Nanomolar Determination of Methyldopa in the Presence of Large Amounts of Hydrochlorothiazide Using a Carbon Paste Electrode Modified with Graphene Oxide Nanosheets and 3‐(4′‐Amino‐3′‐hydroxy‐biphenyl‐4‐yl)‐acrylic Acid

A novel electrochemical sensor for sensitive detection of methyldopa at physiological pH was developed by the bulk modification of carbon paste electrode (CPE) with graphene oxide nanosheets and 3‐(4′‐amino‐3′‐hydroxy‐biphenyl‐4‐yl)‐acrylic acid (3,′AA). Applying square wave voltammetry (SWV), in phosphate buffer solution (PBS) of pH 7.0, the oxidation current increased linearly with two concentration intervals of methyldopa, one is 1.0×10^?8–1.0×10^?6 M and the other is 1.0×10^?6–4.5×10^?5 M. The detection limit (3σ) obtained by SWV was 9.0 nM. The modified electrode was successfully applied for simultaneous determination of methyldopa and hydrochlorothiazide. Finally, the proposed method was applied to the determination of methyldopa and hydrochlorothiazide in some real samples.     

Voltammetric determination of cefpirome at multiwalled carbon nanotube modified glassy carbon sensor based electrode in bulk form and pharmaceutical formulation

A new, simple and low cost voltammetric method for the determination of cefpirome in pharmaceutical preparations has been developed using multiwalled carbon nanotube modified glassy carbon electrode (MWCNT), which showed stable response with enhanced selectivity and sensitivity over the bare glassy carbon electrode. A multiwalled carbon nanotube (MWCNT) modified glassy carbon electrode (GCE) is used for the simultaneous determination of cefpirome by differential pulse voltammetry and square wave voltammetry. Results indicated that cathodic peak of cefpirome is greatly improved at MWCNT modified GC electrode as compared with the bare GC electrode showing excellent electrocatalytic activity towards cefpirome reduction. Linear calibration curves are obtained over the concentration range 100-600 μg mL(-1) in Britton Robinson buffer at pH 4.51 with limit of detection (LOD) and limit of quantification (LOQ) are 0.647 μg mL(-1) and 2.159 μg mL(-1) using SWV and 5.540 μg mL(-1) and 18.489 μg mL(-1) using DPV, respectively. The described method is rapid and can be successfully applied for the determination of cefpirome in bulk form and pharmaceutical formulations.     

Label‐Free Electrochemical Aptasensor for Determination of Chloramphenicol Based on Gold Nanocubes‐Modified Screen‐Printed Gold Electrode

An ultrasensitive label‐free electrochemical aptasensor was developed for selective detection of chloramphenicol (CAP). The aptasensor was made using screen‐printed gold electrode modified with synthesized gold nanocube/cysteine. The interactions of CAP with aptamer were studied by cyclic voltammetry, square wave voltammetry (SWV) and electrochemical impedance spectroscopy. Under optimized conditions, two linear calibration curves were obtained for CAP determination using SWV technique, from 0.03 to 0.10 µM and 0.25–6.0 µM with a detection limit of 4.0 nM. The aptasensor has the advantages of good selectivity and stability and applied to the determination of CAP in human blood serum sample.     

