Simultaneous determination of hydroquinone and catechol based on glassy carbon electrode modified with gold-graphene nanocomposite

We have synthesized a virtually monodisperse gold-graphene (Au-G) nanocomposite by a single-step chemical reduction method in aqueous dimethylformamide solution. The nanoparticles are homogenously distributed over graphene nanosheets. A glassy carbon electrode was modified with this nanocomposite and displayed high electrocatalytic activity and extraordinary electronic transport properties due to its large surface area. It enabled the simultaneous determination of hydroquinone (HQ) and catechol (CC) in acetate buffer solution of pH?4.5. Two pairs of well-defined, quasi-reversible redox peaks are obtained, one for HQ and its oxidized form, with a 43 mV separation of peak potentials (ΔE_p), the other for CC and its oxidized form, with a ΔE_p of 39 mV. Due to the large separation of oxidation peak potentials (102 mV), the concentrations of HQ and CC can be easily determined simultaneously. The oxidation peak currents for both HQ and CC increase linearly with the respective concentrations in the 1.0 μM to 0.1 mM concentration range, with the detection limits of 0.2 and 0.15 μM (S/N?=?3), respectively. The modified electrode was successfully applied to the simultaneous determination of HQ and CC in spiked tap water, demonstrating that the Au-G nanocomposite may act as a high-performance sensing material in the selective detection of some environmental pollutants.

Electrochemical behavior of tartaric acid at CuGeO3 nanowire modified glassy carbon electrode

The electrochemical behavior of tartaric acid at the CuGeO₃ nanowire modified glassy carbon electrode has been investigated by cyclic voltammetry (CV). The results show that two pairs of semireversible electrochemical peaks are observed and can be assigned to the process of oxidation–reduction and adsorption–desorption of tartaric acid at the modified glassy carbon electrode, respectively. The intensity of the CV peaks increases linearly with the increase of the content of tartaric acid in the range of 0.01–5 mM and scan rate ranging from 25–200 mV s^?1. CuGeO₃ nanowire modified glassy carbon electrode exhibits good detection ability for tartaric acid in neutral solution with the detection limit of 8.9 and 7.7 μM for cvp1 and cvp2, respectively, at a signal-to-noise ratio of 3. The CuGeO₃ nanowire modified glassy carbon electrode has good reproducibility and stability.     

Electrochemical Behavior of Hydroquinone at Poly (Acridine Orange)–Modified Electrode and Its Separate Detection in the Presence of o‐Hydroquinone and m‐Hydroquinone

Abstract

Poly (acridine orange) (PAO) film–modified electrode was prepared by the electrooxidation of Acridine orange on a glassy carbon electrode (GCE) for the detection of hydroquinone in the presence of o‐hydroquinone and m‐hydroquinone. The electrochemical behavior of hydroquinone on the modified electrode was investigated with respect to different solution acidity, scan rate, and accumulation time. A pair of sharp and well‐defined peaks was obtained at 0.45 and 0.42 V [vs. a saturated calomel electrode (SCE)] at the PAO film–modified electrode. The potential difference between this pair of cathodic and anodic peaks was decreased to only 30 mV as compared to the 241 mV that was obtained on the bare glassy carbon electrode (GCE). As to o‐hydroquinone and m‐hydroquinone, their corresponding oxidation peaks appeared at 0.55 V and 0.89 V (vs. SCE), respectively. The oxidation potential differences between these three isomers enabled the separate detection of hydroquinone. Under the optimum experimental situation, the oxidation peak current of hydroquinone was proportional to the concentration at the range of 6.8×10^?7–9.6×10^?5 M. The detection limit was been estimated as 3×10^?7 M with 130 s accumulation. This method was applied to the hydroquinone detection in tap water samples.     

Determination of Some Phenothiazines Compounds in Pharmaceuticals and Human Body Fluid by Electrocatalytic Oxidation at a Glassy Carbon Electrode Using Methylene Blue as a Mediator

Abstract

A glassy carbon electrode plus Methylene blue as a mediator was employed to study and sense the electrocatalytic oxidation of phenothiazines, including chlorpromazine, perphenazine, promazine, and fluphenazine, using cyclic voltammetry and chronoamperometry as diagnostic techniques. The electron-transfer coefficient, alpha (= 0.45), for phenothiazines compounds at the surface of glassy carbon electrode was determined using a cyclic voltammetry technique. It was found that under a selected pH (8.6) the peak current due to the oxidation of Methylene blue at the surface of the electrode that occurrs at a potential of about ? 180 mV is proportional to the phenothiazines concentration. Linear analytical curves were obtained in the ranges of 1.0 × 10^?6 ? 2.1 × 10^?4 mol L^?1 for the phenothiazines compounds. The influences of potentially interfering substances on the current response of the system were examined. The method was used for the determination of phenothiazines compounds, including chlorpromazine, perphenazine, promazine, and fluphenazine in human.     

