Surface Of Single-walled Carbon Nanotubes Research Articles

Diameter Dependent Melting and Softening of dsDNA Under Cylindrical Confinement

Carbon nanotubes (CNTs) are considered promising candidates for biomolecular confinement, including DNA encapsulation for gene delivery. Threshold values of diameters have been reported for double-stranded DNA (dsDNA) encapsulation inside CNTs. We have performed all-atom molecular dynamics (MD) simulations of dsDNAs confined inside single-walled CNTs (SWCNTs) at the physiologically relevant temperature of 300 K. We found that the dsDNA can be confined without being denatured only when the diameter of the SWCNT exceeds a threshold value. Below this threshold diameter, the dsDNA gets denatured and melts even at the temperature of 300 K. Our simulations using SWCNTs with chirality indices (20,20) to (30,30) at 300 K found the critical diameter to be 3.25 nm (corresponding to (24,24) chirality). Analyses of the hydrogen bonds (H-bonds), Van der Walls (VdW) energy, and other inter-base interactions show drastic reduction in the number of H-bonds, VdW energy, and electrostatic energies between the bases of dsDNA when it is confined in narrower SWCNTs (up to diameter of 3.12 nm). On the other hand, the higher interaction energy between the dsDNA and the SWCNT surface in narrower SWCNTs assists in the melting of the dsDNA. Electrostatic mapping and hydration status analyses show that the dsDNA is not adequately hydrated and the counter ion distribution is not uniform below the critical diameter of the SWCNT. As properly hydrated counter ions provide stability to the dsDNA, we infer that the inappropriate hydration of counter ions and their non-uniform distribution around the dsDNA cause the melting of the dsDNA inside SWCNTs of diameter below the critical value of 3.25 nm. For confined dsDNAs that do not get denatured, we computed their elastic properties. The persistence length of dsDNA was found to increase by a factor of about two and the torsional stiffness by a factor of 1.5 for confinement inside SWCNTs of diameters up to 3.79 nm, the stretch modulus also following nearly the same trend. Interestingly, for higher diameters of SWCNT, 3.79 nm and above, the dsDNA becomes more flexible, demonstrating that the mechanical properties of the dsDNA under cylindrical confinement depend non-monotonically on the confinement diameter.

Development of a novel ratiometric electrochemical sensor for monitoring β-galactosidase in Parkinson's disease model mice

Rapid, simple, accurate and highly sensitive detection of enzymes is essential for early screening and clinical diagnosis of many diseases. In this study, we report the fabrication of a turn-on ratiometric electrochemical sensor for the in situ determination of β-Galactosidase (β-Gal) based on surface engineering and the design of a molecular probe (Pygal) specific for β-Gal recognition. First, Pygal probe was synthesized and characterized, and then co-assembled with the methylene blue (MB) internal reference probe on the surface of single-wall carbon nanotubes (SWCNT)-modified carbon fiber microelectrode (CFME). The resulting CFME/SWCNT/MB + Pygal sensor is activated in the presence of β-Gal giving one peak at 0.33 V originating from the oxidation of the product of Pygal enzymatic hydrolysis (PyOH). Another oxidation peak attributed to MB appears simultaneously at −0.28 V allowing the construction of a ratiometric electrochemical sensor for β-Gal detection with improved sensitivity and accuracy. The sensor showed a linear response to β-Gal in a wide concentration range from 1.5 to 30 U L−1 and a low detection limit of 0.1 U L−1. Moreover, the sensor demonstrated excellent selectivity against several biologically relevant hydrolases and redox-active molecules. Finally, the combination of excellent electrochemical performance and favorable physicochemical properties of CFME allowed the determination of β-Gal in the whole blood of Parkinson's Disease (PD) model mice. The workflow reported in this study provides a strategy for the design and development of sensors for the in vivo monitoring of other enzymes important for the early diagnosis of different health issues.

