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1.  Synthesis and Characterization of Magnetic Metal-encapsulated Multi-walled Carbon Nanobeads 
Nanoscale Research Letters  2008;3(2):76-81.
A novel, cost-effective, easy and single-step process for the synthesis of large quantities of magnetic metal-encapsulated multi-walled carbon nanobeads (MWNB) and multi-walled carbon nanotubes (MWNT) using catalytic chemical vapour deposition of methane over Mischmetal-based AB3alloy hydride catalyst is presented. The growth mechanism of metal-encapsulated MWNB and MWNT has been discussed based on the catalytically controlled root-growth mode. These carbon nanostructures have been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM and HRTEM), energy dispersive analysis of X-ray (EDAX) and thermogravimetric analysis (TGA). Magnetic properties of metal-filled nanobeads have been studied using PAR vibrating sample magnetometer up to a magnetic field of 10 kOe, and the results have been compared with those of metal-filled MWNT.
doi:10.1007/s11671-008-9116-6
PMCID: PMC3244788
Magnetic metal-filled multi-walled carbon nanobeads (MWNB); Alloy hydride catalyst; Chemical vapour deposition; Magnetization
2.  Asymmetric Flexible Supercapacitor Stack 
Nanoscale Research Letters  2008;3(4):145-151.
Electrical double layer supercapacitor is very significant in the field of electrical energy storage which can be the solution for the current revolution in the electronic devices like mobile phones, camera flashes which needs flexible and miniaturized energy storage device with all non-aqueous components. The multiwalled carbon nanotubes (MWNTs) have been synthesized by catalytic chemical vapor deposition technique over hydrogen decrepitated Mischmetal (Mm) based AB3alloy hydride. The polymer dispersed MWNTs have been obtained by insitu polymerization and the metal oxide/MWNTs were synthesized by sol-gel method. Morphological characterizations of polymer dispersed MWNTs have been carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM and HRTEM). An assymetric double supercapacitor stack has been fabricated using polymer/MWNTs and metal oxide/MWNTs coated over flexible carbon fabric as electrodes and nafion®membrane as a solid electrolyte. Electrochemical performance of the supercapacitor stack has been investigated using cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy.
doi:10.1007/s11671-008-9127-3
PMCID: PMC3244800
Multiwalled carbon nanotubes; Polymers; Metal oxide; Supercapacitor stack; Asymmetry; Solid electrolyte
3.  Effect of Purity and Substrate on Field Emission Properties of Multi-walled Carbon Nanotubes 
Nanoscale Research Letters  2007;2(7):331-336.
Multi-walled carbon nanotubes (MWNT) have been synthesized by chemical vapour decomposition (CVD) of acetylene over Rare Earth (RE) based AB2(DyNi2) alloy hydride catalyst. The as-grown carbon nanotubes were purified by acid and heat treatments and characterized using powder X-ray diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, Thermo Gravimetric Analysis and Raman Spectroscopy. Fully carbon based field emitters have been fabricated by spin coating a solutions of both as-grown and purified MWNT and dichloro ethane (DCE) over carbon paper with and without graphitized layer. The use of graphitized carbon paper as substrate opens several new possibilities for carbon nanotube (CNT) field emitters, as the presence of the graphitic layer provides strong adhesion between the nanotubes and carbon paper and reduces contact resistance. The field emission characteristics have been studied using an indigenously fabricated set up and the results are discussed. CNT field emitter prepared by spin coating of the purified MWNT–DCE solution over graphitized carbon paper shows excellent emission properties with a fairly stable emission current over a period of 4 h. Analysis of the field emission characteristics based on the Fowler–Nordheim (FN) theory reveals current saturation effects at high applied fields for all the samples.
doi:10.1007/s11671-007-9067-3
PMCID: PMC3246377  PMID: 21798103
Multi-walled carbon nanotubes; DyNi2alloy hydride; Spin coating; Dichloro ethane; Graphitized carbon paper; CNT field emitter; Fowler–Nordheim theory
4.  Ecological Uptake and Depuration of Carbon Nanotubes by Lumbriculus variegatus 
Environmental Health Perspectives  2008;116(4):496-500.
Background
Carbon nanotubes represent a class of nanomaterials having broad application potentials and documented cellular uptake and ecotoxicological effects that raise the possibility that they may bioaccumulate in living organisms.
