Detection of hydrogen peroxide produced at a liquid/liquid interface using scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) was used to monitor in situ hydrogen peroxide (H₂O₂) produced at a polarized water/1,2-dichloroethane (DCE) interface. The water/DCE interface was formed between a DCE droplet containing decamethylferrocene (DMFc) supported on a solid electrode and an acidic aqueous solution. H₂O₂ was generated by reducing oxygen with DMFc at the water/DCE interface, and was detected with a SECM tip positioned in the vicinity of the interface using a substrate generation/tip collection mode. This work shows unambiguously how the H₂O₂ generation depends on the polarization of the liquid/liquid interface, and how proton-coupled electron transfer reactions can be controlled at liquid/liquid interfaces.     

Effects of carbon nanofiber composites on electrode processes involving liquid|liquid ion transfer

Composite electrodes were prepared from chemical vapor deposition grown carbon nanofibers consisting predominantly of ca. 100 nm diameter fibers. A hydrophobic sol–gel matrix based on a methyl-trimethoxysilane precursor was employed and composites formed with carbon nanofiber or carbon nanofiber—carbon particle mixtures (carbon ceramic electrode). Scanning electron microscopy images and electrochemical measurements show that the composite materials exhibit high surface area with some degree of electrolyte solution penetration into the electrode. These electrodes were modified with redox probe solution in 2-nitrophenyloctylether. A second type of composite electrode was prepared by simple pasting of carbon nanofibers and the same solution (carbon paste electrode). For both types of electrodes it is shown that high surface area carbon nanofibers dominate the electrode process and enhance voltammetric currents for the transfer of anions at liquid|liquid phase boundaries presumably by extending the triple-phase boundary. Both anion insertion and cation expulsion processes were observed driven by the electro-oxidation of decamethylferrocene within the organic phase. A stronger current response is observed for the more hydrophobic anions like ClO₄⁻ or PF₆⁻ when compared to that for the more hydrophilic anions like F⁻ and SO₄²⁻. Presented at the 4th Baltic Conference on Electrochemistry, Greifswald, March 13–16, 2005     

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.     

Probing TCNQ‐mediated Metal Reduction Reactions at Liquid‐liquid Interface with SECM

Metal reduction at the interface between two immiscible electrolyte solutions (ITIES) has been studied with scanning electrochemical microscopy (SECM). Metal cations in the aqueous phase are reduced by 7,7,8,8‐tetracyanoquinodimethane anion (TCNQ^?) residing in the oil phase, methyl isobutyl ketone (MIBK). TCNQ^? is formed at the SECM tip by reducing TCNQ, which results in a positive feedback loop between the tip and the ITIES when an electron is donated to a metal cation. The effect of the Galvani potential difference on the rate of the interfacial electron transfer was investigated, establishing the potential difference either by an additional substrate electrode in the aqueous phase or by an a common ion in both phases. It is shown that the Galvani potential difference as a driving force does enable TCNQ^? mediated Cu²⁺ reduction. Finite element method (FEM) simulations were run to provide information on the reaction kinetics and stoichiometry.     

Distribution of three ions in the thin film experiment

In the theoretical model it is assumed that a graphite disk electrode is covered by a thin film of the solution of decamethylferrocene and some supporting electrolyte in nitrobenzene and immersed in an aqueous solution of the same electrolyte. Oxidation of decamethylferrocene is accompanied by the transfer of anions of the electrolyte from water into nitrobenzene. The flux of decamethylferrocenium cations at the electrode surface and the flux of anions at the liquid/liquid interface are separated by the thickness of the film, but the electroneutrality is ensured by the migration of ionic species through the film. Theoretical concentration profiles of ionic species in the film are reported for cyclic voltammetry.     

