Amperometric determination of the insecticide fipronil using batch injection analysis: comparison between unmodified and carbon-nanotube-modified electrodes

The electrochemical oxidation of fipronil is investigated on unmodified and multi-walled carbon-nanotube (MWCNT)-modified glassy carbon electrodes (GCEs), and its amperometric determination using batch injection analysis (BIA) is demonstrated. An oxidation peak was observed at 1.5 V in a 0.1 mol L^?1 HClO₄/acetone solution (50:50, v/v) on both surfaces. Although MWCNT-modified GCE provided greater sensitivity, the unmodified GCE showed low RSD value, wider linear range, and reduced adsorption of fipronil or its oxidized products on the electrode surface. A detection limit of 4.7 μmol L^?1 and linear range of 25–300 μmol L^?1 were obtained using a bare GCE. The method was applied in veterinary formulations with results in agreement with those obtained by high-performance liquid chromatography.     

Evaluation of a novel composite based on functionalized multi-walled carbon nanotube and iron phthalocyanine for electroanalytical determination of isoniazid

A novel platform for electroanalysis of isoniazid based on graphene-functionalized multi-walled carbon nanotube as support for iron phthalocyanine (FePc/f-MWCNT) has been developed. The FePc/f-MWCNT composite has been dropped on glassy carbon forming FePc/f-MWCNT/GC electrode, which is sensible for isoniazid, decreasing substantially its oxidation potential to +200 mV vs Ag/AgCl. Electrochemical and electroanalytical properties of the FePc/f-MWCNT/GC-modified electrode were investigated by cyclic voltammetry, electrochemical impedance spectroscopy, scanning electrochemical microscopy, and amperometry. The sensor presents better performance in 0.1 mol L^?1 phosphate buffer at pH 7.4. Under optimized conditions, a linear response range from 5 to 476 μmol L^?1 was obtained with a limit of detection and sensitivity of 0.56 μmol L^?1 and 0.023 μA L μmol^?1, respectively. The relative standard deviation for 10 determinations of 100 μmol L^?1 isoniazid was 2.5%. The sensor was successfully applied for isoniazid selective determination in simulated body fluids.     

Highly sensitive electrocatalytic determination of pyrazinamide using a modified poly(glycine) glassy carbon electrode by square-wave voltammetry

A new voltammetric sensor based on electropolymerization of glycine at glassy carbon electrode (GCE) was developed and applied to determine of pyrazinamide (PZA) by square-wave voltammetry (SWV). The initial cyclic voltammetric studies showed an electrocatalytic activity of poly(Gly)/GCE on redox system of pyrazinamide in 0.1 mol L^?1 phosphate buffer solution pH 7.5, with E _Pc and E _Pa in ?0.85 and ?0.8 V (versus E _Ag/AgCl), respectively. Studies at different scan rates suggest that the redox system of pyrazinamide at poly(Gly)/GCE is a process controlled by diffusion in the interval from 10 to 100 mV s^?1. Square-wave voltammetry-optimized conditions showed a linear response of PZA concentrations in the range from 0.47 to 6.15 μmol L^?1 (R?=?0.998) with a limit of detection (LOD) of 0.035 μmol L^?1 and a limit of quantification (LOQ) of 0.12 μmol L^?1. The developed SWV-poly(Gly)/GCE method provided a good intra-day (RSD?=?3.75 %) and inter-day repeatability (RSD?=?4.96 %) at 4.06 μmol L^?1 PZA (n?=?10). No interference of matrix of real samples was observed in the voltammetric response of PZA, and the method was considered to be highly selective for the compound. In the accuracy test, the recovery was found in the range of 98.2 and 104.0 % for human urine samples and pharmaceutical formulation (tablets). The PZA quantification results in pharmaceutical tablets obtained by the proposed SWV-poly(Gly)/GCE method were comparable to those found by official analytical protocols.     

