Novel nanostructured electrochemical sensor for voltammetric determination of N-acetylcysteine in the presence of high concentrations of tryptophan

A carbon paste electrode chemically modified with multiwall nanotubes and ethynylferrocene (ETFc) was used as a selective and sensitive electrochemical sensor for the determination of minor amounts of N-acetylcysteine (N-AC) in the presence of a high concentration of tryptophan (Trp). Square wave voltammetry (SWV) of N-AC at the modified electrode exhibited linear dynamic range with a detection limit (3 s) of 0.08 μmol?L^?1. The separations of anodic peak potentials of N-AC and Trp reached 400 mV using SWV. With good selectivity and sensitivity, the present method provides a simple method for selective detection of N-AC in real samples such as drug and urine.     

Nanomolar detection limit for determination of norepinephrine in the presence of acetaminophen and tryptophan using carbon nanotube-based electrochemical sensor

A carbon paste electrode modified by carbon nanotubes and a synthesized hydroquinone derivative (abbreviated as DHB) was fabricated. It was used as an electrochemical sensor for simultaneous determination of norepinephrine (NE), acetaminophen (AC), and tryptophan (Trp). Oxidation potential of NE decreased about 220 mV at the modified electrode in comparison with unmodified electrode because of electrocatalysis of oxidation of NE via E? mechanism at the modified electrode. Differential pulse voltammetry was used for obtaining the calibration plot of NE and two linear range of 0.2–20.0 μM and 20.0–1,500.0 μM and an interesting detection limit (3σ) of 40.0 nM were obtained for NE. Also, simultaneous determination of NE, AC, and Trp was described by the proposed sensor and linear range of 20.0–800.0 μM was found for AC and Trp. Finally, the electrochemical sensor was used for the determination of NE, AC, and Trp in mixture.     

Electrochemical and catalytic investigations of epinephrine,acetaminophen and folic acid at the surface of titanium dioxide nanoparticle-modified carbon paste electrode

In the present paper, the use of a carbon paste electrode modified with 1-(4-(1, 3-dithiolan-2-yl)-6, 7-dihydroxy-2-methyl-6, 7-dihydrobenzofuran-3-yl)ethanone (DDE) and TiO₂ nanoparticles prepared by a simple and rapid method was described. The modified electrode showed excellent properties for electrocatalytic oxidization of epinephrine (EP), acetaminophen (AC) and folic acid (FA). The apparent charge transfer rate constant, k _s?=?1.14 s^?1, and transfer coefficient, α?=?0.54, for electron transfer between the modifier and carbon paste electrode were calculated. It has been found that under optimum condition (pH?=?7.0) in cyclic voltammetry, the oxidation of EP occurs at a potential about 280 mV less positive than that of an unmodified carbon paste electrode. The values of transfer coefficients (α?=?0.46), catalytic rate constant (k?=?1.2?×?10⁴ M^?1 s^?1) and diffusion coefficient (D?=?2.70?×?10^?5 cm² s^?1) were calculated for EP. Differential pulse voltammetry (DPV) exhibited two linear dynamic ranges of 0.5 to 50.0 μM and 50.0 to 1,000 μM for EP. This modified electrode is quite effective not only for the detection of EP, AC and FA but also for the simultaneous determination of these species in a mixture. The limit of detection for EP, AC and FA is 0.10, 1.80 and 2.36 μM, respectively.     

Highly sensitive electrochemical detection of dopamine and uric acid on a novel carbon nanotube-modified ionic liquid-nanozeolite paste electrode

A novel biosensor has been constructed by incorporating modified nanosized natural zeolite and 3-hydroxypropanaminium acetate (HPAA) as a novel room temperature ionic liquid, supported on multiwalled carbon nanotube (MWCNTs) and employed for the simultaneous determination of dopamine (DA) and uric acid (UA). A detailed investigation by transmission electron microscopy and electrochemistry is performed in order to elucidate the preparation process and properties of the composites. The voltammetric studies using the modified carbon paste electrode show two well-resolved anodic peaks for DA and UA with a potential difference of 160 mV, revealing the possibility of the simultaneous electrochemical detection of these compounds. The modified carbon paste electrode shows good conductivity, stability, and extraction effect due to the synergic action of HPAA, MWCNTs, and iron ion-doped natrolite zeolite. Under optimized conditions, the peak currents are linear from 8.12?×?10^?7 to 3.01?×?10^?4?mol?L^?1 and from 9.31?×?10^?7 to 3.36?×?10^?4?mol?L^?1 with detection limits of 1.16?×?10^?7 and 1.33?×?10^?7?mol?L^?1 for DA and UA using the differential pulse voltammetric method, respectively. Finally, the modified carbon paste electrode proved to have good sensitivity and stability and is successfully applied for the simultaneous determination of DA and UA in human blood serum and urine samples.     

