On the microelectrode behaviour of graphite–epoxy composite electrodes

Electrodes fashioned from conducting particles embedded in insulating binder show some properties commonly associated with arrays of microelectrodes, viz. independence of current from convective flow and sigmoidal current–voltage curves. We have systematically investigated the electrochemical behaviour of a range of composite formulations. The fraction of the surface area that is conducting and the size of the conducting features have been quantified using the novel technique of conducting atomic force microscopy (C-AFM). We have shown that flow sensitivity and voltammetric behaviour are correlated with results from the C-AFM. The more dilute formulations behave like arrays of microelectrodes, though exhibiting large time constants. The origin of this behaviour is discussed.     

Are the stainless steel electrodes inert?

The electrodes used for electroporation, electrofusion, and electrotransfection, as well as in electrochemotherapy should not interact with the object and release electrolysis products. We studied how far the stainless steel electrodes fulfil these requirements. The results showed that in the case of the exponential electric pulses with amplitude of 3 kV cm(-1) and time constant of 0.5 ms, the stainless steel electrodes released ferrous ions to a concentration of several micromoles per liter. We determined experimentally the dependence of the amount of the released ferrous ions on the pulse parameters and on the ionic strength of the medium. It was shown that the released ferrous ions should not be ignored. They could be of importance for the bleomycin electrochemotherapy.     

β-Sonogel-Carbon electrodes: A new alternative for the electrochemical determination of catecholamines

In this work, a new alternative for the electrochemical determination of catecholamines based on β-cyclodextrin-Sonogel-Carbon electrodes is reported. The incorporation of β-CD and graphite in the preparation of the Sonogel-Carbon material leads to a modification of the electrode surface properties which causes a significant increase in the oxidation peak current of biomolecules such as dopamine, l-epinephrine, d,l-norepinephrine and catechol. This phenomenon might be attributed to the formation of an inclusion complex between β-CD and the catecholamines. The amount of β-CD necessary to form the Sonogel electrode was studied and optimization of electrochemical parameters, perm selectivity and mechanical stability of the sensor are discussed. Scanning electron microscopy and electrochemical impedance spectroscopy measurements were employed to characterize the electrical parameters and the structural properties of the new electrode surface, respectively. Cyclic voltammetry (CV) and Adsorptive differential pulse voltammetry (AdDPV) measurements were also used to explore the electrochemical behaviour of the electrode versus the quoted catecholamines. The β-CD-Sonogel-Carbon electrode offers fast and linear responses towards dopamine, norepinephrine, epinephrine and catechol, with good and low detection limits: 0.164, 0.294, 0.699 and 0.059 μmol L⁻¹, respectively.     

Formation of lithiated gold and its use for the preparation of reference electrodes — an EQCM study

Lithiated gold wires can be used to build reference electrodes with outstanding potential stabilities over several days and even over the course of one year. These electrodes are well suited for investigations in the context of lithium-ion batteries (LIBs). In this work, a detailed procedure for the preparation of such electrodes with tailored mechanical properties, which can be fitted gastight into electrochemical cells using commercially available fittings, is given. The electrochemical lithiation process is studied using the electrochemical quartz crystal microbalance (EQCM) technique, and the differences in lithiation of wire type and thin film type gold electrodes are discussed. All experiments were carried out with two different electrolytes, namely, a LiPF₆ and a lithium bis(trifluoromethane sulfonyl) imide (LiTFSI)-based electrolyte, and we conclude that for a higher lithiation rate and long-term stability, the use of LiTFSI-based electrolyte in the preparation phase is beneficial. The EQCM data provides a better insight in the analysis of film formation processes, like the buildup of the solid electrolyte interphase (SEI) during the lithiation, the rate of deposition of metallic lithium, or additional information on the kinetics of Li-Au alloy formation.

