Electrodeposition of mercury film on electrodes modified with clay minerals

A new type of carbon paste electrode modified with clay mineral and covered with a mercury film is presented in this work. Electrodeposition of the mercury film was performed on the carbon paste electrode modified with montmorillonite. The mercury film was deposited by both electrodeposition in situ and a preliminary electrodeposition. The pre-deposited film of mercury showed to be suitable for anodic stripping voltammetry. An open-circuit sorption of Cd, Pb, and Cu with subsequent anodic stripping voltammetry exhibited higher current responses of metals. Besides the enhanced sensitivity superior separation of the current responses during a simultaneous stripping of metals is expected to be achieved by means of the newly prepared electrode. Presented at the 57th Congress of Chemical Societies, Tatranské Matliare, 4–8 September 2005.     

Bismuth film electrodes for the study of metal thiolate complexation: An alternative to mercury electrodes

A comparative study of the usual static mercury drop electrode (SMDE) and the bismuth film electrode (BiFE) as applied to the analysis of metal complexation by thiol-rich peptides is done. Preliminary experiments on BiFE by differential pulse voltammetry showed that Cd(II) and Pb(II)-ions behave in a similar way as using stripping voltammetry and stripping chronopotentiometry with regard to some splitting effects of the signals. Additionally, on BiFE glutathione (GSH) and some phytochelatins (PC_n) produce quite irregular signals related to the anodic oxidation of bismuth, which restricted the studies to a narrower concentration range than on SMDE. In the presence of both metal ion and peptide the same characteristic signals were observed on BiFE and SMDE, but better resolution was achieved in the first one, allowing a qualitative analysis of the complexation process for the Pb-GSH system which was not possible on SMDE. This suggests that BiFE may be a complementary tool to Hg electrodes, if not a valuable alternative, in the study of metal complexation.     

Organoclay-enzyme film electrodes

This paper aims at showing the interest of organoclays (clay minerals containing organic groups covalently attached to the inorganic particles) as suitable host matrices likely to immobilize enzymes onto electrode surfaces for biosensing applications. The organoclays used in this work were natural Cameroonian smectites grafted with either aminopropyl (AP) or trimethylpropylammonium (TMPA) groups. The first ones were exploited for their ability to anchor biomolecules by covalent bonding while the second category exhibited favorable electrostatic interactions with negatively charged enzymes due to ion exchange properties that were pointed out here by means of multisweep cyclic voltammetry. AP-clay materials were applied to the immobilization of glucose oxidase (GOD) and TMPA-clays for polyphenol oxidase (PPO) anchoring. When deposited onto the surface of platinum or glassy carbon electrodes as enzyme/organoclay films, these systems were evaluated as biosensing electrochemical devices for detection of glucose and catechol chosen as model analytes. The advantageous features of these organoclays were discussed by comparison to the performance of related film electrodes made of non-functionalized clays. It appeared that organoclays provide a favorable environment to enzymes activity, as highlighted from the biosensors characteristics and determination of Michaelis-Menten constants.     

Comparison of constant-current stripping chronopotentiometry and anodic stripping voltammetry in metal speciation studies using mercury drop and film electrodes

Capabilities for heavy metal speciation of anodic stripping voltammetry (ASV) and constant-current stripping chronopotentiometry (SCP) in both mercury drop (HMDE) and mercury film rotating disk (MFE-RDE) electrodes are compared. For this purpose, the Cd(II)–glycine and Cd(II)–polymethacrylate (PMA) systems are used as models of simple labile and macromolecular labile complexes adsorbing onto the electrode, respectively. The results suggest that SCP could be a valuable alternative to the more widespread ASV in this kind of study. Concerning the electrode, the MFE-RDE is less user-friendly than the HMDE, but presents a better definition of both the hydrodynamic conditions during the deposition step and the stripping regime during the oxidation. An important interference in SCP is the dissolved oxygen, which can be minimised by combining relatively large oxidation currents and low stirring rates. Moreover, for Cd–PMA, double peaks have been observed in both ASV and SCP, which seems to be due to the lack of enough ligand excess to complex the metal ions released by the amalgam oxidation. Anyway, this problem can be minimised by optimising the rotation rate of the electrode and ensuring enough ligand excess.     

