Acid stability of carbon paste enzyme electrodes

This note reports on the unusual protection of several enzymes against harsh pH conditions provided by carbon paste electrodes. Both glucose oxidase and polyphenol oxidase carbon paste amperometric biosensors display a remarkable resistance to acid deactivation compared to conventional biosensors prepared by electropolymeric entrapment of enzymes. For example, the carbon paste enzyme electrodes fully retain their activity upon stressing in strongly acidic conditions (pH approximately 2.0-2.5) for prolonged periods, where conventional (polymer-based) biosensors rapidly lose most of their response. Such unusual acid stability of carbon paste enzyme electrodes is attributed to the "pH memory" of enzymes in the hydrophobic paste environment, to the barrier to hydronium ions provided by the pasting liquid and to decreased conformational mobility.     

Modified carbon paste electrode for potentiometric determination of diquat dibromide pesticide in water and urine samples

A carbon paste electrode for diquat dibromide (Dq.2Br) was prepared and fully characterized in terms of composition, usable pH range, response time and temperature. The electrode was applied to the potentiometric determination of diquat ions in water and urine samples with average recoveries of 97.5-104.0% and relative standard deviations of 0.30-4.73%. The electrode is based on the ion pair, namely, diquat-phosphotungstate dissolved in 2-nitrophenyloctyl ether (2-NPOE) as pasting liquid with 1.0% Na-TPB as an additive. The modified electrode showed a near-Nernstian slope of 30.8 mV over the concentration range of 3.8 × 10^− 6to 1.0 × 10^− 3 M with the limit of detection 9.0 × 10^− 7 over the pH range 4.5-9.5. The electrode exhibits good selectivity for Dq cations with respect to a large number of inorganic cations, organic cations, sugars and amino acids. The proposed potentiometric method offers the advantages of simplicity, accuracy, automation feasibility and applicability to turbid and colored sample solutions.     

MWCNT modified carbon paste electrode for sensitive determination of nicotine

ABSTRACT

A new sensor for the determination of nicotine is proposed based on the reduction of Cu(II)–nicotine complex at MWCNT modified carbon paste electrode. In borate buffer (pH 7.0) the reduction peak of Cu(II)–nicotine complex was observed at ? 0.05 V (versus Ag/AgCl). The increment of peak current obtained by deducting the reduction peak current of the Cu(II)–nicotine complex was rectilinear with nicotine concentration in the range of 0.05–30.0 n g mL^?1, with a detection limit of 0.01 ng/mL^?1. The method was applied for the sensitive quantification of nicotine in real samples with the satisfactory results.     

Electrochemical sensor for ranitidine determination based on carbon paste electrode modified with oxovanadium (IV) salen complex

The preparation and electrochemical characterization of a carbon paste electrode modified with the N,N-ethylene-bis(salicyllideneiminato)oxovanadium (IV) complex ([VO(salen)]) as well as its application for ranitidine determination are described. The electrochemical behavior of the modified electrode for the electroreduction of ranitidine was investigated using cyclic voltammetry, and analytical curves were obtained for ranitidine using linear sweep voltammetry (LSV) under optimized conditions. The best voltammetric response was obtained for an electrode composition of 20% (m/m) [VO(salen)] in the paste, 0.10 mol L^? 1 of KCl solution (pH 5.5 adjusted with HCl) as supporting electrolyte and scan rate of 25 mV s^? 1. A sensitive linear voltammetric response for ranitidine was obtained in the concentration range from 9.9 × 10^? 5 to 1.0 × 10^? 3 mol L^? 1, with a detection limit of 6.6 × 10^? 5 mol L^? 1 using linear sweep voltammetry. These results demonstrated the viability of this modified electrode as a sensor for determination, quality control and routine analysis of ranitidine in pharmaceutical formulations.     

Adsorption character for removal Cu(II) by magnetic Cu(II) ion imprinted composite adsorbent

A novel magnetic Cu(II) ion imprinted composite adsorbent (Cu(II)-MICA) was synthesized, characterized and applied for the selective removal Cu(II) from aqueous solution in the batch system. The adsorption-desorption and selectivity characteristics were investigated. The maximum adsorption occurred at pH 5-6. The equilibrium time was 6.0h, and a pseudo-second-order model could best describe adsorption kinetics. The adsorption equilibrium data fit Langmuir isotherm equation well with a maximum adsorption capacity of 46.25mg/g and Langmuir adsorption equilibrium constant of 0.0956L/mg at 298K. Thermodynamic parameters analysis predicted an exothermic nature of adsorption and a spontaneous and favourable process that could be mainly governed by physisorption mechanism. The relative selectivity coefficients of Cu(II)-MICA for Cu(II)/Zn(II) and Cu(II)/Ni(II) were 2.31, 2.66 times greater than the magnetic non-imprinted composite adsorbent (MNICA). Results suggested that Cu(II)-MICA was a material of efficient, low-cost, convenient separation under magnetic field and could be reused five times with about 14% regeneration loss.     