Differential pulse anodic stripping voltammetric determination of Hg2+ at poly(Eriochrome Black T)-modified carbon paste electrode

The electrochemical behavior of Hg²⁺ was investigated in poly(Eriochrome Black T)-modified carbon paste electrode (CPE) using cyclic voltammetry (CV). Poly(Eriochrome Black T) was prepared in an alkaline medium on the surface of the CPE using a solution of Eriochrome Black T with the CV technique. The electrochemical impedance study revealed a better charge transfer kinetics at the modified electrode. The effects of variation of the experimental conditions, such as the concentration of electrolytes, pH, deposition time, and the deposition potential, which maximize current efficiency were studied. The optimum response of Hg²⁺ was observed in 1.0 M KCl solution. The differential pulse anodic stripping voltammetric technique was employed successfully to detect Hg²⁺, which gave a good linear response at low concentration levels of Hg²⁺. The detection limit was found to be 2.2?×?10^?10 M (S/N?=?3), which is comparable with that achieved in multiwall carbon nanotube-modified electrode. The remarkable electroanalytical performance of the modified electrode makes it amenable to employ it successfully as an electrochemical sensor for the determination of hazardous pollutant Hg²⁺ in environmental samples.     

Benzoylferrocene-modified carbon nanotubes paste electrode as a voltammetric sensor for determination of hydrochlorothiazide in pharmaceutical and biological samples

The electrooxidation of hydrochlorothiazide (HCT) at the surface of a benzoylferrocene modified multi-walled carbon nanotube paste electrode was studied using electrochemical approaches. Under the optimized conditions (pH 7.0), the square wave voltammetric peak current of HCT increased linearly with HCT concentration in the ranges of 6.0?×?10^?7 to 3.0?×?10^?4 M. The detection limit was 9.0?×?10^?8 M HCT. The diffusion coefficient (D?=?1.75?×?10^?5 cm²/s) and electron transfer coefficient (α?=?0.45) for HCT oxidation were also determined. The proposed sensor was successfully applied for the determination of HCT in human urine and tablet samples.     

Sensitive voltammetric determination of diclofenac using room-temperature ionic liquid-modified carbon nanotubes paste electrode

An ionic liquid-modified carbon nanotubes paste electrode (IL/CNTPE) has been fabricated using hydrophilic ionic liquid (n-hexyl-3-methylimidazolium hexafluoro phosphate) as a binder. This electrode showed enhanced electrochemical response and strong analytical activity towards the direct electrochemical oxidation of diclofenac (DCF). The electron transfer coefficient, α, and charge transfer resistance (R _ct) of DCF at the modified electrode were calculated. Under optimal conditions at pH 7.0, the anodic peak currents increased linearly with the concentration of DCF in the range of 0.5–300 μmol L^?1 with a detection limit of 0.2 μmol L^?1 (3σ). The interferences of foreign substances were investigated. Differential pulse voltammetry was used to check the applicability of the proposed sensor to the determination of DCF in real samples with satisfactory results.     

A fast and sensitive nanosensor based on MgO nanoparticle room-temperature ionic liquid carbon paste electrode for determination of methyldopa in pharmaceutical and patient human urine samples

In this study, we describe an ionic liquid–MgO nanoparticle modified carbon paste electrode (MgO/NPs/IL/CPE) was used as a simple, fast, and sensitive tool for the investigation of the electrochemical oxidation of methyldopa (MDOP) using voltammetric methods. The MgO/NPs was characterized with different methods such as TEM, SEM, and XRD. The oxidation peak potential of the MDOP at a surface of MgO/NPs/IL/CPE appeared at 450 mV that was about 100 mV lower than the oxidation peak potential at the surface of the traditional carbon paste electrode (CPE) under similar conditions. The electro-oxidation of MDOP occurred in a pH-dependent 2e^? and 2H⁺ process, and the electrode reaction followed a diffusion-controlled pathway. Under optimal conditions at pH 7.0, the anodic peak currents increased linearly with the concentration of MDOP in the range of 0.08–380 μmol L^?1 with a detection limit of 0.03 μmol L^?1 (3σ). The proposed sensor was successfully applied to the determination of MDOP in real samples such as drug and urine.     

