Chemistry of Carbon Nanotubes

A lactate biosensor based on lactate dehydrogenase/nictotinamide adenine dinucleotide (oxidized form) immobilized on a conducting polymer/multiwall carbon nanotube composite film

An amperometric lactate biosensor was developed based on a conducting polymer, poly-5,2'-5',2''-terthiophene-3'-carboxylic acid (pTTCA), and multiwall carbon nanotube (MWNT) composite on a gold electrode. Lactate dehydrogenase (LDH) and the oxidized form of nicotinamide adenine dinucleotide (NAD(+)) were subsequently immobilized onto the pTTCA/MWNT composite film. The modified electrode was characterized by quartz crystal microbalance (QCM), scanning electron microscopy (SEM), and electrochemical experiments. The detection signal was amplified by the pTTCA/MWNT assembly onto which a sufficient amount of enzyme was immobilized and stabilized by the covalent bond formation between the amine groups of enzyme and the carboxylic acid groups of the pTTCA/MWNT film. Experimental parameters affecting the sensor responses, such as applied potential, pH, and temperature, were assessed and optimized. Analytical performances and dynamic ranges of the sensor were determined, and the results showed that the sensitivity, stability, and reproducibility of the sensor improved significantly using pTTCA/MWNT composite film. The calibration plot was linear (r(2)=0.9995) over the range of 5 to 90 microM. The sensitivity was approximately 0.0106 microA/microM, with a detection limit of 1 microM, based on a signal/noise ratio of 3. The applicability of the sensor for the analysis of l-lactate concentration in commercial milk and human serum samples was demonstrated successfully.

Asymmetrically charged carbon nanotubes by controlled functionalization

Surface modification of carbon nanotubes (CNTs) has been widely studied for some years. However, the asymmetric modification of individual CNTs with different molecular species/nanoparticles at the two end-tips or along the nanotube length is only a recent development. As far as we are aware, no attempt has so far been made to asymmetrically functionalize individual CNTs with moieties of opposite charges. In this paper, we have demonstrated a simple, but effective, asymmetric modification of the sidewall of CNTs with oppositely charged moieties by plasma treatment and pi-pi stacking interaction. The as-prepared asymmetrically sidewall-functionalized CNTs can be used as a platform for bottom-up self-assembly of complex structures or can be charge-selectively self-assembled onto and/or between electrodes with specific biases under an appropriate applied voltage for potential device applications.

Adsorption of hydroxyl- and amino-substituted aromatics to carbon nanotubes

The combined effects of hydroxyl/amino functional groups of aromatics and surface O-containing groups of carbon nanotubes on adsorption were evaluated. When normalized for hydrophobicity, 2,4-dichlorophenol and 2-naphthol exhibited a greater adsorptive affinity to carbon nanotubes and graphite (a model adsorbent without the surface O functionality) than structurally similar 1,3-dichlorobenzene and naphthalene, respectively, and 1-naphthylamine exhibited a much greater adsorptive affinity than all other compounds. Results of the pH-dependency experiments further indicated that the hydroxy/amino functional groups of the adsorbates and the surface properties of the adsorbents played a combinational role in determining the significance of the nonhydrophobic adsorptive interactions. We propose that the strong adsorptive interaction between hydroxyl-substituted aromatics and carbon nanotubes/graphite was mainly due to the electron-donating effect of the hydroxyl group, which caused a strong electron-donor-acceptor (EDA) interaction between the adsorbates and the pi-electron-depleted regions on the graphene surfaces of carbon nanotubes or graphite. In addition to the EDA interaction, Lewis acid-base interaction was likely an extra important mechanism contributing to the strong adsorption of 1-naphthylamine, especially on the O-functionality-abundant carbon nanotubes. The findings of the present study might have significant implications for selective removal of environmental contaminants with carbon nanotubes.