Insulin oxidation and determination at carbon electrodes

The redox chemistry of insulin was investigated at glassy carbon (GC) electrodes that were coated with films of chitosan (CHIT) and multiwalled carbon nanotubes (CNT). While bare electrodes deactivated quickly during insulin oxidation, the GC electrodes coated with CHIT and CHIT-CNT films generated stable insulin currents. The GC/CHIT-CNT electrodes were used for investigating the electrooxidation process of insulin and amperometric determination of insulin. The mass spectrometric, electron paramagnetic resonance, and separation studies of electrolyzed insulin solutions suggested that the loss of 4 mass units upon insulin oxidation at CNT could be accounted for by the formation of two dityrosine cross-links intramolecularly. At a potential of 0.700 V and physiological pH 7.40, the GC/CHIT-CNT electrodes displayed a detection limit of approximately 30 nM insulin (S/N = 3), sensitivity of 135 mA M(-1) cm(-2), linear dynamic range from 0.10 to 3.0 microM (R2 = 0.995), and superior operational and long-term stability. The CNT-based electrodes are promising new insulin detectors for diabetes-related studies such as fast chromatographic analysis of therapeutic insulin formulations or evaluation of quality of pancreatic islets prior to their transplantation.     

Monocrystalline diamond paste-based electrodes and their applications for the determination of Fe(II) in vitamins

A new class of electrochemical sensors, namely, electrodes based on diamond paste, was designed using monocrystalline diamond (natural diamond 1 microm and synthetic diamond, 50 microm (synthetic-1) and 1 microm (synthetic-2)) powder and paraffin oil. The characterization of the electrodes was performed using cyclic voltammetry and differential pulse voltammetry. Fe(II) was determined by differential pulse voltammetry (DPV) at 75 mV (vs Ag/AgCl) using all diamond paste-based electrodes. The linear concentration range was between 10(-8) and 10(-4) mol/L for both the natural diamond and synthetic-2 with detection limits of 10(-10) and 10(-9) mol/L, respectively, whereas the linear concentration range for synthetic-1 was between 10(-7) and 10(-3) mol/L with a detection limit of 10(-8) mol/L Fe(II) was determined successfully from four types of pharmaceutical products. The recovery values of Fe(II) in the pharmaceutical products were higher than 98.00% with relative standard deviation values < 5%.     

Carbon nanotube purification: preparation and characterization of carbon nanotube paste electrodes

Paste electrodes have been constructed using single-wall carbon nanotubes mixed with mineral oil. The electrochemical behavior of such electrodes prepared with different percentages of carbon nanotubes has been compared with that of graphite paste electrodes and evaluated with respect to the electrochemistry of ferricyanide with cyclic voltammetry. Carbon nanotubes were purified by a treatment with concentrated nitric acid, then oxidized in air. In addition, electrochemical pretreatments were carried out to increase the selectivity of carbon nanotube electrodes. Performances of carbon nanotube paste and carbon paste electrodes were evaluated by studying such parameters as current peak, deltaEp, anodic and cathodic current ratio, and charge density toward several different electroactive molecules. Data interpretation based on the carbon nanotubes and carbon surface area is presented. Carbon nanotube paste and carbon paste electrodes were tested as H2O2 and NADH probes, and several analytical parameters were evaluated. The oxidative behavior of dopamine was examined at these electrodes. The two-electron oxidation of dopamine to dopaminequinone showed an excellent reversibility in cyclic voltammetry that was significantly better than that observed at carbon paste electrodes.     

Silicon nanowires-based fluorescence sensor for Cu(II)

Si nanowires (SiNWs) were covalently modified by fluorescence ligand, N-(quinoline-8-yl)-2-(3-triethoxysilyl-propylamino)-acetamide (QlOEt) and finally formed an optical sensor to realize a highly sensitive and selective detection for Cu(II). The QlOEt-modified SiNWs sensor has sensitivity for Cu(II) down to 10(-8) M, which is more sensitive than QlOEt alone. Metal ions interferences have no observable effect on the sensitivity and selectivity of QlOEt-modified SiNWs sensor. The SiNWs-based fluorescence sensor is reversible by addition of acid to replace Cu(II). The sensing mechanisms of QlOEt-modified SiNWs to Cu(II) and the rationale for the increase in sensitivity and selectivity of QlOEt-modified SiNWs over QlOEt on Cu(II) are discussed. The current sensor structure may be extendable to other chemo- and biosensors, and even to nanosensors for direct detection of specific materials in intracellular environment.     

