Mercury Microelectrodes Research Articles

Importance of microbial iron reduction in deep sediments of river-dominated continental-margins

Remineralization of organic carbon in deep-sea sediments is thought to proceed primarily via aerobic respiration and sulfate reduction because the supply of nitrate and metal oxides is not usually significant in deep-sea sediments. Dissimilatory metal reduction, on the other hand, may represent a dominant pathway in coastal and continental shelf sediments where delivery of terrigenous Fe(III) and Mn(IV/III) oxides is sufficiently high or where physical mixing processes near the sediment–water interface can result in the reoxidation of Fe2+ or Mn2+. Passive continental margin sediments receiving outflow from large rivers are well-known deposition centers for organic carbon and may also be hotspots for metal-reducing microbial activity, considering the simultaneous rapid deposition of unconsolidated metal oxides of terrigenous origin. Despite its potential, only a few studies have examined the role of microbial metal reduction in Corg remineralization in these environments. To investigate carbon remineralization processes in continental margin sediments, shallow cores across channels and levees in the Congo River fan (~5000m) and Louisiana slope (<1800m) were profiled for the main redox species involved in early diagenesis using a combination of voltammetric gold mercury (Au/Hg) microelectrodes and conventional analyses. Interestingly, metal reduction dominated carbon remineralization processes in the top ~20cm of sediment subject to high deposition, while evidence for sulfate reduction was lacking. These findings suggest that dissimilatory Fe(III) reduction may be more significant than previously thought in continental slope sediments, which may have important implications on carbon cycling in marine environments. In addition, these findings may have geological implications in controlling atmospheric oxygen levels over geological time and the evolution of microbial life on Earth.

Non‐Invasive Probing of Nanoparticle Electrostatics

AbstractElectrostatic interactions between surface‐charged nanoparticles (NPs) and electrodes studied using existing techniques unavoidably and significantly alter the system being analyzed. Here we present a methodology that allows the probing of unperturbed electrostatic interactions between individual NPs and charged surfaces. The uniqueness of this approach is that stochastic NP impact events are used as the probe. During a single impact, only an attomole of the redox species reacts and is released at the interface during each sensing event. As an example, the effect of electrostatic screening on the reduction of negatively charged indigo NPs at a mercury microelectrode is explored at potentials positive and negative of the potential of zero charge. At suitable overpotentials fully driven electron transfer is seen for all but very low (&lt;0.005 M) ionic strengths. The loss of charge transfer in such dilute electrolytes is unambiguously shown to arise from a reduced driving force for the reaction rather than a reduced population of NPs near the electrode, contradicting popular perceptions. Electrostatics were found not to significantly affect the reactivity of the studied NPs. Importantly, the presented technique is general and can be applied to a wide variety of NPs, including metals, metal oxides and organic compounds.

AFM Cantilever with in Situ Renewable Mercury Microelectrode

We report here first results obtained on a novel, in situ renewable mercury microelectrode integrated into an atomic force microscopy (AFM) cantilever. Our approach is based on a fountain pen probe with appropriate dimensions enabling reversible filling with (nonwetting) mercury under changing the applied pressure at a connected mercury supply in a dedicated experimental setup. The fountain pen probe utilizes a special design with vertical pillars inside the channel to minimize mechanical perturbation. In proof of principle experiments, dropping and hanging mercury drop were observed as a function of the applied pressure at the external mercury supply. Electrical conductivity occurred only through the mercury after filling, and the empty fountain pen probe showed excellent electrical insulation. This was demonstrated by chronoamperometric measurements in the electrolyte and by mechanical and electrical contacting of an ITO substrate with a mercury-filled and empty probe in air. Finally, cyclic voltammetry and square wave voltammetry were done in a static mercury electrode fountain pen configuration, demonstrating the principle usability of the mercury probe for electrochemical studies. Our findings are of fundamental importance as they enable further integration of a renewable mercury electrode probe into an AFM setup, which is the subject of ongoing work.

