Electrochemical Sensor Based on Multi‐walled Carbon Nanotube/Gold Nanoparticle Modified Glassy Carbon Electrode for Detection of Estradiol in Environmental Samples

We report a rapid and simple method for sensing estradiol by electro‐oxidation on a multi‐walled carbon nanotube (MWCNT) and gold nanoparticle (AuNP) modified glassy carbon electrode (GCE). Compared with a bare GCE, AuNP/GCE and MWCNT/GCE, the composite modified GCE shows an enhanced response to estradiol in 0.1 M phosphate buffer solution. Experimental parameters, including pH and accumulation time for estradiol determination were optimised at AuNP/MWCNT/GCE. A pH of 7.0 was found to be optimum pH with an accumulation time of 5 minutes. Estradiol was determined by linear sweep voltammetry over a dynamic range up to 20 %mol L^?1 and the limit of detection was estimated to be 7.0×10^?8 mol L^?1. The sensor was successfully applied to estradiol determination in tap water and waste water.     

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.     

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.     

Electrochemical Behavior of 8‐Azaguanine at DNA Langmuir–Blodgett Modified Glassy Carbon Electrode and Its Analytical Application

A highly sensitive electrochemical biosensor for the detection of trace amounts of 8‐azaguanine has been designed. Double stranded (ds)DNA molecules are immobilized onto a glassy carbon electrode surface with Langmuir–Blodgett technique. The adsorptive voltammetric behaviors of 8‐azaguanine at DNA‐modified electrode were explored by means of cyclic voltammetry and square wave voltammetry. Compared with bare glassy carbon electrode (GCE), the Langmuir–Blodgett film modified electrode can greatly improve the measuring sensitivity of 8‐azaguanine. Under the optimum experimental conditions, the Langmuir–Blodgett film modified electrode in pH 3.0 Britton–Robinson buffer solutions shows a linear voltammetric response in the range of 5.0×10^?8 to 1.0×10^?5 mol L^?1 with detection limit 9.0×10^?9 mol L^?1. The method proposed was applied successfully for the determination of 8‐azaguanine in diluted human urine with wonderful satisfactory.     

The Mn‐zeolite/Graphite Modified Glassy Carbon Electrode: Fabrication,Characterization and Analytical Applications

In this work, low‐cost and environmentally friendly natural zeolite exchanged with Mn²⁺ cations was used for the first time to modify the glassy carbon electrode with the aim to obtain a fast and simple sensor for voltammetric determination of paracetamol (PAR). The Mn‐zeolite/graphite modified glassy carbon electrode (MnZG?GCE) was prepared by evaporation of solvent from dispersion of the zeolite/graphite mixture with the polymer in acetone. The electrochemical characteristics of MnZG?GCE were conducted by electrochemical impedance spectroscopy and cyclic voltammetry. Compared with graphite modified GCE (G?GCE), MnZG?GCE exhibited better electrochemical parameters, which confirms the superiority of applying zeolite in the proposed sensor. The optimization of the pH‐value of supporting electrolyte and instrumental parameters were carried out. The peak current was proportional to the concentration of PAR in a phosphate buffer saline of pH 6.0 in the range from 0.029 to 0.69 mg L^?1 (R=0.9997) with limit of detection of 8.8 μg L^?1. Finally, the proposed electrode was successfully applied to determine the paracetamol in pharmaceutical formulation and certified reference materials. The satisfactory recoveries, which ranged from 89.2 to 102.7 %, were obtained for all studied samples. It confirmed the attractiveness of relatively inexpensive, easy to fabricate and non‐toxic MnZG?GCE in determination of PAR in complicated matrixes.     

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.     

Evaluation of the Potentialities of a Carbon Nanotubes/Silicone Rubber Composite Electrode in the Determination of Hydrochlorothiazide

A multiwall carbon nanotube/silicone rubber (MWCNT/SR) composite electrode has been used for the determination of hydrochlorothiazide (HCTZ) in pharmaceutical formulations by differential pulse voltammetry (DPV). The electro-oxidation process was evaluated by cyclic voltammetry, from which it was observed that HCTZ presents an irreversible oxidation peak at 0.82 V vs. saturated calomel electrode (SCE) in the potential range from 0.5 to 1.1 V, in Britton-Robinson buffer pH 7.0 at MWCNT/SR. HCTZ was determined by DPV using a MWCNT/SR 70% (MWCNT, m/m) composite electrode after the optimization of the experimental parameters. The linear range was from 5.0 to 70.0 µ mol L^?1, with a limit of detection (LOD) of 2.6 µ mol L^?1. The HCTZ was determined in pharmaceutical formulations using the proposed composite electrode and the results agreed with those from the official high performance liquid chromatography (HPLC) method within 95% confidence level, according to the t-Student test.     

