Edge Plane Pyrolytic Graphite Electrodes for Halide Detection in Aqueous Solutions

The behavior of chloride, bromide and iodide at edge plane pyrolytic graphite electrodes has been explored in aqueous acid solutions. The voltammetric response in each case has been compared with that of basal plane pyrolytic graphite, glassy carbon and boron‐doped diamond. The electrochemical oxidation of chloride is found to only occur on boron‐doped diamond while the electrochemical reversibility for the oxidation of bromide on edge plane pyrolytic graphite is similar to that seen at glassy carbon whilst being superior to basal plane pyrolytic graphite and boron‐doped diamond. In the case of iodide oxidation, edge plane and basal plane pyrolytic graphite and glassy carbon display similar electrode kinetics but are all superior to boron‐doped diamond. The analytical possibilities were examined using the edge plane pyrolytic graphite electrode for both iodide and bromine where is was found that, based on cyclic voltammetry, detection limits in the order of 10^?6 M are possible.     

Synthesis of nanostructured carbon on graphite electrodes with a supported Co catalyst for preparing anodes for microbial fuel cells

The synthesis of nanostructured carbon (NSC) on graphite electrodes with a supported Co catalyst by C₃ and C₄ alkane pyrolysis in the presence of hydrogen has been investigated. Co(II) hydroxo compounds have been deposited onto graphite, and a Co/graphite catalyst has been prepared by the homogeneous precipitation of divalent cobalt from cobalt nitrate solutions in the presence of urea and compounds containing OH groups, namely, lower alcohols (ethanol, n-propanol, and n-butanol) and polyols (ethylene glycol, glycerol, and sorbitol). The effect of Co catalyst preparation conditions on the pyrolytic activity of the catalyst and on the morphology of the synthesized NSC has been investigated. An active Co/graphite catalyst forms in the presence of an alcohol containing 1-3 OH groups. A fairly uniform NSC layer on the graphite surface is obtained at Co(II) nitrate concentrations of 0.05–0.1 mol/L, a urea concentration of 1 mol/L, and glycerol concentrations of 5–20 vol %. The electrochemical characteristics of the electrodes prepared and those of a microbial fuel cell (MFC) involving an NSC/graphite anode and an activated-sludge microbial consortium have been determined. The maximum power of the MFC under the conditions examined is 4.8 mW per square meter of the anode’s geometric surface area.     

A Study on the Electrochemical and Electrochemiluminescent Behavior of Homogentisic Acid at Carbon Electrodes

The electrochemical oxidation and electrochemiluminescent behavior of homogentisic acid (HGA) has been studied in aqueous solutions over a wide pH range by linear sweep voltammetry, cyclic voltammetry, chronocoulometry at a glassy carbon electrode, by controlled potential electrolysis at a large area spectroscopic graphite electrode, and by spectroelectrochemistry at an optically transparent drilled holes graphite (DHG) electrode in a thin‐layer cell. The studies reveal that the electrochemical oxidation of HGA at carbon electrodes is a reversible process involving two‐electron, two‐proton transfer. In addition to the electrochemical oxidation, the chemical oxidation of HGA by dissolved oxygen was investigated by spectroscopic method combined with voltammetry. It was revealed that HGA is fairly stable in strongly acidic media but readily oxidized by dissolved oxygen in alkaline media giving rise to 1,4‐benzoquinone‐2‐acetic acid, the same product as that of electrooxidation of HGA. This oxidation product is stable in acidic, neutral and weakly alkaline media, but can further degrade in strongly alkaline media yielding oxalate as the final product. The electrochemiluminescent mechanism of HGA in the presence of Ru(bpy)₃²⁺ at a glassy carbon electrode was also investigated in detail, based on which a sensitive ECL method for determination of HGA was developed, and the detection limit was 3.0×10^?8 mol L^?1.     

