Development of Voltammetric Sensor for Determination of Tryptophan Using MWCNTs‐modified Sol‐gel Electrode

In the present work, an electrochemical sensor was developed for simple and sensitive determination of tryptophan (Trp) using multi‐walled carbon nanotubes modified sol‐gel electrode (MWCNTs/SGE). The electrocatalytic oxidation of tryptophan was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the oxidation peak current of Trp at the MWCNTs/SGE was greatly improved compared with that of the bare SGE. Furthermore, at the MWCNTs/SGE, the anodic peak potential of Trp is shifted about 220 mV to more negative value indicated that modified electrode has better electrocatalytic activity for electro‐oxidation of Trp. The anodic peak currents increased linearly with the concentration of tryptophan in the range of 0.2 × 10^?6 to 15 × 10^?6 M with a detection limit of 0.139 × 10^?6 M (at an S/N = 3).     

Efficient Direct‐Methanol Fuel Cell Based on Graphene Quantum Dots/Multi‐walled Carbon Nanotubes Composite

Direct‐methanol fuel cells are proton‐exchange fuel cell in which methanol is used as the fuel. The important advantage of these fuel cells is the simplicity of transport and storage of methanol. In this study, methanol fuel cell electrocatalysts including graphene quantum dots (GQDs), functionalized multi‐walled carbon nanotubes (f‐MWCNTs) and GQDs/f‐MWCNTs composite were synthesized. The structures of synthesized electrocatalysts were highlighted by scanning electron microscope (SEM), raman spectroscopy, UV–vis spectroscopy, fourier transform infrared spectroscopy (FTIR), electrochemical impedance spectroscopy (EIS) and x‐ray diffraction (XRD) method. After that, the effective surface areas (ESA) of GQDs, f‐MWCNTs and GQDs/f‐MWCNTs were calculated. Finally, GQDs/f‐MWCNTs composite modified glassy carbon electrode (GQDs/f‐MWCNTs/GCE) showed highest current signals for methanol oxidation than those of comparable GQDs/GCE and f‐MWCNTs/GCE.     

Functionalization of multi‐walled carbon nanotubes with pramipexole for immobilization of palladium nanoparticles and investigation of catalytic activity in the Sonogashira coupling reaction

Pramipexole drug was attached to the surface of multi‐walled carbon nanotubes (MWCNTs) by reaction of acylated carbon nanotubes with pramipexole for the first time. The modified MWCNTs were characterized using Fourier transform infrared spectroscopy, transmission and scanning electron microscopies and CHNS analysis. The prepared pramipexole–MWCNTs were used for immobilization of palladium nanoparticles as a novel nanocatalyst. After characterization of the final nanocomposite, the pramipexole–MWCNTs/Pd was applied as a novel phosphine‐free recyclable heterogeneous catalyst for Sonogashira reactions. Interestingly, the novel catalyst could be recovered and recycled five times without any significant loss in activity.     

Enhanced Electrochemical Detection of DNA Hybridization Based on Au/MWCNTs Nanocomposites

Abstract

The nanocomposites of gold nanoparticles and multi‐walled carbon nanotubes (MWCNTs) have been applied in the enhanced electrochemical detection of DNA hybridization. Gold nanoparticles coated on MWCNTs uniformly were synthesized by simply one step reaction. Target DNA was detected by the peak current difference of differential pulse voltammetry (DPV) signals of the electroactive indicator methylene blue (MB) before and after hybridization on the Au/MWCNTs modified glass carbon electrode (GCE). Due to the excellent electrical conductivity of the novel matrix, the biosensor revealed high sensitivity with the detection level down to 1.0 pM. Excellently selectivity and reproducibility were also discussed.     