A Sensitive Sensor Based on Single‐walled Carbon Nanotubes: Its Preparation,Characterization and Application in the Electrochemical Determination of Drug Clorsulon in Milk Samples

Within this paper, a glassy carbon electrode modified with single‐walled carbon nanotubes (SWCNTs?GCE) was prepared, and employed for the determination of clorsulon (Clo), which is a frequently used veterinary drug against common liver fluke. The comprehensive topographical and electrochemical characterizations of bare GCE and SWCNTs?GCE were performed by atomic force microscopy, electrochemical impedance spectroscopy, and cyclic voltammetry. Significantly enhanced electrochemical characteristics of SWCNTs?GCE toward a ferrocyanide/ferricyanide redox couple was observed when compared to bare GCE. Further, the prepared sensor was applied for the voltammetric determination of Clo, which was electrochemically investigated for the first time in this work. Voltammetric experiments were performed using square‐wave voltammetry with optimized parameters in phosphate buffer solution, pH 6.8, which was selected as the most suitable medium for the determination of Clo. The corresponding current at approx. +1.1 V increased linearly with Clo concentration within two linear dynamic ranges of 0.75–4.00 μmol L^?1 (R²=0.9934) and 4.00–15.00 μmol L^?1 (R²=0.9942) with a sensitivity for the first calibration range of 0.76 μA L μmol^?1, a limit of detection of 0.19 μmol L^?1, and a limit of quantification of 0.64 μmol L^?1. The developed method was subsequently applied for quantitative analysis of Clo in milk samples with results proving high repeatability and recovery.     

Development of a Molecularly Imprinted Poly(Acrylic Acid)‐MWCNT Nanocomposite Electrochemical Sensor for Tramadol Determination in Pharmaceutical Samples

A new voltammetric sensor based on molecularly imprinted poly(acrylic acid)‐MWCNT nanocomposite (MIP‐MWCNT) drop‐coated onto glassy carbon electrode (GCE) was developed and applied to tramadol (TR) determination in pharmaceutical samples. The voltammetric sensor prepared by suspension of MIP‐MWCNT at 1 : 1 (w/w) ratio show an improved performance compared to unmodified GCE. The electrochemical method is based on preconcentration of tramadol onto MIP‐MWCNT modified GCE surface at ?1.5 V vs Ag/AgCl for 180 s in 0.1 Britton‐Robinson buffer (pH 8.0) at stirred solution. Upon preconcentration, the differential anodic voltammogram was recorded under the optimized condition giving rise to an analytical curve varying from 9.0 up to 30.0 μmol L^?1 (R²=0.997) and limits of detection and quantification of 1.4 and 4.8 μmol L^?1, respectively. The method precision was assessed in terms of intraday (n=6) and interday (two consecutive days) precision, giving relative standard deviations (RSD%) values between 2.8 to 7.4 %. Excipients usually found in pharmaceutical pills (magnesium stearate, microcrystalline cellulose, starch, and silica) and paracetamol were evaluated as potential interferents, however no interference was evidenced in TR determination. The method applicability was evaluated by TR analysis in pharmaceutical samples. Moreover, the method accuracy was attested by comparison of addition and recovery assays with a reference technique (high‐performance liquid chromatography).     

Electroanalytical Determination of Estrone in Seawater Samples Using Functionalized Multiwalled Carbon Nanotubes

In this work, a glassy carbon electrode (GCE) modified with multiwalled carbon nanotubes functionalized with carboxylic groups (MWCNT−COOH) was used to determine the hormone estrone in seawater samples. Modification of the electrode was optimized using three successive 10-μL aliquots of the MWCNT−COOH dispersion in ethanol (1 : 5 mL). The cyclic voltammetry results showed an oxidation peak at 0.59 V with characteristics of an irreversible process, pH dependent and controlled by adsorption of species. The results of square-wave voltammetry showed that the intensities of peak currents for the MWCNT−COOH/GCE were about 2.5 times higher than for GCE. The calibration curve showed a linearity of 0.9981 and a sensitivity of 0.1521 μA/mol L⁻¹. The limits of detection and quantification were 0.117 and 0.392 μmol L⁻¹, respectively. The recovery obtained using seawater samples was 91%, indicating the applicability of the method in marine environments.     