Giving physical insight into the Butler–Volmer model of electrode kinetics: Application of asymmetric Marcus–Hush theory to the study of the electroreductions of 2-methyl-2-nitropropane, cyclooctatetraene and europium(III) on mercury microelectrodes

The asymmetric Marcus–Hush (MH) model for electrode kinetics is applied to the kinetic study of the electroreduction of 2-methyl-2-nitropropane in acetonitrile, cyclooctatetraene in dimethylsulfoxide and europium(III) in aqueous solution, using mercury microhemispheres as working electrodes. This kinetic model includes the possibility of the oxidative and reductive processes having different reorganization energies due to differences between the force constants of the electroactive species.For each redox couple, the response obtained in cyclic and square wave voltammetries can be fitted satisfactorily with the four-parameter asymmetric MH model. From the fitting of the voltammograms the values of the kinetic parameters are extracted and analyzed in terms of physical properties of the electroactive species. A comparison of the asymmetric model against the simpler, phenomenological Butler–Volmer (BV) approach is discussed, as well as a possible physical interpretation for the BV transfer coefficient.

Quantitative weaknesses of the Marcus-Hush theory of electrode kinetics revealed by Reverse Scan Square Wave Voltammetry: The reduction of 2-methyl-2-nitropropane at mercury microelectrodes

The Marcus-Hush and Butler–Volmer kinetic electrode models are compared experimentally by studying the reduction of 2-methyl-2-nitropropane in acetonitrile at mercury microelectrodes using Reverse Scan Square Wave Voltammetry. This technique is found to be very sensitive to the electrode kinetics and to permit critical comparison of the two models. The Butler–Volmer model satisfactorily fits the experimental data whereas Marcus-Hush does not quantitatively describe this redox system.

Cyclic voltammetry in weakly supported media: The reduction of the cobaltocenium cation in acetonitrile – Comparison between theory and experiment

Experimental cyclic voltammetry at a hemispherical mercury microelectrode in acetonitrile solution, containing 3 mM cobaltocenium hexafluorophosphate and different concentrations of supporting electrolyte, is compared with theoretical simulations using the Nernst–Planck–Poisson system of equations, without the assumption of electroneutrality, and is found in to be in good agreement. Deviations from diffusion-only theory are analyzed in terms of migration and potential drop in the solution as a function of the concentration of supporting electrolyte. We are unaware of previous reports in which non-steady-state cyclic voltammetry without supporting electrolyte has been quantitatively and fully simulated, so this work opens up a new area for voltammetry.

Electrochemical studies on liquid properties in extended nanospaces using mercury microelectrodes

We developed a novel nanofluidic chip equipped with mercury microelectrodes, which enables electrochemical measurements to be made in 10-100 nm scale spaces (called extended nanospaces), and evaluated the performances. The effects of both space sizes and concentrations on the conductance (G) values of KCl solutions in extended nanospaces (216-5000 nm) were examined using impedance spectrometry. We found that the experimental G values in the extended nanospaces decreased non-linearly with decreasing KCl concentrations in the range of 10(-2) to 10(-7) M and could be explained by theoretical model taking account of surface charge density of on a glass surface. This was found to result from enhancement of proton concentrations of the confined solution owing to fast proton exchange between SiOH groups on surfaces and water. Moreover, the G values provided the specific resistance and capacitance of KCl solutions in the extended nanospaces. These results showed that the viscosity of KCl solutions increased by size-confinement and that the viscosity of solution in 216 nm-sized extended nanospaces became about 2.8 times as large as that of bulk solution. We concluded that the developed nanofluidic chip becomes a new experimental tool for demonstrating confinement-induced nanospatial electrochemical properties of liquids.

Theoretical and experimental study of Differential Pulse Voltammetry at spherical electrodes: Measuring diffusion coefficients and formal potentials

Rigorous and approximate analytical expressions are deduced for Differential Pulse Voltammetry at spherical electrodes of any size, including microelectrodes, when the electrogenerated species is soluble in the electrolytic solution. From these, we examine the utility of DPV for the determination of diffusion coefficients and formal potentials, establishing the optimum conditions for this purpose. The experimental validation of the theoretical results is reported for mercury microelectrodes of ca. 50 and 10 μm diameters both in aqueous and ionic liquid media.

