Electrodeposition of gold nanoparticles at a solid|ionic liquid|aqueous electrolyte three-phase junction

Gold nanoparticles have been electrodeposited on an electrode through electrogeneration at an ITO|AuCl₄^? solution in an ionic liquid|aqueous electrolyte three-phase junction. The electrodeposition was carried out by inverted double-pulse potential chronoamperometry. The direct reduction of AuCl₄^? ions at the electrode is followed by a counterion transfer through the liquid|liquid interface. Contrary to the electrodeposition from a single ionic liquid phase, scanning electron microscopy reveals that the shape of the resulting nanoparticles is highly angular and well-developed with a diameter of 110 ± 30 nm. Catalytic oxidation of glucose on the modified electrode is demonstrated.     

Thin‐Film Voltammetry of a Lutetium Bisphthalocyanine at Ionic Liquid|Water Interface

At room temperature, tetraoctylphosphonium bromide is a viscous ionic liquid, this gel‐like organic phase can be cast over a basal‐plane graphite electrode (BPGE). Cyclic voltammetry at such a modified electrode, in contact with an aqueous solution have revealed one reversible oxidation and five reversible reduction steps for a Lu^III bisphthalocyanine dissolved in the ionic liquid film, a proof that the highly reactive reduced species were protected from interaction with water in this highly lipophilic phase. It has also been shown that the redox properties are influenced by the ions in the aqueous phase, a property which has been attributed to ion‐pairing effects; obviously, the ion transfers at the organic|aqueous interface has been ignored. Electrochemistry of Lu(III)[(_tBu)₄Pc]₂ (cyclic voltammetry and square wave voltammetry) under similar conditions shows that the nature and concentration of the anion in the aqueous solution in contact with the ionic liquid film influences the potential of the electrode reaction. This can be attributed to variations of the interfacial potential and also because the organic phase is an anion exchanger. Moreover, SWV experiments suggest that the rate of the overall reaction varies with the nature and concentration of the anion of the aqueous electrolyte, which implies that the ion transfer through the organic|aqueous interface is slower than the electron exchange rate of the molecule at the surface of graphite.     

Ion-transfer- and photo-electrochemistry at liquid|liquid|solid electrode triple phase boundary junctions: perspectives

Ion transfer at liquid|liquid junctions is one of the most fundamental processes in nature. It occurs coupled to simultaneous electron transfer at the line junction (or triple phase boundary) formed by the two liquids in contact to an electrode surface. The triple phase boundary can be assembled from a redox active microdroplet deposit of a water-immiscible liquid on a suitable electrode surface immersed into aqueous electrolyte. Ion transfer voltammetry measurements at this type of electrode allow both thermodynamic and kinetic parameters for coupled ion and electron transfer processes to be obtained. This overview summarises some recent advances in understanding and application of triple phase boundary redox processes at organic liquid|aqueous electrolyte|working electrode junctions. The design of novel types of electrodes is considered based on (i) extended triple phase boundaries, (ii) porous membrane processes, (iii) hydrodynamic effects, and (iv) generator-collector triple phase boundary systems. Novel facilitated ion transfer processes and photo-electrochemical processes at triple phase boundary electrodes are proposed. Potential future applications of triple phase boundary redox systems in electrosynthesis, sensing, and light energy harvesting are indicated.     

Photoelectrochemically driven processes at the N,N,N′,N′-tetrahexylphenylenediamine microdroplet|electrode|aqueous electrolyte triple interface

Novel photoelectrochemical processes are observed upon irradiation of the liquid|liquid|solid triple interface at microdroplets of N,N,N′,N′-tetrahexyl-para-phenylenediamine (THPD) deposited onto a basal plane pyrolytic graphite electrode and immersed in aqueous electrolyte solution. In the presence of neutral THPD, cathodic photo responses, and in the presence of THPD⁺, anodic photo responses with anion-dependent characteristics, are observed. A maximum in the photocurrents observed at intermediate coverage of the electrode surface suggests that the triple interface THPD|electrode|aqueous electrolyte is the reaction zone. This is the first report of photoelectrochemical processes at this type of interface. Electronic Publication     

Electrocatalytic Determination of Sulfite at Immobilized Microdroplet Liquid|Liquid Interfaces: The EIC′ Mechanism

