Subdiffusion behavior in poly(3,4-ethylenedioxythiophene): Polystyrene sulfonate (PEDOT:PSS) evidenced by electrochemical impedance spectroscopy

We have obtained impedance measurements of PEDOT:PSS films and propose a novel interpretation for electrical charge transport mechanisms in the polymer. A parabolic subdiffusion model produced excellent fitting for the impedance data, suggesting that anomalous ion diffusion limits charge transfer through the nanoporous film.     

Monitoring bacterial-demineralization of human dentine by electrochemical impedance spectroscopy

Objective

The purpose of this study was two-fold: (1) to monitor bacterial biofilm formation and bacteria-induced demineralization of dentine in situ by using electrochemical impedance spectrum (EIS); (2) to examine the relationship between EIS findings and changes in the chemical composition and ultrastructure of dentine during bacteria-induced demineralization.

Methods

In this study, dentine demineralization was induced by Streptococcusmutans (ATCC 25175) in the presence of sucrose in culture medium and was monitored using two EIS measurement systems (Type A with a working electrode and Type B without a working electrode). Scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) and X-ray diffraction (XRD) were employed to examine the morphology, element contents and crystallinity of hydroxyapatite (HAP) on the dentine surface. Transverse microradiography (TMR) was used to characterize the lesion depth and degree of mineral loss during demineralization.

Results

The resistance of the bulk dentine (R_d) and the apparent resistance of dentine (R_a) measured from the Type A and Type B EIS systems, respectively, decreased gradually with demineralization. The resistance of the biofilm formed on dentine surface was determined by fitting the EIS data with equivalent circuits. The presence of biofilm slightly increased R_a of dentine before demineralization. However, the electrochemical behavior of biofilm did not affect the decreasing impedance of dentine with demineralization. The SEM, EDX, XRD and TMR results demonstrated that the surface and bulk dentine gradually became more porous due to the loss of minerals during demineralization, which in turn resulted in the decrease in R_d and R_a values obtained from EIS systems.

Conclusions

This investigation highlighted EIS as a potential technique to monitor biofilm formation and bacterial-induced demineralization in situ.     

PbTe electrodeposition studied by combined electrochemical quartz crystal microbalance and cyclic voltammetry

Electrodeposition of PbTe thin films from alkaline solutions, and the electrochemical behavior of the related precursors TeO₃^2? and PbEDTA^2?, were studied for the first time by means of an electrochemical quartz crystal microbalance (EQCM) combined with cyclic voltammetry. PbTe was found to form by an induced codeposition mechanism via six electron reduction. The reduction mechanism of Te was complicated and sensitive to the substrate surface. On the other hand, the reduction of the PbEDTA^2? complex to Pb⁰ was a simple two electron reaction.     

Studies of structural disorder of self-assembled thiol monolayers on gold by cyclic voltammetry and ac impedance

Self-assembly of octadecyl mercaptan on gold has been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV experiments show that well-assembled thiol monolayers on gold are essentially free of pinhole defects. Support for the existence of ‘collapsed’ sites in thiol monolayers is obtained by comparing the apparent electron transfer rate constant, which is obtained from CV experiments at a film covered Au electrode, with the theoretical one, which is calculated by assuming electrons tunnel across the collapsed-site-free monolayer. The empirical relationship Z=R_s+1/[σ(jω)^α] is used to describe the film coated Au in inert electrolyte and the degree of disorder of the monolayer structure is estimated from the α value. It is demonstrated that the admittance plane plot of an ideally polarizable electrode with constant phase element behavior is a circular arc which can be used to determine α. Results of EIS experiments show that α increases sharply after the initial adsorption time and approaches its final value slowly. This is attributed to the rapid adsorption step followed by a slow crystallization process in the kinetics of alkanethiol adsorption onto the Au electrode. For a well-assembled thiol monolayer on gold, α can approach 0.99, which means that the number of collapsed sites is not large and the surface of the monolayer Au electrode is rather smooth.     

