Neurotransmitter Biomolecule Transfers Across Liquid/Liquid Interface Through A Thick Organic Membrane-Modified Electrode

Electrochemical studies at liquid/liquid interfaces (L/L, or soft interfaces) have disclosed a biomimetic model to mimic charge transfers at cytomembrane surface. Herein, we reported two neurotransmitter biomolecules of dopamine and adrenalone across the L/L interface by a thick organic membrane-modified electrode. This system comprised polarized electrode/oil and oil/water interfaces in series in which the electron transfer (ET) of redox 7,7,8,8-tetracyanoquinodimethane (TCNQ) at electrode/oil interface drove ion transfer (IT) of biomolecules at oil/water interface. This ET-IT coupled reaction overcame the limitation of polarized potential window at conventional single polarized L/L interface. The crucial design of a thick organic membrane could ensure the generated TCNQ anions maintained at electrode/oil interface during the voltammetry, which could not result in interruptions to biomolecule transfers. Through this system, their Gibbs transfer free energies were accurately determined (44.4 and 39.4 kJ mol^?1 for dopamine and adrealone, respectively). Moreover, facilitated biomolecule transfers were evaluated by crown ionophores where both facilitated numbers and constants were determined simultaneously. Owing to the simple electrochemical setup, this system would hold great potentials in future hydrophilic biomacromolecule transfers, such as DNA, peptides and proteins.     

A comparative study of the anion transfer kinetics across a water/nitrobenzene interface by means of electrochemical impedance spectroscopy and square-wave voltammetry at thin organic film-modified electrodes

The kinetics of the transfer of a series of hydrophilic monovalent anions across the water/nitrobenzene (W/NB) interface has been studied by means of thin organic film-modified electrodes in combination with electrochemical impedance spectroscopy and square-wave voltammetry. The studied ions are Cl-, Br-, I-, ClO4-, NO3-, SCN-, and CH3COO-. The electrode assembly comprises a graphite electrode (GE) covered with a thin NB film containing a neutral strongly hydrophobic redox probe (decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato)) and an organic supporting electrolyte. The modified electrode is immersed in an aqueous solution containing a supporting electrolyte and transferring ions, and used in a conventional three-electrode configuration. Upon oxidation of the redox probe, the overall electrochemical process proceeds as an electron-ion charge-transfer reaction coupling the electron transfer at the GE/NB interface and compensates ion transfer across the W/NB interface. The rate of the ion transfer across the W/NB interface is the limiting step in the kinetics of the overall coupled electron-ion transfer reaction. Moreover, the transferring ion that is initially present in the aqueous phase only at a concentration lower than the redox probe, controls the mass transfer regime in the overall reaction. A rate equation describing the kinetics of the ion transfer that is valid for the conditions at thin organic film-modified electrodes is derived. Kinetic data measured with two electrochemical techniques are in very good agreement.     

Comparative study of the thermodynamics and kinetics of the ion transfer across the liquid/liquid interface by means of three-phase electrodes

A comparative study of the behavior of different sorts of three-phase electrodes applied for assessing the thermodynamics and kinetics of the ion transfer across the liquid/liquid (L/L) interface is presented. Two types of three-phase electrodes are compared, that is, a paraffin-impregnated graphite electrode at the surface of which a macroscopic droplet of an organic solvent is attached and an edge pyrolytic graphite electrode partly covered with a very thin film of the organic solvent. The organic solvent contains either decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato) as a redox probe. The role of the redox probe, the type of the electrode material, the mass transfer regime, and the effect of the uncompensated resistance are discussed. The overall electrochemical process at both three-phase electrodes proceeds as a coupled electron-ion transfer reaction. The ion transfer across the L/L interface, driven by the electrode reaction of the redox compound at the electrode/organic solvent interface, is independent of the type of redox probe. The ion transfer proceeds without involving any chemical coupling between the transferring ion and the redox probe. Both types of three-phase electrodes provide consistent results when applied for measuring the energy of the ion transfer. Under conditions of square-wave voltammetry, the coupled electron-ion transfer at the three-phase electrode is a quasireversible process, exhibiting the property known as "quasireversible maximum". The overall electron-ion transfer process at the three-phase electrode is controlled by the rate of the ion transfer. It is demonstrated for the first time that the three-phase electrode in combination with the quasireversible maximum is a new tool for assessing the kinetics of the ion transfer across the L/L interface.     