Electroanalysis of Carbendazim using MWCNT/Ca‐ZnO Modified Electrode

The present research involves the report on electrochemical deportment of Carbendazim (MBC) at multiwalled carbon nanotubes and calcium‐doped zinc oxide nanoparticles altered nanocomposite based carbon paste electrode (MWCNTs/Ca‐ZnO‐CPE). The modified carbon paste evidenced manifest electrocatalytic behavior for MBC in 0.2 M phosphate buffer (PB) solutions. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and square wave voltammetry (SWV) techniques were used for the analysis. The working electrode assembly exhibits faster electron transfer of MBC with increase in the peak current. At bare CPE, MBC showed maximum peak current of 1.098 μA at potential 0.7568 V whereas at MWCNT/Ca‐ZnO/CPE peak current of 5.203 μA was observed at potential 0.7541 V in 0.2 M PBS of pH 7.0 at the sweep rate of 50 mV s^?1. The synthesized 5 % Ca‐ZnO nanoparticles (NPs) were characterized by X‐ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X‐ray analysis (EDX), and Transmission electron microscopy (TEM) analysis. Various factors influencing the voltammetry of MBC such as pre‐concentration time, pH, sweep rate, and amount of MBC were studied and from the studies we observed that the response was found to be diffusion‐controlled. The concentration variation studies for MBC was watched in the linear working range of 0.01 μM to 0.45 μM and the detection limit was found by SWV technique.     

Gold Nanoparticles Stabilized in β‐Cyclodextrin and Decorated with Laccase Applied in the Construction of a Biosensor for Rutin

This paper describes the synthesis and characterization of gold nanoparticles stabilized in β‐cyclodextrin (AuNP‐CD), which were applied as a platform in the immobilization of laccase (LAC). The AuNP‐CD‐LAC were used in the construction of a new biosensor for rutin determination by square‐wave voltammetry (SWV). Under optimized conditions, the calibration curve showed a linear range for rutin of 0.30 to 2.97 μmol L⁻¹, with a limit of detection of 0.17 μmol L⁻¹. The biosensor demonstrated satisfactory repeatability and electrode‐to‐electrode repeatability (with relative standard deviations of 5.6 and 6.0 %, respectively) and good stability. The biosensor was successfully applied in the determination of rutin in different pharmaceutical samples.     

Simultaneous Electrochemical Determination of Hydroquinone and Bisphenol A using a Carbon Paste Electrode Modified with Silver Nanoparticles

In this paper, a rapid and sensitive modified electrode for the simultaneous determination of hydroquinone (HQ) and bisphenol A (BPA) is proposed. The simultaneous determination of these two compounds is extremely important since they can coexist in the same sample and are very harmful to plants, animals and the environment in general. A carbon paste electrode (CPE) was modified with silver nanoparticles (nAg) and polyvinylpyrrolidone (PVP). The PVP was used as a reducing and stabilizing agent of nAg from silver nitrate in aqueous media. The nAg‐PVP composite obtained was characterized by transmission electron microscopy and UV‐vis spectroscopy. The electrochemical behavior of HQ and BPA at the nAg‐PVP/CPE was investigated in 0.1 mol L⁻¹ B−R buffer (pH 6.0) using cyclic voltammetry (CV) and square wave voltammetry (SWV). The results indicate that the electrochemical responses are improved significantly with the use of the modified electrode. The calibration curves obtained by SWV, under the optimized conditions, showed linear ranges of 0.09–2.00 μmol L⁻¹ for HQ (limit of detection 0.088 μmol L⁻¹) and 0.04–1.00 μmol L⁻¹ for BPA (limit of detection 0.025 μmol L⁻¹). The modified electrode was successfully applied in the analysis of water samples and the results were comparable to those obtained using UV‐vis spectroscopy.     

First Electrochemical Report for Simultaneous Determination of Norepinephrine,Tyrosine and Nicotine Using a Nanostructure Based Sensor

A carbon paste electrode was modified with ZnO nanorods and 3‐(4′‐amino‐3′‐hydroxy‐biphenyl‐4‐yl)‐acrylic acid (3,4′AAZCPE) to cause electrocatalysis of norepinephrine oxidation. It has been found that the oxidation of norepinephrine at the surface of modified electrode occurs at a potential of about 180 mV less positive than that of an unmodified carbon paste electrode. Square wave voltammetry (SWV) exhibits linear dynamic range from 1.0×10^?7 to 8.0×10^?5 M and a detection limit of 3.9×10^?8 M for norepinephrine. In addition, this modified electrode was used for simultaneous determination of norepinephrine, tyrosine and nicotine.     