Monitoring acid-demineralization of human dentine by electrochemical impedance spectroscopy (EIS)

Objective

The purpose of this study was to monitor in situ acid-induced demineralization of dentine by using electrochemical impedance spectroscopy (EIS) and correlate the EIS findings with changes in the chemical composition and ultrastructure of dentine.

Method

EIS was used to monitor the process of demineralization of dentine induced by an acid model. Scanning electron microscopy (SEM) was used to examine the ultrastructure, while energy dispersive X-ray (EDX) and X-ray diffraction (XRD) analysis were employed to investigate the changes in the chemical composition of surface dentine with demineralization.

Results

Two kinds of equivalent circuits that matched the histological structure of dentine were used to fit the EIS data. The parameters R_d, which is the resistance of the bulk of dentine layer and R_ct, which is the charge transfer associated with the penetration of electrolyte into the dentinal tubules, decreased with increasing duration of demineralization. SEM images showed that the smear layer on the dentine and peritubular dentine disappeared with demineralization. The EDX results showed that the content of calcium and phosphorus decreased consistently with the decreasing content of HAP as shown by the XRD results. The findings from this study suggested that the changes in R_d and R_ct determined by the EIS corresponded well with the variation in structure and composition of dentine.

Conclusions

EIS could be employed to monitor structural and chemical compositional changes induced by acid-demineralization on dentine surface.     

镍铬、钴铬合金和纯钛在人工唾液中的耐腐蚀性评价

目的:用电化学交流阻抗谱方法研究钴铬、镍铬合金和纯钛在人工唾液中的耐腐蚀性。方法:用电化学工作站测量出3种金属材料在人工唾液中的电化学阻抗谱(Bode图、Nyquist图),利用ZSimpWin软件对数据进行拟合分析,根据等效电路R(CR)拟合结果来评估3种金属的耐腐蚀性能。结果:从Nyquist图可见,3种材料的容抗弧半径大小依次为纯钛>钴铬合金>镍铬合金;从Bode图见,3种材料均为一个时间常数,即表面均是一个电容层。根据等效电路R(CR),3种材料的电容层无明显区别,但三者的阻抗值均很大,其中纯钛的阻抗值最大,钴铬合金的阻抗值居中,镍铬合金的阻抗值最小。结论:3种材料在人工唾液中均具有良好的耐腐蚀性,其耐腐蚀性能顺序是纯钛>钴铬合金>镍铬合金。     

Kinetics of CH3OH oxidation on PtRu/C studied by impedance and CO stripping voltammetry

The kinetics of the CH₃OH oxidation reaction at 60 °C on well-alloyed platinum–ruthenium supported on carbon (Pt₅₀Ru₅₀/C) was studied by electrochemical impedance spectroscopy and compared with carbon-supported platinum (Pt/C). The reaction rate of the overall CH₃OH oxidation increased with increasing electrode potential for both Pt/C and PtRu/C. In the case of Pt/C, when the electrode potential was E ? 450 mV vs. RHE only a capacitive behavior was observed. Resistive and pseudo-inductive types of behavior were evident above 500 and 600 mV vs. RHE. In the case of PtRu/C, a similar change in behavior was observed, except that the two types of behavior were observed at 200 mV lower electrode potentials than for Pt/C. Correlation of the impedance data with pre-adsorbed carbon monoxide (CO_ad) stripping voltammetry allowed the understanding of the methanol oxidation reaction. The change in the reaction rate of the oxidation of CH₃OH to CO_ad as a function of the electrode potential as well as the promotional effect of Ru was evident from a change in the frequency where the frequency deviated from the ～90° phase angle. The change in the reaction rate of the oxidation of CO_ad to CO₂ as a function of the electrode potential as well as the alloying with Ru was evident from a change in the frequency where the phase angle approached zero.     