Reverse Pulse Voltammetry at spherical electrodes: Simultaneous determination of diffusion coefficients and formal potentials. Application to Room Temperature Ionic Liquids

A rigorous analytical solution for Reverse Pulse Voltammetry ( RPV) at spherical electrodes of all sizes is presented, including the case where the diffusion coefficients of the electroactive species are different. From this solution, the value of RPV for the determination of both diffusion coefficients and the formal potential of the redox couple is demonstrated. The experimental application of the theoretical results is carried out both in ionic liquids and in aqueous solution with mercury microelectrodes of ca. 50 and 10 μm diameters. The former solvent system is of particular interest due to the great importance of these new liquids in which notably unequal diffusion coefficients are commonly found.

Application of thin-shielded mercury microelectrodes in anodic stripping voltammetry

The performance in anodic stripping voltammetry (ASV) of hemispherical mercury microelectrodes, fabricated by electrodeposition of liquid mercury on the surface of Pt microdisks which were surrounded by a rather thick or thin insulating shield, was compared. The Pt microdisks were produced by sealing a wire of 25 μm diameter into a glass capillary, and by coating the cylindrical length of the Pt wire with a cathodic electrophoretic paint. The ratio of the overall tip radius b, to the basal radius of the electrode a, so-called RG = b/ a, was equal to 110 ± 10 and 1.52 ± 0.01 for the thick- and thin-shielded microdisk, respectively. The mercury microelectrodes were characterized by cyclic voltammetry at 1 mV s −1, in 1 mM Ru(NH 3) 6 3+ aqueous solution. The steady-state voltammogram recorded with the thin-shielded mercury microelectrode displayed less hysteresis, while the steady-state current was about 30% higher than that of the thicker one. This was a consequence of the additional flux due to diffusion from behind the plane of the electrode. The flux enhancement, which was operative at the thin-shielded mercury microelectrode during the deposition step in the ASV experiments, allowed recording stripping peaks for Cd and Pb, which resulted about 32% larger than those recorded at the thicker shielded mercury microelectrode, under same experimental conditions. The usefulness of the thin-shielded mercury microelectrode for ASV measurements in real samples was verified by determining the content of heavy metal ions released in the pore water (pH 4.5) of a soil slurry.

Microelectrode voltammetry as a high accuracy method for determination of diffusion coefficients

Stripping voltammetry analysis using a hemispheroidal mercury microelectrode has been previously assessed theoretically as a possible candidate ‘primary method’ for amount of substance measurement. It was shown that the measurement methodology can be described completely by a set of measurement equations when a novel quantification process is employed to measure the steady-state diffusion-limited deposition current over an optimum sampling time, instead of employing an analyte stripping step. The corollary is that, for solutions of known composition, the method may be used for highly accurate determination of diffusion coefficients. This investigation uses experimental data from steady-state diffusion-limited current measurements with sampling periods optimised using Allan deviation techniques, together with a full uncertainty analysis to estimate feasibility and likely accuracy of this method.

Cyclic voltammetry studies of Cd 2+ and Zn 2+ complexation with hydroxyl-terminated polyamidoamine generation 2 dendrimer at a mercury microelectrode

Cyclic voltammetry was used to determine the stability constants for the complexation of cadmium and zinc ions with hydroxyl-terminated polyamidoamine generation 2 dendrimers with an ethylenediamine core (PAMAM G2-OH) at a mercury ultramicroelectrode. Cadmium and zinc ions were found to form poly-metal complexes through the interactions with interior amino groups rather than hydroxyl groups and the maximum number of metal ions per dendrimer or stoichiometry was 4. Complexation is accompanied by a significant decrease in the limiting current and by measuring the changes in the magnitude of cathodic current with the analytical dendrimer:metal ion ratio and fitting the resulting curves, the four cumulative consecutive complexation constants were determined: 1.3 × 10 5 M −1, 1.6 × 10 9 M −2, 3.3 × 10 12 M −3 and 4.6 × 10 14 M −4 for Cd 2+ and 1.0 × 10 6 M −1, 1.6 × 10 9 M −2, 4.6 × 10 14 M −3 and 2.6 × 10 17 M −4 for Zn 2+. The stronger interaction of zinc ions with PAMAM G2-OH compared to cadmium was accompanied with a sharper decrease of the cathodic current with the addition of dendrimer and a larger half-wave potential shift than cadmium. This was rationalized by evaluating the changes of the diffusion coefficients for these two different systems by electrochemical means.

