Electrochemical oxidation of adenosine-5′-triphosphate on a chitosan–graphene composite modified carbon ionic liquid electrode and its determination

In this paper a new electrochemical method was proposed for the determination of adenosine-5′-triphosphate (ATP) based on a chitosan (CTS) and graphene (GR) composite film modified carbon ionic liquid electrode (CTS–GR/CILE). CILE was fabricated by using ionic liquid 1-butyl-3-methylimidazolium dihydrogen phosphate ([BMIM]H₂PO₄) as the binder, which was further modified by GR and CTS composite. The modified electrode exhibited an excellent electrocatalytic activity toward the oxidation of ATP with the increase of the oxidation peak current and the decrease of the oxidation peak potential. The electrochemical parameters of ATP on CTS–GR/CILE were calculated with the electron transfer coefficient (α) as 0.329, the electron transfer number (n) as 2.15, the apparent heterogeneous electron transfer rate constant (ks) as 3.705 × 10^? 5 s^? 1 and the surface coverage (Γ_T) as 9.33 × 10^? 10 mol cm^? 2. Under the optimal conditions the oxidation peak current was proportional to ATP concentration in the range from 1.0 × 10^? 6 to 1.0 × 10^? 3 M with the detection limit of 0.311 μM (S/N = 3). The proposed electrode showed excellent reproducibility, stability, anti-interference ability and further successfully applied to the ATP injection sample detection.     

A selective and sensitive electrochemical determination of catechol based on reduced graphene oxide decorated β-cyclodextrin nanosheet modified glassy carbon electrode

In the present work, we have demonstrated the fabrication of catechol (CC) biosensor based on reduced graphene oxide (rGO) decorated β-cyclodextrin (β-CD) nanosheet immobilized using nafion (Nf) on modified GCE (glassy carbon electrode). The rGO/β-CD nanocomposite is synthesized through sonochemical approach and characterized by spectral (UV–visible, FT-IR, and Raman), analytical techniques (XRD, SEM, SAED, mapping analysis, HR-TEM and EDX) and electrochemical studies. The rGO/β-CD/Nf modified GCE exhibit a prominent electrocatalytic activity towards selective and sensitive determination of CC than other modified electrodes. Besides, the electrochemical sensor was revealed an excellent current response for the determination of CC with wide linear ranges (0.1–0.7 µM), high sensitivity (19.1 µA µM^-1cm²) and very low detection limit (LOD) 0.0012 µM. The excellent reproducibility, selectivity, stability, and sensitivity results are achieved for the determination of CC.     

Highly sensitive amperometric sensor for micromolar detection of trichloroacetic acid based on multiwalled carbon nanotubes and Fe(II)–phtalocyanine modified glassy carbon electrode

A highly sensitive electrochemical sensor for the detection of trichloroacetic acid (TCA) is developed by subsequent immobilization of phthalocyanine (Pc) and Fe(II) onto multiwalled carbon nanotubes (MWCNTs) modified glassy carbon (GC) electrode. The GC/MWCNTs/Pc/Fe(II) electrode showed a pair of well-defined and nearly reversible redox couple correspondent to (Fe(III)Pc/Fe(II)Pc) with surface-confined characteristics. The surface coverage (Γ) and heterogeneous electron transfer rate constant (k_s) of immobilized Fe(II)–Pc were calculated as 1.26 × 10^? 10 mol cm^? 2 and 28.13 s^? 1, respectively. Excellent electrocatalytic activity of the proposed GC/MWCNTs/Pc/Fe(II) system toward TCA reduction has been indicated and the three consequent irreversible peaks for electroreduction of CCl₃COOH to CH₃COOH have been clearly seen. The observed chronoamperometric currents are linearly increased with the concentration of TCA at concentration range up to 20 mM. Detection limit and sensitivity of the modified electrode were 2.0 μM and 0.10 μA μM^? 1 cm^? 2, respectively. The applicability of the sensor for TCA detection in real samples was tested. The obtained results suggest that the proposed system can serve as a promising electrochemical platform for TCA detection.     

