Micellar histidinate hematin complex as an artificial peroxidase enzyme model: Voltammetric and spectroscopic investigations

The UV–vis spectrophotometry and cyclic voltammetry were employed to investigate the incorporation of hematin into histidine (His) in a micellar environment of sodium dodecyl sulfate (SDS). Histidine undergoes a reduction process on silver electrode, while, hematin and sodium dodecyl sulfate are not electroreactive species on this electrode. Electrochemistry of twine-by-twine mixture of His, hematin and SDS on silver electrode shows that the peak potential of His in the presence of SDS or hematin shifts negatively which indicates the interaction of SDS and hematin with His. The interaction of SDS and hematin with His was also confirmed using spectrophotometric measurements. However, the peak potential of His on silver electrode shifts positively in the presence of both SDS and hematin which indicates that in a triple-component solution of His–hematin–SDS a unique species is formed and is electroreactive on silver electrode. In this context, this triple-component solution represents unique absorption band in UV–vis spectra, which is related to the formation of a unique structure of a hemoprotein-like biomimetic catalyst. The catalytic activity of this artificial enzyme formed in triple-component solution was examined with respect to hydrogen peroxide and the apparent Michaelis–Menten (K_m) and catalytic rate (k_cat) constants were evaluated to be 3.31 μM and 0.043 s⁻¹, respectively.     

Nucleation-growth kinetics of the oxidation of silver nanocrystals to silver halide crystals

The electrochemical oxidation of silver nanocrystals to silver halide crystals proceeds by a process of nucleation and growth. The mechanism is confirmed by analyzing chronopotentiograms using a new extension of nucleation theory. The theory makes it possible to derive plots of nucleation-growth currents vs potential, and growth rates vs potential, directly from experimental data. Such plots yield powerful insights into the reaction kinetics. In situ AFM imaging reveals that a few thousand of silver nanocrystals are oxidized to only a few tens of silver halide crystals, without pronounced loss of active material. The mechanism of this remarkable process is described in this paper. In particular, it is shown that the decrease in crystal population proceeds via an oversaturated silver solution, i.e., a process that is mediated by “driven” Ostwald ripening across the electrode surface. At the same time, the low solubility of silver species in bulk solution means that few silver ions escape from the surface. This combination of features explains why the transformation from silver to silver halide is near-stoichiometric yet highly reconstructive.Dedicated to our friend and colleague Professor Dr. Alan Bond on the occasion of his 60th birthday     

Electroanalytical study of fluvoxamine

Fluvoxamine (FVX) can be reduced at a mercury-drop electrode, with a maximum peak current intensity being obtained at a potential of –0.7 V vs. Ag/AgCl, in an aqueous electrolyte solution of pH 2. The compound was determined in a pharmaceutical product and in spiked human serum by square-wave adsorptive-stripping voltammetry (SWAdSV) after accumulation at the electrode surface, under batch conditions. Because the presence of dissolved oxygen did not interfere significantly with the analysis, it was also possible to determine FVX in the pharmaceutical product by use of a flow-injection analysis (FIA) system with SWAdSV detection. The methods developed were validated and successfully applied to the quantification of FVX in a pharmaceutical product. Recoveries between 76 and 89% were obtained in serum analysis. The FIA–SWAdSV method enabled analysis of up to 120 samples per hour at reduced cost, implying the possibility of competing with the chromatographic methods usually used for this analysis.     

Nonlinear Impedance of Liquid In–Ga Electrode in Aqueous Solutions of a Symmetrical Surface-Inactive Electrolyte

