Mass Transport at Microband Electrodes: Transient,Quasi‐Steady‐State,and Convective Regimes

Mass transport at microband electrodes is investigated theoretically and experimentally in unstirred solutions by chronoamperometry and cyclic voltammetry. Because natural convection limits the convection‐free domain up to which diffusion layers may only expand, several regimes of mass transport are identified through simulation by means of a previous model. A zone diagram is established which allows all relative contributions to mass transport to be delineated according to the electrode dimension, timescale of experiment, and amplitude of natural convection. In opposition to the quasi‐steady‐state regime usually expected at microband electrodes under diffusion control, a steady‐state regime always occurs at long enough times. By comparison to microdisk electrodes, a greater influence of natural convection is predicted. These results are validated experimentally by monitoring current responses and mapping steady‐state concentration profiles at microband electrodes.     

Development and characterization of a new conducting carbon composite electrode

A new conducting composite flexible material prepared from cellulose acetate (CA) polymer and graphite has been developed and used for the fabrication of electrodes, which were then characterized by cyclic voltammetry and electrochemical impedance spectroscopy. Scanning electron microscopy (SEM) was used to provide information concerning the morphology of the composite electrode surface. The potential window, background currents and capacitance were evaluated by cyclic voltammetry in the pH range from 4.6 to 8.2. The voltammetry of model electroactive species demonstrates a close to reversible electrochemical behaviour, under linear diffusion control. The electroactive area of the composite electrodes increases after appropriate electrode polishing and electrochemical pre-treatment. The electrodes were used as substrate for the electropolymerisation of the phenazine dye neutral red, for future use as redox mediator in electrochemical biosensors. The composite electrodes were also successfully used for the amperometric detection of ascorbate at 0.0 V vs. SCE, and applied to the measurement of ascorbate in Vitamin C tablets; the sensor exhibits high sensitivity and a low detection limit of 7.7 μM. Perspectives for use as a versatile, mechanically flexible and robust composite electrode of easily adaptable dimensions are indicated.     

Comparison of fundamental and second-harmonic a.c., and normal,derivative and differential pulse linear-sweep and stripping voltammetric methods

Linear-sweep and stripping a.c. and pulse voltammetric methods have been compared for a variety of electrodes and electrode processes. Each of the linear-sweep techniques is readily used systematically because, in contrast to d.c. linear-sweep voltammetry, the theory for reversible electrode processes is basically analogous to that for polarography at a dropping mercury electrode. In stripping analysis, some departures are found at a hanging mercury drop electrode because of spherical diffusion effects. For reversible electrode processes, the limits of detection for a.c. and pulse methods are comparable. However, a.c. methods offer advantages over pulse methods in discriminating against irreversible electrode processes and permit the ready use of faster scan rates. Pulse methods are more sensitive for irreversible electrode process. Normal pulse polarography is particularly favourable in minimizing undesirable phenomena arising from adsorption or deposition of material on electrodes.     

Laser Activated Voltammetry: Mass Transport,Surface Effects and Analytical Applications

The voltammetry of various well-characterized aqueous and nonaqueous electrochemical systems has been studied at platinum and gold disk electrodes under illumination from a 10 Hz pulsed Nd:YAG laser frequency doubled to operate at 532 nm. A simple Nernst diffusion layer model is established to quantify the slight enhancement in mass transport observed as a function of laser intensity in the thermoelastic region where light energy absorbed by the metal is insufficient to cause localized melting or vaporization but does lead to a partial thinning of the diffusion layer thickness through surface heating/vibration. This leads to sigmoidal shaped voltammograms whilst maintaining a clean, reproducible electrode surface. Above the ablation threshold, the minimum laser intensity required to cause electrode damage, atomic force microscopy (AFM) is used to probe the nature of the surface damage and its relationship to the laser intensity. The Nernst-diffusion model is verified by means of potential step chronoamperometric measurements in water and acetonitrile where good agreement with theory is seen for transport across a diffusion layer of a thickness corresponding to that inferred from steady-state voltammetry. Applications of the laser activation technique are illustrated by three systems found to be passivating in aqueous media; the two electron reduction of toluidine blue dye, iodide oxidation and the oxidation of ferrocyanide in the presence of the blood protein, fibrinogen. In all cases clean, reproducible and quantitative voltammetry is seen in contrast to that observed in the absence of laser activation, demonstrating the excellent surface-cleaning effects of LAV.     