Preparation of cerium oxide–MWCNTs nanocomposite bulk modified carbon ceramic electrode: a sensitive sensor for tamoxifen determination in human serum samples

In this study, cerium oxide and multi-walled carbon nanotubes nanocomposite were incorporated into the carbon ceramic electrode (CeO₂–MWCNTs/CCE) as a renewable electrode for the electrocatalytic purposes. To demonstrate capability of the fabricated electrode, determination of tamoxifen as an important anticancer drug with differential pulse voltammetry technique was evaluated in details. Linear range, limit of detection and sensitivity of the developed sensor were found to be 0.2–40 nM, 0.132 nM and 1.478 µA nM^?1 cm^?2, respectively. Ease of production, low cost and high electron transfer rate of the CeO₂–MWCNTs/CCE promises it as a novel electro-analytical tool for determination of important species in real samples.

     

An electrochemical sensor based on a glassy carbon electrode modified with optimized Cu–Fe3O4 nanocomposite for 4-nitrophenol detection

On the basis of toxicity and harmfulness of 4-nitrophenol (4-NP), it is vital to fabricate simple, cheap, selective, and reliable system for trace-level 4-NP sensing detection. In this work, Cu_x–Fe₃O₄ (x?=?0, 0.1, 0.3, 0.5, 0.7) nanocomposites with different Cu loading were prepared. The mixture of the fabricated Cu_x–Fe₃O₄ and conductive carbon black Vulcan XC-72 (VXC-72) nanoparticles, mixed with a mass ratio of 1:1, was applied to modify glassy carbon electrode (GCE) for high-efficiency electrochemical sensitive determination of 4-NP. The results show that the as-prepared Cu_x–Fe₃O₄ nanocomposites present a spherical structure, which is aggregated by many of 20–50 nm crystallites. Compared with the Fe₃O_4, the introduction of Cu in Fe₃O₄ can significantly improve the catalytic reduction performance to 4-NP. Thanks to its minimum particle size and maximum specific surface area, the Cu_0.5–Fe₃O₄ displayed the largest sensitive response to 4-NP. Furthermore, the Cu_0.5–Fe₃O₄@VXC-72/GCE sensor exhibited a good linear relationship between the reduction peak current and the 4-NP concentration in the range of 0.1–4.0 μM and 5–150 μM and a relatively low detection limit of 0.065 μM. The present sensor showed high selectivity, strong anti-interference ability, and good stability to 4-NP. It could be satisfactorily applied for the determination of 4-NP in real water samples, and good recovery rates were found.

     

Development of an amperometric l-lactate biosensor based on l-lactate oxidase immobilized through silica sol–gel film on multi-walled carbon nanotubes/platinum nanoparticle modified glassy carbon electrode

Platinum nanoparticles (Ptnano) were used in combination with multi-walled carbon nanotubes (MWCNTs) for fabricating sensitivity-enhanced electrochemical l-lactate biosensor. The composite film of MWCNTs and Ptnano was dispersed on the surface of the glassy carbon electrode (GCE). l-lactate oxidase (LOD) was immobilized on MWCNTs/Ptnano/GCE surface by adsorption. The resulting LOD/MWCNTs/Ptnano electrode was covered by a thin layer of sol–gel to avoid the loss of LOD in determination and to improve the anti-interferent ability. Moreover, the sol–gel microenviroment contributes to both intensified stability and permselectivity. The cyclic voltammetry results indicated that MWCNTs/Ptnano catalyst displayed a higher performance than MWCNTs. Under the optimized conditions of applied potential 0.5 V, pH 6.4, room temperature, the proposed biosensor showed a large determination range (0.2–2.0 mM), a short response time (within 5 s), a high sensitivity (6.36 μA mM^{− 1}) and good stability (90% remains after 4 weeks). The fabricated biosensor had practically good selectivity against interferences. The results for whole blood samples measured by the present biosensor showed a good agreement with those measured by spectrophotometric method.     

