Single-walled carbon nanotubes grown on natural minerals

Single-walled carbon nanotubes (SWNTs) are successfully grown on magnesite crystal by pyrolysis of methane gas under moderate conditions, demonstrating the possibility of naturally occurring SWNTs.The obtained SWNTs were analyzed by Raman scattering, transmission electron microscope (TEM), and thermogravimetric (TG) measurements. These measurements revealed that high purity SWNTs having diameters of about 1-1.8 nm occur on the surface of natural magnesite sample by the pyrolysis of methane gas at 1073-1173 K. Structural properties and formation mechanism of the obtained SWNTs were discussed.     

Single-walled carbon nanotubes as optical materials for biosensing

In this review, we summarize recent progress in the development of single-walled carbon nanotubes (SWNTs) as optical materials for biosensing applications. First, as optical labels, we discuss the use of SWNTs in Raman-based protein detection. Strong and simple resonance Raman spectroscopy of SWNTs opens up a method of protein microarray with detection sensitivity down to femtomolar range. Also, tunable isotopic SWNT-Raman signature enables the simultaneous detection of multiple analytes in complex fluids. Second, the photoluminescence properties of SWNTs are also explored. We examine fluorescence biosensors that integrate the quenching property of SWNTs and the recognition property of functional nucleic acids. Particularly, SWNTs are established as an efficient signal transduction substrate in different biosensing systems, including the detection of specific proteins and DNA sequences, regulation of singlet oxygen generation and label-free fluorescence assays, and all have exhibited very high selectivity and sensitivity.     

Single-walled carbon nanotubes as a dopant in p-type cuprous oxide films

p-type cuprous oxide (Cu₂O) films doped with single-walled carbon nanotubes (SWCNTs) were synthesized using the two-step process of electrochemical co-deposition and subsequent thermal oxidation. SWCNTs are known to act as a conducting pathway in various SWCNT-based composite materials because of their low electrical resistivity in the longitudinal direction. However, they act as an electron acceptor dopant rather than a carrier pathway in p-type Cu₂O. Results show that the doping of SWCNTs in Cu₂O films increases the hole concentration and decreases the carrier mobility with increasing dopant concentration. As a result, the electrical resistivity decreased from 290 to 0.8 Ω cm as the amount of dopant increased. The electronic energy band structure, as determined by X-ray and ultraviolet photoelectron spectroscopy, revealed that the doping of SWCNTs moved the Fermi levels of Cu₂O films toward the valence band maximum by 0.24 eV. This confirms that the SWCNTs act as an electron acceptor and increase the hole concentration and electrical conductivity of the Cu₂O films, despite the lower mobility.     

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.     

A facile approach to covalently functionalized carbon nanotubes with biocompatible polymer

Biocompatible poly(l-lactic acid) (PLA) was successfully covalently grafted onto the convex surfaces and tips of the multi-walled carbon nanotubes (MWNTs) by one step based on in situ polycondensation of the commercially available l-lactic acid monomers. The functional groups in the carboxylic multi-walled carbon nanotubes (MWNT-COOH) showed active enough for participating the polycondensation of l-lactic acid. The resulting PLA-grafted-MWNTs were characterized with Raman spectroscopy, Fourier-transform IR (FTIR), UV-vis, ¹H NMR, thermogravimetric analyses (TGA) and transmission electron microscopy (TEM). Raman, FTIR and ¹H NMR spectroscopies revealed that the PLA was covalently attached to the MWNT. TGA showed that the grafted PLA content could be controlled by the reaction time. The core/shell structures with MWNT as the “hard” core and the PLA polymer layer as the “soft” shell can be clearly seen through HRTEM.     

Single-walled carbon nanotubes functionalized with polydiphenylamine as active materials for applications in the supercapacitors field

