Impedance spectroscopy of lithium-tin film electrodes

A method of electrochemical impedance spectroscopy was used to study the reversible lithium intercalation from nonaqueous electrolyte into tin films with the thickness of 0.1–1 μm. The impedance spectra of lithium-tin (Li _x Sn) electrodes have a complicated shape depending on the electrode state and prehistory; they reflect the occurrence of several consecutive and parallel processes, including the lithium migration, diffusion, and accumulation. The formation of a solid-electrolyte layer on the surface at Li intercalation into Sn is observed. Equivalent circuits are proposed that adequately model the experimental data on the Li _x Sn electrodes both freshly prepared and after prolonged cycling. Problems associated with the choice of equivalent circuits and determination of their parameters, the accuracy of the diffusion coefficient determination, the trends in the parameters’ variation with electrode potential (composition) are discussed.     

Mechanistic studies on lanthanum fluoride crystal electrodes

A mechanistic study of the fluoride electrode involving the impedance measurement technique was performed. The influence of the reference electrode, the fluoride ion concentration and the convection on the indicatrix were examined. From the results it followed that the linear equivalent model of the fluoride electrode may be represented by three components namely, a parallel R-C network representing the bulk of the membrane, two double layer capacities one on each side of the sensor, each shunted by a charge transfer resistor and finally, a diffusion impedance.     

Impedance studies of porous lanthanum-phosphate-bonded nickel electrodes in concentrated sodium hydroxide solution

The hydrogen evolution reaction (HER) was investigated on a lanthanum-phosphate-bonded nickel (LPBN) powder electrode in 30 wt.% NaOH at 70°C using ac impedance and steady-state polarization techniques. Circuits containing one or two constant-phase elements (CPEs) in parallel with a resistance and corresponding to fractal and porous electrode models were tested in order to interpret the ac impedance data. The experimental impedance spectra were well described by the porous electrode model and the circuit containing two CPEs. The results obtained from the ac impedance and steady-state measurements allowed the mechanism and kinetics of the HER to be evaluated. Comparison of these parameters with those obtained on the polycrystalline nickel electrode in 1 M alkaline solution at 25°C indicates that an increase in activity is principally due to an increase in the real surface area.     

A systematic approach to the impedance of surface layers with mixed conductivity forming on electrodes

Electrode impedance can be evaluated on the basis of the electrode reaction kinetics in many systems, even for complicated electrode reactions. However, when a surface layer is present on the electrode surface, the theoretically well-established impedance model of the electrode reaction is often completed with phenomenological equivalent circuit elements in order to achieve the number of time constants as derived from the electrode impedance spectra measured. In these cases, the meaning of the phenomenological equivalent circuit elements are often unclear, though the presence of these elements is helpful to describe the system throughout the frequency domain used for the measurement. In the present work, an attempt will be shown to separate the effect of the electronic and ionic charge transfer in a surface layer and to identify the appropriate equivalent circuits. Examples are shown from the fields of lithium-ion batteries where a solid electrolyte interface as a surface layer is present at the negative electrode and the contribution of various charge carriers may be of importance.     

Carbon film electrodes for oxidase-based enzyme sensors in food analysis

Carbon film resistor electrodes have been evaluated as transducers for the development of multiple oxidase-based enzyme electrode biosensors. The resistor electrodes were first modified with Prussian Blue (PB) and then covered by a layer of covalently immobilized enzyme. Electrochemical impedance spectroscopy was used to characterize the interfacial behaviour of the Prussian Blue modified and enzyme electrodes; the spectra demonstrated that the access of the substrates is essentially unaltered by application of the enzyme layer. These enzyme electrodes were used to detect the substrate of the oxidase (glucose, ethanol, lactate, glutamate) via reduction of hydrogen peroxide at +50 mV versus Ag/AgCl in the low micromolar range. Response times were 1-2 min. Finally, the glucose, ethanol and lactate electrochemical biosensors were used to analyse complex food matrices—must, wine and yoghurt. Data obtained by the single standard addition method were compared with a spectrophotometric reference method and showed good correlation, indicating that the electrodes are suitable for food analysis.     

