Electroactive Ceramic Carbon Electrode Modified with Hydrophobic Polar Solvent

Ceramic carbon electrode modified with redox probe solution in hydrophobic polar solvent was prepared and studied. The electrode consisting of graphite powder, homogeneously dispersed in hydrophobic silicate matrix, was prepared from the mixture of methyltrimethoxysilane based sol and graphite powder by sol‐gel method. It was immersed in t‐butyloferrocene solution in nitrobenzene. The electrode properties were investigated by cyclic voltammetry and chronoamperometry in KNO₃ solution of different concentration. In most cases linear polarization of the electrode towards positive potentials results in peak shaped voltammogram originating from electrooxidation of t‐butyloferrocene. Its shape changes with time, but after 5–7 scans stable curve is obtained. In all conditions the anodic to cathodic charge ratio is larger than unity. The peak current is proportional to the concentration of the redox probe in organic phase and salt in aqueous phase, whereas the midpeak potential is almost not affected by these factors. It has been concluded, that the electrooxidation of redox probe within hydrophobic silicate matrix is followed by two simultaneous processes: t‐butyloferrocenium cation transfer to the aqueous phase and anion transfer from aqueous phase. Their relative contribution depends on the ratio of concentration of the redox probe in organic phase to concentration of salt in aqueous phase.     

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.     

Electroactive ceramic carbon electrode impregnated with organic liquid

The carbon ceramic electrodes impregnated with hydrophobic organic solvent (toluene, hexadecane, nitrobenzene) containing redox probe (decamethylferrocene) were prepared. The electrode material was obtained by sol–gel process. It consists of graphite powder homogeneously dispersed in hydrophobic silica matrix. After gelation and drying it was filled with organic liquid. The electrochemical properties of the electrode were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. Approximately symmetric cyclic voltammograms were obtained with these electrodes immersed in aqueous electrolyte solution. Their shape and current magnitude and position on the potential scale depends on the organic solvent and the salt present in aqueous phase. It has been concluded that the mechanism of the electrode process involves electron transfer between graphite particle and the redox probe in organic phase, followed by anion transfer from the aqueous phase.     

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.     

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.     

Voltammetric studies of microcrystalline C60 adhered to a carbon electrode surface and placed in contact with aqueous electrolyte containing potassium ions

The reduction of microcrystalline C₆₀ fullerene, adhered at a carbon electrode and immersed in aqueous electrolyte, has been studied under various voltammetric conditions. This work reports mainly the voltammetric studies carried out principally in electrolyte containing potassium ions. Comparison of adherence techniques, such as solvent casting and mechanical transfer methods, are made to assess if the type of adhered techniques has any significant influence on the observed electrochemistry. The solvent casting method is found to produce three peaks in the potential for C₆₀^0/n- redox couple as compared to a single and large peak produced when a mechanical transfer technique is employed. When the reduction potential of microcrystalline C₆₀ in the presence of K⁺ is compared with other cations, such as Li, Na, Rb and Cs, it is observed that the shift of reduction potential follows the change in the hydration energy in the order Cs>Rb>K>Na>Li. In a mixed electrolyte study of CsCl/KCl, the reduction potential and peak shape of C₆₀^0/n- redox couple during cyclic voltammetry is observed to change with concentration of the cations and the observed electrochemistry can be attributed to a cation-exchange mechanism. The reduction of C₆₀ is irreversible in aqueous electrolyte containing alkaline cations as the re-oxidation process does not produce any observed electro-activity. Evidence of the formation of a passive coating of K _n C₆₀ fulleride, which does not appear to undergo dissolution is obtained under cyclic voltametric conditions. This coating remains electrochemically active in the presence of tetrabutylammonium ions in acetonitrile. Scan rate, chronocoulometric, and scanning electron microscopic studies provide evidence of the presence of a surface process involving solid–solid transformation.     

Ion insertion into ionic liquid supported toluene generated by electrochemical redox reaction

Ion transfer across the toluene|water, toluene–ionic liquid mixture|water and ionic liquid|water boundary generated by electrochemical redox reaction of tert-butylferrocene (tBuFc) was studied with the glassy carbon (GC) electrode partially covered by the organic liquid deposit and immersed in the aqueous electrolyte solution. The electrooxidation of the redox probe in toluene deposit is followed by ejection of newly formed cation into the aqueous solution. The same reaction in the toluene–ionic liquid deposit promotes anion insertion. This pathway is also found at the electrode modified with ionic liquid.     