Composites based on polydiphenylamine (PDPA) doped with heteropolyanions of H₃PW₁₂O₄₀ and single-walled carbon nanotubes (SWNTs) were prepared by electrochemical polymerization of diphenylamine (DPA) on carbon nanotube films deposited onto Pt electrodes. HRTEM studies reveal that the electrochemical polymerization leads to the filling the spaces between tubes which compose the bundles, creating a monolithic film on the Pt electrode. The resulting composites were tested as active materials in supercapacitors. Resonant Raman scattering studies showed that the electropolymerization of DPA in the presence of H₃PW₁₂O₄₀ and SWNTs leads to the covalent functionalization of SWNTs with doped PDPA. The covalent functionalization of SWNTs with PDPA doped with H₃PW₁₂O₄₀ heteropolyanions was revealed by FTIR spectroscopy, based on the changes in the vibrational features of PDPA and H₃PW₁₂O₄₀. These changes included i) a down-shift of the PDPA IR bands, which was attributed to the C–H bending vibrational mode of benzene (B), C_aromatic–N, C–C stretching (B) + C–H bending (B) and C–C stretching vibrations of the B ring, from 1174, 1321, 1495 and 1603 cm^− 1 to 1165, 1313, 1487 and 1599 cm^− 1, respectively; ii) a change in the peak positions of IR bands associated with the W = O and P-O-W vibration modes of H₃PW₁₂O₄₀; and iii) a down-shift of the IR band situated in the spectral range 650–725 cm^− 1, which was assigned to the inter-ring deformation vibration mode.The characterization of symmetric solid-state supercapacitors was performed for electrodes prepared from i) SWNTs functionalized with PDPA doped with H₃PW₁₂O₄₀ heteropolyanions, ii) SWNTs electrochemically decorated with H₃PW₁₂O₄₀ heteropolyanions, and iii) PDPA doped with H₃PW₁₂O₄₀ heteropolyanions. Preliminary results indicate high discharge capacitance values of up to 157.2 mF/cm² for SWNTs functionalized with PDPA doped with H₃PW₁₂O₄₀ heteropolyanions. The discharge capacitance of this material is superior to those recorded for SWNTs electrochemically decorated with H₃PW₁₂O₄₀ heteropolyanions (~ 18.2 mF/cm²) and PDPA doped with H₃PW₁₂O₄₀ heteropolyanions (~ 62.1 mF/cm²).     

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.     

Single-walled carbon nanotube buckypaper and mesophase pitch carbon/carbon composites

Carbon/carbon composites consisting of single-walled carbon nanotube (SWCNT) buckypaper (BP) and mesophase pitch resin have been produced through impregnation of BP with pitch using toluene as a solvent. Drying, stabilization and carbonization processes were performed sequentially, and repeated to increase the pitch content. Voids in the carbon/carbon composite samples decreased with increasing impregnation process cycles. Electrical conductivity and density of the composites increased with carbonization by two to three times that of pristine BP. These results indicate that discontinuity and intertube contact barriers of SWCNTs in the BP are partially overcome by the carbonization process of pitch. The temperature dependence of the Raman shift shows that mechanical strain is increased since carbonized pitch matrix surrounds the nanotubes.     

Preparation of biocompatible multi‐walled carbon nanotubes as potential tracers for sentinel lymph nodes

Carbon nanotubes (CNTs) are capable of traversing cellular membranes by endocytosis and are therefore promising materials for use in imaging and drug delivery. Unfortunately, pristine CNTs are practically insoluble and tend to accumulate inside cells, organs and tissues. To overcome the poor dispersibility and toxicity of pristine CNTs, hydrophilic functionalization of CNTs has been intensively investigated. Water‐soluble multi‐walled carbon nanotubes (MWCNTs) were prepared by in situ polymerization of acrylic acid in a poor solvent for poly(acrylic acid) (PAA). The solvent type influenced the grafted density and chain length of PAA. MWCNTs with a high grafted density of PAA (22 wt%) could be well dispersed in water, NaCl aqueous solution (0.9 wt%) and cell culture media. The in vitro cytotoxicity of these MWCNTs for endothelial cells is reasonably low even at high concentration of PAA‐g‐MWCNT (70 µg mL^?1). The experimental results show that the biocompatibility of these MWCNTs is sufficient for biological applications. PAA‐g‐MWCNTs were successfully utilized for lymph node tracing. Experimental results suggest that PAA‐g‐MWCNTs have potential to be used as a vital staining dye, which may simplify the identification of lymph nodes during surgery. Copyright © 2009 Society of Chemical Industry     

Thermo-responsive behavior of hybrid core cross-linked polymer micelles with biocompatible shells