Electrocatalytic oxidation of <Emphasis Type="SmallCaps">l</Emphasis>-tyrosine at carboxylic acid functionalized multi-walled carbon nanotubes modified carbon paste electrode

The electrocatalytic oxidation of l-tyrosine (Tyr) was investigated on a carboxylic acid functionalised multi-walled carbon nanotubes modified carbon paste electrode using cyclic voltammetry and amperometry. The surface morphology of the electrodes was studied using field emission (FE)-SEM images, and the interface properties of bare and modified electrodes were investigated by electrochemical impedance spectroscopy (EIS). The influence of the amount of modifier loading and the variation of the pH of the solution on the electrochemical parameters have been investigated. Cyclic voltammetry was carried out to study the electrochemical oxidation mechanism of Tyr, which showed an irreversible oxidation process at a potential of 637.0 mV at modified electrode. The anodic peak current linearly increased with the scan rate, suggesting that the oxidation of Tyr at modified electrode is an adsorption-controlled process. A good linear relationship between the oxidation peak current and the Tyr concentration in the range of 0.8–100.0 μM was obtained in a phosphate buffer solution at pH 7.0 with a detection limit of 14.0?±?1.36 nM (S/N?=?3). The practical utility of the sensor was demonstrated by determining Tyr in spiked cow’s milk and human blood serum. The modified electrode showed excellent reproducibility, long-term stability and antifouling effects.     

The first electrochemical sensor for determination of mangiferin based on an ionic liquid–graphene nanosheets paste electrode

A novel carbon paste electrode modified with graphene nanosheets and an ionic liquid (n-hexyl-3-methylimidazolium hexafluoro phosphate) was fabricated and used for the electrochemical study of mangiferin for the first time. This modified electrode offers a considerable improvement in voltammetric sensitivity toward mangiferin, compared to the bare electrode. Square wave voltammetry (SWV) exhibits a linear dynamic range from 5.0?×?10^?8 to 2.0?×?10^?4 M and a detection limit of 20.0 nM for mangiferin. Finally, the proposed method was successfully applied to the determination of mangiferin in real samples such as serum and urine.     

Voltammetric sensor for simultaneous determination of ascorbic acid,acetaminophen, and tryptophan in pharmaceutical products

A novel carbon paste electrode modified with carbon nanotubes and 5-amino-2′-ethyl -biphenyl-2-ol was fabricated. The electrochemical study of the modified electrode, as well as its efficiency for electrocatalytic oxidation of ascorbic acid (AA), is described. The electrode was employed to study the electrocatalytic oxidation of AA, using cyclic voltammetry, chronoamperometry, and square-wave voltammetry (SWV) as diagnostic techniques. It has been found that the oxidation of AA at the surface of modified electrode occurs at a potential of about 250 mV less positive than that of an unmodified carbon paste electrode. SWV exhibits a linear dynamic range from 2.0?×?10^?7 to 5.0?×?10^?4 M and a detection limit of 1.0?×?10^?7 M for AA. In addition, this modified electrode was used for simultaneous determination of AA, acetaminophen (AC), and tryptophan (TRP). Finally, the modified electrode was used for determination of AA, AC, and TRP in pharmaceutical products.     

An ionic liquid-type multiwall carbon nanotubes paste electrode for electrochemical investigation and determination of morphine

The direct electrochemistry of morphine on modified multiwall carbon nanotubes using carbon ionic liquid (i.e., 1-butyl-3-methylimidazolium hexafluoro phosphate, ([C4mim]–[PF6])) was studied. It was found that the electrode showed sensitive voltammetric response to morphine. The experimental results suggested that the modified electrode promoted electron transfer reaction for the oxidation of morphine. The electron transfer coefficient and charge transfer resistant (R _ct) of morphine at the modified electrode were calculated. Under the optimized conditions at pH 8.0, the peak current was linear to morphine concentrations over the concentration range of 0.45–450 μmol L⁻¹, using differential pulse voltammetry. The detection limit was 0.14 μmol L⁻¹. The proposed method was successfully applied to the determination of morphine in both ampoules and urine samples.     

