Adsorptive Stripping Voltammetric Determination of 2,4-Dichlorophenol by Laponite Modified Carbon Paste Electrode

A laponite modified carbon paste electrode was prepared, characterized and applied for the 2,4-dichlorophenol (2,4-DCP) voltammetric determination. It takes advantage of the ability of laponite to adsorb phenols, as well as of its availability and very low cost. Kinetic and equilibrium data for 2,4-DCP adsorption by laponite in aqueous dispersions demonstrated that the adsorption process obeyed a pseudo first order kinetic model and was consistent with the formation of adsorbed multilayers on a surface with heterogeneous pore distribution. The composite paste electrode exhibited a heterogeneous surface with 65 % increased surface area and 27 % enhanced catalytic activity compared to the unmodified one. The adsorptive stripping voltammetric determination of 2,4-DCP at an electrode with an optimized graphite:laponite ratio of 55 : 15 w% using a 3 min accumulation time at pH 5.5 was found to be suitable for its quantification in the linear concentration range extended up to 50 μmol L⁻¹ with a sensitivity of 0.56 μA L μmol⁻¹ and a LOD of 0.2 μmol L⁻¹ (S/N=3).The 2,4-DCP electrochemical response was not affected by the presence of some structurally similar phenols, like catechol and p-nitrophenol, while resorcinol, 2-chlorophenol, and 4-chlorophenol presented interferences. The results were validated by 2,4-DCP determination in spiked tap water.     

Sensitive Voltammetric Determination of Methyl Parathion Using a Carbon Paste Electrode Modified with Mesoporous Zirconia

A mesoporous zirconia modified carbon paste electrode was developed for electrochemical investigations of methyl parathion (MP, Phen‐NO₂). The significant increase of the peak currents and the improvement of the redox peak potential indicate that mesoporous zirconia facilitates the electronic transfer of MP. The oxidation peak current was proportional to the MP concentration in the range from 1.0×10⁻⁸ to 1.0×10⁻⁵ mol L⁻¹ with a detection limit of 4.6×10⁻⁹ mol L⁻¹ (S/N=3) after accumulation under open‐circuit for 210 s. The proposed method was successfully applied to the determination of MP in apple samples.     

Preparation of Organophilic Nanohybrid Kaolinite and Application to the Electrochemical Detection of Organic Pesticide

Novel organophilic nanohybrid materials (K‐TDD) were obtained by the grafting of 1,2‐tetradecanediol (TDD) onto the surface of kaolinite (K). XRD, IR, TGA‐DTG, and SEM characterization showed that TDD grafting results in a partial exfoliation of kaolinite layers. This material was used to modify a glassy carbon electrode (GCE/K‐TDD) and applied for the trace analysis of methyl parathion (MP). The signal of MP recorded on GCE/K‐TDD was more intense compared to the unmodified GCE or to one modified with a film of natural kaolinite. Several parameters that can affect the stripping response were systematically investigated to optimize the sensitivity of the organokaolinite‐modified electrode. A linear calibration curve for MP was obtained in the concentration range from 2×10⁻⁶ to 14×10⁻⁶ mol .L⁻¹ in acetate buffer (pH 6), giving a detection limit of 9×10⁻⁸ mol .L⁻¹. The sensitivity of the method was found to be 2.42 μA/μM for the range of concentrations that gives a linear calibration curve. The electrode was shown to be very stable, with the electrochemical response of MP decreasing by only 1.5 % after a series of nine measurements. The interference of various inorganic ions and organic compounds likely to influence the stripping determination of the MP were also examined. The results showed that the GCE/K‐TDD electrode was effective in solutions containing interfering species and could be applied for the quantification of MP pesticide in natural water.     

Fabrication of β‐Cyclodextrin/Glycine/Carbon Nanotubes Electrochemical Neurotransmitters Sensor – Application in Ultra‐sensitive Determination of DOPAC in Human Serum

In this work, an electrochemical sensor was constructed by applying two successive thin layers of glycine‐carbon nanotubes mixture and β‐cyclodextrin (CNTs‐Gly)/CD over glassy carbon electrode surface for some neurotransmitters determination. A host‐guest interaction between CD and neurotransmitters molecules is expected and resulted in enhanced sensitivity, selectivity and stability of sensor response. Other components of the sensor are crucial for the unique electrochemical response. Carbon nanotubes allowed large surface area for glycine distribution that provided hydrogen bonding to CD moieties and contributed to facilitated charge transfer. It was possible to determine 3,4‐dihydroxy phenyl acetic acid (DOPAC) in the linear range of 0.1 μmol L⁻¹ to 80 μmol L⁻¹ with detection limit of 9.40 nmol L⁻¹, quantification limit of 31.5 nmol L⁻¹ and sensitivity of 4.16 μA/μmol L⁻¹. The proposed sensor was applied in synthetic cerebrospinal fluids samples using random standard addition method. Also, the proposed sensor was used to determine DOPAC in presence of common interferences and acceptable recovery results were achieved for its analysis in real blood serum. Figures of merit for (CNTs‐Gly)/CD composite in terms of precision, robustness, repeatability and reproducibility were reported.     