Electrochemical sensing of <Emphasis Type="SmallCaps">D</Emphasis>-penicillamine on modified glassy carbon electrode by using a nanocomposite of gold nanoparticles and reduced graphene oxide

A new electrochemical sensor was developed for determination of D-penicillamine using glassy carbon electrode which had been modified by gold nanoparticles–reduced graphene oxide nanocomposite (AuNPs/RGO/GCE) in aqueous solution. Cyclic voltammetry, transmission electron microscopy and electrochemical impedance spectroscopy were used for characterization of the modified electrode. The results indicated that the kinetic of oxidation reaction of D-penicillamine at the surface of the electrode was controlled by both diffusion and adsorption processes. In 0.1 mol L^?1 phosphate buffer (pH 2.0), the oxidation current increased linearly with concentration of D-penicillamine with a linear range of 5.0 × 10^?6 to 1.1 × 10^?4 mol L^?1 and regression coefficient of R ² = 0.9972. Theoretical detection limit, defined based on 3σ of the blank signal (n = 9) divided by the slope of the linear regression equation, was 3.9 × 10^?6 mol L^?1 D-penicillamine using differential pulse voltammetry. The developed method was successfully applied to the determination of D-penicillamine in pharmaceutical formulation and blood serum samples.     

Synthesis of flower-like cuo hierarchical nanostructures as an electrochemical platform for glucose sensing

Flower-like CuO hierarchical nanostructures were synthesized on copper foil substrate through a simple wet chemical route in alkaline media at room temperature. SEM images collected at different reaction times revealed the transformation of initially formed Cu(OH)₂ nanowires to flower-like CuO nanostructures. The hierarchical structure of the as-prepared CuO showed high electrocatalytic activity towards the oxidation of glucose making it a promising electrode material for the development of non-enzymatic glucose sensor. The amperometric sensor exhibited a wide linear response to glucose ranging from 4.5 × 10^?5 to 1.3 × 10^?3 mol L^?1 (R ² = 0.99317) at fixed potential of 0.3 V. The detection limit was 6.9 × 10^?6 mol L^?1 (LOD = 3σ/s) with a sensitivity of 1.71 μA μmol^?1 cm^?2. Moreover, the developed sensor offers a fast amperometric response, good selectivity and stability.     

Carbon ceramic electrodes modified with mixed oxides SiO<Subscript>2</Subscript>/SnO<Subscript>2</Subscript> for determination of levofloxacin

The preparation of a carbon ceramic electrode modified with SnO₂ (CCE/SnO₂) using tin dibutyl diacetate as precursor was optimized by a 2³ factorial design. The factors analyzed were catalyst (HCl), graphite/organic precursor ratio, and inorganic precursor (dibutyltin diacetate). The statistical treatment of the data showed that only the second-order interaction effect, catalyst × inorganic precursor, was significant at 95% confidence level, for the electrochemical response of the system. The obtained material was characterized by scanning electron microscopy (MEV), X-ray diffraction (XRD), RAMAN spectroscopy, XPS spectra, and voltammetric techniques. From the XPS spectra, it was confirmed the formation of the Si–O–Sn bond by the shift in the binding energy values referred to Sn 3d3/2 due to the interaction of Sn with SiOH species. The incorporation of SnO₂ provided an increment of the electrode response for levofloxacin, with Ipa = 147.0 μA for the ECC and Ipa = 228.8 μA for ECC/SnO₂, indicating that SnO₂ when incorporated into the silica network enhances the electron transfer process. Under the optimized working conditions, the peak current increased linearly with the levofloxacin concentration in the range from 6.21×10^?5 to 6.97×10^?4 mol L^?1 with quantification and detection limits of 3.80×10^?5 mol L^?1 (14.07 mg L^?1) and 1.13×10^?5 mol L^?1 (4.18 mg L^?1), respectively.     