Catalytic Activity of Alkali Metal Cations for the Chemical Oxygen Reduction Reaction in a Biphasic Liquid System Probed by Scanning Electrochemical Microscopy

Chemical reduction of dioxygen in organic solvents for the production of reactive oxygen species or the concomitant oxidation of organic substrates can be enhanced by the separation of products and educts in biphasic liquid systems. Here, the coupled electron and ion transfer processes is studied as well as reagent fluxes across the liquid|liquid interface for the chemical reduction of dioxygen by decamethylferrocene (DMFc) in a dichloroethane-based organic electrolyte forming an interface with an aqueous electrolyte containing alkali metal ions. This interface is stabilized at the orifice of a pipette, across which a Galvani potential difference is externally applied and precisely adjusted to enforce the transfer of different alkali metal ions from the aqueous to the organic electrolyte. The oxygen reduction is followed by H₂O₂ detection in the aqueous phase close to the interface by a microelectrode of a scanning electrochemical microscope (SECM). The results prove a strong catalytic effect of hydrated alkali metal ions on the formation rate of H₂O₂, which varies systematically with the acidity of the transferred alkali metal ions in the organic phase.     

The punctured droplet electrode – A new three-phase electrode with well defined geometry

A new three-phase electrode allows detailed studies on well-defined three-phase junctions. It consists of a nitrobenzene drop of well controlled size. The drop is dispensed from a capillary and is punctured with a microcylinder electrode. The organic liquid contains an electroactive compound (decamethylferrocene) and, importantly, no supporting electrolyte. The aqueous phase may contain various salts. Well-defined and reproducible linear-scan and square-wave voltammograms and chronoamperograms of oxidation of decamethylferrocene were obtained. The dependence of the formal potential determined from the square-wave voltammograms of decamethylferrocene versus the standard potential of transfer of anions present in the aqueous phase was almost perfectly linear. The developed approach allows the formation of two or more three-phase boundaries within one small drop. Since the drop is well exposed, this electrode geometry also gives a potential possibility of optical/spectrophotometric inspection of the reaction products in the organic phase and of examination of the reaction-layer growth.     

应用扫描电化学显微镜研究极化液/液界面上的电子转移反应

将含有氧化还原电对的水溶液滴涂在铂盘电极表面, 然后将该电极插入到1,2-二氯乙烷溶液中, 形成稳定的油/水界面. 液滴中的K3Fe(CN)6和K4Fe(CN)6氧化还原电对既可以作为水相中的参比电对参与控制液/液界面上的电势差, 同时又可以作为水相的电子授受体参与界面上的电子转移反应. 结合扫描电化学显微镜电化学系统的特点, 利用其双恒电位仪分别控制界面电势差和现场扫描的优点, 通过扫描电化学显微镜的渐进曲线得到了不同界面电势差控制的电子转移反应速率常数. 实验结果表明, 应用此方法获得的液/液界面可以被外加电位极化, 在一定的电势差范围内, 反应速率常数与界面电势差的关系遵守Butler-Volmer公式.     

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.     

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.     

Enzymatic formation of ions and their detection at a three-phase electrode

An electrochemical method for the detection of enzymatically created anions is described that uses a thin-film electrode with decamethylferrocene as an electroactive redox probe. The enzymatic oxidation of glucose with enzyme glucose oxidase produces gluconic acid as a final product. The oxidation of decamethylferrocene dissolved in the thin-nitrobenzene film, that is spread on the working graphite electrode and submerged in the aqueous solution containing glucose and glucose oxidase, is followed by the up-take of gluconate anions from the aqueous phase to nitrobenzene. The peak currents of the square-wave voltammetric responses of that system are a linear function of the glucose concentration in the milimolar range from 0.1 mmol/L to 0.7 mmol/L (R²=0.994).     

Three-phase electrochemistry with a cylindrical microelectrode

A new experimental approach is proposed to examine the ion transfer across the boundary of two immiscible liquids. A cylindrical platinum or gold microelectrode is immersed into the two-liquid system in such a way that a part of it is located in one liquid and the other part resides in the second liquid. The organic liquid contained either ferrocene or decamethylferrocene and no supporting electrolyte. The aqueous phase contained various inorganic salts. Well defined and reproducible linear-scan and square-wave voltammograms of oxidation of ferrocene and decamethylferrocene were obtained. The dependence of the formal potential derived from the square-wave voltammograms of decamethylferrocene vs. the standard potentials of transfer of anions present in the aqueous phase was perfectly linear. The developed method is more precise, since the three-phase boundary is better defined compared to placing a drop of organic liquid on the surface of a graphite electrode, and should be applicable to a larger set of organic liquids.     

