Thin‐Layer Cyclic Voltammetric and Scanning Electrochemical Microscopic Study of Antioxidant Activity of Ascorbic Acid at Liquid/Liquid Interface

An electron transfer reaction between ascorbic acid (H₂A) in an aqueous solution and oxidizing agent in an organic solution immiscible with water has been studied by thin‐layer cyclic voltammetry (TLCV) for charge transfer at the interface between two immiscible electrolyte solutions (ITIES). As an antioxidant, H₂A provide electrons through the aqueous/organic interface to reduce Fc⁺ and the procedure has been proved to be a one electron process again. In this work, the first combination of TLCV and scanning electrochemical microscopy (SECM) was achieved and showed a reasonable agreement between the results from the two different approaches. Otherwise, lower concentration ratios Kr of aqueous to organic reactants was adopted, which is given as evidence to the proposed procedure of Barker.     

Cyclic Voltammetry of Ion Transfer for Phenylpropanolamine Hydrochloride at Water|Nitrobenzene Interface

The transfer on phenylpropanolamine ion, PPAH⁺, has been studied at the Interface between Two Immiscible Solutions (ITIES). The polarizable potential range was determined by cyclic voltammetry at the interface between an aqueous solution of lithium chloride (LiCl) and a nitrobenzene (NB) solution of electrolyte tetrabutylammonium tetraphenylborate (TBATPB). The half‐wave potential of ion transfer for phenylpropanolamine accross the water|NB interface was found 465.3 mV. The peak separation, the diffusion coefficient, and the standard ion transfer potential of PPAH⁺ were observed to be 59.1 mV, 1.7 × 10^?6 cm²/s, and 104.6 mV, respectively. The temperature of experiment was kept constantly at 25 ± 1 °C using water flow thermostate.     

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.     

Evaluation of Thin Film Titanium Nitride Electrodes for Electroanalytical Applications

Titanium nitride is a hard and inert conducting material that has yet not been widely used as electrode material for electroanalytical applications although there are highly developed protocols available to produce well adherent micro and nanostructured electrodes. In this paper the possibilities of using titanium nitride thin films for electroanalytical applications is investigated. Scanning electrochemical microscope (SECM) was used for analysis of the redox kinetics of a selected fast redox couple at thin films of titanium nitride (TiN) in different thicknesses. The investigation was carried out by approaching an amperometric ultramicroelectrode (UME) to the TiN film while the soluble redox couple (ferrocenemethanol/ferrociniummethanol) served as mediator in a SECM configuration. The substrate was biased at a potential so that it rereduces the species being produced at the UME, thus controlling the feedback effect. Normalized current–distance curves were fitted to the theoretical model in order to find the apparent heterogeneous standard rate constant (k°) at the sample. The data are further supported by structural investigation of the TiN films using scanning force microscopy and X‐ray photoelectron spectroscopy. It was found that the kinetics are little influenced by prolonged storage in air. The heterogeneous standard rate constants in 2 mM ferrocenemethanol were (0.73±0.05)×10^?3 cm s^?1 for 20 nm TiN thin layer, (1.5±0.2)×10^?3 cm s^?1 for 100 nm TiN thin layer and (1.3±0.2)×10^?3 cm s^?1 for 300 nm TiN thin layer after prolonged storage in air. Oxidative surface treatment (in order to remove organic adsorbates) decreased the kinetics in agreement with a thicker oxide layer on the material. The results suggest that their direct use for amperometric detection of reversible redox systems in particular at miniaturized configurations may be advantageous.     

Hydrodynamic Modulation Voltammetry with a Dual Disk Chopped Flow‐Microjet Electrode (CF‐MJE)

A novel form of hydrodynamic modulation voltammetry (HMV) is described, based on the periodic variation of mass transport in a microjet electrode (MJE) system, in combination with phase‐sensitive detection techniques. In the configuration developed, a jet of solution is fired from a nozzle that is aligned directly over the surface of a dual disk Pt‐Pt ultramicroelectrode (UME). The potential at each electrode is controlled separately. A rotating blade, positioned between the nozzle and the UME probe, is used to periodically interrupt flow to the electrode surface, resulting in modulation of the overall mass transfer rate between two defined extremes. The use of a dual disk UME enables two transport‐limited current signals to be recorded simultaneously, one for the analyte of interest, and the other for a ‘reference species’ (oxygen for the studies described herein). The latter current response corresponds to the variation in mass transport rate in the chopped flow (CF) arrangement and is used as the signal for phase sensitive detection of the analyte current. Studies of potassium hexachloroiridate (III) [IrCl ] oxidation in aqueous solution are used to demonstrate the capabilities of the technique. HMV in the CF‐MJE arrangement allows quantitative concentration measurements, down to at least 5×10^?7 M.     