Investigation of the dielectric,mechanical, and thermal properties of noncovalent functionalized MWCNTs/polyvinylidene fluoride (PVDF) composites

To improve the dispersion of multi‐walled walled carbon nanotubes (MWCNTs) and investigate the effect of dispersant for MWCNTs functionalization on the dielectric, mechanical, and thermal properties of Polyvinylidene fluoride (PVDF) composites, two different dispersants (Chitosan and TritonX‐100) with different dispersion capability and dielectric properties were used to noncovalently functionalize MWCNTs and prepare PVDF composites via solution blending. Fourier transform infrared, X‐Ray diffraction, and Raman spectroscopy indicated that TritonX‐100 and Chitosan were noncovalent functionalized successfully on the surface of MWCNTs. With the functionalization of Chitosan and TritonX‐100, the dispersion of MWCNTs changed in different extent, which was investigated by dynamic light scattering and confocal laser scan microscopy. The dielectric, mechanical, and thermal properties of PVDF composites were also improved. Meanwhile, it was also found that the dielectric properties of PVDF composites are closely related to the dielectric properties of dispersant. High dielectric constant of dispersant contributes to the grant dielectric constant of PVDF composites. The mechanical and thermal properties of MWCNTs/PVDF composites largely depend on the dispersion of MWCNTs in PVDF, interfacial interactions and the residual solvent. Copyright © 2016 John Wiley & Sons, Ltd.     

Cyclic Voltammograms of Ferrocene on Multi‐Walled Carbon Nanotubes (MWCNTs)‐Modified Edge Plane Pyrolytic Graphite (EPPG) Electrode in Room Temperature Ionic Liquids (RTILs) of 1‐Ethyl‐3‐Methylimidazolium Tetrafluoroborate (EMIBF4)

In this work, the electrochemical behavior of ferrocene (Fc) was investigated by cyclic voltammetry (CV) in room temperature ionic liquids (RTILs) of 1‐ethyl‐3‐methylimidazolium tetrafluoroborate (EMIBF₄) on glass carbon (GC), edge plane pyrolytic graphite (EPPG) and multi‐walled carbon nanotube (MWCNTs)‐modified EPPG electrodes, respectively. The results demonstrated that on GC electrode, pairs of well‐defined reversible peaks were observed, while for the electrode of EPPG, the peak potential separation (ΔE_p) is obviously larger than the theoretical value of 59 mV, hinting that the electrode of EPPG is distinguished from the commonly used electrode, consistent with the previous proposition that EPPG has many “defects”. To obtain an improved electrochemical response, multi‐walled carbon nanotubes (MWCNTs) were modified on the electrode of EPPG; the increased peak current and promoted peak potential separation not only proved the existence of “defects” in MWCNTs, but also supported that “creating active points” on an electrode is the main contribution of MWCNTs. Initiating the electrochemical research of Fc on the MWCNTs‐modified EPPG electrode in RTILs and verifying the presence of “defects” on both EPPG and MWCNTs using cyclic voltammograms (CVs) of Fc obtained in RTILs of EMIBF₄, is the main contribution of this preliminary work.     

Electrocatalytic Activity of Nanohybrids Based on Carbon Nanomaterials and MFe2O4 (M=Co,Mn) towards the Reduction of Hydrogen Peroxide

We report the advantages of hybrid nanomaterials prepared with electrogenerated ferrites (MFe₂O₄; M: Co, Mn) and multi‐walled carbon nanotubes (MWCNTs) or thermally reduced graphene oxide (TRGO) on the electro‐reduction of hydrogen peroxide. Glassy carbon electrodes (GCE) modified with these hybrid nanomaterials dispersed in Nafion/isopropanol demonstrated a clear synergism on the catalytic reduction of reduction of hydrogen peroxide at pH 13.00. The intimate interaction between MFe₂O₄ and carbon nanomaterials allowed a better electronic transfer and a facilitated regeneration of M²⁺ at the carbon nanomaterials, reducing the charge transfer resistances for hydrogen peroxide reduction and increasing the sensitivities of the amperometric response.     

