MWNT-modified electrodes for voltammetric determination of lipophilic vitamins

Multi-walled carbon nanotube modified graphite electrodes (MWNT-GEs) have been created for the voltammetric determination of α-tocopherol and retinol. The electrode surface was characterized by atomic force microscopy. The MWNT-GEs presented structured surfaces and a significant (26-fold) increase in roughness over unmodified graphite electrodes (8.2 vs. 0.32?nm for MWNT-GEs and GEs, respectively). Their surfaces consisted of aggregates with a highly regular “thorn-like” structure. α-Tocopherol and retinol were oxidized on the bare GEs and the MWNT-GEs in 0.1?M HClO₄ in acetonitrile. Decreases in the overpotential of 0.2 and 0.04?V for α-tocopherol and retinol, respectively, and increased oxidation currents were observed on the MWNT-GEs in comparison with the unmodified electrodes. The calibration graphs were linear in the range 0.065–2.00?mM for α-tocopherol and 0.05–1.50?mM for retinol. The detection limits were found to be 0.05 and 0.04?mM for α-tocopherol and retinol, respectively. The developed electrodes were applied to determine α-tocopherol and retinol in pharmaceuticals. The results obtained agreed well with coulometric titration data.     

Behaviour of electrodes modified with poly(naphthoquinone)films: Electrocatalytic effect in the reduction of oxygen

Stable (polynaphthoquinone)-coated Pt, Au and graphite electrodes are prepared by electropolymerization. Stable poly(naphthoquinone)-coated Pt, Au and graphite electrodes are prepared by electropolymerization. These modified electrodes catalyse oxygen reduction. Rotating-disk studies in the case of modified Pt electrodes show that the rate-limiting factors are the substrate diffusion in the film and the cross-exchange reaction.     

Decreasing Irreversible Capacity of Graphite Electrodes in Lithium-Ion Batteries by Direct Contact of Graphite with Metallic Lithium

The feasibility of reducing the irreversible capacity of negative graphite electrodes in lithium-ion batteries by a direct contact of such electrodes with lithium in the electrolyte is studied. It is shown that the dynamics of the formation of the passive film on graphite and the degree of the decrease in the irreversible capacity depend on the ratio between weights of graphite and lithium in contact. This method of reducing the irreversible capacity does not diminish the reversible capacity of graphite during the cycling. The irreversible capacity of the initial graphite cycled in 1 M LiPF₆ in a mixture of propylene carbonate and diethyl carbonate at a current density of 20 mA g^–1 is 550–1150 mA h g^–1. The reversible capacity of electrodes cycled in the same conditions reaches 290 mA h g^–1.     

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.     

Etude electrochimique du complexe phenylene-bis(salicylidene iminato)cobalt en solution et du complexe polycondense en solution et fixe sur l'electrode. Application a la reduction electroassistee du chlrure de benzyle

The electrochemical behaviour of CoSaloph (Salopy  phenylbis(salicylidene-iminato) was investigated in solution in organic solvents.The polymer complexes Co(Saloph)_n have been prepared and modified electrodes coated with a film of polymer or with a mixture of graphite and polymer have been prepared and their electrochemical properties compared to those of the monomer.Quantitative electro-assisted reduction of benzyl chloride at controlled potential was performed with both types of complexes. In spite of poor yields, this example demonstrates the possibility of the transposition of homogeneous catalysis with coordination compounds on modified electrodes in the field of organic electro-synthesis.     

Voltammetry of Formaldehyde at Mechanically Renewable Solid Electrodes

The electrochemical behavior of formaldehyde (CH₂O) at solid electrodes made of platinum, gold, silver, cobalt, nickel, copper, and graphite was studied. The working surface of the electrodes was renewed by cutting a thin layer (0.5 m) immediately in the test solution. It was found that, in alkaline solutions, CH₂O was oxidized at all electrodes other than cobalt and graphite ones while scanning the potential to the anode and cathode regions. The peaks of CH₂O oxidation at platinum and gold electrodes using potential scanning in the anode and cathode directions, as well as at nickel, copper, and silver electrodes using potential scanning in the anodic direction, are suitable for analytical purposes.     

Graphite impurities cause the observed ‘electrocatalysis’ seen at C60 modified glassy carbon electrodes in respect of the oxidation of l-cysteine

The recently reported claims of electrocatalysis using C₆₀ film modified glassy carbon electrodes [W.T. Tan, A.M. Bond, S.W. Ngooi, E.B. Lim, J.K. Goh, Anal. Chim. Acta 491 (2003) 181] for the oxidation of l-cysteine are questioned. We show that C₆₀ itself is not electrocatalytic at the potentials concerned but rather it is likely that graphite impurities in the C₆₀ material used by Tan et al. that provide their observed ‘electrocatalysis’.     

