Catalytic Effect on Silver Electrodeposition of Gold Deposited on Carbon Electrodes

A new methodology, based on silver electrocatalytic deposition and designed to quantify gold deposited onto carbon paste electrode (CPE) and glassy carbon electrode (GCE), has been developed in this work. Silver (prepared in 1.0 M NH₃) electrodeposition at ?0.13 V occurs only when gold is previously deposited at an adequate potential on the electrode surface for a fixed period of time. When a CPE is used as working electrode, an adequate oxidation of gold is necessary. This oxidation is carried out in both 0.1 M NaOH and 0.1 M H₂SO₄ at oxidation potentials. When a GCE is used as working electrode, the oxidation steps are not necessary. Moreover, a cleaning step in KCN, which removes gold from electrode surface, is included. To obtain reproducibility in the analytical signal, the surface of the electrodes must be suitably pretreated; this electrodic pretreatment depends on the kind of electrode used as working electrode. Low detection limits (5.0×10^?10 M) for short gold deposition times (10 min for CPE and 5 min for GCE) were achieved with this novel methodology. Finally, sodium aurothiomalate can be quantified using silver electrocatalytic deposition and GCE as working electrode. Good linear relationship between silver anodic stripping peak and aurothiomalate concentration was found from 5.0×10^?10 M to 1.0×10^?8 M.     

Simultaneous Determination of Hydroquinone and Catechol at a Glassy Carbon Electrode Modified with Multiwall Carbon Nanotubes

A simply and high selectively electrochemical method for simultaneous determination of hydroquinone and catechol has been developed at a glassy carbon electrode modified with multiwall carbon nanotubes (MWNT). It was found that the oxidation peak separation of hydroquinone and catechol and the oxidation currents of hydroquinone and catechol greatly increase at MWNT modified electrode in 0.20 M acetate buffer solution (pH 4.5). The oxidation peaks of hydroquinone and catechol merge into a large peak of 302 mV (vs. Ag/AgCl, 3 M NaCl) at bare glassy carbon electrode. The two corresponding well‐defined oxidation peaks of hydroquinone in the presence of catechol at MWNT modified electrode occur at 264 mV and 162 mV, respectively. Under the optimized condition, the oxidation peak current of hydroquinone is linear over a range from 1.0×10^?6 M to 1.0×10^?4 M hydroquinone in the presence of 1.0×10^?4 M catechol with the detection limit of 7.5×10^?7 M and the oxidation peak current of catechol is linear over a range from 6.0×10^?7 M to 1.0×10^?4 M catechol in the presence of 1.0×10^?4 M hydroquinone with the detection limit of 2.0×10^?7 M. The proposed method has been applied to simultaneous determination of hydroquinone and catechol in a water sample with simplicity and high selectivity.     

Electrochemical Analysis of D‐Penicillamine Using a Carbon Paste Electrode Modified with Ferrocene Carboxylic Acid

The electrochemical behavior of D ‐penicillamine (D ‐PA) studied at the surface of ferrocene carboxylic acid modified carbon paste electrode (FCAMCPE) in aqueous media using cyclic voltammetry and double step potential chronoamperometry. It has been found that under optimum condition (pH 7.00), the oxidation of D ‐PA at surface of such an electrode is occurred about 420 mV less positive than that an unmodified carbon paste electrode (CPE). The catalytic oxidation peak current was linearly dependent on the D ‐PA concentration and a linear calibration curve was obtained in the ranges 7.5×10^?5 M – 1.0×10^?3 M and 6.5×10^?6 M?1.0×10^?4 M of D ‐PA with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods respectively. The detection limits (3σ) were determined as 6.04×10^?5 M and 6.15×10^?6 M. This method was also used for the determination of D ‐PA in pharmaceutical preparation (capsules) by standard addition method.     

Acetylferrocene Modified Carbon Paste Electrode; A Sensor for Electrocatalytic Determination of Hydrazine

