A biosensor based on Coriolopsis gallica laccase immobilized on nitrogen-doped multiwalled carbon nanotubes and graphene oxide for polyphenol detection

The use of nanomaterials allows the design of ultrasensitive biosensors with advantages in the detection of organic molecules. Catechol and catechin are molecules that occur naturally in fruits, and their presence in products like dyes and wines affects quality standards. In this study, catechol and catechin were measured at the nanoscale by means of cyclic voltammetry. The oxidation of Coriolopsis gallica laccase immobilized on nitrogen-doped multiwalled carbon nanotubes (Lac/CN_x-MWCNT) and on graphene oxide (Lac/GO) was used to measure the concentrations of catechol and catechin. Nitrogen-doped multiwalled carbon nanotubes (CN_x-MWCNT) were synthesized by spray pyrolysis and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS). Covalently bonded hybrids with laccase (Lac/CN_x-MWCNT and Lac/GO) were generated. Catalytic activity of free enzymes determined with syringaldazine yielded 14 584 UmL⁻¹. With Lac/CN_x-MWCNT at concentrations of 6.4 mmol L⁻¹ activity was 9326 U mL⁻¹, while enzyme activity measured with Lac/GO at concentration of 6.4 mmol L⁻¹ was 9 234 U mL⁻¹. The Lac/CN_x-MWCNT hybrid showed higher stability than Lac/GO at different ethyl alcohol concentrations. The Lac/CN_x-MWCNT hybrid can measure concentrations, not previously reported, as low as 1 × 10⁻⁸ mol L⁻¹ by measuring the electric current responses.     

Synthesis, characterization and properties of carbon nanotubes microspheres from pyrolysis of polypropylene and maleated polypropylene

Microspheres assembled from carbon nanotubes (MCNTs), with the diameters ranging from 5.5 to 7.5 μm, were synthesized by means of pyrolysis of polypropylene and maleated polypropylene in an autoclave. The characterization of structure and morphology was carried out by X-ray diffractometer (XRD), field-emission scanning electron microscopy (FESEM), (high resolution) transmission electron microscope [(HR)TEM)], selected-area electron diffraction (SAED) and Raman spectrum. As a typical morphology, the possible growth process of MCNTs was also investigated and discussed. The results of nitrogen adsorption-desorption indicate that the Brunauer-Emett-Teller (BET) surface area (140.6 m²/g) of the MCNTs obtained at 600 °C is about twice as that (74.5 m²/g) of carbon nanotubes obtained at 700 °C. The results of catalytic experiment show that MCNTs based catalyst has higher catalytic activity than the carbon nanotubes based catalyst for the preparation of methanol and dimethoxy-ethane by oxidation of dimethyl ether.     

The effect of carbon nanotubes microstructures on reinforcing properties of SWNTs/alumina composite

Alumina reinforced with 1 wt% single-wall carbon nanotubes (SWNTs) was fabricated by hot-pressing. The fracture toughness of SWNTs/Al₂O₃ composite reaches 6.40 ± 0.3 MPa m^1/2, which is twice as high as that of unreinforced alumina. Nanoindentation introduced controlled cracks and the damage were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SWNTs reinforcing mechanisms including CNT pullout, CNT fracture, CNT bridging and crack deflection were directly observed, and the relationship between carbon nanotubes microstructures in the matrix and mechanical properties was also discussed in detailed.     

Preparation of high yield multi-walled carbon nanotubes by microwave plasma chemical vapor deposition at low temperature

