Electrocatalytic Reduction of Oxygen at Multi‐Walled Carbon Nanotubes and Cobalt Porphyrin Modified Glassy Carbon Electrode

The multi‐walled carbon nanotubes (MWNTs) modified glassy carbon electrode exhibited electrocatalytic activity to the reduction of oxygen in 0.1 M HAc‐NaAc (pH 3.8) buffer solution. Further modification with cobalt porphyrin film on the MWNTs by adsorption, the resulted modified electrode showed more efficient catalytic activity to O₂ reduction. The reduction peak potential of O₂ is shifted much more positively to 0.12 V (vs. Ag/AgCl), and the peak current is increased greatly. Cyclic voltammetry (CV), transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), were used to characterize the material and the modified film on electrode surface. Electrochemical experiments gave the total number of electron transfer for oxygen reduction as about 3, which indicated a co‐exist process of 2 electrons and 4 electrons for reduction of oxygen at this modified electrode. Meanwhile, the catalytic activities of the multilayer film (MWNTs/CoTMPyP)_n prepared by layer‐by‐layer method were investigated, and the results showed that the peak current of O₂ reduction increased and the peak potential shifted to a positive direction with the increase of layer numbers.     

Electrocatalytic reduction of O2 at pyrolytic graphite electrode modified by a novel copper(II) complex with 2-[bis(2-aminoethyl)amino]ethanol and imidazole ligands

A new complex formed by Cu(II) with 2-[bis(2-aminoethyl)amino]ethanol and imidazole is prepared, and its electrochemical properties are studied. The electrochemical experiments are carried out in deaerated pH 7.0 buffer solution through cyclic voltammetry by scanning the potential from 0.1 to −0.5 V with this copper(II) complex-modified electrode as the working electrode. One redox process is observed, which could be assigned to Cu(II)/Cu(I). The formal potential E _0′ = (E _pa + E _pc)/2, where E _pa and E _pc are anodic and cathodic peak potentials, is −248 mV vs. SCE. A straight line, obtained from the plot of I _pc vs. v, indicated a surface-controlled reaction. The modified electrode is very stable and exhibits catalytic activity for oxygen reduction. The possible mechanism for the catalytic reduction of oxygen is studied by cyclic voltammetry and chronoamperometry. The results show that the dioxygen is reduced via a pathway of four-electron reduction to form water. Chronoamperometric measurements show the potentiality of the use of this working electrode as an amperometric sensor for dissolved dioxygen in aqueous media. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 8, pp. 975–979. The text was submitted by the authors in English.     

Electrochemical Reduction and Voltammetric Determination of Metronidazole Benzoate at Modified Carbon Paste Electrode

Abstract

A metalloporphyrin incorporated carbon paste sensor has been developed for the determination of metronidazole benzoate (MTZB). Zn(II) complex of 5,10,15,20-tetrakis (3-methoxy-4-hydroxy phenyl) porphyrin (TMHPP) was used as the active material. The MTZB gave a well-defined reduction peak at?0.713 V in 0.1 mol l^?1 phosphate buffer solution of pH around 7. Compared with bare carbon paste electrode (CPE), the TMHPP Zn(II) modified electrode significantly enhanced the reduction peak current of MTZB as well as lowered its reduction potential. Under optimum conditions the reduction peak current was proportional to MTZB concentration over the range 1 × 10^?3 mol l^?1 to 1 × 10^?5 mol l^?1. The detection limit was found to be 4.36 × 10^?6 mol l^?1. This sensor has been successfully applied for the determination of MTZB in pharmaceutical formulations and urine samples.     