The electrocatalytic oxidation of hydrazine at a carbon paste electrode spiked with acetylferrocene as a mediator was studied by cyclic voltammetry, differential pulse voltammetry, and chronoamperometry. In contrast to other ferrocenic compounds, acetylferrocene exhibits a chemical irreversible behavior, but it can act as an effective mediator for electrocatalytic oxidation of hydrazine, too. The heterogeneous electron transfer rate constant between acetylferrocene and the electrode substrate (carbon paste) and the diffusion coefficient of spiked acetylferrocene in silicon oil were estimated to be about 3.45×10^?4 cm s^?1 and 4.45×10^?9 cm² s^?1, respectively. It has been found that under the optimum conditions (pH 7.5) the oxidation of hydrazine occurs at a potential of about 228 mV less positive than that of an unmodified carbon paste electrode. The catalytic oxidation peak current of hydrazine was linearly dependent on its concentration and the obtained linear range was 3.09×10^?5 M–1.03×10^?3 M. The detection limit (2σ) has been determined as 2.7×10^?5 M by cyclic voltammetry. Also, the peak current was increased linearly with the concentration of hydrazine in the range of 1×10^?5 M–1×10^?3 M by differential pulse voltammetry with a detection limit of 1×10^?5 M. This catalytic oxidation of hydrazine has been applied as a selective, simple, and precise new method for the determination of hydrazine in water samples.     

Cu(II) Hexacyanoferrate(III) Modified Carbon Paste Electrode; Application for Electrocatalytic Detection of Nitrite

Copper hexacyanoferrate (CuHCF) film‐modified carbon paste electrode (CPE) has been prepared from various electrolytic aqueous solutions using consecutive cyclic voltammetry. The cyclic voltammograms showed the direct deposition of CuHCF films from the mixing of Cu²⁺ and Fe(CN)₆^3? ions and each time with one of the six cations: H⁺, Na⁺, K⁺, NH₄⁺, Mg²⁺, and Al³⁺. The CuHCF film showed a single redox couple that exhibited a cation effect (Na⁺, K⁺, Mg²⁺, and NH₄⁺) and anion effect (Cl^?, NO₃^?, SO₄^2?, ClO₄^?, and BrO₃^?) in the cyclic voltammograms. Voltammetric studies have indicated that in presence of nitrite, the cathodic peak current of CuHCF increases, followed by a decrease in the corresponding anodic current. This indicated that nitrite was reduced by the redox mediator immobilized on the electrode surface via an electrocatalytic mechanism. The process of reduction and its kinetics were investigated by using cyclic voltammetry, differential pulse voltammetry, chronoamperometry and chronocoulometry techniques. The electrocatalytic ability about 800 mV can be seen. The rate constant of the catalytic reduction of nitrite was found to be 7.9×10⁵ cm³ mol^?1 s^?1. Linearity range obtained was 5×10^?5?8.4×10^?3 by cyclic voltammetry and 8×10^?6?1.3×10^?3 and 4×10^?3?2×10^?2 by differential pulse voltammetry.     

Electrocatalytic Oxidation and Determination of Cysteamine by Poly‐N,N‐dimethylaniline/Ferrocyanide Film Modified Carbon Paste Electrode

Functionalized poly‐N,N‐dimethylaniline film was prepared by adsorption of ferrocyanide onto the polymer forming at the surface of carbon paste electrode (CPE) in aqueous solution by using potentiostatic method. The electrocatalytic ability of poly‐N,N‐dimethylaniline/ferrocyanide film modified carbon paste electrode (PDMA/FMCPE) was demonstrated by oxidation of cysteamine. Cyclic voltammetry and chronoamperometry techniques were used to investigate this ability. Results showed that pH 7.00 is the most suitable for this purpose. It is found that the catalytic reaction rate constant, (k_h), is equal to 2.142×10³ M^?1 s^?1 by the data of chronoamperometry. The catalytic reduction peak current was linearly dependent on the cysteamine concentration and the linearity range obtained was 8.00×10^?5 M–1.14×10^?2 M. Detection limit was determined 7.97×10^?5 M (2σ). This method has been successfully employed for quantification of cysteamine in real sample.     

Selective determination of cysteine in the presence of tryptophan by carbon paste electrode modified with quinizarine

A sensitive and selective electrochemical method for the determination of L-cysteine was developed using a modified carbon paste electrode (MCPE) with quinizarine. Cyclic voltammetry was used to investigate the redox properties of this modified electrode at various solution pH values and at various scan rates. The apparent charge transfer rate constant, ks and transfer coefficient for electron transfer between quinizarine and carbon paste electrode (CPE) were calculated as 2.76 s^?1 and 0.6, respectively. This modified carbon paste electrode shows excellent electrocatalytic activity toward the oxidation of L-cysteine in a phosphate buffer solution (pH 7.0). The linear range of 1.0 × 10^?6 to 1.0 × 10^?3 M and a detection limit (3s) of 2.2 × 10^?7 M were observed in pH 7.0 phosphate buffer solutions. In differential pulse voltammetry, the quinizarine modified carbon paste electrode (QMCPE) could separate the oxidation peak potentials of L-cysteine and tryptophan present in the same solution, though at the unmodified CPE the peak potentials were indistinguishable. This work introduces a simple and easy approach to selective detection of L-cysteine in the presence of tryptophan. Also, the modified electrode was employed for the determination of L-cysteine in the real samples such as serum of blood and acetylcysteine tablet.     