Vertically-aligned carbon nanotubes(CNTs) with multi-walled structure were successfully grown on a Fe-deposited Si substrate at low temperature below 330°C by using the microwave plasma chemical vapor deposition of methane and carbon dioxide gas mixture. This is apparently different from the conventional reaction in gas mixtures of hydrogen and methane, hydrogen and acetylene, and hydrogen and benzene ... etc. High quality carbon nanotubes were grown at lower temperature with CO₂ and CH₄ gas mixture than those used by the previous. After deposition, the microstructure morphology of carbon nanotubes was observed with scanning electron microscope and high-resolution transmission electron microscope. The characteristics of carbon nanotubes were analyzed by laser Raman spectroscopy. The results showed the variation of the flow rate ratio of CH₄/CO₂ from 28.5 sccm/30 sccm to 30/30 sccm and the DC bias voltage from –150 V to –200 V, at 300 W microwave power, 1.3–2.0 kPa range of total gas pressure, and substrate temperatures between 300°C and 350°C. Vertically aligned carbon nanotubes with the diameter of about 15 nm and multi-walled structure were illustrated by SEM and HRTEM. However, the highest yield of carbon nanotubes of about 50% was obtained at low temperature below 330°C by MPCVD for the CH₄/CO₂ gas mixture with properly controlled parameters.     

Facile synthesis of MnO2/CNT nanocomposite and its electrochemical performance for supercapacitors

A nanocomposite of manganese dioxide coated on the carbon nanotubes (MnO₂/CNTs) was synthesized by a facile direct redox reaction between potassium permanganate and carbon nanotubes without any other oxidant or reductant addition. The morphology, microstructure and crystalline form of this MnO₂/CNT nanocomposite were characterized by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrochemical properties are characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (GCD). The results show that the facile prepared MnO₂/CNTs nanocomposite shows specific capacitance of 162.2 F g⁻¹ at the current density of 0.2 A g⁻¹ and excellent charge/discharge property with 90% of its specific capacitance kept after 2000 cycles at the current density of 5 A g⁻¹.     

SiOC ceramic nanotubes of ultrahigh surface area

Silicon oxycarbide ceramic nanotubes have been successfully synthesized by inert atmosphere pyrolysis of polysilicone nanotubes using a sacrificial alumina membrane as a template at different pyrolysis temperatures. Scanning electron microscopy images show that the silicon oxycarbide ceramic nanotubes have well-aligned tubular structures. X-ray diffraction patterns and Raman spectra reveal that the obtained silicon oxycarbide ceramic nanotubes are amorphous below 1200 °C and are mainly composed of SiO₂ crystallites and free carbon when the temperature exceeds 1300 °C. Nitrogen-sorption isotherms indicate that the silicon oxycarbide ceramic nanotubes have high Brunauer–Emmett–Teller (BET) specific surface areas (up to 1387 m²/g), large pore volumes (up to 1.82 cm³/g).     

Carbon nanotube-assisted synthesis and high catalytic activity of CeO2 hollow nanobeads

CeO₂ hollow nanobeads have been synthesized facilely using carbon nanotubes as templates by means of a solvothermal treatment combined with controlled calcinations. The properties of CeO₂ hollow nanobeads were characterized by TEM, EDS, XRD and XPS. The obtained CeO₂ hollow nanobeads with polycrystalline face-centered cubic phase have a uniform morphology ranging from 150 to 200 nm in outer diameter and 40 to 60 nm in inner diameter. A possible formation mechanism has been suggested to explain the formation of CeO₂ hollow nanobeads. It is found that CeO₂ hollow nanobeads have an excellent catalytic performance for the CO oxidation.     

Preparation and characterization of titanate nanotubes/carbon composites

Titanate nanotubes/carbon composites(TNT/CCs) were synthesized by allowing carbon-coated TiO₂ (CCT) powder to react with a dense aqueous solution of NaOH at 120 °C for a proper period of time. As-prepared CCT and TNT/CCs were characterized by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectrometry. The processes for formation of titanate nanotubes/carbon composites were discussed. It was found that the TiO₂ particles in TiO₂-carbon composite were enwrapped by a fine layer of carbon with a thickness of about 4 nm. This carbon layer functioned to inhibit the transformation from anatase TiO₂ to orthorhombic titanate. As a result, the anatase TiO₂ in CCT was incompletely transformed into orthorhombic titanate nanotubes upon 24 h of reaction in the dense and hot NaOH solution. When the carbon layers were gradually peeled off along with the formation of more orthorhombic titanate nanotubes at extended reaction durations (e.g., 72 h), anatase TiO₂ particles in CCT were completely transformed into orthorhombic titanate nanotubes, yielding TNT/CCs whose morphology was highly dependent on the reaction time and temperature.     