Electrochemical detection for liquid chromatography using the wall-jet cell/ultramicroelectrode detector

A carbon fibre array and a platinum thin ring ultramicroelectrode were constructed and used in conjunction with a large volume wall-jet cell as an amperometric detector in liquid chromatography. Electrode characterization studies were performed in 5.00 mM K₃Fe(CN)₆ + 1 M KCl aqueous solution. It was found that the wall-jet cell/ultramicroelectrode combination has much reduced flow rate dependence compared to the wall-jet cell/conventional electrode system. LC application in the absence of supporting electrolyte was found to be possible and have good sensitivity using the ultramicroelectrode. Detection limits were 100 pg and 10 ng for the platinum thin ring and the carbon fibre array electrodes respectively.     

Impacts of Forced Convection Generated via High Precision Stirring on Scanning Electrochemical Microscopy Experiments in Feedback Mode

In this study, the effects of forced convection on scanning electrochemical microscopy (SECM) experiments in feedback mode using ferrocenemethanol as redox mediator are presented. Forced convection, which enhances the mass transfer inside the system, was generated via an electrical high precision stirrer integrated into the SECM setup. A thin‐film interdigitated array electrode serving as model substrate was investigated with probe scan curves in z‐direction and SECM imaging in constant height mode utilizing ultramicroelectrodes (UME) with diameters (d_probe) of 25 μm and 12.5 μm. It was found that forced convection increased the overall current during SECM imaging without distorting distinctive features of the imaged structure when working with a 25 μm UME at substrate‐to‐tip distances of 14 μm and 11 μm. Furthermore, the electrochemical contrast was improved under hydrodynamic conditions for a substrate‐to‐tip distance of 11 μm and scan rates of 5 μm s^?1, 10 μm s^?1, 20 μm s^?1 and 40 μm s^?1. When further decreasing the gap between the UME and the substrate to 9 μm almost no effects of the forced convection were observed. Consequently, for a 25 μm UME, forced convection led to higher currents and improved performance during SECM experiments in feedback mode at substrate‐to‐tip distances of 14 μm and 11 μm, whereas no effects were observed for a 12.5 μm UME at a distance of 8 μm.     

Detection of haemoglobin using an adsorption approach at a liquid–liquid microinterface array

The behaviour of haemoglobin (Hb) at the interface between two immiscible electrolyte solutions (ITIES) has been examined for analytical purposes. When Hb is fully protonated under acidic conditions (pH <pI) in the aqueous phase, it undergoes a potential-dependent adsorption and complexation, at the interface, with the anions of the organic phase electrolyte. When utilised as a simple and fast preconcentration step, consisting of adsorbing the protein at the interface, in conjunction with voltammetric desorption, this opens up the ITIES to the adsorptive stripping voltammetry approach. Utilising a 60 s adsorption step and linear sweep voltammetry, a linear response to Hb concentration in aqueous solution over the range 0.01–0.5 μM was achieved. The equation of the best-fit straight line was I _p ? =?7.46 C???0.109, R?=?0.996, where I _p is the peak current (in nanoampere) and C is haemoglobin concentration (in micromolar). The calculated detection limit (3σ) was 48 nM for a 60 s preconcentration period, while the relative standard deviation was 13.3 % for six successive measurements at 0.1 μM Hb. These results illustrate the prospects for simple, portable and rapid label-free detection of biomacromolecules offered by electrochemistry at arrays of liquid–liquid microinterfaces.     

Fabrication of Prussian Blue modified ultramicroelectrode for GOD imaging using scanning electrochemical microscopy

A Prussian Blue (PB) film modified disk ultramicroelectrode (UME) was fabricated by electrochemical deposition technique on a Pt-disk UME. The electrocatalytical reductions of hydrogen peroxide derived from glucose oxidase (GOD) on this modified UME were investigated. The enzymatic biochemical reactivity was imaged by scanning electrochemical microscopy (SECM) utilizing the PB film modified UME. It is evident that sensitivity and spatial resolution for hydrogen peroxide measurement were improved obviously. SECM images obtained clearly revealed the concentration profile of the reaction products around the enzymes. The PB film modified microelectrode is in the nature of simple preparation, high catalytic activity on hydrogen peroxide and substrate selectivity for SECM etc.     

