Thin Mercury Film Electrode Research Articles

Carbon dioxide electrochemical sensor based on lipid cubic phase containing tetraazamacrocyclic complexes of Ni(II)

Liquid-crystalline cubic phases formed by polar lipids in an aqueous medium were used as hosting layers for catalytic tetraazamacrocyclic Ni(II) complexes. The role of the cubic phase is to hold the catalyst close to the electrode surface, to retain its catalytic activity and to provide suitable environment for the catalytic reaction. The properties of the cubic phase Q 224 (monoolein:water:catalyst) were investigated by microscopy with polarized light and electrochemical techniques. Nickel (II) complexes with derivatives of 1,4,8,11-tetraazacyclotetradecane were incorporated into bicontinuous cubic liquid-crystalline layer modifying the thin mercury film or glassy carbon electrode and used for the study of catalytic reduction of carbon dioxide. The reproducibility of the catalytic current of CO 2 reduction was found better than obtained using other methods based on modified electrodes. The linear dependence of catalytic reduction current on the concentration of CO 2 allowed the use of the system for carbon dioxide sensing in the solution. The sensor described in the present report was found useful also in case of CO 2 determination in the gas phase. In such environmentally interesting measurements, CO 2 was absorbed directly by the cubic phase in which the three electrodes: working, counter and reference were placed.

Redox Reactions of and Transformation between Cysteine−Mercury Thiolate and Cystine in Metallothioneins Adsorbed at a Thin Mercury Film Electrode

Voltammetric studies of rabbit liver metallothioneins (MTs) adsorbed onto thin mercury films preformed onto glassy carbon electrodes were carried out in MT-free phosphate buffers in an attempt to shed light on the possible redox-modulated MT metal-transfer processes. The redox behavior of the surface-confined MTs was studied by cyclic voltammetry and differential pulse voltammetry, and the amount of MT adsorption was quantified by a flow-injection quartz crystal microbalance. Two reversible redox waves, with Ep values at −0.63 and −0.91 V, respectively, were observed for the first time. These values and the overall voltammetric characteristics were found to be remarkably analogous to those of cysteine adsorbates at thin mercury films. The peak at Ep = −0.63 V is attributable to the reduction of the Cys−mercury thiolates formed between the adsorbed MTs and the mercury electrode, whereas that at Epc = −0.91 V is assigned to the reduction of Cys-Cys (cystine analogue) present in the portion of the MT adsorbate that is not in direct contact with the mercury film. The two redox waves were observed to be interchangeable through preelectrolysis at a negative potential (e.g., −1.1 V) to reduce the MT adsorbate, or at a more positive potential (e.g., −0.1 V) to oxidize the adsorbed MTs. On the basis of these voltammetric results, we proposed a simple schematic to elucidate the redox reactions of and the transformation between the cysteine−mercury thiolates and cystines that are present in the MT adsorbates under different electrochemical redox conditions. Since the electrochemical reduction of the cystine analogues in the MT adsorbates to the corresponding cysteines (a process facilitating metal complexation) and the reoxidation of the cysteine residues back to cystine analogues (a process causing metal release) are both reversible, it appears that the metal transfer between MT and a substrate might accompany the variation of the redox states of the MT−metal complexes. Our voltammetric studies of MTs thus provide supportive evidence for the mechanism of the MT metal transfer in cytoplasmic milieu proposed by Vallee and co-workers (Maret, W.; Vallee, B. L. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 3478−3482).

Nafion-Coated Mercury Thin Film and Glassy Carbon Electrodes for Electroanalysis: Characterization by Electrochemical Impedance

The different behavior of Nafion-coated mercury thin-film electrodes in anodic stripping voltammetry in nitrate and acetate buffer electrolytes has been probed by electrochemical impedance studies. Investigations concerned glassy carbon electrodes, glassy carbon coated with Nafion, mercury thin film electrodes and mercury thin film electrodes deposited between the glassy carbon substrate and the polymer film, with values of applied potential similar to those employed in anodic stripping voltammetry. Comparison is made with results obtained using Nafion/5 % poly(vinyl sulfonic acid) coatings. In the absence of cationic reducible species, acetate buffer leads to larger impedance values than nitrate electrolyte and correlates well with better characteristics of stripping signals at Nafion-based coatings on mercury thin-film electrodes.