Voltammetric measurements of the interaction of metal complexes with nucleic acids

Cyclic voltammetry and differential-pulse voltammetry at mm-sized electrodes were used to measure the decrease in the rate of diffusion of metal complexes upon binding to DNA and to extract the binding constants and effective binding site sizes. A linear correlation was observed between the site size determined electrochemically and the diameter of the complexes [site size: Cu(phen)2(2+) > Fe(phen)3(2+) > Co(bipy)3(3+) approximately Fe(bipy)3(2+) > Ru(NH3)6(3+)]. The binding constants were found to be influenced by the charge of the metal complex, the nature of ligand and the geometry about the metal centre. Competition experiments, in which differential pulse voltammetry was used to observe the release of bound metal complex on addition of a second DNA-binding molecule to the solution, were sensitive to the nature and location of the binding sites for the two species. Steady-state voltammetric experiments at microelectrodes are shown to have a number of advantages over cyclic voltammetry and differential pulse voltammetry at mm-sized electrodes for determination of binding constants. In particular, the steady-state diffusion limited current is directly proportional to the diffusion coefficient, rather than its square root, which improves the discrimination between DNA-bound and freely diffusing metal complex. Further, the kinetics of the binding process do not affect the steady state measurement, whereas for transient techniques, e.g., cyclic voltammetry, only a range of values can be extracted corresponding to the limits of fast and slow binding kinetics compared to the experimental timescale.     

Electrochemical characterisation of macroporous electrodes: Recent advances and hidden pitfalls

Investigating the intrinsic kinetics of macroporous electrodes by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) is an exceptionally challenging task with a multitude of hidden pitfalls. The classical analysis (e.g. the Randles–?ev?ík or the Nicholson methods) cannot be used for macroporous electrodes as these can only be applied to electrodes which exhibit mass transport controlled by planar semi-infinite diffusion and that have spatially uniform redox activity. Unfortunately, a number of recent articles incorrectly interprete data by applying classical analysis to macroporous electrodes. To address this, the CV and EIS analysis methods, which can be used for macroporous electrodes are reviewed and discussed.     

Additive differential pulse voltammetry,instead of double differential pulse voltammetry

A new electrochemical double pulse potential technique called additive differential pulse voltammetry (ADPV) is proposed. This technique is inspired by the original idea of Birke et al. [Anal. Chem. 53 (1981) 852] of recording two differential pulse (DP) voltammograms and it consists of plotting the sum of these two signals versus the first pulse potential, although in this paper the proposal is to obtain the ADPV signal through just one experiment. ADPV behaves in an identical way to the triple-pulse technique double differential pulse voltammetry (DDPV) for reversible processes when diffusion coefficients are equal for spherical electrodes and for any value of diffusion coefficients in planar electrodes. In the case of reversible electrode processes with amalgamation of reaction product or other more complex processes, ADPV is more advantageous than DDPV. This is due, among other reasons, to the fact that, under these conditions, a double potential step is much simpler to analyse than a triple potential step.     

Oxygen reduction at the surface of glassy carbon electrodes modified with anthraquinone derivatives and dyes

The preparation, electrochemical and catalytic behaviour of glassy carbon electrodes modified by anthra-9,10-quinone, its amino derivatives and dyes were investigated. The stability of the modified electrodes was studied by cyclic voltammetry in acidic and neutral media. The electrocatalytic ability of the modified electrodes for the reduction of dioxygen to hydrogen peroxide was examined by cyclic voltammetry, chronoamperometry and chronocoulometry techniques. The influence of pH on the electrochemical and catalytic behaviour was studied and pH 5.0–8.0 was chosen as the optimum working pH by comparing the shift in oxygen reduction potential. The anthraquinone-adsorbed glassy carbon electrodes possess excellent electrocatalytic abilities for dioxygen reduction with overpotential ranging from 280 to 560 mV lower than that at a plain glassy carbon electrode. Hydrodynamic voltammetric studies were performed to determine the heterogeneous rate constants for the reduction of O₂ at the surface of the modified electrodes, mass specific activity of the anthraquinones used and the apparent diffusion coefficient of O₂ in buffered aqueous O₂-saturated solutions. Studies showed the involvement of two electrons in dioxygen reduction.     

