Voltammetric determination of cholesterol in human blood serum

Cholesterol has a number of important functions in a human body. The disorders of cholesterol biosynthesis increase the risk of the development of cardiovascular diseases and worsens the function of the immune system. This study is devoted to the development of a method for determining the concentration of cholesterol in blood serum using voltammetry. We selected the following working conditions for the determination of cholesterol: a phosphate buffer solution with pH 6.86 was a supporting electrolyte; potential sweep rate was W = 30 mV/s; and recording was carried out in the constant-current mode. An organomodified electrode was used as a working electrode. The peak was observed at +0.98 V. The dependence of the electrooxidation current of cholesterol on its concentration was linear in the range of 0.1–50 μM with a limit of detection of 0.01 μM. The results of the determination of cholesterol in blood serum by voltammetry were compared to those obtained by the fluorimetric method.     

Bioaccumulation and determination of lead using treated-Pennisetum-modified carbon paste electrode

The present work describes the development and application of a carbon paste electrode modified by treated-Pennisetum setosum for the determination of lead(II) by anodic stripping differential pulse voltammetry. Most experiments were performed using the preconcentration/voltammetry/regeneration scheme. The resulting modified electrode offers a preferential uptake of lead(II) from solutions. Operational conditions, such as percentage treated-Pennisetum loading in the carbon paste, pH of electrolyte solution, ionic strength, preconcentration time, voltammetric waveform and interference are characterized and optimized to allow quantitative determination of lead. The electrode surface can be regenerated by immersing the modified electrode in 0.05 mol l⁻¹ hydrochloric acid for 2 min. For the measurement step, the optimum conditions were acetate buffer pH 5.0 and 0.60 ionic strength with the preconcentration time of 5 min. The modified electrode contained 10% (w/w) treated-Pennisetum. The detection limit (3σ) was 0.01 mg l⁻¹ Pb(II). For 16 preconcentration/measurement/renewal cycles, the responses could be reproduced with a 5.39% relative standard deviation. This method has been be successfully applied to the determination of lead(II) in natural water samples using standard addition method.     

Enhanced electrochemical performance of LiFePO4 cathode with the addition of fluoroethylene carbonate in electrolyte

The effect of the fluoroethylene carbonate (FEC) addition in electrolyte on LiFePO₄ cathode performance was investigated in low-temperature electrolyte LiPF₆/EC/PC/EMC (0.14/0.18/0.68). Cyclic voltammetry, electrochemical impedance spectroscopy, and charge/discharge tests were conducted in this work. In the presence of FEC, the polarization of LiFePO₄ electrode decreased both at room and low temperatures. Meanwhile, the exchange current density increased. The rate capability of LiFePO₄ electrode was greatly enhanced as well. The morphology of the solid electrolyte interphase (SEI) on LiFePO₄ surface was modified with the addition of FEC as confirmed by scanning electron microscopy measurement. A compact film with small impedance was formed on LiFePO₄ surface compared to the case of FEC-free. The compositions of the film were analyzed by X-ray photoelectron spectroscopic measurement. The contents of Li _x PO _y F _z , LiF, and the carbonate species generated from solvents decomposition were reduced. The modified SEI promoted the migration of lithium ion through the electrode/electrolyte interphase and enhanced the electrochemical performance of the cathode.     

Rotating electrode d.c. solution conductivity device

In an electrochemical cell with one electrode rotating asymmetrically with respect to a second fixed electrode, a constant current produces an a.c. component of the voltage between the pair at the rotational frequency. This a.c. signal arises from the varying iR drop between the extremes of rotational position and thus is a measure of the conductivity of the cell electrolyte. When such a rotating electrode is added to an operating two-electrode cell (e.g. a plating bath), the a.c. signal derived from the rotating element may be used additionally to map the spatial current distribution in the cell. The filtered and amplified signal in either system has also been used to trigger a counter for digital measurement of electrode rotational speed.     

The renovated silver ring electrode

In this preliminary note, a new type of working electrode – the renovated silver ring electrode (RSRE) – is presented. The main constituents of the RSRE: a specially constructed silver ring electrode, a silver sheet used as silver counter/quasi-reference electrode and a silicon O-ring are fastened together in a polypropylene body. The renovation of this electrode is carried out through mechanical removal of solid contaminants and electrochemical activation in the electrolyte which fills the RSRE body. The effectiveness of the renovation procedure was tested by designating the RF and E_pzc and by recording C_d–t curves. As shown on selected examples, RSRE exhibits good performance in underpotential deposition stripping voltammetry (UPD-SV) applied for the determination of Pb(II) traces in certified reference materials.     

