Insight of electrolyte-free voltammetry at microelectrodes

Distance (w)-changeable Pt wire electrodes and the resistivity of water when voltage over 1 V was applied to the two electrodes.

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High frequency faradaic rectification voltammetry at microelectrodes

An experimental setup for carrying out faradaic rectification measurements at micrometer-sized electrodes under potential control is described. A new method of data analysis is proposed that allows the determination of the standard rate constant and the electron-transfer coefficient of a fast charge transfer process without knowing the impedance of the microelectrode. This method is based on the frequency dependence of the shape of the faradaic rectification voltammograms (i.e., the average width of the peaks and the ratio of the peak heights) rather than on the magnitude of the faradaic rectification signal. The method was tested in the determination of heterogeneous electron transfer kinetics of Fe(CN)6(3-/4-) and Ru(NH3)6(2+/3+) in aqueous solutions on a platinum microelectrode (12.5 microm in radius) and ferrocene/ferrocinum redox couple in a dimethylformamide solution on a gold microelectrode (12.5 microm in radius).     

Reactant adsorption in cyclic staircase voltammetry on spherical microelectrodes. Reversible redox reactions

On spherical microelectrodes, cyclic staircase voltammograms of reversible redox reactions complicated by the reactant adsorption depend on lateral interactions in the monolayer. Attraction forces increase the separation between the main wave and the post-peak, while the repulsion prevents the resolution of responses of the surface and volume reactions. The theoretical relationships between the dimensionless responses and the dimensionless adsorption constant, the relative surface adsorption capacity, the Frumkin coefficient and the electrode sphericity are given.     

Analytical theory of the catalytic mechanism in square wave voltammetry at disc electrodes

A simple analytical expression is presented for the study of the first-order catalytic mechanism using Square Wave Voltammetry (SWV) at disc electrodes. These electrodes are extensively used in electrochemical studies but modelling the electrochemical response at this geometry is complex and usually requires the use of sophisticated numerical methods. By contrast, the analytical solution presented in this work is easy to compute and it is applicable to any size of the disc and for arbitrary kinetics of the catalytic reaction. The effects of the electrode size, the homogeneous rate constants, the frequency and the square wave amplitude on the SWV response are analyzed. Criteria are given for the detection of the steady-state response as well as procedures for the extraction of the catalytic rate constant from the value of the peak current. The theory is applied to obtain the kinetics of the reduction of the anion nitrite by an electrogenerated heteropolyanion [P(W(3)O(10))(4)](4-) at gold microdiscs.     

Magnetic focusing of redox molecules at ferromagnetic microelectrodes

The electrochemical behavior of Fe and Ni disk-shaped microelectrodes (25 to 250 μm radius) in a uniform magnetic field (1 T) is reported. Magnetization of the ferromagnetic microelectrode generates a magnetic field gradient across the depletion layer that can be used to focus paramagnetic molecules toward the electrode surface. The magnetic force acting on the depletion layer is given by F_∇B=(χ/μ)B·∇B, where χ and μ are the volume magnetic susceptibility and magnetic permeability of the depletion layer, respectively, and B is the magnetic field. Magnetic field focusing of the nitrobenzene radical anion at Fe and Ni microelectrodes is demonstrated.     

Underpotential deposition and anodic stripping voltammetry at mesoporous microelectrodes

Using the technique of liquid crystal templating a series of high surface area mesoporous platinum microelectrodes was fabricated. The underpotential deposition of metal ions at such electrodes was found to be similar to that at conventional platinum electrodes. The phenomena of underpotential deposition, in combination with the intrinsic properties of mesoporous microelectrodes (i.e. a high surface area and efficient mass transport) was exploited for the purpose of anodic stripping voltammetry. In particular the underpotential deposition of Ag(+), Pb(2+) and Cu(2+) ions was investigated and it was found that mesoporous microelectrodes were able to quantify the concentration of ions in solution down to the ppb range. The overall behaviour of the mesoporous electrodes was found to be superior to that of conventional microelectrodes and the effects of interference by surfactants were minimal.     

Square wave voltammetry assay of organophosphorus inhibition on cholinesterase in two phases of isooctane/water

A novel method for the detection of pesticides by inhibition of cholinesterase (ChE) from earthworm was developed. Two immiscible phases are employed where the organic phase isooctane contains substrate and the aqueous phase does enzyme. Water insoluble indophenol acetate was hydrolyzed by ChE at the interface of two phases to produce water soluble indophenol. The latter spontaneously penetrates into aqueous solution and gives the change of electrochemical signal. Organophosphorus compounds methyl parathion dissolved in isooctane phase could inhibit the activity of ChE at the interface of two phases and a corresponding inhibition relationship is given in the concentration range of 50 ng/ml–100 μg/ml.     