     

401 — The development of bienzyme glucose electrodes

Amperometric glucose electrodes are constructed on the basis of glucose oxidase and peroxidase. The first two types of electrodes employ a Pt or glassy carbon electrode and a bienzyme membrane. In the third type of electrode peroxidase was adsorbed on the organic metal electrode and the electrode obtained was covered with a glucose oxidase membrane. In the first two types of devices the electron exchange between the peroxidase-active center and the electrode is carried out by potassium ferrocyanide.The electrodes possess a linear dependence of the stationary current on the glucose concentration in the range of 0.01–1 mM. The stationary current is attained in 2–4 min. The sensitivity of the first and second types of electrodes shows little dependence in the potential ranging from 0.1 to 0.3 V (vs. saturated Ag⋎AgCl) and from 0.3 to −0.1 V respectively. The lowest sensitivity of the electrode based on organic metal is displayed at 0.1 V; the potential increase (up to 0.2 V) orthe decrease (to −0.1 V) leads to negligible sensitivity rise.The electrodes (types I and II) retain their activity for more than 100 days, whereas the third type for 4–6 days only. These electrodes possess a high selectivity, showing no response to the ascorbic acid and other electrode-active compounds present in blood plasma.     

Electrochemical properties of doped lantanum–nickelate-based electrodes

This work is devoted to the studying of effects of La₂NiO₄ doping with alkaline-earth elements: Ca, Sr, and Ba (at an amount of 15 mol %) on its structural, electrical, and electrochemical properties. The effects of the alkaline-earth element nature, introduction of the Ce_0.8Sm_0.2O_1.9-electrolyte (SDC) component to the functional layer, and the presence of collecting film onto electrochemical activity of the electrodes contacting the Ce_0.8Sm_0.2O_1.9-electrolyte are examined. The doping was found to increase the La2NiO4 full conductivity due to increase in the hole conductivity. The maximal conductivity (at the sample density of 86–89%) was obtained for the Ca-doped composition: 85 S/сm at 700°C, as compared with 65 S/сm for undoped La₂NiO₄. at the same time, the doping was found to deteriorate the electrodes’ electrochemical activity which is likely to be due to loss of interstitial oxygen as a result of the doping. The using of composite electrodes allows increasing polarization conductivity markedly. For instance, at 700°С the conductivity of La₂NiO₄ is 0.25 S/сm²; of its based composite, 0.67 S/сm².     

On the selection of the optimal plasticizer for calix[n]arene-based ion-selective electrodes: Possible correlation between the ion selectivity and the ‘softness’ of the plasticizer

A criterion for the selection of a suitable plasticizer for calix[n]arene-based ion-selective electrodes is discussed. The cation selectivity of plasticized membranes without the ligand was first measured as a reference. The membranes can be roughly classified into two groups. The first group shows cation selectivity in the order Cs⁺+>K⁺>Na⁺>Li⁺. The membranes in the second group are made of phosphorus plasticizers, which show a selectivity in the reverse order. The plasticizers in the first group featured a linear relationship between the dipole moment of the plasticizer (calculated by a PM3 method) and the ratio of cesium selectivity to lithium selectivity. The linear relationship supports the view that the polar membrane which includes a soft plasticizer with a large dipole moment shows selectivity for Cs⁺, whereas the nonpolar membrane including the soft plasticizer with the small dipole moment shows much lower selectivity for Cs⁺. Next, 2-fluorphenyl-2-nitrophenyl ether (FPNPE) which showed the highest Cs⁺ selectivity and tris(2-ethylhexyl)phosphate (TEHP) which showed the highest Li⁺ selectivity were mixed in an appropriate ratio to make membranes with a different affinity for hard ions. The metal selectivities of several crown-based and calixarene-based ionophores were examined in these membranes. Although a few exceptions exist, the polar soft membrane is favorable when the interfering metal ion is hard, whereas the hard membrane is favorable when the interfering metal ion is soft.     