Kinetic Speciation of Lead and Cadmium in Freshwaters Using Square-Wave Anodic Stripping Voltammetry with a Thin Mercury Film Rotating Disk Electrode

Square-wave anodic stripping voltammetry with a thin mercury film electrode was used to investigate the kinetic speciation of lead and cadmium in freshwaters. Diffusion coefficients and dissociation rate coefficients of lead and cadmium species were determined in an ultrafiltered sample of Rideau River surface water (RRSW). The RRSW sample was first filtered through 0.45 μm cellulose acetate filters and then fractionated into <50 000 D, <10 000 D and < 1000 D molecular weight cut-off (MWCO) fractions by cascade (sequential) ultrafiltration. Diffusion coefficients of lead and cadmium complexes in the RRSW samples were significantly lower (<50%) than those for the lead and cadmium aquo complexes. Two groups of kinetically distinguishable lead and cadmium species were determined in the RRSW samples: one of fast-dissociating complexes and another of slowly-dissociating complexes. Dissociation rate coefficients of the fast-dissociating complexes of lead and cadmium increased with decreasing MWCO fractions. Dissociation rate coefficients of the slowly-dissociating complexes of lead and cadmium had the smallest values in the 10 000 D MWCO and 1000 D MWCO fractions, respectively. The above results agree with a model of the metals forming complexes with humic substances, which are ubiquitous in freshwaters; the polyfunctional and the oligoelectrolytic properties of humic substances mainly determined the thermodynamic stability and kinetic lability of the complexes.

Use of a Spline Function of a Fractional Degree for the Description of the Base Line in the Determination of Platinum by Stripping Voltammetry

The systematic error in measuring the analytical signal of Pt obtained at a mercury thin-film electrode with staircase potential sweep has been studied. The description of the residual current by splines of degree 2.5 reduces the average systematic error by the factor of 4.3 as compared to the linear description. It is shown that splines of degree 2.5 are more stable as compared to the cubic splines, which suggests that the random error arising from the subtraction of the residual current is reduced when the spline degree decreases from 3 to 2.5.

Use of differentiation and smoothing in linear-sweep and staircase stripping voltammetry of some metals

Two methods of signal smoothing were considered: square-wave (moving average) and triangular (weighted average) filters. It was found that smoothing by these two filters did not distort noticeably the signal shape and the linearity of calibration plots. Calibration plots for several series of analytical signals of heavy metals were studied. Analytical signals were obtained by stripping voltammetry using a mercury thin-film electrode and processed using the peak area, the peak height, and the range of the extrema of the first and second peak derivatives. It was shown that, for staircase stripping voltammetry, the linearity of calibration plots decreases substantially in this series of signal processing methods. For linear sweep, calibration plots were linear over the entire concentration range for all signal-processing methods.

Electrochemistry of Azocrown Ethers in Langmuir-Blodgett Monolayers

Monomolecular films of amphiphilic derivatives of crown ethers bearing an azo group in the macrocycle were prepared on surfaces of pure water and transferred onto electrodes using the Langmuir-Blodgett technique. The azocompounds studied were separated into Z and E stereoisomers. Monolayers of both isomers of azocrowns were transferred onto the surfaces of hydrophilic (thin mercury film electrode, TMFE) and hydrophobic (indium-tin oxide, ITO) electrodes. The electrode processes showed more reversible cyclic voltammetry profiles when mercury was used instead of ITO as the electrode substrate. This difference was ascribed to the different orientation of the molecules on the electrode surface.

