced, and covered finally the HOPG surface without overlap at the maximum coverage, 70%. The projected area of the disks was proportional to the anodic charge when the charge was small. The disks were hydrophilic. The double layer capacitance of the oxidized HOPG increased slightly with the anodic charge, implying that the disks should be an electrical insulator. The fully disk-coated HOPG did not block the diffusion-controlled current of the redox species. The layer of the disks must be porous for ions or solutions. The formation of the uniform size may be ascribed to the difference between the density of graphite oxide and that of the basal plane of graphite. The formation of nano-disks and their properties are inconsistent with such an image of intercalation that ions are inserted into layers of graphenes of HOPG.

Hemin in aerated dimethylsulfoxide catalyzes the reduction of dioxygen at glassy carbon electrodes with a gain voltage for the catalysis, 0.5 V. The catalytic rate, observed as the reduction current, increases with an increase in concentrations of dioxygen, but reaches a maximum at 6% of the saturated concentration. The voltammetric peak current has a linear relation with the scan rate. These variations are different from the ordinary catalytic mechanism, in which hemin oxidized by dioxygen might be reused for the electrochemical reduction. The voltammetric peak current in deaerated hemin solution is diffusion controlled, whereas that in aerated solution is represented as a sum of the diffusion current and a surface wave. The catalytic current is caused by hemin incorporated with dioxygen, of which adsorption density is close to an amount of a monolayer. Therefore hemin films are not suitable for continuous reduction of dioxygen. Once the adsorbed layer is electrochemically oxidized to remove the adsorption film, the catalytic reduction wave is retrieved.

1,4-Benzoquinone (BQ) and tetracyanoquinodimethane (TCNQ), which are reduced consecutively to mono-anions and dianions under conventionally voltammetric conditions, cannot be reduced up to the dianions without supporting electrolyte in acetonitrile solution. Effects of the IR -drop and the electrostatic force in low concentrations of supporting electrolyte are examined by use of the first reduction waves. They apply to the analysis of the second waves. Voltammograms without supporting electrolyte at microelectrodes with the diameters less than 0.4 μm do not include noticeably IR -drop. The large overpotential for the anions cannot be explained in terms of the electric migration which has been estimated from solutions of the Nernst–Planck equation and the Laplace equation. With an increase in the conductivity of the solution by addition of electrolyte, the second reduction wave appears, involving the potential shift. The electrolyte does not influence the first reduction waves but varies the second one. The salt effect specific to the second wave is ascribed to non-stoichiometric association of the redox anion with the salt cation.

Rate constants of fast electron transfer reactions of hexachloroiridium, hexaammine ruthenium, aminoferrocene, tetracyanoquinodimethan (TCNQ), ferrocenecarboxylic acid and ferrocene, which have been determined equivocally by fast voltammetry, are attempted to be evaluated with steady-state voltammetric measurements at ultramicroelectrodes. This work is motivated by the experimental feasibility of high current density at ultramicroelectrodes without effects of capacitive currents or solution resistance. The theory of mass transport mentions that current densities by steady-state voltammetry at electrodes 10 μm and 1 μm in diameter correspond, respectively, to 2.1 V s−1 and 210 V s−1 scan rates at large electrodes. However, no potential shift is observed even at electrodes 0.01 μm in diameter for the six species. This fact implies that the heterogeneous rate constants are at least larger than 10 cm s−1. Fast scan voltammetry is carried out at a large electrode for scan rates less than 10 V s−1. Peak-to-peak potentials increase with an increase in the scan rates. This variation is fitted to the mass transport equation complicated by the electrode kinetics for the rate constant of 0.01–0.1 cm s−1. The inconsistency between the two methods is ascribed partly to solution resistance and partly to the resistance relevant to the electric double layer at fast scan voltammetry. The contribution of the solution resistance is demonstrated from the facts that the peak currents vary linearly with the peak potentials, that the slope of the linearity is independent of kinds of redox species, and that the slope corresponding to the conductivity has a linear relationship with concentration of supporting electrolyte.