Facile SILAR Processed Bi2S3:PbS Solid Solution on MWCNTs for High‐performance Electrochemical Supercapacitor

Carbon based composite materials have gained much attention because of fulfilling desirable properties for supercapacitor application. In the featured work, the thin film of Bi₂S₃:PbS solid solution has been synthesized on multi‐walled carbon nanotubes (MWCNTs) by simple successive ionic layer adsorption and reaction (SILAR) method. The nanoparticle morphology provides sufficient electroactive channels for electrolyte ions to penetrate during electrochemical activities. The composite exhibits superior specific capacitance 676 F/g at constant specific current density of 5.56 A/g with fast charge‐discharge cycles. In association of energy storage characteristics, the fabricated symmetric cell exhibits excellent energy density of 13.36 Wh/kg by acquiring power density of 0.83 kW/kg. The superior results of the hybrid electrode promise a novel direction for high performance supercapacitor application.     

Highly Selective and Sensitive Detection of Dopamine in the Presence of Excessive Ascorbic Acid Using Electrodes Modified with C60‐Functionalized Multiwalled Carbon Nanotube Films

Electrochemical detection of dopamine (DA) in the presence of a large excess of ascorbic acid (AA) was investigated with a novel all‐carbon nanocomposite film of C₆₀‐MWCNTs (C₆₀‐functionalized multi‐walled carbon nanotubes) using a bare MWCNTs film as control. Although both films can selectively detect DA from AA by separating their oxidation potentials, the C₆₀‐MWCNTs film shows special selectivity and good sensitivity for detecting DA. On one hand, the C₆₀‐MWCNTs composite film shows a higher activity for DA oxidation with enhanced peak current. On the other hand, the C₆₀‐MWCNTs composite film effectively suppresses the oxidation of AA. Remarkably, it is found that the oxidation current of DA is over 2 times higher than that of AA even when the concentration of AA is about 3 to 4 orders of magnitude higher than that of DA. This offers a tremendous advantage for the simple and clean detection of DA free of the interfering AA signal in a real assay. Cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectrometry are used to characterize the C₆₀‐MWCNTs composite film. These novel properties are interpreted to arise from the facile electron transfer between C₆₀ and MWCNTs in the C₆₀‐MWCNTs nanocomposite film.     

Solid phase extraction of polyhalogenated pollutants from freshwater using chemically modified multi‐walled carbon nanotubes and their determination by gas chromatography

This paper describes the application of pristine and chemically modified multi‐walled carbon nanotubes (MWCNTs) as packing materials for the preconcentration and determination of various polyhalogenated organic pollutants, pentachlorophenol, 2,4,5‐trichlorophenol, 3,3′,4,4′‐tetrachlorobiphenyl, and 2,2′,5,5′‐tetrabromobiphenyl from real water samples based on solid‐phase extraction. MWCNTs were chemically modified by octadecyl amine and polyethylene glycol, separately, and the resulting nano materials were used as packing materials for solid phase extraction. Method development, applicability, and suitability of the above mentioned adsorbents for the solid phase extraction were studied. Method development showed great reproducibility and sensitivity, and low limits of detection within a considerable linear range. The regeneration and reusability of the SPE cartridges were studied using Rideau River (Ottawa, Canada) surface water samples and the results showed that cartridges could be used for three cycles of adsorption/desorption with no loss of efficiency. In general, the results suggested that modification of MWCNTs affords a novel class of adsorbents, which could be used for the SPE of various analytes from aqueous solutions with great efficiency, recovery, reproducibility, sensitivity, and precision, within a wide range of analyte concentrations.     

Heterogeneous Electron Transfer and Oxygen Reduction Reaction at Nanostructured Iron(II) Phthalocyanine and Its MWCNTs Nanocomposites

Electron transfer and oxygen reduction dynamics at nanostructured iron(II) phthalocyanine/multi‐walled carbon nanotubes composite supported on an edge plane pyrolytic graphite electrode (EPPGE‐MWCNT‐nanoFePc) platform have been reported. All the electrodes showed the category 3 diffusional behaviour according to the Davies–Compton theoretical framework. Both MWCNTs and MWCNT‐nanoFePc showed huge current responses compared to the other electrodes, suggesting the redox processes of trapped redox species within the porous layers of MWCNTs. Electron transfer process is much easier at the EPPGE‐MWCNT and EPPGE‐MWCNT‐nanoFePc compared to the other electrodes. The best response for oxygen reduction reaction was at the EPPGE‐MWCNT‐nanoFePc, yielding a 4‐electron process.     

