Electrochemical behavior and determination of the insecticide synergist piperonyl butoxide

Piperonyl butoxide may be reversibly oxidized in acetonitrile at a glassy carbon electrode to a cation radical under short time scale voltammetric conditions, e.g., cyclic voltammetry when the potential scan rate is above 500 mV s(-)(1). During longer time domain experiments, the cation radical decays in a rate-limiting heterolytic bond cleavage step and subsequent transfer of a second electron at the potential of the first process. Additionally, a second oxidation process develops at more positive potentials. One product isolated from the initial oxidation process in an almost quantitative yield, under controlled potential electrolysis conditions, is 6-n-propyl-1,3-benzodioxole-5-carboxaldehyde. This carboxaldehyde is oxidized at the same positive applied potential as the second oxidation process observed in long time domain voltammetric experiments with piperonyl butoxide. The limit of detection for piperonyl butoxide in acetonitrile, using differential pulse voltammetry at a glassy carbon electrode, is 1.6 × 10(-)(6) M (3σ), with a limit of determination of 4.1 × 10(-)(6) M (10σ). Piperonyl butoxide was selectively determined using differential pulse voltammetry with a concentration of 5.11 ± 0.02 g L(-)(1) in a commercial insecticide formulation containing pyrethrins. This result is in good agreement with the manufacturer's stated concentration of 5.07 g L(-)(1). The sample preparation requires only simple dilution of the formulation in an acetonitrile/dichloromethane (95:5) solvent mixture.     

Voltammetric characterization of a N,N'-diphenyl-p-phenylenediamine-loaded screen-printed electrode: a disposable sensor for hydrogen sulfide

The voltammetric response of a 10% (by weight) N,N'-diphenyl-p-phenylenediamine (DPPD) and 90% (by weight) carbon and binder screen-printed electrode has been examined in aqueous media over a range of pH using cyclic voltammetry both in the presence and in the absence of sulfide. In the absence, the screen-printed electrode undergoes an initial oxidative process on the surface of the solid organic particles to form an insoluble layer of the corresponding cation radical salt, DPPD(*)(+)X(-), where X(-) is an anion present in the solution. The charge transfer is thought to occur at the three-phase boundary between solid DPPD, carbon, and the aqueous solution. At higher potentials, a second oxidative wave is observed that is attributed to the oxidation of the bulk DPPD with intercalation of the anion species present to form a solid phase of DPPD(*)(+)X(-). The two voltammetric processes were found to stabilize after repetitive scanning, after which time, sulfide was added to the solution. The voltammetric response was found to respond to sulfide by showing a decrease in both the oxidative and reductive waves, which can be attributed to the sulfide effectively blocking the three-phase boundary. The response was found to be independent of the electrode used and at pH 4 produced a linear range from 20 to 165 microM, and a limit of detection of 7.5 microM for sulfide detection was achieved.     

Preconcentration and Voltammetric Determination of Mercury(II) at a Chemically Modified Glassy Carbon Electrode

In this work, the organic compound 2-mercaptobenzimidazole was covalently bound on the surface of a glassy carbon rod, via silanization, yielding a material capable of selectively complexing Hg(2+) ions. This material was applied as an electrode for voltammetric determination of mercury(II) following its nonelectrolytic preconcentration. After exchanging the medium, the voltammetric measurements were carried out by anodic stripping in the differential pulse mode (pulse amplitude, 50 mV; scan rate, 1.25 mV s(-)(1)) using 10(-)(2) mol L(-)(1) NaSCN solution as supporting electrolyte. An anodic stripping peak was obtained at 0.06 V (vs SCE) by scanning the potential from -0.3 to +0.3 V. After a 5 min preconcentration period in a pH 4.0 Hg(2+) solution, this electrode shows increasing voltammetric response in the range 0.1-2.2 μg mL(-)(1), with a relative standard deviation of 5% and a practical detection limit of 0.1 μg mL(-)(1) (5.0 × 10(-)(7) mol dm(-)(3)). Compared with the conventional stripping approach, this chemically modified glassy carbon electrode procedure presented good discrimination against interference from Cu(II) in up to 10-fold molar excess.     