Poly(2-(methacryloyloxy)ethyl phosphorylcholine)-b-poly(N-isopropylacrylamide-co-2-(N,N-dimethylamino)ethyl methacrylate) (pMPC-b-p(NIPAM/DMA)) was synthesized via reversible addition-fragmentation chain transfer (RAFT) controlled radical polymerization. Below the critical aggregation temperature (CAT), i.e., about 40 °C, the diblock copolymer dissolved in water as a unimer with a hydrodynamic radius (R_h) of ca. 10 nm. Above CAT the diblock copolymers formed polymer micelles with an R_h of ca. 40 nm, composed of a p(NIPAM/DMA) core and biocompatible pMPC shell due to hydrophobic self-aggregation of the thermo-responsive p(NIPAM/DMA) block. The pendent 2-(N,N-dimethylamino)ethyl group of DMA in pMPC-b-p(NIPAM/DMA) reduced HAuCl₄ to form gold nanoparticles (AuNPs) and could attach to their surfaces. The cores of these polymer micelles could be cross-linked above CAT by HAuCl₄, which upon being reduced generated AuNPs as cross-linking points to form core cross-linked (CCL) polymer micelles, as confirmed by UV-vis absorption and dynamic light scattering measurements. The CCL polymer micelles absorbed visible light at 532 nm because of surface plasmon resonances of the AuNPs. The R_h of the CCL polymer micelles remained at ca. 40 nm regardless of temperature.     

Deposition of silver nanoparticles on single wall carbon nanotubes via a self assembled block copolymer micelles

We demonstrate a facile route to decorate the surface of networked single walled carbon nanotubes (SWNTs) with silver nanoparticles (Ag NPs). The method is based on utilization of either spherical poly(styrene-b-4vinylpyridine) (PS-b-P4VP) or cylindrical poly(styrene-b-acrylic acid) (PS-b-PAA) copolymer micelles capable of stabilizing nanotubes in solution and subsequently forming a thin and uniform block copolymer/SWNTs composite film upon spin coating. The selective doping of silver acetate into either P4VP or PAA domains in a thin composite film, followed by thermal treatment, results in the formation of Ag NPs in the cores of micelles. Further heat treatment at 500 °C sufficiently high for degrading both block copolymers allows us to fabricate a thin SWNTs network in which Ag NPs are efficiently deposited on the surface of nanotubes. A sharp surface plasmon absorption band around 400 nm of the networked SWNTs with Ag NPs confirms the presence of Ag NPs with narrow distribution in their size.     

Defects in carbon nanotubes

Carbon nanotubes are quasi one-dimensional nanostructures with unique eletrical prroperties that make them prime candidates for molecular electronics, which is certainly a most promising direction in nanotechnology. Early theoretical works predicted that the electronic properties of "ideal" carbon nanotubes depend on their diameter and chirality. However, carbon nanotubes are probably not as perfect as they were once thought to be. Defects such as pentagons, heptagons, vacancies, or dopant are found to modify drastically the electronic properties of these nanosystems. Irradiation processes can lead to interesting, highly defective nanostructures and also to the coalescence of nanotubes within a rope. The introduction of defects in the carbon network is thus an interesting way to tailor its intrinsic properties, to create new potential nanodevices. The aim of the present Acount is to investigate theoretically the effects of different types of defects on the electronic properties of carbon nanotubes, and to propose new potential applications in nanoelectronics.     

Molecules in carbon nanotubes

This Account focuses on structural and dynamic behavior of molecules encapsulated in carbon nanotubes. The impact of the confinement on the molecular packing, orientation, translation, rotation, and reactivity is demonstrated for a range of fullerene and nonfullerene molecules. These phenomena are described and analyzed using the current understanding of molecule-nanotube and intermolecular interactions.     

Single-walled carbon nanotube growth from liquid gallium and indium

We present the first demonstration of single-walled carbon nanotube growth from liquid gallium and indium catalysts. The nanotubes were grown via thermal chemical vapor deposition from 1 to 3 nm films of gallium and indium, which dissociate into liquid droplets on silicon substrates at high temperatures. The nanotubes were characterized by Raman spectroscopy and atomic force microscopy and are found to have diameters between 1 and 2 nm.     

A novel hybrid of carbon nanotubes/iron nanoparticles: iron-filled nodule-containing carbon nanotubes

A straightforward, one-step method for the preparation of novel carbon nanotube/iron nanoparticle hybrids with some degree of shape control is reported herein. These carbon nanostructures differ from those reported previously: the nanoparticles were not attached to or coated onto the surface of carbon nanotubes but embedded inside the carbon wall. They were synthesized in good yield by thermolysis of ferrocene and thiophene mixtures in a closed steel vessel. The shapes and compositions of these nanostructures can be simply controlled by adjusting the reaction temperature and relative amounts of the precursors. Iron-filled T-junction carbon nanotubes were also obtained easily by this procedure. These iron-filled nodule-containing carbon nanotubes (INCNTs) are either empty or filled with iron or iron carbide (Fe(C)) nanowires. The outer diameters of these nanotubes range from 70 to 150 nm and the lengths reach up to several micrometers. The average size of the Fe(C) nanoparticles (or empty cores) inside the nodules is about 50 nm in diameter. The carbon in the INCNTs is amorphous. Sulfur was found being responsible for the disordered structure and playing a unique role in promoting the growth of INCNTs as well as the formation of T- or Y-junctions.     