Enhancement of thermoelectric property of carbon nanotubes by p-type doping with judicious molecular design of spiro-bifluorene derivatives

Although thermoelectric composites of carbon nanotubes (CNTs) with organic polymers or small organic molecules have witnessed explosive achievements, judicious molecular design still remains a great challenge. Here, p-type doping using spiro-bifluorene derivatives is proposed to significantly improve single-walled CNT (SWCNT) thermoelectric performance. The composite shows an increase of ∼108% in room-temperature power factor relative to the pristine SWCNT (112.6 ± 4.3 μW m-1 K-2), with the maximum of 239.2 ± 4.2 μW m-1 K-2. The corresponding mechanism of effective p-type doping and strong interfacial interaction are elucidated by calculated energy levels, SWCNT surface coating and photophysical spectroscopic measurements. Finally, the thermoelectric device connected in series display an open-circuit voltage and an output power of 9.77 mV and 57.3 nW, respectively at a temperature gradient of 30 K. The results will benefit the exploitation of novel CNT-based TE composites and their applications.

Potentiometric Biosensor Based on Artificial Antibodies for an Alzheimer Biomarker Detection

This paper presents a potentiometric biosensor for the detection of amyloid β-42 (Aβ-42) in point-of-care analysis. This approach is based on the molecular imprint polymer (MIP) technique, which uses covalently immobilised Aβ-42 to create specific detection cavities on the surface of single-walled carbon nanotubes (SWCNTs). The biosensor was prepared by binding Aβ-42 to the SWCNT surface and then imprinting it by adding acrylamide (monomer), N,N′-methylene-bis-acrylamide (crosslinker) and ammonium persulphate (initiator). The target peptide was removed from the polymer matrix by the proteolytic action of an enzyme (proteinase K). The presence of imprinting sites was confirmed by comparing a MIP-modified surface with a negative control (NIP) consisting of a similar material where the target molecule had been removed from the process. The ability of the sensing material to rebind Aβ-42 was demonstrated by incorporating the MIP material as an electroactive compound in a PVC/plasticiser mixture applied to a solid conductive support of graphite. All steps of the synthesis of the imprinted materials were followed by Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). The analytical performance was evaluated by potentiometric transduction, and the MIP material showed cationic slopes of 75 mV-decade−1 in buffer pH 8.0 and a detection limit of 0.72 μg/mL. Overall, potentiometric transduction confirmed that the sensor can discriminate Aβ-42 in the presence of other biomolecules in the same solution.

Selective dispersion of semiconducting single-walled carbon nanotubes with aromatic polyimides

Individually dispersed single-walled carbon nanotubes (SWNTs) with the conductivity of metals or semiconductors only are crucial for their applications in electronics, optics and magnetism. In this work, five aromatic polyimides with varying backbones were initially applied to the study on the dispersion of arc-discharge SWNTs. Spectroscopic characterizations by vis-NIR absorption and Raman scattering confirm that the polyimide molecules possess various selective dispersing ability toward semiconducting SWNTs. The effective dispersion of SWNTs is a result of the spirally wound polyimides around SWNTs dictated by the π-π stacking interactions between the backbone of polyimides and the surface of SWNTs, and the selectivity in semiconducting SWNTs is attributable to the charge transfer from polyimide molecules to SWNTs. The various dispersing behaviors of the polyimides toward SWNTs could be accounted for by the difference in aromatic surface area and flexibility of their backbones.

Impedance Technique-Based Label-Free Electrochemical Aptasensor for Thrombin Using Single-Walled Carbon Nanotubes-Casted Screen-Printed Carbon Electrode.

An impedance technique-based aptasensor for the detection of thrombin was developed using a single-walled carbon nanotube (SWCNT)-modified screen-printed carbon electrode (SPCE). In this work, a thrombin-binding aptamer (TBA) as probe was used for the determination of thrombin, and that was immobilized on SWCNT through π–π interaction. In the presence of thrombin, the TBA on SWCNT binds with target thrombin, and the amount of TBA on the SWCNT surface decreases. The detachment of TBA from SWCNT will be affected by the concentration of thrombin and the remaining TBA on the SWCNT surface can be monitored by electrochemical methods. The TBA-modified SWCNT/SPCE sensing layer was characterized by cyclic voltammetry (CV). For the measurement of thrombin, the change in charge-transfer resistance (Rct) of the sensing interface was investigated using electrochemical impedance spectroscopy (EIS) with a target thrombin and [Fe(CN)6]3− as redox maker. Upon incubation with thrombin, a decrease of Rct change was observed due to the decrease in the repulsive interaction between the redox marker and the electrode surface without any label. A plot of Rct changes vs. the logarithm of thrombin concentration provides the linear detection ranges from 0.1 nM to 1 µM, with a ~0.02 nM detection limit.