Electrochemical Determination of Transmembrane Protein Na+/K+‐ATPase and Its Cytoplasmic Loop C45

Electrochemistry of membrane proteins is complicated by the fact that the studied substances are poorly soluble or insoluble in aqueous environment. The solubilization of proteins using surfactants (detergents) affects the electrochemical analysis or even renders it impossible. In the present study, the electrochemistry of the transmembrane protein Na⁺/K⁺‐ATPase (NKA) and its water‐soluble isolated cytoplasmic loop C45 is described. The proteins were studied using adsorptive transfer cyclic voltammetry and square‐wave voltammetry on basal‐plane pyrolytic graphite electrode (PGE) as well as constant‐current chronopotentiometric stripping analysis on hanging mercury drop electrode (HMDE). The nonionic surfactant octaethylene glycol monododecyl ether (C₁₂E₈) was used for NKA solubilization. Under these conditions the oxidation currents of Tyr and Trp (peak Y: +0.55 V and peak W: +0.7 V, vs. Ag/AgCl/3 M KCl) and catalytic reduction currents (peak H: ?1.8 V) of NKA and C45 loop can be observed. Using the experimental procedures suggested in this study, we were able to investigate the oxidation, reduction and adsorption of NKA and C45 at femtomole level without the necessity of labeling by electroactive markers or techniques based on protein immobilization within the lipid bilayer attached to the electrode surface.     

Electrochemical oxidation of ethyl 3-oxo-3-phenyl-2-phenylhydrazonopropionate

The electrochemical oxidation of ethyl 3-oxo-3-phenyl-2-phenylhydrazonopropionate has been studied in the pH range 3.0–11.0 at a pyrolytic graphite electrode by linear and cyclic sweep voltammetry, coulometry and spectral studies. The results indicate that the 2 e⁻, 2 H⁺ oxidation of this compound gives phenol and ethyl 3-phenyl-2,3-dioxopropionate as the major products of electrooxidation.     

Direct Oxidation of Ascorbic Acid at an Edge Plane Pyrolytic Graphite Electrode: A Comparison of the Electroanalytical Response with Other Carbon Electrodes

The direct electrochemical oxidation of ascorbic acid at an edge plane pyrolytic graphite electrode (EPPG) is investigated and compared with other common carbon‐based electrodes, specifically glassy carbon, boron doped diamond and basal plane pyrolytic graphite. It is found that the EPPG electrode shows a significantly higher degree of electrochemical reversibility than the other electrode substrates giving rise to an analytically optimized limit of detection and sensitivity of 7.1×10^?5 M and 0.065 A M^?1 respectively.     

Cobalt(II) and Cobalt(III) Tri‐tert‐butoxysilanethiolates. Synthesis,Properties, Crystal and Molecular Structures of [Co{μ‐SSi(OBut)3}{SSi(OBut)3}(NH3)]2 and [Co{SSi(OBut)3}2(NH3)4][SSi(OBut)3] Complexes

The heteroleptic neutral tri‐tert‐butoxysilanethiolate of cobalt(II) incorporating ammonia as additional ligand ( 1 ) has been prepared by the reaction of a cobalt(II) ammine complex with tri‐tert‐butoxysilanethiol in water. Complex 1 , dissolved in hexane, undergoes oxidation in an ammonia saturated atmosphere to the ionic cobalt(III) compound 2 . Molecular and crystal structures of 1 and 2 have been determined by single crystal X‐ray structural analysis. 1 forms a dimeric molecule [Co{μ‐SSi(OBu^t)₃}{SSi(OBu^t)₃}(NH₃)]₂ with a folded central Co₂S₂ ring and distorted tetrahedral ligand arrangement at both Co^II atoms (CoNS₃ core). The product 2 is composed of the octahedral Co^III complex cation [Co{SSi(OBu^t)₃}₂(NH₃)₄]⁺ and the tri‐tert‐butoxysilanethiolate anion. Within the crystal two pairs of ions interact by hydrogen bonds forming well separated entities. 1 and 2 are the first structurally characterized cobalt thiolates where metal is also bonded to ammonia and 2 is the first cobalt(III) silanethiolate.     

Gas sensing using edge-plane pyrolytic-graphite electrodes: electrochemical reduction of chlorine

The voltammetric responses of chlorine in aqueous acid solutions have been explored using different carbon-based electrodes. Edge-plane pyrolytic graphite has more electrochemical reversibility than glassy carbon, basal-plane pyrolytic graphite, or boron-doped diamond electrodes. A significant reduction in the overpotential is observed on the edge-plane pyrolytic-graphite electrode in contrast with the other carbon-based electrode substrates. These results suggest that edge-plane pyrolytic graphite can be optimally used as the working electrodes in Clark-cell devices for low-potential amperometric gas sensing of Cl₂.     