Voltammetric Detection of Captopril in a Commercial Drug Using a Gold-Copper Metal-organic Framework Nanocomposite Modified Electrode

We report the application of an electrochemical sensor based on gold-copper metal-organic framework immobilized on the surface of a glassy carbon electrode to the detection of captopril (CAP), an angiotensin-converting enzyme inhibitor. Cyclic voltammetric studies showed that the joint action of gold nanoparticles and copper-1,3,5-benzenetricarboxylate (Cu−BTC) enhanced the electrochemical response to the Cu-captopril complex that is adsorbed onto the surface of the electrode. Release of gold nanoparticles from Au@Cu−BTC not only increased the conductivity of the electrode but also provided a more favorable environment for the deposition of reduced Cu that is catalytically renewed on the electrode surface. The anodic current of the Cu^(II)−CAP oxidation peak varied linearly within two concentration ranges, namely 0.5 to 7.0 μmol L⁻¹ and 10 to 2500 μmol L⁻¹, with a limit of detection of 0.047 μmol L⁻¹. The mean recovery for the determination of captopril in commercial tablets was 100.3 % suggesting that the method has considerable potential for future industrial applications.     

Simultaneous Determination of Lipophilic Vitamin Esters Using Square-wave Voltammetry at the Glassy Carbon Electrode

Simple and rapid voltammetric method for simultaneous determination of all-trans-retinyl acetate (RAc) or all-trans-retinyl palmitate (RPa) and α-tocopheryl acetate (α-TOAc) has been proposed. The respective method was based on the anodic oxidation of the compounds of interest by square-wave voltammetry in acetone with 0.1 mol L⁻¹ LiClO₄ at the glassy carbon electrode. The procedure was also beneficial with respect to simple dissolution of sample directly in the supporting electrolyte. The all-trans-retinyl acetate could be quantified in two linear ranges (3.1–140 μmol L⁻¹ and 140–400 μmol L⁻¹) and α-tocopheryl acetate in linear range 5.3–400 μmol L⁻¹ with detection limits of 0.9 μmol L⁻¹ RAc (or 0.8 μmol L⁻¹ RPa) and of 1.6 μmol L⁻¹ α-TOAc. Selected commercial cosmetic products were analysed achieving satisfactory recoveries.     

Ionic silsesquioxane film immobilized on silica applied in the development of carbon paste electrode for determination of methyl parathion

A mesoporous silica-based hybrid material composed of silica xerogel modified with an ionic silsesquioxane, which contains the 1,4-diazoniabicyclo[2.2.2]octane chloride group, was obtained. The silsesquioxane film is highly dispersed on the surface. This hybrid material was utilized to develop a carbon paste electrode (CPE) for determination of methyl parathion. Transmission FTIR, elemental analysis and N₂ adsorption–desorption isotherms were used for characterization of the material. The electrochemical behavior of methyl parathion was evaluated by cyclic voltammetry and differential pulse voltammetry. It was observed a linear response to methyl parathion in the concentration range from 1.25 × 10^?7 to 2.56 × 10^?6 mol L^?1 by employing the carbon paste electrode, in Britton–Robinson buffer solution (pH 6). The achieved detection limit (3 SD of the blank divided by the slope of calibration curve) was 0.013 µmol L^?1 and sensitivity was 6.3 µA µmol L^?1. This result shows the potentiality of this electrode for application as electrochemical sensor for methyl parathion.     