Does thiol-functionalized viologen monolayer memorize anion present when forming the monolayer?

We investigated the effect of the presence of an additive anion while forming a self-assembled monolayer of a thiol-functionalized redox active species upon the behavior in aqueous solutions. A bromide salt was added in an acetonitrile solution of a thiol-functionalized viologen (viologen-thiol: N-pentyl-N′-(11-mercapto)undecyl-4,4′-bipyridinium bishexafluorophosphate) in which the self-assembled monolayer was formed on a polycrystalline Au electrode. We examined the structure and electrochemical behavior of the resulting monolayer-modified electrode in three different aqueous electrolyte solutions. The viologen-thiol monolayer prepared in the presence of Br⁻ exhibited obviously different behavior in both KBr and KF electrolyte solutions from that prepared in the absence of Br⁻. On the other hand, the difference was minor in KPF₆ solution. The difference of the electrochemical behavior was represented by the coverage of the viologen-thiol and the formal potential of viologen dication/radical cation redox couple. The memory that the viologen-thiol monolayer was formed in the presence of Br⁻ was retained in KF and KBr solutions, though it was erased in the electrolyte solution containing PF₆⁻, which is a softer anion than Br⁻. However, no definitive difference of the film structure was deduced from the electroreflectance study in regard to the monomer content and the average orientation of the viologen moiety.     

A Fullerene/Lipid Electrode Device: Reversible Electron Transfer Reaction of C60 Embedded in a Cast Film of an Artificial Ammonium Lipid on an Electrode in Aqueous Solution

Two reversible one-electron transfers are observed for an electrode device made from C₆₀ and an artificial lipid (see schematic drawing). Cyclic voltammetric studies reveal that the redox couples are unchanged even after 50 cycles, thus indicating that the C₆₀ radical monoanion and the C₆₀ dianion generated in aqueous solution are very stable.     

Scanning electrochemical microscopy study of ion transfer process across water/2-nitrophenyloctylether interface supported by hydrophobic carbon ceramic electrode

A carbon ceramic electrode (CCE) modified with the redox probe—decamethylferrocene solution in hydrophobic organic solvent—2-nitrophenyloctyl ether and immersed into an aqueous solution was studied by scanning electrochemical microscopy (SECM). After the electrochemical oxidation of decamethylferrocene, its cations were detected near the electrode surface in the aqueous phase. This indicates that some fraction of the redox-active cations electrochemically produced in the organic phase is transferred across the liquid/liquid interface. They are reduced at the SECM tip and form a solid deposit. The amount of deposited decamethylferrocene was estimated by the anodic reaction at the tip. It is affected by the substrate–tip distance, deposition time, and electrolyte concentration. The SECM images of unmodified and modified CCEs are consistent with their heterogeneous structure.     

Electrochemical Generation of the 9,10-Antraquinone Radical Anions and Its Use in the Polystyrene Synthesis

The nature of active centers and anionic mechanism of the styrene polymerization during the 9,10-antraquinone electroreduction in the monomer-dimethylacetamide-alkali metal (or ammonium) perchlorate system is studied by voltammetry, ESR, IR- and UV-spectroscopy. It is shown that the potential of electrolysis depends on the supporting electrolyte composition; the association of the supporting electrolyte cation with the organic anion, in turn, affects the mechanism of the polymerization initiation and the macromolecule growth kinetics. The potential of generation of 9,10-antraquinone and the styrene conversion in catholyte increase with increasing radius of the supporting cation in the series Li⁺ <; Na⁺ <; K⁺ <; Rb⁺ <; Cs⁺ <; (C₂H₅)₄N⁺ <; (C₄H₉)₄N⁺.     

Mesoporous platinum hosts for electrodeliquidliquid – Triple phase boundary redox systems

Mesoporous metal hosts are attractive electrode materials for complex electrode reactions, for example those involving a system of two immiscible liquids. Here we show that a solution of tetraphenylporphyrianto manganese (MnTPP) in 4-(3-phenylpropyl)-pyridine (PPP) organic liquid can be immobilized into mesoporous platinum thin films electrodeposited from a liquid crystalline template. When immersed in an aqueous solution, the organic liquid remains immobilized inside the mesoporous platinum framework. Well-defined, stable, and reproducible ion transfer voltammograms are recorded. The effects of mesoporous platinum membrane thickness (volume), scan rate, and the type of aqueous electrolyte anion (for Cl⁻, , , CN⁻, SCN⁻ and ) are investigated. Mesoporous platinum is proposed as a very effective electrode material for liquidliquid anion sensing and for other applications of electrochemically driven liquidliquid redox systems.     