Quasi in situ XPS study of anion intercalation into HOPG from the ionic liquid [EMIM][BF4]

Highly oriented pyrolytic graphite (HOPG) electrodes were electrochemically oxidized in the ionic liquid [EMIM][BF₄]. Both, the electrolyte and the electrode surface were investigated using X-ray photoelectron spectroscopy (XPS) after electrochemical treatment. For that purpose an electrochemical preparation chamber was attached to the ultra high vacuum system allowing for preparation of electrodes in non-aqueous electrolyte and subsequent sample transfer under inert nitrogen atmosphere. The XP-spectra of all species detected on the oxidized HOPG surface show core level shifts towards lower binding energies referring to a Fermi level shift and proving that a graphite intercalation compound was formed. Anion intercalation occurs together with co-intercalation of cations at 2 V vs. carbon quasi-reference electrode and is found to be irreversible. XPS analysis of the ionic liquid prior to and after electrochemical treatment indicates a change in electrolyte composition.     

Bioanalytical Application of the Ordered Mesoporous Carbon Modified Electrodes

An ordered mesoporous carbon modified electrode (OMCE) was prepared by film forming method. The electrochemical behavior of the OMCE was evaluated in connection with the electrochemistry of some electroactive biospecies, such as ascorbic acid (AA), acetaminophenol (AP), cysteine (CySH), dopamine (DA), epinephrine (EP), uric acid (UA), β‐nicotinamide adenine dinucleotide (reduced disodium salt hydrate, NADH), and hydrogen peroxide (H₂O₂) with cyclic voltammetry. Compared with the conventional carbon nanotubes (CNT) and graphite powder (GP) modified electrodes, the OMCE provided the best electrochemical reactivities in all cases associated with decreased over potential, better‐defined peak shape, and higher sensitivity. In addition, the OMC, CNT, and GP modified electrodes were employed as sensitive sensors for H₂O₂ and NADH quantification and as stable platforms for the fabrication of glucose and ethanol biosensors on which the enzymes were immobilized.     

Electrochemical Behaviour of HOPG and CVD‐Grown Graphene Electrodes Modified with Thick Anthraquinone Films by Diazonium Reduction

Highly oriented pyrolytic graphite (HOPG) and graphene grown on Ni (Ni‐Gra) or Cu (Cu‐Gra) by chemical vapour deposition were modified with thick anthraquinone (AQ) films (7?60 nm) by redox grafting of the pertinent diazonium salt. Glassy carbon (GC) electrodes were used for comparison. The AQ‐modified GC electrodes showed excellent blocking properties towards the Fe(CN)₆^3?/4? redox probe, although it was noted that in the case of Ni‐Gra and Cu‐Gra, the blocking ability depended strongly on the underlying substrate. Oxygen reduction studies revealed good electrocatalytic activity of AQ‐modified HOPG, Ni‐Gra, and Cu‐Gra, compared with the bare electrodes.     

Effects of current densities on the formation of LiCoO<Subscript>2</Subscript>/graphite lithium ion battery

The activation characteristics and the effects of current densities on the formation of a separate LiCoO₂ and graphite electrode were investigated and the behavior also was compared with that of the full LiCoO₂/graphite batteries using various electrochemical techniques. The results showed that the formation current densities obviously influenced the electrochemical impedance spectrum of Li/graphite, LiCoO₂/Li, and LiCoO₂/graphite cells. The electrolyte was reduced on the surface of graphite anode between 2.5 and 3.6 V to form a preliminary solid electrolyte interphase (SEI) film of anode during the formation of the LiCoO₂/graphite batteries. The electrolyte was oxidized from 3.95 V vs Li⁺/Li on the surface of LiCoO₂ to form a SEI film of cathode. A highly conducting SEI film could be formed gradually on the surface of graphite anode, whereas the SEI film of LiCoO₂ cathode had high resistance. The LiCoO₂ cathode could be activated completely at the first cycle, while the activation of the graphite anode needed several cycles. The columbic efficiency of the first cycle increased, but that of the second decreased with the increase in the formation current of LiCoO₂/graphite batteries. The formation current influenced the cycling performance of batteries, especially the high-temperature cycling performance. Therefore, the batteries should be activated with proper current densities to ensure an excellent formation of SEI film on the anode surface.     