Resolution of a Mononitrophenol Isomers Mixture by Differential Alternative Pulses Voltammetry

The high resolution second order voltammetric technique, Differential Alternative Pulse Voltammetry (DAPV), was applied for the simultaneous quantification of the mononitrophenol (NP) isomers. Complete resolution of the three isomers was achieved at concentration ratios as high as 1 : 5, employing the corresponding anodic and cathodic peaks appeared on the DAPV curve. The working glassy carbon electrode was modified by a nanocomposite of graphite nanopowder and Au nanoparticles to increase the sensitivity. The linear concentration range was found to be extended up to 125 μmol L^?1 for both the m‐NP and p‐NP isomers, while the o‐NP isomer presented calibration plot of two linear sections: up to 100 μmol L^?1 and up to 225 μmol L^?1. The LOD was found to be as low as 1.5 μmol L^?1, 2.5 μmol L^?1, and 0.5 μmol L^?1 for o‐NP, m‐NP, and p‐NP, respectively. The accuracy of the proposed method was evaluated by quantification of spiked tap waters samples. No interference was observed from a range of phenolic compounds such as phenol, 4‐aminophenol, 4‐chlorophenol, 2,4‐dinitrophenol, and resorcinol.     

Bi‐enzyme Electrochemical Sensor for Selective Determination of Organophosphorus Pesticides with Phenolic Leaving Groups

An amperometric bi‐enzyme sensor for detection of organophosphorus pesticides (OPs) with phenolic leaving groups, which are not electroactive, is presented in this work. The biosensing platform was created by a simple, controllable, and reproducible one‐step electrodeposition onto the surface of a glassy carbon electrode of a chitosan bionanocomposite with entrapped carboxylated multi walled carbon nanotubes, organophosphorus hydrolase (OPH), and horseradish peroxidase (HRP). The OPs determination involved a sequence of OPH and HRP‐catalyzed reactions resulting in phenolic radicals production, which were quantified by registering the current of their reduction at a potential of −50 mV vs. Ag, AgCl/KCl_sat.The developed sensor was applied for the determination of prothiofos, as an example. At optimized conditions (pH 7.25 and H₂O₂ concentration 200 μmol L⁻¹), a LOD as low as 0.8 μmol L⁻¹ was attained, while the linear concentration range was extended from 2.64 μmol L⁻¹ up to 35 μmol L⁻¹. The main advantage of the proposed bi‐enzyme sensor is its selectivity toward the OPs with phenolic leaving groups, excluding the interference of the nitrophenyl‐substituted OPs.     

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.     

Determination of Creatinine in Urine by Voltammetry and Glassy Carbon Electrode Modified with Functionalized Multi-walled Carbon Nanotubes and Copper

A square-wave voltammetric method was developed for the determination of creatinine using glassy carbon electrode functionalized multi-walled carbon nanotubes and doped with copper. Since the creatine-creatinine balance is dependent on pH, the linear responses at either pH 4.0 or 7.0 were evaluated allowing versatility in application, covering analyte concentration from limit of quantification (0.3 μmol L⁻¹) up to 1.200 μmol L⁻¹ (pH 4) or 660 μmol L⁻¹ (pH 7). Electrode is simple to prepare and robust for routine analysis and it was used for the determination of creatinine in urine after simple thin-layer separation procedure to avoid interference from creatine.     

Carbon‐nanotube Modified Screen‐printed Electrode for the Simultaneous Determination of Nitrite and Uric Acid in Biological Fluids Using Batch‐injection Amperometric Detection

A portable electroanalytical system applied for rapid and simultaneous determination of uric acid (UA) and nitrite (NIT) in human biological fluids (urine, saliva and blood) is reported. The system is based on batch‐injection analysis with multiple‐pulse amperometric (BIA‐MPA) detection using screen‐printed electrodes (SPEs) modified with multi‐walled carbon nanotubes. Sample dilution in optimized electrolyte (0.1 mol L⁻¹ Britton‐Robinson buffer pH 2) followed by injection of 100 μL on the electrode surface using an electronic micropipette is performed. UA is detected at +0.45 V and both UA+NIT at +0.70 V. Linear calibration plots for UA and NIT were obtained over the range of 1–500 μmol L⁻¹ with detection limits of 0.05 and 0.06 μmol L⁻¹, respectively. For comparison, a differential‐pulse voltammetric (DPV) method was optimized, and linear calibration plots for UA and NIT were obtained over range of 1–30 μmol L⁻¹ and 1–40 μmol L⁻¹ with detection limits of 0.1 and 0.3 μmol L⁻¹, respectively. BIA‐MPA is highly precise (RSD<1.3 %), fast (160 h⁻¹) and free from sample‐matrix interferences as recovery values ranged from 77 to 121 % for spiked samples (short contact time of sample aliquot with SPE). Contrarily, recovery tests conducted using DPV did not provide adequate recovery values (>150 %), probably due to the longer contact time of the SPE with the biological samples during analysis leading to a severe interference of sample matrices.     