Catalytic Polarographic Waves of 2,2′-Bipyridine Cobalt Complexes in Aqueous Media

Abstract

The electrochemical reduction of 2,2′-bipyridine (bipy) complexes of cobalt (II), [Co(bipy)₃]²⁺, in aqueous medium has been studied with dc tast, normal pulse polarography and controlled-potential coulometry. The cathodic wave in the process [Co(bipy)₃]²⁺/[Co(bipy)₃]⁺ shows catalytic character in the presence of hydrogen ions. The rate constant of the parallel chemical reaction was found to be 2.2 × 10⁴ M^?1. s^?1.     

Synthesis, Crystal Structure and Thermal Decomposition Character of an Environmentally-Friendly Energetic Coordination Compound of [Co(NH3)4(NO3)](NO3)·0.5H2O

A new coordination compound of [Co(NH₃)₄(NO₃)]NO₃·0.5H₂O was synthesized by reacting Co(NO₃)₂ with NH₃·H₂O aqueous solution. It has been characterized by using elemental analysis, FT‐IR technology, and X‐ray single crystal diffraction analysis. It belongs to the monoclinic crystal system and P(2)/n space group with the unit cell: a=0.74618(1) nm, b=2.26114(4) nm, c=1.04282(2) nm, β=92.038(1) °. The central ion Co(II) displays a stable octahedral configuration, which was constructed with the central ion Co(II), four nitrogen atoms from NH₃ molecules and two oxygen atoms from NO^?₃. The outer sphere was NO^?₃ and a common crystalline water molecule, which connected the inner sphere by electrostatic force and the intermolecular force. The intermolecular and intramolecular hydrogen bonds constructed a three‐dimensional network structure, which increased the stability of the whole crystal structure. In addition, thermal decomposition character of the title complex was investigated by using TG‐DTG, DSC and FT‐IR technologies. There are two endothermic peaks and one exothermic one in the process of thermal decomposition, and the decomposition residue at 300°C is the mixture of CoO and Co₂O₃.     

Electrooxidation of Chlorpromazine in Aqueous and Micellar Media and Spectroscopic Studies of the Derived Cationic Free Radical and Dication Species

Summary.  The electrochemical behaviour of chlorpromazine has been examined in phosphate buffers in aqueous as well as micellar media at a pyrolytic graphite electrode surface. Two oxidation peaks were obtained in linear sweep voltammetry of chlorpromazine. The first peak corresponds to the formation of the cationic free radical, which on further 1e⁻-oxidation gives a dication. The spectroscopic changes and kinetics of the cationic free radical and dication species generated during electrooxidation of chlorpromazine were investigated in both media. The decay of the dication was studied chronoamperometrically and was found to follow first-order kinetics with a half-life of ∼25 ms. Surfactants affect both E _p and i _p values. The anionic surfactant SDS has been found to catalyze the reaction of the free radical cation and the dication.     

Oxidation of Sanguinarine and Its Dihydro‐Derivative at a Pyrolytic Graphite Electrode Using Ex Situ Voltammetry. Study of the Interactions of the Alkaloids with DNA

This study describes the oxidation of sanguinarine (SG) and its metabolite dihydrosanguinarine (DHSG) on the surface of a basal‐plane pyrolytic graphite electrode (PGE). Since both alkaloids strongly adsorb onto the surface of pyrolytic graphite, measurements were performed using ex situ voltammetric methods, adsorptive transfer (AdT) cyclic voltammetry (CV) and square‐wave voltammetry (SWV). Oxidation peaks of SG (peak A) and DHSG (peak A*) were observed around the potential of +0.7 V (vs. Ag/AgCl/3 M KCl), depending on the experimental conditions. The voltammetric peaks A and A* are probably related to the oxidation of N‐methylphenanthridine nitrogenous heterocycle of SG and oxidation of DHSG back to SG, respectively. The electrochemical results and optimized AdT SWV were subsequently applied to the study of the interactions of SG and DHSG with DNA in vitro. Analysis of the alkaloid/DNA interactions was based on observing heights of oxidation peaks A and A* after incubation of SG and/or DHSG with supercoiled (sc) DNA [pBSK_(?)]. Electrochemical study of the interactions was supported and complemented with measurements using gel electrophoresis (Topoisomerase I scDNA relaxation assay) and steady‐state and time‐resolved fluorescence spectroscopy. The results suggest that SG intercalates into the double‐stranded structure of scDNA (the SG/base pair ratio is max. 1/4) while increased binding affinity was observed for quaternary cation (SG⁺). DHSG which, unlike SG⁺, does not possess a strictly planar molecular structure, did not show intercalative DNA binding in any of the three methods applied.     

