Simultaneous and Direct Determination of Tryptophan and Tyrosine at Boron‐Doped Diamond Electrode

A facile method for the simultaneous measurement of tryptophan (Trp) and tyrosine (Tyr) was firstly exploited at unmodified boron‐doped diamond (BDD) electrode. The experimental results indicated that by using differential pulse voltammetry, the oxidative peaks of these two kinds of amino acids could be completely separated at BDD electrode. The peak separation of Trp and Tyr was developed to be 0.64 V when Na₂PO₄/NaOH buffer solution with the optimized pH 11.2 was employed. The detection limit of Trp was obtained to be 1×10^?5 M, while that of Tyr was achieved to be 1×10^?6 M. The present method was also evidenced to be available to the determination of real samples of amino acids.     

Selenomethionine‐Catalyzed Nickel Ion Reduction at a Mercury Electrode: Applications in the Analysis of Nutritional Supplements

Selenomethionine (SeMet) is a catalyst for Ni²⁺ reduction at a mercury electrode in a borax buffer at pH around 9 and gives rise to a differential pulse voltammetric peak, A, at ?0.74 V vs. the Ag|AgCl, (3 M KCl) reference electrode. Peak current is directly proportional to SeMet concentration over the concentration range 0.4–10 μM. Alkali and alkali‐earth ions depress to some extent the sensitivity but the current‐concentration relationships remains linear even under these conditions. Differential pulse cathodic stripping voltammetry (DPCSV) in 0.01 M borax results in two partially overlapped peaks. The more negative (A, at about ?0.74 V) is similar to that recorded with no deposition and is due to the catalysis by nonadsorbed SeMet, whereas the more positive one (B, at about ?0.60 V) results from the catalysis by adsorbed SeMet. Only the DPCSV peak A appears if 0.1 M KNO₃ is also present along with 0.01 M borax. Stearic acid, which is present in nutritional supplement tablets, improves the separation of the DPCSV peaks. Consequently, the peak B recorded with 0.01 M borax buffer allows determining SeMet in nutritional supplement tablets by the standard addition method and enables discriminating between the organic and inorganic selenium forms.     

Minicomputer-controlled,background-subtracted anodic stripping voltammetry: evaluation of parameters and performance

A new minicomputer-controlled anodic stripping voltammetry technique is described. The technique uses differential voltammetry at one electrode, and uses a rapid data-averaging algorithm to avoid the need for scan averaging; this allows excellent sensitivities and short analysis times, with adequate reproducibilities. Both linear-scan and staircase waveforms are discussed in conjunction with the technique, and the approximate Roe—Toni theory for linear-scan voltammetry is shown to apply. The system response is investigated for film thickness, scan rate, deposition time, electrode rotation rate, and metal concentration. Results were used to optimize the sensitivity of the technique. The metal ions Cd, Pb and Cu could be determined in solutions as dilute as 1–10 pg/ml^-1 in 15 min. The viability of the technique in solutions showing a large background is also discussed, and a comparison is made with scan-averaged techniques.     

Determination of molybdenum in the presence of 2-(2-benzothiazolylazo)-p-cresol by catalytic-adsorptive stripping voltammetry

The adsorptive collection of the molybdenum (VI) complexed with 2-(2-benzothiazolylazo)-p-cresol (BTAC) coupled with the catalytic current of the adsorbed complex at a static mercury drop electrode yields an ultrasensitive voltammetric procedure for the determination of molybdenum. Optimal experimental conditions were: a stirred acetate buffer ¶0.2 M (pH 3.5) as supporting electrolyte, a BTAC concentration of 1.0 × 10^–6 M as ligand, and a concentration of 0.1 M potassium nitrate as the oxidizing agent. In addition, a preconcentration potential of –0.080 V vs Ag/AgCl (3 M KCl), equilibration time of 15 s, a frequency of 30 Hz, a scan increment of 2 mV, a pulse amplitude of 0.050 mV, and a drop area of 0.032 cm² were used. The cyclic voltammogram was recorded using a staircase wave with a scan rate of 100 mV/s. The forward scan starts at the initial potential of –0.080 V and is reversed at –0.90 V. Using the catalytic current at ～–0.55 V the response to the Mo(VI) was found to be linear over a concentration range of 1.0–10.0 μg/L. The limit of detection is as low as 6.2 × 10^–10 M with 4 min of preconcentration time. The possible interference of other trace ions was investigated. The merits of this procedure are demonstrated using of reference samples.     