Sensitive Voltammetric Determination of Bisphenol A Based on a Glassy Carbon Electrode Modified with Copper Oxide‐Zinc Oxide Decorated on Graphene Oxide

A highly sensitive and selective chemical sensor was prepared based on metallic copper‐copper oxides and zinc oxide decorated graphene oxide modified glassy carbon electrode (Cu?Zn/GO/GCE) through an easily electrochemical method for the quantification of bisphenol A (BPA). The composite electrode was characterized via scanning electron microscopy (SEM), X‐Ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The electrochemical behavior of BPA in Britton‐Robinson (BR) buffer solution (pH 7.1) was examined using cyclic voltammetry (CV). Under optimized conditions, the square wave voltammetry (SWV) response of Cu?Zn/GO/GCE towards BPA indicates two linear relationships within concentrations (3.0 nmol L^?1?0.1 μmol L^?1 and 0.35 μmol L^?1?20.0 μmol L^?) and has a low detection limit (0.88 nmol L^?1). The proposed electrochemical sensor based on Cu?Zn/GO/GCE is both time and cost effective, has good reproducibility, high selectivity as well as stability for BPA determination. The developed composite electrode was used to detect BPA in various samples including baby feeding bottle, pacifier, water bottle and food storage container and satisfactory results were obtained with high recoveries.     

Analytical Potentialities of Carbon Nanotube/Silicone Rubber Composite Electrodes: Determination of Propranolol

A new composite electrode based on multiwall carbon nanotubes (MWCNT) and silicone‐rubber (SR) was developed and applied to the determination of propranolol in pharmaceutical formulations. The effect of using MWCNT/graphite mixtures in different proportions was also investigated. Cyclic voltammetry and electrochemical impedance spectroscopy were used for electrochemical characterization of different electrode compositions. Propranolol was determined using MWCNT/SR 70 % (m/m) electrodes with linear dynamic ranges up to 7.0 µmol L^?1 by differential pulse and up to 5.4 µmol L^?1 by square wave voltammetry, with LODs of 0.12 and 0.078 µmol L^?1, respectively. Analysis of commercial samples agreed with that obtained by the official spectrophotometric method. The electrode is mechanically robust and presented reproducible results and a long useful life.     

Differential pulse voltammetric determination of benznidazole loaded in nanostructured lipid carriers and urine**

Benznidazole (BZN) is the first-choice drug for treating Chagas disease (CD). However, it is not ideal for this purpose as it is highly toxic and has irregular pharmacokinetics due to factors such as its low aqueous solubility. These factors necessitate the development of reliable and effective alternative methods for the analytical determination of BZN in biological and pharmaceutical samples. In this context, we present a new electroanalytical method for quantifying BZN using differential pulse voltammetry (DPV) and a glassy carbon electrode (GCE). This method was applied to urine and a pharmaceutical formulation of BZN incorporated into nanostructured lipid carriers (NLC-BZN). The proposed method provided a linear analytical range of 1.00–10.6 μmol L⁻¹ (R=0.999), with a detection limit of 0.044 μmol L⁻¹ and a quantification limit of 0.13 μmol L⁻¹. The relative standard deviation of the intra-day and inter-day precision was below 2.50 %. Through interference studies, the methodology proved to be selective for BZN, because there was no significant potential interference in any of the samples. The recovery tests showed that the accuracy was within the limits recommended in the literature. Therefore, the developed DPV/GCE method can be successfully applied as an alternative method for detecting BZN in NLC-BZN pharmaceutical formulations and human urine.     

Glassy Carbon Electrode Modified with Layering of Carbon Black/Poly(Allylamine Hydrochloride) Composite for Multianalyte Determination

An electrochemical sensor using glassy carbon electrode modified with carbon black within a poly(allylamine hydrochloride) film is proposed in this work. The novel sensor was characterized by scanning electron microscopy, electrochemical impedance spectroscopy, and cyclic voltammetry using the redox probe Fe(CN)6^3−/4−. The sensor was applied for the simultaneous determination of dopamine (DA), paracetamol (PAR), amlodipine (AML), and rosuvastatin (RSV). The quantification of all four analytes was carried out by linear sweep voltammetry and presented a linear concentration range for all analytes from 1.0 to 90 μmol L⁻¹, with limit of detection of 0.55, 1.3, 5.7, and 3.0 μmol L⁻¹ for DA, PAR, AML, and RSV, respectively. This sensor was successfully applied in the simultaneous determination of these analytes in environmental, pharmaceutical, and biological samples.     

Alumina Polished Glassy Carbon Electrode as a Simple Electrode for Lower Potential Electrochemical Detection of Dopamine in its Sub‐micromolar Level

The electrochemical behaviour of dopamine (DA) at a cleaned and alumina polished glassy carbon electrode (GCE) was studied using cyclic voltammetry (CV). The CV studies revealed that alumina polished GCE (AGCE) shows an enhanced oxidation peak current response with 217 mV negative potential shift towards DA than that of cleaned GCE. The differential pulse voltammetry result shows that the AGCE detects the DA in the linear concentration ranges from 0.15 to 25.25 µmol L^?1. The limit of detection was calculated as 0.046 µmol L^?1 with a sensitivity of 3.74 µA µmol L^?1 cm^?2 for the determination of DA. The fabricated AGCE shows a satisfactory selectivity, practicality along with appreciable repeatability and reproducibility.     

