Adsorption and Reactivity of Chlorogenic Acid at a Hydrophobic Carbon Ceramic Composite Electrode: Application for the Amperometric Detection of Hydrazine

Sol-gel technique was used for construction of a carbon composite electrode. The prepared carbon ceramic electrode was modified with electroless deposition of chlorogenic acid for less than 1 min. The adsorbed thin films of chlorogenic acid on carbon composite electrode show one pair of peaks with a surface confined characteristic, which strongly depends on the solution pH, as anticipated for quinone /hydroquinone functionalities. The modified electrode shows highly catalytic activity toward hydrazine electrooxidation at wide pH range (5–11). Also the rotating modified electrode shows excellent analytical performance for amperometric determination of hydrazine. The detection limit, sensitivity, response time and linear dynamic range are 20 nM, 220 nA / μM, 1 second and 0.1 μM-1 mM, respectively. The catalytic rate constant for hydrazine oxidation at the surface of modified electrode was evaluated by cyclic voltammetry and was found to be around 1.5×10³ M⁻¹s⁻¹in phosphate buffer solution (pH 8). The precision of chronoamperometric measurements was 1–3% for 5 replicate determinations in the concentration range of the linear calibration. The reproducibility of modified CCE was evaluated with 8 successive polishing and modifications and then the anodic peak current was measured (RSD 2%). The advantages of this sensor are excellent catalytic activity, high sensitivity, good reproducibility and simplicity of preparation at short time periods.     

Simultaneous detection of ascorbate and uric acid using a selectively catalytic surface

The direct and selective detection of ascorbate at conventional carbon or metal electrodes is difficult due to its large overpotential and fouling by oxidation products. Electrode modification by electrochemical reduction of diazonium salts of different aryl derivatives is useful for catalytic, analytical and biotechnological applications. A monolayer of o-aminophenol (o-AP) was grafted on a glassy carbon electrode (GCE) via the electrochemical reduction of its in situ prepared diazonium salts in aqueous solution. The o-aminophenol confined surface was characterized by cyclic voltammetry. The grafted film demonstrated an excellent electrocatalytic activity towards the oxidation of ascorbate in phosphate buffer of pH 7.0 shifting the overpotential from +462 to +263 mV versus Ag/AgCl. Cyclic voltammetry and d.c. amperometric measurements were carried out for the quantitative determination of ascorbate and uric acid. The catalytic oxidation peak current was linearly dependent on the ascorbate concentration and a linear calibration curve was obtained using d.c. amperometry in the range of 2-20 μM of ascorbate with a correlation coefficient 0.9998, and limit of detection 0.3 μM. The effect of H₂O₂ on the electrocatalytic oxidation of ascorbate at o-aminophenol modified GC electrode has been studied, the half-life time and rate constant was estimated as 270 s, and 2.57 × 10⁻³ s⁻¹, respectively. The catalytically selective electrode was applied to the simultaneous detection of ascorbate and uric acid, and used for their determination in real urine samples. This o-AP/GCE showed high stability with time, and was used as a simple and precise amperometric sensor for the selective determination of ascorbate.     

Electrodeposited nano-scale islands of ruthenium oxide as a bifunctional electrocatalyst for simultaneous catalytic oxidation of hydrazine and hydroxylamine

For the first time, an electrodeposited nano-scale islands of ruthenium oxide (ruthenium oxide nanoparticles), as an excellent bifunctional electrocatalyst, was successfully used for hydrazine and hydroxylamine electrocatalytic oxidation. The results show that, at the present bifunctional modified electrode, two different redox couples of ruthenium oxides serve as electrocatalysts for simultaneous electrocatalytic oxidation of hydrazine and hydroxylamine. At the modified electrode surface, the peaks of differential pulse voltammetry (DPV) for hydrazine and hydroxylamine oxidation were clearly separated from each other when they co-exited in solution. Thus, it was possible to simultaneously determine hydrazine and hydroxylamine in the samples at a ruthenium oxide nanoparticles modified glassy carbon electrode (RuON-GCE). Linear calibration curves were obtained for 2.0-268.3 μM and 268.3-417.3 μM of hydrazine and for 4.0-33.8 μM and 33.8-78.3 μM of hydroxylamine at the modified electrode surface using an amperometric method. The amperometric method also exhibited the detection limits of 0.15 μM and 0.45 μM for hydrazine and hydroxylamine respectively. RuON-GCE was satisfactorily used for determination of spiked hydrazine in two water samples. Moreover, the studied bifunctional modified electrode exhibited high sensitivity, good repeatability, wide linear range and long-term stability.     