Tailoring SWCNTs surface morphology using PEI for highly selective and stable detection of Cu2+ heavy metal ion: a nanosensing platform

ABSTRACT Novel and facile investigation applying polyethylenimine (PEI) functionalised Single-wall Carbon Nanotubes (SWCNTs) for detection of heavy metal ion, copper (Cu2+) ions has been studied as a nanosensing platform. PEI/SWCNTs shows highly selective and rapid detection of Cu2+ ions since SWCNTs having large surface area shows high porosity and high chemical reactivity towards Cu2+ ions with high stability. FESEM and Raman Spectra measurements have been employed for characterising the nanocomposite and these techniques indicate that the PEI functionalised SWCNTs have been favourably synthesised. The detection of Cu2+ ions on the fabricated PEI/SWCNTs nanocomposite has been carried out by UV-vis spectroscopy. This study illustrates that PEI@SWCNTs nanoplatform is highly sensitive for selective and stable sensing of Cu2+ ions in the range of 0.01 µM to 100 µM with lower limit of detection (LOD) of 6.2 nM in aqueous medium. Selectivity and stability studies suggest that it is a unique and versatile platform for detection of copper ions rather in complex mediums with long-term usability.

Realisation of highly sensitive and selective NO2 detection at room temperature utilizing defect-induced single-walled carbon nanotubes combined with Pt functionalisation

For the discriminative detection of trace amounts of certain gas molecules in ambient environments, the gas response, selectivity, and humidity independence of sensing characteristics should be improved to realise real-time environmental monitoring applications. In this study, we implement a NO2 gas sensor operating at room temperature with excellent selectivity. The fabricated sensor exhibits good NO2 response even under a humid atmosphere between 33% and 76% RH (RH = relative humidity), compared with previously reported carbon-based sensing materials. To synthesise Pt-functionalised defect-induced single-walled carbon nanotubes (SWCNTs), pure SWCNTs are heat treated at 700 °C for 1 h under an Ar atmosphere for defect formation; subsequently, nanosized Pt particles are randomly functionalised on the surfaces of defect-induced SWCNTs via optimal ultraviolet irradiation. The Pt-functionalised defect-induced SWCNTs achieves high NO2 response, low detection limit (67 ppb), strong selectivity, good reversibility, and the ability to detect NO2 under a humid atmosphere. However, they exhibit only moderately improved humidity-immune NO2 sensing characteristics. The NO2 response of both the Pt-functionalised and defect-induced SWCNTs deteriorates significantly in a humid atmosphere (76% RH). These results show that Pt-functionalised defect-induced SWCNTs are a promising sensing material for the detection of NO2 gas in real-time applications.

Guanine-Specific Chemical Reaction Reveals ssDNA Interactions on Carbon Nanotube Surfaces.

Understanding the conformations of physisorbed single-stranded DNA (ssDNA) oligos on single-wall carbon nanotube (SWCNT) surfaces is important for advancing basic nanoscience and for developing applications in biomedicine and quantum information processing. Here we report evidence that the ssDNA strands are partly desorbed from the nanotube surface under common conditions. SWCNT suspensions were prepared in eight ssDNA oligos, each containing 1 guanine and 30 thymine bases but differing in the position of the guanine within the strand. Singlet oxygen exposure then covalently functionalized the guanine to the SWCNT surface, red-shifting the nanotube fluorescence by an amount reflecting the guanine spatial density at the surface. Spectral shifts were greatest for central guanine positions and smallest for end positions. In conjunction with steered molecular dynamics simulations, the results suggest that steric interference between neighboring ssDNA strands on an individual nanotube causes significant dislocation or desorption of the strand ends while central regions remain better wrapped around the nanotube. This effect decreases with decreasing concentrations of free ssDNA.