Experimental data related to the potential dependence of nonlinear characteristics of the electrical double layer at a liquid In–Ga electrode in aqueous solutions of a symmetrical surface-inactive electrolyte are obtained for the first time. It is shown that, as opposed to polycrystalline Cd and Pb electrodes, on a liquid (atomically smooth) In–Ga electrode, as on Hg, there is a clear intersection of the potential dependences of a nonlinear signal for different concentrations of a 1–1-valence surface-inactive electrolyte at one point. The intersection point exactly corresponds to the potential of zero charge of an electrode undistorted by specific adsorption of ions. It is established that, when estimating hydrophilicity of metals by a nonlinear impedance method, most information is provided by the region of average negative charges, rather than by the region near zero charge. It is shown that, as opposed to a linear impedance method, the nonlinear impedance method makes it possible to determine, directly from experiment, quantities that directly characterize the metal–solvent chemisorption interaction in a pure form; at the same time, these quantities are criteria of lyophilic nature of metals. Quantities that characterize the metal–solvent chemisorption interaction, obtained by the linear and nonlinear impedance methods are in good agreement, which confirms the validity of the approach we proposed earlier for separating the difference between reciprocal capacitances of the inner part of the electrical double layer on Hg and metal M, ^Hg _M C ^-1 _i= 1/C ^Hg _i– 1/C ^M _iinto physical (^Hg _M C ^–1)_physand chemical constituents. This coincidence also confirms correctness of numerical values obtained earlier for quantities (^Hg _M C ^–1)_phys.     

Transients of the Open-Circuit Potential Observed during the Interaction of Formic Acid with Preliminarily Adsorbed Oxygen on a Platinized Platinum Electrode

Transients of the open-circuit potential, which are observed during the interaction of formic acid with preliminarily adsorbed oxygen (O_ads) on a Pt/Pt electrode in 0.5 M H₂SO₄, are measured. It is established, by means of the method of cathodic potentiodynamic pulses, that the slowest interaction of formic acid with O_ads occurs in the region of large coverages of the electrode surface by oxygen (θ_O ∼1–0.8). A presumption is put forward that the process rate in this region is defined by a direct reaction of O_ads with molecules of formic acid from the bulk solution. It is shown that the interaction of formic acid with O_ads in the region of intermediate coverages (θ_O ∼ 0.8–0.2) proceeds via a mechanism of “conjugated reactions.” Transients of the open-circuit potential for formic acid are compared to transients for carbon monoxide obtained in analogous conditions. The substantially shorter overall time of potential decay in the case of CO (at the same concentrations) is caused by a faster reaction of CO with adsorbed oxygen in the region of large θ_O. The difference is explained by assuming that the HCOOH adsorption as opposed to CO bears a dissociative character.__________Translated from Elektrokhimiya, Vol. 41, No. 8, 2005, pp. 936–942.Original Russian Text Copyright © 2005 by Manzhos, Maksimov, Podlovchenko.     

Silver nanoparticle assemblies supported on glassy-carbon electrodes for the electro-analytical detection of hydrogen peroxide

Electrochemical detection of hydrogen peroxide using an edge-plane pyrolytic-graphite electrode (EPPG), a glassy carbon (GC) electrode, and a silver nanoparticle-modified GC electrode is reported. It is shown, in phosphate buffer (0.05 mol L^–1, pH 7.4), that hydrogen peroxide cannot be detected directly on either the EPPG or GC electrodes. However, reduction can be facilitated by modification of the glassy-carbon surface with nanosized silver assemblies. The optimum conditions for modification of the GC electrode with silver nanoparticles were found to be deposition for 1 min at –0.5 V vs. Ag from 5 mmol L^–1 AgNO₃/0.1 mol L^–1 TBAP/MeCN, followed by stripping for 2 min at +0.5 V vs. Ag in the same solution. A wave, due to the reduction of hydrogen peroxide on the silver nanoparticles is observed at –0.68 V vs. SCE. The limit of detection for this modified nanosilver electrode was 2.0×10^–6 mol L^–1 for hydrogen peroxide in phosphate buffer (0.05 mol L^–1, pH 7.4) with a sensitivity which is five times higher than that observed at a silver macro-electrode. Also observed is a shoulder on the voltammetric wave corresponding to the reduction of oxygen, which is produced by silver-catalysed chemical decomposition of hydrogen peroxide to water and oxygen then oxygen reduction at the surface of the glassy-carbon electrode.     