Simple and rapid determination of clonidine in pharmaceutical samples by voltammetry using a bare glassy carbon electrode

This work presents, for the first time, the voltammetric behavior of clonidine (CLO) drug and its determination, using an unmodified glassy carbon electrode (GCE). CLO exhibited only an irreversible oxidation process on the GCE, with peak potential at +0.85 V in pH 12 (vs Ag/AgCl). CLO oxidation process is pH-dependent and the electrochemical mechanisms on the GCE were proposed in acidic and basic medium. The determination of CLO was optimized in 0.1 mol L⁻¹ phosphate buffer solution at pH 12.0 using differential pulse voltammetry (DPV), which provides a good linear range (0.65 to 106.00 μmol L⁻¹) and low theoretical limit of detection (0.14 μmol L⁻¹) for the quality control of this drug in pharmaceutical samples. In addition, stable responses of CLO at the GCE were obtained in the same day (RSD = 3.4 %; n = 5) and different days (RSD = 2.0 %; n = 3). Moreover, the determination of CLO in a pharmaceutical formulation using the proposed GCE-DPV method presented good accuracy, since the recovery was close to 100 % and the dosing result was in agreement with an official method (HPLC-UV). The proposed method demonstrates a good analytical performance for CLO determination in pharmaceutical samples, providing a faster, simpler and lower-cost alternative for quality control of CLO than other reported methods.     

Poly(Taurine‐Glutathione)/Carbon Nanotube Modified Glassy Carbon Electrode as a New Levofloxacin Sensor

A new highly sensitive and selective electrochemical levofloxacin sensor based on co‐polymer‐carbon nanotube composite electrode was developed. Taurine and Glutathione were electrochemically co‐polymerized on multiwalled carbon nanotubes modified glassy carbon electrode (Poly(TAU‐GSH)/CNT/GCE) and used as a levofloxacin sensor in pH 6 phosphate buffer solution. The new composite electrode surfaces were characterized by scanning electron microscopy, atomic force microscopy and electrochemical impedance spectroscopy. Under the optimized conditions, two linear segments were obtained for increasing LEV concentrations between 20 nmol L^?1‐1 μmol L^?1 and 1.5 μmol L^?1‐55 μmol L^?1 LEV with a detection limit of 9 nmol L^?1 using amperometry. Poly(TAU‐GSH)/CNT/GCE exhibited high sensitivity, selectivity with good stability. The new sensor was employed for real samples of LEV tablets and urine. Promising results were obtained with good accuracy which were also in accordance with LC‐MS/MS analysis.     

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.     

Functionalised multi-walled carbon nanotubes-modified electrode for sensitive determination of Diuron in seawater samples

ABSTRACT

This work was focused in to develop an electroanalytical method based on a direct modification of a glassy carbon electrode (GCE) by the deposition of successive aliquots of diluted dispersions of functionalised carbon nanotubes (MWCNT-COOH) in ethanol (0.1 mg.mL^?1) aiming the determination of Diuron into seawater samples, a common antifouling substance, using differential pulse voltammetry as electroanalytical technique. The GCE/MWCNT-COOH showed a sensitivity of 2.20 μA/μmol L^?1 about 10 times higher than the unmodified counterpart (GCE) which showed a sensitivity of 0.192 μA/μmol L^?1. The limits of detection and quantificationwere 6.88 × 10^?8 and 2.29 × 10^?7 mol L^?1 for GCE/MWCNT-COOH while for GCE were 7.87 × 10^?7 and 2.62 × 10^?6 mol L^?1, respectively. The applicability was evaluated with spiked detectable amounts of Diuron into seawater samples. The recovery results were between 76% and 119%.     

Determination of Chlorpromazine Hydrochloride with a Layered Double Hydroxide Modified Glassy Carbon Electrode as a Nanocatalyst

We report a simple and sensitive voltammetric sensor for the determination of chlorpromazine (CPZ) based on Ni?Al layered double hydroxide (NiAlLDH) modified glassy carbon electrode (GCE). NiAlLDH was simply electrodeposited on GCE surface in a very short time. The response linear range was 1×10^?3–1×10^?9 mol L^?1, with a detection limit of 1×10^?9 mol L^?1. The NiAlLDH film showed well defined and well separate peaks for dopamine, ascorbic acid, uric acid and CPZ in the same solution. The proposed electrode was used to measure the active pharmaceutical ingredient of CPZ tablet as a real sample.