Single-walled carbon nanotubes functionalized with poly(nile blue A) and their application to dehydrogenase-based biosensors

This paper reports a new type of nanocomposite of poly(nile blue A) with single-walled carbon nanotubes (PNb-SWNTs). This nanocomposite was fabricated by the functionalization of SWNTs with poly(nile blue A), which was formed by electropolymerizing an Nb monomer through the use of cyclic voltammetry. Scanning electron microscopy (SEM), ultraviolet-visible spectroscopy (UV-vis), cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were used to characterize the PNb-SWNTs. The cyclic voltammetric results indicated that PNb-SWNTs were able to electrocatalyze the oxidation of NADH at a very low potential (ca. −80 mV versus SCE) and lead to a substantial decrease in the overpotential by more than 700 mV compared with the bare glassy carbon (GC) electrode. A biosensor, ADH-PNb-SWNT/GC, was developed by immobilizing alcohol dehydrogenase (ADH) onto the PNb-SWNT/GC electrode surface. The biosensor showed electrocatalytic activity toward the oxidation of ethanol with a good stability, reproducibility, and higher biological affinity. Under optimal conditions, the electrochemical response to detect ethanol has the typical characteristics of Michaelis-Menten kinetics with an apparent Michaelis-Menten constant of  ∼ 6.30 mM, and depends linearly on the concentration of ethanol from 0.1 to 3.0 mM (with a correlation coefficient of 0.998), with a detection limit of ∼50 μM (at a signal-to-noise ratio of 3). The facile procedure of immobilizing ADH used in the present work can promote the development of electrochemical research for enzymes (proteins), biosensors, biofuel cells and other bioelectrochemical devices.     

Tailored dispersion of carbon nanotubes in water with pH-responsive polymers

In an effort to control the level of carbon nanotube exfoliation in water, pH-responsive polymers (i.e., weak polyelectrolytes) have been used as stabilizers in water. This noncovalent functionalization of single-walled carbon nanotubes (SWNTs) results in suspensions whose dispersion state can be altered by simply changing pH. In this study poly(acrylic acid), poly(methacrylic acid), poly(allylamine) and polyethyleneimine were used to stabilize aqueous SWNT suspensions. The results indicate that SWNTs stabilized with these polymers show a pH tailorable exfoliation and bundling in water, as evidenced by cryo-TEM images and shifts in suspension viscosity. Composite films prepared by drying these aqueous suspensions suggest that nanotube microstructure in the liquid state is largely preserved in the solid composites, with more bundled/networked structures showing higher electrical conductivity. A stabilization mechanism based upon the results obtained is proposed to explain the exfoliation and aggregation behavior of SWNTs. This method of controlling the microstructure of SWNTs in liquid state with pH could have a significant impact on the ability to tailor the microstructure and properties of composites.     

Shear-controlled electrical conductivity of carbon nanotubes networks suspended in low and high molecular weight liquids

Controlling the electrical conductivity is a critical issue when processing material systems consisting of an insulating matrix filled with conductive particles. We provide experimental evidence that given shear rates result in specific conductivity levels in such different systems as high-viscosity carbon nanotube/polymer melt or low-viscosity carbon nanotube/epoxy fiber suspensions. The steady-state conductivities are independent of the initial state of the dispersion. The observed behavior is modeled phenomenologically by the competition between build-up and destruction of conductive filler network. A particle-level simulation of flowing fiber suspension also reflects the observed behavior. Our results show that properties of particulate suspensions can be controlled by steady shear. They should be considered to obtain reproducible properties in shear-based processing technologies as injection molding or resin transfer molding.     

Nanostructuring electrodes with carbon nanotubes: A review on electrochemistry and applications for sensing

Carbon nanotubes have begun to attract enormous interest in electrochemistry because of their small size and good electrochemical properties. The vast majority of studies thus far have used ensembles of carbon nanotubes to nanostructure macroscopic electrodes either with randomly dispersed nanotubes or with aligned carbon nanotubes. The resultant nanotube modified electrodes have most frequently been used for electro-analytical purposes such as the development of biosensors. This review introduces carbon nanotubes and approaches to nanostructuring electrodes with carbon nanotubes, discusses what is known of their electrochemical properties and briefly outlines some of the exciting applications to which they are being targeted for use.     