Noncovalent assembly of carbon nanotubes and single-stranded DNA: an effective sensing platform for probing biomolecular interactions

In this paper, we report the assembly of single-walled carbon nanotubes (SWNTs) and single-stranded DNA to develop a new class of fluorescent biosensors which are able to probe and recognize biomolecular interactions in a homogeneous format. This novel sensing platform consists of a structure formed by the interaction of SWNTs and dye-labeled DNA oligonucleotides such that the proximity of the nanotube to the dye effectively quenches the fluorescence in the absence of a target. Conversely, and very importantly, the competitive binding of a target DNA or protein with SWNTs for the oligonucleotide results in the restoration of fluorescence signal in increments relative to the fluorescence without a target. This signaling mechanism makes it possible to detect the target by fluorescence spectroscopy. In the present study, the schemes for such fluorescence changes were examined by fluorescence anisotropy and fluorescence intensity measurements for DNA hybridization and aptamer-protein interaction studies.

Light-controlled single-walled carbon nanotube dispersions in aqueous solution

We have succeeded in dispersing single-walled carbon nanotubes (SWNTs) into an aqueous solution of poly(ethylene glycol)-terminated malachite green derivative (PEG-MG) through simple sonication. It was found that UV exposure caused reaggregation of these predispersed SWNTs in the same aqueous medium, as adsorbed PEG-MG photochromic chains could be effectively photocleavaged from the nanotube surface. The observed light-controlled dispersion and reaggragation of SWNTs in the aqueous solution should facilitate the development of SWNT dispersions with a controllable dispersity for potential applications.

Coating multi-walled carbon nanotubes with rare-earth complexes by an in situ synthetic method

Multi-walled carbon nanotubes (MWCNTs) were coated with rare-earth (RE) (Eu(3+), Tb(3+)) complex layers by a simple in situ synthetic method via noncovalently functionalized MWCNTs with sodium dodecyl sulfate. Extensive characterizations of the resulting nanocomposite materials have been performed using scanning electron microscopy (SEM), FT-IR spectra, x-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive x-ray spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS). The photoluminescence properties of the RE-complex-modified MWCNTs were also investigated. Under the excitation of UV light, these samples exhibit characteristic emissions of Eu(3+) or Tb(3+) ions. The presence of the MWCNT framework does not quench the lanthanide-centered luminescence originating from the characteristic electronic transitions within the 4f shell of the RE ions. Furthermore, the existence of the nanotube framework can improve the stability of the RE complex under UV irradiation and elongate the fluorescence lifetime of the RE complex.

One-pot synthesis of carbon nanotube-polyaniline-gold nanoparticle and carbon nanotube-gold nanoparticle composites by using aromatic amine chemistry

Harsh oxidative treatment of single-walled carbon nanotubes (SWNTs) was used to generate carboxyl groups on SWNT sidewalls. The oxidized SWNTs can disperse well in ethanolic solutions containing aniline (or it derivative, 4-dodecylaniline), possibly due to the formation of proton-transfer complexes between carboxyl and amine groups. Addition of HAuCl 4 into the above-mentioned solutions can readily produce SWNT-polyaniline (PANI)-Au nanoparticle (NP) or SWNT-Au NP composites in an in situ one-pot fashion. Transmission electron microscopy, UV-vis spectroscopy and cyclic voltammetry have been employed to characterize the obtained composites. Our findings suggest that the obtained composites and electrodes modified with this material may find interesting applications in electrochemical sensors and conducting polymer coatings.

Multipurpose organically modified carbon nanotubes: from functionalization to nanotube composites

We show that covalent functionalization of carbon nanotubes (CNTs) via 1,3-dipolar cycloaddition is a powerful method for enhancing the ability to process CNTs and facilitating the preparation of hybrid composites, which is achieved solely by mixing. CNTs were functionalized with phenol groups, providing stable dispersions in a range of polar solvents, including water. Additionally, the functionalized CNTs could easily be combined with polymers and layered aluminosilicate clay minerals to give homogeneous, coherent, transparent CNT thin films and gels.