Slow scan voltammetry is useful for extracting Faradaic diffusion-controlled current from capacitive one, because the former and the latter are, respectively, proportional to the square-root of scan rate and the scan rate itself. This report shows clear, reproducible voltammograms of 0.5 μM redox species with the help of sodium alginate (SA) at scan rates less than 0.1 mV s −1 . SA enhances viscosity of solution without any influence on diffusion coefficient of the redox species so that natural convention is suppressed during the slow scan. Therefore, it allowed us to obtain ideal voltammograms reproducibly at slow scans, retaining the Faradaic current. Especially, a wire electrode is demonstrated to be a powerful tool for detecting low concentrations by slow scan voltammetry because of the large electrode area in comparison with the length of the boundary between the electrode and the insulator. A voltammetric detection limit is obtained in the relationship between the scan rates and concentration of redox species.

Since ionic conductivity has a linear relation with the square of the number of charge, ionic latex particles with a huge number of the charge could provide high conductance. It is expected that addition of only a small amount of latex particles into voltammetric solution enhances the conductance so much that voltammograms can be measured, overcoming ohmic drop. Conductivity of latex suspensions of polystyrene–polystyrenesulfonic acid with volume fractions less than 0.02, which were well deionized by centrifugation, was determined by ac-impedance at two parallel wire electrodes. Since the resistance was determined by the dependence of the in-phase component on the electrode distance, it did not include participation of electric double layers or adsorption of latex. The relationship between conductivity and a diffusion coefficient stated that the conductivity of the suspension was provided mainly by diffusion of latex particles with multiple charges rather than that of the counterion. The suspension with [H + ] = 10 −5 M, corresponding to 8.9 × 10 5 number mm −3 , including hydrogen gas showed a voltammetric oxidation peak of hydrogen, whereas hydrochloric acid with [HCl] = 10 −5 M showed a resistive current–potential curve.

Current–voltage curves were obtained at parallel platinum electrodes in the thin layer cell including pure water. They were under the steady state in the voltage domain from 1.0 V to 1.3 V when the distance of the electrodes was less than 100 μm. The solution resistance obtained from the current–voltage curve was much smaller than that predicted from the resistivity of pure water. The reason can be explained in terms of generation and accumulation of hydrogen ion and hydroxide ion before the recombination reaction. These kinetically survived ions decrease the resistance, and enhance the electrolysis rate. We subtract the reaction-controlled current–voltage curves from the overall curves to evaluate ion-included solution resistance. The resistivity of the solution averaged in the cell increased with an increase in the distance between the electrodes. In order to understand the above behavior, we calculated concentration profiles of the ions and potential distribution in the cell on the basis of Nernst–Planck equation including dissociation kinetics of water.

The behavior of electric double layers at polarized interfaces in KCl solutions is revisited in order to examine properties of the constant phase element (CPE). We pay attention specifically to frequency-dependence of both the capacitance and the resistance. Two parallel platinum wires immersed in solution are used as insulator-free electrodes. They avoid stray capacitance or irreproducibility of impedance caused by incompleteness of electric shield of electrodes. The Nyquist plot takes approximately a straight line because the in-phase component, Z 1 is inversely proportional to ac-frequency, similar to the capacitance. Since Z 1 extrapolated to zero separation of the electrodes is non-zero, a resistance is present at the double layer in parallel form. It is not a Faradaic resistance because of absence of any electroactive species. The parallel resistance is inversely proportional to the frequency, whereas the capacitance decreases with a linear relation to logarithm of the frequency. The latter is responsible for the frequency-dependence of the former. The parallel resistance is the apparent one involved inevitably in ac-measurements of the capacitance. Values of the capacitance are independent of concentration of KCl in the domain from 0.1 mM to 3 M.

Sodium alginate solution, being viscous hydrocolloid, was examined voltammetrically in the context of viscous effects by use of a ferrocenyl compound as a redox probe. Voltammograms were almost independent of concentrations of sodium alginate even in a solid-like state. Diffusion coefficients of the ferrocenyl compound did not vary with viscosity evaluated by a viscometer. Ionic conductivity of sodium alginate was also independent of the viscosity. In contrast, diffusion coefficients of the latex particle 0.84 μm in diameter decreased with an increase in the viscosity. The intrinsic viscosity was evaluated. The value is equivalent to the volume as large as 3 m 3 of water in which one mole of the unit of sodium alginate polymer generates the viscosity. Consequently, redox species diffuse through water portions circumventing the alginate network. As an application, sodium alginate solutions were used for long-time chronoamperometry. The current was decayed with the time even of 1500 s without any effect of natural convection, whereas that in the aqueous solution was irreproducible for times longer than 20 s. The other application was electroplating of silver. The silver surface deposited in sodium alginate had no morphology by the SEM image, while the surface plated without sodium alginate showed morphology even on an optical microscopy scale.