Differential Pulse Voltammetric Determination and Application of Square‐Wave Voltammetry of yRNA on a CPB‐Cellulose Modified Electrode

Electrochemical behavior of remarkably low levels of Ribonucleic acid yeast (yRNA) is studied through differential pulse voltammetry (DPV), and kinetic parameters of the electrochemical reaction have also been calculated through square‐wave voltammetry (SWV), based on immobilization of yRNA on the surface of a CPB‐cellulose modified electrode. YRNA/ CPB‐cellulose/ITO conductive glass electrode is demonstrated by Infrared reflect (IR) and electrochemical impedance spectroscopy (EIS). The oxidation peak potential of yRNA shifts negatively with increasing pH. The peak currents decrease gradually in successive scans and no corresponding reduction peaks occur, indicating that oxidation process of yRNA is completely irreversible. Variables influencing DPV response of yRNA, such as pH, pulse amplitude and electrolyte concentration, are explored and optimized. Peak currents are proportional to the concentration of yRNA in the range of 0.1 μg mL^?1–1.0 μg mL^?1, and the linear regression coefficient equals 0.9923. The detection limit for yRNA is 1.0×10^?10 g mL^?1. Interferences of L ‐cysteine, L ‐alanine, Hemoglobin, Uridine 5′‐monophosphate, Guanosine 5′‐monophosphate, Adenosine 5′‐triphosphate and some metal ions (Co³⁺, Cr³⁺, Ni²⁺, Hg²⁺, Zn²⁺, etc) are negligible. The methods adopted here are more sensitive and selective than currently applied techniques and overcome the drawback of absorption spectroscopy arising from a strong interference due to other UV‐absorbing substances.     

A Sensitive Electrochemical Sensor for Voltammetric Determination of Ganciclovir Based on Au‐ZnS Nanocomposite

An electrode modified with ZnS and gold nanoparticles (Au‐ZnS NPs) is introduced for highly sensitive voltammetric determination of ganciclovir (GCV). Surface structure and topography of the modified electrode was studied by SEM, EDX and XRD techniques. Electrochemical oxidation of GCV was investigated by cyclic (CV) and square wave voltammetry (SWV) in Briton‐Robinson buffer solution (pH 1.5). The results showed that electrochemical oxidation of GCV at the Au‐ZnS modified glassy carbon electrode (GCE) is irreversible and exhibited diffusion controlled electrode process over the pH range from 1.0 to 6.0. The oxidation potential peak and pH relationship showed that electrons and protons were transferred simultaneously over the electrochemical oxidation process. Using the proposed sensor, the linear calibration curves were obtained in the ranges of 0.04–1.50 μM and 1.5–70.0 μM with detection limit of 0.01 μM GCV by SWV technique. The modified electrode was successfully applied as a sensitive, reproducible and repeatable sensor for determination of the trace amount of GCV in human serum, urine and cymevene vials. Reasonable results were obtained from comparing the measurements of the real samples by the new sensor to high performance liquid chromatography (HPLC) as a standard method.     

Square wave voltammetry for analytical determination of paracetamol using cobalt microparticles film modified platinum electrode

Cobalt microparticles (Co MPs) modified Pt electrode is simply and conveniently fabricated. The electrochemical properties of paracetamol (PCT) at the prepared modified electrode are investigated using cyclic voltammetry (CV) and square wave voltammetry (SWV) measurements. Based on these techniques, a sensitive and rapid electrochemical method is developed for the determination of PCT. The result indicates that the oxidation of PCT is strongly improved at the Co MPs/Pt electrode as compared with the bare Pt electrode, with relatively high sensitivity, stability and life time. The determination of PCT on the Co MPs/Pt with square wave voltammetry displays a high sensitivity of 101 μA/mM and a low detection limit of 0.42 μM (S/N = 3) in the range (0.5–100 μM). The sensitivity of the modified electrode for the detection of PCT is almost 17 times greater than on the bare Pt electrode. The proposed method is successfully applied to the PCT determination in tablets.     