Electrochemical and microscopic studies of surface-confined DNA

We have studied the micropatterning and characterization of the organic monolayers using cyclic voltammetry (CV), scanning electrochemical microscopy (SECM), atom force microscopy, and AC impedance, and have determined the electrochemical parameters, i.e., the apparent reaction rate constant (K _f) and the coverage of the electrode surface (θ). CV and SECM experiments demonstrated that the surface of the modified electrode represents an insulating substrate for ferricyanide. Using the high sensitivity of the electron transfer of ferricyanide to the modification of the gold surface with DNA, we selected this reaction as a probe to study the different modification stages at this modified electrode. SECM images obtained from bare, partially modified, and totally modified electrodes showed very good resolution with different topographies or null according to the extent of modification. Based on a comparison with the results of the experiments, a reasonable agreement can be obtained, which means a conjunction of these techniques.     

On the diffusion of ferrocenemethanol in room-temperature ionic liquids: an electrochemical study

The electrochemical behaviour of ferrocenemethanol (FcMeOH) has been studied in a range of room-temperature ionic liquids (RTILs) using cyclic voltammetry, chronoamperomery and scanning electrochemical microscopy (SECM). The diffusion coefficient of FcMeOH, measured using chronoamperometry, decreased with increasing RTIL viscosity. Analysis of the mass transport properties of the RTILs revealed that the Stokes-Einstein equation did not apply to our data. The "correlation length" was estimated from diffusion coefficient data and corresponded well to the average size of holes (voids) in the liquid, suggesting that a model in which the diffusing species jumps between holes in the liquid is appropriate in these liquids. Cyclic voltammetry at ultramicroelectrodes demonstrated that the ability to record steady-state voltammograms during ferrocenemethanol oxidation depended on the voltammetric scan rate, the electrode dimensions and the RTIL viscosity. Similarly, the ability to record steady-state SECM feedback approach curves depended on the RTIL viscosity, the SECM tip radius and the tip approach speed. Using 1.3 μm Pt SECM tips, steady-state SECM feedback approach curves were obtained in RTILs, provided that the tip approach speed was low enough to maintain steady-state diffusion at the SECM tip. In the case where tip-induced convection contributed significantly to the SECM tip current, this effect could be accounted for theoretically using mass transport equations that include diffusive and convective terms. Finally, the rate of heterogeneous electron transfer across the electrode/RTIL interface during ferrocenemethanol oxidation was estimated using SECM, and k(0) was at least 0.1 cm s(-1) in one of the least viscous RTILs studied.     

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.     

Electrochemical and electrocatalytic behaviors of Safranin O/Nafion film deposited on the glassy carbon electrode

The glassy carbon (GC) electrode modified with Nafion and Safranin O (SFO) was prepared and its electrochemical properties were investigated. The SFO molecules were strongly and irreversibly adsorbed on the Nafion — modified GC surface. The electrochemical behavior and mechanism for interactions of the SFO molecules with the Nafion film were investigated through cyclic voltammetric method. The electrocatalytic reduction of nitric oxide was performed at this modified electrode by cyclic voltammetric and hydrodynamic amperometric techniques. The Nafion membrane played a duel role as a matrix for the SFO immobilization and also helped to partition the nitric oxide from the solution phase. The diffusion coefficient of NO at the SFO/Nafion/GC modified electrode was calculated using chronoamperometry. The dependence of response currents on the concentration of NO was examined and was linear in the range of 0.05–1.9 mM of NO.     

