Electrochemical and impedance spectroscopy studies of various diazonium salts on a glassy carbon electrode

Abstract  

The derivatization of a glassy carbon electrode surface was achieved with and without electrochemical reduction of various diazonium salts in acetonitrile solutions. The surfaces were characterized, before and after their attachment, by cyclic voltammetry and electrochemical impedance spectroscopy to evidence the formation of a coating on the carbon surface. The results were indicative of the presence of substituted phenyl groups on the investigated surface. Also, the effects of diazonium thin films at the surface of a glassy carbon electrode, modification time, and salt concentration on their electrochemical responses in the presence of the Fe(CN)₆^3−/4− probe were investigated. Electrochemical impedance measurements indicated that the kinetics of electron transfer is slowed down when the time and the concentration used to modify the glassy carbon electrode are increased. We therefore modified a glassy carbon surface via its derivatization with and without electrochemical reduction of various diazonium salts in acetonitrile solution.     

Electrochemical impedance spectroscopy at single-walled carbon nanotube network ultramicroelectrodes

Electrochemical impedance spectroscopy (EIS), coupled with chemical vapour deposition (CVD) grown single-walled carbon nanotube (SWNT) network disk-shaped ultramicroelectrodes (UMEs), gives stable, very well-defined and highly reproducible EIS responses for electrolysis of a simple outer sphere redox couple (FcTMA^+/2+). The resulting EIS data can be fitted accurately using a simple electrical circuit model, enabling information on double-layer capacitance, diffusion coefficient of the electroactive species and the rate constant of ET (k⁰) to be extracted in a single EIS experiment. These values are replicated for a range of mediator concentrations and UME sizes (in the range 25–100 μm diameter) demonstrating the robustness of the method. These initial studies bode well for impedance based electroanalysis using SWNT network UMEs.     

Electrochemical impedance study of hydrogen evolution on Bi(001) electrode in the HClO4 aqueous solutions

Electrochemical impedance spectroscopy has been applied for investigation of the hydrogen evolution kinetics at the electrochemically polished Bi(001) plane, and the complicated reaction mechanism (slow adsorption and charge-transfer steps) has been established. The charge-transfer resistance and adsorption capacitance values depend noticeably on the electrode potential applied. The adsorption resistance is maximal in the region of electrode potential E _min = −0.65 V vs. (Hg|Hg₂Cl₂|4 M KCl), where the minimal values of constant phase element (CPE) coefficient Q have been calculated. The fractional exponent α _CPE values of the CPE close to unity (α _CPE ≥ 0.94 and weakly dependent on the electrode potential and pH of solution () have been obtained, indicating the weak deviation of Bi(001)|HClO₄ + H₂O interface from the ideally flat capacitive electrode. Q differs only very slightly from double-layer capacitance C _dl values in the whole region of potentials and , investigated.     

Electrochemical impedance spectroscopy in interfacial studies

A fundamental aim in the field of electrochemistry is to investigate electron transfer events caused by electrode processes, which are more commonly described as redox reactions. In this short review, an overview of the use of electrochemical redox reactions in the realm of organic synthesis is given. These reactions can be divided into three subcategories: cathodic reduction, anodic oxidation, and a paired approach. This short review illustrates the basic schemes of these reactions and introduces representative examples that have been reported in the past 2 years, with a particular emphasis on the development of novel reactions.     

Electrochemical impedance spectroscopy analysis for oxygen reduction reaction in 3.5% NaCl solution

The electrochemical impedance spectroscopy (EIS) at different potentials has been used to study the oxygen reduction reaction (ORR) in 3.5% NaCl solution on glassy carbon (GC) electrode in this work. Results show that ORR consists of three two-electron reaction steps and both superoxide ion (O₂ ^–) and hydrogen peroxide (H₂O₂), which are produced by ORR, obstruct the diffusion of oxygen to the surface of the electrode and make the EIS results change into a transmissive finite diffusion process with the real part contraction and a reflective finite diffusion process from a semi-infinite diffusion process. The values of electron transfer resistance (R _t) and diffusion resistance (R _d) were calculated from EIS. O₂ ^– influenced strongly on the R _t values and induced a maximum at −0.45 V.     