Analytical Modelling of Electron-coupled Ion Transfers with Immobilized vs Soluble Redox Transducer at Thick Film-modified Electrodes

Analytical theoretical solutions are deduced for the current-potential response, concentration profiles and interfacial potentials of electron-coupled ion transfers in the cyclic voltammetry with thick film-modified electrodes. The theory covers a wide variety of possible situations, namely, the redox transducer can be either immobilized on the working electrode or freely diffusing in the organic film, and the ion transfer can be either simple or complicated by homogeneous chemical processes. A comparative and comprehensive study of each case is performed, establishing the key parameters that define the behaviour of the system, as well as guidelines to analyse its electrochemical signal.     

A TTF–TCNQ Electrode as a Voltammetric Analogue of an Ion‐Selective Electrode

A TTF‐TCNQ/PVC composite electrode is proposed as a voltammetric cation and anion sensor. The electrode relies on the principle that, during redox processes involving the TCNQ^0/? couple for cations and the TTF^+/0 couple for anions, electrolyte ions are included into lattice sites in the charge neutralization process. This voltammetric ion‐sensor provides results that are similar to those of sensors based on two electrodes (viz. one modified with TCNQ for cations and another modified with TTF for anions) but with some practical advantages over them.     

Shuttling mechanism of ion transfer at the interface between two immiscible liquids

The transfers of hydrophilic ions between aqueous and organic phases are ubiquitous in biological and technological systems. These energetically unfavorable processes can be facilitated either by small molecules (ionophores) or by ion-transport proteins. In absence of a facilitating agent, ion-transfer reactions are assumed to be "simple", one-step processes. Our experiments at the nanometer-sized interfaces between water and neat organic solvents showed that the generally accepted one-step mechanism cannot explain important features of transfer processes for a wide class of ions including metal cations, protons, and hydrophilic anions. The proposed new mechanism of ion transfer involves transient interfacial ion paring and shuttling of a hydrophilic ion across the mixed-solvent layer.     

Multiple Active Sites: Lithium Storage Mechanism of Cu‐TCNQ as an Anode Material for Lithium‐Ion Batteries

Recently, carboxylate metal‐organic framework (MOF) materials were reported to perform well as anode materials for lithium‐ion batteries (LIBs); however, the presumed lithium storage mechanism of MOFs is controversial. To gain insight into the mechanism of MOFs as anode materials for LIBs, a self‐supported Cu‐TCNQ (TCNQ: 7,7,8,8‐tetracyanoquinodimethane) film was fabricated via an in situ redox routine, and directly used as electrode for LIBs. The first discharge and charge specific capacities of the self‐supported Cu‐TCNQ electrode are 373.4 and 219.4 mAh g^?1, respectively. After 500 cycles, the reversible specific capacity of Cu‐TCNQ reaches 280.9 mAh g^?1 at a current density of 100 mA g^?1. Mutually validated data reveal that the high capacity is ascribed to the multiple‐electron redox conversion of both metal ions and ligands, as well as the reversible insertion and desertion of Li⁺ ions into the benzene rings of ligands. This work raises the expectation for MOFs as electrode materials of LIBs by utilizing multiple active sites and provides new clues for designing improved electrode materials for LIBs.     

Elucidation of the mechanism of redox grafting of diazotated anthraquinone

Redox grafting of aryldiazonium salts containing redox units may be used to form exceptionally thick covalently attached conducting films, even in the micrometers range, in a controlled manner on glassy carbon and gold substrates. With the objective to investigate the mechanism of this process in detail, 1-anthraquinone (AQ) redox units were immobilized on these substrates by electroreduction of 9,10-dioxo-9,10-dihydroanthracene-1-diazonium tetrafluoroborate. Electrochemical quartz crystal microbalance was employed to follow the grafting process during a cyclic voltammetric sweep by recording the frequency change. The redox grafting is shown to have two mass gain regions/phases: an irreversible one due to the addition of AQ units to the substrate/film and a reversible one due to the association of cations from the supporting electrolyte with the AQ radical anions formed during the sweeping process. Scanning electrochemical microscopy was used to study the relationship between the conductivity of the film and the charging level of the AQ redox units in the grafted film. For that purpose, approach curves were recorded at a platinum ultramicroelectrode for AQ-containing films on gold and glassy carbon surfaces using the ferro/ferricyanide redox system as redox probe. It is concluded that the film growth has its origin in electron transfer processes occurring through the layer mediated by the redox moieties embedded in the organic film.     