A nano-structured Ni(II)–chelidamic acid modified gold nanoparticle self-assembled electrode for electrocatalytic oxidation and determination of methanol

A nano-structured Ni(II)–chelidamic acid (2,6-dicarboxy-4-hydroxypyridine) film was electrodeposited on a gold nanoparticle–cysteine–gold electrode. The morphology of Ni(II)–chelidamic acid gold nanoparticle self‐assembled electrode was investigated by scanning electron microscopy (SEM). Electrocatalytic oxidation of methanol on the surface of modified electrode was studied by cyclic voltammetry and chronoamperometry methods. The hydrodynamic amperometry at a rotating modified electrode at constant potential versus reference electrode was used for detection of methanol. Under optimized conditions the calibration plots are linear in the concentration range 0–50 mM with a detection limit of 15 μM. The formed matrix in our work possessed a 3D porous network structure with a large effective surface area, high catalytic activity and behaved like microelectrode ensembles. The modified electrode indicated reproducible behavior and a high level stability during the experiments, making it particularly suitable for analytical purposes.     

Simulated strength and structure of carbon–carbon reinforced composite

The atomistic simulations of carbon nanotube (CNT) – carbon reinforced composite material are reported. The studied composite samples are obtained by impregnating certain amounts of CNTs (3,3) and (6,6) into a pristine graphite matrix. The addition of CNTs is found to be of significant usefulness for the CNT–reinforced composites, since it allows to achieve extreme lightness and strength. Being impregnated into graphite matrix, CNTs are able to increase the critical component of its initially highly anisotropic Young modulus by 2–8 times. The linear thermal expansion coefficients do not exceed 10⁻⁶ to 10⁻⁵ K⁻¹, making this material applicable for novel aviation and space vehicles. The degree of dispersion of CNTs within graphite matrix is found to drastically influence composite properties.     

Synthesis and characterization of chitosan–carbon nanotube composites

Acid functionalized single walled carbon nanotubes were covalently grafted to chitosan by first reacting the oxidized carbon nanotubes with thionyl chloride to form acyl-chlorinated carbon nanotubes which are subsequently dispersed in chitosan and covalently grated to form composite material, CNT–chitosan, 1, which was washed several times to remove un-reacted materials. This composite has been characterized by FTIR, 13C NMR, TGA, SEM and TEM and has been shown to exhibit enhanced thermal stability. The reaction of 1, with poly lactic acid has also been accomplished to yield CNTchitosan–g-poly(LA), 2 and fully characterized by the above techniques. Results showed covalent attachment of chitosan and chitosan–poly lactic acid to the carbon nanotubes.     

Mechanical properties and oxidation behaviors of carbon/carbon composites with C–TaC–C multi-interlayer

To improve the mechanical properties and oxidation-resistance properties, a C–TaC–C multi-interlayer structure was introduced in carbon/carbon (C/C) composites by chemical vapor infiltration. Compared with conventional C/C composites, a higher fracture toughness and strength have been achieved by using the C–TaC–C multi-interlayer. In addition, the composites also exhibit a higher preliminary oxidation temperature and a lower mass loss at high temperatures. The oxidation rate of the composites increases with temperature increasing in the range of 700–1300 °C, reaching a maximum value at 1300 °C, then decreases in 1300–1400 °C. A hexagonal structure of Ta₂O₅ phase is obtained when being oxidized at 700–800 °C, and it transforms to an orthorhombic phase at temperatures above 900 °C. The structures of C–TaC–C multi-interlayer are intact without cracks or porosities after being oxidized at 700–800 °C. In 900–1300 °C, the composites are oxidized uniformly with the formation of pores. At temperatures above 1300 °C, there are oxidation and non-oxidation regions with the oxidation process being controlled by diffusion.     