Electrochemical oxidation of 4,5,6,7-tetrahydro-1H-pyrazolo[3,4-d] pyrimidine-4,6-dione (oxipurinol) at the pyrolytic graphite electrode

The electrochemical oxidation of 4,5,6,7-tetrahydro-1H-pyrazolo[3,4-d] pyrimidine-4,6-dione (oxipurinol) at the pyrolytic graphite electrode (PGE) has been studied. Oxipurinol exhibits up to three voltammetric oxidation peaks at the PGE between pH 1–12. The first pH-dependent peak (peak I_a) is proposed to be an initial, irreversible 2e-2H⁺ reaction to give 5,6-dihydro-4H-pyrazolo[3,4-d] pyrimidine-4,6-dione. This primary product further reacts by two routes. The major route, accounting for ca. 90% of the latter compound, involves a Michael addition of water followed by further electrochemical oxidation and hydrolysis to give 5,6-dihydro-5,6-dihydroxy-5-carboxy-6-diazenouracil. The minor route involves further electrochemical oxidation of 5,6-dihydro-4H-pyrazolo[3,4-d]-pyrimidine-4,6-dione in a 2e-2H⁺ reaction to give 4,5,6,7-tetrahydro-3H-pyrazolo[3,4-d]-pyrimidine-3,4,6-trione.Decomposition and, generally, additional electrochemical reactions of 5,6-dihydro-5,6-dihydroxy-5-carboxy-6-diazenouracil result in the formation of alloxan, parabanic acid, 6-diazo-isobarbituric acid and 5′-hydroxy-5-carboxy-6,6′-azouracil. The two latter compounds have never previously been reported. Decomposition of 4,5,6,7-tetrahydro-3H-pyrazolo[3,4-d]pyrimidine-3,4,6-trione results in formation of uracil-5-carboxylic acid.Detailed reaction schemes have been proposed to explain the observed electrochemistry and the formation of the observed products.     

Insights into the electro-oxidation of hydrazine at single-walled carbon-nanotube-modified edge-plane pyrolytic graphite electrodes electro-decorated with metal and metal oxide films

Electrochemistry of edge-plane pyrolytic graphite electrodes (EPPGEs) modified with Aldrich single-walled carbon nanotubes (SWCNTs) electro-decorated with metal (Ni, Fe and Co) and their oxides have been studied. The morphology and identity of the metallic dispersions were examined by scanning electron microscopy and energy-dispersive spectroscopy. We show that SWCNTs serve as efficient conducting carbon material for electronic communication between metal films and the underlying carbon electrode. By using cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques, it is proved that both EPPGE-SWCNT-Ni and EPPGE-SWCNT-Fe exhibit comparable electrochemical response in buffered aqueous solution (pH 7.0) and towards electro-oxidation of hydrazine in Na₂SO₄ solution. The impedance spectra of these SWCNT-metal hybrids were complicated and follow electrical equivalent circuit model typical of adsorption-controlled charge transfer kinetics. Hydrazine impedance spectra exhibited inductive loop, characteristic of Faradaic current being governed by the occupation of an intermediate state. On the other hand, the EIS data obtained in a simple redox probe, [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻, showed that EPPGE-SWCNT and EPPGE-SWCNT-Ni followed electrical equivalent circuit models typical of partial charge transfer or adsorption-controlled kinetics with some resemblance to the behaviour of electrolyte–insulator–semiconductor sensors.     