Voltammetric determination of antibacterial drug gemifloxacin in solubilized systems at multi-walled carbon nanotubes modified glassy carbon electrode

A sensitive electroanalytical method for determination of gemifloxacin in pharmaceutical formulation has been investigated on the basis of the enhanced electrochemical response at multi-walled carbon nanotubes modified glassy carbon electrode in the presence of CTAB. Solubilized system of different surfactants including SDS, Tween-20 and CTAB were taken for the study of electrochemical behaviour of gemifloxacin at modified electrode. The reduction peak current increases in the presence of CTAB while other surfactants show opposite effect. The modified electrode exhibits catalytic activity, high sensitivity, stability and is applicable over wide range of concentration for the determination of gemifloxacin. The mechanism of electrochemical reduction of gemifloxacin has been proposed on the basis of CV, SWV, DPV and coulometeric techniques. The proposed squarewave voltammetric method shows linearity over the concentration range 2.47-15.5 μg/mL. The achieved limits of detection (LOD) and quantification (LOQ) are 0.90 ng/mL and 3.0 ng/mL respectively.     

Voltammetric Determination of 2′‐Deoxyadenosine and Adenine in Urine of Patients with Hepatocellular Carcinoma Using Fullerene‐C60‐modified Glassy Carbon Electrode

A fullerene‐C₆₀‐modified glassy carbon electrode has been examined for the simultaneous determination of 2′‐deoxyadenosine (2′‐dAdo) and adenine in human blood and urine using Osteryoung square‐wave voltammetry (OSWV) at pH 7.2. Compared to bare glassy carbon electrode (GCE), the modified electrode displays a shift of the oxidation potential in the negative direction with significant increase in the peak current for both the analytes. At modified electrode well‐defined anodic peaks at potential of 1248 mV and 994 mV are observed for 2′‐dAdo and adenine respectively. Linear calibration curves were obtained within the concentration range 10 nM to 100 μM for both the compounds in 0.1 M phosphate buffer solution (PBS) with the limit of detection 0.8×10^?8 M and 0.95×10^?8 M for 2′‐dAdo and adenine respectively. The analytical utility of the present method is demonstrated by quantitative detection of 2′‐dAdo and adenine in human urine of normal subjects as well as in patients with hepatocellular carcinoma. Interfering effect of some coexisting metabolites has also been reported.     

Square wave and elimination voltammetric analysis of azidothymidine in the presence of oligonucleotides and chromosomal DNA

Azidothymidine (AZT, 3'-azido-3'-deoxythymidine, Zidovudine, Retrovir) is an approved and widely used antiretroviral drug for the treatment of human immunodeficiency virus (HIV) infection. Dynamic electrochemical methods have been employed for the fast and inexpensive determination of this drug in natural samples. The electrochemical signal of AZT, resulting from the reduction of azido group, was studied by square wave voltammetry (SWV), linear sweep voltammetry (LSV) and elimination voltammetry with linear scan (EVLS) using a hanging mercury drop electrode (HMDE). This paper explores the possibility of determining AZT in the presence of native (dsDNA) or denatured calf thymus DNA (ssDNA), and/or some synthetic oligodeoxynucleotides (ODNs). The detection limit of AZT in the absence and in the presence of ssDNA (10 microg/ml) is 1 and 250 nM, respectively. It was found that the signal of AZT is not substantially affected by the presence of DNA. We can therefore assume that the electrons are transferred through the adsorption layer of nucleic acids. By using the elimination procedure, both irreversible reduction signals of AZT and DNA are augmented. Moreover, the elimination signal in the peak-counterpeak form may indicate the adsorption of the analytes on the electrode surface preceding an electron transfer.     