A simple and efficient electrochemical sensor for folic acid determination in human blood plasma based on gold nanoparticles–modified carbon paste electrode

Folic acid (FA) is a water soluble vitamin that exists in many natural species. The lack of FA causes some deficiencies in human body, so finding a simple and sensitive method for determining the FA is important. A new chemically modified electrode was fabricated for determination of FA in human blood plasma using gold nanoparticles (AuNPs) and carbon paste electrode (CPE). Gold nanoparticles–modified carbon paste electrode (AuNPs/CPE) was characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The experimental parameters such as pH, scan rate (ν) and amount of modifier were studied by cyclic voltammetry and the optimized values were chosen. The electrochemical parameters such as diffusion coefficient of FA (D_FA), electrode surface area (A) and electron transfer coefficient (α) were calculated. Square wave voltammetry as an accurate technique was used for quantitative calculations. A good linear relation was observed between anodic peak current (i_pa) and FA concentration (C_FA) in the range of 6 × 10^? 8 to 8 × 10^? 5 mol L^? 1, and the detection limit (LOD) achieved 2.7 × 10^? 8 mol L^? 1, that is comparable with recently studies. This paper demonstrated a novel, simple, selective and rapid sensor for determining the FA in the biological samples.     

Application of ionic liquid–TiO2 nanoparticle modified carbon paste electrode for the voltammetric determination of benserazide in biological samples

An ionic liquid–TiO₂ nanoparticle modified carbon paste electrode (IL–TiO₂/CPE) was used as a fast and sensitive tool for the investigation of the electrochemical oxidation of benserazide using voltammetry. This modified electrode has been fabricated using hydrophilic ionic liquid (n-hexyl-3-methylimidazolium hexafluoro phosphate) as a binder. The modified electrode offers a considerable improvement in voltammetric sensitivity toward benserazide, compared to the bare electrode. Using differential pulse voltammetry (DPV), the electrocatalytic oxidation peak current of benserazide shows a linear calibration curve in the range of 1.0–600 μmol L^? 1 benserazide. The limit of detection was equal to 0.4 μmol L^? 1. The relative standard deviation (RSD%) for eight successive assays of 10 μmol L^? 1 benserazide was 1.1%. Finally, the proposed method was successfully applied to the determination of benserazide in real samples such as blood serum and urine.     

Application of graphene-ionic liquid-chitosan composite-modified carbon molecular wire electrode for the sensitive determination of adenosine-5′-monophosphate

In this paper, a graphene (GR) ionic liquid (IL) 1-octyl-3-methylimidazolium hexafluorophosphate and chitosan composite-modified carbon molecular wire electrode (CMWE) was fabricated by a drop-casting method and further applied to the sensitive electrochemical detection of adenosine-5′-monophosphate (AMP). CMWE was prepared with diphenylacetylene (DPA) as the modifier and the binder. The properties of modified electrode were examined by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. Electrochemical behaviors of AMP was carefully investigated with enhanced responses appeared, which was due to the presence of GR-IL composite on the electrode surface with excellent electrocatalytic ability. A well-defined oxidation peak of AMP appeared at 1.314 V and the electrochemical parameters were calculated by electrochemical methods. Under the selected conditions, the oxidation peak current of AMP was proportional to its concentration in the range from 0.01 μM to 80.0 μM with the detection limit as 3.42 nM (3σ) by differential pulse voltammetry. The proposed method exhibited good selectivity and was applied to the detection of vidarabine monophosphate injection samples with satisfactory results.     

Modelling of carbon–carbon composite manufacturing processes

This paper is devoted to the modelling of technological processes of manufacturing of siliconized carbon–carbon composites. The developed model describes the changes that occur in the properties of the composites (strength, elastic moduli, shrinkage) during the technological cycle of manufacturing and also the residual stresses generated in composite structures. It is shown that the level of the residual stresses and the character of changes in the properties of carbon–carbon composites essentially differ from those of polymer–matrix composites.     

Electrochemical horseradish peroxidase biosensor based on dextran–ionic liquid–V2O5 nanobelt composite material modified carbon ionic liquid electrode

Direct electrochemistry of horseradish peroxidase (HRP) was realized in a dextran (De), 1-ethyl-3-methylimidazolium ethylsulphate ([EMIM]EtOSO₃) and V₂O₅ nanobelt composite material modified carbon ionic liquid electrode (CILE). Spectroscopic results indicated that HRP retained its native structure in the composite. A pair of well-defined redox peaks of HRP appeared in pH 3.0 phosphate buffer solution with the formal potential of ?0.213 V (vs. SCE), which was the characteristic of HRP heme Fe(III)/Fe(II) redox couple. The result was attributed to the specific characteristics of De–IL–V₂O₅ nanocomposite and CILE, which promoted the direct electron transfer rate of HRP with electrode. The electrochemical parameters of HRP on the composite modified electrode were calculated and the electrocatalysis of HRP to the reduction of trichloroacetic acid (TCA) was examined. Under the optimal conditions the reduction peak current increased with TCA concentration in the range from 0.4 to 16.0 mmol L^?1. The proposed electrode is valuable for the third-generation electrochemical biosensor.     