Electrocatalytic properties of [Ru(bpy)(tpy)Cl]PF6 at carbon ceramic electrode modified with nafion sol-gel composite: application to amperometric detection of l-cysteine

A two-step sol-gel technique was used here to prepare a carbon ceramic electrode modified with nafion and [Ru(bpy)(tpy)Cl]PF₆. This involves two steps: first, forming a bulk-modified carbon ceramic electrode with nafion, and then immersing the electrode into a Ru-complex solution (electroless deposition) for a short period of time (5-25 s). Cyclic voltammograms of the resulting surface-modified carbon ceramic electrode show stable and a well-defined redox couple due to Ru(II)/Ru(III) system with surface-confined characteristic. l-Cysteine (CySH) has been chosen as a model to elucidate the electrocatalytic ability of Ru-complex nafion sol-gel composite electrode. Not only the modified electrode shows excellent catalytic activity toward l-cysteine electrooxidation in pH range 3-9, but the antifouling effect of nafion film also increases the reproducibility of results in comparison with CCE modified with homogeneous mixing of graphite powder and Ru-complex (one step sol-gel method). Under the optimized conditions in amperometry method, the concentration calibration range, detection limit and sensitivity were 0.1-100 μM, 20 nM and 50 nA/μM, respectively. The advantages of this modified electrode are good reproducibility, excellent catalytic activity, simplicity of preparation and especially its antifouling properties towards l-cysteine and its oxidation products. Additionally, it is promising as a detector in flow system or chromatography systems.     

A glucose biosensor using methyl viologen redox mediator on carbon film electrodes

A new methyl viologen-mediated amperometric enzyme electrode sensitive to glucose has been developed using carbon film electrode substrates. Carbon film electrodes from resistors fabricated by pyrolytic deposition of carbon were modified by immobilization of glucose oxidase through cross-linking with glutaraldehyde in the presence of bovine serum albumin. The mediator, methyl viologen, was directly immobilised with the enzyme together with Nafion cation-exchange polymer. The electrochemistry of the glucose oxidase/methyl viologen modified electrode was investigated by cyclic voltammetry and by electrochemical impedance spectroscopy. The biosensor response to glucose was evaluated amperometrically; the detection limit was 20 μM, the linear range extended to 1.2 mM and the reproducibility of around 3%. When stored in phosphate buffer at 4 °C and used every day, the sensor showed good stability over more several weeks.     

Preparation and layer-by-layer self-assembly of positively charged multiwall carbon nanotubes

The report described a method of more stably dispersing oxidized carbon nanotubes (CNTs) by forming complex with polycation and the layer-by-layer self-assembly behavior of the complex with polyanion was studied. The properties of the self-assembled multilayer film containing carbon nanotubes were studied. Cyclic voltammetry, UV-vis-NIR spectroscopy, electrochemical impedance spectroscopy and scanning electron microscopy were used for characterization of film assembly. UV-vis-NIR spectroscopy and cyclic voltammetry study indicated the uniform growth of the film. Electrochemical impedance spectroscopy results showed that incorporating of carbon nanotubes in the polyelectrolyte multilayers decreased in the electron-transfer resistance R_ct, indicating more favorable electrochemical reaction interface. The electrocatalytic property of the multilayer modified electrode to NADH was investigated mainly with different numbers of the bilayers and the results showed that along with the increase of the assembled bilayers the overpotential of NADH oxidation decreased. The detection limit could reach 6 μM at a detection potential of 0.4 V.     