Electrochemical Study of 2,4‐Dinitrophenylhydrazine as Derivatization Reagent and Aldehydes at Carbon Glassy Electrode

A selective method based on derivatization with 2,4‐dinitrophenylhydrazine (DNPH) is described for the determination of several carbonyl compounds. The factors affecting the derivatization reaction of aldehyde and DNPH were investigated. The product of the derivatization reaction has been characterized by UV/Vis spectrophotometry, NMR, infrared spectroscopy and cyclic voltammetry. Then, an electrochemical study for the determination of aldehyde‐2.4‐dinitrophenylhydrazone was performed at glassy carbon electrode (GCE) using square wave voltammetry (SWV). After the optimization of experimental parameters, the limits of detection (at 3σ ) obtained for all aldehyde‐2,4‐DNPH were varied from 15.82 to 78.39 μmol L⁻¹ and relative standard deviations were between 1.8 and 4.5%. Finally, the proposed method was applied to determine the aldehydes concentration in drinking water and orange juice samples with satisfactory results.     

Multi‐Walled Carbon Nanotubes (MWCNT)‐Ionic Liquid‐Modified Carbon Paste Electrode: Probing FurazolidoneDNA Interactions and DNA Determination

The interactions of furazolidone (Fu) with double‐stranded calf thymus DNA (dsDNA) on the multi‐walled carbon nanotubes‐ionic liquid‐modified carbon paste electrode (MWCNT‐IL‐CPE) have been studied by cyclic voltammetry. In the presence of DNA, the cathodic peak current of Fu decreased and the peak potential shifted to a positive potential, indicating the intercalative interaction of Fu with DNA. The binding constant of Fu with DNA and stoichiometric coefficient has been determined according to the Hill's model. This electrochemical method was further applied to the determination of DNA. Two linear calibration curves were obtained for DNA detection in the concentration ranges of 0.03–0.10 and 0.10–4.0 μg l^?1 with a detection limit of 0.027 μg l^?1. The method was successfully applied to analyze Fu in serum samples.     

Voltammetric Determination of Phenoxybenzyl‐Type Insecticides at Chemically Modified Conducting Polymer‐Carbon Nanotubes Coated Electrodes

The electrochemical reduction of three common insecticides such as cypermethrin (CYP), deltamethrin (DEL) and fenvalerate (FEN) was investigated at glassy carbon electrode (GCE), multiwalled carbon nanotubes modified GCE (MWCNT‐GCE), polyaniline (herein called as modifier M₁) and polypyrrole (herein called as modifier M₂) deposited MWCNT/GCE using cyclic voltammetry. Influences of pH, scan rate, and concentration were studied. The surface morphology of the modified film was characterized by scanning electron microscopy (SEM) and X‐ray diffraction analysis (XRD). A systematic study of the experimental parameters that affect differential pulse stripping voltammetry (DPSV) was carried out and the optimized experimental conditions were arrived at. The calibration plots were linear over the insecticide's concentration range 0.1–100 mg L^?1 and 0.05–100 mg L^?1 for all the three insecticides at MWCNT‐GCE and MWCNT(M₁)‐GCE respectively. The MWCNT(M₂)‐GCE performed well among the three electrode systems and the determination range obtained was 0.01–100 mg L^?1 for CYP, DEL and FEN. The limit of detection (LOD) was 0.35 μg L^?1, 0.9 μg L^?1 and 0.1 μg L^?1 for CYP, DEL and FEN respectively on MWCNT(M₂)‐GCE modified system. Suitability of this method for the trace determination of insecticide in spiked soil sample was also determined.     

Voltammetry of a Novel Antimycobacterial Agent 1‐Hydroxy‐N‐(4‐nitrophenyl)naphthalene‐2‐carboxamide in a Single Drop of a Solution