Vertically aligned α-MnO<Subscript>2</Subscript> nanosheets on carbon nanotubes as cathodic materials for aqueous rechargeable magnesium ion battery

In this work, vertically aligned α-MnO₂ nanosheets on carbon nanotubes are synthesized simply by a solution process and the electrochemical performance as host materials of magnesium ion is tested in aqueous solution. Cyclic voltammetry analysis confirms the enhanced electrochemical activity of carbon nanotube-supported samples. Moreover, carbon nanotubes skeleton could reduce the charge transfer resistant of the cathode materials, which is confirmed by electrochemical impedance spectroscopy. Furthermore, when tested as magnesium ion batteries cathodic electrode, the α-MnO₂/carbon nanotube sample registers a prominent discharge capacity of 144.6 mAh g^?1 at current density of 0.5 A g^?1, which is higher than the discharge capacity of α-MnO₂ (87.5 mAh g^?1) due to the synergistic effect of insertion/deinsertion reaction and physical adsorption/desorption process. After the 1000th cycle, a remarkable discharge capacity of 48.3 mAh g^?1 is collected for α-MnO₂/carbon nanotube at current density of 10 A g^?1, which is 85% of the original. It is found that the carbon skeleton not only improved the capacity but also enhanced the cycling performance of the α-MnO₂ electrode significantly. Therefore, α-MnO₂/carbon nanotube is a very promising candidate for further application in environmentally benign magnesium ion batteries.     

An electrochemical sensor based on carbon nanotubes and a new Schiff base for selective determination of dopamine in the presence of uric acid, folic acid, and acetaminophen

In this paper, the electrochemical properties of a carbon paste electrode modified by a synthesized Schiff base, 2,2′-[1,4-phenylenediyl-bis(nitrilomethyl-idene)]-bis(4-hydroxyphenol), and carbon nanotubes were studied by cyclic voltammetry. The modified electrode was used as an electrochemical sensor for catalytic oxidation of dopamine (DA). Differential pulse voltammetry (DPV) of DA by this electrochemical sensor exhibited two linear dynamic ranges with a detection limit (3σ) of 0.42 μM. Also, the selectivity of the prepared electrochemical sensor was checked for determination of DA in the presence of uric acid (UA), folic acid (FA), and acetaminophen (AC). The DPV results indicate that the proposed sensor can be used for simultaneous determination of DA, UA, and FA and also simultaneous determination of DA and AC. Finally, the proposed electrochemical sensor was used for determinations of these substances in real sample.     

Sensitive voltammetric determination of cysteamine using promazine hydrochloride as a mediator and modified multi-wall carbon nanotubes carbon paste electrodes

This paper presents a sensitive electrochemical method for the determination of cysteamine (CA) using promazine hydrochloride-modified multi-wall carbon nanotubes carbon paste electrode (PrH/MWCNTs CPE). Because of the good electrochemical activity of MWCNTs and the acceptable performance of promazine hydrochloride (PrH) as an electrocatalytic mediator, the modified electrode significantly enhanced the sensitivity for the detection of CA in comparison to the bare carbon paste electrode (CPE). All chemical parameters such as pH of solution, concentration of PrH and kinetic parameters of the system were investigated. Linear sweep voltammetric (LSV) method was used to follow the electrocatalytic effect of CA on the current–potential response of PrH. Under optimum conditions, the obtained net peak current ?I _p(I _sample???I _blank) was linear with CA concentrations in two dynamic ranges of 2.0–346.5 μmol l^?1 (?I _p?=?(0.0195?±?0.0043)C _CA?+?(0.7648?±?0.0397) (r ²?=?0.9948)) and 346.5–1,912.5 μmol l^?1 (?I _p?=?(0.0100?±?0.0026)C _CA?+?(3.8981?±?0.0828) (r ²?=?0.9911)) with a detection limit of 0.8 μmol l^?1. Finally, the PrH/MWCNTs CPE was successfully applied for the determination of CA in urine and drug samples with satisfactory results.     

Voltammetric determination of uric acid in the presence of ascorbic acid and dopamine using chitosan/ionic liquid composite electrode

A chitosan/ionic liquid composite electrode was prepared and used to determine uric acid (UA) in the presence of a large excess of ascorbic acid (AA) and dopamine (DA) by linear sweep voltammetry (LSV). The modified electrode shows large peak separations between DA, AA, and UA. Due to the existence of chitosan and ionic liquid in the composite, the modified electrode exhibits strong electrochemical catalytic activity toward the oxidation of UA. Under optimal conditions, the peak current is linearly dependent on the UA concentration in the range of 5?×?10^?7–2?×?10^?4 M in the presence of 5?×?10^?4 M AA and 5?×?10^?5 M DA with a correlation coefficient of 0.9978, and the detection limit is 5?×?10^?8 M at a signal-to-noise ratio of 3. With good sensitivity and stability, the constructed sensor was applied in the determination of UA in human serum samples and satisfactory results were obtained.     