New folate-functionalized biocompatible block copolymer micelles as potential anti-cancer drug delivery systems

The main objective of this study was to synthesize novel folic acid-functionalized diblock copolymer micelles and evaluate their solubilization of two poorly water-soluble anti-tumor drugs, tamoxifen and paclitaxel, which suffer from low water solubility and/or poor hydrolytic stability. The diblock copolymer consisted of a permanently hydrophilic block comprising 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) residues and a pH-sensitive hydrophobic block comprising 2-(diisopropylamino)ethyl methacrylate (DPA) residues. Folic acid (FA) was conjugated to the end of the MPC block so that this group was located on the micelle periphery. Tamoxifen- and paclitaxel-loaded micelles were prepared from FA-MPC-DPA copolymers prepared with two different block compositions that were designed to produce optimal solubilization of each drug. Their drug-loading capacities and aqueous stabilities were determined by high performance liquid chromatography. The hydrodynamic diameters of tamoxifen- and paclitaxel-loaded FA-MPC-DPA micelles ranged from 30 to 60 nm, as judged by dynamic light scattering (DLS) and transmission electron microscopy (TEM) studies. Finally, tamoxifen and paclitaxel release profiles were evaluated in phosphate buffer solution at pH 7.4 and 5. These studies demonstrated that FA-MPC-DPA micelles acted as useful drug carriers, leading to relatively slow release of both tamoxifen and paclitaxel into aqueous solution over a period of 7 days. In addition, rapid release can be triggered by lowering the solution pH to 5, which leads to protonation of the DPA block and hence rapid micellar dissociation.     

Single-walled carbon nanotube/Nafion composites as methanol sensors

Single-walled carbon nanotube (SWCNT)/Nafion composite films were fabricated on an interdigitated electrode by using a simple casting method. Nafion acts as a polymer backbone to give stable and homogeneous cast thin films. The potential use of the composites as sensors of methanol concentration in water was investigated. The composites are operative even at ambient temperatures, and respond quickly to concentration changes. The resistance increases significantly with increasing concentration between 0.5 M and 4 M. The composites may be useful as a material to measure the concentration of methanol for direct fuel cells.     

Light scattering evidence of selective protein fouling on biocompatible block copolymer micelles

Selective protein fouling on block copolymer micelles with well-known potential for tumour-targeting drug delivery was evidenced by using dynamic light scattering measurements. The stability and interaction of block copolymer micelles with model proteins (BSA, IgG, lysozyme and CytC) is reported for systems featuring a hydrophobic (poly[2-(diisopropylamino)-ethyl methacrylate]) (PDPA) core and hydrophilic coronas comprising poly(ethylene oxide)/poly(glycerol monomethacrylate) (PEO-b-PG2MA) or poly[2-(methacryloyloxy)ethyl phosphorylcholine] (PMPC). The results revealed that protein size and hydrophilic chain density play important roles in the observed interactions. The PEO(113)-b-PG2MA(30)-b-PDPA(50) nanoparticles are stable and protein adsorption is prevented at all investigated protein environments. The successful protein-repellent characteristic of these nanoparticles is attributed to a high hydrophilic surface chain density (>0.1 chains per nm(2)) and to the length of the hydrophilic chains. On the other hand, although PMPC also has protein-repellent characteristics, the low surface chain density of the hydrophilic shell is supposed to enable interactions with small proteins. The PMPC(40)-b-PDPA(70) micelles are stable in BSA and IgG environments due to weak repulsion forces between PMPC and the proteins, to the hydration layer, and particularly to a size-effect where the large BSA (R(H) = 4.2 nm) and IgG (R(H) = 7.0 nm) do not easily diffuse within the PMPC shell. Conversely, a clear interaction was observed with the 2.1 nm radius lysozyme. The lysozyme protein can diffuse within the PMPC micellar shell towards the PDPA hydrophobic core in a process favored by its smaller size and the low hydrophilic PMPC surface chain density (～0.049 chains per nm(2)) as compared to PEO-b-PG2MA (～0.110 chains per nm(2)). The same behavior was not evidenced with the 2.3 nm radius positively charged CytC, probably due to its higher surface hydrophilicity and the consequent chemical incompatibility with PDPA.