The aim of this study is the development of a miniaturized voltammetric method for the determination of an antimycobacterial agent 1‐hydroxy‐N‐(4‐nitrophenyl)naphthalene‐2‐carboxamide (HNN) in a single drop (20 μL) of a solution by cathodic and anodic voltammetry at a glassy carbon electrode. Cyclic voltammetry was used to investigate its redox properties followed by the optimization of differential pulse voltammetric determination in a regular 10 mL volume. The optimal medium for the analytical application of both cathodic and anodic voltammetry was found to be Britton‐Robinson buffer pH 7.0 and dimethyl sulfoxide (9 : 1, v/v). HNN gave one cathodic peak at around −0.6 V and one anodic peak at around +0.2 V vs. Ag|AgCl (3 mol L⁻¹ KCl) reference electrode. Determination of HNN in a 10 mL volume gave the limit of quantification around 10 nmol L⁻¹ by both adsorptive stripping anodic and cathodic voltammetry. Afterwards, miniaturized voltammetric methods in a single drop of solution (20 μL) were investigated. This approach requested some modifications of the cell design and voltammetric procedures. A novel method of removing dissolved oxygen in a single drop had to be developed and tested. Developed miniaturized voltammetric methods gave parameters comparable to the determination of HNN in 10 mL. The applicability of the miniaturized method was verified by the determination of HNN in a drop of a bacterial growth medium.     

Electrochemical Characteristics and Electrosensing of an Antiviral Drug,Entecavir via Synergic Effect of Graphene Oxide Nanoribbons and Ceria Nanorods

A sensitive electrochemical sensor was fabricated based on ceria‐graphene oxide nanoribbons composite (CeO₂‐GONRs) for an antiviral drug, entecavir (ETV). It was characterized by SEM, EDAX, AFM, IR and Raman spectroscopic techniques. The electrochemical behaviour of ETV was investigated by cyclic voltammetric, differential pulse voltammetric (DPV), linear sweep voltammetric (LSV) and square wave voltammetric (SWV) methods at CeO₂‐GONRs modified glassy carbon electrode. Good linearity was observed between the peak current and concentration of ETV in the range of 0.51 ‐ 100 μM with a detection limit of 0.042 μM in DPV method, 2.1 – 61.1 μM with a detection limit of 0.7 μM in LSV method and 0.1 ‐ 80 μM with a detection limit of 68.1 nM in SWV method. The proposed sensitive DPV method was successfully applied for the determination ETV in tablets and biological samples.     

Lignosulfonate‐Modified Electrode for Electrocatalytic Reduction of Acidic Nitrite

Sodium lignosulfonate (LS) undergoes oxidative electropolymerization on a glassy carbon (GC) electrode from sulfuric acid solution to form a chemically modified electrode exhibiting anionic character and redox activity. Cyclic voltammetry reveals the existence of two redox systems at E°′ values of +0.29 and +0.53 V, respectively. Peak currents are proportional to the scan rate as expected for surface confined systems. The GC|poly‐LS electrode shows electrocatalytic activity toward the reduction of acidic nitrite. When operating in a constant potential amperometric mode (at 0.0 V, vs. Ag/AgCl), a linear relationship between nitrite concentration and reduction current is observed over the range of 1 to 250 μM. The detection limit reaches 0.3 μM (S/N=3). The electrode may be practically applied for nitrite determination in human saliva.     

Flower‐like Ni(II)‐based Metal‐organic Framework‐decorated Ag Nanoparticles: Fabrication,Characterization and Electrochemical Detection of Glucose

Glucose concentration monitoring is important for the prevention, diagnosis and treatment of diabetes. In this work, a composite material of AgNPs/MOF‐74(Ni) was prepared for electrochemical determination of glucose. AgNPs/MOF‐74(Ni) was characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X‐ray photoelectron spectroscopy (XPS). The electrochemical properties of the glassy carbon electrodes modified with the AgNPs/MOF‐74(Ni) composites were characterized by cyclic voltammetry (CV) and current‐time curve (I‐t curve) with three electrode system. The determination of glucose with the electrode modified by AgNPs/MOF‐74(Ni) has a linear range of 0.01～4 mM with the correlation coefficient (R²) of 0.994. The detection limit is 4.7 μM (S/N=3) and the sensitivity is 1.29 mA ? mM^?1 ? cm^?2. In addition, this sensing system possesses reasonable reproducibility and stability. The good performance of electrochemical determination for glucose is attributed to the concerted effect of silver nanoparticles and MOF‐74(Ni) on the promotion of glucose oxidation