Iron porphyrin-modified electrodes: influence of the method of modification on the stability and electroactivity in oxidation of sulfite or hydrogensulfite in ethanol–water solutions

The aim of this work is to study four types of modification of a glassy carbon electrode by Fe(III)-tetrakis(p-tetraaminophenyl)porphyrin and determine the influence of the method of immobilization of the complex on glassy carbon in electrocatalytic properties for the sulfite and hydrogensulfite oxidation in ethanol–water. The first modification was deposition of a drop of solution containing the porphyrin on a glassy carbon electrode and evaporation of the solvent (dry-drop method). The second method was immersion of the electrode at 54°C in a solution of dimethylformamide containing the porphyrin for 2 h. The third method consisted of the same heating treatment but after formation of a chemical bond of 4-aminopyridine on the glassy carbon surface, which acts as an axial ligand for the first layer of porphyrin. The fourth method involves electropolymerization of the porphyrin on the electrode surface. Important differences in stability, the potential where the oxidation wave begins and selectivity of the electrode to sulfite or hydrogensulfite were observed. The behavior of the polymer-modified electrode is different in water compared to ethanol–water.     

An investigation of the sulphur(−II)/sulphur(0) system on bold electrodes

Sulphur deposited on gold by the anodic oxidation of sulphur(−II) species in solution has been studied by X-ray photoelectron spectroscopy. The initial layer behaved as gold sulphide. Multilayers of sulphur had a lower volatility and a smaller electron binding energy than bulk elemental sulphur, indicating that there is interaction with the underlying gold or gold sulphide.The anodic oxidation of sulphur(−II) to sulphur, and the reverse process, has been investigated on gold using the rotating ring disc electrode technique. Polysulphide ions were formed as intermediates in both processes. Polysulphides were also produced by chemical reaction of deposited sulphur with sulphur(−II) species in solution. The polysulphide intermediates were identified as S²⁻₅ at pH 13, a mixture of species with average stoichiometry S²⁻_3.3 at pH 9.2 and S²⁻₂, possibly HS⁻₂, at pH 6.8.     

DNA-modified electrodes (Ⅶ)——Preparation and characterization of DNA-bonded and DNA-adsorbed SAM/Au electrodes

Two kinds of DNA-modified electrodes were prepared by covalent and adsorptive immobilization of DNA onto self-assembled monolayers of 2, 2'-dithiodiethanol on gold electrodes and characterized by cyclic voltammetry, Xray photoelectron spectroscopy and scanning tunneling microscopy. The results suggest that the methods are satisfactory for the immobilization of DNA on electrodes.     

Improvement of the electrochemical properties of “as-grown” boron-doped polycrystalline diamond electrodes deposited on tungsten wires using ethanol

The electrochemical properties of boron-doped diamond (BDD) polycrystalline films grown on tungsten wire substrates using ethanol as a precursor are described. The results obtained show that the use of ethanol improves the electrochemistry properties of “as-grown” BDD, as it minimizes the graphitic phase upon the surface of BDD, during the growth process. The BDD electrodes were characterized by Raman spectroscopy, scanning electronic microscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The boron-doping levels of the films were estimated to be ∼10²⁰ B/cm³. The electrochemical behavior was evaluated using the and redox couples and dopamine. Apparent heterogeneous electro-transfer rate constants were determined for these redox systems using the CV and EIS techniques. values in the range of 0.01–0.1 cm s⁻¹ were observed for the and redox couples, while in the special case of dopamine, a lower value of 10⁻⁵ cm s⁻¹ was found. The obtained results showed that the use of CH₃CH₂OH (ethanol) as a carbon source constitutes a promising alternative for manufacturing BDD electrodes for electroanalytical applications.     

Temporal stability of standard potentials of silver–silver chloride reference electrodes

The four-year evolution of standard potential of a silver?Csilver chloride reference electrode (denoted further in the text as an Ag/AgCl electrode) is presented together with some suggestions for the improvement of pH primary measurement procedure.     