Implementation of a Statistically Supported Heuristic Approach to Alternating Current Voltammetric Harmonic Component Analysis: Re-evaluation of the Macrodisk Glassy Carbon Electrode Kinetics for Oxidation of Ferrocene in Acetonitrile

Sinusoidal large amplitude ac voltammetric techniques gene-rate very large data sets. When analyzed in the frequency domain, using a Fourier transform (FT)-band filtering- inverse FT sequence, the data may be resolved into the aperiodic dc, fundamental, second, and higher order ac harmonics. Each of these components exhibit a different level of sensitivity to electrode kinetics, uncompensated resistance and capacitance. Detailed simulations illustrate how the heuristic approach for evaluation of each data subset may be implemented and exploited in the assessment of the electrode kinetics for the fast Fc ? Fc(+) + e (Fc = ferrocene) oxidation process at a glassy carbon macrodisk electrode. The simulations presented in this study are based on the Butler-Volmer model and incorporate consideration of the uncompensated resistance (R(u)), double-layer capacitance (C(dl)), rate constant (k(0)), and charge transfer coefficient (α). Error analysis of the heuristically evaluated simulation-experiment comparison is used to assist in establishing the best fit of data for each harmonic. The result of the heuristic pattern recognition type approach for analysis of the oxidation of ferrocene (0.499, 0.999, and 5.00 mM) at a glassy carbon macrodisk electrode in acetonitrile (0.1 M Bu(4)NPF(6)) implies that k(0) ≥ 0.25 cm s(-1) on the basis of analysis of the first 4 harmonics and plausibly lies in the range of 0.25-0.5 cm s(-1) with α = 0.25-0.75 when analysis of the next four harmonics is undertaken. The k(0) value is significantly faster then indicated in most literature reports based on use of dc cyclic voltammetry under transient conditions at glassy carbon macrodisk electrode. The data analysis with a sinusoidal amplitude of 80 mV is conducted at very low frequency experiments of 9 Hz to minimize contribution from electrode heterogeneity, frequency dispersion, and adsorption, all of which can complicate the response for the oxidation of Fc in acetonitrile at a glassy carbon electrode.     

Direct electrochemistry of cytochrome c at a glassy carbon electrode modified with single-wall carbon nanotubes

The electrochemistry of horse heart cytochrome c was studied by cyclic voltammetry at a glassy carbon electrode modified with single-wall carbon nanotubes (SWNTs). A pair of well-defined redox waves was obtained in cytochrome c aqueous solution at an activated SWNT film-modified electrode. The optimal conditions for activating the SWNT film-modified electrode has been determined. The electrode reaction of cytochrome c is a diffusion-controlled process. The peak current increases linearly with the concentration of cytochrome c in the range from 3.0 x 10(-5)-7.0 x 10(-4) M. The detection limit is 1.0 x 10(-5) M. The activated SWNT film was characterized by scanning electron microscopy. Furthermore, interaction of cytochrome c with adenine was characterized by electrochemical and spectral methods.     