Prediction of simultaneous adsorption of Cu(II) and Pb(II) onto activated carbon by conventional Langmuir type equations

Removal of Cu(II) and Pb(II) by adsorption onto activated carbon was examined in single- and binary-component aqueous solutions representative of contaminated solutions containing heavy metals. Reversibility of adsorption of the heavy metals on the activated carbon was evaluated by desorption experiments. The number of the maximum adsorption sites and adsorption equilibrium constants of Cu(II) and Pb(II) were estimated by the results of single-component systems assuming the Langmuir adsorption model. The adsorption sites per gram of activated carbon resulted in similar values for Cu(II) and Pb(II) from the isotherms. The adsorption constant for Pb(II) was nearly 1.8 times greater than that of Cu(II). Rate constants of adsorption and desorption were also estimated from the kinetic analysis. Using the single set of common parameters obtained from the single-component systems, the experimental results for a binary-component system were quantitatively predicted. Competitive adsorption of Cu(II) and Pb(II) on the same adsorption sites was confirmed by both experimental and predicted results of adsorption in the binary mixture.     

Chemically modified activated carbon with 1-acylthiosemicarbazide for selective solid-phase extraction and preconcentration of trace Cu(II), Hg(II) and Pb(II) from water samples

A new sorbent 1-acylthiosemicarbazide-modified activated carbon (AC-ATSC) was prepared as a solid-phase extractant and applied for removing of trace Cu(II), Hg(II) and Pb(II) prior to their determination by inductively coupled plasma optical emission spectrometry (ICP-OES). The separation/preconcentration conditions of analytes were investigated, including effects of pH, the shaking time, the sample flow rate and volume, the elution condition and the interfering ions. At pH 3, the maximum static adsorption capacity of Cu(II), Hg(II) and Pb(II) onto the AC-ATSC were 78.20, 67.80 and 48.56 mg g⁻¹, respectively. The adsorbed metal ions were quantitatively eluted by 3.0 mL of 2% CS(NH₂)₂ and 2.0 mol L⁻¹ HCl solution. Common coexisting ions did not interfere with the separation. According to the definition of IUPAC, the detection limits (3σ) of this method for Cu(II), Hg(II) and Pb(II) were 0.20, 0.12 and 0.45 ng mL⁻¹, respectively. The relative standard deviation under optimum conditions is less than 4.0% (n = 8). The prepared sorbent was applied for the preconcentration of trace Cu(II), Hg(II) and Pb(II) in certified and water samples with satisfactory results.     

Isotherm,kinetic, and thermodynamic studies for sorption of Cu(II) and Pb(II) by activated carbon prepared from Leucaena plant wastes

ABSTRACT

Leucaena biomass waste was used for preparation of activated carbon by chemical activation using KOH and pyrolysis in a muffle furnace at 800°C for 30 min. Leucaena activated carbon (LAC) as a sorbent material was used for removal of Cu (II) and Pb (II) ions from aqueous solutions. The present study reveals that LAC is efficient sorpent, fast kinetics, as easy to handle, and small amount of secondary sludge produced. The isotherm, kinetics, and thermodynamics of Cu (II) and Pb (II) sorptions by LAC were investigated. The isothermal data were found to be correlated with the Langmuir model better than the Freundlich model. The maximum sorption capacity (Q₀) for the studied sorbent toward Pb (II) and Cu (II) was 32.18 and 7.89 mg/g, respectively. The experimental data show that the external diffusion and intra-particular diffusion are significant in the determination of the sorption. The thermodynamic parameters indicated that the sorption processes were spontaneous and endothermic in nature.     

A novel multi-walled carbon nanotube (MWNT)-based nanocomposite for PEFC electrodes

A novel nanocomposite comprising MWNTs and mixed-conducting polymeric components (electronic and ionic) is prepared, characterized and investigated as a support for platinum (Pt). Nanocomposite of MWNTs and poly (3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT?CPSS) is prepared by in situ polymerization and characterized using Fourier?CTransform infrared spectroscopy (FT?CIR), thermogravimetric analysis (TGA) in conjunction with scanning electron microscopy (SEM). Atomic force microscopy (AFM) studies are also carried out to characterize the surface topography of MWNTs/PEDOT?CPSS nanocomposite. X-ray diffraction (XRD) studies reveal that MWNTs/PEDOT?CPSS nanocomposite provides better backbone for the improved dispersion of Pt as evidenced by the reduced Pt crystallite size over MWNTs/PEDOT?CPSS nanocomposite compared to MWNTs. Electrochemical characterization studies performed with Pt/nanocomposite and Pt/MWNTs demonstrate the superior catalytic activity of Pt/nanocomposite under reduced Nafion loadings in relation to Pt/MWNTs. It is observed that mixed conducting nanoporous network of MWNTs/PEDOT?CPSS composite structure promotes the catalytic activity of Pt by enhancing catalyst utilization.     