Sensitive determination of the endocrine disruptor bisphenol A at ultrathin film based on nanostructured hybrid material SiO<Subscript>2</Subscript>/GO/AgNP

In this work, we described an electrochemical sensor using a nanocomposite based on graphene oxide (GO), silver nanoparticles (AgNP), and disordered mesoporous silica (SiO₂), which was used for the determination of bisphenol A in water samples. Initially, the hybrid material SiO₂/GO was synthesized via sol-gel process, subsequently decorated with AgNP with an approximate 20 nm particle size prepared directly on the surface of the SiO₂/GO using N, N-dimethylformamide (DMF) as an agent reducer. A glassy carbon electrode was modified with SiO₂/GO/AgNP and used in developing a sensitive electrochemical sensor for the determination of bisphenol A in phosphate buffer 0.1 mol L^?1 (pH 7.0). The detection limit was 45.2 nmol L^?1 with a linear response range between 1.0 × 10^?7 and 2.6 × 10^?6 mol L^?1 and a sensitivity of 1.27 × 10^?7 A mol^?1 L. Finally, the optimized electrochemical sensor was used for the quantitation of endocrine interfering in natural waters.     

Application of a sensitive nanocomposite-based electrochemical sensor for voltammetric determination of dicyclomine hydrochloride in real samples

In the present study a glassy carbon electrode, modified with nanocomposite of gold nanoparticles/multiwalled carbon nanotubes (GNPs/MWCNTs/GCE), was used for determination of dicyclomine hydrochloride (DcCl). The results showed that synergetic effects of GNPs and MWCNTs highly improved electrochemical response and sensitivity of the sensor. The electrochemical oxidation of DcCl was investigated by cyclic voltammetry and differential pulse voltammetry. Also, scanning electron microscopy and energy dispersive x-ray spectroscopy were used to evaluate microstructure of electrochemical sensor. The effect of various experimental parameters including pH and scan rate on the voltammetric response of DcCl were investigated. Under the optimal conditions linear response was observed in range of 1.0–1.2 × 10² µmol L^?1 for DcCl. The lower detection limit was found to be 0.40 µmol L^?1 for DcCl. The investigated method showed good stability, reproducibility and repeatability. The proposed sensor was successfully applied to the determination of DcCl in real samples.     

Electrochemical oxidation behavior of 8-azaguanine at graphene-Nafion composite film-modified glassy carbon electrode

A simple but highly sensitive electrochemical sensor for the determination of 8-azaguanine based on graphene-Nafion nanocomposite film-modified glassy carbon electrode (G-Nafion/GCE) was reported. The electrochemical behaviors of 8-azaguanine at G-Nafion/GCE were investigated by cyclic voltammetry (CV), square wave voltammetry (SWV), chronoamperometry (CA), and chronocoulometry (CC). The results showed that the electrochemical sensor exhibited excellent electrocatalytic activity to 8-azaguanine. 8-Azaguanine can be effectively accumulated at G-Nafion/GCE and produce a sensitive anodic peak, due to the synergetic functions of graphene and Nafion. Under the selected conditions, the modified electrode in pH 1.98 Britton-Robinson buffer solution showed a linear voltammetric response to 8-azaguanine within the concentration range of 5.0 × 10^?8～3.0 × 10^?5 mol L^?1, with the detection limit of 1.0 × 10^?8 mol L^?1. And, the method was also applied to detect 8-azaguanine in spiked human urine with wonderful satisfactory results.     

Determination of allura red using composites of water-dispersible reduced graphene oxide-loaded Au nanoparticles based on ionic liquid

A facile route for producing reduced graphene oxide (RGO)-loaded Au nanoparticles based on ionic liquids (IL) has been proposed, in which the as-prepared RGO can be dispersed stably in water. With the assistance of IL, Au nanoparticles were uniformly and densely absorbed on the surfaces of the IL functionalised reduced graphene oxide (IRGO), forming a new composite of IRGO/Au with high dispersibility. This IRGO/Au composite enhanced its electrochemical signal obviously in the measurement of allura red in foods and exhibited a wider linear response ranging from 0.297 (0.0006 μmol L^?1) to 99.3 μg L^?1 (0.2 μmol L^?1) with lower detection limit of 0.213 μg L^?1 (0.00043 μmol L^?1) at a signal-to-noise ratio of 3. To further study the practical applicability of the proposed sensor, the modified electrode was successfully applied to detect allura red in five kinds of common foods and the assay results were in a good agreement with the reference values detected by HPLC.     