Determination of selenium(IV) by a photooxidized 3,3'-diaminobenzidine/perfluorinated polymer mercury film electrode

A new, and easily fabricated, chemically modified electrode for the determination of selenium(IV) was examined by cathodic square-wave stripping voltammetry. This new electrode consisted of an anion-exchange perfluorinated polymer (Tosflex) coated thin mercury film electrode containing photooxidized 3,3'-diaminobenzidine (ODAB). The coating solution of Tosflex and ODAB was spin-coated on a glassy carbon electrode followed by electroplating of a thin film of mercury. During the preconcentration, ODAB was reduced electrochemically and selenium was accumulated simultaneously onto the electrode by interacting with the reduced ODAB. After a 5-min preconcentration period, linear response was observed from 0.5 to 50 ppb selenium, and the detection limit was 0.1 ppb. The proposed method does not require a darkened room, which was required in many of the previous methods involving 3,3'-diaminobenzidine. In addition, the resistance to interference from surface-active compounds was improved by incorporating Tosflex in the film.

Modified electrode voltammetric sensors for trace metals in environmental samples

Nafion-modified mercury thin film electrodes have been investigated for the analysis of trace metals in environmental samples of waters and effluent by batch injection analysis with square wave anodic stripping voltammetry. The method, involving injection over the detector electrode of untreated samples of volume of the order of 50 microlitres has fast response, blocking and fouling of the electrode is minimum as shown by studies with surface-active components. Comparison is made between glassy carbon substrate electrodes and carbon fibre microelectrode array substrates, the latter leading to a small sensitivity enhancement. Application to analysis of river water and industrial effluent for labile zinc, cadmium, lead and copper ions is demonstrated in collected samples and after acid digestion.

Determination of Cu, Pb, Cd, and Zn in river sediment extracts by sequential injection anodic stripping voltammetry with thin mercury film electrode

Determination of Cu, Pb, Cd and Zn was performed in sediment extracts obtained according to the three steps sequential extraction procedure proposed by the European Community Standards, Measurements and Testing Program. The metal content was determined by anodic stripping voltammetry with a thin mercury film electrode controlled by a sequential injection (SIA) system. The proposed method improved the reproducibility of conventional anodic stripping voltammetry, as well as the sample throughput, allowing analysis of 30 to 45 samples per hour. The influence of flow rate and sample volume was studied to achieve an adequate sensitivity for the leachate studied. No interferences due to adsorption of organic matter, colloids, or complexes with slow rate of dissociation were observed. The intermetallic formation of Cu-Zn was avoided by forming the mercury film in presence of Ga(III) ions in the SIA system, resulting in low consumption of reagent in comparison to flow injection or continuous flow systems. Results were in good agreement with those obtained by Induced Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES).

Coulometric determination of lead(II) and lead(IV) in high-T c superconductors containing copper and bismuth

The behavior of Pb(II) and Pb(IV) was studied by voltammetry in supporting electrolytes containing 0.1 M NaOH or 0.1 M NaOH + 0.1 M Na3Cit at a graphite and a mercury thin-film electrodes. The best conditions were found for the coulometric determination of Pb by the oxidation of Pb(0) to Pb(II) in the presence of 5- to 20-fold amounts of Bi(III) and Cu(II) within the range 0.2–2 mg Pb(II) with an error and a relative standard deviation of less than 0.5%. Along with the procedure proposed previously for the determination of Pb(II) and Pb(IV) present simultaneously using a platinum gauze electrode, this procedure was applied to the determination of Pb(II) and Pb(IV) in samples of high-Tc. Cu-Bi superconductors. For samples of high-Tc. Bi-Pb-A-Cu-0 superconductors (A = Ba or Sr-Ca) containing from 2 to 12% Pb(II), the relative standard deviation was less than 0.5%; for 0.6–7% Pb(IV), it was 1-0.2%.

Complexation of metal ions by azocrown ethers in Langmuir–Blodgett monolayers

Monomolecular films of amphiphilic derivatives of crown ethers bearing an azo group in the macrocycle were formed on the surfaces of pure water or aqueous solutions of metal cations and transferred onto a thin mercury film electrode using the Langmuir–Blodgett technique. The properties of the monolayer modified electrodes were studied by voltammetry. The azocrown molecules assembled in monolayers at the air–solution and electrode–solution interfaces were found to interact with selected alkali metal cations. This was revealed by the changes of the parameters of the surface–pressure and surface–potential isotherms recorded during the formation of monolayers at the air–solution interface, and by the electrochemical behaviour of the monolayer covered electrodes. The stability constants of the complexes formed by Na+, K+ and Ca2+ cations with the neutral (Kox) and reduced (Kred) forms of the macrocyclic ligand in the monolayer, and the reaction coupling efficiency (Kox/Kred), were calculated.