Fast-scan voltammetry of cyclic nitroxide free radicals

Fast-scan cyclic voltammetry at carbon fiber microelectrodes is used to detect the cyclic nitroxide 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical (TEMPO) and three analogs. The electrochemical behavior of the TEMPO analogs at unmodified carbon fiber electrodes is found to differ greatly from their behavior at glassy carbon electrodes. After the electrode is coated with the polymer Nafion, the electrodes exhibit increased sensitivity to TEMPO and 4-amino-TEMPO. Voltammograms of the nitroxides at Nafion-coated electrodes indicate that the oxidized form (oxoammonium ion) and the free radical form have greatly different mobilities through the polymeric coating. Response times to changes in nitroxide concentration vary from subsecond at bare electrodes (all four analogs) and 4-hydroxy-TEMPO at modified electrodes to 1-3 s for TEMPO and 4-amino-TEMPO at modified electrodes. The detection limit for 4-amino-TEMPO is 50 μM at an unmodified electrode and 5 μM at a Nafion-coated carbon fiber electrode. The sensitivity of the Nafion-modified electrode to TEMPO, 4-hydroxy-TEMPO, and 4-amino-TEMPO can be improved by choosing a resting potential at which the oxoammonium ion form of the nitroxide is preconcentrated into the Nafion film. Using fast-scan cyclic voltammetry and the modified carbon fiber electrodes, the reaction of two nitroxide free radicals with ascorbate can be monitored. This work shows that fast-scan voltammetry at microelectrodes is a sensitive method that can be used to follow reactions of cyclic nitroxide free radicals in solution.     

Electroanalysis using macro-, micro-, and nanochemical architectures on electrode surfaces. Bulk surface modification of glassy carbon microspheres with gold nanoparticles and their electrical wiring using carbon nanotubes

Gold nanoparticles (approximately 30-60 nm in diameter) were deposited onto the surface of glassy carbon microspheres (10-20 microm) through electroless plating to produce bulk (i.e., gram) quantities of nanoparticle surface-modified microspheres. The gold nanoparticle-modified powder was then characterized by means of scanning electron microscopy and cyclic voltammetry. The voltammetric response of a macroelectrode consisting of a film of gold nanoparticle-modified glassy carbon microspheres, bound together and "wired-up" using multiwalled carbon nanotubes (MWCNTs), was investigated. We demonstrate that by intelligently exploiting both nano- and microchemical architectures and wiring up the electroactive centers using MWCNTs in this way, we can obtain macroelectrode voltammetric behavior while only using approximately 1% by mass of the expensive gold material that would be required to construct the equivalent gold film macrodisk electrode. The potential utility of electrodes constructed using chemical architectures such as this was demonstrated by applying them to the analytical determination of arsenic(III) concentration. An optimized limit of detection of 2.5 ppb was obtained.     

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%.     

Determination of pentachlorophenol at carbon nanotubes modified electrode incorporated with beta-cyclodextrin

A facile and reliable electrochemical technique at beta-cyclodextrin incorporated carbon nanotubes modified glassy carbon electrode (beta-CD/CNTs/GCE) was proposed for determination of pentachlorophenol (PCP). The electrochemical behavior of PCP at the beta-CD/CNTs/GCE was investigated by cyclic voltammetry and linear sweep voltammetry. The beta-CD/CNTs/GCE showed good analytical performance characteristics in electrocatalytic oxidation of PCP, compared with the simple carbon nanotube modified electrode (CNTs/GCE) and bare glassy carbon electrode (GCE). After accumulation for 5 min on beta-CD/CNTs/GCE, the peak current increased linearly with the concentration of PCP in the range from 8.0 x 10(-7) to 1.04 x 10(-5) mol/L. The detection limit was 4.0 x 10(-8) mol/L at 3 sigma level. The proposed electrode presented good repeatability for the determination of PCP in artificial wastewater, and the recovery was 97%-103%. This modified electrode combined the advantages of carbon nanotubes and supramolecular cyclodextrin, leading to new capabilities for electrochemical detection of PCP.     