A novel and simple biosensor based on poly(indoleacetic acid) film and its application for simultaneous electrochemical determination of dopamine and epinephrine in the presence of ascorbic acid

A novel and simple biosensor based on poly(indoleacetic acid) film-modified electrode (PIAA/CPE) was fabricated by electrochemical polymerization of indoleacetic acid on a carbon paste electrode (CPE) through cyclic voltammetry. The resulting electrode was characterized by scanning electron microscopy, and the electrochemical behaviors of dopamine (DA) and epinephrine (EP) at the electrode were studied. It was illustrated that PIAA/CPE had excellent electrochemical catalytic activities toward DA and EP. The anodic peak currents (I _pa) were dramatically enhanced by about seven-fold for DA and ten times for EP at PIAA/CPE. Thus, the determinations of DA and EP were carried out using PIAA/CPE successfully. The linear responses were obtained in the range of 3.0?×?10^?7～7.0?×?10^?4 and 1.0?×?10^?6 ～8.0?×?10^?4 mol L^?1 with the detection limits (3σ) of 1?×?10^?7 and 4?×?10^?7 mol L^?1 corresponding with DA and EP, respectively. Moreover, the cathodic peaks of DA and EP were well-separated with a potential difference about 325 mV in pH 5.3 phosphate-buffered saline, so simultaneous determination of DA and EP was carried out in this paper. Additionally, the interference studies showed that the PIAA/CPE exhibited excellent selectivity in the presence of ascorbic acid (AA). With good selectivity and sensitivity, the present method has been successfully applied to the determination of DA and EP in pharmaceutical samples.     

Voltammetric determination of droxidopa in the presence of carbidopa using a nanostructured base electrochemical sensor

A novel carbon paste electrode modified with ZnO nanorods and 2-(4-oxo-3-phenyl-3,4-dihydroquinazolinyl)-N′-phenyl-hydrazinecarbothioamide (2PHCZNCPE) was fabricated and employed to study the electrocatalytic oxidation of droxidopa, using cyclic voltammetry, chronoamperometry and square wave voltammetry as diagnostic techniques. It has been found that the oxidation of droxidopa at the surface of modified electrode occurs at a potential of about 435 mV less positive than that of an unmodified carbon paste electrode. Square wave voltammetry exhibits a linear dynamic range from 7.0 × 10^–8 to 3.0×10^?4 M and a detection limit of 45.0 nM for droxidopa. Finally this modified electrode was used for simultaneous determination of droxidopa and carbidopa.     

Wide linear range nanomaterial/ionophore-based electrode used for determination of lead in environmental and biological samples with differential pulse voltammetry

A new chemically modified carbon paste electrode is fabricated to determine lead ion concentration in its trace level in aqueous media with differential pulse voltammetry (DPV). The best performance is obtained by the carbon paste electrode composition including 20% of dithiodibezoic acid (DDA), 80% of high purity graphite powder and 60?µL of colloidal gold nanoparticle (AuNP) solution. The proposed electrode has a wide linear calibration response from 1?×?10^?9 to 6?×?10^?5 M with a detection limit of 6.6?×?10^?10?M, at pH 3.5. Seven replicate determination of 5?×?10^?8?M of lead ion concentration gives a relative standard deviation of 3.33%. The modified sensor is applied to determine lead contents in some environmental and biological Samples with satisfactory results.     

Nanostructured Base Electrochemical Sensor for Simultaneous Quantification and Voltammetric Studies of Levodopa and Carbidopa in Pharmaceutical Products and Biological Samples

A novel carbon paste electrode modified with carbon nanotubes and 5‐amino‐2′‐ethyl‐biphenyl‐2‐ol (5AEB) was fabricated. The electrochemical study of the modified electrode, as well as its efficiency for electrocatalytic oxidation of levodopa (LD) and carbidopa (CD), is described. Cyclic voltammetry (CV) was used to investigate the redox properties of this modified electrode at various scan rates. The apparent charge transfer rate constant, k_s, and transfer coefficient, a, for electron transfer between 5AEB and CPE were calculated as 17.3 s^?1 and 0.5, respectively. Square wave voltammetry (SWV) exhibits a linear dynamic range from 2.5×10^?7 to 2.0×10^?4 M and a detection limit of 9.0×10^?8 M for LD.     