The study of inspection on SiC coated carbon–carbon composite with subsurface defects by lock-in thermography

An investigation into the effect of size on the quantitative estimation of defect depth in a SiC coated carbon–carbon (C/C) composite has been undertaken by lock-in thermography. A dedicated 3-D thermal modeling has been introduced, and an efficient numerical algorithm based on finite-difference splitting method in time domain (FDSM-TD) is applied to solve the thermal model. The heat transfer partial differential equation (PDE) and mathematic morphological algorithms are used to filter the phase angle data noise. The diameter of a defect had an appreciable effect on the observed phase angle which consequently has significant implications with regard to estimating the defect depth. Phase angle contrast measurements for a range of defects in a 6.0 mm SiC coated C/C composite specimen indicate that an optimal excitation frequency of 0.525 Hz is available for defect detection. Results obtained with an excitation frequency of 0.525 Hz are used to discuss the limitations of determining the defect size and depth.     

Determination of chlorpyriphos in broccoli using a voltammetric acetylcholinesterase sensor based on carbon nanostructure–chitosan composite material

The widespread use of organophosphate pesticides (OPPs) in agriculture leads to residue accumulation in the environment, which is dangerous to human health and disrupts the ecological balance. This study reports results obtained by a standard chromatographic method (high performance liquid chromatography, HPLC) and an amperometric method for chlorpyriphos (CPF) determination in broccoli. Reversed-phase HPLC with UV–VIS detection was used for the separation, and the mobile phase was acetonitrile–water (75:25 v/v). The electrochemical experiments were performed in phosphate buffer solutions at pH = 7.4, with an incubation time of 10 min. The response of the sensor was a linear function of CPF concentration from 10^? 10 to 10^? 7 M with a corresponding equation: y = 0.1467x + 1.4472 (R² = 0.9959) and a detection limit of 1.58 × 10^? 10 M. The biosensor methodology was used to analyze CPF directly in broccoli that had been previously spiked with CPF solution.     

Preparation and corrosion behavior of Ni and Ni–graphene composite coatings

The graphite oxide was synthesized using the Hummers method, and then it was reduced by hydrazine hydrate to obtain graphene. It was characterized with UV (ultra violet), IR (infra red), XRD (X-ray diffraction) spectra and SEM (scanning electron microscope) images. The composite coating of Ni–graphene on mild steel specimens was obtained by the electrodeposition technique. The composite coating was subjected to various electrochemical tests to know its corrosion behavior and compared with pure Ni coating. The EIS (electrochemical impedance spectroscopy) was performed to confirm the corrosion resistance property. The composite film was studied by recording its XRD and SEM. The crystallite size, texture coefficients and hardness of coating was measured.     

Process and mechanical properties of carbon/carbon–silicon carbide composite reinforced with carbon nanotubes grown in situ

A hierarchical C_f/C–SiC composite was fabricated via in situ growth of carbon nanotubes (CNTs) on fiber cloths following polymer impregnation and pyrolysis process. The effects of CNTs grown in situ on mechanical properties of the composite, such as flexural strength, fracture toughness, crack propagation behavior and interfacial bonding strength, were evaluated. Fiber push-out test showed that the interfacial bonding strength between fiber and matrix was enhanced by CNTs grown in situ. The propagation of cracks into and in fiber bundles was impeded, which results in decreased crack density and a “pull-out of fiber bundle” failure mode. The flexural strength was increased while the fracture toughness was not improved significantly due to the decreased crack density and few interfacial debonding between fiber and matrix, although the local toughness can be improved by the pull-out of CNTs.