Determination of Trace Copper by Adsorptive Voltammetry Using a Multiwalled Carbon Nanotube Modified Carbon Paste Electrode

A new method for the determination of trace copper was described. A multiwalled carbon nanotube modified carbon paste electrode was prepared and the adsorptive voltammetric behavior of copper‐alizarin red S (ARS) complex at the modified electrode was investigated. By use of the second‐order derivative linear sweep voltammetry, it was found that in 0.04 mol/L acetate buffer solution (pH 4.2) containing 4×10^?6 mol/L ARS, when accumulation potential is 0 mV, accumulation time is 60 s and scan rate is 100 mV/s, the complex can be adsorbed on the surface of the electrode, yielding one sensitive reduction peak at ?172 mV (vs. SCE). The peak current of the complex is proportional to the concentration of Cu(II) in the range of 2.0×10^?11–4.0×10^?7 mol L^?1 with a detection limit (S/N=3) of 8.0×10^?12 mol/L (4 min accumulation). The proposed method was successfully applied to the determination of copper in biological samples with satisfactory results, the recoveries were found to be 96%–102%.     

Electrochemical Behavior of o‐Nitrophenol at Hexagonal Mesoporous Silica Modified Carbon Paste Electrodes

A hexagonal mesoporous silica (HMS) modified carbon paste electrode (CPE) was fabricated and characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods (ferrocene as a probe). The electrochemical behavior of nitrophenol (o‐NP) at the HMS modified electrode (HMSCPE) was investigated. Compared with CPE, a well‐defined reduction peak and a remarkably peak current response was observed. It is indicated that mesoporous HMS exhibited remarkable enhancement effects on the electrochemical reduction of o‐NP. The electrochemical reduction mechanism was also discussed. Consequently, a simple and sensitive electrochemical method was proposed for the determination of o‐NP, which was used to determine o‐NP in waste water samples.     

Renewable New Copper Complex Bulk‐Modified Carbon Paste Electrode: Preparation,Electrochemistry, and Electrocatalysis

A conductive carbon paste electrode (CPE) comprised of a new copper‐complex of [Cu₂(Dpq)₂(Ac)₂(H₂O)₂](ClO₄)₂?H₂O (Dpq=dipyrido[3,2‐d : 2′,3′‐f]quinoxaline, Ac=acetate) and carbon powder, was fabricated by the direct mixing method. The electrochemical behavior and electrocatalysis of the new copper‐complex modified CPE (Cu‐CPE) have been studied in detail. Cyclic voltammograms showed that the Cu‐CPE had a favorable electrochemical response of a reversible redox couple of Cu(II)/Cu(I). The Cu‐CPE showed good electrocatalytic activity toward the reduction of the bromate, nitrite and hydrogen peroxide. The electrocatalytic reduction peak current of KBrO₃, KNO₂ and H₂O₂ showed a linear dependent on their concentrations. All of the results revealed that the Cu‐CPE had a good reproducibility, remarkable long term stability and especially good surface renewability by simple mechanical polishing in the event of surface fouling, which is important for practical application.     

Electrocatalytic Oxidation of Dopamine at a Nanocuprous Oxide‐Methylene Blue Composite Glassy Carbon Electrode

Cuprous oxide nanowhisker was prepared by using cetyltrimethyl ammonium bromide (CATB) as soft template, and was characterized by XRD and TEM methods. The electrochemical properties of nano‐Cu₂O and nano‐Cu₂O‐methylene blue (MB) modified electrode were studied. The experimental results indicate that nano‐Cu₂O shows a couple of redox peaks corresponding to the redox of Cu(II)/Cu(I), the peak currents are linear to the scan rates which demonstrate that the electrochemical response of Cu₂O is surface‐controlled. The composite nano‐Cu₂O‐Nafion‐MB modified electrode shows a trend of decrease of peak currents corresponding to the Cu (II)/Cu (I). However, the electrocatalytic ability of nano‐Cu₂O‐MB composite film to dopamine increases dramatically. At this composite electrode, dopamine shows a couple of quasireversible redox peaks with a peak separation of 106 mV, the peak current increases about 8 times and the oxidation peak potential decreases about 200 mV as compared to that at bare glassy carbon electrode. The peak currents change linearly with concentration of dopamine from 1×10^?7 to 3.2×10^?4 mol/L, the detection limit is 4.6×10^?8 mol/L. The composite electrode can effectively eliminate the interference of ascorbic acid and has better stability and excellent reproducibility.     