High Sensitive Sensor Based on Carbon Nanotube Electrode for Determination of Lanthanum in the Presence of Calcon Carboxylic Acid

In this paper we have investigated the electrochemical activity of lanthanum chloride (La (III)) in the presence of calcon carboxylic acid (CCA) using a multi-walled carbon nano tube/carbon paste electrode (CNT/CPE). The peak current increases linearly with increasing of the La (III) concentration. For this purpose, a few electrochemical methods such as cyclic, differential pulse voltammetry, linear sweep and hydrodynamic voltammetry, and chronoamperometry were used. The results show that calcon carboxylic acid as a ligand was useful for determination of La (III) and was able to improve its sensitivity. Cyclic voltammetry was used for study of reduction reaction of La (III) at the surface of modified electrode. The electrochemical parameters for La (III) at the surface of CNT/CPE, such as diffusion coefficient (D/ cm² s ^?1 = 5.26 × 10^?6), the electron transfer coefficient, (α = 0. 43), and the reduction rate constant, (k/ M s^?1 = 2.33 (±0.015) × 10²), were determined using voltammetry methods, which with the detection limit of La (III) by differential pulse voltammetry was found to be 1.3 nM. The combination of CCA with CNT as mediators in carbon paste electrode showed that this electrode is capable, sensitive, and simple to quantify La (III) in real samples with an average recovery of 97.64%.     

An electroanalytical study of the anticancer drug dacarbazine

The electrochemical behaviour of dacarbazine [5-(3,3-dimethyl-1-triazenyl) imidazole-4-carboxamide; DTIC] was investigated by Tast and differential pulse polarography (d.p.p.) at the dropping mercury electrode, by cyclic and differential pulse voltammetry at the hanging mercury drop electrode and by anodic voltammetry at the glassy carbon electrode. Calibration graphs were obtained for 2×10^?8?2×10^?5 M DTIC by d.p.p., for 5×10^?9?1×10^?5 M by adsorptive stripping voltammetry ar a hanging mercury drop electrode, and for 1?10×10^?5 M by high-performance liquid chromatography with oxidative amperometric detection at a glassy carbon electrode. The methods are compared and applied to determine DTIC added to blood serum after a simple clean-up procedure.     

Determination of Esomeprazole on an Electropolymerized L‐arginine and β‐cyclodextrin Modified Screen Printed Carbon Electrode

The formation of an inclusion complex of the proton‐pump inhibitor (PPI) drug esomeprazole (ESO) with ß‐cyclodextrin (ß‐CD) has been investigated and proven by cyclic voltammetry (CV). The formation constant of the complex was determined. Thereafter, an electropolymerized β‐CD and L‐arginine (L‐arg) modified screen printed carbon electrode (P‐β‐CD‐L‐arg/SPCE) was developed for the determination of ESO using differential pulse adsorptive stripping voltammetry (DPAdSV). A significant enhancement of the peak current was observed when applying an accumulation step due to the effect of adsorption. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) further indicated that the polymer of β‐CD and L‐arg efficiently improved the electron transfer kinetic between analyte and electrode surface. Under the optimized conditions, the oxidation peak current was linearly proportional to the concentration of the drug in the range of 1.0×10^?8 to 1.0×10^?5 M. The DPAdSV method was successfully used to determine the concentrations of the drug in spiked human serum samples.     

L‐Cysteine Voltammetry at a Carbon Paste Electrode Bulk‐Modified with Ferrocenedicarboxylic Acid

The electrochemical behavior of L ‐cysteine studied at the surface of ferrocenedicarboxylic acid modified carbon paste electrode (FDCMCPE) in aqueous media using cyclic voltammetry, differential pulse voltammetry and double potential step chronoamperometry. It has been found that under optimum condition (pH 8.00) in cyclic voltammetry, the oxidation of L ‐cysteine occurs at a potential about 200 mV less positive than that of 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 analytical curves were obtained in the ranges of 3.0×10^?5 M–2.2×10^?3 M and 1.5×10^?5 M–3.2×10^?3 M of L ‐cysteine concentration with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods respectively. The detection limits (3σ) were determined as 2.6×10^?5 M and 1.4×10^?6 M by CV and DPV methods.     