Reversible reorganization of alkyl ester groups grafted on glassy carbon electrode: Induction by a redox probe

The oxidation of the tetrabutylammonium salt of adipic acid monomethyl ester was performed to extend the concept that the oxidation of carboxylates with terminal functional groups with electrons in π-orbitals induces the covalent grafting of carbon surfaces. When 1,4-benzoquinone was used as a redox probe to identify the compaction level of the grafted film, the voltammetric behaviour of this redox probe changed by cycling. Once a reproducible and quasi-reversible wave was obtained, the repose of the electrode in the electrolyte solution during a few minutes allowed the recovery of the original voltammogram of the redox probe on the modified electrode. This behaviour is cyclic and can be understood as the result of a reversible reorganization effect of the organic film. This reorganization phenomenon is affected by the water content in the electrochemical cell, which can be explained by hydrogen bonds forming between the ester groups present in the structure film and water.     

Cyclic Voltammetry of Hexachloroiridate(IV): An Alternative to the Electrochemical Study of the Ferricyanide Ion

An instrumental analysis experiment on the cyclic voltammetry of hexachloroiridate(IV) is described in this paper. The hexachloroiridate(IV)/hexachloroiridate(III) redox couple allows the analytical chemistry student to study the behavior of electrochemically reversible electron transfer with no complications. The cyclic voltammetric response of hexachloroiridate(IV)/hexachloroiridate(III) is compared with the ferricyanide/ ferrocyanide redox couple, which has been known to exhibit quasireversible electron transfer as a result of film formation on the electrode surface. Considerations regarding the stability of the hexachloroiridate(IV) ion in 0.1 M KNO₃ are also addressed.     

A novel label-free electrochemical aptasensor for thrombin based on the {nano-Au/thionine}n multilayer films as redox probes

Herein, a novel label-free electrochemical aptasensor based on direct immobilization of the redox probes on an electrode surface was reported. Gold electrode coated Nafion was firstly modified with redox probe-thionine (Thi) through ion exchange adsorption. Then, with the help of chemisorption and electrostatic adsorption, negatively charged nano-Au and positively charged Thi were layer-by-layer (LBL) self-assembled onto the modified electrode surface, which formed {nano-Au/Thi⁺}_n multilayer films for improving the amount of redox probes and immobilizing thiolated thrombin aptamers (TBA). In the presence of target thrombin (TB), the TBA on the multilayer film could catch the TB onto the electrode surface, which resulted in a barrier for electro-transfer, leading to decrease of the current. The proposed method avoided the cubsome redox probe labeling process, increased the amount of redox probe and reduced the distance between the redox probe and electrode surface. Thus, the approach showed a high sensitivity and a wider linearity to TB in the range from 0.12 nM to 46 nM with a detection limit of 40 pM.     

Experimental study of the interplay between long-range electron transfer and redox probe permeation at self-assembled monolayers: evidence for potential-induced ion gating

Evidence for the competition between long-range electron transfer across self-assembled monolayers (SAMs) and incorporation of the redox probe into the film is reported for the electroreduction of Ru(NH(3)) at hydroxyl- and carboxylic-acid-terminated SAMs on a mercury electrode, by using electrochemical techniques that operate at distinct time scales. Two limiting voltammetric behaviors are observed, consistent with a diffusion control of the redox process at mercaptophenol-coated electrodes and a kinetically controlled electron transfer reaction in the presence of neutral HS-(CH(2))(10)-COOH and HS-(CH(2))(n)()-CH(2)OH (n = 3, 5, and 10) SAMs. The monolayer thickness dependence of the standard heterogeneous electron transfer rate constant shows that the electron transfer plane for the reduction of Ru(NH(3)) at hydroxyl-terminated SAMs is located outside the film | solution interface at short times. However, long time scale experiments provide evidence for the occurrence of potential-induced gating of the adsorbed structure in some of the monolayers studied, which takes the form of a chronoamperometric spike. Redox probe permeation is shown to be a kinetically slow process, whose activation strongly depends on redox probe concentration, applied potential, and chemical composition of the intervening medium. The obtained results reveal that self-assembled monolayers made of mercaptobutanol and mercaptophenol preserve their electronic barrier properties up to the reductive desorption potential of a fully grown SAM, whereas those of mercaptohexanol, mercaptoundecanol, and mercaptoundecanoic acid undergo an order/disorder transition below a critical potential, which facilitates the approach of the redox probe toward the electrode surface.     