AgNP-decorated SBA-15 for MWCNT Paste Modified Electrode: A Sensor for Simultaneous Voltammetric Determination of Paracetamol and Sulfamethoxazole

A new electrochemical sensor based on a carbon nanotube paste electrode modified with a Santa Barbara Amorphous material (SBA-15) decorated with silver nanoparticles, namely CNT/SBA/Ag-PE, was developed. It was successfully applied for individual and simultaneous determination of both paracetamol (PC) and sulfamethoxazole (SMZ) medicines. The electrode exhibited a linear dynamic range of 0.12–110 μmol L⁻¹ for paracetamol and 0.06–70 μmol L⁻¹ for sulfamethoxazole, and detection limits of 38 and 19 nmol L⁻¹, respectively. The proposed sensor offered high sensitivity, fast response time and the potential for detecting both drugs simultaneously. The CNT/SBA/Ag-PE enabled the simultaneous determination of PC and SMZ in urine samples with high recovery rates.     

Sensitive Determination of 17β-Estradiol using a Magneto Sensor Based on Magnetic Molecularly Imprinted Polymer

The analysis of 17β-estradiol with high sensitivity and selectivity is extremely relevant to control the impacts that this compound can cause on health and the environment. Thus, we describe the development and application of a magneto carbon paste electrode based on magnetic molecularly imprinted polymer (MCPE-MMIP) for determination of 17β-estradiol. The analyte adsorbed on the MMIP was immobilized on the electrode surface by magnetic capture. The morphological and structural characterization of the obtained MMIP suggests that the material was effectively synthesized. MCPE-MMIP showed an improvement in the sensitivity for 17β-estradiol detection when compared to electrode configurations in the absence of this material. The optimum conditions (0.10 mol L⁻¹ phosphate buffer pH 7.0) were reached by differential pulse adsorptive stripping voltammetry (DPAdSV), in which the method presented linearity ranged from 0.06 to 175 μmol L⁻¹ with limits of detection and quantification of 0.02 and 0.06 μmol L⁻¹, respectively. The proposed sensor was applied effectively in the analysis of 17β-estradiol in river water and raw milk samples, exhibiting excellent recovery values (between 96.20 and 104 %), which were confirmed by HPLC analysis.     

Development of Electrochemical Platform Based on Molecularly Imprinted Poly(methacrylic acid) Grafted on Iniferter-modified Carbon Nanotubes for 17β-Estradiol Determination in Water Samples

This paper describes the development of a new electrochemical sensor for 17β-estradiol (E2) determination based on glassy carbon electrode (GCE) modified with molecularly imprinted polymer grafted onto iniferter-multiwall carbon nanotubes surface (MIP-MWCNT) and dihexadecyl-hydrogen-phosphate (DHP). The electrochemical method was based on closed-circuit preconcentration of E2 in 0.1 mol L⁻¹ phosphate buffer (pH 7.0) during 500 s. Upon preconcentration, E2 was determined by differential pulse voltammetry (DPV) exhibiting a limit of detection of 0.01 μmol L⁻¹. The sensor exhibited higher selectivity toward E2 and it was applied for E2 determination in natural water samples, with accuracy attested by HPLC-DAD.     

Application of a Tetraamino Cobalt(II) Phthalocyanine Modified Screen Printed Carbon Electrode for the Sensitive Electrochemical Determination of L-Dopa in Pharmaceutical and Biological Samples

A novel synthesized tetraamino cobalt(II) phthalocyanine monomer was used for the fabrication of a sensor by electrochemical polymerization. A disposable electrochemical sensor based on the use of a screen printed carbon electrode covered with an electropolymerized film of tetraamino cobalt(II) phthalocyanine for the determination of L-dopa in pharmaceutical tablets and biological samples was described. Cyclic voltammetry and electrochemical impedance spectroscopy were performed for the characterization of the bare and modified electrode. For the electrochemical detection of L-dopa differential pulse voltammetry was used. The proposed method exhibits a good response towards electrooxidation of L-dopa in the linear concentration range: from 0.1 to 1000.0 μmol L⁻¹ in BRB pH=2.0, with a detection limit of 0.03 μmol L⁻¹ and from 1 to 1000 μmol L⁻¹ in PBS pH=7.4, with a detection limit of 0.33 μmol L⁻¹. Due to the fact that the developed sensor was applied in two different types of real samples, two buffer media were used, BRB pH=2.0 for pharmaceutical and urine samples and PBS pH=7.4 for whole blood samples. The proposed pCoTAPc/SPCE was successfully applied for the determination of L-dopa in pharmaceutical tablets, urine and in whole blood samples with satisfactory results.     