Amperometric Sensor for Detection of the Reduced Form of Nicotinamide Adenine Dinucleotide Using a Poly(pyronin B) Film Modified Electrode

An electrochemical method for the preparation of poly(pyronin B) film was proposed in this paper. A poly(pyronin B) (poly(PyB)) film modified glassy carbon electrode (GCE) has been fabricated via an electrochemical oxidation procedure and applied to the electrocatalytic oxidation of reduced form of nicotinamide adenine dinucleotide (NADH). The poly(PyB) film modified electrode surface has been characterized by atomic force microscope (AFM), scanning electron microscope (SEM), electrochemical impedance spectroscopy (EIS), UV‐visible absorption spectrophotometry (UV‐vis) and cyclic voltammetry (CV). These studies have been used to investigate the poly(PyB) film, which demonstrates the formation of the polymer film and the excellent electroactivity of poly(PyB) in neutral and even in alkaline media. Due to its potent catalytic effects towards the electrooxidation of NADH at lower potential (0.0 V), poly(PyB) film modified electrode can be used for the selective determination of NADH in real samples because of dopamine, ascorbic acid and uric acid oxidation can be avoided at this potential. The catalytic peak currents are linearly dependent on the concentrations of NADH in the range of 1.0×10^?6 to 5.0×10^?4 mol/L with correlation coefficients of 0.999. The detection limits for NADH is 0.5×10^?6 mol/L. Poly(PyB) modified electrode also shows good stability and reproducibility due to the irreversible attachment of polymer film at GCE surface.     

Selective determination of dopamine with a cibacron blue/poly-1,5-diaminonaphthalene composite film

A selective detection method for dopamine (DA) was developed by incorporating cibacron blue (F3GA) into poly-1,5-diaminonaphthalene (PDAN) layer. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) were employed to characterize the modified surfaces. The modified electrode was effective in selectively facilitating the electron transfer of DA and blocking the interferences of negatively charged species attributed to the sulfonate groups in the F3GA/PDAN composite film. This method enabled the determination of DA in the presence of various interfering species, including ascorbic acid (AA), in a phosphate buffer solution (pH 7.4). The modified electrode demonstrated good performance in the detection of DA in a concentration range of 5.0-100 μM, with a detection limit (k = 3) of 0.1 ± 0.01 μM. The application was conducted for the determination of DA in a human urine sample and the sensor was proven to be rapid, has excellent selectivity, and stable amperometric response.     

Grafted Azure A modified electrodes as disposable β-nicotinamide adenine dinucleotide sensors

We report the in situ generation of aryl diazonium cations of Azure A, a redox-active phenothiazine dye, by reaction between the corresponding aromatic aminophenyl group and sodium nitrite in 0.1 M HCl. The subsequent electrochemical reduction of these dye diazonium salts gives rise to conductive electrografted films onto screen-printed carbon (SPC) electrodes. The resulting Azure A films have a very stable and reversible electrochemical response and exhibit potent and persistent electrocatalytic behavior toward NADH oxidation. We have optimized the electrografting conditions in order to obtain SPC modified electrodes with high and stable electrocatalytic response. The kinetic of the reaction between the NADH and the redox active centers in the Azure A film has been characterized using cyclic voltammetry and single step chronoamperometry. The catalytic currents were proportional to the concentration of NADH giving rise to linear calibration plots up to a concentration of 0.5 mM with a detection limit of 0.57 ± 0.03 μM and a sensitivity of 9.48 A mol cm⁻² μM⁻¹. The precision of chronoamperometric determinations was found to be 2.3% for five replicate determinations of 3.95 μM NADH. The great stability of such modified electrodes makes them ideal for their application in the development of biosensing platforms based on dehydrogenases.     