Supramolecular Assemblies of Nucleoside Functionalized Carbon Nanotubes: Synthesis,Film Preparation,and Properties

Nucleoside‐functionalized multi‐walled carbon nanotubes ( N‐MWCNTs ) were synthesized and characterized. A self‐organization process using hydrogen bonding interactions was then used for the fabrication of self‐assembled N‐MWCNTs films free of stabilizing agents, polymers, or surfactants. Membranes were produced by using a simple water‐dispersion‐based vacuum‐filtration method. Hydrogen‐bond recognition was confirmed by analysis with IR spectroscopy and TEM images. Restoration of the electronic conduction properties in the N‐MWCNTs membranes was performed by removing the organic portion by thermal treatment under an argon atmosphere to give d‐N‐MWCNTs . Electrical conductivity and thermal gravimetric analysis (TGA) measurements confirmed the efficiency of the annealing process. Finally, oxidative biodegradation of the films N‐MWCNTs and d‐N‐MWCNTs was performed by using horseradish peroxidase (HRP) and low concentrations of H₂O₂. Our results confirm that functional groups play an important role in the biodegradation of CNT by HRP: N‐MWCNTs films were completely biodegraded, whereas for d‐N‐MWCNTs films no degradation was observed, showing that the pristine CNT undergoes minimal enzyme‐catalyzed oxidation This novel methodology offers a straightforward supramolecular strategy for the construction of conductive and biodegradable carbon nanotube films.     

Preparation of HMWCNT/PLLA nanocomposite scaffolds for application in nerve tissue engineering and evaluation of their physical,mechanical and cellular activity properties

This study was aimed to prepare biodegradable and porous nanocomposite scaffolds with microtubular orientation structure as a model for nerve tissue engineering by thermally induced phase separation (TIPS) method using dioxane as the solvent, crystalline poly (L‐lactic acid) (PLLA) and multi‐walled carbon nanotubes (MWCNTs). In order to overcome dispersion of MWCNTs in the PLLA matrix, heparinization of MWCNTs was performed. Solvent crystallization, oriented structure, the mean pore diameter and porosity percentage of the scaffolds were controlled by fundamental system parameters including temperature‐gradient of the system, polymer solution concentration and carbon nanotube content. Scanning Electron Microscopy (SEM), ImageJ, software and dynamic mechanical thermal analysis (DMTA) were used to investigate the structural and mechanical properties. TEM observation was carried out for characterization of nanotube dispersion in PLLA. It was found that the scaffolds containing heparinized multi‐walled carbon nanotubes (HMWCNTs) exhibited higher storage modulus, better carbon nanotube (CNT) dispersion and tubular orientation structure than those with non heparinized MWCNTs. In‐vitro studies were also conducted by using murine P₁₉ cell line as a suitable model system to analyze neuronal differentiation over a 2‐week period. Immunofluorescence and DAPI staining were used to confirm the cells' attachment and differentiation on the PLLA/HMWCNT nanocomposite scaffolds. Based on the results, we can conclude that the PLLA/HMWCNT scaffolds enhanced the nerve cell differentiation and proliferation, and therefore, acted as a positive cue to support neurite outgrowth. Copyright © 2015 John Wiley & Sons, Ltd.     