Decoupling the trade-off between thermoelectric and mechanical performances for polymer composites via interfacial regulation

Despite the explosive achievements in thermoelectric (TE) composites, their mechanical (ME) performance has received little attention relative to the significant enhancement of the TE performances. Furthermore, there always exists a trade-off between TE and ME performances, i.e., both are difficult to be improved simultaneously. Herein, we realize the decoupling of the trade-off between TE and ME performances for flexible polycarbonate (PC)/single-walled carbon nanotube (SWCNT) composites via interfacial adjustment. The barrel-shaped droplet-like PC granules are observed on the surfaces of the acid-treated SWCNTs (a-SWCNTs), indicating high wettability and interfacial affinity. The obtained PC/a-SWCNT composite displays a maximum power factor (PF) value of 6.2 μW m−1 K−2, which distinctly overmatches the PC/pristine SWCNT (p-SWCNT) with a PF value of 4.8 μW m−1 K−2. In addition, the composites reveal the increments of 22% and 16% in the fracture strength and tensile modulus over the PC/p-SWCNT composite, respectively. This decoupling of the trade-off is ascribed to the high electrical conductivity of a-SWCNTs and the strong interfacial interaction between PC and a-SWCNTs. The present work provides an easy and promising methodology to decouple the intractable trade-off issue of TE and ME performances and benefits their practical TE applications.

Quantum defects as versatile anchors for carbon nanotube functionalization.

Single-wall carbon nanotubes (SWCNTs) are used in diverse applications that require chemical tailoring of the SWCNT surface, including optical sensing, imaging, targeted drug delivery and single-photon generation. SWCNTs have been noncovalently modified with (bio)polymers to preserve their intrinsic near-infrared fluorescence. However, demanding applications (e.g., requiring stability in biological fluids) would benefit from a stable covalent linkage between the SWCNT and the functional unit (e.g., antibody, fluorophore, drug). Here we present how to use diazonium salt chemistry to introduce sp3 quantum defects in the SWCNT carbon lattice to serve as handles for conjugation while preserving near-infrared fluorescence. In this protocol, we describe the straightforward, stable (covalent), highly versatile and scalable functionalization of SWCNTs with biomolecules such as peptides and proteins to yield near-infrared fluorescent SWCNT bioconjugates. We provide a step-by-step procedure covering SWCNT dispersion, quantum defect incorporation, bioconjugation, in situ peptide synthesis on SWCNTs, and characterization, which can be completed in 5-7 d.

Molecular dynamics simulations reveal single-stranded DNA (ssDNA) forms ordered structures upon adsorbing onto single-walled carbon nanotubes (SWCNT)

Replica exchange molecular dynamics were used to observe the adsorption of single-stranded DNA (ssDNA) onto the surface of single-walled carbon nanotubes (SWCNTs). The assembly of these systems has garnered interest as a method by which SWCNTs can be separated based on chirality. While the exact mechanism of separation is yet unknown, it is hypothesized that the structure of the ssDNA layer pays an important role. Characterization of such an adsorbed layer has been a matter of recent work with the focus being on atomic level detail or such as base-stacking and hydrogen bonding. In this manuscript, we detail a new observation of ssDNA organization and demonstrate how it can be used to infer additional information about the way in which such biopolymers wrap around the cylindrical SWCNT.

Theoretical Study of the Adsorption Mechanism of Ioversol Contrast Agent on the Single-Walled Carbon Nanotube Surface

Theoretical Study of the Adsorption Mechanism of Ioversol Contrast Agent on the Single-Walled Carbon Nanotube Surface

Adsorption-desorption nano-aptasensors: fluorescent screening assays for ochratoxin A.