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.

Langmuir–Blodgett films of azocrown ethers on electrodes — voltammetric recognition of isomers

Abstract Monolayers of amphiphilic derivatives of crown ethers bearing electroactive azo groups in the macrocycle were transferred from the air ∣ water interface onto indium–tin oxide (ITO) or thin mercury film electrodes (TMFE) using the Langmuir–Blodgett technique. Differences in the electrochemical reversibility of the systems observed on these two electrodes were explained by different orientation of the azocrown molecules on the hydrophobic (TMFE) and hydrophilic (ITO) electrode surfaces. The electrochemical studies of the monolayer modified electrodes proved that the organization of the monolayer and the redox properties of the azocrown molecules depend on the geometry around the NN moiety. Voltammetry allowed Z - and E -isomers of the azocrowns to be recognized and their isomerization processes in monolayers to be followed. The effects of pH and of the alkali metal cations on the stability of the isomers were studied. Isomerization to the more stable E -isomer was hindered in alkaline solutions and under these conditions well separated reduction peaks of the E - and Z -isomers were obtained.

Novel polymer-modified electrodes for batch injection sensors and application to environmental analysis

Various polymer coatings have been investigated for the protection of mercury thin-film electrodes in the square wave anodic stripping voltammetry of environmental samples using batch injection analysis, with injection of untreated samples of volume 50 μl directly over the sensing electrode. Polymer coatings studied include those with controlled porosity, and cation-exchange polymers based on sulphonated polymers. Of the polymers tested, films of ca. 1 μm thickness made from Nafion® mixed with 5% poly(vinyl sulphonic acid) were found to give the best results in tests with the model surfactants Triton-X-100 detergent, sodium dodecyl sulphate and protein standard. The validity of the approach is demonstrated by application to real samples.

Optimization of Mercury Thin Film Electrodes for Sono-ASV Studies

The feasibility of using Nafion-coated mercury thin film electrodes on glassy carbon, iridium and platinum as substrates for anodic stripping voltammetric determination of cadmium and lead in the presence of ultrasound is examined. Glassy carbon was found to be the most appropriate substrate material for these electrodes. Nafion-coated glassy carbon mercury thin film electrodes, NCGCMFE's, were mechanically resistant under sonication, as shown by AFM imaging. These electrodes showed higher sensitivity (up to 170 times higher for cadmium depending upon deposition times) for the determination of cadmium and lead than glassy carbon mercury thin film electrodes formed without Nafion. A major factor contributing to this behavior appeared to be the greater mechanical stability of the mercury thin film provided by the Nafions-coating under conditions of high mass transport provided by ultrasound. In the case of iridium and platinum-based mercury thin film electrodes, which showed lower sensitivity than NCGCMFE's (14 and 1.4 times more sensitive than mercury films on iridium and platinum, respectively, for the determination of cadmium), the polymer-film became detached after short sonication times, precluding their use for analytical purposes.

Mechanism of interaction of in situ produced nitroimidazole reduction derivatives with DNA using electrochemical DNA biosensor

The electrochemical reduction of nitroimidazoles follows a complex mechanism and in theory the nitro group is able to receive up to six electrons to form the corresponding amine. Under anaerobic and low oxygen pressure conditions metronidazole follows a reduction mechanism similar to that of nitrobenzene. Electrochemical reduction of nitroimidazole derivatives shows two reduction waves in aqueous acid media, the first involving four electrons and corresponding to the reduction of the nitro group to form the intermediate hydroxylamine (-NHOH) and the second involving two electrons and corresponding to the reduction of the hydroxylamine to amine (-NH2). Using three different electrode materials (bare glassy carbon electrode, mercury thin film electrode, and DNA biosensor), it has been verified that hydroxylamine formation involves four electrons and is pH dependent. This chapter shows that the DNA biosensor developed provides a new perspective to the research and study of the mechanism of action of nitroimidazoles with DNA.

