Solidified floating organic drop microextraction for determination of trace amounts of zinc in water samples by flame atomic absorption spectrometry

A glassy carbon electrode was prepared that was coated with a composite film containing electropolymerized poly(amidosulfonic acid) and multi-walled carbon nanotubes. It was used to study the electrochemical response of procaine by differential pulse voltammetry. The results indicate that the electrode exhibits a remarkable improvement in the oxidation peak of procaine, and this led to a simple and sensitive method for the electroanalytical determination of procaine. The peak current is proportional to the concentration of procaine from 80 nM to 1.0 µM. The detection limit is 25 nM (S/N?=?3). The modified electrode was successfully applied to the direct determination of procaine in pharmaceutical formulations.     

Aptamer-based Biosensor for Detection of Phenylalanine at Physiological pH

A simple, sensitive aptamer-based biosensor for the detection of phenylalanine is developed using the electrochemical transduction method. For this proposed aptasensor, a 5-thiol-terminated aptamer is covalently attached onto a gold electrode. At the first time, the electrode was evaluated as an electrochemical aptasensor for determination of phenylalanine in aqueous solutions. This sensor was tested in a Tris–HCl buffer with physiological pH?=?7.4 by cyclic voltammetry and differential pulse voltammetry. The detection limit and sensitivity of the modified electrode toward phenylalanine were estimated to be 1 nM (S/N?=?3) and 0.367 μA nM^?1, respectively. The linear range of the signal was observed between 1 and 10 nM of phenylalanine with 0.9914 correlation factor. The herein-described approach is expected to promote the exploitation of aptamer-based biosensors for protein assays in biochemical and biomedical studies.     

Sensing Lorazepam with a glassy carbon electrode coated with an electropolymerized-imprinted polymer modified with multiwalled carbon nanotubes and gold nanoparticles

We report on an electrode for the amperometric determination of lorazepam. A glassy carbon electrode was coated with a molecular imprint made by electropolymerization of ortho-phenylenediamine and filled with multiwalled carbon nanotubes and gold nanoparticles, which enhances the transmission of electrons. The sensor was studied with respect to its response to hexacyanoferrate (III) as a probe and by electrochemical impedance spectroscopy, cyclic voltammetry and square wave voltammetry. The linear response range to Lorazepam is from 0.5 nM to 1.0 nM and from 1.0 nM to 10.0 nM, with a detection limit of 0.2 nM (at an S/N of 3). The electrode was successfully applied to determine Lorazepam in spiked human serum.

Voltammetric behavior of acebutolol on pencil graphite electrode: highly sensitive determination in real samples by square-wave anodic stripping voltammetry

In this work, an electrochemical investigation of acebutolol (ACE), a beta-blocker drug, was carried out in alkaline medium using pencil graphite (PG) electrode. In cyclic voltammetry, the compound displayed a reversible and adsorption-controlled oxidation peak. By using square-wave anodic stripping voltammetry, the oxidation peak current observed at +0.78 V showed a linear relationship with concentration at 0.4–7 nM interval in Britton–Robinson buffer (pH 10.0) and a detection limit of 0.09 nM. The relative standard deviation of 4.72% for the concentration level of 2.0 nM (n = 11) was also calculated. The PG electrode that is used for the first time in this method was successfully applied to determine the ACE in pharmaceutical formulations and urine.     