Measurement of the forward and back rate constants for electron transfer at the interface between two immiscible electrolyte solutions using scanning electrochemical microscopy (SECM): Theory and experiment

A new numerical model is developed for the scanning electrochemical microscopy (SECM) feedback mode for reversible electron transfer (ET) processes at the interface between two immiscible electrolyte solutions (ITIES). Results from this model were compared with data obtained using an earlier SECM feedback model in which the back reaction was not considered, to identify when the latter will be important. The dependence of the ET rate constant for the oxidation of 7,7,8,8-tetracyanoquinodimethane radical anion (TCNQ⁻) in 1,2-dichloroethane (DCE) by aqueous ferricyanide on the interfacial potential drop (Δ_w^oφ) was studied using SECM. The Δ_w^oφ value was varied by changing the concentration of NaClO₄ in the aqueous phase while a fixed concentration of organic electrolyte, tetra-n-hexylammonium perchlorate, was used in the DCE phase. The results obtained were compared to earlier published studies on the forward reaction between TCNQ in DCE and aqueous ferrocyanide. Both the forward and back ET rate constants were found to depend strongly on the interfacial potential drop, with measured ET coefficients in the region of 0.5–0.6. A similar ET rate constant was observed at zero driving force for both the forward and back reactions. These experimental results suggest that the Butler–Volmer model applies to ET at the ITIES, when the driving force for the reaction is low, and under conditions of relatively high ionic strength in both the aqueous and organic phases.     

Fabrication of Active Horseradish Peroxidase Micropatterns with a High Resolution by Scanning Electrochemical Microscopy

We used a new reactive species OH^? to fabricate active horseradish peroxidase (HRP) micropatterns with a high resolution by scanning electrochemical microscopy (SECM) coupled with a carbon fiber disk electrode as the SECM tip. In this method, except for active HRP micropatterns predesigned other regions on a HRP‐immobilized substrate were deactivated by OH^? generated at the tip held at ?1.7 V in 1.0 mol/L KCl containing 2.0×10^?3 mol/L benzoquinone (BQ) (pH 8.0). The feedback mode of SECM with a tip potential of ?0.2 V was used to characterize the active HRP micropatterns in 1.0 mol/L KCl containing 2.0×10^?3 mol/L BQ and 2.0×10^?3 mol/L H₂O₂.     

Investigation of Active and Inactivated Yeast Cells by Scanning Electrochemical Impedance Microscopy

Scanning electrochemical microscopy (SECM), electrochemical impedance spectroscopy (EIS) and scanning electrochemical impedance microscopy (SEIM) were used to investigate electrochemical activity of active and inactivated yeast Saccharomyces cerevisiae cells. SEIM experiment was performed using a unique electrochemical impedance spectrometer with a fast Fourier transform (FFT‐EIS) function, which enabled simultaneously perturb/evaluate electrochemical system at 50 frequencies. This allowed very quick observing the differences between impedance spectra, which were taken every few seconds. Therefore, we were able to apply SEIM for relatively fast determination of electrochemical impedance dependence on the distance between ultramicroelectrode (UME) and surface modified by immobilized yeast cells. It was determined that electrochemical activity and ‘breathing’ (a consumption of dissolved oxygen) of yeast can be electrochemically observed when the distance between UME and surface of yeast cells is in the range from 0 μm to 25 μm. Therefore, 25 μm is the maximum distance suitable for efficient investigation of yeast cell activity when experiments are performed in FFT‐SEIM mode. Charge transfer resistance of active and inactivated yeast cells was determined using EIS. It was calculated that charge transfer resistance of active yeast cells is 1.5 times lower than that of inactivated yeast cells. Lipophilic vitamin K₃ (Vit‐K₃) and hydrophilic vitamin K₁ (Vit‐K₁) were mixtured and used as redox mediators for charge transfer from yeast cells.     