New method for a graphical portrayal of the electrochemical impedance

A new method of a graphical representation of the electrode impedance is proposed. The method allows one to construct the impedance of a complicated circuit by summing up impedance plots for series and parallel branches. Rules for constructing complex representations are presented. New shapes of impedance plots for some of the most typical circuits are analyzed. The method opens additional possibilities for analyzing how the experiment accuracy impacts the limits of discovery of individual elements of complicated circuits. The method gives grounds to introduce the notion of the information potential of an experimental setup. Conditions for a complete decidability of equivalent circuits are shown by the example of a system that comprises several parallel branches consisting of the Frumkin-Melik-Gaikazyan impedances.     

Compact and diffuse double layer interaction in unsupported system small-signal response

Previous exact results for the small-signal impedance of an unsupported electrode/material/electrode system which include effects of the finite size of charge carriers are simplified and discussed. The material contains non-recombining charges of opposite sign with the positive one immobile and uniformly distributed. General boundary conditions which encompass the range from no electrode reaction to ohmic electrode behavior are employed. In the presence of an electrode reaction, the interaction of the compact and diffuse double layers leads to considerably more complexity in the equivalent circuit than might appear in simple treatments of the supported case, in which the diffuse double layer capacitance is neglected or the compact double layer and diffuse double layer capacitances are placed in series. Two different approximate equivalent circuits made up of frequency-independent elements are found which yield remarkable agreement with the exact results over the entire frequency range of interest. The first involves the ordinary approximate circuit (OAC) previously found in the absence of compact layer effects plus a series compact layer contribution involving a parallel resonant circuit with quality factor at resonance which may approach unity. Pseudo-inductance effects are found to be extremely significant in this representation. The second approximate equivalent circuit, simpler and almost as accurate as the first, has the same form as the original OAC but with its reaction element values altered by the presence of the compact layer. For non-Butler-Volmer electrode kinetics an upper limit is found for the experimentally determinable apparent reaction rate constant, a feature of practical importance for thin films or membranes. The response of thin films and membranes, including compact layer effects, can very readily be erroneously confused with pure bulk response, yielding entirely incorrect values for the geometrical capacitance and bulk resistance of the material.     

The use of electrochemical impedance spectroscopy (EIS) in the evaluation of the electrochemical properties of a microbial fuel cell

Electrochemical impedance spectroscopy (EIS) has been used to determine several electrochemical properties of the anode and cathode of a mediator-less microbial fuel cell (MFC) under different operational conditions. These operational conditions included a system with and without the bacterial catalyst and EIS measurements at the open-circuit potential of the anode and the cathode or at an applied cell voltage. In all cases the impedance spectra followed a simple one-time-constant model (OTCM) in which the solution resistance is in series with a parallel combination of the polarization resistance and the electrode capacitance. Analysis of the impedance spectra showed that addition of Shewanella oneidensis MR-1 to a solution of buffer and lactate greatly increased the rate of the lactate oxidation at the anode under open-circuit conditions. The large decrease of open-circuit potential of the anode increased the cell voltage of the MFC and its power output. Measurements of impedance spectra for the MFC at different cell voltages resulted in determining the internal resistance (R(int)) of the MFC and it was found that R(int) is a function of cell voltage. Additionally, R(int) was equal to R(ext) at the cell voltage corresponding to maximum power, where R(ext) is the external resistance that must be applied across the circuit to obtain the maximum power output.     

Determination of diffusion coefficient of lithium in substituted LiMn1.95Cr0.05O4 spinel using impedance technique

Electrodiffusion properties of chromium-substituted lithium-manganese spinel Li _x Mn_1.95Cr_0.05O₄ intended for application as a cathodic material for lithium-ion batteries is studied. The studies are carried out at 25°C using the electrochemical impedance spectroscopy technique in alkyl-carbonate electrolyte. In the analysis of impedance spectra, the apparatus of electric equivalent circuits was employed to determine surface layer resistances, double electric layer capacitance, differential intercalation capacity, chemical diffusion coefficient D of lithium, and other electrode characteristics. The issues of substantiating the choice of electric equivalent circuits and correct interpretation of their elements are discussed; dependences of the calculated model parameters on the electrode potential (lithium concentration in the electrode) are analyzed. The chemical diffusion coefficient of Li⁺ in Li _x Mn_1.95Cr_0.05O₄ found on the basis of the impedance spectra is in the range of 10^?9 to 10^?12 cm²/s under electrode potential variation in the range of 3.5–4.5 V (vs. Li/Li⁺) with a pronounced minimum of D in the middle of this range. Repeated cycling of the electrode is accompanied by a gradual increase in resistance of the solid-electrolyte interphase (SEI).     