Sensitive Electrochemical Aptasensor for Thrombin Detection Based on Graphene Served as Platform and Graphene Oxide as Enhancer

A sensitive electrochemical aptasensor was developed with conductive graphene served as platform and inert graphene oxide (GO) as enhancer. An electrodeposited nano-Au layer was firstly formed on conductive graphene modified glass carbon electrode surface for further immobilizing of electrochemical redox probe hexacyanoferrates nanoparticles (NiHCFNPs). Subsequently, another nano-Au layer was formed for immobilizing of thrombin aptamer (TBA). In the presence of thrombin, the TBA on the electrode surface could bind with thrombin, which made a barrier for electrons and inhibited the electro-transfer, resulting in the decreased electrochemical signals of NiHCFNPs. Owing to the non-conductivity property of graphene oxide, further decreased electrochemical signals of NiHCFNPs could be obtained via the sandwich reaction with GO-labeled TBA. According to the signal changes before the thrombin recognition and after sandwich reaction, trace detection of thrombin could be achieved. As a result, the proposed approach showed a high sensitivity and a wider linearity to thrombin in the range from 0.005 nM to 50 nM with a detection limit of 1 pM.     

Determination of Rutin by a Graphene-Modified Glassy Carbon Electrode

A reduced graphene oxide-modified glassy carbon electrode for sensitive detection of rutin is reported. The modified electrode was obtained by one-step electrochemical reduction of graphene oxide on the bare glassy carbon electrode. In the presence of graphene, an enhanced electrochemical response for rutin appeared with a pair of well-defined anodic and cathodic peaks in pH 3.0 phosphate buffer. Under the optimized conditions, the anodic peak currents exhibited a linear relationship with rutin concentration from 0.1 to 2.0 µM with a detection limit of 23.2 nM. The modified electrode was employed to the analysis of tablets (with satisfactory recovery of 19.96 mg/per tablet) and Flos Sophorae. The graphene-modified electrode exhibited high sensitivity, good stability, and selectivity for the determination of rutin.     

Electrochemical behavior of bisphenol A at glassy carbon electrode modified with gold nanoparticles, silk fibroin, and PAMAM dendrimers

A glassy carbon electrode was modified with a composite made from gold nanoparticles and silk fibroin whose surface was further modified with amino-terminated G4 poly(amidoamine) dendrimer. This electrode shows distinct electrochemical response to bisphenol A (BPA). Electrochemical impedance spectroscopy was used to characterize the surface. The electrode displayed improved adsorption capacity and an increased response to BPA, compared to a surface without modification. Under the optimal detection conditions, the respeonse is linear in the concentration range from 1 nM to 1.3 μM, the correlation coefficient is 0.9991, and the detection limit is 0.5 nM (at an S/N of 3). The method was applied to the determination of BPA in water samples, and the recovery was in the range from 97% to 105%.     

Sensitive detection of enteropathogenic E. coli using a bfpA gene-based electrochemical sensor

We have developed a sensitive assay for enteropathogenic E. coli (EPEC) by integrating DNA extraction, specific polymerase chain reaction (PCR) and DNA detection using an electrode modified with the bundle-forming pilus (bfpA) structural gene. The PCR amplified products are captured on the electrode and hybridized with biotinylated detection probes to form a sandwich hybrid containing two biotinylated detection probes. The sandwich hybridization structure significantly combined the numerous streptavidin alkaline phosphatase on the electrode by biotin-streptavidin connectors. Electrochemical readout is based on dual signal amplification by both the sandwich hybridization structure and the enzyme. The electrode can satisfactorily discriminate complementary and mismatched oligonucleotides. Under optimal conditions, synthetic target DNA can be detected in the 1 pM to 10 nM concentration range, with a detection limit of 0.3 pM. EPEC can be quantified in the 10 to 10⁷ CFU mL^?1 levels within 3.5 h. The method also is believed to present a powerful platform for the screening of pathogenic microorganisms in clinical diagnostics, food safety and environmental monitoring.