Surface-enhanced Raman spectroscopic study of the metal—sulfur bonding: Adsorption of N,N-dimethyl-thioformamide on copper and silver electrodes

Surface-enhanced Raman spectroscopy has been used to study the adsorption of N,N-dimethyl-thioformamide (DMTF) on Cu and Ag electrodes. Similar to the cases of thiourea and thioacetamide, the predominantly CS stretching band at 975 cm⁻¹ of DMTF is shifted to lower frequencies by about 30–40 cm⁻¹ upon adsorption on Cu and Ag surfaces. We conclude that DMTF is S-bonded to the metal surfaces. The frequency shifts of the CS stretch resemble those observed for the organometallic complexes of DMTF.     

The electrochemical behaviour of polycrystalline silver electrodes in Na2CO3 solution and the effect of ClO− 4 ions

The electrochemical behaviour of polycrystalline silver electrodes in Na₂CO₃ solutions was studied under potentiodynamic and potentiostatic conditions and complemented with X-ray diffraction analysis. Potentiodynamic E/i anodic curves exhibit active passive transition prior to an oxygen evolution reaction. The active region involves a small peak AI followed by a major peak AII before the passive region. Peak AI is assigned to the formation of an Ag₂O layer while peak AII is due to the formation of an Ag₂CO₃ layer. The height of the anodic peaks increases with increasing Na₂CO₃ concentration, scan rate and temperature. The effect of increasing additions of NaClO₄ on the electrochemical behaviour of Ag in Na₂CO₃ solutions was investigated. The perchlorate ions stimulate the active dissolution of Ag, presumably as a result of the formation of soluble AgClO₄ salt. In the passive region, ClO⁻ ₄ ions tend to break down the dual passive film, leading to pitting corrosion at a certain critical pitting potential. The pitting potential decreases with ClO⁻ ₄ concentration. Potentiostatic current/time transients showed that the formation of Ag₂O and Ag₂CO₃ layers involves a nucleation and growth mechanism under diffusion control. However, in the presence of ClO⁻ ₄ ions, the incubation time for pit initiation decreases on increasing the anodic potential step. Received: 3 July 1998 / Accepted: 10 March 1999     

Padé approximation of ECE and DISP processes at channel electrodes

A two-point Padé approximation is derived for ECE and DISP1 reactions occurring at channel electrodes for all rate constants. These are shown to be in excellent agreement with previous studies.     

460 — Electrochemical characteristics of tantalum electrodes used in culture medium

These study presents new information concerning the electrochemical conditions of tantalum electrodes in a culture system which has been used to deliver direct current to peripheral nerve ganglia. Current and potential measurements of both cathode and anode have shown that this system is well-defined and reproducible, and that the potential of the anode when it is tantalum drives the potential of either the platinum or tantalum cathode. Voltammetric measurements have shown that the working potential of the cathode during the discharge of a 1.4 V battery is associated with an electrochemical process involving oxygen or hydrogen peroxide. Some hypotheses concerning the electrical and electrochemical behavior of the system have been addressed.     

Electrochemical behavior of β2-agonists at graphite nanosheet modified electrodes

In this paper, the voltammetric characteristics of several β₂-agonists including salbutamol, ractopamine, bamethane, isoxsuprine, ritodrine, fenoterol, terbutaline, metaproterenol, clenbuterol, clenproperol, mabuterol, cimaterol, cimbuterol and brombuterol were comparatively evaluated using graphite nanosheet (GN) modified glassy carbon (GC) electrodes. All the compounds can be oxidized at GN modified electrodes with enhanced peak current and reduced peak potential compared with naked GC electrodes. The electrochemical behaviors of the compounds are different due to different substituent groups on the aromatic rings. For the first time, an ECE process was observed for salbutamol and its analogues. The capability of determining β₂-agonists individually or simultaneously from aqueous solution using differential pulse voltammetry and amperometry with the developed electrode was also investigated.