Kinetics of copper(II)‐catalyzed oxidation of S(IV) by atmospheric oxygen in ammonia buffered solutions

To understand the chemistry of Cu(II)–NH₃–S(IV)–O₂ system, the kinetics of the oxidation of sulfur(IV) catalyzed by amminecopper(II) complexes has been studied in the ammonia‐buffered solutions. Sulfur(IV) is oxidized to sulfate. The complexes, Cu(NH₃)²⁺, Cu(NH₃)₂²⁺, and Cu(NH₃)²⁺₃ appear to be equally reactive and Cu(NH₃)₄²⁺ appears to be unreactive. The kinetics obey the rate law: where α₁ and γ₁ are the rate constants for O₂‐dependent and O₂‐independent pathways, respectively, for Cu(NH₃)²⁺, Cu(NH₃)²⁺₂, and Cu(N H₃)₃²⁺ complexes, which appear to be equally reactive. The values of α₁ and γ₁ were found to be (1.32 ± 0.21) × 10⁶ L² mol^?2 s^?1 (1.74 ± 0.40) × 10⁹ L³ mol^?3 s^?1respectively at 30°C. The reaction rate is not influenced by the presence of ethanol—a free radical scavenger, so a nonradical mechanism has been proposed. The results of this study are useful in understanding the atmospheric chemistry of aqueous phase autoxidation of dissolved sulfur dioxide in copper(II)–ammonia–sulfur(IV)–oxygen system at high pH. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 379–392, 2011     

Epoxidation of cyclohexene with H2O2 over efficient water‐tolerant heterogeneous catalysts composed of mono‐substituted phosphotungstic acid on co‐functionalized SBA‐15

A series of Keggin‐type heteropolyacid‐based heterogeneous catalysts (Co‐/Fe‐/Cu‐POM‐octyl‐NH₃‐SBA‐15) were synthesized via immobilized transition metal mono‐ substituted phosphotungstic acids (Co‐/Fe‐/Cu‐POM) on octyl‐amino‐co‐functionalized mesoporous silica SBA‐15 (octyl‐NH₂‐SBA‐15). Characterization results indicated that Co‐/Fe‐/Cu‐POM units were highly dispersed in mesochannels of SBA‐15, and both types of Brønsted and Lewis acid sites existed in Co‐/Fe‐/Cu‐POM‐octyl‐NH₃‐SBA‐15 catalysts. Co‐POM‐octyl‐NH₃‐SBA‐15 catalyst showed excellent catalytic performance in H₂O₂‐mediated cyclohexene epoxidation with 83.8% of cyclohexene conversion, 92.8% of cyclohexene oxide selectivity, and 98/2 of epoxidation/allylic oxidation selectivity. The order of catalytic activity was Co‐POM‐octyl‐NH₃‐SBA‐15 > Fe‐POM‐octyl‐NH₃‐SBA‐15 > Cu‐POM‐octyl‐NH₃‐SBA‐15. In order to obtain insights into the role of ‐octyl moieties during catalysis, an octyl‐free catalyst (Co‐POM‐NH₃‐SBA‐15) was also synthesized. In comparison with Co‐POM‐NH₃‐SBA‐15, Co‐POM‐octyl‐NH₃‐SBA‐15 showed enhanced catalytic properties (viz. activity and selectivity) in cyclohexene epoxidation. Strong chemical bonding between ‐NH₃⁺ anchored on the surface of SBA‐15 and heteropolyanions resulted in excellent stability of Co‐POM‐octyl‐NH₃‐SBA‐15 catalyst, and it could be reused six times without considerable loss of activity.     

Co2O3‐NH2‐MCM‐41 Decorated Graphite as an Effective Electrode: Synthesis,Characterization and its Application for Electro‐catalytic Oxidation of Acid Red 1