Electrochemical impedance spectroscopy study of proteolysis using unmodified gold nanoparticles

Proteases are involved in numerous cell functions and abnormal proteolysis may lead to a diversity of serious diseases. Herein, a simple electrochemical method is developed to study proteolysis by employing unmodified gold nanoparticles (AuNPs). Substrate of a protease is modified on a gold disk electrode, forming a barrier for electrochemical species and reflecting a significant charge transfer resistance (R_ct). After the proteolysis process, the substrate can be cleaved coupled with the decline of R_ct. The electrical properties of the substrate residues on the electrode may also change, leading to the subsequent adsorption of AuNPs. Due to the excellent electrical conductivity of AuNPs, R_ct can be further decreased, which can be used to reveal the proteolysis process. The proposed method allows the determination of the model protease, trypsin, with desirable sensitivity and specificity. It may also hold great potential use in the study of other proteolysis processes and some biomedical applications in the future.     

Electrochemical impedance spectroscopy of polynucleotide adsorption

The dependence of the impedance of the electrode double layer of mercury electrode on frequency around the potentials of the tensammetric peaks of single-stranded and double-helical polynucleotides and DNA was studied. From the frequency dependence of the impedance of the electrode double layer represented in a complex plane impedance plot, the electric equivalent circuit of the electrode covered with adsorbed DNA layer was determined. It was concluded that the desorption of denatured ssDNA is accompanied by higher dielectric losses than the desorption of native dsDNA. This can be explained by the higher flexibility of ssDNA compared to the dsDNA. The capacitance peak of single-stranded polyadenylic acid (poly A) observed at pH 8 around -1.3 V splits at low frequencies in two peaks.     

Electrochemical impedance spectroscopy of ZnO nanostructures

The paper reports on the use of electrochemical impedance spectroscopy to determine the doping character and carrier density of freshly prepared and annealed ZnO nanostructures. The ZnO nanostructures were obtained by chemical oxidation of metallic Zn in a 5% N,N-dimethylformamide (DMF) aqueous solution at 95 °C for 24 h. The as-grown nanostructured ZnO samples display a high donor density of 3.71 ± 0.88 × 10²¹ cm^?3. Annealing at 100 and 200 °C did not have any effect on the donor density while thermal annealing at 300 °C in air for 1 h induced a decrease in the doping concentration without affecting the surface morphology.     

Electrochemical impedance analysis of nanoporous TiO_2 electrode at low bias potential

<正>TiO_2 colloid was prepared by the sol-gel method and was bladed on transparent conduction glass to made nanoporous electrode. The impedance performance of TiO_2 electrode was studied at various bias potential.A simplified equivalent circuit was proposed to investigate the charge transport impedance of TiO_2 film and good fitting results were obtained.     

Electrochemical behavior of the system ferricyanide-ferrocyanide at a graphite-epoxy composite electrode

Cyclic and direct voltammetry with linear potential sweep has been used for the investigation of the dependence of the reversibility and reduction current in the system Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ on the concentrations of LiCl, NaCl, KCl, and CsCl solutions. The electrode was made of a graphite-epoxy composite and activated by mechanically cutting a surface layer directly in the solution and deactivated by the long-term storage in the air. The selected type of the graphite electrode and the method used to activate its surface provides the reversibility and diffusion control of the electrode process in the system Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ regardless of the composition of the supporting solution. In the case of the deactivated electrode, the degree of irreversibility of this process depends on the form and concentration of metal chloride in the supporting electrolyte and the diffusion transfer is complicated by the adsorption of compounds formed between the ferricyanide and the cation of the supporting solution.     