Mercury metal dispersed spontaneously into the aqueous solution as micro-droplets when it came in quiescent contact with the solution. The self-dispersion was observed by means of the dynamic light scattering method, thermogravimetry and voltammetry. The mercury microdroplets generated in water had ca. 0.3 lm in diameter. The diameter and the concentration of the droplets were stable. The concentration of mercury might be 1–3 lmol dm � 3 if the droplets were to be dissolved homogeneously in water. The concentration exceeds the environmental limits by three orders of magnitude. The dispersed mercury was confirmed not to be ions or oxide of mercury but to be metal droplets. The self-dispersion of mercury belongs to the self-emulsification at liquid|liquid interfaces without surfactants.

When hydrogen gas is bubbled into water, it may well be present as stabilized bubbles rather than hydrated hydrogen molecules, as in the spontaneous emulsification at oil|water interfaces without surfactant. On this prediction, we used dynamic light scattering (DLS) to find bubbles 0.4–0.5 μm in diameter, which were stable for more than 9 h. The intensity of the scattering light, which was proved to be proportional to concentrations of polystyrene latex suspensions, was also kept in solution in contact with hydrogen gas atmosphere. The bubbles were stable even at 50 g (gravity) by centrifugation. Voltammograms of the bubble-included solution had the oxidation peak, of which current was proportional to the intensity of DLS. Concentration of hydrogen in solution was evaluated accurately by comparing voltammetric currents at a regular electrode and a small electrode. The oxidation of hydrogen should be caused by the hydrated hydrogen which was supplied by dissolution of bubbles. Kinetic data of the dissolution were obtained at microelectrodes by using the advantage of extracting kinetics from diffusion currents. Voltammetric currents at microelectrodes were smaller by 10 times than those predicted from diffusion of hydrated hydrogen. Therefore, the oxidation is controlled by the dissolution rate at the high current density. The rate was estimated to be 2 × 10−8 mol s−1 cm−2, which was converted to the linear transfer rate, 0.4 mm s−1, at gas|water interface.

Diameters of invisible microelectrodes have been estimated from steady-state diffusion-controlled currents of a known concentration of redox species on the assumption of a disk form. However, geometry of the disk is often deformed by polishing the electrode surface obliquely against a polishing pad, by malleability of metal, and/or by distortion of metal wire. Then, the exposed surface is close to an ellipse with rough circumference. The diameter estimated from the steady-state current should be an average value among a major radius, a minor radius, and circumference length. In order to obtain a way of the average, we obtained here voltammetric steady-state currents at elliptic electrodes which were fabricated by polishing glass-coated platinum wire obliquely. Values of the diffusion-controlled currents at the elliptic electrode with smooth edge agreed with the theoretical values with 4% error. The steady-state current at a deformed electrode was approximately proportional to the square root of the area of the electrode rather than the length of the edge, as opposed to the conventional concept of the edge effect on the current. Even if electrode geometry is uncertain, the diameter evaluated from the steady-state current corresponds to the square root of the area.

