Enantioselective recognition of penicillamine enantiomers on bovine serum albumin-modified glassy carbon electrode

As a natural chiral selector, bovine serum albumin (BSA) has been used to recognize penicillamine (Pen) enantiomers through electrochemical methods. The recognition and assay rely on the stereoselectivity of BSA embedded in ultrathin Al₂O₃ sol–gel film coated on the surface of glassy carbon electrode (BSA/GCE). The enantioselective interaction between Pen enantiomers and BSA was monitored by cyclic voltammetry and electrochemical impedance spectroscopy measurements, from which larger response signals were obtained from d-Pen. The factors influencing the performance of the modified biosensor were also investigated. The association constant (K) was calculated to be 1.93?×?10⁴?L?mol^?1 for d-Pen and 1.20?×?10³?L?mol^?1 for l-Pen. A good linear response was exhibited with the concentration of Pen enantiomers by BSA/GCE over the range of 1?×?10^?8–1?×?10^?1?mol?L^?1 with a detection limit of 3.31?×?10^?9?mol?L^?1.     

Sensitive electrochemical determination of Sudan II in food samples using a poly(aminosulfonic acid) modified glassy carbon electrode

In this paper, a novel poly(aminosulfonic acid) modified glassy carbon electrode (PASA/GCE) for the determination of Sudan II was fabricated through electrochemical polymerizat ion. The electrochemical behavior of Sudan II at the modified electrode was studied by cyclic voltammetry. Results show that the modified electrode exhibits excellent electrocatalytic activity toward the electrochemical redox reaction of Sudan II. Under optimal experimental conditions, the oxidation peak current is linearly proportional to the concentration of Sudan II in the ranges of 4.0 × 10^?8 to 1.0 × 10^?6 mol L^?1 and 1.0 × 10^?6 to 1.2 × 10^?5 mol L^?1. The linear regression equations are i _pa(A) = 2.87c + 3.74 × 10^?6, r = 0.9977 and i _pa(A) = 0.78c + 6.11 × 10^?6, r = 0.9982, respectively, and the detection limit is 4.0 × 10^?9 mol L^?1. The novel method shows good recovery, reproducibility and sensitivity for the voltammetric determination of Sudan II in food samples.     

Adsorption voltammetry of the vanadium-2-(5′-bromo-2′-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) system

The behaviour of the vanadium(V) complex with 5-Br-PADAP at a mercury electrode was investigated in HOAcNaOAc. The adsorption phenomena were observed by linear-sweep voltammetry. The mechanism of the electrode reaction was found to be the irreversible reduction of the V(V) in the complex adsorbed on the surface of the electrode to the V(IV) complex with 5-Br-PADAP. In 0.02 mol l^?1 HOAc-0.012 mol l^?1 NaOAc (pH 4.5) and 1 × 10^?6 mol l^?1 5-Br-PADAP, the detection limits of linear-sweep adsorption voltammetry and 1.5th-order derivative adsorption voltammetry are 5 × 10^?10 and 2.5 × 10^?11 mol l^?1 , respectively. The method was applied to samples of ore (Geological Deposit).     

Study of the solubility of antimony in mercury by cyclic and stripping voltammetry

The solubility of antimony in mercury has been studied by cyclic and stripping voltammetry in solutions of Sb(III) in 4 M HCl. The solubility has been evaluated at 8.57×10^?4 mol l^?1 or 1.27×10^?3 at. % at 25°C which agrees well with solubility values derived from data reported by other authors at different temperatures. A solubility heat (H_sol) of 16.7 kJ mol^?1 was calculated.     

Polymer-modified glassy carbon electrode for the electrochemical detection of quinine in human urine and pharmaceutical formulations

Glassy carbon electrode was modified by electropolymerization of 4-amino-3-hydroxynaphthalene sulfonic acid. Cyclic voltammetric study of quinine showed higher current response at the modified electrode compared to the bare and activated glassy carbon electrodes in pH 7.0 phosphate buffer solution. Under optimized conditions, a calibration curve was obtained by square wave voltammetry at the modified electrode. The linear relationship between the peak current and the concentration of quinine in the range of 1.0?×?10^?7 to 1.0?×?10^?5 M was I _pa (in microamperes)?=?6.26C (in micromolars)?+?0.2997 (R ²?=?0.999). The detection limit calculated (S/N?=?3) was 1.42?×?10^?8 M, which is much lower than similar reports. The method was successfully applied for the determination of quinine in spiked human urine, and pharmaceutical formulations and recovery values >90 % were obtained.     