Theoretical investigation of methane adsorption onto boron nitride and carbon nanotubes

Methane adsorption onto single-wall boron nitride nanotubes (BNNTs) and carbon nanotubes (CNTs) was studied using the density functional theory within the generalized gradient approximation. The structural optimization of several bonding configurations for a CH₄ molecule approaching the outer surface of the (8,0) BNNT and (8,0) CNT shows that the CH₄ molecule is preferentially adsorbed onto the CNT with a binding energy of −2.84 kcal mol⁻¹. A comparative study of nanotubes with different diameters (curvatures) reveals that the methane adsorptive capability for the exterior surface increases for wider CNTs and decreases for wider BNNTs. The introduction of defects in the BNNT significantly enhances methane adsorption. We also examined the possibility of binding a bilayer or a single layer of methane molecules and found that methane molecules preferentially adsorb as a single layer onto either BNNTs or CNTs. However, bilayer adsorption is feasible for CNTs and defective BNNTs and requires binding energies of −3.00 and −1.44 kcal mol⁻¹ per adsorbed CH₄ molecule, respectively. Our first-principles findings indicate that BNNTs might be an unsuitable material for natural gas storage.     

Production of empty and iron-filled multiwalled carbon nanotubes from iron–phthalocyanine polymer and their electromagnetic properties

A facile production of multiwalled carbon nanotubes (MCNTs) using iron-phthalocyanine polymer as the only carbon source with two kinds of metallic catalysts (Fe(CO)₅ and nano-iron) has been compared here. SEM, TEM and XRD were employed to figure the detailed structures of the carbon nanotubes. Consequently, catalyst played a key role in the formation of MCNTs: nano-iron resulted in iron-filled CNTs while Fe(CO)₅ led to empty CNTs. Both of these two CNTs were long and straight, with ~100 nm in diameter and several tens of micrometers in length. Moreover, dielectric and magnetic properties were carried to further study synthesized carbon nanotubes. The results showed that the empty MCNTs had better dielectric properties than iron-filled MCNTs although the iron-filled CNTs exhibited the magnetic saturation of ~3.5 emu/g and coercive force of ~594.0 Oe, which is much higher than empty MCNTs.     

Bulk preparation and characterization of mesoporous carbon nanotubes by catalytic decomposition of cyclohexane on sol–gel prepared Ni–Mo–Mg oxide catalyst

Multiwalled carbon nanotubes were synthesized using Ni–Mo–Mg oxide catalyst prepared by sol–gel technique. Carbon nanotubes were formed in situ by the reduction of nickel oxide (NiO) and molybdenum oxide (MoO₃) to Ni and Mo by a gas mixture of nitrogen, hydrogen and cyclohexane at 750 °C. Scanning Electron Microscopy (SEM) was used to confirm the formation of carbon nanotubes (CNTs). The pore size distribution of carbon nanotubes (CNTs) was investigated by N₂ adsorption and desorption. It was found that the pore size fell into the mesopore range: 2 < d < 50 nm. Interpretation was also made using Raman spectroscopy, Diffuse reflectance spectroscopy, X-ray diffraction and ESR spectra. This method is found to produce a very high yield weighing over 20 times of the catalyst. Based on the experimental conditions and results obtained a possible growth mechanism of the carbon nanotubes is proposed.     

Preparation of ZnS nanotubes via surfactant micelle-template inducing reaction

Semiconductor ZnS nanotubes, with outer diameters in the range of 37–52 nm and lengths up to 3 microns, have been successfully synthesized from solutions containing a nonionic surfactant, Triton X-100 (t-octyl-(OCH₂CH₂) _x OH, x = 9,10). The as-synthesized nanotubes have been characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM). The growth mechanism of nanotubes has also been discussed.     