The Solvent Effect on H2O2 Generation at Room Temperature Ionic Liquid|Water Interface

H₂O₂ is a versatile chemical and can be generated by the oxygen reduction reaction (ORR) in proton donor solution in molecular solvents or room temperature ionic liquids (IL). We investigated this reaction at interfaces formed by eleven hydrophobic ILs and acidic aqueous solution as a proton source with decamethylferrocene (DMFc) as an electron donor. H₂O₂ is generated in colorimetrically detectable amounts in biphasic systems formed by alkyl imidazolium hexafluorophosphate or tetraalkylammonium bis(trifluoromethylsulfonyl)imide ionic liquids. H₂O₂ fluxes were estimated close to liquid|liquid interface by scanning electrochemical microscopy (SECM). Contrary to the interfaces formed by hydrophobic electrolyte solution in a molecular solvent, H₂O₂ generation is followed by cation expulsion to the aqueous phase. Weak correlation between the H₂O₂ flux and the difference between DMFc/DMFc⁺ redox potential and 2 electron ORR standard potential indicates kinetic control of the reaction.     

Electrochemical Systems Based on Sol-Gel Silica Matrix Impregnated with Organic Solvent

Two electrochemical systems based on sol-gel silica matrix impregnated with organic solvent were prepared and studied. The first one is composed of tetramethylorthosilicate based material filled with ferrocene solution in polar solvent: propylene carbonate. Electrodes are immersed in this solid electrolyte during all stages of sol-gel process. Despite of the lack of the extra added salt, by using ultramicroelectrode, undistorted electrochemical signal corresponding to the electrooxidation of the ferrocene was obtained. Its diffusion coefficient within the sol-gel matrix depends on the time elapsed after gelation and it is not much below that in salt solution in the same solvent. The second system is based on graphite dispersion in hydrophobic sol-gel silicate matrix. This material was filled with mixture of liquid butylferrocene and hexadecane. After immersion in aqueous salt solution it serves as working electrode. The electrochemical signal corresponding to the electrooxidation of the butylferrocene within organic phase was obtained. Probably the electrode process occurs at three phase (carbon/organic phase/aqueous phase) junction and it is accompanied by anion transfer through the liquid-liquid interface.     

Ex Situ Scanning Electrochemical Microscopy (SECM) Investigation of Bismuth‐ and Bismuth/Lead Alloy Film‐Modified Gold Electrodes in Alkaline Medium

Scanning electrochemical microscopy (SECM) in feedback mode was employed to characterise the reactivity and microscopic peculiarities of bismuth and bismuth/lead alloys plated onto gold disk substrates in 0.1 mol L^?1 NaOH solutions. Methyl viologen was used as redox mediator, while a platinum microelectrode was employed as the SECM tip. The metal films were electrodeposited ex situ from NaOH solutions containing either bismuth ions only or both bismuth and lead ions. Approach curves and SECM images indicated that the metal films were conductive and locally reactive with oxygen to provide Bi³⁺ and Pb²⁺ ions. The occurrence of the latter chemical reactions was verified by local anodic stripping voltammetry (ASV) at the substrate solution interface by using a mercury‐coated platinum SECM tip. The latter types of measurements allowed also verifying that lead was not uniformly distributed onto the bismuth film electrode substrate. These findings were confirmed by scanning electron microscopy images. The surface heterogeneity produced during the metal deposition process, however, did not affect the analytical performance of the bismuth coated gold electrode in anodic stripping voltammetry for the determination of lead in alkaline media, even in aerated aqueous solutions. Under the latter conditions, stripping peak currents proportional to lead concentration with a satisfactory reproducibility (within 5 % RSD) were obtained.     

Scanning electrochemical microscope observation of defects in a hexadecanethiol monolayer on gold with shear force-based tip-substrate positioning

Scanning electrochemical microscopy (SECM) was used for imaging of n-hexadecanethiol-modified Au surfaces. In these studies, small defects were observed in the monolayer when a submicrometer electrode was used as an SECM tip, although a cyclic voltammogram of a Au disk electrode showed that the surface of the Au was completely covered with n-hexadecanethiol. The dependence of the SECM images on the potential of the Au electrode was also examined. A comparison of the current at the Au electrode and the tip current in the SECM images showed that direct electron transfer through the monolayer was dominant, rather than electron transfer at the defects. The size of the defects was estimated from the tip current to be 1-100 nm, under the assumption that the defects were small compared to the SECM probe.