When a nitrobenzene (NB) phase came in quiescent contact with a water phase, water droplets were formed spontaneously near the oil|water interfaces (K. Aoki, M. Li, J. Chen, T. Nishiumi, Electrochem. Commn. 11 (2009) 239). We reported here quantitative data of the NB-droplets by use of UV-absorbance, dynamic light scattering (DLS) and voltammetry with a help of ferrocene. The supernatant separated spontaneously from the oil–water mixture contained 15.5 mmol dm −3 evaluated by UV spectra, whereas the centrifuged supernatant did 9.3 mM. The difference suggested the presence of NB-droplets, the diameter of which ranged 0.15–0.5 μm obtained by DLS. Ferrocene was dissolved deliberately in the aqueous solution and the NB solution up to saturation. The voltammograms in the ultrasonicated supernatant exhibited diffusion-controlled redox peaks of ferrocene, which should be supplied from both dissolved ferrocene and ferrocene-dissolved NB droplets. The former was 1/3.6 times of the latter by the comparison with the current of ferrocene-saturated aqueous solution. Applying the expression for the diffusion-controlled peak current of a big particle, we estimated the number concentration of NB droplet to be 1.1 × 10 14 dm −3 . This value is equivalent to the average distance, L = 2.1 μm, between neighboring two droplets, corresponding to the diffusional traveling time, L 2 / D = 1.3 s, which would be long enough for collision of droplets to stabilize the emulsions.

Uniformly sized poly( N -isopropylacrylamide- co -acrylic acid) (NIPA/AA) hydrogel particles were synthesized in order to modify an electrode with a carboxyl group-immobilized one-particle layer. They were spheres with a common diameter under given conditions, which ranged from 0.5 μm to 1.0 μm depending on the pH, ionic concentration and temperature. They were swollen at low salt concentrations, low temperatures and high pH of aqueous suspensions. One particle contained 6.0 × 10 8 carboxyl groups, the concentration of which was ca. 1 M, exhibiting p K a = 5.2. The particles were adsorbed spontaneously on the platinum surface in a one-particle layer with regular arrangement. The particle-adsorbed electrode showed the voltammetric peak for the reduction of hydrogen ions which were dissociated from the carboxylic groups of the adsorbed layer. The amount of the dissociated hydrogen ions per particle was 19% of the loaded amount of carboxyl groups, whereas the non-gelized particles had 3% of the loaded amount. The film worked as a source of supply of hydrogen ions for the electrochemical reduction of 1,4-naphthoquinone. It provided acidic environment of pH 4.5 for the reduction in the neutral solution.

Voltammetric currents of the ferrocenyl derivative in highly viscous poly(ethylene glycol) solution (PEG) at microelectrodes were independent of their viscosity, contrary to the prediction from the diffusion-controlled current combined with the Stokes–Einstein equation. The usage of microelectrodes generally prevents deformation of voltammograms by poor conductivity. However, the voltammograms even at the disk electrode 10 μm in diameter and slow scan rates exhibited hysteresis, because the diffusion coefficient in PEG was smaller than in water owing to high viscosity of the solution. The diffusion coefficients evaluated from the peak current through the diffusion equation at microelectrode were of the order of 10 −7 cm 2 s −1 for the viscosity ranging from 0.1 to 160 Pa s. They are at most by 1000 times larger than the values calculated from the viscosities by the Stokes–Einstein equation. A practical significance of this result is to suggest a possibility of using highly viscous PEG as solid-like solvents without large restriction of mass transport. A possible diffusion model is that the redox species diffuses through lowly viscous local domains, avoiding collision with the backbone of the polymer network.

Two Pt disks with 1.6 and 0.1 mm diameter, whose sizes were accurately evaluated, were applied to voltammetric diffusion-controlled currents to determine the concentrations of redox species. Since voltammetric peak currents at a regular electrode, Ip, and limiting currents at a microelectrode, IL, are proportional to D 1/2 and D, respectively, the ratio Ip 2 /IL is independent of the diffusion coefficient but dependents on the concentration. We used this fact to determine the concentrations of ferrocenyl derivatives, hemin, hexacyanoferrate(II), dioxygen gas and hydrogen gas. New insights are not only the absolute determination of concentrations but also the sensitive detection of adsorption of electrochemical products which blocked the voltammetric currents.

The reduction of the emeraldine form of polyaniline film into leucoemeraldine, which corresponds to the conversion of an electric conductor into an insulator, shifted in the positive direction with increasing scan rate and film thickness. Similar dependence was found in the diffusion-controlled voltammograms of dispersed polyaniline latex particles with eight diameters ranging from 0.2 to 7.5 μm. The particles were synthesized by coating dispersed polystyrene latex with polyaniline. These variations were explained in terms of electric percolation of the conducting species to the electrode. The theoretical expression for the Nernst equation was derived on the assumption that the percolated and the un-percolated conducting species took inner potentials of the electrode and the solution phase, respectively. The conducting species does not participate in the determination of the equilibrium potential, though it participates in the Faradaic current. The cathodic peak potential shifted in the negative direction with an increase in particle size, solution viscosity, and film thickness, as predicted from the derived Nernst equation.