The use of Nafion-coated thin mercury film electrodes for the determination of the dissolved copper speciation in estuarine water

Differential pulse anodic stripping voltammetry (DPASV) using a Nafion-coated thin mercury film electrode (NCTMFE) was implemented to determine the dissolved copper speciation in saline estuarine waters containing high concentrations of dissolved organic matter (DOM). The study used model ligands and estuarine water from San Francisco Bay, California, USA to demonstrate that the NCTMFE is more effective at distinguishing between electrochemically inert and labile copper species when compared to the conventional thin mercury film electrode (TMFE). Copper titration results verify that the NCTMFE better deals with high concentrations of DOM by creating a size-exclusion barrier that prevents DOM from interacting with the mercury electrode when performing copper speciation measurements. Pseudovoltammograms were used to illustrate that copper complexes found in natural waters were more apt to be electrochemically inert at the NCTMFE relative to the TMFE when subjected to high negative overpotentials. Copper speciation results using the NCTMFE from samples collected in San Francisco Bay estimated that >99.9% of all copper was bound to strong copper-binding ligands. These L₁-class ligands exceeded the concentration of total dissolved copper in all samples tested and control the equilibrium of ambient [Cu²⁺] in the San Francisco Bay estuary.     

Fast ultrasound-assisted treatment of urine samples for chronopotentiometric stripping determination of mercury at gold film electrodes

This work describes an efficient, fast, and reliable analytical methodology for mercury determination in urine samples using stripping chronopotentiometry at gold film electrodes. The samples were sonicated in the presence of concentrated HC1 and H₂O₂ for 15 min in order to disrupt the organic ligands and release the mercury. Thirty samples can be treated over the optimized region of the ultrasonic bath. This sample preparation was enough to allow the accurate stripping chronopotentiometric determination of mercury in the treated samples. No background currents and no passivation of the gold film electrode due to the sample matrix were verified. The samples were also analyzed by cold vapour atomic absorption spectrometry (CV-AAS) and good agreement between the results was verified. The analysis of NIST SRM 2670 (Toxic Metals in Freeze-Dried Urine) also validated the proposed electroanalytical method. Finally, this method was applied for mercury evaluation in urine of workers exposed to hospital waste incinerators.     

Voltammetric determination of ubiquinone adsorption at mercury electrodes

Adsorption isotherms of the ubiquinones UQ₄, UQ₆, UQ₉ and UQ₁₀ have been determined by two independent voltammetric techniques at the stationary mercury/solution interface. While the redox behavior is essentially the same for all homologues investigated, the molar free enthalpy of adsorption Δ_AG increases steadily with the number of isoprenic units from UQ₄ to UQ₁₀. In case of UQ₁₀, the experiments have been carried out with three different potentiostatic systems. This allows one to estimate the accuracy of the methods which are direct ones and do not require calibration.     

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.     

Poly(ester sulphonic acid) coated mercury thin film electrodes: characterization and application in batch injection analysis stripping voltammetry of heavy metal ions

Mercury-thin film electrodes coated with a thin film of poly(ester sulphonic acid) (PESA) have been investigated for application in the analysis of trace heavy metals by square wave anodic stripping voltammetry using the batch injection analysis (BIA) technique. Different polymer dispersion concentrations in water/acetone mixed solvent are investigated and are characterised by electrochemical impedance measurements on glassy carbon and on mercury film electrodes. The influence of electrolyte anion, acetate or nitrate, on polymer film properties is demonstrated, acetate buffer being shown to be preferable for stripping voltammetry applications. Although stripping currents are between 30 and 70% less at the coated than at bare mercury thin film electrodes, the influence of model surfactants on stripping response is shown to be very small. The effect of the composition of the modifier film dispersion on calibration plots is shown; however, detection limits of around 5 nM are found for all modified electrodes tested. This coated electrode is an alternative to Nafion-coated mercury thin film electrodes for the analysis of trace metals in complex matrices, particularly useful when there is a high concentration of non-ionic detergents.     

氨苄青霉素降解产物在汞膜电极上的伏安法研究

考察了氨苄青霉素在酸、碱条件下的降解产物于玻碳汞膜电极上的伏安行为 ,降解产物在醋酸缓冲溶液 ( p H5.5)的底液中均产生了灵敏的阴极还原峰 ,以碱降解条件为好。已将方法应用于模拟样品的分析。     

Signal stability of Nafion-coated thin mercury film electrodes for stripping voltammetry

The signal stability of the Nafion-coated thin mercury film electrode (NCTMFE) was studied by using cadmium and lead as test analytes and differential pulse anodic stripping voltammetry as detection method. In particular, the effect of the casting solvent and the curing procedure employed in the preparation of the polymer film was examined. Best results were obtained with N,N-dimethylacetamide as casting solvent and a two-step curing procedure in which the polymer was evaporated to dryness at 55 degrees and cured at 105 degrees with a hot-air gun. Mercury plating was performed ex situ. An NCTMFE prepared in this manner has a better signal stability than ex situ-plated as well as in situ-plated conventional mercury film electrodes.     