Simultaneous Determination of Dopamine and Ascorbic Acid Using the Nano‐Gold Self‐Assembled Glassy Carbon Electrode

Electrochemical behavior of dopamine (DA) was investigated at the gold nanoparticles self‐assembled glassy carbon electrode (GNP/LC/GCE), which was fabricated by self‐assembling gold nanoparticles on the surface of L ‐cysteine (LC) modified glassy carbon electrode (GCE) via successive cyclic voltammetry (CV). A pair of well‐defined redox peaks of DA on the GNP/LC/GCE was obtained at E_pa=0.197 V and E_pc=0.146 V, respectively. And the peak separation between DA and AA is about 0.2 V, which is enough for simultaneous determination of DA and AA. The peak currents of DA and AA were proportional with their concentrations in the range of 6.0×10^?8–8.5×10^?5 mol L^?1 and 1.0×10^?6–2.5×10^?3 mol L^?1, with the detection limit of 2.0×10^?8 mol L^?1 and 3.0×10^?7 mol L^?1 (S/N=3), respectively. The modified electrode exhibits an excellent reproducibility, sensibility and stability for simultaneous determination of DA and AA in human serum with satisfactory result.     

Determination of coumarins by voltammetric techniques in micellar and emulsified media

An electroanalytical study of the oxidation processes of umbelliferone and hymecromone at a glassy carbon electrode in micellar solution and emulsified medium by different voltammetric techniques is described. The non-ionic surfactant Triton X-405 in acetate-buffered medium at pH 4.8 was found to be the most suitable. Different ranges of linearity were obtained in the micellar solutions, depending on the technique used; the limits of determination for differential pulse voltammetry (DPV) at a stationary electrode were 2.9×10^?6 mol l^?1 and 3.3×10^?6 mol l^?1 for umbelliferone and hymecromone, respectively. In the emulsified medium formed with a mixture of toluene and ethyl acetate (3:2), the oxidation processes yielded similar results. With DPV, linear calibration plots were obtained in the ranges 1.0×10^?5–9.0×10^?7 mol l^?1 umbelliferone and 1.0×10^?5–2.0×10^?6 mol l^?1 hymecromone. The media used are predominantly aqueous so that special reference electrodes and solvent purification are not needed.     

Simultaneous determination of dopamine and serotonin by use of covalent modification of 5-hydroxytryptophan on glassy carbon electrode

An electrochemical biosensor was fabricated by covalent modification of 5-hydroxytryptophan (5-HTP) on the surface of glassy carbon electrode (GCE). The electrode, denoted as 5-HTP/GCE, was characterized by X-ray photoelectron spectroscopy, cyclic voltammetry and differential pulse voltammetry. For comparison, tryptophan modified GCE (TRP/GCE) and serotonin modified GCE (5-HT/GCE) were prepared by the same method. It was found that electrocatalytic ability of these electrodes was in the order of 5-HTP/GCE?>?TRP/GCE?>?5-HT/GCE for the oxidation of dopamine (DA) and 5-HT. The sensor was effective to simultaneously determine DA and 5-HT in a mixture. It can resolve the overlapping anodic peaks into two well-defined voltammetric peaks at 0.24 and 0.39 V (versus SCE). The linear response is in the range of 5.0?×?10^?7–3.5?×?10^?5 mol L^?1 with a detection limit of 3.1?×?10^?7 mol L^?1 for DA, and in the range of 5.0?×?10^?6–3.5?×?10^?5 mol L^?1 with a detection limit of 1.7?×?10^?6 mol L^?1 for 5-HT (s/n?=?3), respectively.     

The electrochemistry of uric acid at a gold electrode modified with L-cysteine, and its application to sensing uric urine

The electrochemistry of uric acid at a gold electrode modified with a self-assembled film of L-cysteine was studied by cyclic voltammetry and differential pulse voltammetry. Compared to the bare gold electrode, uric acid showed better electrochemical response in that the anodic peak current is stronger and the peak potential is negatively shifted by about 100 mV. The effects of experimental conditions on the oxidation of uric acid were tested and a calibration plot was established. The differential pulse response to uric acid is linear in the concentration range from 1.0?×?10^?6 to ~?1.0?×?10^?4 mol?L^?1 (r?=?0.9995) and from 1.0?×?10^?4 to ~?5.0?×?10^?4 mol?L^?1 (r?=?0.9990), the detection limit being 1.0?×?10^?7 mol?L^?1 (at S/N?=?3). The high sensitivity and good selectivity of the electrode was demonstrated by its practical application to the determination of uric acid in urine samples.