Electrochemical Study of Interaction Between Clozapine and DNA and Its Analytical Application

Abstract

The interaction between clozapine (CLZ) as an orally administrated antipsychotic drug with double stranded calf thymus DNA (dsDNA) was investigated at electrode surface using differential pulse voltammetry (DPV). Activated carbon paste electrode (CPE) was modified with dsDNA and used for monitoring the changes of the characteristics peak of CLZ in 0.05 M acetate buffer (pH 4.3). The adsorptive stripping voltammetry on dsDNA‐modified carbon paste electrode (dsDNA‐CPE) was used for determination of very low concentration of CLZ. Under optimal conditions, the oxidation peak current is proportional to CLZ concentration in the range of 7×10^?9?1.2×10^?6 mol l^?1 with a detection limit of 1.5×10^?9 mol l^?1 for 180 s accumulation time by DPV. The proposed dsDNA‐CPE was successfully used for determination of CLZ in human serum samples with recovery of 97.0±2.5%.     

Electrocatalytic Determination of Vitamin C Using Calixarene Modified Carbon Paste Electrodes

Effect of in situ complexation of some ions with variable valencies, like Co(II), Ni(II), Mn(II), Cu(II) and Pb(II) on the the electrooxidation of Vitamin C (L ‐ascorbic acid) was studied by cyclic voltammetry using carbon paste electrodes modified with p‐tert‐butylcalix[4]arene and p‐tert‐butylcalix[6]arene in perchloric acid, acetic acid and ammonium acetate media. Pb(II) was found to bind strongly with p‐tert‐butylcalix[4]arene in acetate medium, resulting in its being retained at the electrode surface and catalyzing the oxidation of ascorbic acid. The overpotential was reduced by about 200 mV with an increase in the peak currents. Linearity was observed over the range of 0.07–400 ppm with a detection limit of 30 ppb by differential pulse voltammetry. Interferences of some common substances like sugars and amino acids were studied and the modified electrode was used for the determination of vitamin C in commercial samples.     

On the Adsorption and Reduction of the Herbicide Picloram on Mercury and Carbon Electrodes

At concentrations higher than 2?10^?4 M , and below pH 3, the cyclic voltammograms of picloram (=4‐amino‐3,5,6‐trichloropyridine‐2‐carboxylic acid) on Hg electrodes show two prepeak systems (named I and II attending to the proximity to the main reductions peak), which can be attributed to the weak adsorption of reactant and the strong adsorption of the product at the electrode surface. The system II is due to the uncharged form of picloram, and system I to the picloram protonated at the pyridine N‐atom. Small amounts of the surfactant Triton X‐100 (=α‐[4‐(1,1,3,3‐tetramethylbutyl)phenyl]‐ω‐hydroxypoly(oxyethane‐1,2‐diyl)) cause the disappearance of system I, the shift of system II, and also affect the intensities and widths of anodic and cathodic peaks but not the charge passed in each peak. Thus, the adsorption process responsible for the appearance of system I is inhibited by the presence of Triton; by contrast, the process corresponding to system II is only modified by the surfactant, becoming an electrochemical process occurring at the potentials corresponding to system II, which is more reversible than that observed in the absence of Triton. The addition of Triton permitted the analysis of the main reduction process. Convolution voltammetry of the main reduction peak is consistent with the loss of a Cl‐atom in equilibrium which occurs after a reversible electron transfer and is followed by the reductions of both species present in the equilibrium (Scheme 2). This is also the reduction mechanism on a glassy carbon electrode but the electron transfer on the carbon electrode increases with respect to the mercury electrodes; in addition, the loss of the Cl‐atom does not take place on the electrode surface. From the recording of differential capacity–potential curves, it was concluded that picloram is adsorbed on the carbon electrode; but this adsorption is too weak to induce the appearance of prepeak systems.     