Optimisation of mercury film deposition on glassy carbon electrodes: evaluation of the combined effects of pH, thiocyanate ion and deposition potential

The combined effects of pH, thiocyanate ion and deposition potential in the characteristics of thin mercury film electrodes plated on glassy carbon surfaces are evaluated. Charges of deposited mercury are used as an experimental parameter for the estimation of the effectiveness of the mercury deposition procedure. The sensitivity of the anodic stripping voltammetry (ASV) method for the determination of lead at in situ and at ex situ formed thin mercury films are also examined. It was concluded that, in acidic solutions (pH 2.5-5.7) and fairly negative deposition potentials, e.g. −1.3 to −1.5 V, thiocyanate ion promotes the formation of the mercury film, in respect both to the amount of deposited mercury and to the mercury deposition rate. Also, the mercury coatings produced in thiocyanate solutions are more homogeneous, as depicted by microscopic examinations. In the presence of thiocyanate there is no obvious advantage of using high concentrations of mercury and/or high deposition times for the in situ and ex situ preparation of the mercury film electrodes. The optimised thin mercury film electrode ex situ prepared in a 5.0 mM thiocyanate solution of pH 3.4 was successfully applied to the ASV determination of lead and copper in acidified seawater (pH 2). The limit of detection (3σ) was 6×10⁻¹¹ M for lead and 2×10⁻¹⁰ M for copper for a deposition time of 5 min. Relative standard deviations (R.S.D.s) of <1.2% were obtained for determinations at the nanomolar of concentration level.     

Applications of small iridium-based mercury electrodes to high concentrations of depolarizers

Voltammetric and microscopic data on the analytical properties of small iridium-based (127- μm diameter) mercury film electrodes are presented. A method of estimating and controlling the size of the deposit is described. The voltammetric behaviour of electrodes with mercury deposits varying from a film to a near-hemisphere is discussed for linear-scan, differential- and normal- pulse voltammetry. The systems Pb²⁺/Pb, Fe³⁺/Fe²⁺ and Co²⁺/Co, are used to show that, by changing the size of the deposit, either time-dependent or almost time-independent (steady-state) voltammograms can be obtained under the same instrumental conditions. Possible applications to the determination of high concentrations of depolarizers are outlined; voltammetric data for the determination of lead in the range 10–120 mM are presented. The evaluation of diffusion coefficients is discussed.     

Two-dimensional condensation of nucleic acid components at mercury film and gold electrodes

The adsorption of cytidine at the mercury film electrodes and at the Au (111) single crystal electrode has been investigated. Some kinetic aspects such as the influence of pH and temperature on the formation or dissolution of cytidine adlayer on the pyrolytic graphite electrode covered by a mercury film or on the Au (111) have been studied.     

A glucose biosensor using methyl viologen redox mediator on carbon film electrodes

A new methyl viologen-mediated amperometric enzyme electrode sensitive to glucose has been developed using carbon film electrode substrates. Carbon film electrodes from resistors fabricated by pyrolytic deposition of carbon were modified by immobilization of glucose oxidase through cross-linking with glutaraldehyde in the presence of bovine serum albumin. The mediator, methyl viologen, was directly immobilised with the enzyme together with Nafion cation-exchange polymer. The electrochemistry of the glucose oxidase/methyl viologen modified electrode was investigated by cyclic voltammetry and by electrochemical impedance spectroscopy. The biosensor response to glucose was evaluated amperometrically; the detection limit was 20 μM, the linear range extended to 1.2 mM and the reproducibility of around 3%. When stored in phosphate buffer at 4 °C and used every day, the sensor showed good stability over more several weeks.     

Nafion-coated mercury thin film electrodes for batch-injection analysis with anodic stripping voltammetry

Batch-injection analysis exhibits the advantages of rapid and simple electroanalysis of microlitre samples. Nafion-coated mercury thin film electrodes have been evaluated for use in batch-injection analysis with anodic stripping voltammetry (BIA-ASV). The advantages of Nafion-coated electrodes in reducing electrode contamination by components of complex matrices are combined with the analysis of small microlitre sample volumes. The measurement of traces of lead and cadmium is used to illustrate the approach. An optimised procedure for formation of Nafion-coated mercury thin film electrodes is evolved. The relative sensitivity for BIA-ASV at electrodes with and without Nafion coatings is 0.9 and 0.8 for cadmium and lead respectively; detection limits are 2 x 10(-9) M and 4 x 10(-9) M. Studies were done concerning the influence of surfactants and their effect was found to be much less with the Nafion film coating. Applications to real environmental samples are demonstrated.     