Computational electrochemistry: Lattice Boltzmann simulations of voltammetry at microelectrodes

This paper describes the development of a two-dimensional lattice Boltzmann method for the simulation of mass transport at a range of microelectrode geometries using potential step and linear sweep voltammetric techniques. The simulations of the current response for microband and micro hemicylinder geometries were compared with previously published analytical solutions. Excellent agreement between the numerical models and the analytical solutions was observed for a series of experimental parameters. To illustrate the flexibility of the lattice Boltzmann method the simulation of the current response for a range of electrode geometries, distorted from microband electrode geometry to micro hemicylinder electrode, is also described.     

Mercury-coated platinum microelectrodes for steady-state voltammetry in aqueous solutions at high temperature

Sphere-cap mercury microelectrodes fabricated on a platinum disk substrate were tested in aqueous solutions over the temperature range 295–353 K. The performance of these electrodes was assessed in solutions containing Ru(NH₃)₆Cl₃ and LiClO₄ as supporting electrolyte. From the steady-state limiting current obtained at three different mercury microelectrodes, the diffusion coefficient of the electroactive species at different temperatures was determined. It was found that the diffusion coefficient values were consistent with the Stokes–Einstein and activation energy models. The results obtained also allowed us to conclude that sphere-cap mercury microelectrodes can be useful for elevated temperature electroanalysis in aqueous solutions.     

Microwave induced jet boiling investigated via voltammetry at ring-disk microelectrodes

High intensity microwave radiation is (self-)focused at metal electrodes immersed in aqueous electrolyte solutions to generate highly localized superheating and convection effects. It is shown that, for an electrode pointing downward, low intensity microwave radiation causes density driven convective flow (upward), which at the onset of boiling abruptly switches to a fast jet of liquid moving away from the electrode surface (downward). This "jet-boiling" phenomenon allows extremely high rates of mass transport and mixing to be realized at the electrode surface. Cyclic voltammograms obtained at electrodes placed into a microwave field show very strong mass transport enhancement effects. Cyclic voltammograms recorded at a Pt/Pt ring-disk electrode system (r(1) = 25 microm, r(2) = 32 microm, r(3) = 32.4 microm) in the presence of microwave radiation are employed to further explore mass transport effects under microwave conditions. Mass transport coefficients, collection efficiencies, and temperatures are determined as a function of microwave intensity.     

Molecularly imprinted polymer solid-phase extraction coupled to square wave voltammetry at carbon fibre microelectrodes for the determination of fenbendazole in beef liver

A molecularly imprinted polymer was developed and used for solid-phase extraction (MISPE) of the antihelmintic fenbendazole in beef liver samples. Detection of the analyte was accomplished using square wave voltammetry (SWV) at a cylindrical carbon fibre microelectrode (CFME). A mixture of MeOH/HAc (9:1) was employed both as eluent in the MISPE system and as working medium for electrochemical detection of fenbendazole. The limit of detection was 1.9 × 10⁻⁷ mol L⁻¹ (57 μg L⁻¹), which was appropriate for the determination of fenbendazole at the maximum residue level permitted by the European Commission (500 μg kg⁻¹ in liver). Given that the SW voltammetric analysis could not be directly performed in the sample extract as a consequence of interference from some sample components, a sample clean-up with a MIP for selectively retaining fenbendazole was performed. The MIP was synthesized using a 1:8:22 template/methacrylic acid/ethylene glycol dimethacrylate ratio. A Britton–Robinson Buffer of pH 9.0 was selected for retaining fenbendazole in the MIP cartridges, and an eluent volume of 5.0 mL at a flow rate of 2.0 mL min⁻¹ was chosen in the elution step. Cross-reactivity with the MIP was observed for other benzimidazoles. The synthesized MIP exhibited a good selectivity for benzimidazoles with respect to other veterinary drugs. The applicability of the MISPE-SWV method was tested with beef liver samples, spiked with fenbendazole at 5,000 and 500 μg kg⁻¹. Results obtained for ten different liver samples yielded mean recoveries of (95 ± 12)% and (96 ± 11)% for the upper and lower concentration level, respectively.     

The dual reciprocity method: simulation of potential step voltammetry at microelectrodes

The dual reciprocity method (DRM) is presented as an efficient and powerful method for the analysis of electrochemical processes occurring under diffusional transport control. This article outlines the theory and numerical details required to apply the DRM for modelling chronoamperometry at microband and microcylinder electrodes. The results obtained are found to be in good agreement with analytical approximations for the two geometries. The benefits of the DRM approach are discussed, including the reduction in dimensionality brought about by the formulation procedure which permits a two dimensional diffusion problem to be modelled using a one dimensional boundary mesh. The flexibility of the method for tackling geometries of complex topography are noted.     