In vivo biocompatibility of ultra-short single-walled carbon nanotube/biodegradable polymer nanocomposites for bone tissue engineering

Scaffolds play a pivotal role in the tissue engineering paradigm by providing temporary structural support, guiding cells to grow, assisting the transport of essential nutrients and waste products, and facilitating the formation of functional tissues and organs. Single-walled carbon nanotubes (SWNTs), especially ultra-short SWNTs (US-tubes), have proven useful for reinforcing synthetic polymeric scaffold materials. In this article, we report on the in vivo biocompatibility of US-tube reinforced porous biodegradable scaffolds in a rabbit model. US-tube nanocomposite scaffolds and control polymer scaffolds were implanted in rabbit femoral condyles and in subcutaneous pockets. The hard and soft tissue response was analyzed with micro-computed tomography (micro CT), histology, and histomorphometry at 4 and 12 weeks after implantation. The porous US-tube nanocomposite scaffolds exhibited favorable hard and soft tissue responses at both time points. At 12 weeks, a three-fold greater bone tissue ingrowth was seen in defects containing US-tube nanocomposite scaffolds compared to control polymer scaffolds. Additionally, the 12 week samples showed reduced inflammatory cell density and increased connective tissue organization. No significant quantitative difference in polymer degradation was observed among the various groups; qualitative differences between the two time points were consistent with expected degradation due to the progression of time. Although no conclusions can be drawn from the present study concerning the osteoinductivity of US-tube nanocomposite scaffolds, the results suggest that the presence of US-tubes may render nanocomposite scaffolds bioactive assisting osteogenesis.

Structural defects play a major role in the acute lung toxicity of multiwall carbon nanotubes: physicochemical aspects

Carbon nanotubes (CNT) have been reported to elicit toxic responses in vitro and in vivo, ascribed so far to metal contamination, CNT length, degree of oxidation, or extent of hydrophilicity. To examine how structural properties may modulate the toxicity of CNT, one preparation of multiwall CNT has been modified (i) by grinding (introducing structural defects) and subsequently heating either in a vacuum at 600 degrees C (causing reduction of oxygenated carbon functionalities and reduction of metallic oxides) or in an inert atmosphere at 2400 degrees C (causing elimination of metals and annealing of defects) and (ii) by heating at 2400 degrees C in an inert atmosphere and subsequently grinding the thermally treated CNT (introducing defects in a metal-deprived carbon framework). The presence of framework and surface defects, metals, and oxygenated functionalities was monitored by means of a set of techniques, including micro-Raman spectroscopy, adsorption calorimetry, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, and atomic emission spectroscopy. Contrary to traditional toxicants, such as asbestos, CNT may quench rather than generate oxygenated free radicals. The potential of the modified CNT to scavenge hydroxyl radicals was thus evaluated by means of electron spin resonance spectroscopy (spin trapping). The original ground material exhibited a scavenging activity toward hydroxyl radicals, which was eliminated by heating at 2400 degrees C but restored upon grinding. This scavenging activity, related to the presence of defects, appears to go paired with the genotoxic and inflammatory potential of CNT reported in the companion paper. Thus, defects may be one of the major factors governing the toxic potential of CNT.

Carbon nanotubes introduced into the abdominal cavity of mice show asbestos-like pathogenicity in a pilot study

Carbon nanotubes have distinctive characteristics, but their needle-like fibre shape has been compared to asbestos, raising concerns that widespread use of carbon nanotubes may lead to mesothelioma, cancer of the lining of the lungs caused by exposure to asbestos. Here we show that exposing the mesothelial lining of the body cavity of mice, as a surrogate for the mesothelial lining of the chest cavity, to long multiwalled carbon nanotubes results in asbestos-like, length-dependent, pathogenic behaviour. This includes inflammation and the formation of lesions known as granulomas. This is of considerable importance, because research and business communities continue to invest heavily in carbon nanotubes for a wide range of products under the assumption that they are no more hazardous than graphite. Our results suggest the need for further research and great caution before introducing such products into the market if long-term harm is to be avoided.