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.     

四磺酸酞菁铜—DDAB表面活性剂薄膜电极对卤代乙酸的电化学催化

四磺酸酞菁铜阴离子（ＣｕＰｃＴｓ＾４－）在水溶液中可借助离子交换进入阳离子表面活性剂双十二烷基二甲基溴化铵（ＤＤＡＢ）薄膜电极，从而形成ＣｕＰｃＴＳ＾４－－ＤＤＡＢ薄膜电极，循环伏安实验表明，在ＫＢｒ溶液中，该薄膜电极有一对良好的还原氧化峰，阴阳极峤电位分别为－０．７８Ｖ和－０．６５Ｖ（ｖｓ．ＳＣＥ）．探讨了该薄膜电极的电化学行为，特别是对其各种卤代乙酸的电化学催化，对其机理进行了推测。     

Simple electrochemical method for deposition and voltammetric inspection of silver particles at the liquid-liquid interface of a thin-film electrode

A novel experimental methodology for depositing and voltammetric study of Ag nanoparticles at the water-nitrobenzene (W-NB) interface is proposed by means of thin-film electrodes. The electrode assembly consists of a graphite electrode modified with a thin NB film containing decamethylferrocene (DMFC) as a redox probe. In contact with an aqueous electrolyte containing Ag(+) ions, a heterogeneous electron-transfer reaction between DMFC((NB)) and Ag(+)((W)) takes place to form DMFC(+)((NB)) and Ag deposit at the W-NB interface. Based on this interfacial reaction, two different deposition strategies have been applied. In the uncontrolled potential deposition protocol, the electrode is immersed into an AgNO(3) aqueous solution for a certain period under open circuit conditions. Following the deposition step, the Ag-modified thin-film electrode is transferred into an aqueous electrolyte free of Ag(+) ions and voltammetrically inspected. In the second protocol the deposition was carried out under controlled potential conditions, i.e., in an aqueous electrolyte solution containing Ag(+) ions by permanent cycling of the electrode potential. In this procedure, DMFC((NB)) is electrochemically regenerated at the electrode surface, hence enabling continuation and voltammetric control of the Ag deposition. Hence, the overall electrochemical process can be regarded as an electrochemical reduction of Ag(+)((W)) at the W-NB interface, where the redox couple DMFC(+)/DMFC acts as a mediator for shuttling electrons from the electrode to the W-NB interface. Ag-particles deposited at the W-NB interface affect the ion transfer across the interface, which provides the basis for voltammetric inspection of the metal deposit at the liquid-liquid interface with thin-film electrodes. Voltammetric properties of thin-film electrodes are particularly sensitive to the deposition procedure, reflecting differences in the properties of the Ag deposit. Moreover, this methodology is particularly suited to inspect catalytic activities of metal particles deposited at the liquid-liquid interface toward heterogeneous electron-transfer reactions occurring at the at the liquid-liquid interface.     

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.     

Nanoparticle films as electrodes: voltammetric sensitivity to the nanoparticle energy gap

Electron-transfer reactions of redox solutes at electrode/solution interfaces are facilitated when their formal potentials match, or are close to, the energy of an electronic state of the electrode. Metal electrodes have a continuum of electronic levels, and redox reactions occur without restraint over a wide span of electrode potentials. This paper shows that reactions on electrodes composed of films of metal nanoparticles do have constraints when the nanoparticles are sufficiently small and molecule-like so as to exhibit energy gaps, and resist electron transfers with redox solutes at potentials within the energy gap. When solute formal potentials are near the electronic states of the nanoparticles in the film, electron-transfer reactions can occur. The electronic states of the nanoparticle film electrodes are reflected in the formal potentials of the electrochemical reactions of the dissolved nanoparticles at naked metal electrodes. These ideas are demonstrated by voltammetry of aqueous solutions of the redox solutes methyl viologen, ruthenium hexammine, and two ferrocene derivatives at films on electrodes of 1.1 nm core diameter Au nanoparticles coated with protecting monolayers of phenylethanethiolate ligands. The methyl viologen solute is unreactive at the nanoparticle film electrode, having a formal potential lying in the nanoparticle's energy gap. The other solutes exhibit electron transfers, albeit slowed by the electron hopping resistance of the nanoparticle film. The nanoparticles are not linked together, being insoluble in the aqueous medium; a small amount of an organic additive (acetonitrile) facilitates observing the redox solute voltammetry.     