The Electrochemical Behavior in Aqueous Media of Conducting Polymers: I: The Methanol Soluble Fraction Obtained on the Cu(II) Catalyzed Polymerization of 2,5-Dibromo-3-Methylthiphene

Abstract

The methanol soluble amorphous fraction obtained on the CuCl₂ induced polymerization of 2,5-dibromo-3-methylthiophene exhibits redox waves on cyclic voltammetric potential polarization for a copper redox couple (for the metal ion incorporated in the polymer matrix) and a redox couple associated with the polymer matrix itself. Furthermore, when used as an electrode for the ferri/ferrocyanide couple in the electrolyte phase it acts as a well behaved electrode surface. Electrodes made from this material show a remarkable stability in aqueous media.     

Phosphate and arsenate electro-insertion processes into a N,N,N′,N′-tetraoctylphenylenediamine redox liquid

The electro-insertion of ions is a well-known phenomenon, which allows the transfer of anions or cations across phase boundaries to be monitored and driven electro-chemically. Extremely hydrophilic anions, such as phosphate and arsenate, are not usually observed to undergo electro-insertion. It is shown here that at organic redox liquid|water|electrode triple interfaces these anions can be forced electro-chemically to transfer into organic media.The transfer process of phosphate anions from aqueous buffer solutions into organic microdroplets of the redox liquid N,N,N^′,N^′-tetraoctylphenylenediamine (TOPD) is pH and concentration sensitive. It is shown that phosphate is transferred in the form of PO₄HK⁻ in the presence of phosphate buffer. Two distinct potential regions are identified and attributed to (i) interfacial redox processes at the liquid|liquid interface associated with deprotonation and (ii) bulk redox processes associated with anion transfer from the aqueous to the organic phase.The comparison of phosphate and arsenate electro-insertion processes suggests that arsenate is less hydrophilic and transferred into the organic phase preferentially.     

Evolution of Anolyte Composition in the Oxidative Electrolysis of Sodium Bromide in a Sulfuric Acid Medium

The oxidation of a 10 mM aqueous solution of sodium bromide in a sulfuric acid medium on the surface of a platinum electrode in a cell with separated spaces was studied. The process is important in view of the use of the bromine–bromide redox couple in redox flow batteries. The study was performed by cyclic voltammetry, potentiostatic chronoamperometry with optical absorption spectrum recording, and measurements of the potential of the redox reference electrode. A numerical procedure for processing the experimental spectra of the solution was developed to separate them into the spectrum of molecular bromine and the residual signal. The latter was attributed to the absorption of the tribromide anion based on the literature data. The experimental dependences of the Br₂ and Br₃^- concentrations for the oxidative electrolysis of the NaBr solution in the sulfuric acid medium agreed well with the theoretical predictions. The current efficiency of bromine formation was evaluated.

     

The Effect of Ionic Liquid Covalent Bonding to Sol‐Gel Processed Film on Ion Accumulation and Transfer

Accumulation of electroactive anions into a silicate film with covalently bonded room temperature ionic liquid film deposited on an indium tin oxide electrode was studied and compared with an electrode modified with an unconfined room temperature ionic liquid. A thin film containing imidazolium cationic groups was obtained by sol‐gel processing of the ionic liquid precursor 1‐methyl‐3‐(3‐trimethoxysilylpropyl)imidazolium bis(trifluoromethylsulfonyl)imide together with tetramethylorthosilicate on the electrode surface. Profilometry shows that the obtained film is not smooth and its approximate thickness is above 1 μm. It is to some extent permeable for a neutral redox probe – 1,1′‐ferrocene dimethanol. However, it acts as a sponge for electroactive ions like Fe(CN)₆^3?, Fe(CN)₆^4? and IrCl₆^3?. This effect can be traced by cyclic voltammetry down to a concentration equal to 10^?7 mol dm^?3. Some accumulation of the redox active ions also occurs at the electrode modified with the ionic liquid precursor, but the voltammetric signal is significantly smaller compare with the bare electrode. The electrochemical oxidation of the redox liquid t‐butyloferrocene deposited on silicate confined ionic liquid film is followed by the expulsion of the electrogenerated cation into an aqueous solution. On the other hand, the voltammetry obtained with the electrode modified with t‐butyloferrocene solution in the ionic liquid precursor exhibits anion sensitive voltammetry. This is explained by anion insertion into the unconfined ionic liquid deposit following t‐butylferricinium cation formation.     