Objectives
Radioactively labeled nanotubes were synthesized using a novel methane chemical vapor deposition procedure. Single-walled carbon nanotubes (SWNTs), multiwalled carbon nanotubes (MWNTs), and pyrene were spiked to sediment samples, and the respective uptake and depuration of these nanotubes and pyrene were assessed by the oligochaete, Lumbriculus variegatus.
Results
14C-labeled carbon nanotubes were developed for these experiments to overcome significant previous limitations for quantifying nanotube materials in environmental and biological media. Biota-sediment accumulation factors for SWNTs and MWNTs were observed to be almost an order of magnitude lower than those for pyrene, a four-ringed polycyclic aromatic hydrocarbon (PAH). The depuration behaviors of the oligochaete suggested that the nanotubes detected in these organisms were associated with sediments remaining in the organism guts and not absorbed into cellular tissues as was the pyrene. The results suggest that, unlike PAHs, purified carbon nanotubes do not readily absorb into organism tissues.
doi:10.1289/ehp.10883
PMCID: PMC2290976  PMID: 18414633
bioaccumulation; carbon nanotubes; ecotoxicology; environmental risks; fullerenes; Lumbriculus variegatus; MWNT; nanomaterials; nanotechnology; SWNT
5.  Delivery of small interfering RNAs in human cervical cancer cells by polyethylenimine-functionalized carbon nanotubes 
Nanoscale Research Letters  2013;8(1):267.
Carbon nanotubes are capable of penetrating the cell membrane and are widely considered as potential carriers for gene or drug delivery. Because the C-C and C=C bonds in carbon nanotubes are nonpolar, functionalization is required for carbon nanotubes to interact with genes or drugs as well as to improve their biocompatibility. In this study, polyethylenimine (PEI)-functionalized single-wall (PEI-NH-SWNTs) and multiwall carbon nanotubes (PEI-NH-MWNTs) were produced by direct amination method. PEI functionalization increased the positive charge on the surface of SWNTs and MWNTs, allowing carbon nanotubes to interact electrostatically with the negatively charged small interfering RNAs (siRNAs) and to serve as nonviral gene delivery reagents. PEI-NH-MWNTs and PEI-NH-SWNTs had a better solubility in water than pristine carbon nanotubes, and further removal of large aggregates by centrifugation produced a stable suspension of reduced particle size and improved homogeneity and dispersity. The amount of grafted PEI estimated by thermogravimetric analysis was 5.08% (w/w) and 5.28% (w/w) for PEI-NH-SWNTs and PEI-NH-MWNTs, respectively. For the assessment of cytotoxicity, various concentrations of PEI-NH-SWNTs and PEI-NH-MWNTs were incubated with human cervical cancer cells, HeLa-S3, for 48 h. PEI-NH-SWNTs and PEI-NH-MWNTs induced cell deaths in a dose-dependent manner but were less cytotoxic compared to pure PEI. As determined by electrophoretic mobility shift assay, siRNAs directed against glyceraldehyde-3-phosphate dehydrogenase (siGAPDH) were completely associated with PEI-NH-SWNTs or PEI-NH-MWNTs at a PEI-NH-SWNT/siGAPDH or PEI-NH-MWNT/siGAPDH mass ratio of 80:1 or 160:1, respectively. Furthermore, PEI-NH-SWNTs and PEI-NH-MWNTs successfully delivered siGAPDH into HeLa-S3 cells at PEI-NH-SWNT/siGAPDH and PEI-NH-MWNT/siGAPDH mass ratios of 1:1 to 20:1, resulting in suppression of the mRNA level of GAPDH to an extent similar to that of DharmaFECT, a common transfection reagent for siRNAs. Our results indicate that the PEI-NH-SWNTs and PEI-NH-MWNTs produced in this study are capable of delivering siRNAs into HeLa-S3 cells to suppress gene expression and may therefore be considered as novel nonviral gene delivery reagents.