Enhanced electrocatalytic oxidation of hydroxylamine on Pt/polypyrrole composite modified glassy carbon electrode

A sensitive hydroxylamine sensor is developed by electrodeposition of Pt nanoparticles on pre-synthesized polypyrrole nanoparticles modified glassy carbon electrode. The modified electrode presents distinctly electrocatalytic activity toward hydroxylamine oxidation. The kinetic parameters such as the overall numbers of electrons involved in hydroxylamine oxidation, the electron transfer coefficient, standard heterogeneous rate constant, and diffusion coefficient are evaluated. The current response increases linearly with increasing hydroxylamine concentrations and exhibits two wide linear ranges of 5.0?×?10^?7–1.1?×?10^?3 and 1.1?×?10^?3–18.8?×?10^?3 M with a detection limit of 0.08 μM (s/n?=?3). The proposed electrode presents excellent operational and storage ability for determining hydroxylamine. Moreover, the sensor shows good sensitivity, selectivity, and reproducibility properties.     

Establishment and practical application of the electron transfer model in lithium-air batteries

The electron transfer failure model is proposed to analyze the effects of discharge products on the air electrode performance in lithium-air batteries. In order to understand how the air electrode products are obtained during the discharge cycle, three kinds of carbon-based air electrode are constructed, and discharged products are analyzed by powder XRD and SEM techniques. It can be concluded that the air electrode is covered with the products, indicating that the discharge products closely depended on the surface states of carbon materials. EIS studies show that the value of R _ct value changes due to the deposition of discharge products on carbon materials. Finally, all present studies show that the air electrode failure is mainly caused by the difficulty of charge transfer.     

Selective and sensitive determination of dopamine via suppressing ascorbic acid using an N-(1-H-indole-3yl) methylene thiazole-2-amine thin film-modified glassy carbon electrode

The selective determination of dopamine (DA) was performed using a glassy carbon (GC) electrode modified with N-(1-H-indole-3yl) methylene thiazole-2-amine (IMT2A). IMT2A was deposited on the GC electrode by cyclic voltammetry. This modified electrode demonstrated an electrocatalytic effect on the oxidation of DA in the presence of uric acid (UA) and ascorbic acid (AA) using differential pulse voltammetry (DPV) method in 0.1 M phosphate buffer solution (PBS) of pH 7. Selective determination was realized in elimination of AA response on the IMT2A-modified electrode. The oxidation peak currents increased linearly with two concentration intervals of DA at pH 7 phosphate buffer. One of them is 0.25–9.15 μM, and the other is 9.15–95.1 μM. The limit of detection (LOD) was found as 0.086 μM. The proposed electrode was applied to the determination of DA in pharmaceutical preparations and human urine sample with satisfactory results.

     

The influence of α-ZrP coat on proton transfer resistance of cation exchange membrane-solution interface

In this work, the effect of surface modification on proton transfer resistance of the membrane and/or membrane surface interface is investigated. Cation exchange membranes, PE01 and Nafion1135, were modified by zirconium phosphate (ZrP) and inorganic-organic composite membranes were prepared. α-ZrP (α-Zr(HPO₄)₂ · H₂O) was deposited on the composite membrane surface as verified by XRD and SEM. Zeta potential of the composite membrane decreases with the increase of immersion time, indicating that the deposition α-ZrP results in a decrease of membrane surface charge density. The proton transfer resistance measured by AC impedance technique in 0.05 mol/L H₂SO₄ solution shows an interesting result: the proton transfer resistance of membrane-solution (Z_e) sharply reduces while proton transfer resistance of membrane (Z_m) increases slightly with the immersion time for both of the membranes. The slight increase of Z_m is due to the deposit of ZrP on membrane surface, and the sharp decrease of Z_e is attributed to the decrease in the static electrical interaction strength between counterions and the charged groups of the membrane, which is caused by interact of α-ZrP with the ion exchange groups resulting in a reduction of membrane surface charge density. The equivalent circuit models for electrode/solution/membrane/solution/electrode system and electrode/solution system were examined. The results observed in this work seem very interesting in the ion transfer study between phase interfaces.