Supported Cu(II) polymer catalysts for aqueous phenol oxidation

Supported Cu(II) polymer catalysts were used for the catalytic oxidation of phenol at 30 degrees C and atmospheric pressure using air and H(2)O(2) as oxidants. Heterogenisation of homogeneous Cu(II) catalysts was achieved by adsorption of Cu(II) salts onto polymeric matrices (poly(4-vinylpyridine), Chitosan). The catalytic active sites were represented by Cu(II) ions and showed to conserve their oxidative activity in heterogeneous catalysis as well as in homogeneous systems. The catalytic deactivation was evaluated by quantifying released Cu(II) ions in solution during oxidation, from where Cu-PVP(25) showed the best leaching levels no more than 5 mg L(-1). Results also indicated that Cu-PVP(25) had a catalytic activity (56% of phenol conversion when initial Cu(II) catalytic content was 200 mg L(Reaction)(-1)) comparable to that of commercial catalysts (59% of phenol conversion). Finally, the balance between activity and copper leaching was better represented by Cu-PVP(25) due to the heterogeneous catalytic activity had 86% performance in the heterogeneous phase, and the rest on the homogeneous phase, while Cu-PVP(2) had 59% and CuO/gamma-Al(2)O(3) 68%.     

Determination of Cu2+ using poly(2-aminothiazole)/ multi-walled carbon nanotubes composite film modified glassy carbon electrodes

2-Aminothiazole was electropolymerized by cyclic voltammetry (CV) on the multi-walled carbon nanotubes (MWCNTs) modified glassy carbon electrode (GCE) surface. Poly(2-aminothiazole)/MWCNTs/GCE was used for determination of copper ions. The anodic peak currents of copper ions evaluated by differential pulse stripping voltammetry (DPSV) are linear with the concentrations in the range from 1.0 x 10(-7) M to 2.0 x 10(-5) M with a linear coefficiency of 0.9985. The detection limit is 2.0 x 10(-9) M calculated for a signal-to-noise ratio of 3 (S/N = 3). The proposed method was applied successfully to the determination of copper ions in drinking water, and the recovery was 96%.     

Electrochemiluminescent detection of chlorpromazine by selective preconcentration at a lauric acid-modified carbon paste electrode using tris(2,2'-bipyridine)ruthenium(II)

A new detection scheme for the determination of adsorbable coreactants of Ru(bpy)3(2+) electrochemiluminescent reaction is presented. It is based on selective preconcentration of coreactant onto an electrode, followed by Ru(bpy)3(2+) electrochemiluminescent detection. The coreactant employed is chlorpromazine. It was sensitively detected after 5-min preconcentration onto a lauric acid-modified carbon paste electrode. The linear concentration range was found to occur from 1 x 10(-8) to 3 x 10(-6) mol L-1 with a detection limit of 3.1 x 10(-9) mol L-1. The total analysis time is less than 10 min. As a result of selective preconcentration and medium exchange, such remarkable selectivity is achieved that reproducible quantitation of chlorpromazine in urine is possible.     

Some advantages of carbon paste electrodes in the morphological study of finely divided iron oxides

The electroreduction of finely divided trivalent iron oxides using a carbon paste electrode occurs according to a mechanism involving the dissolution of the solid and then the reduction of solvated Fe³⁺ ions into Fe²⁺ ions. The voltammetric curves exhibit two peaks. The first is due to the reduction of Fe³⁺ ions that come from dissolution of the smallest particles at the beginning of the experiment. The second is characteristic of reduction of the solid; its shape, position and amplitude are influenced by morphology. Theoretical aspects are considered. It appears that the use of a carbon paste electrode for the study of divided solids containing an electroreactive ion can easily give information either on the particle size distribution, if the dissolution kinetics of the compound are known, or on the dissolution kinetics of monodisperse solids or solids whose particle size distribution is well defined.     

Direct electrochemical determination of ascorbic acid by a cobalt(II) tetra-neopentyloxy phthalocyanine-multi-walled carbon nanotubes glassy carbon electrode

A cobalt(II) tetra-neopentyloxy phthalocyanine-multi-walled carbon nanotubes (CoTNPPc–MWNTs) composite was synthesized and characterized by UV–Vis spectra and transmission electron microscopy. The CoTNPPc–MWNTs glassy carbon electrode (CoTNPPc–MWNTs/GCE) was prepared by drop coating. The electrocatalytic performance of the chemically modified electrode was investigated for oxidation of ascorbic acid (AA). It was found that in phosphate buffer solution at pH = 6.60, the chemically modified electrode exhibited excellent electrocatalytic activity toward the oxidation of AA. The oxidation peak current increased linearly with the concentration of AA in the range of 10 μM–1.6 mM within the detection limit of 5 μM and low response time of 4 s.