Polystyrene/poly(acrylic acid) latex particles 0.26 μm in diameter, which were synthesized by emulsion copolymerization of styrene and acrylic acid, were immobilized on the platinum electrode in a uniform film. The film did not block the diffusion current of a ferrocene derivative in the solution. The voltammograms showed the reduction of hydrogen ion, which was supplied not only from the solution but also from the dissociation of carboxyl acid of the adsorbed latex. The peak currents at pH less than 4.5 were proportional to the thickness of the latex film, and were supplied by dissociation from the latex. In contrast, those at pH more than 5.5 were independent of the film thickness, and were controlled by the dissociation kinetics of the carboxyl acid within the first particle layer. Expressions for the voltammetric currents with the CE mechanism were derived theoretically. The theory predicts that the reaction rate constant can be evaluated from the lower deviation from the proportionality of the peak currents to scan rates, as in consistent with the experimental results. The rate constant was by one order smaller than the homogeneous rate constant of acrylic acid. pH-Titration demonstrated the electrostatic interaction of –COO−, like in self-assembled carboxylate films. The average concentration of carboxylate in the latex was ca. 2.7 M. The dissociation was limited to the surface domain of the latex.

Voltammetry of ferrocenyl derivative, known as a reversible redox species, was made in viscous solutions of polyethylene glycols in order to find effects of the viscosity on voltammograms, especially deviation from the ordinary diffusion-controlled behavior. Voltammetric peak currents at high scan rates and in high viscous solutions were deviated lower from the proportionality to the square-root of the scan rates. The deviation could not be explained in terms of participation of sluggish electron transfer reactions or solution resistance, because the peak potential difference corrected by the solution resistance were close to 60 mV. The deviation was similar to the memory effect of diffusion. Diffusion coefficients and the solution resistance were evaluated from the proportionality of the peak current and peak potential shifts, respectively. These values obeyed Stokes–Einstein equation for the viscosity less than 0.3 Pa s. Dilatancy was observed at viscosity more than 0.3 Pa s.

Polyaniline was synthesized with the chemical oxidation in the presence of poly(vinylpyrrolidone) as a surfactant for small-sizing polyaniline grains. It was dispersed in glycerin including sulfuric acid to yield electrically conducting solution. Voltammograms of the solution by the two-electrode measurement showed the proportionality of the Ohm's law in the cell voltage from −0.2 to 0.2 V. Voltammograms by the three-electrode measurement in the domain 0.0–1.5 V versus Ag|AgCl were different from the Ohmic law and Faradaic current of polyaniline, because of a mixture of both the contributions. The conductance did not vary with the volume fraction of polyaniline up to 20% and enhanced drastically until 35% with large fluctuation, and then increased proportionally with the volume fraction. This variation was explained in terms of the electric percolation with a threshold at the volume fraction of 20–35%. The mixture with over 45% fraction was actually solid. The suspension was applied to confirming the relation between the Ohmic current at a microelectrode and its diameter. The current showed a linear relation with the electrode diameter with a small intercept of the diameter, of which value corresponded to the size of polyaniline grains.

Electrochemical properties at the carboxyl latex-coated electrode were examined in the context of accumulation or blocking of redox species in solution. The latex suspension was synthesized by copolymerization of styrene with acrylate, and was composed of spherical particles 0.27 μm in diameter. The deionized suspension showed iridescence. The latex particles were adsorbed strongly on a glass and a platinum surface in a two-dimensionally crystallized layer. One particle had 7.9 × 105 carboxyl groups, which were distributed in the inside of the particle at average concentration of 0.14 M. The latex-coated electrode was electrochemically active for the redox reactions of (ferrocenylmethyl)trimethyl ammonium (FcTMA) and Fe(CN)64- in aqueous solution. The voltammetric currents of FcTMA at pH more than 4 were larger than those without the film, whereas those at pH less than 3 were slightly smaller. The peak current was diffusion-controlled of FcTMA. The accumulation and the blocking were explained quantitatively in terms of Donnan equilibrium of FcTMA with immobilized carboxyl groups, depending on the dissociation constant of the carboxyl group, concentration of salt and pH.