Fast high-performance liquid chromatographic analysis of mianserin and its metabolites in human plasma using monolithic silica column and solid phase extraction

A fast high-performance liquid chromatography (HPLC) method was developed and validated for the simultaneous determination of mianserin (MIAN) and its metabolites desmethylmianserin (DMM), 8-hydroxymianserin (HM) and mianserin-N-oxide (MNO) in human plasma. Each compound, together with internal standard (propranolol) was extracted from the plasma matrix using solid phase extraction. Chromatographic resolution of the analytes was performed on a Chromolith Speed Rod monolithic silica column ( mm i.d.) under isocratic conditions using a mobile phase of 74:26 (v/v) 25 mM phosphate buffer (pH 5.3 adjusted with phosphoric acid): acetonitrile. The elution of the analytes were monitored at 292 mm and conducted at ambient temperature. Because of high column efficiency the mobile phase was pumped at a flow rate of 3.5 ml/min. The total run time of the assay was 5 min. The method was validated over the range of 10-200 ng/ml for MIAN, 10-150 ng/ml for DMM, 20-300 ng/ml for HM and 25-500 ng/ml for MNO. The method proved to be precise (within-run precision ranging from 1.6 to 6.9% R.S.D. and between-run precision ranging from 1.3 to 7.2% R.S.D.) and accurate (within-run accuracies ranged from 1.4 to 6.4% and between-run accuracies ranging from 1.5 to 4.5%). The mean absolute recoveries were 95.7, 94.8, 99.6, and 102.6% for MIAN, DMM, HM and MNO, respectively. The limit of quantitation (LOQ) for MIAN and DMM was 10 ng/ml and for HM and MNO were 20 and 25 ng/ml in human plasma, respectively. The limit of detection (LOD) for MIAN, DMM, HM and MNO was 5, 2.5, 10 and 15 ng/ml, respectively. The described method demonstrates the feasibility for employing monolithic columns to effect rapid bioanalytical HPLC analysis for the quantitative determination of MIAN and its major metabolites in human plasma.     

Electrochemical behavior and determination of fluphenazine at multi-walled carbon nanotubes/(3-mercaptopropyl)trimethoxysilane bilayer modified gold electrodes

A novel multi-walled carbon nanotubes/(3-mercaptopropyl)trimethoxysilane (MPS) bilayer modified gold electrode was prepared and used to study the electrochemcial behavior of fluphenazine and determine it. Fluphenazine could effectively accumulate at this electrode and produce two anodic peaks at about 0.78 V and 0.93 V (versus SCE). The peak at about 0.78 V was much higher and sensitive, thus it could be applied to the determination. Various conditions were optimized for practical application. Under the selected conditions (i.e. 0.05 M pH 3.5 HCOOH-HCOONa buffer solution, 5 μl 1 mg ml⁻¹ multi-walled carbon nanotubes for Φ=2.0 mm electrode, accumulation at open circuit for 180 s), the anodic peak current was linear to fluphenazine concentration in the range from 5×10⁻⁸ to 1.5×10⁻⁵ M with correlation coefficient of 0.9984, the detection limit was 1×10⁻⁸ M. For a 1×10⁻⁵ M fluphenazine solution, the relative standard deviation of peak current was 2.51% (n=8). This method was successfully applied to the determination of fluphenazine in drug samples and the recovery was 96.4-104.4%. The electrode could be easily regenerated and exhibited some selectivity, but some surfactants reduced the peak current greatly. The modified electrode was characterized by alternating current impedance and electrochemical probe.     