Sensitive detection of sulfanilamide by redox process electroanalysis of oxidation products formed in situ on glassy carbon electrode

This paper reported a simple method for sulfanilamide determination by redox process electroanalysis of oxidation products (SFDox) formed in situ on glassy carbon electrode. The CV experiments showed a reversible process after applied E _acc = + 1.06 V and t _acc = 1 s, in 0.1 mol L^?1 BRBS (pH = 2.0) at 50 mV s^?1. Different voltammetric scan rates (from 10 to 450 mV s^?1) suggested that the redox peaks of SFDox on the glassy carbon electrode (GCE) is an adsorption-controlled process. Square-wave voltammetry (SWV) method optimized conditions showed a linear response to SFD from 3.00 to 250.0 μmol L^?1 (R = 0.998) with a limit of detection of 0.638 μmol L^?1 and limit of quantification of 2.0 μmol L^?1. The developed the SWV method was successfully used in the determination of SFD pharmaceutical formulation and human serum. The SFD quantification results in pharmaceutical obtained by SWV-GCE were comparable to those found by official analytical protocols.     

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.     

A new sensor architecture based on carbon Printex 6L to the electrochemical determination of ranitidine

A glassy carbon electrode (GCE) modified with carbon Printex 6L (Printex6L/GCE) as a novel sensor is proposed. A morphological study was carried out using scanning electron microscopy, and an electrochemical characterization of the proposed electrode was performed by cyclic voltammetry (CV) using [Fe(CN)₆]^4? as a redox probe. With the incorporation of the carbon Printex 6L film onto the GCE surface, the [Fe(CN)₆]^4? analytical signal was substantially increased and the difference between the oxidation and reduction potentials (ΔE _p) decreased, a characteristic of the electrocatalytic effect. Furthermore, the use of carbon Printex 6L film resulted in an 84 % increase in the oxidation current and a 123 % increase in the reduction current. Faster charge transfer was observed at the proposed electrode/electrolyte interface during CV when compared with GCE. The Printex6L/GCE was tested for ranitidine (RNT) sensing and showed a decrease in the working potential and an increase in the analytical signal, when compared with GCE, again demonstrating an electrocatalytic effect. Under optimized experimental conditions, the developed square-wave adsorptive anodic stripping voltammetry (SWAdASV) method presented an analytical curve that was linear in RNT concentration range from 1.98 × 10^?6 to 2.88 × 10^?5 mol L^?1 with a detection limit of 2.44 × 10^?7 mol L^?1. The developed Printex6L/GCE was successfully applied to the determination of RNT concentrations in human body fluid samples (urine and serum).     

Fabrication of a Sensitive Electrochemical Biosensor for Detection of DNA Hybridization Based on Gold Nanoparticles/CuO Nanospindles Modified Glassy Carbon Electrode

In this work, a sensitive electrochemical DNA biosensor for the detection of sequence‐specific target DNA was reported. Firstly, CuO nanospindles (CuO NS) were immobilized on the surface of a glassy carbon electrode (GCE). Subsequently, gold nanoparticles (Au NPs) were introduced to the surface of CuO NS by the electrochemical deposition mode. Probe DNA with SH (HS‐DNA) at the 5′‐phosphate end was covalently immobilized on the surface of the Au NPs through Au? S bond. Scanning electron microscopy (SEM) was used to elucidate the morphology of the assembled film, and electrochemical impedance spectroscopy technique (EIS) was used to investigate the DNA sensor assembly process. Hybridization detection of DNA was performed with differential pulse voltammetry (DPV) and the methylene blue (MB) was hybridization indicator. Under the optimal conditions, the decline of reduction peak current of MB (ΔI) was linear with the logarithm of the concentration of complementary DNA from 1.0×10^?13 to 1.0×10^?6 mol·L^?1 with a detection limit of 3.5×10^?14 mol·L^?1 (S/N=3). In addition, this DNA biosensor has good selectivity, and even can distinguish single‐mismatched target DNA.     