Kinetics of oxidation of certain pyrimidines by Ce(IV)

The kinetics of the oxidation of certain biologically important pyrimidine bases (Uracil, Thymine, and 6-Methyluracil) by Ce(IV) in aqueous H₂SO₄ has been investigated. A first-order dependence of rate each on [Ce(IV)], [pyrimidine], and an inverse first-order dependence on [H₂SO₄] has been observed. Rate and activation parameters for the oxidation of these pyrimidines have been computed. A suitable rate law and a mechanism consistent with the kinetic observations and product analysis have been proposed. © 1996 John Wiley & Sons, Inc.     

A highly sensitive nonenzymatic glucose sensor based on nickel oxide–carbon nanotube hybrid nanobelts

In this study, we demonstrated a highly sensitive electrochemical sensor for the determination of glucose in alkaline aqueous solution by using nickel oxide single-walled carbon nanotube hybrid nanobelts (NiO–SWCNTs) modified glassy carbon electrode (GCE). The hybrid nanobelts were prepared by the deposition of SWCNTs onto the Ni(SO₄)_0.3(OH)_1.4 nanobelt surface, followed by heat treatment at different temperatures ranging from 400 °C to 600 °C. The NiO–SWCNTs hybrid nanobelts modified electrode prepared at 500 °C displays enhanced electrocatalytic activity towards glucose oxidation, revealing a synergistic effect between the NiO and the deposited SWCNTs. The as-fabricated nonenzymatic glucose sensor exhibits excellent glucose sensitivity (2,980 μA cm^?2 mM^?1), lower detection limit (0.056 μM, signal/noise [S/N] ratio?=?3), and wider linear range (0.5–1,300 μM). Moreover, the sensor has been successfully used for the assay of glucose in serum samples with good recovery, ranging from 96.4 % to 102.4 %.     

The electrochemical behavior of PtRu- and Pt-modified zeolite X in alkaline solution

Zeolite NaX was modified by Pt and Pt/Ru nanodispersed metallic clusters. The procedure of impregnation with acetylacetonate salt/acetone solution was applied. Scanning electron microscope analysis confirmed partial zeolite framework destruction. According to energy dispersive X-ray analysis, Pt/Ru ratio in sample was about 1. Electrochemical behavior of PtRu- and Pt-modified zeolites was investigated in alkaline solutions, 5 mM NaOH?+?1 M Na₂SO₄ and 0.1 M NaOH. The shape of cyclic voltammograms of 13XPtRu electrode, recorded in slightly alkaline solution, was greatly affected by the presence of hydrogen that remained in the sample after synthetic procedure. Oxygen reduction reaction (ORR) was investigated in an O₂-saturated aqueous 0.1-M NaOH solution. The obtained Tafel slopes indicated ORR mechanism that involves one-electron discharge-determining step. According to Koutecky–Levich slope, the oxygen reduction reaction followed 4e^? mechanism on both 13XPtRu and 13XPt electrode. The onset of ORR on 13XPtRu electrode was shifted toward more positive potentials in comparison to 13XPt electrode.     

Electrochemical deposition of platinum nanoparticles in multiwalled carbon nanotube–Nafion composite for methanol electrooxidation

Platinum nanoparticles were successfully deposited within a multiwalled carbon nanotube (MWCNT)–Nafion matrix by a cyclic voltammetry method. A Pt(IV) complex was reduced to platinum nanoparticles on the surface of MWCNTs. The resulting Pt nanoparticles were characterized by scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The Pt–MWCNT–Nafion nanocomposite film-modified glassy carbon electrode had a sharp hydrogen desorption peak at about −0.2 V vs. Ag/AgCl (3 M) in a solution of 0.5 M H₂SO₄, which is directly related to the electrochemical activity of the Pt nanoparticles presented on the surface of MWCNTs. The electrocatalytic properties of the Pt–MWCNT–Nafion nanocomposite-modified glassy carbon electrode for methanol electrooxidation were investigated by cyclic voltammetry in a 2 M CH₃OH + 1 M H₂SO₄ solution. In comparison with the Pt-coated glassy carbon electrode and the Pt–Nafion modified glassy carbon electrode, the Pt–MWCNT–Nafion-modified electrode had excellent electrocatalytic activity toward methanol electrooxidation. The stability of the Pt–MWCNT–Nafion nanocomposite-modified electrode had also been evaluated.     