doi:10.1186/1556-276X-8-267
PMCID: PMC3683344  PMID: 23742156
SWNTs; MWNTs; PEI; Small interfering RNA; Gene delivery
6.  Characterization of Electrosynthesized Conjugated Polymer-Carbon Nanotube Composite: Optical Nonlinearity and Electrical Property 
The effects of multi-walled carbon nanotube (MWNT) concentration on the structural, optical and electrical properties of conjugated polymer-carbon nanotube composite are discussed. Multi-walled carbon nanotube-polypyrrole nanocomposites were synthesized by electrochemical polymerization of monomers in the presence of different amounts of MWNTs using sodium dodecylbenzensulfonate (SDBS) as surfactant at room temperature and normal pressure. Field emission scanning electron microscopy (FESEM) indicates that the polymer is wrapped around the nanotubes. Measurement of the nonlinear refractive indices (n2) and the nonlinear absorption (β) of the samples with different MWNT concentrations measurements were performed by a single Z-scan method using continuous wave (CW) laser beam excitation wavelength of λ = 532 nm. The results show that both nonlinear optical parameters increased with increasing the concentration of MWNTs. The third order nonlinear susceptibilities were also calculated and found to follow the same trend as n2 and β. In addition, the conductivity of the composite film was found to increase rapidly with the increase in the MWNT concentration.
doi:10.3390/ijms13010918
PMCID: PMC3269728  PMID: 22312294
conducting polymers; carbon nanotubes; optical properties; electrical characterization
7.  Fabrication of a Microbial Biosensor Based on QD-MWNT Supports by a One-Step Radiation Reaction and Detection of Phenolic Compounds in Red Wines 
Sensors (Basel, Switzerland)  2011;11(2):2001-2012.
An Acaligense sp.-immobilized biosensor was fabricated based on QD-MWNT composites as an electron transfer mediator and a microbe immobilization support by a one-step radiation reaction and used for sensing phenolic compounds in commercial red wines. First, a quantum dot-modified multi-wall carbon nanotube (QD-MWNT) composite was prepared in the presence of MWNT by a one-step radiation reaction in an aqueous solution at room temperature. The successful preparation of the QD-MWNT composite was confirmed by XPS, TEM, and elemental analysis. Second, the microbial biosensor was fabricated by immobilization of Acaligense sp. on the surface of the composite thin film of a glassy carbon (GC) electrode, which was prepared by a hand casting method with a mixture of the previously obtained composite and Nafion solution. The sensing ranges of the microbial biosensor based on CdS-MWNT and Cu2S-MWNT supports were 0.5–5.0 mM and 0.7–10 mM for phenol in a phosphate buffer solution, respectively. Total concentration of phenolic compounds contained in commercial red wines was also determined using the prepared microbial immobilized biosensor.
doi:10.3390/s110202001
PMCID: PMC3274030  PMID: 22319395
microbial biosensor; quantum dots; one-step radiation reaction; electron transfer supports; phenolic compounds; red wines
8.  In situ Polymerization of Multi-Walled Carbon Nanotube/Nylon-6 Nanocomposites and Their Electrospun Nanofibers 
Nanoscale Research Letters  2008;4(1):39-46.
Multiwalled carbon nanotube/nylon-6 nanocomposites (MWNT/nylon-6) were prepared by in situ polymerization, whereby functionalized MWNTs (F-MWNTs) and pristine MWNTs (P-MWNTs) were used as reinforcing materials. The F-MWNTs were functionalized by Friedel-Crafts acylation, which introduced aromatic amine (COC6H4-NH2) groups onto the side wall. Scanning electron microscopy (SEM) images obtained from the fractured surfaces of the nanocomposites showed that the F-MWNTs in the nylon-6 matrix were well dispersed as compared to those of the P-MWNTs. Both nanocomposites could be electrospun into nanofibers in which the MWNTs were embedded and oriented along the nanofiber axis, as confirmed by transmission electron microscopy. The specific strength and modulus of the MWNTs-reinforced nanofibers increased as compared to those of the neat nylon-6 nanofibers. The crystal structure of the nylon-6 in the MWNT/nylon-6 nanofibers was mostly γ-phase, although that of the MWNT/nylon-6 films, which were prepared by hot-pressing the pellets between two aluminum plates and then quenching them in icy water, was mostly α-phase, indicating that the shear force during electrospinning might favor the γ-phase, similarly to the conventional fiber spinning.
doi:10.1007/s11671-008-9199-0
PMCID: PMC2893824  PMID: 20596470
In situ polymerization; Nylon-6; Nanofibers; Carbon nanotube; Nanocomposite
9.  In situ Polymerization of Multi-Walled Carbon Nanotube/Nylon-6 Nanocomposites and Their Electrospun Nanofibers 
Nanoscale Research Letters  2008;4(1):39-46.