Electrocatalytic oxidation of some biologically important compounds at conducting polymer electrodes modified by metal complexes

Conducting poly(3-methylthiophene) electrodes were electrochemically prepared. The resulting polymer films were modified with an inorganic complex, ferrocene. The incorporation of the ferrocene/ferrocenium moiety into the polymer film resulted in enhanced charge transfer towards the oxidation of some organic molecules of biological interest. The electrochemical response of the complex-containing polymer electrode was compared to that of the unmodified polymer electrode and that of the substrate. Apparent diffusion coefficients of the redox species were estimated from the cyclic voltammetric data for different biological molecules at the ferrocene-containing polymer electrode. Infra-red spectroscopic measurements for the “as-grown” films revealed the presence of the inorganic complex within the polymer. The modified polymer electrode showed noticeable enhancement for the charge transfer across the film interface and can be used as an electrochemical sensor for biological compounds. Received: 3 June 1997 / Accepted: 7 July 1997     

Introducing hydrophilic carbon nanoparticles into hydrophilic sol-gel film electrodes

A hydrophilic carbon nanoparticle–sol-gel electrode with good electrical conductivity within the sol-gel matrix is prepared. Sulfonated carbon nanoparticles with high hydrophilicity and of 10–20 nm diameter (Emperor 2000) are co-deposited onto tin-doped indium oxide substrates employing a sol-gel technique. The resulting carbon nanoparticle-sol-gel composite electrodes are characterized as a function of composition and salt (KCl) additive. Scanning electron microscopy and voltammetry in the absence and in the presence of a solution redox system suggest that the composite electrode films can be made electrically conducting and highly porous to promote electron transport and transfer. The effect of the presence of hydrophilic carbon nanoparticles is explored for the following processes: (1) double layer charging, (2) diffusion and adsorption of the electrochemically reversible solution redox system 1,1′-ferrocenedimethanol, (3) electron transfer to the electrochemically irreversible redox system hydrogen peroxide, and (4) electron transfer to the redox liquid tert-butylferrocene deposited into the porous composite electrode film. The extended electrochemically active hydrophilic surface area is beneficial in particular for surface sensitive processes (1) and (3), and it provides an extended solid|organic liquid|aqueous solution boundary for reaction (4). The carbon nanoparticle–sol-gel composite electrodes are optimized to provide good electrical conductivity and to remain stable during electrochemical investigation.     

薄层循环伏安法研究硝基苯/水界面上离子诱导的电子转移反应

应用薄层循环伏安法研究了硝基苯/水两相界面间,且有共同离子四丁基铵TBA+存在于两相中,在有机相中的四氰化二甲基苯醌(TCNQ)与水相中的K4Fe(CN)6之间发生的反向电子转移反应。在直径为0.64cm的裂解石墨电极上用2μL硝基苯溶液使之自然扩散在电极表面形成薄层的有机相,并以此作为工作电极。对电极为铂丝(0.5mm),参比电极为Ag/AgCl电极,均置于总体积为2mL的水相中。由于共同离子TBA+的诱导,在硝基苯/水界面间,在已氧化的TCNQ+阳离子(在有机相中)与[Fe(CN)6]4-阴离子(在水相中)之间发生了反向电子转移反应。试验证明:在一定条件下,通过改变两相中共同离子的浓度,可使一些不能发生的两相界面的电子转移反应得以发生;这类电子转移反应系受界面电位差所控制。此外,还测得了在恒定的共同离子浓度比值的条件下,此两相界面电子转移反应的表观速率常数(k)为0.135cm.s-1.mol-1。