UV-assisted grafting of polymers: A method towards biocompatible carbon nanotubes

A strategy for covalent grafting of biocompatible polymers onto sidewalls of multi-walled carbon nanotubes (MWNTs) via UV-initiated free-radical polymerization is presented. The effects of the irradiation doze(time) and monomer/MWNTs ratio on the stability of the corresponding aqueous dispersions were investigated. It was found that stable dispersions of MWNTs modified with polyacrylamide, poly(N-isopropylacrylamide), poly[poly(ethylene glycol) methacrylate] and poly(sodium methacrylate) can be obtained by irradiation with UV light for at least 5 min at an irradiation dose rate of 5.7 J/cm² min at a minimum monomer/CNTs ratio of 200:1. Biocompatibility of polymer-modified MWNTs was assessed using the standard MTT-dye reduction assay and compared to pristine MWNTs. As a rule, all polymer-functionalized nanotubes examined in this study were non-cytotoxic up to concentration 150 μg/mL and, remarkably, MWNTs-g-PNIPAAm did not exhibit cytotoxicity even at the highest concentration studied (300 μg/mL). MWNTs modified with stimuli-sensitive polymers underwent a reversible transition from well-dispersed nanotubes in water to precipitate triggered by changes in temperature or pH.     

Fabrication of efficient field emitters with thin multiwalled carbon nanotubes using spray method

Thin multiwalled carbon nanotube (t-MWCNTs)-based field emitters are made by use of a spray method. The number of tube walls is between 2 and 6, with the corresponding outer diameters between 3 and 6 nm. They were dispersed in dichloroethane and sprayed onto metal-deposited indium tin oxide glass. After heat treatment, they were found to be tightly adhered to metal electrode. Excellent field emission characteristics were exhibited, with a large field enhancement factor and low turn-on voltage, comparable to those of singlewalled CNTs. However, the t-MWCNTs demonstrated a significantly lower degradation rate than SWCNTs in the emission current. This high emission stability was attributed to their stable edge structures, similar to conventional large-diameter MWCNTs. Therefore, t-MWCNTs could be utilized as an alternative material for field emitters.     

In situ electropolymerization of conducting polypyrrole/carbon nanotubes composites on stainless steel: Role of carbon nanotubes types

In situ electropolymerization was used to prepare polypyrrole-oxidized multi-walled carbon nanotubes and polypyrrole-oxidized single-walled carbon nanotubes composites on a stainless steel surface from 0.1 M oxalic acid by using cyclic voltammetry. The electropolymerization process was investigated and discussed, and the results showed that the addition of the oxidized carbon nanotubes greatly enhanced the electropolymerization process, especially in the case of oxidized single walled carbon nanotubes. The results also showed that increasing the pyrrole monomer concentration leads to increasing the amount of polypyrrole electrodeposited, and this is more pronounced in the presence of the carbon nanotubes. The electropolymerization process was mainly under diffusion control as the process was inhibited by increasing the scan rate. In general, the presence of oxidized carbon nanotubes improved the electropolymerization of the polypyrrole and greatly enhanced its thermal and morphological properties.     

Respiratory toxicity of carbon nanotubes: How worried should we be?

The synthesis of carbon nanotubes (CNTs) and their actual and potential industrial uses have attracted the attention of many scientists world wide but relatively little attention has been paid, so far, to their potential detrimental effects on human health and the environment. Here, we briefly sketch the main determinants of the lung toxicity of inhaled particles and outline the existing experimental methods to explore their toxic potential to the lung. We then summarize the first available studies that have examined the respiratory toxicity of CNTs in experimental animals. Although data are still fragmentary and subject to criticisms, e.g. because of the non-physiological mode of administration used, the results indicate that if CNTs reach the lungs they can exert serious toxicity, manifested in experimental animals as inflammatory and fibrotic reactions. These reports represent a cause of concern for human health and indicate that strict preventive and protective measures should be taken to limit inhalation exposure to CNTs in occupational settings.     