One pot synthesis of poly(5-hydroxyl-1,4-naphthoquinone) stabilized gold nanoparticles using the monomer as the reducing agent for nonenzymatic electrochemical detection of glucose

Monodispersed and highly stable gold nanoparticles with a diameter between 8 and 9 nm were synthesized in a weakly alkaline medium by chemical reduction of AuCl₄⁻ using 5-hydroxyl-1,4-naphthoquinone, and stabilized by the simultaneously formed poly(hydroxyl-1,4-naphthoquinone). The electrochemical properties of the resultant poly(hydroxyl-1,4-naphthoquinone) stabilized gold nanoparticles (AuNQ NPs) and its electrocatalytic activity for glucose oxidation in alkaline media were then investigated using a range of techniques, including dc cyclic, rotating disk electrode and Fourier transformed large amplitude ac voltammetry. The results demonstrate that these AuNQ NP modified electrodes exhibit excellent catalytic activity toward glucose oxidation in the potential region where the premonolayer oxidation process occurs. The overall catalytic glucose oxidation process was found to be mass transport controlled under the experimental conditions employed, allowing measurements to be conducted with a high reproducibility. The AuNQ NP modified electrodes showed a high sensitivity of 183 μA mM⁻¹ cm⁻² with a wide linear dynamic range of 0.5–50 mM and a detection limit of 61 μM. However, despite its excellent tolerance toward ascorbic acid, significant interference from uric acid was found with this AuNQ NP modified electrode.     

Use of nickel implanted boron-doped diamond thin film electrode coupled to HPLC system for the determination of tetracyclines

The electrochemical analysis of tetracyclines was investigated using nickel-implanted boron-doped diamond thin film electrode (Ni-DIA) by cyclic voltammetry and high performance liquid chromatographic with amperometry. Cyclic voltammetry was used to study the electrochemical oxidation of tetracyclines. Comparison experiments were carried out utilizing as-deposited BDD and glassy carbon electrodes. Ni-DIA electrode provided well-resolved oxidative irreversible cyclic voltammograms and the highest current signals among the electrode studied. High performance liquid chromatography (HPLC) with amperometric detection was also studied. The chromatography was performed using a commercially available Inertsil C18 column, with the mobile phase being: 80% phosphate buffer (pH 2.5)-20% acetonitrile and detected at 1.55 V. The methods were validated over the concentration range 0.05-100 ppm with the overall average recoveries from 83.3 to 102.5% and R.S.D. of less than 10%. The proposed method was further applied to analyse shrimp samples.     

Amperometric and voltammetric detection of hydrazine using glassy carbon electrodes modified with carbon nanotubes and catechol derivatives

A simple procedure was developed to prepare a glassy carbon (GC) electrode modified with carbon nanotubes (CNTs) and catechol compounds. First, 25 microL of DMSO-CNTs solutions (0.4 mg/mL) was cast on the surface of GC electrode and dried in air to form a CNTs film. Then the GC/CNTs modified electrode immersed into a chlorogenic acid, catechine hydrate and caffeic acid solution (electroless deposition) for a short period of time (2-80s). The cyclic voltammogram of the modified electrode in aqueous solution shows a pair of well-defined, stable and nearly reversible redox couple (quinone/hydroquinone) with surface confined characteristics. The combination of unique electronic and electrocatalytic properties of CNTs and catechol compounds results in a remarkable synergistic augmentation on the response. The electrochemical reversibility and stability of modified electrode prepared with incorporation of catechol compound into CNTs film was evaluated and compared with usual methods for attachment of catechols to electrode surfaces. The transfer coefficient (alpha), heterogeneous electron transfer rate constants (k(s)) and surface concentrations (Gamma) for GC/CNTs/catechol compound modified electrodes were calculated through the cyclic voltammetry technique. The modified electrodes showed excellent catalytic activity, fast response time and high sensitivity toward oxidation of hydrazine in phosphate buffer solutions at pH range 4-8. The modified electrode retains its initial response for at least 2 months if stored in dry ambient condition. The properties of modified electrodes as an amperometric sensor for micromolar or lower concentration detection of hydrazine have been characterized.     