Voltammetric Determination of Methylparaban in Cosmetics Using a Multi-Wall Carbon Nanotubes/Nafion Composite Modified Glassy Carbon Electrode

A novel voltammetric sensor using multi-wall carbon nanotubes (MWNTs) coupled with Nafion modified glassy carbon electrode (GCE) was developed for the detection of methylparaben (MP). The sensor exhibited good electrocatalytic activity toward the oxidation of MP in the phosphate buffer solution (PBS, pH 6.5). It displayed good sensitivity, repeatability, reproducibility, and long-term stability. Under the optimized conditions, the anodic peak current was linear with the concentration of MP in the range of 3 × 10^?6 mol L^?1 to 1 × 10^?4 mol L^?1. The detection limit was 1 × 10^?6 mol L^?1. The proposed method was successfully applied to determine MP in cosmetics with satisfactory results.     

Fabrication of a nanostructured TiO2/carbon nanotube composite electrode for voltammetric and impedimetric determination of NTO explosive in the water and soil samples

An electrochemical oxidation route was developed for sensitive and selective assay of nitrotriazolone (NTO) explosive in some environmental samples on a multi-walled carbon nanotube (MWCNTs)/TiO₂ nanocomposite paste electrode, for prevention of the analytical interference of conventional reducible energetic compounds. Detailed evaluations were made for the electrochemical behaviour of NTO on the modified electrode by adsorptive stripping voltammetry, electrochemical impedance spectroscopy (EIS) and chronoamperometry techniques in the pH range of 2.0–10.0. Parameters such as diffusion coefficient constant of NTO were calculated, and various experimental conditions were also optimised. Under optimal conditions the calibration curve had two linear dynamic ranges of 130.0–3251.5 μg L^?1 and 6.5–26.0 mg L^?1 with a detection limit of 26.0 μg L^?1 (0.2 μmol L^?1) and precision of <3%. This electrochemical sensor was further applied to determine NTO in real soil and water samples with satisfactory results.     

Exfoliated graphite nanoplatelets and gold nanoparticles based electrochemical sensor for determination of levodopa

This paper describes a rapid, accurate, and sensitive method for the determination of levodopa in a pharmaceutical sample using a glassy carbon electrode modified with a hybrid nanocomposite constituted of exfoliated graphite nanoplatelets dispersed in a suspension of gold nanoparticles in carboxymethylcelullose (AuNP-CMC-xGnP/GCE). The nanocomposite was characterized by scanning electron microscopy, transmission electron microscopy, UV-Vis spectroscopy, and zeta potential. Electrochemical characterization of the proposed sensor by cyclic voltammetry and electrochemical impedance spectroscopy indicated that the nanocomposite used for the electrode modification facilitated electron transfer. Using square-wave voltammetry (SWV) under optimized conditions (0.50% (m/v) of AuNP-CMC-xGnP, 0.1 mol L^?1 sulfuric acid, frequency 30 Hz, pulse amplitude 50 mV, and scan increment 6.0 mV), the calibration curve showed a linear range for levodopa from 5 to 50 μmol L^?1, with a limit of detection of 0.5 μmol L^?1. The sensor demonstrated good repeatability and electrode-to-electrode repeatability, with relative standard deviations of 2 and 4%, respectively. The proposed method was successfully applied to quantify levodopa in a pharmaceutical sample by SWV, showing good accuracy. Recoveries of 98 to 107% demonstrated that the method is suitable for practical applications. Therefore, the proposed sensor represents a useful tool for rapid and accurate determination of levodopa.     