Influence of the block geometry on the voltammetric response of partially blocked electrodes: Application to interfacial liquid–liquid kinetics of aqueous vitamin B12S with random arrays of femtolitre microdroplets of dibromocyclohexane

The cyclic voltammetric response of electrodes modified with catalytically reactive blocks is simulated using finite difference methods. The responses of three different models using various block geometries are studied. The results are used to determine kinetic parameters of coupled liquid|liquid interfacial reactions. First, we examine the liquid–liquid reaction between aqueous vitamin B_12S and pure trans-dibromocyclohexane (DBCH) microdroplets immobilized on a basal plane pyrolytic graphite (bppg) surface, immersed in an aqueous solution of vitamin B₁₂. Second, cyclic voltammetry on electrodes modified with microdroplets of DBCH diluted in dodecane is employed to determine the apparent bimolecular interfacial rate constant for the initial step in the DBCH(oil)/B_12S(aq) reaction. The results are compared with a previous SECM/ITIES study of a similar reaction.     

Scanning Electrochemical Microscopy as an In Vitro Technique for Measuring Convective Flow Rates Across Dentine and the Efficacy of Surface Blocking Treatments

Scanning electrochemical microscopy (SECM) is shown to be a powerful technique for both the measurement of local solution velocities through human dentine slices, in vitro, and for assessing quantitatively the effect of surface treatments on the flow process. SECM employs a small ultramicroelectrode (micron dimensions) as an imaging probe to provide information on the topography and transport characteristics of dentine, with high spatial resolution. In these studies the dentine sample is a membrane in a two compartment cell, which contains solutions of identical composition, including a redox active mediator (Fe(CN) . In the absence of an applied pressure, the transport‐limited current response at the probe electrode is due to diffusion of Fe(CN) to the UME, which depends on the probe to sample separation. Under an applied hydrostatic pressure, hydrodynamic flow across the sample enhances mass transport to the UME. With this methodology it was possible to accurately measure effective fluid velocities, by recording tip currents with and without pressure, and assess the efficacy of potential flow retarding agents for the treatment of dentinal hypersensitivity. For native dentine, the solution velocity was found to vary dramatically with location on the sample. The application of a glycerol monooleate ‐ base paste treatment to the surface of dentine was found to lower local flow velocities significantly. This electroanalytical methodology is simple to implement and is generally applicable to assessing the efficacy and mode of action of a wide variety of potential fluid flow retarding agents.     

In situ electrochemical studies for Li+ ions dissociation from the LiCoO2 electrode by the substrate-generation/tip-collection mode in SECM

This work presents the transportation of Li⁺ ions at the interface of a charging LiCoO₂ electrode through the substrate-generation/tip-collection (SG/TC) feedback mode of scanning electrochemical microscopy (SECM). The TC current, due to the reduction of the ethylene carbonate (EC) supermolecule, is collected more strongly at 1.8 V than that of the Li⁺(DEC) _n at 2.5 V near at the substrate because of the increased concentration of the supermolecule Li⁺(EC)_m, which means that the electrolyte is not uniformly distributed over the substrate. The smooth SG/TC current loop is formed at the probe position optimized by the probe scan curve technique between the LiCoO₂ substrate with 4.0 V and the probe with 1.8 V, which is applied to analyze the Li⁺ ion transport at the interface of the LiCoO₂ electrode. Moreover, the LiCoO₂ substrate, which has a flat surface, is imaged to the nonuniform surface electrochemically by the SECM. We infer that these experimental techniques will help analyze transporting Li⁺ ions at the interface and the electrochemical uniformity of the electrode.     

纳米尺度扫描电化学显微镜的探针制备及在电催化氧和氢反应研究中的应用

扫描电化学显微镜是一种在检测样品表面物理形貌的同时能提供丰富的电化学信息的扫描探针技术,由于超微电极的引入,它可以高时空分辨率地探究各类样品的物理形貌和电化学性能之间的构效关系. 随着现代纳米科技的不断发展,扫描探针的尺寸也逐渐从亚微米发展到纳米级别. 与此同时,高效优选各类氧反应和氢反应电催化材料,明晰其电化学反应过程和性能是二十一世纪绿色新能源转换存储系统（如可再生燃料电池、金属空气电池等）的重要研究方向. 本文首先概括了可应用于扫描电化学显微镜的纳米级扫描探针的制备及发展,之后着重介绍了近四年纳米尺度扫描电化学显微镜在电催化氧反应和氢反应研究中的一些最新研究进展. 最后以点窥面,对未来纳米尺度扫描电化学显微镜的未来发展趋势作了展望.     

