Differential pulse voltammetric determination of N-acetylcysteine by the electrocatalytic oxidation at the surface of carbon nanotube-paste electrode modified with cobalt salophen complexes

The preparation and electrochemical performance of the carbon nanotube-paste electrode modified with salophen complexes of cobalt(III) perchlorate, with various substituents on the salophen ligand, as well as their electrocatalytic activity toward the oxidation of N-acetylcysteine (NAC) is investigated. Several Schiff base complexes containing various nucleophilic and electrophilic functional groups were prepared, and their electrochemical characteristics for the electro-oxidation of NAC were evaluated using cyclic and differential pulse voltammetry (CV and DPV). The results revealed, the modified electrodes show an efficient and selective electrocatalytic activity toward the anodic oxidation of NAC among biologically important compounds in buffered solutions at pH of 7.0. The best voltammetric responses were obtained for a carbon-paste electrode (CPE) modified with a salophen complex containing para-methoxy groups on its salicylaldehyde ring. The analytical response of the modified electrode for response to other sulfhydryl compounds (e.g., cysteine, penicillamine, captopril and mercaptopropionyl glycine) in comparison to NAC was investigated by CV and DPV methods. The DPV method was applied as a sensitive method for the quantitative detection of the trace amounts of NAC. A linear dynamic range from 1 × 10⁻⁷ to 1 × 10⁻⁴ M with calibration sensitivity of 0.0646 μA/μM is resulted for NAC in DPV measurements. The detection limit was 5 × 10⁻⁸ M, which is remarkably lower than those reported previously for NAC using other modified electrodes. The results of voltammetric determinations show a very good reproducibility, and the R.S.D. for the slope of the calibration curve, based on 10 measurements in a period of two months, was <3.9%. The detection system provides very stable electrochemical responses toward NAC, makes it very suitable for using in pharmaceutical and clinical measurements.     

Lead (II) carbon paste electrode based on derivatized multi-walled carbon nanotubes: Application to lead content determination in environmental samples

For the first time a novel derivatized multi-walled carbon nanotubes-based Pb²⁺ carbon paste electrode is reported. The electrode with optimum composition, exhibits an excellent Nernstian response to Pb²⁺ ion ranging from 5.9 × 10⁻¹⁰ to 1.0 × 10⁻² M with a detection limit of 3.2 × 10⁻¹⁰ M and a slope of 29.5 ± 0.3 mV dec⁻¹ over a wide pH range (2.5-6.5) with a fast response time (25 s) at 25 °C. Moreover, it also shows a high selectivity and a long life time (more than 3 months). Importantly, the response mechanism of the proposed electrode was investigated using AC impedance technique. Finally, the electrode was successfully applied for the determination of Pb²⁺ ion concentration in environmental samples, e.g. soils, waste waters, lead accumulator waste and black tea, and for potentiometric titration of sulfate anion.     

Anodic stripping voltammetric determination of copper(II) using a functionalized carbon nanotubes paste electrode modified with crosslinked chitosan

The development and application of a functionalized carbon nanotubes paste electrode (CNPE) modified with crosslinked chitosan for determination of Cu(II) in industrial wastewater, natural water and human urine samples by linear scan anodic stripping voltammetry (LSASV) are described. Different electrodes were constructed using chitosan and chitosan crosslinked with glutaraldehyde (CTS-GA) and epichlorohydrin (CTS-ECH). The best voltammetric response for Cu(II) was obtained with a paste composition of 65% (m/m) of functionalized carbon nanotubes, 15% (m/m) of CTS-ECH, and 20% (m/m) of mineral oil using a solution of 0.05 mol L⁻¹ KNO₃ with pH adjusted to 2.25 with HNO₃, an accumulation potential of −0.3 V vs. Ag/AgCl (3.0 mol L⁻¹ KCl) for 300 s and a scan rate of 100 mV s⁻¹. Under these optimal experimental conditions, the voltammetric response was linearly dependent on the Cu(II) concentration in the range from 7.90 × 10⁻⁸ to 1.60 × 10⁻⁵ mol L⁻¹ with a detection limit of 1.00 × 10⁻⁸ mol L⁻¹. The samples analyses were evaluated using the proposed sensor and a good recovery of Cu(II) was obtained with results in the range from 98.0% to 104%. The analysis of industrial wastewater, natural water and human urine samples obtained using the proposed CNPE modified with CTS-ECH electrode and those obtained using a comparative method are in agreement at the 95% confidence level.     

A selective modified bentonite–porphyrin carbon paste electrode for determination of Mn(II) by using anodic stripping voltammetry

A novel voltammetric sensor based on chemically modified bentonite–porphyrin carbon paste electrode (MBPCE) has been introduced for the determination of trace amount of Mn(II) in wheat flour, wheat rice and vegetables. In this method Mn(II) gives well-defined voltammetric peak at the pH range of 3.5–7.5. For the preliminary screening purpose, the catalyst was prepared by modification of bentonite with porphyrin and characterized by thermogravimetric method (TG) and UV–vis spectroscopy. The detection limit (three times signal-to-noise) with 4 min accumulation is 1.07 × 10⁻⁷ mol L⁻¹ Mn(II). The peak currents increases linearly with Mn(II) concentration over the range of 6.0 × 10⁻⁷ to 5.0 × 10⁻⁴ mol L⁻¹ (r² = 0.9959). Statistical treatment of the results gave a relative standard deviation lower than 2.30%. The chemical and instrumental parameters have been optimized and the results showed that 1000-fold excess of the additive ions had not interferences on the determination of Mn(II).     