Multiwalled carbon nanotube/nylon-6 nanocomposites (MWNT/nylon-6) were prepared by in situ polymerization, whereby functionalized MWNTs (F-MWNTs) and pristine MWNTs (P-MWNTs) were used as reinforcing materials. The F-MWNTs were functionalized by Friedel-Crafts acylation, which introduced aromatic amine (COC6H4-NH2) groups onto the side wall. Scanning electron microscopy (SEM) images obtained from the fractured surfaces of the nanocomposites showed that the F-MWNTs in the nylon-6 matrix were well dispersed as compared to those of the P-MWNTs. Both nanocomposites could be electrospun into nanofibers in which the MWNTs were embedded and oriented along the nanofiber axis, as confirmed by transmission electron microscopy. The specific strength and modulus of the MWNTs-reinforced nanofibers increased as compared to those of the neat nylon-6 nanofibers. The crystal structure of the nylon-6 in the MWNT/nylon-6 nanofibers was mostly γ-phase, although that of the MWNT/nylon-6 films, which were prepared by hot-pressing the pellets between two aluminum plates and then quenching them in icy water, was mostly α-phase, indicating that the shear force during electrospinning might favor the γ-phase, similarly to the conventional fiber spinning.
doi:10.1007/s11671-008-9199-0
PMCID: PMC2893824  PMID: 20596470
In situ polymerization; Nylon-6; Nanofibers; Carbon nanotube; Nanocomposite
10.  Synthesis of Novel Porphyrin and its Complexes Covalently Linked to Multi-Walled Carbon Nanotubes and Study of their Spectroscopy 
Nanoscale Research Letters  2009;4(6):578-583.
Novel covalent porphyrin and its complexes (Co2+, Zn2+) functionalized multi-walled carbon nanotubes (MWNTs) have been successfully synthesized by the reaction of the carboxyl on the surface of MWNTs which was synthesized to use carbon radicals generated by the thermal decomposition of azodiisobutyronitrile (AIBN) with 5-p-hydroxyphenyl-10,15,20-triphenyl-porphyrin and its complexes (Co2+, Zn2+). Three resulting nanohybrids were characterized by spectroscopy (FT-IR, Raman, and UV-vis), TGA, and TEM. The quality of porphyrin attached to the MWNTs was determined from thermogravimeric analysis (TGA) of the MWNTs, which showed a weight loss of about 60%. The Raman and absorption spectroscopy data showed that the electronic properties of modified MWNTs were mostly retained, without damaging their one-dimensional electronic properties. From fluorescence measurements, it was observed that the porphyrin and its complexes (Co2+, Zn2+) were nearly quenched by MWNTs, indicating that this covalently modified mode facilitated the effective energy or electron transfer between the excited porphyrin moiety and the extended π-system of MWNTs.
doi:10.1007/s11671-009-9282-1
PMCID: PMC2893826  PMID: 20596471
Synthesis; Characterization; Multi-walled carbon nanotubes; Porphyrin; Complexes; Covalent functionalization; Spectroscopy
11.  Synthesis of Novel Porphyrin and its Complexes Covalently Linked to Multi-Walled Carbon Nanotubes and Study of their Spectroscopy 
Nanoscale Research Letters  2009;4(6):578-583.
Novel covalent porphyrin and its complexes (Co2+, Zn2+) functionalized multi-walled carbon nanotubes (MWNTs) have been successfully synthesized by the reaction of the carboxyl on the surface of MWNTs which was synthesized to use carbon radicals generated by the thermal decomposition of azodiisobutyronitrile (AIBN) with 5-p-hydroxyphenyl-10,15,20-triphenyl-porphyrin and its complexes (Co2+, Zn2+). Three resulting nanohybrids were characterized by spectroscopy (FT-IR, Raman, and UV-vis), TGA, and TEM. The quality of porphyrin attached to the MWNTs was determined from thermogravimeric analysis (TGA) of the MWNTs, which showed a weight loss of about 60%. The Raman and absorption spectroscopy data showed that the electronic properties of modified MWNTs were mostly retained, without damaging their one-dimensional electronic properties. From fluorescence measurements, it was observed that the porphyrin and its complexes (Co2+, Zn2+) were nearly quenched by MWNTs, indicating that this covalently modified mode facilitated the effective energy or electron transfer between the excited porphyrin moiety and the extended π-system of MWNTs.
doi:10.1007/s11671-009-9282-1
PMCID: PMC2893826  PMID: 20596471
Synthesis; Characterization; Multi-walled carbon nanotubes; Porphyrin; Complexes; Covalent functionalization; Spectroscopy
12.  High temperature in-situ observations of multi-segmented metal nanowires encapsulated within carbon nanotubes by in-situ filling technique 
Nanoscale Research Letters  2012;7(1):448.