Application of Sphere Cap Mercury Microelectrodes and Scanning Electrochemical Microscopy (SECM) for Heavy Metal Monitoring at Solid/Solution Interfaces

AbstractIn this paper, a combination of anodic stripping voltammetry (ASV) at mercury microelectrodes and scanning electrochemical microscopy (SECM) was employed to establish the concentration profiles of heavy metal ions at the soil/water interface of a natural sediment and bauxitic‐soil sample. Air dried soil samples were equilibrated with aqueous solutions containing 0.5 M NaCl plus acetate buffers or HClO4 at different concentrations to allow the achievement of different pH either in the aqueous phase or in the soil slurry. It was verified that mixing acidified aqueous solution and the soil without stirring led to concentration profiles displaying a maximum at the water/soil interface. This was explained as due to pH gradients that originated at the soil/water interface and within the soil slurries. On the other hand, mixing the soils and water solutions, under stirring for 24–48 h, provided almost uniform pH throughout the system. This allowed obtaining uniform metal ion concentrations within the soil slurries. For the evaluation of the concentrations, the anodic charge involved in the stripping step was utilized. The mercury microelectrode was standardized in a 0.5 M NaCl aqueous solution by performing calibration plots over the concentration range 0.1–500 μM. To evaluate the concentrations of the metal ions in the soil slurries, a procedure was developed to obtain the apparent diffusion coefficients of the metal ions, which depended on the physical properties of the soil such as porosity and tortuosity. The microanalytical methodology proposed here can be utilized for predicting the mobilization of trace elements from soils that comes into contact with acid rain.

The use of microelectrodes with AGNES

Absence of gradients and nernstian equilibrium stripping (AGNES) is a new electroanalytical technique designed to determine free heavy metal ion concentrations in solutions. AGNES had been applied, up to date, with conventional equipment such as the hanging mercury drop electrode (HMDE). Due to their much smaller volume, microelectrodes can reach a given preconcentration factor within a much shorter deposition time, so their use for AGNES has been evaluated in this work. For the particular case of the mercury microelectrode deposited onto an Ir disk (radius around 5 μm), AGNES has been successfully used for speciation purposes in the system Pb + PDCA (pyridinedicarboxylic acid). However, due to a relatively large capacitive current, which decays slowly, the limit of quantification for such microelectrodes has only been reduced by one half with respect to that of the HMDE.

Stripping Voltammetry as a Possible Primary Method for Amount of Substance

Stripping voltammetry analysis using a hemispheroidal mercury microelectrode is assessed as a possible candidate "primary method" for amount of substance measurement. It has been shown that this measurement methodology can be described completely by a set of measurement equations. Novel proposals are made to improve the accuracy and traceability of the quantification process by measuring the limiting deposition current over an optimum time instead of employing an analyte stripping step. The range of application and the limitations of the technique are discussed, and the requirement for underpinning traceability from electrical standards is shown. It is concluded that the lack of amount-independent diffusion coefficient data limits the status of the technique to a high-accuracy "definitive method" at present

On-chip generated mercury microelectrode for heavy metal ion detection

In this paper, the on-chip generated mercury microelectrode for heavy metal ion detection has been presented. A mercury droplet (approximately 150 microm in diameter) is on-chip generated to form a microelectrode through the mercury microfluidics control. The mercury microelectrode is used to electrochemically detect the heavy metal ions. The sample solutions with different concentration of heavy metal ions (Pb(2+) and Cd(2+)) have been successfully detected on the mercury droplet microelectrode using the square wave stripping voltammetry.

Remote Stripping Analysis of Lead and Copper by a Mercury‐Coated Platinum Microelectrode