The determination of copper at thin film electrodes by constant current stripping potentiometry

A comparison of the determination of copper by constant current stripping potentiometry (CCSP) at mercury and gold films has been carried out. The preferred solution conditions for the mercury film study were determined to be 0.1M ammonium acetate at pH 4.5 and 0.1M HCl for the gold film study. The influence of chloride on the stripping signal was investigated and it was found that for the mercury film conditions, well-formed stripping signals could be obtained up to a chloride concentration of 0.5 M which permitted the ready determination of copper in seawater. With the gold film, high chloride concentrations affected both the film stability and the glassy carbon surface and repeatable results were difficult to obtain. The optimized CCSP methods were applied to various aqueous samples including tap water, seawater, TCLP (acetic acid) extracts as well as TCLP extracts using groundwater and ocean water. Based on the results obtained for these various matrices, it was concluded that there are several advantages favoring the mercury film. The interference from organic components in the sample matrix on the general applicability of CCSP for the determination of copper at either a mercury or gold film is discussed.     

Sensitive detection of cyclophosphamide using DNA-modified carbon paste, pencil graphite and hanging mercury drop electrodes

The interaction of cyclophosphamide (CP) with calf thymus double-stranded DNA (dsDNA) and thermally denatured single-stranded DNA (ssDNA) immobilized at the carbon paste (CPE) and pencil graphite electrodes (PGE), was studied electrochemically based on oxidation signals of guanine and adenine using differential pulse voltammetry (DPV).As a result of the interaction of CP with DNA, the voltammetric signals of guanine and adenine increased in the case of dsDNA while a slight increase was observed in ssDNA. The effect of experimental parameters such as the interaction time between CP and DNA forms and the concentration of CP, were studied using DPV with CPE and PGE. Additionally, reproducibility and detection limits were determined using both electrodes. A comparison of the analytical performance between CPE and PGE was done. Our results showed that these two different DNA biosensors could be used for the sensitive, rapid and cost effective detection of CP itself as well as of CP-DNA interaction.Furthermore, the interaction of CP with dsDNA and ssDNA was studied in solution and at the electrode surface by means of alternating current voltammetry (ACV) in 0.3 M NaCl and 50 mM sodium phosphate buffer (pH 8.5) supporting electrolyte, using a hanging mercury drop electrode (HMDE) as working electrode.The conclusions of this study were mainly based on tensammetric peaks I (at −1.183 V) and II (−1.419 V) of DNA. This study involved the interaction of CP with surface-confined and solution phase DNA where experimental parameters, such as the concentration of CP and the interaction time, were studied. By increasing the concentration of CP, an increase of peak II was observed in both ds and ssDNA, while an increase of peak I was observed only in the case of dsDNA. An overall conclusion of the study using HMDE was that the interaction of CP with surface-confined DNA significantly differed from that with solution phase DNA. The increase of peaks I and II was lower in the case of interaction of CP with surface-confined DNA, probably due to steric positioning of DNA at the electrode surface.     

Reliability of background current subtraction in anodic stripping voltammetry at mercury film electrodes

The effectiveness and accuracy of the correction for background current in subtractive anodic stripping voltammetry at rotating mercury film electrodes are discussed. The effects of different experimental parameters on the subtracted baseline are examined. Long deposition periods, at extreme potentials, result in large errors in the background correction. The incomplete background correction is attributed mainly to changes in the morphology of the working electrode. Different approaches for obtaining the subtractive stripping response are compared. Errors are reduced when subtractive differential pulse stripping is used at different convection rates, resulting in a detection limit of 2 × 10^-9 M cadmium with 1-min deposition. Cadmium, lead, and zinc ions at nano molar concentrations were used as test ions.     

Differential pulse anodic stripping voltammetry detection of metallothionein at bismuth film electrodes

As a representation of metalloproteins, metallothionein (MT), which plays important biological and environmental roles such as in the metabolism and detoxification of some metals, was detected at bismuth film electrode (BiFE) by differential pulse anodic stripping voltammetry (DPASV). In pH 2–5.5, two well-defined anodic peaks were produced and attributed to the Zn²⁺ and Cd²⁺ inherent to MT. The calibration plot of DPASV peak currents for Cd²⁺ inherent to MT versus MT concentrations showed a good linearity with a detection limit of 3.86 × 10⁻⁸ mol/L for MT. As a non-toxic excellent electrode material, BiFE shows good performance for detecting MT, and is expected to find further applications in the studies of many other metalloproteins.