Direct electrochemical behavior of cytochrome c, and its determination on phytic acid modified electrode

Phytic acid (PA) with its unique structure was attached to a glassy carbon electrode (GCE) to form PA/GCE modified electrode which was characterized by electrochemical impedance. The electrochemical behavior of cytochrome c (Cyt c) on the PA/GCE modified electrode was explored by cyclic voltammetry and differential pulse voltammetry. The Cyt c displayed a quasi-reversible redox process on PA modified electrode pH 7.0 phosphate buffer solution with a formal potential (E ^0′) of 57 mV (versus Ag/AgCl). The peak currents were linearly related to the square root of the scan rate in the range of 20–120 mV·s^?1. The electron transfer rate constant was determined to be 12.5 s^?1. The PA/GCE modified electrode was applied to the determination of Cyt c, in the range of 5?×?10^?6 to 3?×?10^?4 M, the currents increase linearly to the Cyt c concentration with a correlation coefficient 0.9981. The detection limit was 1?×?10^?6 M (signal/noise?=?3).     

Electrocatalytic Response of Dopamine at an Iron Nanoparticles–Nafion–Modified Carbon Paste Electrode

Abstract

Iron nanoparticles (INPs) were dispersed in Nafion solution to obtain a homogeneous INP-Nafion dispersion, and then a drop of this dispersion was cast on the surface of a carbon paste electrode (CPE) to fabricate an INP-Nafion-modified electrode. The electrochemical behavior of dopamine (DA) at this modified electrode was studied by cyclic voltammetry in a pH 7.0 Britton-Robinson (B-R) buffer solution. The result showed that the modified CPE exhibited an obvious electrocatalytical response toward DA, with the anodic and cathodic peak potentials shifted negatively and positively respectively, and great enhance of the peak currents at the scan rate of 100 mV s^?1. The effects of carbon paste constitution, amount of the dispersion, pH, and scan rate were investigated. Under the optimum experimental conditions, the peak currents determined by differential pulse voltammetry showed an excellent linear relationship with DA concentration in the range from 10 to 110 µM with the detection limit of 3.3 µM. In addition, ascorbic acid and some other possible interferents did not interfere with the voltammetric sensing of DA, and this method also had good stability and reproducibility.     

Design of a New Carbon Paste Electrode Modified with TiO2 Nanoparticles to Use in an Electrochemical Study of Codeine and Simultaneous Determination of Codeine and Acetaminophen in Human Plasma Serum Samples

A new electrochemical sensor was fabricated via TiO₂ nanoparticles onto a carbon paste electrode. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) studied the response of the modified electrode toward codeine. The effects of pH, modifier amount, pulse amplitude, and scan rate of potential have been examined. Using DPV, we could measure simultaneously codeine and acetaminophen in one mixture. The detection limits of 0.018 and 0.050 µmol L^?1 were achieved for codeine and acetaminophen, respectively. The electrooxidation pathway, transfer coefficient, and standard rate constant, are estimated. The proposed voltammetric sensor was successfully applied to determination of codeine and acetaminophen in human plasma serum samples.     

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.     

Some distinguishing tests for α-amino acids and a specific spot test for lysine

Tests are described for differentiation between neutral, basic, and acidic amino acids based on the formation of nickel dimethylglyoxime precipitate from an equilibrium mixture containing Ni²⁺, dimethylglyoxime and Ni²⁺, dimethylglyoxime-glycine. A highly sensitive and specific test for lysine is described, based on the formation of nickel dimethylglyoxime precipitate in a drop of the equilibrium mixture, by the reaction of the volatile amino aldehyde, produced by the oxidative deamination-decarboxylation of lysine. Other basic amino acids, i.e., arginine and histidine do not interfere, and the test can be applied for lysine in a mixture with other amino acids.     