An electrochemical sensor based on 1-benzyl-4-ferrocenyl-1H-[1,2,3]-triazole/carbon nanotube; detection of D-penicillamine in the presence of tryptophan

A glassy carbon electrode modified with 1-benzyl-4-ferrocenyl-1H-[1,2,3]-triazole (BFT) and carbon nanotubes have been applied to the electrocatalytic oxidation of D-penicillamine (D-PA) which reduced the overpotential by about 470 mV with obviously increase the current response. Due to its strong electrocatalytic activity towards D-PA, the modified electrode can resolve the overlapped voltammetric waves of D-PA and tryptophan (TRP) into two well-defined voltammetric peaks with peak-to-peak separation in potentials of about 270 mV. This property allows to selective determination of D-PA in the presence of TRP. The transfer coefficient (a) for the electrocatalytic oxidation of D-PA and diffusion coefficient of this substance under the experimental conditions were also investigated. In phosphate buffer solution (PBS) of pH 8.0, the oxidation current increased linearly with two concentration intervals of D-PA, one is 1.0 to 10.0 μM and, the other is 10.0 to 800.0 μM. The detection limit (3σ) obtained by square wave voltammetry (SWV) was 0.1 μM. The proposed method was successfully applied to the determination of D-PA, and TRP in real samples.     

Anodic stripping voltammetry of arsenic(III) using gold nanoparticle-modified electrodes

A novel method for the detection of arsenic(III) in 1 M HCl at a gold nanoparticle-modified glassy carbon electrode has been developed. The gold nanoparticles were electrodeposited onto the glassy carbon electrode via a potential step from +1.055 to -0.045 V vs SCE for 15 s from 0.5 M H2SO4 containing 0.1 mM HAuCl4. The resulting electrode surfaces were characterized with both AFM and cyclic voltammetry. Anodic stripping voltammetry of arsenic(III) on the modified electrode was performed. After optimization, a LOD of 0.0096 ppb was obtained with LSV.     

Electrochemical oxidation of histamine and serotonin at highly boron-doped diamond electrodes

The electrochemistry of histamine and serotonin in neutral aqueous media (pH 7.2) was investigated using polycrystalline, boron-doped diamond thin-film electrodes. Cyclic voltammetry, hydrodynamic voltammetry, and flow injection analysis (FIA) with amperometric detection were used to study the oxidation reactions. Comparison experiments were carried out using polished glassy carbon (GC) electrodes. At diamond electrodes, highly reproducible and well-defined cyclic voltammograms were obtained for histamine with a peak potential at 1.40 V vs SCE. The voltammetric signal-to-background ratios obtained at diamond were 1 order of magnitude higher than those obtained for GC electrodes at and above 100 microM analyte concentrations. A linear dynamic range of 3-4 orders of magnitude and a detection limit of 1 microM were observed in the voltammetric measurements. Well-defined sweep rate-dependent voltammograms were also obtained for 5-hydroxytryptamine (5-HT). The characteristics of the voltammogram indicated lack of adsorption of its oxidation products on the surface. No fouling or deactivation of the electrode was observed within the experimental time of several hours. A detection limit of 0.5 microM (signal-to-noise ratio 13.8) for histamine was obtained by use of the FIA technique with a diamond electrode. A remarkably low detection limit (10 nM) was obtained for 5-HT on diamond by the same method. Diamond electrodes exhibited a linear dynamic range from 10 nM to 100 microM for 5-HT determination and a range of 0.5-100 microM for histamine determination. The FIA response was very reproducible from film to film, and the response variability was below 7% at the actual detection limits.     

Simultaneous determination of serotonin and dopamine at the PEDOP/MWCNTs-Pd nanoparticle modified glassy carbon electrode

Electrochemical determination of dopamine (DA) and serotonin (5-HT) have been studied at a modified glassy carbon electrode (GCE) in 0.1 M phosphate buffer solution (PBS) using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) at pH 7.4, all over the interfering biomolecule ascorbic acid (AA). The GCE was modified by palladium-functionalized, multi-walled carbon nanotubes (MWCNTs-Pd) with electrochemical deposition of poly 3,4-ethylenedioxy pyrrole (PEDOP), denoted as PEDOP/MWCNTs-Pd/GCE, and investigated by SEM and EIS experiments. The highly electrocatalytic activity of the modified electrode toward 5-HT and DA was demonstrated from the sensitive and well-separated voltammetric experiment. The oxidation peaks found were 0.165 and 0.355 mV for DA and 5-HT, respectively. The composite film shows a significant accumulation effects on two species, as well as the mutual interference among the analytes. This biosensor was best in response compared to other modified electrodes made in the same lab. The lowest detection limits were found to be 5.0 x 10(-9) and 1.0 x 10(-8) for 5-HT and DA, respectively. The respective linear ranges were determined as 1.0 x 10(-7) to 2.0 x 10(-4) and 1.0 x 10(-7) to 2.0 x 10(-4) for 5-HT and DA.     