Melt‐crystallization mechanism of poly(ethylene terephthalate)/multi‐walled carbon nanotubes prepared by in situ polymerization

A series of poly(ethylene terephthalate)/multi‐walled carbon nanotubes (PET/MWCNTs) nanocomposites were prepared by in situ polymerization using different amounts of multi‐walled carbon nanotubes (MWCNTs). The polymerization of poly(ethylene terephthalate) (PET) was carried out by the two‐stage melt polycondensation method. The intrinsic viscosity (IV) of the composites is ranged between 0.31 and 0.63 dL/g depending on the concentration of the MWCNTs. A decrease of IV was found by increasing MWCNTs content. This is due to the reactions taking place between the two components leading to branched and crosslinked macromolecules. These reactions are, mainly, responsible for thermal behavior of nanocomposites. The melting point of the nanocomposites was shifted to slightly higher temperatures by the addition till 0.55 wt % of MWCNTs while for higher concentration was reduced. The degree of crystallinity in all nanocomposites was, also, reduced by increasing MWCNTs amount. However, from crystallization temperature, it was found that MWCNTs till 1 wt % can enhance the crystallization rate of PET, whereas at higher content (2 wt %), the trend is the opposite due to the formation of crosslinked macromolecules. From the extended crystallization analysis, it was proved that MWCNTs act as nucleating agents for PET crystallization. Additionally, the crystallization mechanism due to the existence of MWCNT becomes more complicated because two mechanisms with different activation energies are taking place in the different degrees of crystallization, depending on the percentage of MWCNT. The effect of molecular weight also plays an important role. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1452–1466, 2009     

Electrochemical DNA Biosensors Based on Palladium Nanoparticles Combined with Carbon Nanotubes

Palladium nanoparticles, in combination with multi‐walled carbon nanotubes (MWCNTs), were used to fabricate a sensitivity‐enhanced electrochemical DNA biosensor. MWCNTs and palladium nanoparticles were dispersed in Nafion, which were used to modify a glassy carbon electrode (GCE). Oligonucleotides with amino groups at the 5′ end were covalently linked onto carboxylic groups of MWCNTs on the electrode. The hybridization events were monitored by differential pulse voltammetry (DPV) measurement using methylene blue (MB) as an indicator. Due to the ability of carbon nanotubes to promote electron‐transfer and the high catalytic activities of palladium nanoparticles for electrochemical reaction of MB, the sensitivity of presented electrochemical DNA biosensors was remarkably improved. The detection limit of the method for target DNA was 1.2×10^?13 M.     

Combined effect of graphene oxide and MWCNTs on microwave absorbing performance of epoxy composites

Currently, carbon nanotube (CNT) ‐based composites have been considered as microwave absorbers because of the fascinating properties of CNTs. In this work, multi‐walled CNTs (MWCNTs) and graphene oxide (GO) ‐based epoxy composites (i.e. MWCNT/EPr and GO‐MWCNT/EPr), with sample thickness of 2 mm, were prepared to study microwave absorbing properties in the frequency band of 8–18 GHz. Uniform dispersion of MWCNTs in the organic solvent and polymer matrix was achieved by preparation of GO. The test for electromagnetic parameters, i.e. complex permittivity and the permeability of the samples, was carried out with vector network analyzer (VNA) using reflection‐transmission waveguides. Results showed that GO‐MWCNT/EPr composites have better absorption capability than MWCNT/EPr composites. The improved reflection loss for the composites with 0.4 wt% and 0.6 wt% of GO (out of total filler loading 6 wt%) were ?14.32 dB and ?14.29 dB, respectively. The improvement in reflection loss and absorption bandwidth for GO‐MWCNTs composites suggested that MA features are synergistically effected by GO and MWCNTs. Further skin depth and shielding effectiveness terms are studied to observe overall mechanism of electromagnetic (EM) shielding which showed that multiple reflections also play a role in EM shielding. Copyright © 2015 John Wiley & Sons, Ltd.     

Insight into the Origin of the Positive Effects of Imidazolium Salt on Electrocatalytic Activity: Ionic Carbon Nanotubes as Metal‐Free Electrocatalysts for Oxygen Reduction Reaction

We report remarkable metal‐free electrocatalytic activities of the imidazolium salt‐functionalized ionic multi‐walled carbon nanotubes (IM‐f‐MWCNTs) in the oxygen reduction reaction (ORR). The electrocatalytic activity can be attributed to the induced polarization of the π‐electrons of CNTs, thus accelerating interfacial electron transfer. The zwitterionic MWCNTs functionalized with poly(vinylimidazolium sulfonate) have a more positive surface charge and exhibit a better electrocatalytic activity than the poly(vinylbutylimidazolium chloride)‐functionalized MWCNTs. The IM‐f‐MWCNTs showed better fuel selectivity than the commercial Pt/C electrocatalyst.     