Construction of a biointerface for glucose oxidase through diazonium chemistry and electrostatic self-assembly technique

In this study, a new procedure for the fabrication of biosensors was developed. The method is based on the covalent attachment of nitrophenyl groups to the electrode surface via diazonium salt reaction followed by their conversion to amine moieties through electrochemical reduction and electrostatic layer-by-layer (LbL) assembly technique. In this procedure, highly stable iron oxide (Fe₃O₄) nanoparticles (IONPs), chitosan (CHIt), GOx, and Nile blue (NB) were assembled on the surface of aminophenyl modified glassy carbon electrode (AP/GCE) by LbL assembly technique. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the interfaces. The surface coverage of the active GOx and Michaelis–Menten constant (K _M) of the immobilized GOx were Γ?=?3.38?×?10^?11 mol cm^?2 and 2.54 mM, respectively. The developed biosensor displayed a well-defined amperometric response for glucose determination with high sensitivity (8.07 μA mM^?1) and low limit of detection (LOD) of 19.0 μM. The proposed approach allows simple biointerface regeneration by increasing pH which causes disruption of the ionic interactions and release of the electrostatic attached layers. The biosensor can then be reconstructed again using fresh enzyme. Simple preparation, good chemical and mechanical stabilities, and easy surface renewal are remarkable advantages of the proposed biosensor fabrication procedure.     

Electrochemical sensor for determination of aflatoxin B1 based on multiwalled carbon nanotubes-supported Au/Pt bimetallic nanoparticles

A sensitive and selective imprinted electrochemical sensor for the determination of aflatoxin B₁ (AFB₁) was constructed on a glassy carbon electrode by stepwise modification of functional multiwalled carbon nanotubes (MCNTs), Au/Pt bimetallic nanoparticles (Au/PtNPs), and a thin imprinted film. The fabrication of a homogeneous porous poly o-phenylenediamine (POPD)-grafted Au/Pt bimetallic multiwalled carbon nanotubes nanocomposite film was conducted by controllable electrodepositing technology. The sensitivity of the sensor was improved greatly because of the nanocomposite functional layer; the proposed sensor exhibited excellent selectivity toward AFB₁ owing to the porous molecular imprinted polymer (MIP) film. The surface morphologies of the modified electrodes were characterized using a scanning electron microscope. The performance of the imprinted sensor was investigated by cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy in detail. A linear relationship between the sensor response signal and the logarithm of AFB₁ concentrations ranging from 1?×?10^?10 to 1?×?10^?5 mol L^?1 was obtained with a detection limit of 0.03 nmol L^?1. It was applied to detect AFB₁ in hogwash oil successfully.     

Preparation of Molecularly Imprinted Electrochemical Sensor for Detection of Vincristine Based on Reduced Graphene Oxide/Gold Nanoparticle Composite Film

An innovative molecularly imprinted electrochemical sensor was fabricated based on reduced graphene oxide (RGO) and gold nanocomposite (Au) for rapid detection of vincristine (VCR). The RGO‐Au composite membrane was obtained via direct one‐step electrodeposition technique of graphene oxide (GO) and chloroauric acid (HAuCl₄) on the surface of a glassy carbon electrode (GCE) by means of cyclic voltammetry (CV) in the potential range between ?1.5 and 0.6 V in phosphate buffer solution (PBS) of pH 9.18, which is capable of effectively utilizing its superior electrical conductivity, larger specific surface area due to its synergistic effect between RGO and Au. The molecularly imprinted polymers (MIPs) were synthesized on the RGO‐Au modified glassy carbon electrode surface with VCR as the template molecular, methyl acrylic acid (MAA) as the functional monomer, and ethylene glycol maleic rosinate acrylate (EGMRA) as a cross‐linker. The performance of the sensor was investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) in detail. Under the optimum conditions, the fabricated sensor exhibited a linear relationship between oxidation peak current and VCR concentration over the range of 5.0×10^?8–5.0×10^?6 mol·L ^minus;1 with a correlation coefficient of 0.9952 and a detection limit (S/N=3) of 2.6×10^minus;8 mol·L^minus;1. The results indicated that the imprinted polymer films exhibited an excellent selectivity for VCR. The imprinted sensor was successfully used to determine VCR in real samples with recoveries of 90% –120% by using the standard addition method.