Anodic stripping voltammetry of copper at insonated glassy carbon-based electrodes: application to the determination of copper in beer

The suitability of ultrasound-assisted anodic stripping voltammetry (sono-ASV) for the detection of total copper content in beer using both mercury thin film and glassy carbon electrodes has been investigated. An immersion horn probe is introduced into a small thermostatted conventional three electrode cell (20 cm3) opposite the working electrode: an ex situ mercury plated Nafion®-coated mercury film electrode or a bare glassy carbon electrode. Minimal sample pre-treatment is required which consists of acidification of the beer with dilute nitric acid and out-gassing with argon. After the deposition of copper (as the metal or its amalgam) on the electrode in the presence of ultrasound, a square wave scan is employed to get the analytical signal. In the absence of ultrasound, electrode passivation by organic species and lower rates of mass transport prevent the observation of any measurable signals. In situ cavitational cleaning of the electrode by insonation maintains the electrode activity. Total copper content levels in the range of 100 to 300 µg Cu L–1 were determined by sono-ASV using both electrode substrates and showed excellent agreement with values provided by an independent method. This highlights the validity of the sono-ASV method as a useful electroanalytical technique in hostile media.

Zeolite/Polymer-Modification of a Mercury Electrode. Effects on the Cu2+ Interference in the Stripping Determination of Zn2+

The present study was carried out in order to investigate whether zeolite 13X (aperture 7.4 A˚) modification of a mercury electrode could provide discrimination against the interference from Cu2+ in the stripping determination of Zn2+ (utilizing the zeolite ion exchange capability, and the possibility for small differences in size and shape of hydrated Zn2+ and Cu2+ and hence the differences in zeolite affinity to these species). For this purpose a chemically modified electrode (CME) was constructed from a glassy carbon electrode, which was first coated with a zeolite 10X containing polymer matrix then subjected to mercury plating. The analytical characteristics of the CME were subsequently compared with those of a conventional thin mercury film electrode (TMFE). The comparison, which was based on analogous stripping experiments carried out using first the TMFE, then the CME, showed that stripping signals for Zn2+ as well as Cu2+ were obtained with both electrodes. However, the intermetallic compound interference observed for the CME Zn-response was much less pronounced than that encountered for the TMFE Zn-response, thus indicating that the zeolite modification provided a marked discrimination towards deposition of Cu2+. Subsequent standard addition experiments with 60 s depositions at −1200 mV revealed that the Zn-stripping signal obtained with the CME was linear for 0–3.5 μM Zn2+ in the presence of 10 μM Cu2+.

Linear Scan Stripping Voltammetry at Glassy-Carbon Based Thin Mercury Film Electrodes for Determination of Trace Aluminium in Dialysis Fluids.

Linear scan cathodic stripping voltammetry at glassy-carbon based thin mercury film electrodes is a simple and inexpensive alternative for determining trace Al(III) in dialysis fluids. The efficiency of a variety of ligands (SVRS, Cupferron and Blue Black Eriochrome R) was evaluated comparing their voltamperometric response by application of a linear scan mode, after preconcentration into the mercury film electrode as Al(III) complexes. The best results were obtained when Cupferron was used as ligand, since its stripping current compares favourably in terms of sensitivity and resolution. The sharply defined cathodic peak at -1.3 V, corresponding to the reduction of the interfacial accumulated complex, could be used for quantitation. The response is linear up to 50 μg/l; correlation coefficient, 0.995. The relative standard deviation (at 20 μg/l level) is 3.5%, a detection limit of 0.5μg/l was estimated from the signal to noise characteristics of the response for 5 μg/l, which compares favourably among the reported electroanalytical methods for aluminium. We calibrated the method under conditions of use estimating the trace aluminium exposure in patients undergoing dialytic treatment.