Adsorptive Stripping Differential Pulse Voltammetric Determination of Risperidone with a Multi‐Walled Carbon Nanotube‐Ionic Liquid Paste Modified Glassy Carbon Electrode

A new composite electrode has been fabricated based on coating multi‐walled carbon nanotubes (MWCNTs) and n‐octylpyridinum hexafluorophosphate (OPPF₆) ionic liquid composite on a glassy carbon (GC) electrode (OPPF₆‐MWCNTs/GCE). This electrode shows very attractive electrochemical performances for electrooxidation of risperidone (RIS) compared to conventional electrodes using carbon and mineral oil, notably improved sensitivity and stability. The oxidation peak potentials in cyclic voltammogram of RIS on the OPPF₆‐MWCNTs/GCE was occurred around 230 mV vs. SCE at Britton–Robinson (B–R) buffer (pH 4.0) at scan rate of 100 mV s^?1. The electrochemical parameters such as diffusion coefficient (D), charge transfer coefficient (α) and the electron transfer rate constant (k/_s) were determined using cyclic voltammetry. Under the optimized conditions, the peak current was linear to risperidone concentration over the concentration range of 10–200 nM with sensitivity of 0.016 μA/nM^?1 using differential pulse voltammetry. The detection limit was 6.54 nM (S/N = 3). The electrode also displayed good selectivity and repeatability. In the presence of clozapine (CLZ) the response of RIS kept almost unchanged. Thus this electrode could find application in the determination of RIS in some real samples. The analytical performance of the OPPF₆‐MWCNTs/GCE was demonstrated for the determination of RIS in human serum and pharmaceutical samples.     

In situ Growth of a Cobalt‐based Metal‐organic Framework on Multi‐walled Carbon Nanotubes for Simultaneously Detection of Hydroquinone and Catechol.

In the present study, we report a facile method for preparing a porous MWCNTs/ZIF‐67 nanocomposite with the help of a morphology‐maintained ZIF‐67 in situ growth on multi‐walled carbon nanotubes. Interesting, the MWCNTs/ZIF‐67 nanocomposite demonstrated excellent electrochemical activity for hydroquinone (HQ) and catechol (CC) attribute to the effective interconnections ZIF‐67 crystals and MWCNTs. The analytical curves for HQ and CC obtained by differential pulse voltammetry (DPV) were linear in the range from 0.5 to 100 μM. Benefitting from the excellent conductivity of MWCNTs as well as the high surface area and porosity of ZIF‐67, the advanced nanocomposite displayed good reproducibility, high selectivity and excellent stability, and was successfully employed to assay the content of dihydroxybenzene isomers in the lake water samples.     

Synthesis and properties of iron oxide coated carbon nanotubes hybrid materials and their use in epoxy coatings

Multi‐walled carbon nanotubes (MWCNTs) were acidified with nitration mixture, and the Fe₂O₃‐MWCNTs (iron oxide coated multi‐walled carbon nanotubes) hybrid material via sol‐gel method then verified the results through scanning electron microscope, X‐ray diffraction, and thermal gravimetric analysis. We modified the hybrid material with silane coupling agent (KH560), Fe₂O₃‐MWCNTs/epoxy, MWCNTs/epoxy composites coating, and the pure epoxy coatings were respectively prepared. The properties of the composite coatings were tested through the electrochemical workstation (electrochemical impedance spectroscopy), shock experiments, and thermal gravimetric analysis. Finally, we used scanning electron microscope to observe the surface conditions of the coatings. The results show that Fe₂O₃‐MWCNTs have good dispersion in the epoxy resin, and the Fe₂O₃‐MWCNTs/epoxy composite coatings have enhanced mechanical properties and corrosion resistance. Copyright © 2015 John Wiley & Sons, Ltd.