Laser activation voltammetry: selective removal of reduced forms of methyl viologen deposited on glassy carbon and boron-doped diamond electrodes

The effect of high-intensity laser pulses on the reduction of methyl viologen at glassy carbon electrodes in aqueous solution is investigated using laser activation voltammetry (LAV) under both channel flow and no-flow conditions and compared with the effect of conventional variable-temperature voltammetry. The reduction proceeds in two consecutive one-electron steps, and the neutral two-electron-reduction product of methyl viologen is shown by voltammetry and in situ optical microscopy to form two types of deposits, amorphous and crystalline, on the electrode surface. Laser activation voltammetry using a 10 Hz pulsed Nd-YAG 532 nm laser is shown to remove the deposits from the electrode surface at different laser intensities: the amorphous material is more easily ablated than the crystalline deposit. By conventional variable-temperature voltammetry, it is shown that the two stripping peaks disappear as the temperature is increased. However, with conventional heating, the opposite ease of removal is detected compared to the case of laser activation voltammetry: the stripping response associated with the crystalline material disappears at lower temperatures compared to that for the amorphous material. In the presence of high-intensity laser pulses (>0.17 W cm(-2)), glassy carbon surfaces are damaged and the voltammetric characteristics become poor. It is shown that, by the employment of a thin-film boron-doped diamond electrode grown using a chemical vapor deposition procedure on a tungsten substrate, much higher laser intensities can be applied and well-defined LAV signals can be obtained without deactivation of the electrode.     

延胡索乙素在碳纳米管修饰玻碳电极上的电化学检测

本文建立了延胡索乙素在碳纳米管修饰玻碳(CNT/GC)电极上的电化学检测方法。在pH6.20的Michaelis.缓冲液中,用方波伏安法研究了延胡索乙素在CNT/GC电极上的电化学行为。延胡索乙素在+0.90v(vs.Ag/AgCl)左右产生一个灵敏的阳极氧化峰,峰电流与延胡索乙素的浓度在8.0×10-7～5.0×10-5mol/L范围内呈良好线性关系,最低检测限达3.0×10-7mol/L。该法简单、快速、灵敏,可应用于生药材和中成药中延胡索乙素的测定。     

Preparation of multiwalled carbon nanotubes-platinum-Nafion-glucose oxidase nanobiocomposite and its application to glucose biosensor

Platinum (Pt) nanoparticles were electrodeposited within multiwalled carbon nanotubes-Nafion-glucose oxidase (MWNTs-Nafion-GOx) nanobiocomposite by a potentiostatic method. The morphology and nature of the resulting MWNTs-Pt-Nafion-GOx nanobiocomposite were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). The electrocatalytic properties of the MWNTs-Pt-Nafion-GOx nanobiocomposite film modified glassy carbon electrode were characterized by cyclic voltammetry and amperometry in the presence of hydrogen peroxide. The glucose biosensor sensitivity was strongly influenced by the deposits of Pt nanoparticles and amount of GOx concentration within the MWNTs-Pt-Nafion-GOx nanobiocomposite film. The optimized glucose biosensor displayed a sensitivity of 640 nA mM(-1), a linear range of up to 4 mM, a detection limit of 4 microM, and a response time of less than 4 s at an operating potential of +500 mV versus Ag/AgCl (3 M KCl).     