Application of multivariate optimization method in nanomolar simultaneous determination of morphine and codeine in the presence of uric acid using a glassy carbon electrode modified with a hydroxyapatite-Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticle/multiwalled carbon nanotubes composite

The electrochemical oxidation of morphine (MO) and codeine (COD) has been investigated by the application of a novel glassy carbon electrode modified with a hydroxyapatite-Fe₃O₄ nanoparticles/multiwalled carbon nanotubes composite (HA-FeNPs-MWCNTs/GCE). The modified electrode worked as an efficient sensor for simultaneous determination of MO and COD in the presence of uric acid. Response surface methodology was utilized to optimize the voltammetric response of the modified electrode for the determination of MO and COD. The amount of HA-FeNPs in the modifier matrix (%HA-FeNPs), the solution pH and the accumulation time were chosen as the three important operating factors through the experimental design methodology. The central composite design as a response surface approach was applied for obtaining the optimum conditions leading to maximum oxidation peak currents for MO and COD. The differential pulse voltammetry results showed that the obtained anodic peak currents were linearly proportional to concentration in the range of 0.08–32 µM with a detection limit (S/N = 3.0) of 14 nM for MO and in the range of 0.1–28 µM and with a detection limit of 22 nM for COD. The proposed method was successfully applied to determine these compounds in human urine and blood serum samples.     

Highly sensitive and selective voltammetric detection of mercury(II) using an ITO electrode modified with 5-methyl-2-thiouracil, graphene oxide and gold nanoparticles

We have developed an electrochemical sensor for highly selective and sensitive determination of Hg(II). It is based on the specific binding of 5-methyl-2-thiouracil (MTU) and Hg(II) to the surface of an indium tin oxide (ITO) electrode modified with a composite made from graphene oxide (GO) and gold nanoparticles (AuNPs). This leads to a largely enhanced differential pulse voltammetric response for Hg(II). Following optimization of the method, a good linear relationship (R?=?0.9920) is found between peak current and the concentration of Hg(II) in the 5.0–110.0 nM range. The limit of detection (LOD) is 0.78 nM at a signal-to-noise ratio of 3. A study on the interference by several metal ions revealed no interferences. The feasibility of this method was demonstrated by the analyses of real water samples. The LODs are 6.9, 1.0 and 1.9 nM for tap water, bottled water and lake water samples, respectively, and recoveries for the water samples spiked with 8.0, 50.0 and 100.0 nM were 83.9–96.8 %, with relative standard deviations ranging from 3.3 % to 5.2 %.

Electrochemical biosensor for sensitively simultaneous determination of dopamine, uric acid, guanine, and adenine based on poly-melamine and nano Ag hybridized film-modified electrode

An electrochemical sensor for simultaneous determination of dopamine (DA), uric acid (UA), guanine (G), and adenine (A) has been constructed by copolymerizing melamine monomer and Ag ions on a glassy carbon electrode (GCE) with cyclic voltammetry. The poly-melamine and nano Ag formed a hybridized film on the surface of the GCE. The morphology of the film was characterized by scanning electron microscope. The electrochemical and electrocatalytic properties of this film were characterized by cyclic voltammetry, linear sweep voltammetry, and square wave voltammetry (SWV). In 0.1 M phosphate buffer solution (pH 4.5), the modified electrode resolved the electrochemical response of DA, UA, G, and A into four well-defined voltammetric oxidation peaks by SWV; the oxidation peak current of DA, UA, G, and A increased 13-, 6-, 7-, and 9-fold, respectively, compared with those at the bare GCE and the SWV peak currents of DA, UA, G, and A with linear concentrations in the ranges of 0.1–50, 0.1–50, 0.1–50, and 0.1–60 μM, respectively. Based on this, a method for simultaneous determination of these species in mixture was setup. The detection limits were 10 nM for DA, 100 nM for UA, 8 nM for G, and 8 nM for A.     