Amperometric sensing of NADH and ethanol using a hybrid film electrode modified with electrochemically fabricated zirconia nanotubes and poly (acid fuchsin)

We report on a glassy carbon electrode (GCE) modified with a film of chitosin containing acid fuchsin (AF) adsorbed onto zirconia nanotubes. The mixture was polymerized by cyclic voltammetric scannings in the potential range from - 0.8?V to +1.3?V in buffer solution to produce a hybrid film electrode (nano-ZrO₂/PAF/GCE). The morphology of the hybrid film electrode surface was characterized by scanning electron microscopy. Its electrochemical properties were studied via electrochemical impedance spectroscopy. The electrochemical response of nicotinamide adenine dinucleotide (NADH) was investigated by differential pulse voltammetry and amperometry. The results indicated that the nano-ZrO₂/PAF/GCE possesses well synergistic catalytic activity towards NADH. Compared to an unmodified GCE, the oxidation overpotential is negatively shifted by 224?mV, and the oxidation current is significantly increased. Under optimal conditions, the amperometric response is linearly proportional to the concentration of NADH in the 1.0 – 100.0?μM concentration range. Ethanol also can be determined by amperometry if alcohol dehydrogenase and NADH are added to the sample. Two linear relationships between current and alcohol concentration were obtained. They cover the range from 0.03 to 1.0?mM, and from 1.0 to 12.0?mM.

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.     

Application of a palladium hexacyanoferrate film-modified aluminum electrode to electrocatalytic oxidation of hydrazine.

A palladium hexacyanoferrate (PdHCF) film as an electrocatalytic material was obtained at an aluminum (Al) electrode by a simple electroless dipping method. The modified Al electrode demonstrated a well-behaved redox couple due to the redox reaction of the PdHCF film. The PdHCF film showed an excellent electrocatalytic activity toward the oxidation of hydrazine. The electrocatalytic oxidation of hydrazine was studied by cyclic voltammetry and rotating disk electrode voltammetry techniques. A calibration graph obtained for the hydrazine consisted of two segments (localized at concentration ranges 0.39-10 and 20-75 mM). The rate constant k and transfer coefficient alpha for the catalytic reaction and the diffusion coefficient of hydrazine in the solution D, were found to be 3.11 x 10(3) M(-1) s(-1), 0.52 and 8.03 x 10(-6) cm2 s(-1) respectively. The modified electrode was used to amperometric determination of hydrazine in photographic developer. The interference of ascorbic acid and thiosulfate were investigated and greatly reduced using a thin film of Nafion on the modified electrode. The modified electrode indicated reproducible behavior and a high level of stability during electrochemical experiments, making it particularly suitable for analytical purposes.     

Electrocatalytic oxidation of hydrazine at poly(4,5-dihydroxy-1,3-benzenedisulfonic acid) multiwall carbon nanotubes modified-glassy carbon electrode: Improvement of the catalytic activity

A stable electroactive thin film of poly(4,5-dihydroxy-1,3-benzenedisulfonic acid) was electrochemically deposited at the surface of multiwall carbon nanotubes-glassy carbon electrode. The electrocatalytic oxidation of hydrazine has been studied at the surface of the modified electrode using cyclic voltammetry, chronoamperometry and linear sweep voltammetry as diagnostic techniques. The modified electrode exhibits good electrocatalytic activity for the oxidation of hydrazine with a good sensitivity. Linear calibration range was in the wide concentration range of 10–3540 μM hydrazine with a detection limit of 1.8 μM and a sensitivity of 85.3 nA/μM. A Tafel plot, derived from voltammograms, indicated a one-electron transfer process to be the rate-limiting step and the overall number of electrons involved in the catalytic oxidation of hydrazine was found to be four. The influences of potentially interfering substances were studied. The diffusion coefficient of hydrazine was also evaluated. Finally, the proposed modified electrode was used for the determination of hydrazine in spiked water samples.     