Electrocatalytic Characteristics of Ferrocenecarboxylic Acid Modified Carbon Paste Electrode in the Oxidation and Determination of L‐Cysteine

The electrochemical behavior of L ‐cysteine studied at the surface of ferrocenecarboxylic acid modified carbon paste electrode (FCMCPE) in aqueous media using cyclic voltammetry and double step potential chronoamperometry. It has been found that under optimum condition (pH 7.00) in cyclic voltammetry, the oxidation of L ‐cysteine is occurs at a potential about 580 mV less positive than that an unmodified carbon paste electrode. The kinetic parameters such as electron transfer coefficient, α and catalytic reaction rate constant, K′_h were also determined using electrochemical approaches. The electrocatalytic oxidation peak current of L ‐cysteine showed a linear dependent on the L ‐cysteine concentration and linear calibration curves were obtained in the ranges of 10^?5 M–10^?3 M and 4.1×10^?8 M–3.7×10^?5 M of L ‐cysteine concentration with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods respectively. The detection limits (2δ) were determined as 2.4×10^?6 M and 2.5×10^?8 M by CV and DPV methods. This method was also examined for determination of L ‐cysteine in some samples, such as Soya protein powder, serum of human blood by using recovery and standard addition methods.     

Poly(ortho‐toluidine) Modified Carbon Paste Electrode: A Sensor for Electrocatalytic Reduction of Nitrite

The electrocatalytic reduction of nitrite has been studied by poly(ortho‐toluidine) films modified carbon paste electrode (P‐OT/MCPE). Cyclic voltammetry and chronoamperometry techniques were used to investigate the suitability of poly(ortho‐toluidine) as a mediator for the electrocatalytic nitrite reduction in aqueous solution with various pH. Results showed that pH 0.00 is the most suitable for this purpose. In the optimum pH, the reduction of nitrite occurs at a potential about 600 mV more positive than unmodified carbon paste electrode. The catalytic reaction rate constant, (k_h), was calculated 8.68×10² M^?1 s^?1 by the data of chronoamperometry. The catalytic reduction peak current was linearly dependent on the nitrite concentration and the linearity range obtained was 5.00×10^?4 M–1.90×10^?2 M. Detection limit has been found to be 3.38×10^?4 M (2σ). This method has been successfully employed for quantification of nitrite in real sample.     

Electrocatalytic Determination of Ascorbic Acid at the Surface of 2,7‐Bis(ferrocenyl ethyl)fluoren‐9‐one Modified Carbon Paste Electrode

A chemically modified carbon paste electrode with 2,7‐bis(ferrocenyl ethyl)fluoren‐9‐one (2,7‐BFEFMCPE) was employed to study the electrocatalytic oxidation of ascorbic acid in aqueous solution using cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The diffusion coefficient (D=1.89×10^?5 cm² s^?1), and the kinetic parameter such as the electron transfer coefficient, α (=0.42) of ascorbic acid oxidation at the surface of 2,7‐BFEFMCPE was determined using electrochemical approaches. It has been found that under an optimum condition (pH 7.00), the oxidation of ascorbic acid at the surface of such an electrode occurs at a potential about 300 mV less positive than that of an unmodified carbon paste electrode. The catalytic oxidation peak currents show a linear dependence on the ascorbic acid concentration and linear analytical curves were obtained in the ranges of 8.0×10^?5 M–2.0×10^?3 M and 3.1×10^?5 M–3.3×10^?3 M of ascorbic acid with correlation coefficients of 0.9980 and 0.9976 in cyclic voltammetry and differential pulse voltammetry, respectively. The detection limits (2δ) were determined to be 2.9×10^?5 M and 9.0×10^?6 M with cyclic voltammetry and differential pulse voltammetry, respectively. This method was also examined for determination of ascorbic acid in pharmaceutical preparations.     

Determination of Ascorbic Acid in the Presence of Uric Acid and Folic Acid by a Nanostructured Electrochemical Sensor Based on a TiO2 Nanoparticle Carbon Paste Electrode

A carbon paste electrode (CPE), modified with novel hydroquinone/TiO₂ nanoparticles, was designed and used for simultaneous determination of ascorbic acid (AA), uric acid (UA) and folic acid (FA). The magnitude of the peak current for modified TiO₂-nanoparticle CPE (MTNCPE) increased sharply in the presence of ascorbic acid and was proportional to its concentration. A dynamic range of 1.0–1400.0 μM, with the detection limit of 6.4 × 10^?7 M for AA, was obtained using the DPV technique (pH = 7.0). The prepared electrode was successfully applied for the determination of AA, UA, and FA in real samples.     