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.     

Square wave voltammetry for analytical determination of paracetamol using cobalt microparticles film modified platinum electrode

Cobalt microparticles (Co MPs) modified Pt electrode is simply and conveniently fabricated. The electrochemical properties of paracetamol (PCT) at the prepared modified electrode are investigated using cyclic voltammetry (CV) and square wave voltammetry (SWV) measurements. Based on these techniques, a sensitive and rapid electrochemical method is developed for the determination of PCT. The result indicates that the oxidation of PCT is strongly improved at the Co MPs/Pt electrode as compared with the bare Pt electrode, with relatively high sensitivity, stability and life time. The determination of PCT on the Co MPs/Pt with square wave voltammetry displays a high sensitivity of 101 μA/mM and a low detection limit of 0.42 μM (S/N = 3) in the range (0.5–100 μM). The sensitivity of the modified electrode for the detection of PCT is almost 17 times greater than on the bare Pt electrode. The proposed method is successfully applied to the PCT determination in tablets.     

Square wave voltammetry of nitrofurans in aqueous media using a carbon fiber microelectrode

The nitrofuran compounds exhibit a wide spectrum of biological action, and the understanding of their reduction mechanism can contribute to a better comprehension of their biological action. In this sense, the electrochemical reduction of nitrofurans has been studied through square wave voltammetry (SWV) in aqueous media, having a carbon fiber microelectrode as working electrode. One single cathodic peak in acidic media is registered for all studied drugs (nitrofurazone, nitrofurantoin, and furazolidone), being the hidroxylamine derivative formation linearly dependent on pH. In alkaline media, the peaks reduction is not pH-dependent, but a split of the original into two distinct peaks is observed. The evaluation of the results obtained in unprecedented form by SWV for different alkaline pH media, suggests that the charge transfer process for the R-NO₂ ^?? formation is followed by irreversible chemical reaction, which chacraterizes an EC mechanism.     

Multiple square wave voltammetry for analytical determination of paraquat in natural water, food, and beverages using microelectrodes

This paper reports on the use of multiple square wave voltammetry (MSWV) for analytical determination of paraquat herbicide at gold microelectrode (Au-ME) in different samples of natural water, food, and beverages. In this work, the MSWV consisted in a sequence of four pairs of potential pulse in the same step and the interval potential evaluated was of the 0.0 V at −1.2 V versus Ag/AgCl 3.0 mol L⁻¹. The paraquat herbicide presented two reduction peaks, in −0.69 V and −0.99 V, with profile of the redox process totally reversible, and the use of multiple pulses allowed a detection of nanomolar levels after the optimization of experimental and voltammetric conditions. Analytical curves were constructed for pulse potential frequency of 250 s⁻¹, pulse amplitude of 50 mV, scan increment of 2 mV and pulse number of 8 pulses in a same step. The two reduction peaks showed that the peak currents were found to be directly proportional to the pesticide concentration in the range comprised between 5.0 × 10⁻⁷ mol L⁻¹ and 1.04 × 10⁻⁵ mol L⁻¹. With this, it was possible to determine detection limits (DL), which resulted in 0.044 μg L⁻¹ (0.044 ppb) and 0.146 μg L⁻¹ (0.146 ppb), respectively, for peak 1 and peak 2. DL results, obtained using MSWV, were 2–3 orders of magnitude lower (10⁻² to 10⁻³) less than those observed for traditional square wave voltammetry or published in literature, clearly pointing to the advantages arising from the possibility of using a MSWV for analytical purposes in contaminated matrices. In addition, the proposed methodology was applied in different samples of natural water, food and beverages without pre-treatment or pre-concentration step, where a recovery measurement indicated that the methodology could be employed to analyze paraquat in such matrices.     

Square wave adsorptive stripping voltammetry of molybdenum(VI) in continuous flow at a wall-jet mercury film electrode sensor

An adsorptive stripping voltammetry method for the determination of traces of molybdenum(VI) in flowing solution at a wall-jet electrode sensor has been developed. After adsorption of a molybdenum complex on a wall-jet mercury film electrode, the complex is reduced by a square wave scan. More satisfactory results were obtained using 8-hydroxyquinoline as a complexing agent in nitrate medium than using Toluidine Blue in oxalic acid. Enhanced sensitivity was achieved by optimizing adsorption time and square wave parameter values. The detection limit of Mo(VI) was found to be at the nanomolar level. Interference of some other metallic species in the determination of nanomolar Mo(VI) was also investigated: Cu(II), Zn(II), Mn(II) do not interfere at 10 muM, whereas 1 muM FeEDTA(-) causes an increase in peak current. This iron interference was removed effectively with citric acid.     

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(NH₃)₆³⁺ 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.