Dissolution, characterization and photofunctionalization of carbon nanotubes

A simple method to disperse carbon nanotubes (CNTs) has been achieved, which gives two photofunctionalized CNTs, hydrazine nanotubes (h-CNTs) and 1,3,4-oxadiazole nanotubes (o-CNTs). Results from FTIR, ¹H NMR spectroscopy and TEM observations showed that the functionalization was successful. The modified nanotubes can dissolve in most of the nonpolar organic solvents and no precipitate was observed in the solution of the nanotubes even after 2 months. The functionalized nanotubes showed photo-electronic properties, which is due to the attachment of the function groups to them as proved by steady-state fluorescence spectroscopy. Both h-CNTs and o-CNTs showed good thermal stability below 300 °C and might be used as functional materials.     

Facile synthesis route of graphene-like structures from multiwall carbon nanotubes

Graphene-like nanostructures were synthesized from multiwall carbon nanotubes through chemical exfoliation route in mild conditions. For this purpose multiwall carbon nanotubes were synthesized by Chemical Vapor Deposition method using Al-Fe-Co catalyst and treated with KMnO₄. The obtained nanostructures were characterized by Raman spectroscopy, XRD, FTIR, EDX, SEM and TEM methods. FTIR results show that, treating the carbon nanotubes with KMnO₄ decorates their surface with oxygen containing functional groups. XRD and Raman spectroscopy results reveal that the outermost layers of the nanotubes were exfoliated during the treatment. The formation of graphene-like nanostructures was confirmed by SEM and TEM methods. The novelty of this work is the first time use of this type of mild and cheap condition for obtaining graphene-like nanostructures from MWCNTs without any other intermediate treatment.     

Calorimetric study of carbon nanotubes and aluminum

Differential scanning calorimeter (DSC) was used to investigate apparent activation energy and reaction order of the reaction of carbon nanotubes and aluminum by Kissinger equation and Crane equation under non-isothermal condition. The reaction product was examined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experimental results show that carbon nanotubes react with aluminum and form Al₄C₃ phases with needle shape. The peak temperature of the reaction of carbon nanotubes and aluminum is found to depend on the heating rate during the continuous heating. Apparent activation energy and reaction order of the reaction of carbon nanotubes and aluminum are 194.01 and 0.92 kJ/mol, respectively.     

Preparation and characterization of yttria-stabilized zirconia nanotubes

Yttria-stabilized zirconia (YSZ) nanotubes were synthesized by the sol–gel method using porous anodic alumina oxide (AAO) as the templates. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersion X-ray (EDX) spectrum and selected area electron diffraction (SAED) techniques were used to characterize the morphology and crystalline structure of the prepared YSZ nanotubes. The length and the diameter of the YSZ nanotubes are 50 μm and 200 nm, respectively, which are in good agreement with the dimensions of the template pores, while the wall thickness of the nanotubes depends on the impregnation time. XRD and SAED measurements indicate that the obtained YSZ nanotubes after sintering at 1073 K possess a polycrystalline structure and a cubic crystal phase. Brunauer–Emmett–Teller (BET) measurement shows that the YSZ nanotubes have a surface specific area of around 40.5 m² g⁻¹ that is higher than that corresponding to the YSZ nanopowders.     

Multi-walled carbon nanotubes/TiO2 composite nanofiber by electrospinning

TiO₂/carbon nanotubes (CNTs) composite nanofibers were prepared by sol-gel processing followed by electrospinning technique. Phase pure titania/CNT nanofiber of 100–150 nm diameters were obtained by high temperature calcinations of the inorganic organic composite fibers. The inclusion of nanotubes with TiO₂ was confirmed by FT-IR and Raman spectra and corresponding morphology and crystallinity were observed by SEM, TEM, and XRD analysis.     