AbstractThe performance of a remote stripping sensor based on mercury microelectrodes (MM‐RS) for the in situ detection of trace metals in aquatic systems, was investigated. The submersible device employed here consists of a single mercury‐coated platinum disk microelectrode assembled in a two‐electrode cell configuration, and connected remotely by a 30 m long shielded cable. First, the MM‐RS device is characterized in Ru(NH3)$\rm{ {_{6}^{3+}}}$ and Pb2+ synthetic aqueous solutions by applying cyclic voltammetry and anodic stripping voltammetry (ASV), respectively. The results obtained show that the small electrode dimensions and the related low currents involved, the long remote connection cable or the use of a two‐electrode system do not cause noise effects or uncompensated resistance problems in the measurements. Using square‐wave voltammetry in the stripping step, linear calibration graphs for Pb2+ ions over the concentration range 1×10−9−5×10−7 M were obtained, and a detection limit, DL, of 0.15 nM was found. The relative standard deviation (RSD), at 5×10−8 M Pb2+ level, was within 5%. The effect of humic acid and of sodium dodecylsulfate surfactants on the stripping responses was also investigated. The performance of the submersible MM‐RS system was tested for the in situ monitoring of the labile fraction of lead and copper on a site of the Lagoon of Venice. In situ Pb2+ and Cu2+ concentrations were monitored for about 8 hours, by leaving the sensor immersed in the lagoon waters (2 m depth) and recording the response every hour. Under these field conditions, reliable in situ data for the labile fraction of these metal ions with a satisfactory precision, the RSD being within 7 and 9 % for lead and copper, respectively, were obtained.

Numerical simulations of linear scan anodic stripping voltammetry at a modified square array of hemispherical microelectrodes located in a thin-layer cell

The finite element method has been used to simulate LSASV measurement at a loosely packed square array of recessed microdiscs and of hemispherical mercury microelectrodes located in a thin-layer cell. The mercury volume (radius=7.5 μm), the protective gel (thickness=300 μm) and the solution medium (thickness=200 μm) have been meshed. The concentrations of the reduced and oxidised species were linked both by the Nernst equation and the equality of the fluxes at the mercury|gel interface. Both overlapping of adjacent diffusion layers and depletion occurring in this finite box decrease the amount of species being reduced during the deposition step. However, the peak width ( W 1/2=39 mV) and peak potential ( E p− E ′0=−34.6±1 mV) of the resulting stripping current are independent of the deposition time. During the stripping step, the metal concentration at the surface of the electrode is much higher than the initial concentration. The time to reach a uniform concentration in the gel again after the voltammetric reoxidation step has been determined, for both single microelectrode and a square array, as a function of the deposition time, thus giving the maximum frequency of successive measurements that should be used. Agreement between simulated and experimental results confirms that diffusion is the predominant transport process occurring in such thin-layer cell. In addition, comparison with analytical expressions has been done whenever possible.

Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 4. The kinetic current regime

The theory for depletive stripping chronopotentiometry at scanned deposition potential (SSCP) has been extended to include the case of kinetic currents. Invoking the Koutecký–Koryta approximation, i.e., assuming a spatially discontinuous transition from labile to non-labile behaviour, allows a rigorous expression to be obtained for the full SSCP wave. Theoretical predictions are supported by experimental data for the quasi-labile Cd–NTA system at a conventional HMDE and a mercury microelectrode. The characteristic features of the SSCP wave for systems influenced by homogeneous kinetics allow dynamic speciation characteristics to be unambiguously identified and discriminated from those due to irreversibility in the electrochemical reaction or heterogeneity in the chemical speciation.

Oxidation of mercury microelectrodes in complexing media in the presence and absence of supporting electrolyte:: Formation of thiocyanate complexes

Normal pulse- and staircase-voltammetric oxidation of a mercury microelectrode in thiocyanate media both with and without supporting electrolyte was investigated. An equation describing the normal pulse voltammetric limiting currents flowing through a nearly spherical segment microelectrode was developed theoretically and verified experimentally. The diffusion coefficients of mercury(II) complexes containing different numbers of the thiocyanate anions were determined. A general time-dependent digital simulation scheme was developed for calculating the current for the oxidation of mercury(0) in the presence of ligands and excess supporting electrolyte and for any pulse voltammetric technique. The time-dependent mass balance equation recently developed for a system, in which diffusion coefficients of reactant and products are different, was applied. A brief review was given on the standard potentials of various mercury systems available in the literature. A general procedure for the evaluation of the stability constants of metal complexes based on the shape of the voltammograms was described and applied to the case of thiocyanato complexes of Hg(II). The stability constant results agreed well with those of Nyman and Alberts [Anal. Chem. 32 (1960) 207]. Also discussed was the determination of the formation constant β 1 for Hg(SCN) +.