Stripping Voltammetry of Mercury(II) with a Chemically Modified Carbon Paste Electrode Containing Silica Gel Functionalized with 2,5‐Dimercapto‐1,3,4‐thiadiazole

The accumulation voltammetry of mercury(II) was investigated at a carbon paste electrode chemically modified with silica gel functionalized with 2,5‐dimercapto‐1,3,4‐thiadiazole (DTTPSG‐CPE). The repetitive cyclic voltammogram of mercury(II) solution in the potential range ?0.2 to +0.8 V (vs. Ag/AgCl), (0.02 mol L^?1 KNO₃ ; v=20 mV s^?1) show two peaks one at about 0.0 V and other at 0.31 V. However, the cathodic wave peak, around 0.0 V, is irregular and changes its form in each cycle. This peak at about 0.0 V is the reduction current for mercury(II) accumulated in the DTTPSG‐CPE. The anodic wave peak at 0.31 V is well‐defined and does not change during the cycles. The resultant material was characterized by cyclic and differential pulse anodic stripping voltammetry performed with the electrode in differents supporting electrolytes. The mercury response was evaluated with respect to pH, electrode composition, preconcentration time, mercury concentration, “cleaning” solution, possible interferences and other variables. The precision for six determinations (n=6) of 0.05 and 0.20 mg L^?1 Hg(II) was 2.8 and 2.2% (relative standard deviation), respectively. The method was satisfactory and used to determine the concentration of mercury(II) in natural waters contaminated by this metal.     

Renewable Surface Sol‐gel Derived Carbon Ceramic Electrode Modified with Copper Complex and Its Application as an Amperometric Sensor for Bromate Detection

A sol‐gel technique was used for the preparation of a three dimensional carbon composite electrode modified with [Cu(bpy)₂]Br₂ complex. A reversible redox couple of Cu(II)/Cu(I) is observed at the electrode surface. The electrochemical behavior and stability of the modified electrode was characterized by cyclic voltammetry. The charge transfer coefficient (α) and charge transfer rate constant (K_s) for the modified electrode were determined by cyclic voltammetry, which were found to be 0.46 and 14.2 s^?1, respectively. The modified electrode showed excellent catalytic activity toward bromate reduction at significantly reduced overpotentials and can be used successfully for amperometric detection of bromate. Under the optimized conditions, the calibration plots are linear in the concentration range 0.5 μM ?200μM. Detection limit (signal to noise is 3) and sensitivity were found to be 0.1 μM and 20 nA / μM, respectively. These analytical parameters compare favorably with those obtained with modern analytical techniques. The modified carbon ceramic electrode doped with Cu‐Complex shows a good reproducibility, a short response time (t<2 s), remarkable long term stability (>4 months) and especially good surface renewability by simple mechanical polishing (RSD for 6 successive polishing is 1.5%).     

Electrode reactions of palladium(II) in chloride solution at carbon paste electrodes modified with derivatives of N-benzoylthiourea

The study of the electrode reactions of palladium(II) at non-modified carbon paste electrodes (CPEs) in chloride solution has revealed the existence of a chloropalladate(II) complex at the electrode surface. The complex is formed during the application of anodic potentials after preceding palladium deposition. In the present paper the electrode reactions of Pd^II at CPEs modified with some N′,N′-disubstituted derivatives of N-benzoylthiourea [as selective ligands for palladium(II)] are studied in chloride solution by cyclic voltammetry. Two reduction peaks are observed in the cathodic scans recorded after deposition of palladium and anodization of the electrode. From the results it is concluded that [in addition to the chloropalladate(II) complex, observed at the non-modified electrode] a second palladium complex is formed at positive potentials. The formation of the palladium(II) complex of the N-benzoylthiourea derivatives by ligand exchange at the electrode surface is assumed. The ligand exchange itself occurs without charge transfer across the electrode|solution interface; therefore, it cannot be detected electrochemically. After palladium deposition and anodic treatment a pronounced "inverse" peak (i.e., an anodic peak in the cathodic scan) with peak currents up to 100 μA is observed at about +0.8 V. Its peak current increases with the amount of deposited palladium and the number of cycles. The reactions at the electrode surface are discussed. The results of the study reveal the existence of two different surface complexes of palladium(II) at ligand-modified CPEs, but the surface reactions could not be elucidated in detail. Electronic Publication     

Sensing System Integrating Lanthanum Hydroxide Nanowires with Copper(II) Ion for Uracil and its Application