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%.     

Stripping voltammetry method for determination of manganese as complex with oxine at the carbon paste electrode with and without modification with montmorillonite clay

Oxine (8-hydroxyquinoline) was used as an efficient and selective ligand for stripping voltammetry trace determination of Mn(II). A validated square-wave adsorptive cathodic stripping voltammetry method has been developed for determination of Mn(II) selectively as oxine complex using both the bare carbon paste electrode (CPE) and the modified CPE with 7 % (w/w) montmorillonite-Na clay. Modification of carbon paste with montmorillonite clay was found to greatly enhance its adsorption capacity. Limits of detection of 45 ng l^?1 (8.19?×?10^?10 mol L^?1) and 1.8 ng l^?1 (3.28?×?10^?11 mol L^?1) Mn(II) were achieved using the bare and modified CP electrodes, respectively. The achieved limits of detection of Mn(II) as oxine complex using the modified CPE are much sensitive than the detection limits obtained by most of the reported electrochemical methods. The developed stripping voltammetry method using both electrodes was successfully applied for trace determination of Mn(II) in various water samples without interferences from various organic and inorganic species.     

Electrocatalytic Oxidation and Voltammetric Determination of L‐Cysteic Acid at the Surface of p‐Bromanil Modified Carbon Paste Electrode

The electrochemical properties of L ‐cysteic acid studied at the surface of p‐bromanil (tetrabromo‐p‐benzoquinone) modified carbon paste electrode (BMCPE) in aqueous media by cyclic voltammetry (CV) and double step potential chronoamperometry. It has been found that under optimum condition (pH 7.00) in cyclic voltammetry, the oxidation of L ‐cysteic acid at the surface of BMCPE occurs at a half‐wave potential of p‐bromanil redox system (e.g., 100 mV vs. Ag|AgCl|KCl_sat), whereas, L ‐cysteic acid was electroinactive in the testing potential ranges at the surface of bare carbon paste electrode. The apparent diffusion coefficient of spiked p‐bromanil in paraffin oil was also determined by using the Cottrell equation. The electrocatalytic oxidation peak current of L ‐cysteic acid exhibits a linear dependency to its concentration in the ranges of 8.00×10^?6 M–6.00×10^?3 M and 5.2×10^?7 M–1.0×10^?5 M using CV and differential pulse voltammetry (DPV) methods, respectively. The detection limits (2σ) were determined as 5.00×10^?6 M and 4.00×10^?7 M by CV and DPV methods. This method was used as a new, selective, rapid, simple, precise and suitable voltammetric method for determination of L ‐cysteic acid in serum of patient's blood with migraine disease.     

Application of mesoporous SiO2-Modified carbon paste electrode for voltammetric determination of epinephrine

Mesoporous SiO₂ of SBA-15 is reported to modify carbon paste electrodes for detecting epinephrine (EP). Carbon paste electrodes modified with synthesized SBA-15 show high sensitivity for voltammetric determination of EP, which is attributed to the strong adsorption ability of SBA-15 to EP and large surface area of the working electrode resulted from SBA-15 modification. The effects of pH value, amount of SBA-15 and scan rate were investigated. Under optimum conditions, the anodic peak current of EP was proportional to its concentration over the range from 1.0 × 10^?6 to 6.0 × 10^?5 mol L^?1 and the limit of detection was 6.0 × 10^?7 mol L^?1. The results show mesoporous SiO₂-modified carbon paste electrodes, particularly SBA-15-modificd electrodes, create new opportunities for sensitive, simple and suitable method in the electrochemical analysis of EP.     

Nano Au/TiO2 hollow microsphere membranes for the improved sensitivity of detecting specific DNA sequences related to transgenes in transgenic plants

Gold nanoparticles (nano Au)/titanium dioxide (TiO₂) hollow microsphere membranes were prepared on the carbon paste electrode (CPE) for enhancing the sensitivity of DNA hybridization detection. The immobilization of nano Au and TiO₂ microsphere was investigated with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The hybridization events were monitored with EIS using [Fe(CN)₆]^3?/4? as indicator. The sequence-specific DNA of the 35S promoter from cauliflower mosaic virus (CaMV35S) gene was detected with this DNA electrochemical sensor. The dynamic detection range was from 1.0×10^?12 to 1.0×10^?8 mol/L DNA and a detection limit of 2.3×10^?13 mol/L could be obtained. The polymerase chain reaction (PCR) amplification of the terminator of nopaline synthase (NOS) gene from the real sample of a kind of transgenic soybean was also satisfactorily detected.