Electrochemical sensing platform based on the use of ZnONPs and MWCNTs as CPE modifiers for a selective and sensitive determination of polyamine spermine in the urine sample

A simple electroanalytical procedure has been developed for the determination of polyamine spermine using the ZnONPs-MWCNTs-CPE sensor. Spermine has been irreversibly oxidized on a modified electrode at a potential of +0.92 V versus Ag/AgCl (KCl, 3.5 mol L⁻¹). The developed sensor demonstrated a respective linear response in borate buffer pH 8.5 from 2 μmol L⁻¹ to 100 μmol L⁻¹, with a detection limit of 0.300 μmol L⁻¹ and quantification limit of 0.998 μmol L⁻¹. The proposed sensor showed high reproducibility (RSD = 2.58 %), stability, robustness, and no obvious interference effects of several inorganic ions and organic molecules. The obtained results demonstrated excellent performance during the determination of the spermine in human urine samples with satisfactory recovery results (98.41–101.34 %), offering promising opportunities for practical clinical analysis.     

Acetylcholinesterase Immobilization on 3‐Mercaptopropionic Acid Self Assembled Monolayer for Determination of Pesticides

Studies on the immobilization of acetylcholinesterase onto a SAM gold electrode and the use of the fabricated biosensor for the determination of carbaryl and parathion are presented. The influence of pH, ionic strength, enzyme loading and concentration of glutaraldehyde on the response of the biosensor was investigated . The amperometric biosensor developed in this study provided linearity to parathion and carbaryl in the 2.0 a 30.0×10^?6 mol L^?1 concentration range. The detection limits under the optimum working conditions were found to be 9.3 μg L^?1 for parathion and 9.0 μg L^?1 for carbaryl. The enzyme electrode was found to be stable for 7 days.     

Simple and Fast Determination of Warfarin in Pharmaceutical Samples Using Boron‐doped Diamond Electrode in BIA and FIA Systems with Multiple Pulse Amperometric Detection

This paper proposes the use of the boron‐doped diamond electrode (BDDE) in flow and batch injection analysis (FIA and BIA) systems with multiple‐pulse amperometric (MPA) detection for the determination of warfarin (WA) in pharmaceutical formulations. The electrochemical behavior of WA obtained by cyclic voltammetry (CV) in 0.1 mol L⁻¹ phosphate buffer shows an irreversible oxidation process at +1.0 V (vs Ag/AgCl). The MPA was based on the application of two sequential potential pulses as a function of time on BDDE: (1) for WA detection at +1.2 V/100 ms and; (2) for electrode surface cleaning at −0.2 V/200 ms. Both hydrodynamic systems (FIA‐MPA and BIA‐MPA) used for WA determination achieved high precision (with relative standard deviations around 2 %, n =10), wide linear range (2.0−400.0 μmol L⁻¹), low limits of detection (0.5 μmol L⁻¹) and good analytical frequency (94 h⁻¹ for FIA and 130 h⁻¹ for BIA). The WA determination made by the proposed methods was compared to the official spectrophotometric method. The FIA‐MPA and BIA‐MPA methods are simple and fast, being an attractive option for WA routine analysis in pharmaceutical industries.     

Electrochemical deposition of silicate–cetyltrimethylammonium bromide nanocomposite film on glassy carbon electrode for sensing of methyl parathion

A novel electrochemical sensor for methyl parathion based on silicate– cetyltrimethylammonium bromide nanocomposite film has been fabricated by electro-assisted deposition onto glassy carbon electrode in one-step via an electrochemical modulation of pH at the electrode/solution interface to promote controlled gelification of tetraethylorthosilicate sol, and was characterized with scanning electron microscopy, X-ray diffraction, and electrochemical impedance spectroscopy. The electrochemical sensing of methyl parathion on the film-modified electrode was investigated applying cyclic voltammetry and square wave voltammetry. Compared to the unmodified electrode, the shapes of the redox peaks were improved and the peak currents significantly increased. Experimental parameters such as deposition time, pH value, and accumulation conditions have been optimized. A linear relationship between the peak current and methyl parathion concentration was obtained in the range from 1.0 × 10⁻⁷ to 1.0 × 10⁻⁴ mol L⁻¹ with a detection limit of 1.04 × 10 ⁻⁸ mol L⁻¹ (S/N = 3) after accumulation at 0 V for 120 s. The film electrode shows great promise for determination of methyl parathion in real samples.