The reduction of 1,4-benzoquinone (BQ) in acetonitrile was blocked in steady-state microelectrode voltammetry although it has been believed to be a sluggish electron transfer reaction. Ferrocene was added to the BQ solution with equi-concentration in order to confirm the diffusion-control step and to evaluate accurately the kinetic effect from potential differences. Fast scan voltammograms showed the negative potential shifts both of the cathodic and the anodic peaks with an increase in the scan rate. In contrast, microelectrode voltammetry showed that the ratio of the steady-state limiting current for BQ to that for ferrocene decreased with a decrease in the electrode radii. The halfwave potential of the reduction of BQ did not vary with the radii within error. These features are largely deviated from the Butler–Volmer kinetic behavior. The electrode surface after the long term electro-reduction was coated with a precipitate. The proposed reaction mechanism is formation of films by follow-up chemical reactions of BQ associated with slight potential shift. The film blocks diffusion of BQ. This model was theoretically formulated and elucidated the experimental results. � 2007 Elsevier B.V. All rights reserved.

Stabilised liquid|liquid interfaces between an organic 4-(3-phenylpropyl)-pyridine (PPP) phase and an aqueous electrolyte phase are obtained in the presence of suitable nanoparticles. The use of nanoparticulate stabilisers (ca. 30nm diameter laponite or 9-18nm diameter carbon) in "Pickering" emulsion systems allows stable organic microdroplets to be formed and these are readily deposited onto conventional tin-doped indium oxide (ITO) electrodes. In contrast to the electrically insulating laponite nanoparticles, conducting carbon nanoparticles are shown to effectively catalyse the simultaneous electron transfer and ion transfer process at triple phase boundary junctions. Anion transfer processes between the aqueous and organic phase are driven electrochemically at the extensive triple phase junction carbon nanoparticle|4-(3-phenylpropyl)-pyridine|aqueous electrolyte. The organic phase consists of a redox active reagent 5,10,15,20-tetraphenyl-21H,23H-porphinato manganese(III) (MnTPP + ), 5,10,15,20-tetraphenyl-21H,23H-porphinato iron(III) (FeTPP + ), or proto-porphyrinato-IX iron(III) (hemin) dissolved in 4-(3-phenylpropyl)-pyridine (PPP). The composition of the aqueous electrolyte phase determines the reversible potential for the Nernstian anion transfer process. The methodology is shown to be versatile and, in future, could be applied more generally in liquid|liquid electroanalysis.

Ferrocene solubilized with poly(vinylpyrrolidone) in aqueous KCl solution exhibited a well-defined voltammetric peak at 1.33 V vs. Ag∣AgCl at a platinum electrode. The wave was attributed to the oxidation of chloride to chlorine, demonstrated by smell of chlorine, by a view of formation of gas bubbles, by coloration through the reaction with diethyl-p-phenylene diamine, and by the increase in the anodic current with the concentration of chloride. Since no wave was observed in the ferrocene-free solution or KCl-free solution in this potential domain, the reaction mechanism was suggested to be the oxidation of chloride into chlorine catalyzed by micellar ferrocene. The potential at the foot of the wave (1.08 V) was less positive that the standard potential of Cl2/Cl−, and hence the reaction may be useful for enhancing the energetic efficiency at chlor-alkali industry. The value of the peak current was one-sixth the theoretical diffusion-controlled current, and was proportional to the square-root of the potential scan rate. Keywords: Chlorine, Catalytic current, Micellar ferrocene, Surfactant, Poly(vinylpyrrolidone)