Liquid|liquid interfaces provide a natural boundary and a reactive interface where an organic phase is in contact with an aqueous analyte. The selectivity of ion transfer processes at liquid|liquid interfaces can help to provide sensitivity, introduce reactive reagents, or allow analyte accumulation at the electrode surface. In this study, microdroplet deposits of the organic liquid 4‐(3‐phenylpropyl)‐pyridine (PPP) with the ferrocenylmethyl‐dodecyldimethylammonium⁺ (FDA⁺) redox system are deposited onto a basal plane pyrolytic graphite electrode and employed to transfer anions from the aqueous into the organic phase. A clear trend of more hydrophobic anions transferring more readily (at more negative potentials) is observed and an ESI‐mass spectrometry method is developed to confirm the transfer. Subsequently, the electrocatalytic oxidation of sulfite, SO₃^2?, within the organic phase and in the presence of different electrolyte anions is investigated. Competition between sulfite transfer and inert anion transfer occurs. The electrocatalytic sulfite oxidation is suppressed in the presence of PF₆^? and occurs most readily in the presence of the hydrophilic nitrate anion. The resulting process can be classified as an electrocatalytic EIC′‐process (E: electron transfer; I: ion transfer; C: chemical reaction step). The effectiveness of the electrocatalytic process is limited by i) competition during anion transfer and ii) the liquid|liquid interface acting as a diffusion barrier. The analytical sensitivity of the method is limited to ca. 100 μM SO₃^2? (or ca. 8 ppm) and potential approaches for improvement of this limit are discussed.     

A new access to Gibbs energies of transfer of ions across liquid|liquid interfaces and a new method to study electrochemical processes at well-defined three-phase junctions

Droplets of polar and nonpolar aprotic solvents containing dissolved electroactive species can be easily attached to paraffin-impregnated graphite electrodes. When the electrode with the attached droplet is introduced into an aqueous electrolyte solution, the electrochemical reactions of the dissolved species can be elegantly studied. Provided the droplet does not contain a dissolved electrolyte, the electrochemical reaction will be confined to the very edge of the three-phase junction droplet|graphite|aqueous electrolyte. When a neutral species is oxidised, two pathways are possible: the oxidised species can remain in the droplet and anions will be transferred from the aqueous solution to the organic solvent, or the oxidised species may leave the droplet and enter the aqueous solution. Depending on the nature of the dissolved species, the nature of the organic solvent, the presence or absence of appropriate anions and cations in the two liquid phases, very different reaction pathways are possible. The new approach allows studies of ion transfer between immiscible solvents to be performed with a three-electrode potentiostat. Electrochemical determinations of the Gibbs energy of ion transfer between aqueous and nonpolar nonaqueous liquids are possible, whereas conventional ion transfer studies require the presence of a dissociated electrolyte in the organic phase. The new method considerably widens the spectrum of accessible ions.     

Ion Transport Across Liquid|Liquid Interfacial Boundaries Monitored at Generator‐Collector Electrodes

Anion transfer processes at a liquid|liquid interface were studied with an interdigitated gold band array electrode. The organic phase, 4‐(3‐phenylpropyl)‐pyridine containing Co(II)phthalocyanine, was immobilised as random droplets at the electrode surface and then immersed into aqueous electrolyte. Oxidation of Co(II)phthalocyanine at the generator electrode was shown to be associated with anion transfer from the aqueous into the organic phase. The corresponding back reduction at the collector electrode with anion expulsion was delayed by the anion/cation diffusion time across the interelectrode gap. A working curve based on a finite difference numerical simulation model was employed to estimate the apparent diffusion coefficients for anions in the organic phase (PF₆^?4^?3^?). Potential applications in ion analysis are discussed.     

The voltammetric behaviour of solid 2,2-diphenyl-1-picrylhydrazyl (DPPH) microparticles

The electrochemical behaviour of 2,2-diphenyl-1-picrylhydrazyl (DPPH) microparticles, attached to a graphite electrode and adjacent to an aqueous electrolyte solution, has been studied by cyclic voltammetry. DPPH exhibits one reversible redox couple with a formal potential of 0.340 V versus Ag|AgCl (pH=7.0). At more positive potentials, a redox couple appears with a formal potential E_f=0.733 V versus Ag|AgCl. The oxidation at this potential is followed by an irreversible chemical reaction generating a product which gives a redox couple with a formal potential at 0.177 V versus Ag|AgCl. The reduction process of this couple is followed by a slow chemical reaction in the course of which DPPH is reformed.     

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.     