Multi-segmented one-dimensional metal nanowires were encapsulated within carbon nanotubes (CNTs) through in-situ filling technique during plasma-enhanced chemical vapor deposition process. Transmission electron microscopy (TEM) and environmental TEM were employed to characterize the as-prepared sample at room temperature and high temperature. The selected area electron diffractions revealed that the Pd4Si nanowire and face-centered-cubic Co nanowire on top of the Pd nanowire were encapsulated within the bottom and tip parts of the multiwall CNT, respectively. Although the strain-induced deformation of graphite walls was observed, the solid-state phases of Pd4Si and Co-Pd remain even at above their expected melting temperatures and up to 1,550 ± 50°C. Finally, the encapsulated metals were melted and flowed out from the tip of the CNT after 2 h at the same temperature due to the increase of internal pressure of the CNT.
doi:10.1186/1556-276X-7-448
PMCID: PMC3438045  PMID: 22873841
Carbon nanotubes; in-situ filling method; Metal nanowires; Encapsulation; Transmission electron microscopy (TEM); Environmental TEM; Melting temperature
13.  Conducting composite materials from the biopolymer kappa-carrageenan and carbon nanotubes 
Summary
Conducting composite films containing carbon nanotubes (CNTs) were prepared by using the biopolymer kappa-carrageenan (KC) as a dispersant. Rheological studies indicated that 0.5% w/v was the appropriate KC concentration for dispersing CNTs. Our results showed that multiwalled nanotubes (MWNTs) required less sonic energy than single-walled nanotubes (SWNTs) for the dispersion process to be complete. Films prepared by vacuum filtration exhibited higher conductivity and improved mechanical characteristics compared to those prepared by evaporative casting. All composite films displayed sensitivity to water vapour, but MWNT films were more sensitive than SWNT films.
doi:10.3762/bjnano.3.48
PMCID: PMC3388366  PMID: 23016146
biopolymers; carbon nanotubes; carrageenan; composite materials; conductivity; mechanical; rheology
14.  Characterization of electrospun nanocomposite scaffolds and biocompatibility with adipose-derived human mesenchymal stem cells 
Electrospun nanocomposite scaffolds were fabricated by encapsulating multi-walled carbon nanotubes (MWNT) in poly (lactic acid) (PLA) nanofibers. Scanning electron microscopy (SEM) confirmed the fabrication of nanofibers, and transmission electron microscopy identified the alignment and dispersion of MWNT along the axis of the fibers. Tensile testing showed an increase in the tensile modulus for a MWNT loading of 0.25 wt% compared with electrospun nanofibrous mats without MWNT reinforcement. Conductivity measurements indicated that the confined geometry of the fibrous system requires only minute doping to obtain significant enhancements at 0.32 wt%. Adipose-derived human mesenchymal stem cells (hMSCs) were seeded on electrospun scaffolds containing 1 wt% MWNT and 0 wt% MWNT, to determine the efficacy of the scaffolds for cell growth, and the effect of MWNT on hMSC viability and proliferation over two weeks in culture. Staining for live and dead cells and DNA quantification indicated that the hMSCs were alive and proliferating through day 14. SEM images of hMSCs at 14 days showed morphological differences, with hMSCs on PLA well spread and hMSCs on PLA with 1% MWNT closely packed and longitudinally aligned.
PMCID: PMC2673972  PMID: 17722553
adipose-derived human mesenchymal stem cells; multi-walled carbon nanotubes; bone tissue engineering; poly (lactic acid); electrospinning; nanocomposites
15.  Supercapacitance from Cellulose and Carbon Nanotube Nanocomposite Fibers 
ACS Applied Materials & Interfaces  2013;5(20):9983-9990.