Temperature dependence of the polarization resistance of the O<Subscript>2</Subscript>,Au/La<Subscript>0.88</Subscript>Sr<Subscript>0.12</Subscript>Ga<Subscript>0.82</Subscript>Mg<Subscript>0.18</Subscript>O<Subscript>2.85</Subscript> electrode system

The impedance of a porous gold electrode in contact with solid electrolyte La_0.88Sr_0.12Ga_0.82Mg_0.18O_2.85 and the effect of the manufacture conditions on its polarization resistance are studied at 600–800°C in air. The overall oxygen reaction rate on a gold electrode is described as the sum of two partial constituents, namely, the oxygen exchange at the gas/electrolyte interface at the gold/gas/electrolyte triple-phased boundary.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 190–197.Original Russian Text Copyright © 2005 by Shkerin, Sokolova, Khlupin, Beresnev.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

Adsorption of ethyl xanthate anions on a silver electrode. An in-situ FTIR study

The adsorption of ethyl xanthatc ion (EX⁻) on a silver electrode in sodium sulphate, phosphate buffer (pH 6.8) and borate buffer (pH 9.2) solutions was studied in situ using Fourier transform IR reflection spectroscopy. The measurements were carried out using a thin-layer flow cell, which allows a continuous supply of the electroactive species into the thin layer. The voltammetric behaviour of the silver electrode in EX⁻-containing solution is characterized by the formation of silver ethyl xanthate (AgEX), which is preceded by a prewave due to chemisorption of ethyl xanthate. An IR band at 1220 cm ⁻¹, assigned to chemisorbed ethyl xanthate, was observed in spectra obtained in the chemisorption region. All IR bands characteristic of AgEX were observed in spectra of a silver electrode surface covered by a few monolayers of AgEX. The in-situ IR measurements also showed that the adsorption of EX⁻ starts directly on the positive side of the potential of zero charge of silver (−0.7 V/SHE).     

Linear scan stripping voltammetry of copper(II) at the chemically modified carbon paste electrode

A new chemically modified electrode (CME), -benzoinoxime (CUPRON) modified carbon paste electrode, for determining copper(II) is reported because of its excellent selectivity and sensitivity. The electrode is made by mixing a quantity of CUPRON (25%, w/w) with graphite powder (50%, w/w) and paraffin oil (25%, w/w). The CME preferentially deposits copper from the pH 8.5 NH₃-NH₄Cl buffer solution containing copper(II) under an open circuit and most of metal ions do not interfere with the measurements. The detection limit (S/N of three) for determining Cu(II) is 3 × 10^–10 g/ml after 10 min accumulation in fast linear scan stripping voltammetric measurement. Linear calibration curves are obtained for Cu(II) concentration ranged from 1 × 10^–8 M to 1 × 10^–6 M. The response can be maintained with relative standard deviation of 6.0% in a 5 × 10^–6 M Cu(II) solution after eight accumulation/measurement/ regeneration cycles at the same electrode surface. The effect resulted from carbon paste preparation, reduction potential, electrode renewal, electrolyte and solution pH, preconcentration time, concentration dependence, possible interference and other variables has been evaluated. As for application, the CME demonstrates its high sensitivity and copper-selectivity in complex composition samples, such as anodic mud and polluted water.     

Precise and accurate determination of halogens in organic compounds using an amalgamated gold electrode

Precise and accurate titration of halogens in organic and inorganic materials has been studied. The halogen was titrated with coulometrically generated silver and determined potentiometrically by using an amalgamated gold indicator electrode. An argentometric titration curve using the amalgamated gold indicator electrode showed a large jump in potential at the endpoint as well as in mercurometric determination. Sodium carboxymethyl cellulose (CMC) and aluminum nitrate were added as they are most effective for clarifying the sample solution and eliminating adsorption of the halide ion on the silver halide generated. The electrolytes used were 0.5 M nitric acid containing 0.1% CMC and 0.2 M aluminum nitrate. Approximately 0.2 mmol of chloride, bromide, and iodide ions were titrated with high precision and accuracy. Their standard deviations were 0.03, 0.02, and 0.02%, respectively, with no apparent systematic error. The precise determination of halogens, employing oxygen flask combustion and dissolution of the halide salts in the electrolyte, was carried out with ±0.1% absolute error using samples weighing between 20 and 30 mg.     

Electrokinetic Potential of Particles in Suspensions and Donnan Equilibrium in a Thin Gel Film

The influence exerted by the thickness of a gel layer on the difference of the electric potentials of phases and on the distribution of the potential near the phase boundary are considered in terms of the classical theory of the Donnan equilibrium in a two-phase system constituted by a gel and an electrolyte solution. The results obtained make it possible to explain a number of experimental facts.__________Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 2, 2005, pp. 224–228.Original Russian Text Copyright © 2005 by Bibik.     