Tin oxide nanoparticles-polymer modified single-use sensors for electrochemical monitoring of label-free DNA hybridization

In this study, SnO₂ nanoparticles (SNPs)-poly(vinylferrocenium) (PVF⁺) modified single-use graphite electrodes were developed for electrochemical monitoring of DNA hybridization. The surfaces of polymer modified and polymer-SNP modified pencil graphite electrodes (PGEs) were firstly characterized by using SEM analysis. The electrochemical behaviours of these electrodes were also investigated using the differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) techniques. The polymer-SNP modified PGEs were then tested for the electrochemical sensing of DNA based on the changes at the guanine oxidation signals. Experimental parameters, such as; different modifications in DNA oligonucleotides, DNA probe concentrations were examined to obtain more sensitive and selective electrochemical signals for nucleic acid hybridization. After optimization studies, DNA hybridization was investigated in the case of complementary of hepatitis B virus (HBV) probe, mismatch (MM), and noncomplementary (NC) sequences.     

Preparation of substituted rare-earth metal diphthalocyanine complexes and study of their electrochemical behavior

Rare-earth metal diphthalocyanine complexes with R substituents, R₈Pc₂MH (where Pc=phthalocyanine; R=propoxy, t-butyl; M=Er, Lu), were prepared. Their films on basal plane pyrolytic graphite and on indium-tin oxide Nesa glass electrodes in several aqueous electrolytes were investigated by voltammetric analysis and in situ electronic absorption spectroscopy at controlled potentials. These modified electrodes showed multi-color electrochromic behavior as a result of multi-step redox reaction. The film color turned from original green through orange to red in an oxidation process, and through dark blue to purplish in a reduction process.     

Exploring the electrocatalytic sites of carbon nanotubes for NADH detection: an edge plane pyrolytic graphite electrode study

The electrocatalytic properties of multi-walled carbon nanotube modified electrodes toward the oxidation of NADH are critically evaluated. Carbon nanotube modified electrodes are examined and compared with boron-doped diamond and glassy carbon electrodes, and most importantly, edge plane and basal pyrolytic graphite electrodes. It is found that CNT modified electrodes are no more reactive than edge plane pyrolytic graphite electrodes with the comparison with edge plane and basal plane pyrolytic graphite electrodes allowing the electroactive sites for the electrochemical oxidation of NADH to be unambiguously determined as due to edge plane sites. Using these highly reactive edge plane sites, edge plane pyrolytic graphite electrodes are examined with cyclic voltammetry and amperometry for the electroanalytical determination of NADH. It is demonstrated that a detection limit of 5 microM is possible with cyclic voltammetry or 0.3 microM using amperometry suggesting that edge plane pyrolytic graphite electrodes can conveniently replace carbon nanotube modified glassy carbon electrodes for biosensing applications with the relative advantages of reactivity, cost and simplicity of preparation. We advocate the routine use of edge plane and basal plane pyrolytic graphite electrodes in studies utilising carbon nanotubes particularly if 'electrocatalytic' properties are claimed for the latter.     

Electrocatalytic Oxidation of NADH by Oxidized s‐Adenosyl‐L‐Methionine (SAMe): Application to NADH and SAMe Determinations

s‐Adenosyl‐L ‐methionine (SAMe) is an adenosine analogue with therapeutical activity against affective disorders and liver dysfunctions. It can be oxidized on graphite electrode yielding a strongly adsorbed electroactive oxidation product for which a quinone‐imine structure is proposed. This compound is capable of electrocatalyzing the NADH oxidation at low potentials, lowering the overvoltage by about 300 mV. An amperometric method for NADH determination at +0.1 V (Ag|AgCl|KCl_sat) is developed using an oxidized‐SAMe‐modified electrode in pH 9. Linear calibration plots were obtained with a detection limit of 2.4 nM. The electrode response time and the relative standard deviation of the slope of the calibration plot for 5 different modified electrodes were 12 s and 5.6% respectively. The catalytic scheme also provides the first method to determine SAMe itself by adsorptive differential pulse voltammetry. The linear range was found to be 42.4–424 nM with a reproducibility of 6.9%. The method was applied to SAMe determination in a pharmaceutical formulation.     

Electrocatalytic Oxidation and Flow-Injection Determination of Sulfur-Containing Amino Acids at Graphite Electrodes Modified with a Ruthenium Hexacyanoferrate Film

The electrochemical behavior of sulfur-containing amino acids (cysteine, cystine, and methionine) at graphite electrodes modified with a ruthenium(III) hexacyanoferrate(II) film was studied. Glassy carbon and carbon paste were used as graphite materials. The electrocatalytic oxidation of amino acids at a modified electrode resulted in a decrease in the oxidation potentials of amino acids and an increase in the currents of their oxidation peaks as compared to those observed at an unmodified electrode. The voltammetric characteristics and hydrodynamic conditions for detecting the maximum catalytic current were found. A procedure is proposed for the electrocatalytic determination of cysteine, cystine, and methionine at a carbon-paste electrode modified with an inorganic film of ruthenium(III) hexacyanoferrate(II) under the conditions of flow-injection analysis.     