Dynamic electrochemical impedance spectroscopy of the adsorption and relaxation of polyaniline chains during potentiodynamic redox transformations

A new approach to an equivalent circuit analysis of impedance spectra of a conducting polyaniline (PAni) layer is presented. Film properties are often modeled by constant-phase elements (CPE), which are closer to experimental results than capacitors. We want to take an insight into the CPE. We propose a novel equivalent circuit based on the Frumkin-Melik-Gaikazyan adsorption model, as we suppose that the PAni layer molecules may behave like other adsorbing and relaxing organic compounds. Measurements are performed using recently developed experimental technique—dynamic electrochemical impedance spectroscopy. Obtained spectra are analyzed using the proposed equivalent circuits. Characteristics of several parameters are analyzed and discussed. Published in Russian in Elektrokhimiya, 2007, Vol. 43, No. 9, pp. 1111–1119. The text was submitted by the authors in English.     

New triiodomercurate-modified carbon paste electrode for the potentiometric determination of mercury

A new tetrazolium-triiodomercurate-modified carbon paste electrode has been described for the sensitive and selective determination of mercury. The electrode shows a stable, near-Nernstian response for 1×10⁻³ to 6×10⁻⁶ M [HgI₃]⁻ at 25 °C over the pH range of 4.0-9.0, with an anionic slope of 55.5±0.4 mV. The lower detection limit is 4×10⁻⁶ M with a fast response time of 30-50 s. Selectivity coefficients of a number of interfering anions and iodo complexes of some metal ions have been estimated. The interference from many of the investigated ions is negligible. The determination of 1-200 μg/ml of mercury in aqueous solutions shows an average recovery of 98.5% and a mean relative standard deviation of 1.6% at 50.0 μg/ml. The direct determination of mercury in spiked wastewater, metal amalgams and dental alloy gave results that compare favorably with those obtained by the cold vapor atomic absorption spectrometric method. Potentiometric titration of mercury and phenylmercury acetate with standard potassium iodide has been monitored using the developed triiodomercurate-carbon paste electrode (CPE) as an end point indicator electrode.     

Determination of Organophosphorus Insecticides in Water by C-18 Solid Phase Extraction and Quantitative TLC

Abstract

A simple, rapid, and sensitive method for the determination of sarafloxacin (A-55620) in fish serum using enrofloxacin as internal standard is described. The serum sample and internal standard enrofloxacin are loaded onto the extraction column packed with C2 sorbent material. The column is rinsed and then eluted. The detection limit is 5 ng/g, the recovery rate varying from 92 to 100 % .     

Square-wave voltammetric (SWV) determination of Captopril in reconstituted serum and pharmaceutical formulations

A simple, fast and sensitive square-wave voltammetric (SWV) method for the determination of trace amounts of Captopril in pharmaceutical formulation and reconstituted serum is reported. A three-electrode system containing the static mercury drop electrode (SMDE) working electrode, Pt auxiliary electrode and Ag/AgCl reference electrode was used throughout. Sodium sulfite was used as both supporting electrolyte and oxygen removing agent. No nitrogen purging is needed for oxygen removal from sample solution. Calibration graph showed good linearity in the concentration range of 0.5-50.0 μg mL⁻¹ of Captopril and regression coefficient of 0.9957 is obtained. R.S.D. for eight replicate measurements and LOD of the proposed method are 1.2% and 6.28 × 10⁻³ μg mL⁻¹, respectively. The effect of various parameters (equilibration time, scan increment, pulse height, drop size, frequency and sodium sulfite concentration) on the determination were investigated. The procedure was successfully applied to the determination of Captopril in pharmaceutical formulation and reconstituted serum.     

High-performance liquid chromatographic method for the simultaneous determination of enrofloxacin and its primary metabolite ciprofloxacin in canine serum and prostatic tissue

A simple and sensitive high-performance liquid chromatographic method was developed for the determination of enrofloxacin and ciprofloxacin in canine serum and prostatic tissue. Sample preparation consisted of mixing canine serum with a 1:1 dilution of acetonitrile and 0.1 M sodium hydroxide followed by ultrafiltration through a 10,000 molecular mass cut-off filter. Prostatic tissue was sonicated with the same solution prior to ultrafiltration. Separation of these two quinolones in the ultrafiltrate was accomplished by ion-paired liquid chromatography using a reversed-phase analytical column eluted with an acetonitrile-methanol-water solution. Enrofloxacin and ciprofloxacin were detected by a photometric ultraviolet-visible detector set at 278.6 nm and confirmed by a photodiode array detector operating from 230 to 360 nm. The limits of detection for enrofloxacin and ciprofloxacin were 4 and 2 ng/ml, respectively.     