Aqueous-organic biphasic redox-chemistry of high-hydride content rhodium clusters: Towards immobilisation of redox-switchable H2 binding materials on a surface

Water-insoluble phosphine-surface-stabilised rhodium clusters (R = cyclohexyl, Ar^F = (3,5-CF₃)₂C₆H₃) can be immobilised onto suitable glassy carbon, graphite, or tin-doped indium oxide (ITO) electrode surfaces. When immersed into alcohol/aqueous electrolyte media, they display stable voltammetric responses consistent with the formation of organic microdroplet | aqueous electrolyte two-phase conditions which are similar to those observed in corresponding anhydrous (CH₂Cl₂) solution phase. This results in an electrode surface that is straightforwardly modified with cluster materials that can uptake or release dihydrogen depending on their redox-state. The results presented herein lead to a slight re-evaluation of the originally reported hydrogen store and release cycle for this system.     

The determination of standard Gibbs energies of transfer of cations across the nitrobenzene|water interface using a three-phase electrode

A three-phase electrode consisting of a droplet of a nitrobenzene solution of iron(III) tetraphenyl porphyrine chloride (Fe(III)-TPP-Cl) attached to a graphite electrode and immersed in an aqueous electrolyte solution was applied to determine the standard Gibbs energies of transfer of cations between water and nitrobenzene. The reduction of Fe(III)-TPP-Cl prompts the transfer of the cations from the aqueous to the organic phase. The system is chemically and electrochemically reversible.     

Influence of nanoporous carbon electrode thickness on the electrochemical characteristics of a nanoporous carbon|tetraethylammonium tetrafluoroborate in acetonitrile solution interface

Electrochemical characteristics for the nanoporous carbon|Et₄NBF₄+acetonitrile interface have been studied by cyclic voltammetry and impedance spectroscopy methods. The influence of the electrolyte concentration and thickness of the nanoporous electrode material on the shape of the cyclic voltammetry and impedance curves has been established and the reasons for these phenomena are discussed. A value of zero charge potential, depending slightly on the structure and concentration of the electrolyte, the region of ideal polarizability and other characteristics have been established. The nanoporous nature of the carbon electrodes introduces a distribution of resistive and capacitive elements, giving rise to complicated electrochemical behaviour. Analysis of the complex plane plots shows that the nanoporous carbon|Et₄NBF₄+acetonitrile electrolyte interface can be simulated by an equivalent circuit, in which two parallel conduction paths 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 or of the partial charge transfer (i.e. adsorption stage limited) process. The values of the characteristic frequency depend on the electrolyte concentration and electrode potential, i.e. on the nature of the ions adsorbed at the surface of the nanoporous carbon electrode. The value of the solid state phase resistance established is independent of the thickness of the electrode material.     

Interfacial behaviour of a quantum well electrode|electrolyte: EER spectra of an SQW GaAs|AlxGa1−xAs electrode in HQ + BQ non-aqueous electrolyte

The quantum-confined Stark effect and the Franz-Keldysh oscillation of a single quantum well (SQW) GaAs|Al_xGa_1−xAs electrode were studied in non-aqueous hydroquinone + benzoquinone solution with electrolyte electroreflectance spectroscopy. By investigation of the relation of the quantum-confined Stark effect and the Franz-Keldysh oscillation with applied external bias, the interfacial behaviour of an SQW electrode was analysed.     

Abrasively immobilised multiwalled carbon nanotube agglomerates: a novel electrode material approach for the analytical sensing of pH.

We demonstrate for the first time that agglomerates of multiwalled carbon nanotubes (MWCNTs) can be formed in which the binder in the agglomerate is itself a redox-active molecular solid. Two separate agglomerates were formed by dissolving 9,10-phenanthraquinone (PAQ) or 1,2-napthaquinone (NQ) in acetone together with MWCNTs and adding an excess of aqueous solution to cause precipitation of agglomerates, approximately 10 microns in dimension, which consist of bundles of nanotubes running into and throughout the amorphous molecular solid that binds the agglomerate together. The nature of this structure, when immobilised on a substrate electrode and in contact with aqueous electrolyte solutions, gives rise to many three-phase boundaries, electrolyte|agglomerate|conductor, which is advantageous to the solid-state analytical electrochemistry of such a material as it imparts a larger electroactive surface area than other modified carbon electrodes. The two agglomerates each gave a voltammetrically measurable response to changes in pH; when abrasively immobilised on a basal plane pyrolitic graphite electrode a plot of peak potential against pH produced a linear response for both MWCNT-PAQ and MWCNT-NQ agglomerates over the pH range pH 1-12 and over the temperature range 20-70 degrees C.     