Hemoglobin co-immobilized with silver–silver oxide nanoparticles on a bare silver electrode for hydrogen peroxide electroanalysis

Hemoglobin (Hb) and silver–silver oxide (Ag–Ag₂O) nanoparticles were co-immobilized on a bare silver electrode surface by cyclic voltammetry, and were characterized by UV–vis reflection spectroscopy, scanning electron microscopy, and electrochemical impedance spectroscopy. The immobilized Hb was shown to maintain its biological activity well. Direct electron transfer between Hb and the resulting electrode was achieved without the aid of any electron mediator. The reduction currents to hydrogen peroxide (H₂O₂) at co-immobilized electrodes showed a linear relationship with H₂O₂ concentration over a concentration range from 6.0?×?10^?6 to 5.0?×?10^?2 mol L^?1, and a detection limit of 2.0?×?10^?6 mol L^?1 (S/N?=?3).     

A novel electrochemical sensor based on molecularly imprinted polymers for caffeine recognition and detection

A sensitive and selective electrochemical sensor based on molecularly imprinted polymers (MIPs) was developed for caffeine (CAF) recognition and detection. The sensor was constructed through the following steps: multiwalled carbon nanotubes and gold nanoparticles were first modified onto the glassy carbon electrode surface by potentiostatic deposition method successively. Subsequently, o-aminothiophenol (ATP) was assembled on the surface of the above electrode through Au–S bond before electropolymerization. During the assembled and electropolymerization processes, CAF was embedded into the poly(o-aminothiophenol) film through hydrogen bonding interaction between CAF and ATP, forming an MIP electrochemical sensor. The morphologies and properties of the sensor were characterized by scanning electron microscopy, cyclic voltammetry, and differential pulse voltammetry. The recognition and determination of the sensor were observed by measuring the changes of amperometric response of the oxidation-reduction probe, [Fe(CN)₆]^3?/[Fe(CN)₆]^4?, on modified electrode. The results demonstrated that the prepared sensor had excellent selectivity and high sensitivity for CAF, and the linear range was 5.0?×?10^?10?~?1.6?×?10^?7?mol?L^?1 with a detection limit of 9.0?×?10^?11?mol?L^?1 (S/N?=?3). The sensor was also successfully employed to detect CAF in tea samples.     

Preparation of an amperometric sensor for norfloxacin based on molecularly imprinted grafting photopolymerization

A sensitive amperometric sensor for norfloxacin (NF) was introduced. The receptor layer was prepared by molecularly imprinted photopolymerization of acrylamide and trimethylolpropane trimethacrylate on the surface of a gold electrode. The binding mechanism of the molecularly imprinted polymer was explored by ultraviolet (UV) and infrared (IR) spectroscopy. The chemosensor was characterized by cyclic voltammetry (CV), differential pulse voltammetry (DPV), electrochemical impedance (EI), and scanning electron microscopy (SEM). The electrode prepared by photopolymerization has a better recognition ability to template molecules than that of electropolymerization and NIP. Some parameters affecting sensor response were optimized. Norfloxacin was detected by measurements of an amperometric i–t curve. The linear relationships between current and logarithmic concentration are obtained from 1.0?×?10^?9 to 1.0?×?10^?3?mol?L^?1. The detection limit of the sensor was 1.0?×?10^?10?mol?L^?1. The proposed method is sensitive, simple, and cheap, and is applied to detect NF in human urine successfully.

Construction of a modified carbon paste electrode based on TiO2 nanoparticles for the determination of gallic acid

A modified carbon paste electrode was prepared by incorporating the TiO₂ nanoparticles in the carbon paste matrix. The electrochemical behavior of gallic acid (GA) is investigated on the surface of the electrode using cyclic voltammetry and differential pulse voltammetry. The surface morphology of the prepared electrode was characterized using the scanning electron microscopy. The results indicate that the electrochemical response of GA is improved significantly at the modified electrode compared with the unmodified electrode. Furthermore, the capabilities of electron transfer on these two electrodes were also investigated by electrochemical impedance spectroscopy. Under the optimized condition, a linear dynamic range of 2.5?×?10^?6 to 1.5?×?10^?4?mol?L^?1 with detection limit of 9.4?×?10^?7?mol?L^?1 for GA is obtained in buffered solutions with pH 1.7. Finally, the proposed modified electrode was successfully used in real sample analysis.     