Reinforcement of polymers with carbon nanotubes. The role of an ordered polymer interfacial region. Experiment and modeling

Significant increases in the Young's modulus of nanotube–polymer composites have been associated with the formation of an ordered polymer layer coating the nanotubes. Polyvinyl alcohol (PVA) is known to display nanotube-induced ordering. It is used here as a model matrix to investigate how the polymer coating influences the mechanical reinforcement of the composite material. Young's modulus and calorimetry measurements were carried out on films of PVA-based composites reinforced with different types of nanotubes. An unmistakable correlation between polymer ordering and reinforcement was found. This is supported by the introduction of a model capable of establishing, on quantitative grounds, how the ordered phase affects the increase in the Young's modulus. Rather than acting as intrinsically stiffer reinforcing agents, our results suggest that the major role played by the nanotubes in improving the mechanical properties of composites is to nucleate an ordered polymer coating. It is the presence of this stiff ordered phase that dominates the reinforcement mechanism.     

Fabrication of superhydrophobic surfaces with non-aligned alkyl-modified multi-wall carbon nanotubes

Films of superhydrophobic multi-wall carbon nanotubes (MWCNTs) have been obtained by using alkyl-modified MWCNTs (MWCNT(COOC₁₈H₃₇)_n) and a simple and effective preparation method. The films show both a high contact angle and a small sliding angle for water droplets. A particular characteristic is that on the superhydrophobic surface the alkyl-modified MWCNTs are not intentionally aligned, thus avoiding the preparation techniques using aligned carbon nanotubes to produce the same effect.     

Synthesis of carbon nanocapsules and carbon nanotubes by an acetylene flame method

The fabrication of carbon nanocapsules and carbon nanotubes (CNTs) using an acetylene flame method was investigated. Carbon nanocapsules, a graphitic structure of nanoparticles with a hollow core, were synthesized using catalyst-free acetylene flames while CNTs were formed with the presence of cobalt-based catalysts in addition to acetylene flames. When the synthesis of these materials was carried out, the results showed that a massive amount of high-purity carbon nanocapsules with a particle size in the range of 15-30 nm can be produced with the acetylene flame method. The CNTs produced were multi-walled carbon nanotubes measuring a few micrometers in length and 20-30 nm in diameter. The acetylene flame method holds great potential for the cost-effective production of CNTs as well as carbon nanocapsules.     

Third-order optical nonlinearity in single-wall carbon nanotubes

The third-order optical nonlinearity in carbon nanotubes (CNT) exposed to an intensive external electromagnetic field has been investigated. The analysis is based on the quantum kinetic equations for the density matrix of π-electrons in CNT. In the regime of weak driving field, the kinetic equations have been solved by the perturbation method and the third-order nonlinear polarizability of different achiral CNTs has been calculated. The theory elaborated has been used for the evaluation of nonlinear susceptibility of CNT-based composites. Comparison with available experimental data has been presented. In the case of high intensive driving field a nonperturbative numerical simulation of the process has been carried out in the time domain. The axial current density in CNT has been calculated. Excitation of π-plasmons in CNTs has been analyzed.     

Fabrication and mechanical properties of multi-walled carbon nanotubes doped AlN ceramics prepared by spark plasma sintering

In this study, multi-walled carbon nanotubes (MWCNTs) are uniformly dispersed in aluminium nitride (AlN) powders, and the MWCNTs-doped AlN ceramics are sintered at 1500 °C with a holding time of 5 min by spark plasma sintering using Y₂O₃ as the sintering additive. The effects of the MWCNTs content on the microstructure and mechanical properties of the as-obtained ceramic composites are investigated. The results reveal that many submicron pores are generated when protecting the structure of the CNTs, thereby reducing the density of the AlN ceramic. However, the gradual filling of the grain gap may compensate for the strengthening after CNT doping. The relative density and hardness reach the maximum values of 89.6% of the theoretical density and 7.0 ± 0.3 GPa, respectively, at the doping amount of 2.5 wt%.     