Multiwalled carbon nanotube (MWNT)/cellulose composite nanofibers have been prepared by electrospinning a MWNT/cellulose acetate blend solution followed by deacetylation. These composite nanofibers were then used as precursors for carbon nanofibers (CNFs). The effect of nanotubes on the stabilization of the precursor and microstructure of the resultant CNFs were investigated using thermogravimetric analysis, transmission electron microscopy and Raman spectroscopy. It is demonstrated that the incorporated MWNTs reduce the activation energy of the oxidative stabilization of cellulose nanofibers from ∼230 to ∼180 kJ mol–1. They also increase the crystallite size, structural order, and electrical conductivity of the activated CNFs (ACNFs). The surface area of the ACNFs increased upon addition of nanotubes which protrude from the fiber leading to a rougher surface. The ACNFs were used as the electrodes of a supercapacitor. The electrochemical capacitance of the ACNF derived from pure cellulose nanofibers is demonstrated to be 105 F g–1 at a current density of 10 A g–1, which increases to 145 F g–1 upon the addition of 6% of MWNTs.
doi:10.1021/am403622v
PMCID: PMC3807724  PMID: 24070254
cellulose; supercapacitor; nanocomposites
16.  Noncovalently Modified Carbon Nanotubes with Carboxymethylated Chitosan: A Controllable Donor-Acceptor Nanohybrid 
We report here the modification of multiwalled carbon nanotubes (MWNTs) with a kind of polysaccharide, carboxymethylated chitosan (cmCs), and their potential usage as donor-acceptor nanohybrids. The modified composites (cmCs/MWNTs) were characterized by high-resolution TEM, FT-IR, TGA and time-resolved spectroscopy. The time-resolved spectroscopic experiments revealed that interfacial electron transfer readily takes place between MWNTs and surface immobilized cmCs chains. The forward electron transfer is fast (< 20 ns) while the backward recombination is slow. The recombination process strongly depends on the chain length of carboxylmethylated chitosan, i.e. a shorter recombination lifetime (~1.1 μs) for the shorter-chain cmCs coated MWNTs against that of the longer-chain cmCs coated MWNTs (~3.5 μs). The results demonstrated that the cmCs/MWNTs composite may be applied as a controllable donor-acceptor nanohybrid.
PMCID: PMC2635665  PMID: 19325737
multiwalled carbon nanotubes; carboxymethylated chitosan; charge separation; donor-accepter nanohybrid
17.  Increased paclitaxel cytotoxicity against cancer cell lines using a novel functionalized carbon nanotube 
Potential applications of carbon nanotubes have attracted many researchers in the field of drug delivery systems. In this study, multiwalled carbon nanotubes (MWNTs) were first functionalized using hyperbranched poly citric acid (PCA) to improve their hydrophilicity and functionality. Then, paclitaxel (PTX), a potent anticancer agent, was conjugated to the carboxyl functional groups of poly citric acid via a cleavable ester bond to obtain a MWNT-g-PCA-PTX conjugate. Drug content of the conjugate was about 38% (w/w). The particle size of MWNT-g-PCA and MWNT-g-PCA-PTX was approximately 125 and 200 nm, respectively. Atomic force microscopy and transmission electron microscopy images showed a curved shape for MWNT-g-PCA and MWNT-g-PCA-PTX, which was in contrast with the straight or linear conformation expected from carbon nanotubes. It seems that the high hydrophilicity of poly citric acid and high hydrophobicity of MWNTs led to conformational changes from a linear state to a curved state. Paclitaxel can be released from the MWNT-g-PCA-PTX conjugates faster at pH 6.8 and 5.0 than at pH 7.4, which was suitable for the release of the drug in tumor tissues and tumor cells. In vitro cytotoxicity studies were evaluated in the A549 and SKOV3 cell lines. MWNT-g-PCA had an insignificant cytotoxic effect on both cell lines. MWNT-g-PCA-PTX had more of a cytotoxic effect than the free drug over a shorter incubation time (eg, 24 hours versus 48 hours), which suggests improved cell penetration of MWNT-g-PCA-PTX. Therefore, paclitaxel conjugated to MWNT-g-PCA is promising for cancer therapeutics.
doi:10.2147/IJN.S17336
PMCID: PMC3084317  PMID: 21556345
multiwalled carbon nanotubes; functionalization; anticancer; drug delivery; nanoparticles
18.  Enzymatic Degradation of Multi-Walled Carbon Nanotubes 
The journal of physical chemistry. A  2011;115(34):9536-9544.