In situ tracing the process of human enamel demineralization by electrochemical impedance spectroscopy (EIS)

OBJECTIVE: The purpose of this study was to in situ characterize the demineralization (namely dissolution of hydroxyapatite) on the surface of the human enamel using electrochemical impedance spectroscopy (EIS) technology. METHODS: Fresh human third molars extracted without visible evidence of caries, were used in this study. After they were immersed in a demineralizing solution prepared from lactic acid and carboxy methyl cellulose sodium (Na-CMC) buffering at pH 4, demineralization happened on their surfaces. EIS of the specimens were measured at a series of immersed interval. X-ray diffractometer (XRD) were used to distinguish the microstructure of the surface layer of the specimens. The depositions that appeared in the demineralizing solution after 46 h immersion were analyzed by fourier transform infrared spectrometer (FTIR). RESULTS: XRD analysis revealed that the percentage of intensity (I%) of HAP gradually decreased with the elapsed immersing time, which indicated the dissolution of HAP columns of enamel. Nyquist spectra were fitted with an equivalent circuit characterized by some parameters, such as Q and R(p) (error<0.1%). Changes of the parameters' values revealed that the rate of demineralization accelerated at the forepart of the demineralization, but slowed down beyond 70 h immersion. CONCLUSIONS: The results suggested that EIS was proved to be a useful method for in situ investigating and in vivo detecting the demineralization of the enamel.     

Redox switching hysteresis in polyaniline–acetate systems: a search of molecular factors important for the dynamics of the polymer reaction

The CV redox hysteresis of three polyaniline–aqueous trihalogenoacetic acid systems has been analyzed. The polymer phases studied involve monocarboxylate monoanion bases characterized by similar pK_a values in water to narrow the differences in molecular interactions. The hysteresis diminishes by 70 mV with size of the anions – from trifluoroacetate, through trichloroacetate, to tribromoacetate. The diffusion coefficients of the trihalogenoacetate anions in the oxidized polyaniline do not differ significantly. Their values are of the order of 10^?10 cm²/s, which is one order of magnitude lower than for the chloride anion in polyaniline. Ex situ 1750–1050 cm^?1 Raman spectra of polyaniline–trihalogenoacetates show small noticeable systematic changes that correlate with the hysteresis behavior. Electrochemical impedance spectroscopy provides evidence for the chemical rate process involved in the redox switching of the polyaniline–tribromoacetate system.     

Investigation of potential inversion in the reduction of 9,10-dinitroanthracene and 3,6-dinitrodurene

In the case of two-electron reduction of a compound, potential inversion refers to the situation where introduction of the second electron occurs with greater ease than the first. That is E⁰₁?E⁰₂<0 where E⁰₁ and E⁰₂ are the standard potentials for the two steps of reduction. The extent of potential inversion in the reduction of 9,10-dinitroanthracene, 1, and 3,6-dinitrodurene, 2, has been assessed by steady-state microelectrode voltammetry, cyclic voltammetry under conditions of near-reversible behavior, cyclic voltammetry under conditions of quasireversible behavior and electrochemical impedance spectroscopy (EIS). The studies were conducted in acetonitrile at 298 K. Cyclic voltammetry under quasireversible conditions and EIS were most sensitive to small changes in E⁰₁?E⁰₂. The value of E⁰₁?E⁰₂ for 1 was found to be ?107 mV and that for 2 was ?280 mV.     

Analysis of the impedance spectra of Pt|poly(o-phenylenediamine) electrodes – hydrogen adsorption and the brush model of the polymer films

Electrochemical impedance spectra obtained for poly(o-phenylenediamine) films on platinum in contact with 0.5 M H₂SO₄ solution were analyzed on the basis of the brush model elaborated for conducting polymer films on electrodes. It is assumed that the polymer film consists of short and long polymer chains forming bundles in such a way that only a small part of the metal substrate is covered by the polymeric material. The analysis of the impedance spectra taken at several potentials in the region of hydrogen adsorption on platinum supports the validity of the model. The model has been tested also by varying the film thickness and the roughness of the platinum substrate.     