A comparison of electrochemically pre-treated and spark-platinized carbon fiber microelectrode. Measurement of 8-oxo-7,8-dihydro-2′-deoxyguanosine in human urine and plasma

A novel method of carbon fiber microelectrode activation using spark discharge was demonstrated and compared to conventional electrochemical pretreatment by potential cycling. The spark discharge was performed at 800 V between the microelectrode connected to positive pole of the power supply and platinum counter electrode. Spark discharge led both to trimming of the fiber tip into conical shape and to the modification of carbon fiber microelectrode with platinum, as proven by scanning electron microscopy and electron dispersive X-ray spectroscopy. After the characterization of electrochemical properties using ferricyanide voltammetry, the activated electrodes were used for electrochemical analysis of 8-oxo-7,8-dihydro-2′-deoxyguanosine, an oxidative stress marker. Subnanomolar detection limits (0.55 nmol L⁻¹) in high-performance liquid chromatography were achieved for spark platinized electrodes incorporated into the flow detection cell.     

Simulation of electrochemical behavior of partially blocked electrodes under linear potential sweep conditions

The paper presents nonlinear model which stands for effective digital simulation of electrochemical behavior of partially blocked electrodes under linear potential sweep and cyclic voltammetry conditions. The model is based on a system of diffusion equations, also involving the Nernst diffusion layer. The mass transport is assumed to be regular in the entire diffusion space. The influence of the thickness of the resist layer on the behavior of the partially blocked electrodes is investigated. The agreement between the theoretical results and experimental ones is obtained to be admirable for several model electrodes with different blocking degree.     

Characterization of Pt nanoparticles deposited onto carbon nanotubes grown on carbon paper and evaluation of this electrode for the reduction of oxygen

Multiwalled carbon nanotubes (MWCNTs) were grown on the fibers of a commercial porous carbon paper used as carbon-collecting electrodes in fuel cells. The tubes were then covered with Pt nanoparticles in order to test these gas diffusion electrodes (GDEs) for oxygen reduction in H2SO4 solution and in H2/O2 fuel cells. The Pt nanoparticles were characterized by cyclic voltammetry, transmission electron microscopy, and X-ray photoelectron spectroscopy. The majority of the Pt particles are 3 nm in size with a mean size of 4.1 nm. They have an electrochemically active surface area of 60 m2/g Pt for Pt loadings of 0.1-0.45 mg Pt/cm2. Although the electroactive Pt surface area is larger for commercial electrodes of similar loadings, Pt/MWCNT electrodes largely outperform the commercial electrode for the oxygen reduction reaction in GDE experiments using H2SO4 at pH 1. On the other hand, when the same electrodes are used as the cathode in a H2/O2 fuel cell, they perform only slightly better than the commercial electrodes in the potential range going from approximately 0.9 to approximately 0.7 V and have a lower performance at lower voltages.     

Recent Advances in Voltammetry

Recent progress in the theory and practice of voltammetry is surveyed and evaluated. The transformation over the last decade of the level of modelling and simulation of experiments has realised major advances such that electrochemical techniques can be fully developed and applied to real chemical problems of distinct complexity. This review focuses on the topic areas of: multistep electrochemical processes, voltammetry in ionic liquids, the development and interpretation of theories of electron transfer (Butler–Volmer and Marcus–Hush), advances in voltammetric pulse techniques, stochastic random walk models of diffusion, the influence of migration under conditions of low support, voltammetry at rough and porous electrodes, and nanoparticle electrochemistry. The review of the latter field encompasses both the study of nanoparticle-modified electrodes, including stripping voltammetry and the new technique of ‘nano-impacts’.     

Poly(Methyl Red) Modified Glassy Carbon Electrodes: Electrosynthesis,Characterization, and Sensor Behavior

Poly(methyl red), PMR, was electropolymerized on glassy carbon electrodes by potential cycling in 50 mM phosphate buffer solution at pH 7.0 and 8.0 and Britton Robinson buffer solution in the pH range 7.0‐11.0. The electrochemical behavior of PMR modified electrodes was investigated by cyclic voltammetry in Britton Robinson buffer solution at different pHs from 5.0 to 10.0 and found that the best PMR film formation was obtained at pH 9.0. Uric acid was quantitatively determined at PMR modified electrodes by cyclic voltammetry and differential pulse voltammetry in Britton Robinson buffer at pH 5.0. Both methods presented a linear dependence between the anodic peak current and the concentration of uric acid in the range of 0.4 to 60 μM and 0.08 to 100 μM with the limits of detection of 0.038 and 0.009 μM for cyclic voltammetry and differential pulse voltammetry, respectively. Poly(methyl red) as redox mediator allowed the determination of uric acid without any interferences from the substances in serum samples.     