Determination of chromium(VI) in dialysis fluids by alternating current and differential pulse voltammetry

Alternating current (ACV) and differential pulse voltammetry (DPV) are employed for the determination of Cr(VI) in dialysis fluids, using 0.1 mol/1 dibasic ammonium citrate as supporting electrolyte (pH 5.9). A three-electrode cell was used: The working electrode was a long-lasting sessile-drop mercury electrode (LLSDME) with a drop time of 240 to 300 s. Precision, expressed as relative standard deviation (s _r%), and accuracy, expressed as relative recovery (R%) are also reported.     

Voltammetric behavior and determination of bismuth on sodium humate modified carbon paste electrode

The preconcentration and voltammetric behavior of Bi^III on a sodium humate modified carbon paste electrode was studied by means of cyclic voltammetry (CV) and differential pulse stripping voltammetry (DPSV). The proposed measurement involves an initial nonelectrolytic preconcentration step in which Bi^III is complexed by the surface modifier in a solution of 0.05 M KNO₃-0.0106 M HNO₃ (pH 2.0) and a subsequent electrochemical scan step in which the preconcentrated Bi^III was reduced and then oxidized promptly in supporting electrolyte of 0.5 M HNO₃. The resulting DPSV anodic current was proportional to the concentration of Bi^III ion over the range of 4.78 × 10⁻⁸–1.44 × 10⁻⁵ M. The detection limit was 4.78 × 10⁻⁸ M. The proposed method was used to determine bismuth in various samples. Various factors affecting the electrode behavior were also investigated at the same time.     

Charge transfer between two immiscible electrolyte solutions: Part VI. Polarographic and voltammetric study of picrate ion transfer across the water/nitrobenzene interface

The transfer of the picrate ion across the interface between two immiscible electrolyte solutions, 0.05 M LiCl in water and 0.05 M tetrabutylammonium tetraphenylborate in nitrobenzene was investigated by electrolysis with the electrolyte dropping electrode and by cyclic voltammetry. Under the conditions of the experiments the charge-transfer process is controlled solely by diffusion. The maximum which appears on the polarogram of the picrate ion close to the limiting current can be suppressed by the addition of a surface-active substance (gelatine). The diffusion coefficients of the picrate ion in the aqueous and nitrobenzene phase were determined from the limiting polarographic current and from the peak current on the cyclic voltammogram. The value of the formal potential of the charge-transfer reaction, which was calculated from the half-wave potential or from the peak potential, is in good agreement with that inferred from the extraction data.     

An enzyme electrode based on electromagnetic entrapment of the biocatalyst bound to magnetic beads

A new type of biosensor is described. It is based on the use of a Clark-type oxygen electrode placed in an electromagnetic field. The biomolecules used in this sensor are immobilized on small magnetic beads which are added to the system when needed and removed as required. By varying the strength of the magnetic field, a homogeneous distribution of particles at the electrode tip is achieved. The electrode is used for the determination of glucose with immobilized glucose oxidase as well as cells of Saccharomyces cerevisiae.     

Background subtraction for rapid scan voltammetry

Methods for the elimination of residual current in fast scan (> 300 V s⁻¹) voltammetry have been evaluated at microvoltammetric electrodes. Staircase voltammetry is found to give only a modest improvement in the ratio of the faradaic current to the residual current. Supeior results are obtained when the residual current, obtained in supporting electrolyte solution, is digitally subtracted from the voltammogram obtained in the presence of the electroactive species. The subtraction process is facilitated by conducting the whole experiment in a flow injection apparatus so that the electrode remains in solution while the two voltammograms are obtained. Undistorted voltammograms have been obtained for the reduction of anthracene (∼ 2 mM) at scan rates up to 10,000 V s⁻¹. At 300 V s⁻¹, virtually undistorted voltammograms are obtained for the reduction of Tl⁺ at low concentration (20 μM) in aqueous solution at a mercury microvoltammetric electrode. Analog offset of the residual current was used to improve the dynamic range. The limits of this technique are caused by the electronic noise in the ideal case where flat residual current curves are obtained.     