The Electro‐Oxidation of Formaldehyde at a Boron‐Doped Diamond Electrode

Abstract

The characteristics of the boron‐doped diamond (BDD) electrode in this work were studied by atomic force microscope (AFM), scanning electron microscopy, and Raman spectroscopy. The electro‐oxidation of formaldehyde at the BDD electrode in 0.5 M K₂SO₄ with different pH was studied by cyclic voltammetry and amperometry. There is no significant oxidation peak of formaldehyde in acidic solution because the oxidation of formaldehyde is at the potential range of water discharge. However, in neutral solution, there is a well‐defined oxidation peak at about +2.2 V vs. Ag/AgCl. The relation between the response current and formaldehyde concentration is linear behavior at the concentration range from 50 to 600 µM. Besides, in neutral solution, the oxidation of formaldehyde is dominated by indirect oxidation at lower formaldehyde concentration, and it is dominated by direct oxidation at higher concentration. Finally, in alkaline solution, the oxidation of formaldehyde is dominated by indirect oxidation caused by a powerful oxidant and is related to the ratio of the amounts of formaldehyde and OH molecules at the BDD electrode surface.     

Sensitive Hydrazine Electrochemical Biosensor Based on a Porous Chitosan–Carbon Nanofiber Nanocomposite Modified Electrode

A novel electrochemically-based biosensor was developed for the determination of hydrazine by modifying a glassy carbon electrode with an aqueous dispersion of carboxylic group-functionalized carbon nanofiber/chitosan solution, and then absorbing hemoglobin on the surface of chitosan-carbon nanofibers. Nafion was used to coat the hemoglobin membrane. The interactions of hemoglobin and the nafion/chitosan-carbon nanofibers were investigated by ultraviolet-visible absorption, infrared, and circular dichroism spectroscopies. The results indicated that the native structure of hemoglobin was retained post-immobilization. The circular dichroism results showed that the α-helical structure of hemoglobin was preserved though a small change was observed in the presence of the nafion/chitosan-carbon nanofibers. The modified nanofibers were further characterized by scanning electron microscopy, electron impendence spectroscopy, and cyclic voltammetry. The electrocatalytic mechanism of hemoglobin to the oxidation of hydrazine was investigated and an irreversible diffusion-controlled electrode process was obtained. The electron transfer rate constant (k_s), transfer coefficient (α), and Michaelis–Menten constant (K_m) were also evaluated. The peak current of the catalytic oxidation was linear with hydrazine concentration from 3.722 × 10^?5 to 1.601 × 10^?3 molar with a correlation coefficient of 0.995. The detection limit was estimated to be 2.7 micromoles per liter. The sensitivity, stability, and reproducibility of the nafion/hemoglobin/chitosan-carbon nanofiber/glassy carbon electrode for the oxidation of hydrazine were also investigated.     

Simple and sensitive determination of hydroxyl radical in atmosphere based on an electrochemically activated glassy carbon electrode

The hydroxyl radical (?OH) plays important roles in environment and health problems. However, the short life time and low concentrations of ?OH limited its detection. In this work, a simple method has been successfully performed for the sensitive detection of hydroxyl radical based on an activated glassy carbon electrode (AGCE).4-hydroxybenzoic acid (4-HBA) was used as a trapping agent for ?OH radicals, leading to the production of electroactive 3,4-dihydroxybenzoic acid (3,4-DHBA). Different procedures including polarisation and cyclic voltammetry in acid or base solutions have been used to activate the glassy carbon electrodes. The electrochemical behaviours of 3,4-DHBA on these activated electrodes were studied and compared. Experimental results showed that the glassy carbon electrode polarised in H₂SO₄ (AGCE-P/H₂SO₄) has the greatest sensitivity and reproducibility to 3,4-DHBA. 3,4-DHBA performed a linear relationship from 1.0 × 10^?7 to 1.0 × 10^?4 M on the AGCE-P/H₂SO₄. The detection limit was down to 6.2 × 10^?8 M. This method has been successfully applied for the detection of hydroxyl radical levels in atmosphere without separation and purification process.     