Voltammetric Characterization of Micro‐ and Submicrometer‐Electrode Arrays of Conical Shape for Electroanalytical Use

Densely packed micro‐ and submicrometer electrode arrays of platinum and gold (the nominal number, N, of electrodes in each array varies between 225 and 3600) are fabricated by photolithographic technique and vapor deposition processes of metal films. The electrodes are conical‐shaped and only their apexes are exposed to the electrolytic solution. The electrode arrays are characterized electrochemically in Ru(NH₃)₆Cl₃ aqueous solutions by using cyclic voltammetry at low scan rates, to establish the number of electrochemically active electrodes (N_ac) in each array; the geometric characterization is performed by scanning electron microscopy. All the investigated arrays provide steady‐state voltammograms, indicating diffusionally independent behavior of each microelectrode. The number of microelectrodes that are active in the fabricated arrays depends on microelectrode density. In particular, for the arrays with N=3600 and N=225, the fraction of active sites is about 45% and 90%, respectively. The analytical performance of some of the Pt version of the arrays is tested in hydrogen peroxide solutions, allowing verifying that linear calibration plots over the concentration range (0.1–20 mM) are obtained. This dynamic range is larger than that typically recorded at smooth polycrystalline platinum electrodes (0.5–5 mM), and the better performance is attributed to both the higher aspect ratio of the cone geometry and the higher mass transport associated to each microelectrode of the array. Reproducibility (within 3.5%, r.s.d.) and long‐term stability (within 5%, r.s.d., after 8 h continuous use) of the electrode systems are satisfactory. A low detection limit, based on the signal to noise ratio equal to 3, of 0.05 mM is found, which is adequate for a rapid monitoring of H₂O₂ in real samples and industrial processes.     

Kinetics of electrochemical lithium intercalation into thin tungsten (VI) oxide layers

The methods of galvanostatic intermittent titration, cyclic voltammetry, and electrode impedance spectroscopy are used to study the behavior of tungsten (VI) oxide film electrodes free of binding and conducting additives in the course of reversible lithium intercalation from nonaqueous electrolyte at 25°C. The studies are performed for electrodes with different degrees of crystallinity at the variation of the lithium concentration in intercalate from zero to 0.017 mol/cm³. Lithium diffusion coefficient is in the range of 10^?11–10^?16 cm²/s. The concentration dependences of the intercalation-layer transport parameters are analyzed, the equivalent circuit versions are discussed, and results obtained by different methods are compared.     

Development of screen-printed carbon electrodes,chemically modified with cobalt phthalocyanine,for electrochemical sensor applications

A method is described for the production of screen-printed graphite electrodes and also for similar electrodes chemically modified with the electrocatalyst cobalt phthalocyanine. Using cyclic voltammetry, the electrochemical behaviour of these electrodes towards ascorbic acid, reduced glutatione and coenzyme A (CoA-SH) was investigated. The modified electrodes were found to give significant decreases in the over-potential required for the oxidation of these species at carbon electrodes. The useful electrochemical window for the unmodified carbon film electrodes was ?1.08 V to +0.85 V vs. SCE, using 1 μA background current cut-off points.Amperometry in stirred solutions was used to investigate the hydrodynamic behaviour of the electrodes and their calibration performance. The limits of detection for ascorbic acid and reduced glutathione at the modified films were 5 × 10^?8 and 1 × 10^?7 M, respectively. The calibration graphs were also linear up to 2 mM concentrations of both analytes. Using differential-pulse voltammetry, linear calibration graphs were obtained for both species up to 2.5 mM. This technique was also used to assess the reproducibility of the electrode manufacture; the coefficient of variation was 2.8% for 1.49 mM ascorbic acid and 6.9% for 0.92 mM reduced glutathione.     