Abstract

A sensing system for uracil was constituted by using lanthanum hydroxide nanowires (LNW) as a modifier to obtain LNW modified carbon paste electrode (LNW/CPE) and by introducing copper(II) ion into supporting electrolyte to transform electroinactive uracil to electroactive uracil‐Cu(II) complex. The voltammetric behaviors of uracil in the presence of Cu(II) ion at LNW/CPE were investigated. A reduction peak of the uracil‐Cu(II) complex at ?0.18 V was the two‐electron reduction of Cu(II) ion in the uracil‐Cu(II) complex; while a new oxidation peak at 0.22 V was the one‐electron oxidation of the uracil‐Cu(I) complex. Additionally, the voltammetric responses of all the complexes at LNW/CPE were more sensitive than that at carbon and multiwall carbon nanotube paste electrodes, which resulted from both the large surface effect of LNW and the chemical coordination of uracil with La(III) ion in LNW. With the sensitive oxidation peak of the uracil‐Cu(I) complex at LNW/CPE, a linear range of 4.0×10^?9?3.0×10^?8 mol/l for uracil was obtained along with a detection limit of 2.0×10^?10 mol/l. The proposed system was evaluated by the determination of uracil derivatives, anticancer drug 5‐flurouracil, in pharmaceutical preparations.     

Measurement of Dissolved Oxygen in Biological Fluids by Using a Modified Carbon Paste Electrode

A modified carbon paste electrode was constructed for the determination of dissolved oxygen using diamino‐o‐benzoquinone (DABQ) as the modifier. The electrochemical behavior of the electrode in citrate buffer (pH 2.0) was studied. In the presence of dissolved oxygen (DO) both cathodic and anodic peak currents decreased, indicating a chemical reaction between modifier and O₂. The decrease in peak current was linearly proportional to the amount of dissolved oxygen in the concentration range of 252–1260 μM of DO. The electrode was utilized in the determination of DO in urine samples. The relative error and RSD of the method were 1.6% and 4.1%, respectively. The electrode was applied more than two months for the determination of DO without any significant divergence in its voltammetric response.     

Carboxylate ion dependency in the Cu(II) catalysed asymmetric Henry reaction: Structural characterisation of a tridentate Schiff base complex containing a coordinated carboxylic acid

Six tridentate Schiff base ligands containing tertiary butyl or benzyl substituents were prepared from chiral amino alcohols and salicylaldehyde derivatives. The ligands were employed as catalysts for the Cu(II) catalysed asymmetric Henry reaction. It was discovered that when different carboxylate salts were used instead of copper acetate as the Cu(II) salt, significant changes in the enantioselectivity of the reactions were observed. Addition of Cu(OAc)₂ to the ligand prepared from salicylaldehyde and α,α‐diphenyl‐tert ‐leucinol resulted in the formation of dark green crystals. X‐ray structural analysis of these crystals showed that a square planar monomeric complex had been formed rather than the expected dimer. In the structure, the copper(II) centre is bonded to the tridentate ONO ligand and an acetate ion. There is a strong hydrogen bond between the protonated alcoholic oxygen of the Schiff base ligand and the uncoordinated acetate oxygen atom. These results, taken together, indicate that the carboxylate anion may be an important part of the active intermediate when this type of copper complex is used as a catalyst in the asymmetric Henry reaction.     

Direct Electrochemistry of Catalase at a Gold Electrode Modified with Single‐Wall Carbon Nanotubes

The direct electrochemistry of catalase (Ct) was accomplished at a gold electrode modified with single‐wall carbon nanotubes (SWNTs). A pair of well‐defined redox peaks was obtained for Ct with the reduction peak potential at ?0.414 V and a peak potential separation of 32 mV at pH 5.9. Both reflectance FT‐IR spectra and the dependence of the reduction peak current on the scan rate revealed that Ct adsorbed onto the SWNT surfaces. The redox wave corresponds to the Fe(III)/Fe(II) redox center of the heme group of the Ct adsorbate. Compared to other types of carbonaceous electrode materials (e.g., graphite and carbon soot), the electron transfer rate of Ct redox reaction was greatly enhanced at the SWNT‐modified electrode. The peak current was found to increase linearly with the Ct concentration in the range of 8×10^?6–8×10^?5 M used for the electrode preparation and the peak potential was shown to be pH dependent. The catalytic activity of Ct adsorbates at the SWNTs appears to be retained, as the addition of H₂O₂ produced a characteristic catalytic redox wave. This work demonstrates that direct electrochemistry of redox‐active biomacromolecules such as metalloenzymes can be improved through the use of carbon nanotubes.     