Electrochemical Behavior and Voltammetric Determination of Diclofenac at a Multi-Walled Carbon Nanotube-Ionic Liquid Composite Modified Carbon Ceramic Electrode

This work describes the electrochemical behavior of diclofenac on the surface of a carbonceramic electrode (CCE) modified with multi-walled carbon nanotubes (MWCNT) and an ionic liquid (IL) composite. The MWCNT-IL composite showed an enhancement effect in the electro-oxidation of diclofenac with respect to a bare carbon ceramic electrode. Based on the experimental outcomes, a possible mechanism for the electro-oxidation of diclofenac is proposed and discussed. Under the optimized experimental conditions, the MWCNT-IL CCE showed a linear response to diclofenac over the concentration range 50 nM–20 µM with a detection limit of 27 nM. The developed diclofenac sensor showed good stability, sensitivity, and reproducibility in the measurement of diclofenac in human blood plasma samples.     

Amperometric biosensor for bisphenol A based on a glassy carbon electrode modified with a nanocomposite made from polylysine, single walled carbon nanotubes and tyrosinase

We have prepared a nanocomposite consisting of single-walled carbon nanotubes and polylysine. It was characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and by UV/vis and FTIR spectroscopy. Tyrosinase was covalently immobilized on the nanocomposite, and the resulting bioconjugate deposited on a glassy carbon electrode to form a biosensor for bisphenol A. The biosensor was characterized by scanning electron microscopy and electrochemical impedance spectroscopy. Under optimized experimental conditions, the biosensor gives a linear response to bisphenol A in the 4.00 nM to 11.5 μM concentration range. Its sensitivity is 788 mA M^?1 cm^?2, and the lower detection limit is 0.97 nM (at an S/N of 3). The biosensor shows good repeatability, reproducibility and long-term stability. In a preliminary practical application, it was successfully applied to the determination of bisphenol A in leachates of plastic spoons.

3,4,9,10-Perylene Tetracarboxylic Acid Noncovalently Modified Multiwalled Carbon Nanotubes: Synthesis,Characterization, and Application for Electrochemical Determination of 2-Aminonaphthalene

Carbon nanotubes have been intensively studied for their diverse applications but are insoluble in water. In this paper, 3,4,9,10-perylene tetracarboxylic acid noncovalently modified multiwalled carbon nanotubes were prepared by a facile approach and applied successfully for electrochemical determination of 2-aminonaphthalene. Infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, and electrochemical methods were used to characterize the hybridized nanotubes. The results reveal that the hybrids exhibit high dispersibility in water, and a glassy carbon electrode modified by the hybrids displayed a higher electrochemical response toward 2-aminonaphthalene than bare glassy carbon and multiwalled carbon nanotube–glassy carbon electrodes with a linear dynamic range of 15.0–500.0 nM and a detection limit of 4.5 nM. The modified hybrid electrode was successfully applied for the determination of 2-aminonaphthalene in water.     

Sensitive and selective stripping voltammetric determination of copper(II) using a glassy carbon electrode modified with amino-reduced graphene oxide and β-cyclodextrin

We describe a square wave anodic stripping voltammetric (SWASV) platform for the determination of Cu(II). It is based on the use of amino-reduced graphene oxide (NH₂-rGO) and β-cyclodextrin (β-CD) that were self-assembled on the surface of a glassy carbon electrode (GCE). The hydrophilicity and electrochemical performance of the resulting modified GCE were investigated by measurement of static contact angles, cyclic voltammetry and electrochemical impedance spectroscopy. Cu(II) was reduced at −1.1 V and then reoxidized at −0.012 V. Under optimum experimental conditions, the modified GCE exhibited excellent SWASV response in that the stripping peak currents (when sweeping between −0.3 and +0.25 V) depends on the concentration of Cu(II) in the 30 nM to 100 μM range. The limit of detection is 2.8 nM (at 3σ/slope). The modified GCE displaying good reproducibility, is stable, highly sensitive and selective. It was successfully applied to the determination of Cu(II) in synthetic and real water samples. The fast electron transfer rate and simple preparation of the NH₂-rGO/β-CD composite makes it a promising electrode material for applications in sensing of heavy metal ions.

Amino-modified rGO and β-cyclodextrin form an attractive material for use in an electrochemical platform for highly sensitive and selective determination of Cu(II).