Because of their unique properties, carbon nanotubes and in particular multi-walled carbon nanotubes (MWNTs) have been used for the development of advanced composite and catalyst materials. Despite their growing commercial applications and increased production, the potential environmental and toxicological impacts of MWNTs are not fully understood; however, many reports suggest that they may be toxic. Therefore, a need exists to develop protocols for effective and safe degradation of MWNTs. In this article, we investigated the effect of chemical functionalization of MWNTs on their enzymatic degradation with horseradish peroxidase (HRP) and hydrogen peroxide (H2O2). We investigated HRP/H2O2 degradation of purified, oxidized, and nitrogen-doped MWNTs and proposed a layer-by-layer degradation mechanism of nanotubes facilitated by side wall defects. These results provide a better understanding of the interaction between HRP and carbon nanotubes and suggest an eco-friendly way of mitigating the environmental impact of nanotubes.
doi:10.1021/jp112324d
PMCID: PMC3113459  PMID: 21348486
19.  Electrochemical properties of double wall carbon nanotube electrodes 
Nanoscale Research Letters  2007;2(2):87-93.
Electrochemical properties of double wall carbon nanotubes (DWNT) were assessed and compared to their single wall (SWNT) counterparts. The double and single wall carbon nanotube materials were characterized by Raman spectroscopy, scanning and transmission electron microscopy and electrochemistry. The electrochemical behavior of DWNT film electrodes was characterized by using cyclic voltammetry of ferricyanide and NADH. It is shown that while both DWNT and SWNT were significantly functionalized with oxygen containing groups, double wall carbon nanotube film electrodes show a fast electron transfer and substantial decrease of overpotential of NADH when compared to the same way treated single wall carbon nanotubes.
doi:10.1007/s11671-006-9035-3
PMCID: PMC3245571
Double wall carbon nanotube; Single wall carbon nanotube; Electrochemistry; Cyclic voltammetry; NADH
20.  Electromechanics in MoS2 and WS2: nanotubes vs. monolayers 
Scientific Reports  2013;3:2961.
The transition-metal dichalcogenides (TMD) MoS2 and WS2 show remarkable electromechanical properties. Strain modifies the direct band gap into an indirect one, and substantial strain even induces an semiconductor-metal transition. Providing strain through mechanical contacts is difficult for TMD monolayers, but state-of-the-art for TMD nanotubes. We show using density-functional theory that similar electromechanical properties as in monolayer and bulk TMDs are found for large diameter TMD single- (SWNT) and multi-walled nanotubes (MWNTs). The semiconductor-metal transition occurs at elongations of 16%. We show that Raman signals of the in-plane and out-of-plane lattice vibrations depend significantly and linearly on the strain, showing that Raman spectroscopy is an excellent tool to determine the strain of the individual nanotubes and hence monitor the progress of nanoelectromechanical experiments in situ. TMD MWNTs show twice the electric conductance compared to SWNTs, and each wall of the MWNTs contributes to the conductance proportional to its diameter.
doi:10.1038/srep02961
PMCID: PMC3797429  PMID: 24129919
21.  Facile approach to prepare multi-walled carbon nanotubes/graphene nanoplatelets hybrid materials 
Nanoscale Research Letters  2013;8(1):243.
A facile approach was developed to prepare multi-walled carbon nanotubes/graphene nanoplatelets hybrid materials through covalent bond formation. First, poly(acryloyl chloride) was grafted onto oxidized multi-walled carbon nanotubes through the reaction between the acyl chloride groups of poly and the hydroxyl groups of oxidized multi-walled carbon nanotubes. Second, the remaining acyl chloride groups of poly were allowed to react with the hydroxyl groups of hydroxylated graphene nanoplatelets. Scanning electron microscopy and transmission electron microscopy data showed that the multi-walled carbon nanotubes and graphene nanoplatelets were effectively connected with each other. And Fourier transform infrared spectroscopy data indicated the formation of covalent bonds between carbon nanotubes and graphene nanoplatelets. Conformational changes were monitored by Raman spectroscopy. This novel kind of carbon hybrid materials may have the potential application in a wide field, especially in increasing the toughness and strength of the matrix resin.
doi:10.1186/1556-276X-8-243
PMCID: PMC3659076  PMID: 23680189
Multi-walled carbon nanotubes; Graphene; Hybrid materials; Poly(acryloyl chloride); Microstructure
22.  Electrochemical Reduction of CO2 to Organic Acids by a Pd-MWNTs Gas-Diffusion Electrode in Aqueous Medium 
The Scientific World Journal  2013;2013:424617.