The study of lithium insertion–deinsertion processes into composite graphite electrodes by in situ atomic force microscopy (AFM)

Li insertion–deinsertion into composite graphite electrodes, comprising synthetic graphite flakes (6 μm average size), polyvinylidene difluoride binder (PVdF), and copper current collectors, in commonly used alkyl carbonate solutions were studied by in situ atomic force microscopy (AFM). In this study, we were able to probe by in situ AFM the behavior of practical, composite graphite electrodes in ethylene carbonate–dimethyl carbonate (EC–DMC) solutions containing salts such as LiAsF₆ and LiPF₆ during entire lithiation–delithiation cycles. These in situ micro/nanomorphological studies could probe surface film formation on the graphite particles, as well as periodic volume changes in the graphite flakes during Li insertion–deinsertion cycles. These cyclic volume changes can explain the capacity fading of graphite electrodes upon prolonged cycling, in Li-ion batteries. While the overall morphology of these electrodes remains steady upon cycling in the appropriate solutions (in which the Li–C electrodes are efficiently passivated), there is a continuous problem in the extent of accommodation of the small volume changes in the graphite particles upon lithiation–delithiation, by the surface films. It is suggested that graphite electrodes fail during prolonged cycling due to small scale, continuous reactions of the active mass with solution species, which gradually increase their impedance and decrease the content of the lithium stored in the electrodes.     

Impedance spectroscopy study of passive layers on the surface of lead-tin and lead-tin-calcium alloys anodically oxidized in 4.8 M sulfuric acid solution

The nature of passive films, which were formed at various potentials in 4.8 M H₂SO₄ solution on the lead-tin and lead-tin-calcium alloys, is studied by the method of impedance spectroscopy. At the potentials of 1.3 and 1.7 V, the electrode impedance is presented by the equivalent circuit, which corresponds to the formation of a bilayer film consisting of lead(II) sulfate and oxide on the electrode surface. Lead(II) oxide, which forms under the layer of lead sulfate, determines a high resistance of passive layer on the electrodes of lead alloys under investigation. An introduction of tin into the lead alloys significantly decreases the resistance of passive layers. An addition of calcium to the lead-tin alloy raises the impedance of the system. At a potential of 2.05 V, a single-layer compact passive film forms on the electrodes of the test lead alloys. It consists predominantly of lead oxides PbO _x (1 < x ≤ 2), which exhibit a higher electron conductivity. An introduction of tin into the lead alloys decreases the resistance of formed films; calcium has almost no effect on the resistance of passive film under these conditions.     

Application of thin film mercury electrodes and solid amalgam electrodes in electrochemical analysis of the nucleic acids components: detection of the two-dimensional phase transients of adenosine

The optical diffractive (DOE)-based sensor was used to the study of the optical roughness of different carbon/graphite electrodes modified by mercury film (MFEs) and solid amalgam-alloy electrodes (S-MeAEs). The electrode surfaces were visualised by an optical metallurgical microscope. The adsorption of adenosine at the MFEs and S-MeAEs has been investigated by capacitance measurement. Some kinetics aspects, such as the influence of the surface morphology, nature of the substrate and thickness of the mercury film and amalgam-alloy on the formation of two-dimensional (2D) physisorbed adenosine adlayer on the MFEs and S-MeAEs, were studied.     

Polyynes and cyanopolyynes synthesis from the submerged electric arc: about the role played by the electrodes and solvents in polyynes formation

The products of the electric arc between graphite electrodes have been investigated by high performance liquid chromatography-diode-array detector (HPLC-DAD) analysis in various media: distilled water, liquid nitrogen, methanol, ethanol, n-hexane and benzene. In distilled water, hydrogen capped polyynes H-(CC)_n-H were the unique products demonstrating that carbon is supplied by the graphite electrodes while hydrogen is supplied by the solvent plasmalysis (in this case water plasmalysis). Arcing graphite electrodes in liquid nitrogen produces cyanopolyynes: NC-(CC)_n-CN demonstrating that in this case the end groups of the polyyne chains are supplied by molecular nitrogen plasmalysis caused by the electric arc. Graphite arcing in methanol and ethanol produces very clean solutions (by-products negligible or absent) of hydrogen-capped polyynes with C₈H₂ as the main product accounting for more than 70 mol percent of the total polyyne concentration. By replacing graphite electrodes with titanium electrodes in methanol or in ethanol, polyynes are not formed at all; only trace amounts of polycyclic aromatic hydrocarbons (PAHs) were detected. When arcing with graphite electrodes is conducted in n-hexane or in benzene, polyyne formation is accompanied by a significant production of PAH, especially in benzene. These results have been rationalized in terms of carbonization or coking tendency of a given solvent. The effect of using titanium electrodes in place of graphite electrodes has been investigated also in n-hexane and in benzene as well as the effects of very high electric current intensity employed to ignite and sustain the submerged electric arc.