Development of cysteine-modified screen-printed electrode for the chronopotentiometric stripping analysis of cadmium(II) in wastewater and soil extracts

Screen-printed carbon electrodes were fabricated with amino acid functionality by using in situ co-deposition of mercury and cysteine. The three-electrode configuration (graphite carbon working electrode, carbon counter electrode and silver/silver chloride reference electrode) incorporating a cysteine-modified working electrode exhibited good sensitivity towards cadmium(II). Several experimental variables affecting the sensor stripping response were characterised and optimised. These include cysteine and mercury concentrations, deposition time, deposition potential and stripping current. Surface analysis was also conducted using scanning electron microscopy (SEM) in order to characterize the electrode surface during cadmium analysis. The stripping chronopotentiometric response for cadmium(II) was linear in the concentration range 0.4–800 g L^–1 when a deposition time of 2 min was used. A detection limit of 0.4 g L^–1 was obtained using 0.025 M Tris–HCl buffer containing 0.1 M KCl (pH 7.4) as the supporting electrolyte. The analytical utility of the cysteine-modified sensor was demonstrated by applying it to cadmium analysis in various wastewater and soil samples collected from a contaminated site and extracted using acetic acid. The results obtained using the developed electrodes agreed satisfactorily with the values achieved using atomic absorption spectrometry and inductively coupled plasma mass spectrometry analysis. These results demonstrate the feasibility of using this type of sensor for cadmium analysis.     

Does thiol-functionalized viologen monolayer memorize anion present when forming the monolayer?

We investigated the effect of the presence of an additive anion while forming a self-assembled monolayer of a thiol-functionalized redox active species upon the behavior in aqueous solutions. A bromide salt was added in an acetonitrile solution of a thiol-functionalized viologen (viologen-thiol: N-pentyl-N′-(11-mercapto)undecyl-4,4′-bipyridinium bishexafluorophosphate) in which the self-assembled monolayer was formed on a polycrystalline Au electrode. We examined the structure and electrochemical behavior of the resulting monolayer-modified electrode in three different aqueous electrolyte solutions. The viologen-thiol monolayer prepared in the presence of Br⁻ exhibited obviously different behavior in both KBr and KF electrolyte solutions from that prepared in the absence of Br⁻. On the other hand, the difference was minor in KPF₆ solution. The difference of the electrochemical behavior was represented by the coverage of the viologen-thiol and the formal potential of viologen dication/radical cation redox couple. The memory that the viologen-thiol monolayer was formed in the presence of Br⁻ was retained in KF and KBr solutions, though it was erased in the electrolyte solution containing PF₆⁻, which is a softer anion than Br⁻. However, no definitive difference of the film structure was deduced from the electroreflectance study in regard to the monomer content and the average orientation of the viologen moiety.     

Adsorption of urea on a polycrystalline silver electrode; comparison of electrochemical and radiometric methods

The adsorption of urea on a polycrystalline silver electrode was studied by radiometry and impedance spectroscopy. The differential capacity of the silver electrode in 0.01 M NaClO₄ solution containing urea in concentrations from 10^–6 to 5×10^–4 M has been determined. The isotherms of urea adsorption, found from the capacitance and radiometric measurements have been compared. The experimental data were described by the Langmuir isotherm, and the Gibbs energy of adsorption was calculated. The urea adsorption takes place in the entire range of the applied potential. The process is reversible with respect to the electrode potential and the bulk urea concentration.Dedicated to Professor Gyorgy Horanyi on the occasion of his 70th birthday     

Variations in the Overall Charge and Open-Circuit Potential of a Palladium Electrode during the Adsorption of Iodine and Iodide Anions

Transients of the current and the open-circuit potential during the adsorption of I^– and I₂ on Pd/Pt (supporting electrolyte 0.5 M H₂SO₄) are measured and so are potentiodynamic and galvanostatic charging curves, in the presence and absence of adlayers. Dependences of the overall electrode charge on potential are constructed. After the adsorption of both I^– and I₂, values of the electrode surface charges at potentials of the double-layer region turn negative and considerably differ in their magnitude. The latter is connected with different character of adlayers, which is shown by experiments on the adsorption of silver atoms at underpotentials after a preliminary adsorption of I^– and I₂ on the electrode surface. Certain discrepancies between experimentally-found and theoretically-expected values of transients are attributed to a substantial increase in the irreversibility of sorption and desorption of hydrogen and oxygen in the presence of iodine atoms adsorbed on the surface.     