Electrocatalytic oxidation and voltammetric determination of ciprofloxacin employing poly(alizarin red)/graphene composite film in the presence of ascorbic acid,uric acid and dopamine

A glassy carbon electrode modified with poly(alizarin red)/electrodeposited graphene (PAR/EGR) composite film was prepared and applied to detect ciprofloxacin (CPFX) in the presence of ascorbic, uric acid and dopamine. The morphology and interface property of PAR/EGR films were examined by scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The electrocatalytic oxidation of CPFX on AR/EGR was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The linearity ranged from 4 × 10⁻⁸ to 1.2 × 10⁻⁴ M with a detection limit (S/N = 3) of 0.01 μM. The modified electrode could be applied to the individual determination of CPFX as well as the simultaneous determination of CPFX, ascorbic acid, uric acid and dopamine. This method proved to be a simple, selective and rapid way to determine CPFX in pharmaceutical preparation and biological media.     

Voltammetric and amperometric determination of ascorbic acid at a chemically modified carbon fibre microelectrode

A carbon fibre microelectrode was modified by electrodeposition of a thin copper-heptacyanonitrosylferrate (CuHNF) film on its surface. The film showed the ability to catalyse electrochemical oxidation of ascorbic acid. The catalytic reaction was limited either by diffusion, or by the electrochemical reaction of the catalyst. A linear, cyclic voltammetric response for ascorbic acid was obtained between 5.0 x 10(-5) and 6.0 x 10(-3)M. By amperometric measurements the linear range could be extended to 8.0 x 10(-6)-2.0 x 10(-3)M. The modified electrode showed good stability and reproducibility.     

Electrochemistry and electrocatalytic activity of catechin film on a glassy carbon electrode toward the oxidation of hydrazine

A very stable electroactive film of catechin was electrochemically deposited on the surface of activated glassy carbon electrode. The electrochemical behavior of catechin modified glassy carbon electrode (CMGCE) was extensively studied using cyclic voltammetry. The properties of the electrodeposited films, during preparation under different conditions, and the stability of the deposited film were examined. The charge transfer coefficient (α) and charge transfer rate constant (k _s) for catechin deposited film were calculated. It was found that the modified electrode exhibited excellent electrocatalytic activity toward hydrazine oxidation and it also showed a very large decrease in the overpotential for the oxidation of hydrazine. The CMGCE was employed to study electrocatalytic oxidation of hydrazine using cyclic voltammetry, rotating disk voltammetry, chronoamperometry, amperometry and square-wave voltammetry as diagnostic techniques. The catalytic rate constant of the modified electrode for the oxidation of hydrazine was determined by cyclic voltammetry, chronoamperometry and rotating disk voltammetry and was found to be around 10⁻³ cm s⁻¹ . In the used different voltammetric methods, the plot of the electrocatalytic current versus hydrazine concentration is constituted of two linear segments with different ranges of hydrazine concentration. Furthermore, amperometry in stirred solution exhibits a detection limit of 0.165 μM and the precision of 4.7% for replicate measurements of 40.0 μM solution of hydrazine.     

Amperometric detection of hydrazine by cyclic voltammetry and flow injection analysis using ruthenium modified glassy carbon electrodes

Glassy carbon electrodes modified with (5-amino-1,10-phenanthroline)bis(bipyridine)ruthium(II) chloride hydrate, [(bpy)₂Ru(5-phenNH₂)]Cl₂·H₂O, are shown to oxidize hydrazine with excellent sensitivity. The presence of an amine group on the ruthenium complex facilitates electropolymerization onto the electrode surface. Using cyclic voltammetry, a large catalytic current is observed upon oxidation of hydrazine in phosphate buffer (pH 5.0), compared to the current obtained from the ruthenium-modified electrode with no hydrazine present. The sensitivity of cyclic voltammetry is sufficient for obtaining a linear calibration curve for hydrazine over the range of 10⁻⁵ to 10⁻² M. Hydrodynamic amperometry was used to determine the working potential for flow injection analysis. The limit of detection for hydrazine was determined to be 8.5 μM using FIA. The thickness of these films was shown to increase linearly with the number of electropolymerization cycles, in the range of 1000-2500 nm for 5-20 cycles, respectively, using Rutherford backscattering spectrometry (RBS). RBS analysis also suggests that the film is multilayered with the outermost layers containing a high ruthenium concentration, followed by layers where the concentration of ruthenium decreases linearly and approaches zero at the electrode surface.