A novel microbial biosensor based on Circinella sp. modified carbon paste electrode and its voltammetric application

Microbial biosensors have been developed for voltammetric determination of various substances. This paper describes the development of a new biosorption based microbial biosensor for determination of Cu²⁺. The developed biosensor is based on carbon paste electrode consisting of whole cells of Circinella sp. Cu²⁺ was preconcentrated on the electrode surface at open circuit and then cathodically detected with the reduction of Cu²⁺. The voltammetric responses were evaluated with respect to percentage cell loading in the carbon paste, preconcentration time, pH of preconcentration solution, scan rate and interferences. The optimum response was realized by biosensor constructed using 5 mg of dry cell weight per 100 mg of carbon paste in pH 5.5 preconcentration solution. Under the optimum experimental conditions, the developed microbial biosensor exhibited an excellent current response to Cu²⁺ over a linear range from 5.0 × 10⁻⁷ to 1.0 × 10⁻⁵ M (r² = 0.9938) with a detection limit of 5.4 × 10⁻⁸ M (S/N = 3). The microbial biosensor had good sensitivity and reproducibility (R.S.D. 4.3%, n = 6). Finally, the applicability of the proposed microbial biosensor to voltammetric determination of Cu²⁺ in real sample was also demonstrated and validated with atomic absorption spectrophotometric (AAS) method.     

Selective voltammetric determination of electroactive neuromodulating species in biological samples using iron(II) phthalocyanine modified multi-wall carbon nanotubes paste electrode

Iron(II) phthalocyanine (FePc) modified multi-wall carbon nanotubes paste electrodes (MWCNTPEs) were used as voltammetric sensors to selectively detect dopamine (DA) in the presence of serotonin (5-HT). The electrochemical behavior of DA at the new modified electrode was investigated using CV. The enhanced current response of DA indicates that FePc modification of the MWCNTPE surface results in a high catalytic activity for the redox reaction of DA. Differential pulse voltammetry was applied in detection of DA and 5-HT at FePc-MWCNTPE. The method parameters were optimized. Detection limit of 2.05 × 10⁻⁷ M was obtained for DA by using the electrocatalytic oxidation signal corresponding to the Fe^II/Fe^III redox process. The separation between the peak potentials of DA and 5-HT is 170 mV which is large enough for the simultaneously, selective determination of the two chemical species in their mixtures. There was no electrochemical response for ascorbic acid (AA) added in the sample. The monoamine neurotransmitter measuring method has been tested in analyzing deproteinized serum samples.     

On the complex formation of CdCl2 with 1-furoylthioureas: Preconcentration and voltammetric behavior of Cd(II) at carbon paste electrodes modified with 3-monosubstituted and 3,3-disubstituted derivatives

Two series of 1-furoylthioureas were used as modifiers of carbon paste electrodes (CPE). The preconcentration of Cd(II) at CPE modified with these 3-monosubstituted and 3,3-disubstituted 1-furoylthioureas and its subsequent voltammetric determination was studied with Cd(II) test solution. The preconcentration of cadmium at the surface of the modified CPE varied with the steric and electronic nature of the present CS neighboring groups. This is in accord with the changes observed in the Raman spectra of the ligands after complexation with CdCl₂, and with the chemical yields of the complexes obtained.     

聚L-苯丙氨酸/铁氰根修饰电极的制备及其对Vc的测定

制备了聚L-苯丙氨酸/铁氰根修饰电极,研究了Vc在该修饰电极上的电化学行为,建立了一种测定Vc的新方法.该方法简便准确.检测下限可达2.4x 10-7mol/L,在4.0x 10-7～2.0x 10-2mol/L的浓度范围内,峰电流与Vc的浓度呈良好的线性关系.该电极用于测定Vc片中Vc的含量,结果令人满意.     

Direct amperometric determination of l-ascorbic acid (Vitamin C) at octacyanomolybdate-doped-poly(4-vinylpyridine) modified electrode in fruit juice and pharmaceuticals

The glassy carbon electrode (GCE) modified with Mo(CN)₈⁴⁻-incorporated-poly(4-vinylpyridine) (PVP/Mo(CN)₈⁴⁻), which has been recently shown to possess several attractive attributes as an efficient electrocatalytic electrode for l-ascorbic acid oxidation and its estimation, is used for l-ascorbic acid estimation directly in orange fruit juice and Celin tablet in a 0.1 M H₂SO₄ acid solution without any special treatment. Constant potential amperometry at 570 mV (saturated calomel electrode, SCE) in stirred solutions is used for this purpose. A good correlation is attained with the official titrametric method. To understand the possible electrocatalytic reaction mechanism for the electro-oxidation of l-ascorbic acid, calibration graphs over the range 1 × 10⁻⁵ to 1 × 10⁻² mol dm⁻³ l-ascorbic acid are compared for the three electrodes, ca. PVP/Mo(CN)₈⁴⁻, undoped PVP, and GCE; the curvature at high ascorbic acid concentration for the PVP/Mo(CN)₈⁴⁻ electrode is explained in terms of Michaelis–Menten (MM) saturation kinetics. The apparent MM constant (K_M), the maximum catalytic current (i_M), the complex decomposition rate constant (k_c), and the heterogeneous modified electrode rate constant (k^′_ME) are calculated from three different approaches. A reasonably high value of ≈1 × 10⁻² cm s⁻¹ is obtained for k^′_ME, indicating efficient l-ascorbic acid mediation at the PVP/Mo(CN)₈⁴⁻ electrode, thus accounting for quite a high sensitivity of this modified film electrode compared to several other modified electrodes.