Graphene Quantum Dots Incorporated into β-cyclodextrin: a Novel Polymeric Nanocomposite for Non-Enzymatic Sensing of L-Tyrosine at Physiological pH

Graphene quantum dot-β-cyclodextrin modified glassy carbon electrode was used as a new nanosensor for determination of L-tyrosine (L-Tyr). It was found that graphene quantum dot-β-cyclodextrin has been stably electrodeposited on glassy carbon electrode modified by simple technique. The cyclic voltammograms of the modified electrode in an aqueous solution displayed a pair of well-defined, stable and irreversible reductive/oxidation redox systems. The apparent electron transfer rate constant (k_s) and transfer coefficient (α) determined by cyclic voltammetry were approximately equal to 8.0 s^–1 and 0.7, respectively. The modified electrode showed excellent catalytic activity towards the oxidation of L-Tyr at positive potential in buffer solution. The nanosensor also displayed fast response time, high sensitivity, low detection limit and a remarkably positive potential oxidation of L-Tyr that decreased the effect of interferences in analysis.     

Electrochemical polymerization of toluidine blue o and its electrocatalytic activity toward NADH oxidation

A stable electroactive thin film of poly(toluidine blue o) (PTOB) has been deposited on the surface of a glassy carbon electrode by cyclic voltammetry from an aqueous solution containing toluidine blue o (TOB). Cyclic voltammograms of PTOB indicate the presence of two redox couples and the formal potential shifts linearly in the negative direction with increasing solution pH with a slope of 58 and 54 mV per pH unit for couple I and couple II, respectively. The PTOB modified glassy carbon electrode shows electrocatalytic activity toward NADH oxidation in phosphate buffer solution (pH 7.0), with an overpotential ca. 470 mV lower than that of the bare electrode. The catalytic rate constant of the modified glassy carbon electrode for the oxidation of NADH is determined by cyclic voltammetry and rotating disk electrode measurements. The experimental results indicate that the electrode can be used as a detector for NADH determination with a linear range of 5.0×10⁻⁶ to 2.0×10⁻³ mol l⁻¹ and the detection limits of (5.0±0.3)×10⁻⁷ mol l⁻¹ at optimal conditions.     

Electrochemical Determination of Anti-Hyperlipidemic Drug Ezetimibe Based on its Oxidation on Solid Electrodes

Ezetimibe is the first of a new class of drugs that selectively inhibits cholesterol absorption in the small intestine and reduces plasma LDL cholesterol. In this study, electrochemical oxidation of ezetimibe was investigated on carbon based electrodes and a single and irreversible peak at both electrodes was observed. A linear response was detected between 2 × 10^?6 and 8 × 10^?5 M with glassy carbon electrode and between 2 × 10^?6 and 2 × 10^?4 M with a boron-doped diamond electrode in 0.1 M H₂SO₄ supporting electrolyte. The proposed methods were successfully applied for the determination of ezetimibe from pharmaceutical dosage forms and human serum samples.     

Investigation on Concentrated Ⅴ（Ⅳ）/Ⅴ（Ⅴ） Redox Reaction by Rotating Disc Voltammetry

The kinetic characteristics of the concentrated Ⅴ（Ⅳ）/Ⅴ（Ⅴ） couple have been studied at a glassy carbon electrode in sulfuric acid using rotating-disc electrode and cyclic voltammetry. The kinetics of the Ⅴ（Ⅳ）/Ⅴ（Ⅴ） redox couple reaction was found to be electrochemically quasi-reversible with the slower kinetics for the Ⅴ（Ⅴ） reduction than that for the Ⅴ（Ⅳ） oxidation. And, dependence of diffusion coefficients and kinetic parameters of Ⅴ（Ⅳ） species on the Ⅴ（Ⅳ） and H2SO4 concentration was investigated. It is shown that the concentration of active species Ⅴ（Ⅳ） should be over 1 mol·L^-1 for the redox flow battery application. Further, with increasing the Ⅴ（Ⅳ） and H2SO4 concentration, the diffusion coefficients of Ⅴ（Ⅳ） were gradually reduced whereas its kinetics was improved considerably, especially in the case of Ⅴ（Ⅳ） and H2SO4 up to 2 and 4 mol·L^-1.     