Simultaneous Voltammetric Determination of Nitrites and Nitrates in Waters

A rapid procedure was developed for the simultaneous voltammetric determination of nitrites and nitrates from a single sample of potable, mineral, or underground water. Nitrites were first determined upon the anodic polarization of a graphite electrode in a sodium sulfate supporting electrolyte. Next, ascorbic acid and a copper salt were added, and the solution was acidified to an optimum pH value. A copper electrode was formedin situ, and the nitrate signal was recorded as the first derivative using cathodic sweep. The procedure does not require sample preparation or oxygen removal from the solution with a nitrogen flow. The copper–graphite electrode was regenerated electrochemically in the same solution. The analytical range for nitrite ions was 0.2–10.0 mg/L in the presence of any amount of nitrates. Nitrates were quantitatively determined in the range 0.5–5 mg/L. Common cations and anions in waters did not interfere with the determination. The procedure is suitable for the analysis of potable, mineral, underground, and surface waters. The procedure was verified by the added–found method and by the photometric procedure GOST (State Standard) 18826-73.     

Potentiostatic voltammetry at spherical electrodes and microelectrodes in the presence of product

An explicit analytical solution for the complete anodic–cathodic I–E–t response of a reversible electrode process in presence/absence of amalgamation under potentiostatic conditions is presented. To obtain this solution we have taken into account the electrode curvature, using in the case of amalgamation, Koutecký’s approximation (i.e. the finite electrode volume has not been considered). Explicit expressions for the concentration profiles and the surface concentrations have been also deduced. All the results are applicable to electrodes of any radius including ultramicroelectrodes when both species are soluble in the electrolytic solution for any value of the ratio D_O/D_R. When amalgamation takes place, analytical results were compared with the numerical ones deduced by using the rigorous condition of null net flux in the electrode centre, pointing out that Koutecký’s approximation remains valid even for electrode radius and time values at which the diffusion layer reaches the electrode centre. For high electrode sphericities the I–t curves present a cross-linking for applied potential values higher than the equilibrium potential which is the cause of the appearance of a “peak” near to the anodic limit of the I–E curve.     

Current Production Processes on Metallic Electrodes in Contact with Oxygen-Conducting Solid Electrolytes with Face-Centered Cubic Lattice of the Fluorite Type

The data concerning the processes of current production that occur in the electrode system O₂, Pt/O²⁻, which are available from relevant literature sources, are considered. Major progress in the understanding of the kinetics of an oxygen reaction has been attained with the application of model electrodes and cells, as well as at the expense of the application of such physical methods as a radiotracer method and atomic force microscopy. A discussion of model notions on the specific features of the occurrence of an oxygen reaction, which are formed on the basis of consideration of experimental results is the major subject matter of this communication. It is claimed that modernization of an existing model with allowance made for the fact of the existence, on the surface of an electrolyte with a face-centered cubic lattice, of a layer with another structure gives one a chance to also describe experimental results that cannot be described in the framework of an existing model.__________Translated from Elektrokhimiya, Vol. 41, No. 7, 2005, pp. 787–803.Original Russian Text Copyright © 2005 by Shkerin.     

Accumulation and voltammetric measurement of silver at zeolite-containing carbon-paste electrodes

Silver-sensitive chemically modified electrodes are constructed by incorporating zeolites into a conventional carbon-paste mixture. Ion-exchange preconcentration is followed by medium-exchange to an electrolyte solution where the surface-bound silver is measured. The silver response is evaluated with respect to preconcentration time, electrode composition, silver concentration, pH, “cleaning” solution, possible interferences and other variables. Short preconcentration times permit convenient measurements down to the sub-mg l^?1 concentration level. The surface is renewed by placing the electrode in a sodium carbonate solution. For six accumulation/measurement/renewal cycles with a 1 mg l^?1 sample, the relative standard deviation was 3.9%. Similar measurements of mercury are also illustrated.