Development and application of the tartrazine voltammetric sensors based on molecularly imprinted polymers

A modified glassy carbon electrode was prepared as an electrochemical voltammetric sensor based on molecularly imprinted polymer film for tartrazine (TT) detection. The sensitive film was prepared by copolymerization of tartrazine and acrylamide on the carbon nanotube-modified glassy carbon electrode. The performance of the imprinted sensor was investigated by cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy in detail. Under the optimum conditions, two dynamic linear ranges of 8?×?10^?8 to 1?×?10^?6?mol?L^?1 and 1?×?10^?6 to 1?×?10^?5?mol?L^?1 were obtained, with a detection limit of 2.74?×?10^?8?mol?L^?1(S/N?=?3). This sensor was used successfully for tartrazine determination in beverages.     

On the mechanism of the Co2(CO)8 catalyzed hydroformylation of olefins in hydrocarbon solvents. A high pressure UV and IR study

High pressure IR and UV spectroscopic experiments confirm the Heck and Breslow mechanism of the hydroformylation of 1-octene and cyclohexene with Co₂(CO)₈ as the starting catalyst. The major repeating unit is HCo(CO)₄, which is formed via the reaction of acylcobalt tetracarbonyl with H₂. The rates are 6.7 × 10^?4 mol l^?1 min^?1 and 8.8 × 10^?5 mol l^?1 min^?1 for 1-octene and cyclohexene, respectively at 80°C and 95 bar CO/H₂ = 1 in methylcyclohexane. The alternative reaction of RCOCo(CO)₄ with HCo(CO)₄ is only a minor pathway, with rates of 1.8 × 10^?5 mol l^?1 min^?1 and 1.1 × 10^?5 mol l^?1 min^?1 for 1-octene and cyclohexene, respectively. It represents an exit from the catalytic cycle. The activation of the catalyst precursor Co₂(CO)₈ is the slowest step of the reaction.     

Electrochemical oxidation determination and voltammetric behaviour of 4-nitrophenol based on Cu2O nanoparticles modified glassy carbon electrode

Cu₂O nanoparticles (nano-Cu₂O) modified glassy carbon electrode (GCE) was fabricated and used to investigate the electrochemical behaviour of 4-nitrophenol (4-NP) by cyclic voltammetry (CV), chronoamperometry (CA), chronocoulometry (CC) and differential pulse voltammetry (DPV). Compared with GCE, a remarkable increase in oxidation peak current was observed. It indicates that nano-Cu₂O exhibits remarkable enhancement effect on the electrochemical oxidation of 4-NP. Under the optimised experimental conditions, the oxidation peak currents were propotional to 4-NP concentration in the range from 1.0?×?10^?6 to 4.0?×?10^?4?mol?L^?1 with a detection limit of 5.0?×?10^?7?mol?L^?1 (S/N?=?3). The fabricated electrode presented good repeatability, stability and anti-interference. Finally, the proposed method was applied to determine 4-NP in water samples. The recoveries for these samples were from 94.60% to 105.5%.     

Development of a Novel Automatic Potentiometric System for Determination of Selenium and Its Application in Pharmaceutical Formulations and Anodic Slime

Poly(vinyl chloride) polymeric membrane sensors containing Sn(IV) phthalocyanine dichloride (SnPC) and Co(II) phthalocyanine (CoPC) as novel electroactive materials dispersed in o‐nitrophenyl octylether (o‐NPOE) as a plasticizer are examined potentiometrically with respect to their response toward selenite (SeO₃^2?) ions. Fast Nernstian response for SeO₃^2? ions over the concentration ranges 7.0×10^?6–1.0×10^?3 and 8.0×10^?6–1.0×10^?3 mol L^?l at pH 3.5–8.5 with lower detection limit of 5.0×10^?6 and 8.0×10^?6 mol L^?1 and calibration slopes of ?25.4 and ?29.7 mV decade^?1 are obtained with SnPC and CoPC based membrane sensors, respectively. The proposed sensors reveals by the modified separate solution method (MSSM) a good selectivity over different anions which differ significantly from the classical Hofmeister series. A segmented sandwich membrane method is used to determine complex formation constants of the ionophores in situe in the solvent polymeric sensing membranes. Membrane incorporating CoPC in a tubular flow detector is used in a two channels flow injection set up for continuous monitoring of selenite at a frequency of ca. 50 samples h^?1. Direct determination of selenium in pharmaceutical formulations and anodic slime gives results in good agreement with data obtained using standard ICP method.     