A study of the electrocatalytic oxidation of methanol on a nickel–salophen-modified glassy carbon electrode

Nickel–salophen-modified glassy carbon electrodes prepared by transferring one drop of Ni–salophen complex solution on the electrode surface. This modified electrode has been used for the electrocatalytic oxidation of methanol in alkaline solutions with various methods such as cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy. The electrooxidation was observed as large anodic peaks, and early stages of the cathodic direction of potential sweep around 20 mV vs. Ag|AgCl|KCl_sat. A mechanism based on the electrochemical generation of Ni (Ш) active sites and their subsequent consumptions by methanol have been discussed. EIS studies were employed to unveil the charge transfer rate as well as the electrical characteristics of the catalytic surface. For the electrochemical oxidation of methanol at 5.0 M concentration, charge transfer resistance of nearly 0.936 kΩ was obtained, while the resistance of the electrocatalyst layer was about 111.6 Ω.     

Narrow span coulometric analysis of neptunium

A narrow span (E _o ±0.1 V) controlled potential coulometric method has been developed for the determination of neptunium in 1M H₂SO₄ with a RSD of 0.2%. The main advantage of this method over the existing coulometric methods is that it can tolerate up to a 5-fold excess of plutonium. The method involves carrying out the electrolysis to about 97% and calculating by an iterative computation the formal electrode potential in situ, which is used to calculate the total amount present in the sample. The method consists in oxidation of all the neptunium to Np (VI) by Ce(IV), destruction of excess Ce(IV) and reduction of Np(VI) to Np(V) by NaNO₂, destruction of excess nitrite by sulfamic acid followed by coulometric titration of Np(V) to Np(VI).     

Study on Electrocatalytic Oxidation of L‐Cysteine at Glassy Carbon Electrode by (FcM)TMA and Its Electrochemical Kinetics

The electrocatalytic oxidation of L ‐cysteine by (ferrocenylmethyl)trimethylammonium at a glassy carbon electrode in 0.1 M Na₂SO₄ aqueous solution has been studied. The rate constant for the catalytic reaction was evaluated as (4.28±0.05)×10³ M^?1 s^?1 by chronoamperometry. Experimental conditions, which maximize the current efficiency of the electrocatalytic oxidation, such as pH value and the concentration of the catalyst, were also investigated. The experimental results of electrocatalytic kinetics of L ‐cysteine oxidation on GCE in the presence of (ferrocenylmethyl)trimethylammonium obviously support the reaction mechanism proposed and the rate determining step assumed in scheme described in this work.     

Electrocatalytic oxidation of NADH at mesoporous carbon modified electrodes

The electrochemical oxidation of β-nicotinamine adenine dinucleotide (NADH) was investigated at a glassy carbon electrode modified with carbon mesoporous materials (CMM). Due to the large surface area and electro-catalytic properties of CMM, the overpotential of the electrodes toward the oxidation of NADH is decreased by 595 mV in aqueous solution at neutral pH. The anodic peak currents increase steadily with the concentration of NADH in the range from 2 µM to 1.1 mM, the detection limit being 1.0 µM at pH 7.2 and a potential of +0.3 V vs. SCE. The apparent Michaelis-Menten constant is ～21.5 μM. The results enable NADH to be sensed at a low potential and are promising with respect to the design of dehydrogenase-based amperometric biosensors.     

High-performance electrochemical sensor based on electrodeposited iron oxide nanoparticle: catecholamine as analytical probe

In this paper, we report the synthesis and electrocatalytic activity of electrodeposited Fe₂O₃ nanoparticles modified on a glassy carbon electrode as highly sensitive sensors for determination of catecholamines. Results showed that the Fe₂O₃ nanoparticles on a glassy carbon electrode exhibit excellent catalytic activity toward catecholamines oxidation, including levodopa, dopamine, and epinephrine, resulting in a marked lowering in the peak potential and considerable improvement of the peak current as compared to the electrochemical activity at the bare glassy carbon electrode. The electrochemical characterizations of catecholamines were performed using cyclic voltammetry, chronoamperometry, and differential pulse voltammetry techniques. The electrocatalytic currents increase linearly with the levodopa, dopamine, and epinephrine concentrations in the ranges of 0.0625–1000, 0.25–1500, and 0.125–1000 µM, respectively, and the detection limits (3σ) were 24 ± 2, 14 ± 2, and 12 ± 2 nM, respectively.