Electrochemical Reduction of Nickel(II)—Dihydroxyanthraquinone Sulfonate: How Complex Formation Affects Biochemical Behavior?

Electrochemical behavior of the Ni^II complex of sodium 1,4-dihydroxy-9,10-anthraquinone-2-sulfonate Na₂[Ni^II(LHSO₃)₂Cl₂] · 2H₂O was studied by cyclic voltammetry in non-aqueous and aqueous buffer. The aim of the present work was to understand the behavior of the semiquinone radical anion formed after reduction of the free quinone centre of a ligand for which the other quinone centre binds a metal ion (Ni^II), not having a stable low oxidation state. In dimethylformamide solution an additional reduction peak appeared at a much more negative potential due to adsorption. The complex Na₂[Ni^II(LHSO₃)₂Cl₂] · 2H₂O undergoes diffusion controlled one-electron reduction unlike the two-electron reduction observed for most hydroxy-9,10-anthraquinones inspite of having two quinone centres in the complex (one on each ligand). In aqueous media a kinetic effect was observed during electron transfer at the electrode surface. The values of the electron transfer rate constants during the reduction of the complex at the electrode were evaluated and found to be dependent on the scan rate. Electrochemical behavior of the chosen hydroxy-9,10-anthraquinone (NaLH₂SO₃) was significantly modified in the complex with quinone. This result is important for biochemical studies, since NaLH₂SO₃ resembles the core moiety of anthracycline antitumor drugs. Several studies have reported that in case of anthracyclines complexation can help to reduce the formation of semiquinone moieties, leading to a lesser generation of superoxide responsible for drug’s cardiotoxicity. The electrochemical behavior of the title complex actually justifies such claims and tries to explain why a reduction in semiquinone formation helps to decrease the formation of superoxide.     

Determination of Amino Acids at a Silver Oxide/Silver Phosphate Electrode and the Analysis of Structure‐Response Relationships

When a silver electrode is conditioned in a solution of 0.5 M sodium hydroxide with added sodium phosphate and using a dual pulse (500 mV/750 mV vs. Ag/AgCl), a stable silver(I)/silver(II) oxide surface is formed. It has been previously shown that various moieties react with the silver(II) oxide in a chemical oxidation at the outer surface of the oxide layer. This is then followed by re‐oxidation of the silver with the generation of current at approximately 500 mV relative to the silver/silver chloride electrode. Previously we found the need to remove carbon dioxide from the base and condition the electrode in a solution containing phosphate ion in order to provide mechanical stability to the oxide layer. We have previously shown this electrode to be applicable to the detection of a variety of carbohydrates. The applicability of the silver oxide/silver phosphate electrode to the post‐chromatographic amperometric detection of amino acids was investigated. Calibration studies of amino acids representative of the various classes demonstrated good sensitivity and linearity in the 1–100 μM range. Responses of amino acids were measured using glucose as an external standard, in order to correct for variability of the oxide layer. Relative responses of the amino acids ranged from 3 down to 0.1. Correlation with structure suggested the importance of absorption in determining the rate of oxidation. Comparison of arginine with n‐benzoyl‐L ‐arginine ethyl ester indicated that side chains as well as the backbone amino group can be oxidized. A Levitch plot of alanine was shown to be linear from approximately 30 to 300 radians per second spin rate at a scan rate of 50 mV per second. Application to post‐chromatographic detection was demonstrated.     

An Electrochemical Sensor Based on Ni(II) Complex and Multi Wall Carbon Nano Tubes Platform for Determination of Glucose in Real Samples