Imaging the stomatal physiology of somatic embryo-derived peanut leaves by scanning electrochemical microscopy

The stomatal physiology, chlorophyll distribution and photosynthetic activity of somatic embryo (SE)- and seedling-derived peanut plants grown in vitro (test tube-grown) and extra vitrum (soil-grown) are investigated using scanning electrochemical microscopy (SECM). This SECM imaging is performed in two different feedback modes, corresponding to oxygen evolution and chlorophyll distribution. More specifically, the oxygen evolution profiles of the in vitro leaves indicate important differences in leaf anatomy between the SE- and seedling-derived leaves. On the other hand, the chlorophyll distribution images show individual stomata of size ca. 27 ± 5μm. Further studies on senescing (aged) leaves reveal interesting voltammograms that vary widely over the stomatal complexes and the surrounding tissues, probably due to the release of electroactive metabolites during chlorophyll breakdown when the leaves turn yellow. Thus, the present investigation could open up new opportunities for characterizing botanical systems using electroanalytical techniques. In addition, it could provide further insights into various areas of current relevance, including signal transduction, cell fate/differentiation and developmental biology. Schematic representation of SECM imaging used in this investigation. The SECM probe is a Pt UME disk (25 μm diameter) embedded in an insulating glass sheath so that the ratio of the diameter of the death to that of the electrode surface (RG) is 7. RE denotes the reference electrode Ag/AgCl, sat. KCl and CE refers to the counter electrode, a Pt wire. Oxygen evolving from the leaf surface during photosynthesis diffuses into the electrolyte (0.1 M KCl) and gets reduced at the Pt UME, biased to a potential of −0.5 V, at a diffusion-limited rate to produce a change in the tip-current     

New adsorptive square wave method for trace determination of prilocain in the flow injection system by a fast fourier analysis

The determination of prilocain, used to manage tonic-clonic seizures, has been carried out at micro gold electrode (Au UME) using continuous fast Fourier transform square wave voltammetry. The Au UME electrode exhibited an effective response towards prilocaine adsorption. The peak current was also found to be significantly increased. The determination was carried out in phosphate containing electrolyte in the pH of 2.0 and a well-defined change on the peak current were noticed. The peak current was found to be linearly dependent on concentration of prilocain in the concentration range 5.0 × 10⁻⁷–1.0 × 10⁻¹¹ M with a detection limit of 5.0 × 10⁻¹² M. This paper describes development of a new analysis system to determine of prilocain by a novel square wave voltammetry method to perform a very sensitive method. The method used for determination of prilocain by measuring the changes in admittance voltammogram of a gold ultramicroelectrode (in 0.05 M H₃PO₄ solution) caused by adsorption of the prilocain on the electrode surface. Variation of admittance in the detection process is created by inhibition of oxidation reaction of the electrode surface, by adsorbed of prilocain. Furthermore, signal-to-noise ratio has significantly increased by application of discrete Fast Fourier Transform (FFT) method, background subtraction and two-dimensional integration of the electrode response over a selected potential range and time window. Also in this work some parameters such as SW frequency, eluent pH, and accumulation time were optimized. The relative standard deviation at concentration 5.0 × 10⁻⁸ M is 5.8% for 5 reported measurements.     

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.     

Imaging Local Proton Fluxes through a Polycarbonate Membrane by Using Scanning Electrochemical Microscopy and Functionalized Alkanethiols

A new application of scanning electrochemical microscopy (SECM) to probe the transport of protons through membranes is described. Herein, a probe ultramicroelectrode (UME) is modified with a self‐assembled monolayer (SAM) of 11‐mercaptoundecanoic acid to qualitatively image areas within different pH regions above a track‐etched membrane. The current response of the modified electrode in the presence of potassium hexacyanoferrate as electroactive component is different in acidic and alkaline solutions. Depending on the pH value of the solution, the SAM‐covered electrode exposes either a neutral or a negatively charged insulating monolayer at pH 3 or 7, respectively, which leads to an increase/decrease in the faradaic current due to electrostatic interactions between the neutral/charged surface and the charged redox mediator. Therefore, local pH changes in the close vicinity of a membrane‐like substrate lead to different current responses recorded at the tip electrode when scanning above the surface.