Electrochemical oxidation of l-cysteine mediated by a fullerene-C60-modified carbon electrode

Use of a glassy carbon electrode modified by adhered microcrystals of fullerene-C₆₀ mediates the oxidation of cysteine in the presence of aqueous potassium-containing electrolytes. Under conditions of cyclic voltammetry, the potential for the oxidation of cysteine is lowered by approximately 100 mV and current is enhanced significantly relative to the situation prevailing when a bare glassy carbon electrode is used. Additional mediation occurs when the potential range covered includes that of C₆₀/C₆₀ⁿ⁻ redox couples. The sensitivity under condition of cyclic voltammetry is significantly dependent on pH, temperature and C₆₀ dosage. Excellent analytical and/or recovery data are obtained with vitamin pill, cassamino acid (hydrolyzed casein) and for a range of beverages.     

Boronic acid-facilitated α-hydroxy-carboxylate anion transfer at liquid/liquid electrode systems: the EICrev mechanism

The transfer of the α-hydroxy-carboxylates of glycolic, lactic, mandelic and gluconic acid from the aqueous electrolyte phase into an organic 4-(3-phenylpropyl)-pyridine (PPP) phase is studied at a triple-phase boundary electrode system. The tetraphenylporphyrinato complex MnTPP dissolved in PPP is employed to drive the anion transfer reaction and naphthalene-2-boronic acid (NBA) is employed as a facilitator. In the absence of a facilitator, the ability of α-hydroxy-carboxylates to transfer into the organic phase improves, consistent with hydrophobicity considerations giving relative transfer potentials (for aqueous 0.1 M solution) of gluconate>glycolate>lactate>mandelate. In the presence of NBA, a shift of the reversible transfer potential to more negative values is indicating fast reversible binding (the mechanism for the electrode process is EIC_rev) and the binding constants are determined as K _glycolate = 2 M⁻¹, K _mandelate = 60 M⁻¹, K _lactate = 130 M⁻¹ and K _gluconate = 2,000 M⁻¹. The surprisingly strong interaction for gluconate is rationalised based on secondary interactions between the gluconate anion and NBA.     

双亲性C60衍生物水相聚集行为

富勒烯C₆₀几乎不溶于水中,从而阻碍了对富勒烯的进一步研究和潜在应用。双亲性C₆₀衍生物在水相中自组装形成聚集体,在水相具有一定的溶解度,其特殊的结构及性能引起了科学家的广泛关注。本文对双亲性C₆₀衍生物在水相中聚集行为的研究现状及研究进展进行了详细系统的介绍。本文第一部分主要阐述了双亲性C₆₀衍生物的结构,根据修饰到C₆₀表面的功能基类型对双亲性C₆₀衍生物进行了分类。第二部分主要阐述了双亲性C₆₀衍生物在水相的聚集行为以及pH值、溶剂极性、浓度、温度和抗衡离子等因素对聚集行为的影响。     

Electrochemistry and electrochromic properties of phenanthroline-iron(II/III) complex films

New films of the iron complexes with bis((2-hydroxyphenyl)methylaminosulfonyl)bathophenanthroline(HPBP) and bis((2-aminophenyl)methylaminosulfonyl)bathophenanthroline(APBP) ligands are prepared on the electrode surfaces by electrochemical polymerization. The resulting film-coated electrode shows a well-defined reversible voltammogram corresponding to the redox reaction of the Fe(II/III) complexes and an electrochromic change from red(absorption maximum: 540 nm) to colorless. The response rate of the color change to a potential step was found to be correlated to the apparent diffusion coefficient(Dapp) for the homogeneous charge-transport process within the film. The Dapp values estimated are (3-4) × 10⁻⁹cm²s⁻¹ for the [Fe(APBP)₃] film and(1-2) × 10⁻⁸cm²s⁻² for the [Fe(HPBP)₃] film, respectively, by potential-step chronoamperometric and chronocoulometric methods. The result of electrochemical quartz crystal microbalance(EQCM) measurements⁴) and dependence of the formal potential of the metal complex of the Fe(II/III) redox couple with activity of the supporting electrolyte anion in NaClO₄ aqueous solution showed that anion, cation, and solvent move simultaneously across the polymer film/solution interface during the redox reaction. A piezoelectric admittance measurement⁴⁾ of the poly[Fe(APBP)₃] coated quartz crystal electrode showed that the viscosity of the film is affected by the oxidation state of iron.