In the present paper, a stable and selective non‐enzymatic sensor is reported for determination of glucose (Glc) by using a carbon paste electrode modified with multiwall carbon nanotubes and Ni(II)‐SHP complex as modifier in an alkaline solution. This modified electrode showed impressive activity for oxidation of glucose in NaOH solution. Herein, Ni(II)‐SHP acts as a suitable platform for oxidation of glucose to glucolactone on the surface of the modified electrode by decreasing the overpotential and increasing in the current of analyte. Under the optimum conditions, the rate constant and electron transfer coefficient between electrode and modifier, were calculated to be 1.04 s⁻¹ and 0.64, respectively. The anodic peak currents indicated a linear dependency with the square root of scan rate and this behavior is the characteristic of a diffusion controlled process. So, the diffusion coefficient of glucose was found to be 3.12×10⁻⁶ cm² s⁻¹ due to the used number of transferred electron of 1. The obtained results revealed two linear ranges (5 to 190.0 μM (R²=0.997), 210.0 to 700.0 μM (R²=0.999)) and the detection limit of 1.3 μM for glucose was calculated by using differential pulse voltammetry (DPV) method. Also, the designed sensor was used for determination of glucose in the blood serum and urine samples. Some other advantages of Ni(II)‐SHP/CNT/CPE sensor are remarkable reproducibility, stability and selectivity which can be related to using nanomaterial of carbon nanotubes due to enhancement of electrode surface area.     

Voltammetric behavior and determination of bismuth on sodium humate modified carbon paste electrode

The preconcentration and voltammetric behavior of Bi^III on a sodium humate modified carbon paste electrode was studied by means of cyclic voltammetry (CV) and differential pulse stripping voltammetry (DPSV). The proposed measurement involves an initial nonelectrolytic preconcentration step in which Bi^III is complexed by the surface modifier in a solution of 0.05 M KNO₃-0.0106 M HNO₃ (pH 2.0) and a subsequent electrochemical scan step in which the preconcentrated Bi^III was reduced and then oxidized promptly in supporting electrolyte of 0.5 M HNO₃. The resulting DPSV anodic current was proportional to the concentration of Bi^III ion over the range of 4.78 × 10⁻⁸–1.44 × 10⁻⁵ M. The detection limit was 4.78 × 10⁻⁸ M. The proposed method was used to determine bismuth in various samples. Various factors affecting the electrode behavior were also investigated at the same time.     

Imprinted Polymer – Modified Hanging Mercury Drop Electrode for Differential Pulse Adsorptive Stripping Voltammetric Analysis of a Diquat Herbicide

An imprinted polymer modified hanging mercury drop electrode (HMDE) in Model 303A system in conjunction with a PAR Model 264A Polarographic Analyzer/Stripping Voltammeter has been used for the selective analysis of a diquat herbicide viz., 5,6‐dihydropyrazino[1,2,3,4‐[lmn]‐1,10‐phenanthrolinium dichlorides in differential pulse cathodic stripping voltammetry mode. Complex aqueous samples (drinking water and agricultural soil suspension), spiked with a diquat herbicide, were directly analyzed by the adsorptive accumulation of the analyte over the working electrode (accumulation potential ?0.8 V (vs. Ag/AgCl), accumulation time 120 s, pH 7.0, supporting electrolyte 0.1 M KCl, scan rate 10 mV s^?1, pulse amplitude 25 mV). The limit of detection for diquat herbicide was found to be 0.34 nmol L^?1 (0.1 ppb, RSD 2%, S/N=2).     

Binding of Copper(II) to Pectins by Electrochemical Methods

The interaction between Cu(II) and pectin extracted from citrus fruit was studied in KNO₃ 0.10 mol dm^?3 at 25 °C and pH 5.5, using ion selective electrode potentiometry and voltammetry, namely differential pulse polarography and square‐wave voltammetry. Although many independent variables may affect Cu(II)‐polymer interactions such as charge density, polymer concentration and copper to polymer concentration ratio, a good fitting was observed for the model with ML and ML₂ complex species, when M:L total concentration (mol dm^?3) ratio varies from 0.2 to 2.7 and the ligand concentration is in the range (0.2 to 1) g dm^?3, i.e., (0.4 to 2)×10^?3 mol COO^? dm^?3. The complex parameters found in these conditions were log β_CuL=3.5±0.1 and log β_CuL2= 8.0±0.2. For lower total ligand and total metal ion concentrations, used in voltammetry, the interaction Cu(II)‐pectin is affected by a cooperative mode (increase of metal ion‐ligand affinity) when the total metal ion concentration increases and by an anti‐cooperative mode when the total ligand concentration increases, possibly due to different conformations of the polymer.     

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.