Studies on the redox behaviour of some polythiophene derivatives by impedance spectroscopy in symmetrical and asymmetrical configurations

The generation and transport of ionic and electronic charges in three polythiophene derivatives, namely poly(3-methyl thiophene), poly(thiophene-3-methanol) and poly(3-thienyl methacrylate), have been studied using impedance spectroscopy in both asymmetrical (Pt ∣ polymer film ∣ electrolyte) and symmetrical (Pt ∣ polymer film ∣ metal contact) configurations. The symmetrical measurements are to our knowledge reported for the first time in the literature for these polymer films. The homogeneous film model proposed by Vorotyntsev was found to describe adequately the impedance spectra for poly(3-methyl thiophene) in the asymmetrical configuration. For the two other polymers, the use of the ordinary Nernst–Planck equations to describe the charge transport seems to be an oversimplification. The impedance spectra of the systems studied in the symmetrical configuration were found to be described adequately by an extension of the homogeneous film model for the metal ∣ film ∣ metal contact. On the basis of the calculated impedance parameters, estimates for the diffusion coefficient and concentration of ionic and electronic charge carriers in the film as a function of potential are obtained. The results are discussed in connection with previous dc resistance and FTIR/UV–vis spectroscopic measurements of the same materials.     

Two-electon-transfer redox systems: Part 6. Two-electron oxidation of hexakis(benzylthio)benzene—a study by electrolysis and cyclic voltammetry

The anodic oxidation of the oligo-thioether hexakis(benzylthio)benzene in a dichloromethane+NBu₄PF₆ electrolyte is investigated by several complementary techniques: cyclic voltammetry, ESR spectroscopy, and macroscopic electrolysis (including fractional electrolysis in combination with the determination of open circuit potentials, i.e. ‘potentiometric titration’). While ESR spectroscopy proves the formation of a π-delocalized radical cation, the electrochemical techniques indicate further oxidation to a dicationic stage, which undergoes follow-up reactions. Analysis of fractional electrolysis results clearly shows strong potential compression for the two electron transfer steps with a difference of formal potentials ΔE°=23 mV. This value is succesfully used to simulate the cyclic voltammograms of the starting compound.     

Study on the electrochemical behavior of epinephrine at a poly(3-methylthiophene)-modified glassy carbon electrode

A poly(3-methylthiophene) (P3MT)-modified glassy carbon electrode (GCE) has been prepared in undeaerated acetonitrile solution under ambient conditions. The electrochemical behavior of epinephrine was studied systematically at the P3MT-modified GCE using cyclic voltammetry. The results indicate that the number and height of redox peaks of epinephrine are strongly dependent on the pH value of the phosphate buffer solution (PBS). Three peaks, one irreversible oxidation peak and one pair of reversible redox peaks, can be observed in neutral PBS while only one pair of redox peaks appear in 0.5 M sulfuric acid solution. In pH 4.0 PBS, the oxidation of epinephrine gives one more cathodic peak than that reported in the literature; in other words, six peaks or three pairs of redox peaks can be obtained due to the oxidation of epinephrine and the following chemical reaction. The effects of other conditions on the oxidative behavior of epinephrine were examined and the oxidative mechanism of epinephrine at the P3MT-modified GCE was discussed.     

The relationship between the electrochemical,mineralogical and flotation characteristics of pyrrhotite samples from different Ni Ores

The common sulphide mineral pyrrhotite (Fe_(1−x)S) occurs in varying crystallographic forms with each type exhibiting subtly different physical and chemical properties. Accounts in the literature suggest that the processing behaviour of magnetic and non-magnetic pyrrhotite differ from each other in their flotation properties, although limited agreement exists in terms of which is the more reactive pyrrhotite form. Since the flotation process is based on the surface properties of the sulphide minerals and pyrrhotite is a metallic conductor, it is of interest to characterise the surface properties of pyrrhotite with respect to its electrochemical state.     