Gibbs free energy of liquid drops on conical fibers

Small drops can move spontaneously on conical fibers. As a drop moves along the cone, it must change shape to maintain a constant volume, and thus, it must change its surface energy. Simultaneously, the exposed surface area of the underlying cone must also change. The associated surface energies should balance each other, and the drop should stop moving when it reaches a location where the free energy is a minimum. In this paper, a minimum Gibbs free energy analysis has been performed to predict where a drop will stop on a conical fiber. To obtain the Gibbs free energies of a drop at different locations of a conical fiber, the theoretical expressions for the shape of a droplet on a conical fiber are derived by extending Carroll's equations for a drop on a cylindrical fiber. The predicted Gibbs free energy exhibits a minimum along the length of the cone. For a constant cone angle, as the contact angle between the liquid and the cone increases, the drop will move toward the apex of the cone. Likewise, for a constant contact angle, as the cone angle increases, the drop moves toward the apex. Experiments in which water and dodecane were placed on glass cones verify these dependencies. Thus, the final location of a drop on a conical fiber can be predicted on the basis of the geometry and surface energy of the cone, the surface tension and volume of the liquid, and the original location where the drop was deposited.     

Effect of Surface Structure on Behavior of RuO2 Electrodes in Sulfuric Acid Aqueous Solution

The electrochemical properties of RuO₂ electrodes are studied by means of cyclic voltammetry, potential step, and impedance measurements in aqueous 0.5 mol/dm³ H₂SO₄ as a supporting electrolyte solution and applying the hanging electrolyte meniscus technique. Two types of the electrodes are used: bulk as-grown single-crystal having (101) exposed interface and 500 nm film produced by reactive RF magnetron sputtering at 450°C. The surface structure of the RuO₂ electrodes prepared by different techniques is defined from X-ray LAUE backscattering. Experimental data show that, for both RuO₂ electrodes, the charging components include a slow diffusion-controlled contribution, due to proton injection–ejection. The diffusion characteristics and the diffusion coefficients for the same electrodes are estimated and reported.     

Characterization of amperometry for in vivo measurement of dopamine dynamics in the rat brain

Constant potential amperometry with Nafion-coated carbon-fiber electrodes has been evaluated as a technique for in vivo detection of the neurotransmitter dopamine. The results of this technique have been compared to results obtained with fast-scan cyclic voltammetry at the same electrode during release of dopamine into the extracellular space of the brain during electrical stimulation of neurons. The data indicate that constant potential amperometry is a viable technique for detecting low concentrations of dopamine. Dopamine permeates the film more quickly with constant-potential amperometry than with repeated fast-scan cyclic voltammetry as predicted by diffusion equations. For the case of cyclic voltammetry, it is demonstrated that the temporal delay caused by diffusion through Nafion film can be removed by deconvolution procedures. Despite the suitability of constant potential amperometry as an in vivo monitoring technique, it does have several disadvantages when compared to fast-scan cyclic voltammetry. The diffusion layer extends outside of the Nafion film making determination of concentration based on in vitro calibrations more difficult to interpret. The reported concentrations are larger than obtained by cyclic voltammetry, a technique with the diffusion layer restricted to the Nafion film, and this result is likely an underestimation of the effect of the catalytic reaction between the o-quinone of dopamine and ascorbate. Amperometry was found to provide only slightly improved signal-to-noise ratios than cyclic voltammetry despite the use of greater filtering. This was because the advantage of dopamine accumulation in the film was lost. In addition, the small magnitude of the amperometric signal makes it more susceptible to electrical interference.     

Poly(3-methylthiophene)/palladium sub-micro-modified sensor electrode. Part II: Voltammetric and EIS studies, and analysis of catecholamine neurotransmitters, ascorbic acid and acetaminophen

Promising voltammetric sensors based on the modification of Pt and poly(3-methylthiophene) (PMT) electrodes with Pd nanoparticles were achieved for the determination of catecholamine neurotransmitters, ascorbic acid and acetaminophen. Electrochemistry of the indicated compounds was studied at these electrodes and interesting electrocatalytic effects were found. Furthermore, simple, easily prepared one electrochemical step Pd-modified Pt electrode (Pt/Pd) is reported for the first time. Cyclic voltammetry (CV) and chronocoulometry (CC) were used for the determination of the apparent diffusion coefficients in different electrolytes at these electrodes and the values are in the range from 10⁻⁴ to 10⁻⁵ cm² s⁻¹. Furthermore, it was found that the method of polymer formation had a substantial effect on the synergism between the polymer film and the loaded metal particles towards the oxidation of dopamine (DA) in different supporting electrolytes. This was confirmed by the CV, CC and EIS (electrochemical impedance spectroscopy) as well as SEM (Scanning Electron Microscopy) results. Pt and PMT electrodes modified with Pd nanoparticles showed excellent results for the simultaneous determination of tertiary and quaternary mixtures of the studied compounds.