Redox and acid-base chemistry of 7,7,8,8-tetracyanoquinodimethane, 7,7,8,8-tetracyanoquinodimethane radical anion, 7,7,8,8-tetracyanoquinodimethane dianion, and dihydro-7,7,8,8-tetracyanoquinodimethane in acetonitrile

The chemistry and electrochemistry of TCNQ (7,7,8,8-tetracyanoquinodimethane), TCNQ(?-), TCNQ(2-), and H(2)TCNQ in acetonitrile (0.1 M Bu(4)NPF(6)) solution containing trifluoroacetic acid (TFA) has been studied by transient and steady-state voltammetric methods with the interrelationship between the redox and the acid-base chemistry being supported by simulations of the cyclic voltammograms. In the absence of acid, TCNQ and its anions undergo two electrochemically and chemically reversible one-electron processes. However, in the presence of TFA, the voltammetry is considerably more complex. The TCNQ(2-) dianion is protonated to form HTCNQ(-), which is oxidized to HTCNQ(?), and H(2)TCNQ which is electroinactive over the potential range of -1.0 to +1.0 V versus Ag/Ag(+). The monoreduced TCNQ(?-) radical anion is weakly protonated to give HTCNQ(?), which disproportionates to TCNQ and H(2)TCNQ. In acetonitrile, H(2)TCNQ deprotonates slowly, whereas in N,N-dimethylformamide or tetrahydrofuran, rapid deprotonation occurs to yield HTCNQ(-) as the major species. H(2)TCNQ is fully deprotonated to the TCNQ(2-) dianion in the presence of an excess concentration of the weak base, CH(3)COOLi. Differences in the redox and acid-base chemistry relative to the fluorinated derivative TCNQF(4) are discussed in terms of structural and electronic factors.     

MWCNTs/Cu(OH)2 nanoparticles/IL nanocomposite modified glassy carbon electrode as a voltammetric sensor for determination of the non-steroidal anti-inflammatory drug diclofenac

This paper describes the development and utilization of a new nanocomposite consisting of Cu(OH)₂ nanoparticles, hydrophobic ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate (EMIMPF₆) and multiwalled carbon nanotubes for glassy carbon electrode modification. The nanocomposite was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) along with energy-dispersive X-ray spectroscopy (EDX). The modified electrode was used for electrochemical characterization of diclofenac. Using differential pulse voltammetry, the prepared sensor showed good sensitivity and selectivity with low overpotential for the determination of diclofenac in the range from 0.18 to 119 μM, with a detection limit of 0.04 μM. Electrochemical studies suggested that the MWCNTs/Cu(OH)₂ nanoparticles/IL nanocomposite modified electrode provided a synergistic augmentation on the voltammetric behavior of electrochemical oxidation of diclofenac, which was indicated by the improvement of anodic peak current.     

Preparation of nano-sized Pb2+ imprinted polymer and its application as the chemical interface of an electrochemical sensor for toxic lead determination in different real samples

In this work, a new nano-structured ion imprinted polymer (IIP) was synthesized by copolymerization of methacrylic acid-Pb(2+) complex and ethylene glycol dimethacrylate according to the precipitation polymerization. Methacrylic acid acted as both functional monomer and complexing agent to create selective coordination sites in a cross-linked polymer. A carbon paste electrode modified with IIP-nanoparticles was used for fabrication of a Pb(2+) sensitive electrode. Differential pulse stripping voltammetry method was applied as the determination technique, after open circuit sorption of Pb(2+) on the electrode and its reduction to metallic form. The IIP modified electrode showed a considerably higher response, compared to the electrode embedded with non-imprinted polymer (NIP). This indicated that the suitable recognition sites were created in the IIP structure in the polymerization stage. Various factors, effective on the response behavior of the electrode, were investigated and optimized. The introduced sensor showed a linear range of 1.0 × 10(-9) to 8.1 × 10(-7)M and detection limit of 6.0 × 10(-10)M (S/N=3). The sensor was successfully applied for the trace lead determination in different samples.