Effect of (Mn + Cr) addition on the microstructure and thermal stability of spray-formed hypereutectic Al–Si alloys

This study aims to investigate the tensile mechanical behavior and fracture toughness of vinyl-ester/polyester hybrid nanocomposites containing various types of nanofillers, including multi- and double-walled carbon nanotubes with and without amine functional groups (MWCNTs, DWCNTs, MWCNT-NH₂ and DWCNT-NH₂). To prepare the resin suspensions, very low contents (0.05, 0.1 and 0.3 wt.%) of carbon nanotubes (CNTs) were dispersed within a specially synthesized styrene-free polyester resin, conducting 3-roll milling technique. The collected resin stuff was subsequently blended with vinyl-ester via mechanical stirring to achieve final suspensions prior to polymerization. Nanocomposites containing MWCNTs and MWCNT-NH₂ were found to exhibit higher tensile strength and modulus as well as larger fracture toughness and fracture energy compared to neat hybrid polymer. However, incorporation of similar contents of DWCNTs and DWCNT-NH₂ into the hybrid resin did not reflect the same improvement in the corresponding mechanical properties. Furthermore, experimentally measured elastic moduli of the nanocomposites containing DWCNTs, DWCNT-NH₂, MWCNTs and MWCNT-NH₂ were fitted to Halphin–Tsai model. Regardless of amine functional groups or content of carbon nanotubes, MWCNT modified nanocomposites exhibited better agreement between the predicted and the measured elastic moduli values compared to nanocomposites with DWCNTs. Furthermore, Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) were used to reveal dispersion state of the carbon nanotubes within the hybrid polymer and to examine the CNT induced failure modes that occurred under mechanical loading, respectively. Based on the experimental findings obtained, it was emphasized that the types of CNTs and presence of amine functional groups on the surface of CNTs affects substantially the chemical interactions at the interface, thus tuning the ultimate mechanical performance of the resulting nanocomposites.     

Carbon nanotubes rooted montmorillonite (CNT-MM) reinforced nanocomposite membrane for PEM fuel cells

Nafion based nanocomposite membranes containing montmorillonite-carbon nanotubes (a binary hybrid material) were produced to develop high performance polymer electrolyte fuel cells. Multi walled carbon nanotubes were grown over 20 and 25 wt% iron loaded montmorillonite catalysts by CVD using acetylene as the carbon precursor. Growth experiments were carried out at optimised conditions to obtain highly selective crystalline carbon nanotubes. X-ray diffraction spectra of the catalysts were recorded for the structural characterisation and definition of particle size. The carbon nanotubes obtained were examined by various physico chemical characterisation studies such as SEM, TEM, Raman spectroscopy and TG analyses to understand the morphology and crystallinity of the CNTs. The MM-CNT hybrid material with I_D/I_G ratio of Raman spectral band as 0.53 represents the high selectivity towards CNTs. Thus the hybrid material produced was considered as the best nanofiller to develop polymer nanocomposites. Nafion based nanocomposite membranes were prepared by adding MM-CNT as nanofiller by solution casting method. A better dispersion of MM-CNT into the Nafion matrix was observed and the addition of the MM-CNT improved the thermal stability of the Nafion membrane.     

Carbon nanotubes synthesis in microwave plasma torch at atmospheric pressure

Well aligned multi-walled carbon nanotubes were synthesized at atmospheric pressure using a microwave plasma torch on silicon substrates with silicon oxide buffer layer and catalyst overlayer in the mixture of argon, hydrogen and methane. Iron or nickel was used as catalysts. The optimum substrate temperature for the deposition on Si/SiO₂/Fe substrates was about 970 K. In this case SEM micrographs of the deposits revealed a presence of vertically aligned nanotubes with the diameters around 15 nm. TEM micrographs showed a presence of amorphous carbon particles in the samples and some defects in the wall structure of the produced nanotubes. In Raman spectra two peaks at 1332 and 1584 cm⁻¹ were observed. The CNTs were also synthesized on the substrates without SiO₂ buffer layer but their quality was lower. The synthesis with Ni instead of Fe catalyst required lower temperature and the alignment of the nanotubes was worse. The deposition process was monitored by optical emission spectroscopy. Atomic lines of hydrogen and argon, an emission of CN due to a presence of nitrogen impurities from atmosphere, a weak molecular band of CH and strong C₂ emission were detected in the spectra.