Progress towards monodisperse single-walled carbon nanotubes

The defining characteristic of a nanomaterial is that its properties vary as a function of its size. This size dependence can be clearly observed in single-walled carbon nanotubes, where changes in structure at the atomic scale can modify the electronic and optical properties of these materials in a discontinuous manner (for example, changing metallic nanotubes to semiconducting nanotubes and vice versa). However, as most practical technologies require predictable and uniform performance, researchers have been aggressively seeking strategies for preparing samples of single-walled carbon nanotubes with well-defined diameters, lengths, chiralities and electronic properties (that is, uniformly metallic or uniformly semiconducting). This review highlights post-synthetic approaches for sorting single-walled carbon nanotubes - including selective chemistry, electrical breakdown, dielectrophoresis, chromatography and ultracentrifugation - and progress towards selective growth of monodisperse samples.

Conductive macroporous composite chitosan-carbon nanotube scaffolds

Multiwalled carbon nanotubes (MWCNTs) were used as doping material for three-dimensional chitosan scaffolds to develop a highly conductive, porous, and biocompatible composite material. The porous and interconnected structures were formed by the process of thermally induced phase separation followed by freeze-drying applied to an aqueous solution of 1 wt % chitosan acetic acid. The porosity was characterized to be 97% by both mercury intrusion porosimetry measurements and SEM image analysis. When MWCNTs were used as a filler to introduce conductive pathways throughout the chitosan skeleton, the solubilizing hydrophobic and hydrophilic properties of chitosan established stable polymer/MWCNT solutions that yielded a homogeneous distribution of nanotubes throughout the final composite matrix. A percolation theory threshold of approximately 2.5 wt % MWCNTs was determined by measurement of the conductivity as a function of chitosan/MWCNT ratios. The powder resistivity of completely compressed scaffolds also was measured and was found to be similar for all MWCNT concentrations (0.7-0.15 Omega cm powder resistivity for MWCNTs of 0.8-5 wt %) and almost five times lower than the 20 k Omega cm value found for pure chitosan scaffolds.

Covalent assembly and micropatterning of functionalized multiwalled carbon nanotubes to monolayer-modified Si(111) surfaces

Multiwalled carbon nanotubes (MWNTs) covalently bound to monocrystalline p-type Si(111) surfaces have been prepared by attaching soluble amine-functionalized MWNTs onto a preassembled undecanoic acid monolayer using carbodiimide coupling. SEM analysis of these functionalized surfaces shows that the bound MWNTs are parallel to the surface rather than perpendicular. The voltammetric and electrochemical impedance spectroscopy measurements reveal that the electron transfer at the MWNT-modified surface is faster than that observed at a MWNT-free alkyl monolayer. We have also demonstrated that it is possible to prepare MWNT micropatterns using this surface amidation reaction and a "reagentless" UV photolithography technique. Following this approach, MWNT patterns surrounded by n-dodecyl areas have been produced and the local electrochemical properties of these micropatterned surfaces have been examined by scanning electrochemical microscopy. In particular, it is demonstrated that the MWNT patterns allow a faster charge transfer which is consistent with the results obtained for the uniformly modified surfaces.

Detecting and treating cancer with nanotechnology

Nanotechnology offers many opportunities for enhanced diagnostic and therapeutic medicine against cancer and other diseases. In this review, the special properties that result from the nanoscale size of quantum dots, metal colloids, superparamagnetic iron oxide, and carbon-based nanostructures are reviewed and interpreted against a background of the structural and electronic detail that gives rise to their nanotechnologic behavior. The detection and treatment of cancer is emphasized, with special attention paid to the biologic targeting of the disease. The future of nanotechnology in cancer research and clinical practice is projected to focus on 'theranostic' nanoparticles that are both diagnostic and therapeutic by design.