Pd-multiwalled carbon nanotubes (Pd-MWNTs) catalysts for the conversion of CO2 to organic acids were prepared by the ethylene glycol reduction and fully characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) technologies. The amorphous Pd particles with an average size of 5.7 nm were highly dispersed on the surface of carbon nanotubes. Functional groups of the MWNTs played a key role in the palladium deposition. The results indicated that Pd-MWNTs could transform CO2 into organic acid with high catalytic activity and CO2 could take part in the reduction reaction directly. Additionally, the electrochemical reduction of CO2 was investigated by a diaphragm electrolysis device, using a Pd-MWNTs gas-diffusion electrode as a cathode and a Ti/RuO2 net as an anode. The main products in present system were formic acid and acetic acid identified by ion chromatograph. The selectivity of the products could be achieved by reaction conditions changing. The optimum faraday efficiencies of formic and acetic acids formed on the Pd-MWNTs gas-diffusion electrode at 4 V electrode voltages under 1 atm CO2 were 34.5% and 52.3%, respectively.
doi:10.1155/2013/424617
PMCID: PMC3885284  PMID: 24453849
23.  Effects of the electrical conductivity and orientation of silicon substrate on the synthesis of multi-walled carbon nanotubes by thermal chemical vapor deposition 
Nanoscale Research Letters  2013;8(1):110.
We studied the effects of the electrical conductivity and orientation of silicon substrate on both catalytic Fe thin film and the structure and morphology of multi-walled carbon nanotube (MWNT) grown by low-pressure chemical vapor deposition. Both p-type Si(100) and Si(111) substrates with three different doping concentrations (high, low, undoped) were used to evaluate the formation of catalytic nanoparticles and the growth of MWNTs. The morphology of catalytic nanoparticles such as size and density was characterized by field-emission scanning electron microscopy, Cs-corrected energy-filtered transmission electron microscopy, and X-ray photoelectron spectroscopy. Structural characteristics of MWNTs grown on different combinations of silicon substrate orientation and electrical conductivities (σ) were also systematically analyzed. Based on the experimental results, growth modes of MWNTs could be controlled by choosing an appropriate combination of σ and orientation of Si substrates.
doi:10.1186/1556-276X-8-110
PMCID: PMC3626923  PMID: 23445774
Multi-walled carbon nanotube; Catalytic nanoparticle; Substrate effect
24.  Functionalization of carboxylated multiwall nanotubes with imidazole derivatives and their toxicity investigations 
Imidazoles and their derivatives are compounds with chemotherapeutic applications. In this study, we investigated the chemical functionalization of carboxylated multiwalled carbon nanotubes (MWNT–COOH) by 1,2-phenylendiamine. Multiwalled nanotube (MWNT)–benzimidazole was obtained by an MWNT–amide reaction with POCl3 after 72 hours, which was confirmed by Fourier transform infrared, scanning electron microscopy, thermal gravimetric analysis, and elemental analysis. These functionalizations were chosen due to -NH2 and NHCO active sites in MWNT–amide for future application. Toxicity assays with fibroblast cells and MTT test for measurement of viable cell numbers were also performed. Cellular results did not show any toxicity change in modified samples from that of the reference samples.
doi:10.2147/IJN.S13629
PMCID: PMC2990384  PMID: 21116331
functionalization; 1,2-phenylendiamine; carboxylated multiwall nanotubes; toxicity
25.  Characterization of Multienzyme-Antibody-Carbon Nanotube Bioconjugates for Immunosensors 
Characterization studies of a multi-enzyme-antibody-carbon nanotube bioconjugate designed for the amplification of electrochemical immunosensing are described. Secondary antibodies for prostate specific antigen (PSA) were covalently linked to highly carboxylated multi-walled carbon nanotube (CNT) along with multiple horseradish peroxidase (HRP) enzyme labels. These bioconjugates provide ultra-sensitive amperometric detection of PSA on a single-wall carbon nanotube forest sandwich immunosensor platform. A single layer of HRP on the surface of the CNT was suggested by images from atomic force microscopy (AFM) and transmission electron microscopy (TEM). HRP on the bioconjugate surface was visualized by confocal microscopy using in-situ HRP-catalyzed polymerization yielding a fluorescent product, and HRP activity was estimated in a conventional assay. Binding of quantum-dot labeled PSA to antibodies on the bioconjugate was used for visualization by TEM. Combining TEM and enzyme activity results gave estimates of ~82 HRPs and 30 ± 15 secondary antibodies per 100 nm of antibody-HRP-CNT bioconjugate.
doi:10.1166/jnn.2008.J016
PMCID: PMC2735079  PMID: 19441303
Nanotube Bioconjugate; Protein Immunosensor; Atomic Force Microscopy; Electron Microscopy; Confocal Microscopy; Quantum Dots

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