Band structure and absorption spectrum of double-walled zigzag carbon nanotubes in an electric field

The electronic structure of the (9, 0)-(18, 0) double-walled zigzag carbon nanotubes in the presence of a uniform transverse electric field is studied by the tight-binding model. The electric field could induce the semiconductor-metal transition, change the direct gap into the indirect gap, alter the subband curvatures, destroy the double degeneracy, produce the new band-edge states, make more subbands group around the Fermi level, and widen the π-band width. Such effects are directly reflected in density of states and optical excitation spectra. The absorption spectra exhibit a lot of prominent peaks, mainly owing to the rich one-dimensional energy subbands. The intensity, the number, and the frequency of absorption peaks are strongly modulated by the electric field. The modulation of electronic and optical properties is amplified by the parallel magnetic field. The predicted electronic and optical properties can be, respectively, verified by the conductance measurements and the optical spectroscopy.     

Anchorage of carbon nanotubes grown on carbon fibres

A novel reinforcing material based on the concept of an uniform 3-dimensional distribution of carbon nanotubes directly grown on yarns of carbon fibres has been developed. This material shows a potential for applications in polymeric matrix composites, combining the properties of carbon nanotubes with those of a traditional reinforcement.In view of the dipping process of the CNT coated fibres into a polymeric matrix, a good anchorage of CNT to the fibre surface is mandatory. Carbon fibres coated with metallic clusters and CNT were immersed into different liquids (deionised water, ethanol, n-butanol, acetone) and processed with different treatments (immersion, magnetic stirring, centrifugation and ultrasonic bath) in order to test their behaviour in different stressing environments. The morphological features of the samples were characterised by SEM both before and after the tests, demonstrating a good adhesion of the three-component material, which was not destroyed even after the most aggressive test.     

碳纳米管的功能化研究进展

碳纳米管是一种新型的炭材料，具有独特的结构，特殊的力学、电学、磁学及光学性能，已引起人们的广泛关注。本文介绍了近年来对碳纳米管功能化处理及其应用等方面的最新进展，讨论了碳纳米管的纯化、修饰、填充及功能化碳纳米管的应用，并对今后的研究方向作了展望。     

Electrooxidation of DNA at glassy carbon electrodes modified with multiwall carbon nanotubes dispersed in polyethylenimine

This work reports the electrochemical response of the complex between dsDNA and PEI formed in solution and at the surface of glassy carbon electrodes (GCE) modified with a dispersion of multi-walled carbon nanotubes in polyethylenimine (CNT-PEI). Scanning Electron Microscopy and Scanning Electrochemical Microscopy demonstrate that the dispersion covers the whole surface of the electrode although there are areas with higher density of CNT and, consequently, with higher electrochemical reactivity. The adsorption of DNA at GCE/CNT-PEI is fast and it is mainly driven by electrostatic forces. A clear oxidation signal is obtained either for dsDNA or a heterooligonucleotide of 21 bases (oligoY) at potentials smaller than those for the oxidation at bare GCE. The comparison of the behavior of DNA before and after thermal treatment demonstrated that the electrochemical response highly depends on the 3D structure of the nucleic acid.     

Magnetic separation of Fe catalyst from single-walled carbon nanotubes in an aqueous surfactant solution

We report an efficient technique to separate ferromagnetic catalyst particles from an aqueous surfactant solution of single-walled carbon nanotubes (SWNTs) by the use of a 1.3 T permanent magnet. High resolution transmission electron microscopy (HRTEM) demonstrates that SWNTs are coated with a surfactant layer that stabilises the aqueous dispersions of SWNTs. The residual quantities of Fe catalyst (∼3%) can be effectively removed from a colloid solution of SWNTs in a magnetic field while absorbance spectra of the initial and purified solutions show that the nanotube diameter distribution remains unchanged.