The graphite electrode decorated with Co₂O₃‐NH₂‐MCM‐41 was successfully fabricated and the potential for applying this electrode for electro‐catalytic oxidation of Acid Red 1 (AR1) was investigated. The Co₂O₃‐NH₂‐MCM‐41 was characterized by Scanning Electron Microscope (SEM), Brunauer‐Emmett‐Teller (BET), X‐ray diffraction (XRD) and Fourier transform infrared spectra (FTIR). Electrochemical measurements including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to investigate electrochemical activity of graphite anode with Co₂O₃‐NH₂‐MCM‐41. The electro‐catalytic oxidation process was carried out via varying different parameters such as voltage, electrolyte pH, electrolyte concentration, current density and interelectrode distance. The results revealed the maximum removal ratio of AR1 was 99.8 %. The AR1 solution was tested during the degradation process by CV analysis at different scan rates, UV‐Vis spectral analysis and gas chromatography−mass spectrometry (GC/MS). The linear relationship between peak current and scan rates indicated an adsorption controlled process for AR1 degradation, UV‐Vis analysis revealed that the degradation process took place through reactions such as destruction of azo groups, benzene ring, naphthalene ring and so on, GC/MS analysis demonstrated that AR1 was finally destructed to small molecules by analyzing intermediates during degradation process.     

Electrochemistry of Levo‐Thyroxin on Edge‐Plane Pyrolytic Graphite Electrode: Application to Sensitive Analytical Determinations

The electrochemical response of sodium levo‐thyroxin (T₄) at the surface of an edge plane pyrolytic graphite (EPPG) electrode is investigated using cyclic voltammetric technique in the presence of 0.1 M HCl as supporting electrolyte. T₄ underwent totally irreversible oxidation at this system and a well‐defined peak at 821 mV was obtained. Compared to the signals obtained in the optimized conditions at bare glassy carbon and carbon paste electrodes, the oxidation current of T₄ at an EPPG electrode was greatly enhanced. The electrochemical process of T₄ was explored and the experimental conditions were optimized. The oxidation peak current represented a linear dependence on T₄ concentration from 0.01 to 10 µM. The detection limit of 3 nM (S/N=3) was obtained for 250 s accumulation at 0.3 V. Determination of T₄ in a synthetic serum sample demonstrated that this sensor has good selectivity and high sensitivity.     

Electrochemical Sensor Based on Oxidation of 2,8‐Dihydroxyadenine to Monitor DNA Damage in Calf Thymus DNA

A novel and reliable direct electrochemical method has been established to monitor DNA damage in acid hydrolyzed calf thymus DNA, based on the determination of 2,8‐dihydroxyadenine (2,8‐DHA). A single‐wall carbon nanotubes (SWCNT) modified edge plane pyrolytic graphite electrode (EPPGE) has been used as a sensor to monitor the DNA damage. 2,8‐DHA the main in vivo adenine oxidation product undergoes oxidation at ～395 mV at SWCNT modified EPPGE using square wave voltammetry (SWV). The sensor exhibits potent and persistent electron‐mediating behavior. A well‐defined oxidation peak for the oxidation of 2,8‐DHA was observed at modified electrode with lowering of peak potential and increase in peak current as compared to bare EPPGE. At optimal experimental conditions, the catalytic oxidative peak current was responsive with the 2,8‐DHA concentrations ranging from 0.05 nM to 100 nM. The detection limit was 3.8×10^?11 M and limit of quantification was 1.27×10^?10 M. The modified electrode exhibited high stability and reproducibility.     

Individual and Simultaneous Voltammetric Determination of Cd(II), Cu(II) and Pb(II) Applying Amino Functionalized Fe3O4@Carbon Microspheres Modified Electrode

Amino‐functionalized Fe₃O₄@carbon microspheres (NH₂?Fe₃O₄@C) were prepared and the electrochemical sensor was constructed using NH₂?Fe₃O₄@C modified glassy carbon electrodes (GCE) to determine toxic heavy metals in aqueous solution. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the structure and phase of NH₂?Fe₃O₄@C. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results indicate that NH₂?Fe₃O₄@C modified GCE possesses large active area and excellent electron transfer. Under optimized electrochemical condition, Cd(II), Pb(II) and Cu(II) were determined using NH₂?Fe₃O₄@C modified GCE. The electrode through amino functionalization exhibits higher sensitivity and lower detection limit toward Cd(II) and Cu(II) due to the acid‐base pairing interaction between the electron‐rich ?NH₂ ligand and the electron‐deficient heavy metal ions. Compared with other similar results reported in the literature, the NH₂?Fe₃O₄@C modified electrode exhibits wider linear response range while with comparable lower detection limit. It also exhibits excellent stability, reproducibility and anti‐interference ability.