Modification of carbon ceramic electrode prepared with sol-gel technique by a thin film of chlorogenic acid: application to amperometric detection of NADH

The carbon ceramic electrode prepared with sol-gel technique is modified by a thin film of chlorogenic acid (CGA). By immersing the carbon ceramic electrode in aqueous solution of chlorogenic acid at less than 2 s a thin film of chlorogenic acid adsorbed strongly and irreversibly on the surface of electrode. The cyclic voltammetry of the resulting modified CCE prepared at optimum conditions shows a well-defined stable reversible redox couple due to hydroquinone/quinone system in both acidic and basic solutions. The modified electrode showed excellent electrocatalytic activity toward NADH oxidation and it also showed a high analytical performance for amperometric detection of NADH. The catalytic rate constant of the modified carbon ceramic electrode for the oxidation of NADH is determined by cyclic voltammetry measurement. Under the optimised conditions the calibration curve is linear in the concentration range 1-120 μm. The detection limit (S/N = 3) and sensitivity are 0.2 μM and 25 nA μM⁻¹.The results of six successive measurement-regeneration cycles show relative standard deviations of 2.5% for electrolyte solution containing 1 mM NADH, indicating that the electrode renewal gives a good reproducible and antifouling surface. The advantages of this amperometric detector are: high sensitivity, excellent catalytic activity, short response time t < 2 s, remarkable long-term stability, simplicity of preparation at short time and good reproducibility.     

The online removal of dissolved oxygen from aqueous solutions used in voltammetric techniques by the chromatomembrane method

The applicability of the chromatomembrane method for the removal of dissolved oxygen from solvents used in voltammetric measurements was investigated. The chromatomembrane cell combined with a flow-through system allows an online deaeration of solutions. These experiments employed a mercury film electrode as working electrode and differential pulse anodic stripping voltammetry as the measuring method. Different eluents with adequate supporting electrolyte (without analyte) were measured to determine the background current, whether any contribution of oxygen is detectable. Voltammograms of eluents deaerated with the chromatomembrane method are compared to those of eluents purged with nitrogen for several minutes immediately before the measurement. No differences in the background currents can be observed when defined flow rates of eluent and nitrogen are maintained. Determinations of cadmium and lead even indicate the high efficiency of this method.     

Effect of inert gas flow on hydrogen underpotential deposition measurements in polymer electrolyte fuel cells

The effect of inert gas flow rate on hydrogen underpotential deposition (H_upd) measurements in polymer electrolyte fuel cells (PEFCs) was investigated using a novel experimental technique. The method combines local voltammetric measurements in PEFCs with the use of sectioned electrodes. The results give experimental proof that the high inert gas flow rate usually employed in voltammetric measurements in PEFCs at the working electrode results in high hydrogen reduction currents in both the cathodic and the anodic sweep, which hampers an accurate determination of the electrochemically active surface area (ECA). Strong spatial inhomogeneities occur at low potentials as a consequence of formation and accumulation of molecular hydrogen along the flow field. The results show that the flow of inert gas should be minimized or even stopped during a measurement to allow molecular hydrogen to accumulate at the working electrode and to provide uniform conditions along the flow field.     

Voltammetric Determination of Aflatoxin B1

For the first time the possibility of voltammetry used for the determination of aflatoxin B1 on a glassy carbon electrode was shown. The effect of pH of a supporting electrolyte on the analytical signal of aflatoxin B1 has been investigated and it was shown that there is a more pronounced peak with a maximum current at pH of 5.33. The most favorable supporting electrolyte for a linear range of detectable concentrations of aflatoxin B1 – 0.1 M (NH₄)₂SO₄ was determined. The results of research on the development of conditions of voltammetric measurement of aflatoxin B1 are presented.     

Cathodic stripping voltammetry of 2-thioorotic acid at the hanging mercury drop electrode

Controlled adsorptive accumulation of 2-thioorotic acid (6-carboxy-2-thiouracil) on the hanging mercury drop electrode provides the basis for the direct stripping measurement of that compound in the nanomolar concentration level. Differential pulse voltammetry, following 3 min preconcentration, yields a detection limit of 5.0×10^-10 M 2-thioorotic acid. The cathodic stripping response is evaluated with respect to experimental parameters such as preconcentration time and potential, bulk concentration and others. Best results are obtained using a 0.001 M NaOH electrolyte.Two different methods of cathodic stripping voltammetry can be proposed for the determination of 2-thioorotic acid and the reproducibility of these methods is studied.     