Electrochemical impedance spectroscopy study of the adsorption behaviour of α-lactalbumin and β-casein at stainless steel

The electrocatalytic reduction of tert-butyl hydroperoxide and cumyl hydroperoxide at platinum and glassy carbon disc microelectrodes has been investigated in N,N-dimethylformamide and 1,2-dichloroethane in the presence of a small amount of iron (III) chloride in the solution. In the course of a negative scan, metallic iron was electrodeposited on the electrode, making possible the reduction of the hydroperoxide at the electrode surface. During the repetitions of the recordings, an unusual reproducibility of the reduction signal was observed. This reproducibility is due to an in situ reconstitution of the active surface of the electrode that allows the electrode reaction to take place, at each scan, on a clean non-contaminated new electrode surface.     

Short-time preparation and electrochemical properties of a single layer of tetraoctylammonium bromide capped Au nanoparticles on dithiol self-assembled monolayer-modified Au electrode

Short time immobilization of densely packed tetraoctylammonium bromide (TOAB) stabilized gold nanoparticles (AuNPs) were established on a Au electrode modified with a self-assembled monolayer (SAM) of 1,6-hexanedithiol (HDT) or 1,4-benzenedimethanethiol (BDMT). The quartz crystal microbalance experiment showed densely packed TOAB–AuNPs single layer formation on both SAMs was achieved within 20 min. AFM images demonstrated that the immobilized TOAB–AuNPs on the SAMs were densely packed and the AuNPs film thickness was 6–7 nm. The electronic communication between the immobilized AuNPs and the underlying bulk electrode was confirmed by cyclic voltammetry and electroreflectance spectroscopy. A reversible electron transfer reaction was observed for both [Fe(CN)₆]^4−/3− and [Ru(NH₃)₆]^2+/3+ at TOAB–AuNPs immobilized on HDT (Au/HDT/AuNPs) and BDMT (Au/BDMT/AuNPs) modified electrodes. The electroreflectance spectra show a red-shifted strong positive-going plasmon resonance bands at 551 nm and 584 nm, respectively, for the Au/BDMT/AuNPs and Au/HDT/AuNPs electrodes. The observed reversible redox response for the solution redox species and red-shifted plasmon resonance bands for the immobilized AuNPs again indicated that the AuNPs were immobilized on the SAMs in a densely packed manner. An advantage of TOAB–AuNPs modified electrode prepared by short time immersion over citrate-stabilized AuNPs modified electrode was demonstrated by the enhanced oxidation of ascorbic acid (AA) at these electrodes. The oxidation of AA was shifted to 90 mV less positive potential with higher oxidation current at TOAB–AuNPs modified electrode when compared to citrate-stabilized AuNPs modified electrode.     

An improved algorithm for the numerical simulation of cyclic voltammetric curves affected by the ohmic potential drops and its application to the kinetics of bis(biphenyl)chromium(I) electroreduction

Previously published algorithms for the modeling of cyclic voltammetric curves affected by ohmic potential drops, in terms of the classical explicit finite differences method, are discussed briefly. A fast and efficient numerical procedure suitable for such simulations is described. This approach exhibits high numerical stability for both high scan rates and large uncompensated ohmic resistances. The procedure is based on the calculation of the faradaic current as a root of the non-linear equation, with a simultaneous calculation of the capacitive current. Comparison of the cyclic voltammograms obtained using this method with those calculated using alternative published procedures proves its validity. For comparison with the experimental data, the cyclic voltammetric response for the reduction of bis(biphenyl)chromium(I) tetraphenylborate in N,N-dimethylformamide is shown and the kinetic parameters of this process are fitted. Compared to earlier modelings, they show better concordance with the results of studies at microelectrodes. In conjunction with earlier successful applications of the analogous numerical procedure to the realistic modeling of electrochemical oscillations and multistability at a constant external voltage, the algorithm presented appears to be one of the most applicable methods of calculation of the electrochemical responses affected by the ohmic potential drops.