Ferrocenylmethanol acrylate (FcMA)-immobilized polystyrene latex particles were synthesized by copolymerizing FcMA and styrene in the presence of polystyrene latex suspensions with eight different diameters ranging from 0.084 to 1.7 μm. The amounts of the ferrocenyl moiety loaded on one particle were proportional to the radii, a . The proportionality suggests the uniform distribution of the ferrocenyl moiety over the particle, of which concentration was 0.18 M. The aqueous suspensions, which were stable in the presence of a surfactant, exhibited reversible voltammetric waves for the ferrocenyl moiety. The peak current was controlled by diffusion of the latex particles. The efficiency of the reaction was obtained from the ratio of the observed current to the theoretical one which was estimated from the number of the ferrocenyl moieties and the diffusion coefficient of the particle by the Stokes–Einstein relation. The ratio was proportional to a −0.47 , whereas it might be a 0 for an ideal particle without any size effect. This relation was explained in terms of the contribution of rotational diffusion of redox particles.

Palladium spherical particles 0.23m in diameter were synthesized by reducing palladium acetate with hydrazine in the presence of surfactant, with an aim of exhibiting both easy separation by filtration and easy dispersion for a catalyst. The particles in the suspension were sedimented slowly but not aggregated. The suspension showed voltammetric redox waves. The anodic wave was ascribed to the oxidation of Pd to Pd 2+ , whereas the cathodic one was to the reduction of the palladium acetate moiety to Pd. The current ratio of the anodic peak to the cathodic one 4:1, was close to the ratios by the partial chemical oxidation with permanganate and by the thermogravimetry, suggesting the composition of 80% palladium metal and 20% palladium acetate in the molar ratio. Heating the palladium particles at 300 ◦ C yielded palladium metal. The decomposition proceeded to the first-order reaction with the activation energy of 40 kJ mol −1 . The particle catalyzed the reduction of methylene blue with hydrazine. The reaction rate was of the first-order with respect to methylene blue. The rate constant was proportional to the geometrical surface area of the palladium particle, suggesting a surface catalysis.

Quenching of the electro-chemiluminescence of the ruthenium complex was applied for detection of NO at the flow injection apparatus equipped with a homemade gas pressure pump with low noise and a flow-through electrogenerated chemiluminescence (ECL) cell. NO quenched the ECL emission of Ru(bpy)32+ at the potential of the oxidation of Ru(bpy)32+ efficiently to set the detection limit of NO at 0.1% of concentration of Ru(bpy)32+. The decrease in ECL intensity is linearly dependent on the logarithmic concentration of NO in the range of 5.7×10−7 M to 1.8×10−5 M. Voltammetric and spectrometric analysis showed that NO was not coordinated to Ru(bpy)32+/3+, but catalyzed the quenching reaction.

When a ferrocene (Fc)-included nitrobenzene droplet was set on an electrode immersed in an aqueous solution, the electrochemical oxidation of Fc at the interface of nitrobenzene∣water∣electrode gave rise to the convection, which flowed from the top of the hemispherical nitrobenzene droplet on the electrode to the three-phase boundary both in the nitrobenzene and aqueous phases. The flow velocity was determined by tracing the dispersed carbon powders with a video microscope. The convection was ascribed to two asymmetries of the flow: the dragging force on nitrobenzene by the diffusing species (Fc) toward the three-phase boundary without any counter diffusion of the product (Fc + ), and the difference in the frictional force near the oil∣electrode boundary and the oil∣water boundary. The convective diffusion equation was combined with Navier–Stokes’ equation associated with the diffusion force of Fc in the oil phase. The combined equations were solved semi-quantitatively on the assumptions of the one-dimensional stream model. The solution explained the potential dependence of the flow velocity.

Redox-active hollow spheres were prepared through extracting a polystyrene core from the latex particle (PSPAAFc) composed of the core and the polyallylamine shell including ferrocenyl carboxylic amide. The suspension of the hollow spheres showed anodic and cathodic voltammetric peaks, which were nearly reversible and diffusion-controlled. The current was 3 times as large as the current for the suspension of the filled PSPAAFc. This value agreed with the theoretical one evaluated from the diameter (1.28 microm), the number of ferrocenyl moieties per particle, 1.2 x 108, by UV spectroscopy, and the diffusion coefficient obtained from the Stokes-Einstein equation. This fact indicates the reaction of the whole loaded charge, in contrast to the partial charge transfer of PSPAAFc. The dynamic flattening motion was observed to support the reaction of the whole charge.