Electrochemical SPM investigation of the solid electrolyte interphase film formed on HOPG electrodes

Solid electrolyte interphase (SEI) film formation on graphite electrodes was studied on highly oriented pyrolytic graphite (HOPG) in nonaqueous electrolyte by in situ electrochemical atomic force microscopy (AFM). For potentials negative to 0.7 V versus Li|Li⁺ a SEI film is formed on the HOPG electrode surface. After the first cycle the film is rough and covers the surface of the HOPG electrode only partially. After the second cycle the HOPG surface is fully covered by a compact film. The thickness of the SEI film was measured by increasing the pressure of the AFM tip and thus scraping a part of the electrode surface. In this way a thickness of about 25 nm was found for the SEI film formed after two scan cycles between 3 and 0.01 V versus Li|Li⁺.     

Anion sensitive voltammetry of fullerene C60 dissolved in 1,2-dichlorobenzene deposit in contact with aqueous electrolyte

Electroreduction of C₆₀ dissolved in hydrophobic solvent deposited on the electrode surface was studied. A microliter amount of C₆₀ and tetrahexylammonium perchlorate solution in 1,2-dichlorobenzene was deposited on basal plane pyrolytic graphite electrode and this electrode was immersed into an aqueous solution. The voltammetry shows three consecutive reduction–oxidation steps. The redox potential of first electroreduction step is sensitive on anion but not on cation present in the aqueous phase. This parameter also depends on electrolyte concentration in the aqueous and organic phase. It is proposed that electroreduction of C₆₀ is preceded by anion exchange and followed by anion expulsion to the aqueous phase. Similar anion effect on the redox potential is also observed for unsupported deposit indicating importance of initial partitioning of electrolyte into the organic phase.     

The electrochemical oxidation of white phosphorus at a three-phase junction

The oxidation of white phosphorus has been studied with graphite rods of varying diameters, which were partially embedded in solid white phosphorus placed on the bottom of a cell filled with aqueous electrolyte solution. During the first scan in linear sweep voltammetry, the current measured at various potentials is directly proportional to the length of the three-phase junction (tpj) line phosphorus|graphite|electrolyte solution. The currents are orders-of-magnitude larger than predicted for a reaction that is strictly confined to a tpj line of atomic size and length of 4–16 mm as used in the experiments. A model is presented which explains the reaction layer as built up of vitreous low-valent phosphorus oxides possessing electron hopping capabilities and ionic conductivity. It is assumed that the reaction starts at the tpj line, however, quickly converts to a reaction where electrons are transferred across an expanding face graphite|phosphorus oxides and where ions are transferred across an expanding face phosphorus oxides|electrolyte solution.     

Electrochemical Studies of Vitamin K1 Microdroplets: Electrocatalytic Hydrogen Evolution

The voltammetry of a basal-plane pyrolytic graphite electrode modified with a random ensemble of unsupported microdroplets of vitamin K₁ is investigated when the electrode is immersed in aqueous electrolytes. It is shown that in dilute acidic solutions, electroreduction occurs in a single two-electron two-proton process to yield the corresponding hydroquinone at the electrode|vitamin K₁ microdroplet|aqueous-electrolyte three-phase boundary. On addition of ionic alkali-metal salts to the aqueous acidic phase, the electrochemical reduction of vitamin K₁ to the quinol is accompanied by catalytic hydrogen evolution within and alkali-metal-cation insertion into the organic microdroplets. In strongly alkaline solutions, electrochemical reduction of vitamin K₁ at the triple-phase junction is proposed as being a single two-electron process with concomitant uptake of alkali-metal cations in order to maintain electroneutrality within the oil phase. Surprisingly, the relative ease of cation insertion into the oil phase is demonstrated to be governed by the degree of ion-pair formation rather than by the Gibbs transfer energy of the cation across the liquid|liquid interface.     

Photosensing Device Composed of Bilayer Membranes of Polymer-Pendant Ru(bpy)3 2+ and Prussian Blue

A photosensing device was fabricated based on a photochemical electron transfer reaction. A graphite electrode was coated with bilayer membranes of polymer-pendant Ru(bpy)₃ ²⁺ and Prussian Blue (PB) to give a bilayer-coated device. It was irradiated with visible light in an aqueous electrolyte and the induced photocurrent was measured as a function of the applied potential. The excitation of PB was responsible for the anodic photocurrent, while the excitation of the Ru complex mainly induced the cathodic photocurrent by an electron-transfer mechanism.     

Partial oxidation of glucose by a Pt|WO3 electrode

Electrochemical oxidation of (+)-glucose by a Pt|WO₃ electrode in sulphuric acid medium was investigated by HPLC and in-situ FTIR spectroscopy. The oxidation on Pt|WO₃ electrode was found to be a multi-step and multi-electron process. The major oxidation products were identified to be glucono-δ-lactone, CO₂ and a probable five-member ring lactone. Based on these observations, the mechanism of glucose oxidation on Pt|WO₃ was discussed.