     

Biosensor for bisphenol A leaching from baby bottles using a glassy carbon electrode modified with DNA and single walled carbon nanotubes

We have developed a biosensor for highly sensitive and selective determination of the endocrinic disruptor bisphenol A (BPA). It is based on glassy carbon electrode modified with calf thymus DNA and a composited prepared from single walled carbon nanotubes (SWNT) and Nafion. The interaction between BPA and DNA was studied by voltammetry. The binding constant was determined to be 3.55?×?10³ M^?1, and the binding site has a length of 4.3 base pairs. These electrochemical studies provide further information for a better understanding of the toxicity and carcinogenicity of BPA. Under optimal conditions, the biosensor displays a linear electrochemical response to BPA in the 10 nM to 20 μM concentration range, with a detection limit as low as 5.0 nM (at an S/N of 3). The method was successfully applied to the quantification of BPA in leachates from plastic baby bottles. Recoveries range from 94.0 % to 106.0 % which underpins the excellent performance of this SWNT-based DNA sensor.

Biosensor for atrazin based on aligned carbon nanotubes modified with glucose oxidase

A sensitive method was developed for the determination of the hebicide atrazine. It based on the use of glucose oxidase that is self-assembled on aligned carbon nanotubes on the surface of a copper electrode. The surface morphology and electrochemical properties of the electrode were characterized by field emission scanning electron microscopy and cyclic voltammetry. The effects of buffer solution and incubation time on the response of the electrode were investigated. Response to atrazine is linear in the range from 0.58 µM to 42 µM, and the detection limit is 39 nM. The performance of the biosensor was verified by determination of atrazine in environmental water samples.     

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.     

Fabrication of Porous Pseudo-Carbon Paste Electrode as a Novel High-Sensitive Electrochemical Biosensor

Abstract

Porous pseudo-carbon paste electrode (PPCPE) as a novel high-sensitive electrochemical biosensor was fabricated by mixing polymethyl methacrylate (PMMA) microspheres for use as the template, graphite powders for the filler, and pyrrole as the precursor of the polymer which acted as the paste. After the polymerization of pyrrole catalyzed by Fe³⁺, the PMMA microspheres were removed to form PPCPE. The pore size was determined by SEM observations, with diameters ranging from 2 to 5 µm. The anodic stripping voltammetry response of DNA indicated that the adsorption of oligonucleotide on PPCPE was enhanced. The PPCPE was easy to preserve and had a good reusability in comparison with that of a pure carbon paste electrode (CPE) and a carbon nanotube-modified carbon paste electrode (CNTPE). The detection limits of guanine and adenine were 20 nM and 8 nM, respectively.     

Electrochemical tyrosine sensor based on a glassy carbon electrode modified with a nanohybrid made from graphene oxide and multiwalled carbon nanotubes

We report on a glassy carbon electrode that was modified with a composite made from graphene oxide (GO) and multiwalled carbon nanotubes (MWCNT) that enables highly sensitive determination of L-tyrosine. The sensor was characterized by transmission electron microscopy and electrochemical impedance spectroscopy, and its electrochemical properties by cyclic voltammetry, chronocoulometry and differential pulse voltammetry. The GO/MWCNT hybrid exhibits strong catalytic activity toward the oxidation of L-tyrosine, with a well defined oxidation peak at 761 mV. The respective current serves as the analytical information and is proportional to the L-tyrosine concentration in two ranges of different slope (0.05 to 1.0 μM and 1.0 to 650.0 μM), with limits of detection and quantification as low as 4.4 nM and 14.7 nM, respectively. The method was successfully applied to the analysis of L-tyrosine in human body fluids. The excellent reproducibility, stability, sensitivity and selectivity are believed to be due to the combination of the electrocatalytic properties of both GO and MWCNT. They are making this hybrid electrode a potentially useful electrochemical sensing platform for bioanalysis.