Electrochemical impedance spectroscopy of tethered bilayer membranes

The electrochemical impedance spectra (EIS) of tethered bilayer membranes (tBLMs) were analyzed, and the analytical solution for the spectral response of membranes containing natural or artificially introduced defects was derived. The analysis carried out in this work shows that the EIS features of an individual membrane defect cannot be modeled by conventional electrical elements. The primary reason for this is the complex nature of impedance of the submembrane ionic reservoir separating the phospholipid layer and the solid support. We demonstrate that its EIS response, in the case of radially symmetric defects, is described by the Hankel functions of a complex variable. Therefore, neither the impedance of the submembrane reservoir nor the total impedance of tBLMs can be modeled using the conventional elements of the equivalent electrical circuits of interfaces. There are, however, some limiting cases in which the complexity of the EIS response of the submembrane space reduces. In the high frequency limit, the EIS response of a submembrane space that surrounds the defect transforms into a response of a constant phase element (CPE) with the exponent (α) value of 0.5. The onset of this transformation is, beside other parameters, dependent on the defect size. Large-sized defects push the frequency limit lower, therefore, the EIS spectra exhibiting CPE behavior with α ≈ 0.5, can serve as a diagnostic criterion for the presence of such defects. In the low frequency limit, the response is dependent on the density of the defects, and it transforms into the capacitive impedance if the area occupied by a defect is finite. The higher the defect density, the higher the frequency edge at which the onset of the capacitive behavior is observed. Consequently, the presented analysis provides practical tools to evaluate the defect density in tBLMs, which could be utilized in tBLM-based biosensor applications. Alternatively, if the parameters of the defects, e.g., ion channels, such as the diameter and the conductance are known, the EIS data analysis provides a possibility to estimate other physical parameters of the system, such as thickness of the submembrane reservoir and its conductance. Finally, current analysis demonstrates a possibility to discriminate between the situations, in which the membrane defects are evenly distributed or clustered on the surface of tBLMs. Such sensitivity of EIS could be used for elucidation of the mechanisms of interaction between the proteins and the membranes.     

Electrochemical study of potassium isobutyl xanthate at a mercury electrode.

Polarographic studies of potassium isobutyl xanthate at a mercury electrode reveal that the product of an anodic reaction is strongly adsorbed at the mercury surface, as indicated by a prewave. The adsorbed film greatly affects the characteristics of the anodic wave of xanthate in an aqeous medium. The current of total wave is proportional to the concentration of xanthate from 0.32 to 1.6 mM.     

Electrochemical study of PEDOT-PSS-MDB-modified electrode and its electrocatalytic sensing of hydrogen peroxide

A method to fabricate poly(3,4-ethylene dioxythiophene)-poly(4-styrene sulfonate)-Meldola Blue (PEDOT-PSS-MDB)-modified electrodes had been disclosed. Firstly, the PEDOT-PSS-film-modified electrode was electrochemically prepared. Then, the PEDOT-PSS was treated as a matrix to immobilize electroactive mediator, Meldola Blue (MDB), by means of an electrostatic interaction to form the proposed film, PEDOT-PSS-MDB. Electrochemical properties of the proposed film exhibited surface confinement and pH dependence. The PEDOT-PSS-MDB electrode could electrocatalytically reduce hydrogen peroxide (H₂O₂) with a low overpotential and showed a linear response to H₂O₂ in the concentration range of 5 to 120 μM, detection limit of 0.1 μM, and sensitivity of 353.9 μA mM⁻¹ cm⁻² (S/N = 3). By comparison, the electrocatalytic activity of PEDOT-PSS-MDB electrode was found superior to that of PEDOT-PSS and MDB-PSS electrodes. It also has competitive potential as compared with other mediators, through the use of HRP to determine H₂O₂. Moreover, the potential interferents such as ascorbic acid, dopamine, uric acid, and glucose were also studied for H₂O₂ determination by the proposed film.     

Electrochemical study of a soluble niobium anode in molten salts

Electrochemical studies of niobium were performed in various molten salts, including NaClKClKF, LiClNaFKF and LiFNaFKF. The cathodic current efficiency increased with increasing number of fluoride components, as did the quality of the metal deposited at the cathode. Thus, the order of cathodic current efficiency was NaClKCl < LiClNaClKCl < NaClKClKF < LiClNaFKF < LiFNaFKF, using equimolar mixtures of the salts. The possible electrode processes were determined to be

and the overall reaction was

in terms of m₁and m₂ mol.     

Electrochemical pressure impedance spectroscopy applied to the study of polymer electrolyte fuel cells

The development of novel in-situ diagnostic techniques allows new insight into the internal working of polymer electrolyte fuel cells (PEFCs) so that improved performance can be realised. Electrochemical impedance spectroscopy (EIS) is a widely used characterisation technique that takes advantage of the dynamic relationship between current and voltage to deconvolute critical mechanisms and sources of performance loss occurring with different time constants. Here, we apply electrochemical pressure impedance spectroscopy (EPIS) which examines the transfer function relating reactant gas pressure modulation to the electrical response of the fuel cell. A sinusoidally oscillating perturbation is applied to the cathode backpressure using a loudspeaker arrangement and the resulting voltage perturbation is monitored. It is shown that the technique can be used to separate the explicit effect of water management from reactant starvation when a PEFC is operated under different reactant humidification conditions.