基于纳米金/壳聚糖/石墨烯的癌胚抗原阻抗型免疫传感器

研究了在PBS缓冲介质中,一种检测癌胚抗原的新型免标记阻抗型免疫传感器的制备及应用,基于石墨烯、纳米金在玻碳电极表面组装制备传感器,通过循环伏安法、交流阻抗法对制备的传感器进行表征。在优化的实验条件下,该免疫传感器的阻抗值随着检测溶液中癌胚抗原(CEA)浓度的增大而增大,并在0.1～85 ng/mL CEA范围内呈线性关系,回归方程为△Ret=1605.55+39.26ρ;检测限为0.04 ng/mL(R=0.9992)。该免疫传感器可用于临床上对CEA的检测。     

Biohybrid microarrays--impedimetric biosensors with 3D in vitro tissues for toxicological and biomedical screening

To investigate the effectiveness of potential anticancer therapeutics or therapies, efficient screening methods are required. On the one hand, multicellular 3D aggregates (spheroids) are a powerful in vitro model for simulating the in vivo situation and on the other hand, planar electrode structures are generally highly suitable for automation and parallel testing. Here, the detection of the effect of active substances on spheroids positioned on electrodes of substrate integrated electrode arrays is exemplarily investigated. As a 3D tissue model a reaggregation system of T47D clone 11 tumor cells is used. The effect of cytotoxins (DMSO, Triton X-100) on spheroids can be detected by recording the effective impedance of planar electrodes covered by spheroids. The equivalent circuit model parameter of electrodes covered by cytotoxin treated spheroids are determined from recorded impedance spectra and compared to the parameter of electrodes covered by control spheroids as well as not covered electrodes. Spheroids on electrodes mainly influence the electrode impedance in the frequency range of 10 kHz to 1 MHz. The results are discussed in view of an optimal electrode/spheroid-interface for sensing the effects of therapeutics with high sensitivity.     

Ethanol electrooxidation on Pd/C nanoparticles in alkaline media

Carbon-supported Pd nanoparticles were prepared by microwave heating-glycol reduction method, and characterized by a wide array of experimental techniques including X-ray diffraction spectroscopy(XRD) and transmission electron microscopy(TEM). The electrooxidation behaviors of ethanol on the Pd/C electrode in alkaline media were investigated using cyclic voltammetry(CV), chronoamperometry(CA), electrochemical impedance spectroscopy(EIS) and single cell performance methods. Pd/C electrode for ethanol oxidation showed high electro-catalytic activity and long term stability. However, it is observed that the current density decreases with the increasing of the potential and negative impedance presents in the potential from-0.1 to0.1 V. The decreasing current density and the negative impedance could be due to the adsorbed intermediates species that inhibited the further oxidation of ethanol. Based on the chemical reaction analysis and EIS spectra, equivalent circuits relating to various potential zones have been obtained. These results reveal the dynamic adsorption of intermediates species on Pd surfaces. Significantly, it is clarified that the adsorption behavior begins from the maximum catalysis of electro-catalysis and ends in the formation of the palladium(II) oxide layer on the electrode surface.     

Biohybrid microarrays – Impedimetric biosensors with 3D in vitro tissues for toxicological and biomedical screening

To investigate the effectiveness of potential anti-cancer therapeutics or therapies, efficient screening methods are required. On the one hand, multicellular 3D aggregates (spheroids) are a powerful in vitro model for simulating the in vivo situation and on the other hand, planar electrode structures are generally highly suitable for automation and parallel testing. Here, the detection of the effect of active substances on spheroids positioned on electrodes of substrate integrated electrode arrays is exemplarily investigated. As a 3D tissue model a reaggregation system of T47D clone 11 tumor cells is used. The effect of cytotoxins (DMSO, Triton X-100) on spheroids can be detected by recording the effective impedance of planar electrodes covered by spheroids. The equivalent circuit model parameter of electrodes covered by cytotoxin treated spheroids are determined from recorded impedance spectra and compared to the parameter of electrodes covered by control spheroids as well as not covered electrodes. Spheroids on electrodes mainly influence the electrode impedance in the frequency range of 10 kHz to 1 MHz. The results are discussed in view of an optimal electrode/spheroid-interface for sensing the effects of therapeutics with high sensitivity.