Determination of Benzoyl Peroxide Levels in Wheat Flour and Pharmaceutical Preparations by Differential Pulse Voltammetry in Nonaqueous Media

Abstract

Benzoyl peroxide (BP) was determined by differential pulse voltammetry (DPV) using a glassy carbon electrode in a dichloromethane‐acetic acid (1.5×10^?2 mol l^?1) solution and tetrabutyl ammonium perchlorate (0.01 mol l^?1) as the supporting electrolyte. The peak potential was ?0.045 V (vs. Ag/AgCl). There was a good linear relationship between the peak current and the benzoyl peroxide concentration in the range of 2.5×10^?6–1.0×10^?4 mol l^?1. The detection limit of the method was 2.5×10^?7 mol l^?1. The recovery was 94.8–106.0%. The samples of wheat flour and the pharmaceutical preparations for the treatment of acne vulgaris were directly detected with desired results. The reaction mechanism of benzoyl peroxide on the electrode was also discussed, which was two electrons and two protons irreversible reaction.     

One-step construction of an electrode modified with electrodeposited Au/SiO2 nanoparticles, and its application to the determination of NADH and ethanol

A new electrode was developed by one-step potentiostatic electrodeposition (at ?2.0 V for 20 s) of Au/SiO₂ nanoparticles on a glassy carbon electrode. The resulting electrode (nano-Au/SiO₂/GCE) was characterized by scanning electronic microscopy, X-ray photoelectron spectroscopy and electrochemical techniques. The electrochemical behavior of dihydronicotinamide adenine dinucleotide (NADH) at the nano-Au/SiO₂/GCE were thoroughly investigated. Compared to the unmodified electrode, the overpotential decreased by about 300 mV, and the current response significantly increased. These changes indicated that the modified electrode showed excellent catalytic activity in the oxidation of NADH. A linear relationship was obtained in the NADH concentration range from 1.0?×?10^?6 to 1.0?×?10^?4 mol?L^?1. In addition, amperometric sensing of ethanol at the nano-Au/SiO₂/GCE in combination with alcohol dehydrogenase and nicotinamide adenine dinucleotide was successfully demonstrated. A wide linear response was also found for ethanol in the range from 5.0?×?10^?5 to 1.0?×?10^?3 mol?L^?1 and 1.0?×?10^?3 to 1.0?×?10^?2 mol?L^?1, respectively. The method was successfully applied to determine ethanol in beer and biological samples.     

The Electrochemical Behavior of α‐Ketoglutarate at the Hanging Mercury Drop Electrode in Acidic Aqueous Solution and Its Practical Application in Environmental and Biological Samples

The voltammetric behavior of α‐ketoglutarate (α‐KG) at the hanging mercury drop electrode (HMDE) has been investigated in acetate buffer solution. Under the optimum experimental conditions (pH 4.5, 0.2 M NaAc‐HAc buffer solution), a sensitive reductive wave of α‐KG was obtained by linear scan voltammetry (LSV) and the peak potential was ?1.18 V (vs. SCE), which was an irreversible adsorption wave. The kinetic parameters of the electrode process were α=0.3 and k_s=0.72 1/s. There was a linear relationship between peak current i_{p, α‐KG} and α‐KG concentration in the range of 2×10^?6–8×10^?4 M α‐KG. The detection limit was 8×10^?7 M and the relative standard deviation was 2.0% (C_α‐KG=8×10^?4 M, n=10). Applications of the reductive wave of α‐KG for practical analysis were addressed as follows: (1) It can be used for the quantitative analysis of α‐KG in biological samples and the results agree well with those obtained from the established ultraviolet spectrophotometric method. (2) Utilizing the complexing effect between α‐KG and aluminum, a linear relationship holds between the decrease of peak current of α‐KG Δi_p and the added Al concentration Cequation/tex2gif-inf-5.gif in the range of 5.0×10^?6–2.5×10^?4 M. The detection limit was 2.2×10^?6 M and the relative standard deviation was 3.1% (Cequation/tex2gif-inf-6.gif=4×10^?5 M, n=10). It was successfully applied to the detection of aluminum in water and synthetic biological samples with satisfactory results, which were consistent with those of ICP‐AES. (3) It was also applied to study the effect of Al^III on the glutamate dehydrogenase (GDH) activity in the catalytically reaction of α‐KG+NH +NADH?L ‐glutamate+NAD⁺+H₂O by differential pulse polarography (DPP) technique. By monitoring DPP reductive currents of NAD⁺ and α‐KG, an elementary important result was found that Al could greatly affect the activity of GDH. This study could be attributed to intrinsic understanding of the aluminum's toxicity in enzyme reaction processes.