Multi‐Walled Carbon Nanotubes (MWCNT)‐Ionic Liquid‐Modified Carbon Paste Electrode: Probing FurazolidoneDNA Interactions and DNA Determination

The interactions of furazolidone (Fu) with double‐stranded calf thymus DNA (dsDNA) on the multi‐walled carbon nanotubes‐ionic liquid‐modified carbon paste electrode (MWCNT‐IL‐CPE) have been studied by cyclic voltammetry. In the presence of DNA, the cathodic peak current of Fu decreased and the peak potential shifted to a positive potential, indicating the intercalative interaction of Fu with DNA. The binding constant of Fu with DNA and stoichiometric coefficient has been determined according to the Hill's model. This electrochemical method was further applied to the determination of DNA. Two linear calibration curves were obtained for DNA detection in the concentration ranges of 0.03–0.10 and 0.10–4.0 μg l^?1 with a detection limit of 0.027 μg l^?1. The method was successfully applied to analyze Fu in serum samples.     

Graphene Oxide‐Modified Electrode Coated with in‐situ Antimony Film for the Simultaneous Determination of Heavy Metals by Sequential Injection‐Anodic Stripping Voltammetry

The proposed chemically modified electrode was graphene oxide that was synthesized via Hummer's method followed by reduction of antimony film by in‐situ electrodeposition. The experimental process could be concluded in three main steps: preparation of antimony film, reduction of analyte ions on the electrode surface and stripping step under the conditions of square wave anodic stripping voltammetry (SWASV). A simple and rapid approach was developed for the determination of heavy metals simultaneously based on a sequential injection (SI), an automated flow‐based system, coupled with voltammetric method using antimony‐graphene oxide modified screen‐printed carbon electrode (SbF‐GO‐SPCE). The effects of main parameters involved with graphene oxide, antimony and measurement parameters were also investigated. Using SI‐SWASV under the optimal conditions, the proposed electrode platform has exhibited linear range from 0.1 to 1.5 M. Calculated limits of detection were 0.054, 0.026, 0.060, and 0.066 μM for Cd(II), Pb(II), Cu(II) and Hg(II), respectively. In addition, the optimized method has been successfully applied to determine heavy metals in real water samples with acceptable accuracy of 94.29 – 113.42 % recovery.     

Electrocatalytic Oxidation of Hydrogen Peroxide on Poly(m‐toluidine)‐Nickel Modified Carbon Paste Electrode in Alkaline Medium

The poly(m‐toluidine) film was prepared by using the repeated potential cycling technique in an acidic solution at the surface of carbon paste electrode. Then transition metal ions of Ni(II) were incorporated to the polymer by immersion of the modified electrode in a 0.2 M NiSO₄, also the electrochemical characterization of this modified electrode exhibits stable redox behavior of the Ni(III)/Ni(II) couple. The electrocatalytic ability of Ni(II)/poly(m‐toluidine)/modified carbon paste electrode (Ni/PMT/MCPE) was demonstrated by electrocatalytic oxidation of hydrogen peroxide with cyclic voltammetry and chronoamperometry methods in the alkaline solution. The effects of scan rate and hydrogen peroxide concentration on the anodic peak height of hydrogen peroxide oxidation were also investigated. The catalytic oxidation peak current showed two linear ranges with different slopes dependent on the hydrogen peroxide concentration and the lower detection limit was 6.5 μM (S/N=3). The catalytic reaction rate constant, (k_h), was calculated 5.5×10² M^?1 s^?1 by the data of chronoamperometry. This modified electrode has many advantages such as simple preparation procedure, good reproducibility and high catalytic activity toward the hydrogen peroxide oxidation. This method was also applied as a simple method for routine control and can be employed directly without any pretreatment or separation for analysis cosmetics products.