Electrochemical behavior of silver thin films interfaced with yttria-stabilized zirconia

Thin silver films (100–800 nm) were deposited by physical vapor deposition (PVD) on yttria-stabilized zirconia solid electrolyte. The electric percolation as a function of the film thickness was studied during deposition and annealing using a two-electrode in-situ resistance measurement technique. Electrical percolation was achieved in as-deposited films greater than 5.4?±?0.4 nm; however, thermal treatment (550 °C in air) resulted in film dewetting for Ag films as thick as 500 nm and formation of electronically isolated Ag nanoparticles, as was confirmed by SEM and XPS. In thermally treated samples, stable electronic conductivity associated with a continuous percolated network was only observed in samples greater than 600 nm in thickness. The effect of polarization on the electrochemical reactions at the three-phase (electrode-gas-electrolyte) and two-phase (electrode-electrolyte) boundaries of the electrode was investigated by solid electrolyte cyclic voltammetry (SECV) at 350 °C and P _O2?=?6 kPa. With the application of positive potential, silver oxide (Ag₂O) was found to form along the three-phase boundary and then extends within the bulk of the electrode with increasing anodic potentials. By changing the hold time at positive potential, passivating oxide layers are formed which results in a shift in favor of the oxygen evolution reaction at the working electrode. This oxide forms according to a logarithmic rate expression with thick oxides being associated with decrease in current efficiency for subsequent oxide formation.     

Electrocatalytic Oxidation of Sulfite on a Nickel Aquapentacyanoferrate Modified Electrode: Application for Simple and Selective Determination

An electroactive metal cyanometallate complex, nickel aquapentacyanoferrate (NAPCF) was synthesized and characterized using XRD and UV‐vis spectral studies. The solid complex was then mechanically immobilized on the surface of a paraffin impregnated graphite electrode (PIGE) and the NAPCF modified electrode was characterized using cyclic voltammetry. The dependence of the modified electrode was tested in terms of supporting electrolyte, scan rate and pH of the medium. The electrocatalytic oxidation of sulfite at the modified electrode was investigated by cyclic voltammetry, hydrodynamic voltammetry and chronoamperometry techniques. It was found that the NAPCF modified electrode efficiently exhibited electrocatalytic activity for the oxidation of sulfite with relatively high sensitivity, selectivity and long life of activity. Based on the electrocatalytic oxidation, the NAPCF modified electrode was used as a sensor for the determination of sulfite. The linear working range for the determination of sulfite was 2.78×10^?6 M to 3.00×10^?3 M with a detection limit of 9.26×10^?7 M. The electrode was applied for the determination of sulfite in real samples satisfactorily.     

Operando discrimination of fast and slow active grains within a cycling electrode for lithium battery

The impact of the formation of an electrolyte based decomposition layer on the distribution of the active grain kinetics has been investigated operando within a cycling electrode for lithium battery. It is demonstrated, from fitting procedure of the electrochemical responses and from operando XRD experiments, that as the passive film forms, slow reacting grains appear within the working electrode and operate simultaneously with faster grains. Slow grains are macroscopically distributed, presumably at the electrolyte side of the electrode. Discrimination of the electrochemical response pertaining to each type of grains allows rationalizing their contribution to the capacity fading dynamics.     

Semidifferential electroanalysis with a solid working electrode

Semidifferential electroanalysis is described for hexacyanoferrate(III), dichromate, copper(II), p-aminophenol, p-benzoquinone, m-dinitrobenzene, guanine, guanosine, adenine, and adenosine at a stationary solid working electrode. Nearly symmetrical, peaked curves are obtained for the electrode processes of all the samples investigated. The predicted dependence of peak height and peak potential on concentration, electrode area, and potential scan rate are confirmed-experimentally for the glassy carbon disk electrode. It is demonstrated that the technique with the solid working electrode provides higher sensitivity and better resolution than ordinary linear sweep voltammetry. The sensitivity is somewhat worse than in differential pulse voltammetry, but the technique has the advantage of Speed.     

Differential ion-selective membrane electrode-based potentiometric gas-sensing cells with enhanced gas sensitivity

A novel design for devising static and automated flow-through type potentiometric gas-sensing systems with enhanced response slopes is described. The approach involves the use of two working gas-sensing electrode half-cells in a differential measurement arrangement. One of the half-cells employs a pH-sensitive polymeric membrane electrode to sense pH changes from diffusing analyte gas within a suitable inner electrolyte solution housed behind an outer gas-permeable membrane. The second working half-cell is fabricated with an anion- or cation-selective membrane electrode that responds selectively to the conjugate acid or base ionic form of the analyte gas trapped within an inner buffer solution housed behind a similar gas-permeable membrane. When the two internal solutions of the half-cells are in electrolytic contact, the differential response of the resulting gas-sensing scheme is significantly enhanced compared with the response of a conventional single working electrode/reference electrode type gas cell. For the model analyte gas ammonia, response slopes observed for both static and flow-through measurement schemes approach the 118 mV decade^?1 predicted by theory. To demonstrate its analytical utility, the flow-through arrangement was used to determine ammonia-N concentrations in bioreactor media with good correlation with conventional electrode and enzymatic methods. The prospects of fabricating similar differential detection arrangements for CO₂, NO₂ and SO₂ are discussed.