In this study, a FRET-based fluorescent aptasensor for the detection of ochratoxin A (OTA) was optimized based on the quenching efficiency of single-walled carbon nanotubes (SWCNTs) and the binding affinity of aptamers. OTA aptamers were conjugated with quantum dots and adsorbed to the surface of both acid-modified and unmodified SWCNTs. The maximum fluorescence quenching efficiency of the SWCNTs were compared. Acid-modified SWCNTs (amSWCNTs) have moderate quenching efficiency, providing an optimal sensitivity for qualitative fluorescence-enhancement biosensor assays. The binding parameters of the QD-modified OTA aptamers (1.12.2 and A08min) on the surface of amSWCNTs were compared. Based on our results, the A08min aptamer is a better candidate for OTA detection. Using the A08min aptamer, the SWCNT method had a limit of detection (LOD) of 40 nM. The amSWCNT method had a significantly lower LOD of 14 nM. Turn-on fluorescent nano-aptasensors are emerging as an effective diagnostic tool for simple detection of mycotoxins. Nanocomplexes designed for the detection of mycotoxins in solution and paper-based tests have proven to be useful.

Experimental and theoretical studies of the interaction of Penicillamine with SWCNT (6,0) as a drug delivery system

In this paper the interaction of Penicillamine (PCA) with single-walled carbon nanotube (SWCNT)(6,0) has been studied experimentally and theoretically. The element mapping (MAP) analysis has been indicated that the drug was adsorbed and distributed throughout the SWCNT (6,0) surface. The obtained FTIR spectrum confirms the presence of PCA on the surface of nanotube. The images of Field Emission Scanning Electron Microscope (FE-SEM) clearly demonstrate that the compound is fused in irregular films of nanometer size in which composed of very small spherical particles. The results of Energy Dispersive X-ray Analysis (EDAX) also, have been confirmed the combination of PCA with nanotube. TEM images have been clearly indicated the possibility of physical interaction between the CNT and the functional group of drug. These results have confirmed that the interaction of PCA with nanotube can be considered as a physical adsorption. The results obtained by the computational have confirmed the experimental data.

Computational study on noncovalent interactions between (n, n) single-walled carbon nanotubes and simple lignin model-compounds.

Composites of carbon nanotubes (CNTs) and lignin are promising and potentially cheap precursors of-to this day-expensive carbon fibers. Since the control of the CNT-lignin interface is crucial to maximize fiber performance, it is imperative to understand the fundamental noncovalent interactions between lignin and CNT. In the present study a density functional theory study is conducted to investigate the fundamental noncovalent interaction strength between metallic (n, n) single-walled CNT (SWCNT) and simple lignin model molecules. In particular, the respective adsorption energies are used to gauge the strength of interaction classes (ππ interaction, CHπ hydrogen bonding and OH-related hydrogen bonding. From the data, substituent-dependent interaction trends as well as class- and curvature-dependent interaction trends are derived. Overall, we find that most of the interaction strength trends appear to be strongly influenced by geometry: flat orientation of the test molecules relative to the (n, n) SWCNT surface and small (n, n) SWCNT curvature-that is, large diameter enhances the CHπ and ππ interactions.

Dendron-Polymer Hybrids as Tailorable Responsive Coronae of Single-Walled Carbon Nanotubes.

Functional composite materials that can change their spectral properties in response to external stimuli have a plethora of applications in fields ranging from sensors to biomedical imaging. One of the most promising types of materials used to design spectrally active composites are fluorescent single-walled carbon nanotubes (SWCNTs), noncovalently functionalized by synthetic amphiphilic polymers. These coated SWCNTs can exhibit modulations in their fluorescence spectra in response to interactions with target analytes. Hence, identifying new amphiphiles with interchangeable building blocks that can form individual coronae around the SWCNTs and can be tailored for a specific application is of great interest. This study presents highly modular amphiphilic polymer-dendron hybrids, composed of hydrophobic dendrons and hydrophilic polyethylene glycol (PEG) that can be synthesized with a high degree of structural freedom, for suspending SWCNTs in aqueous solution. Taking advantage of the high molecular precision of these PEG-dendrons, we show that precise differences in the chemical structure of the hydrophobic end groups of the dendrons can be used to control the interactions of the amphiphiles with the SWCNT surface. These interactions can be directly related to differences in the intrinsic near-infrared fluorescence emission of the various chiralities in a SWCNT sample. Utilizing the susceptibility of the PEG-dendrons toward enzymatic degradation, we demonstrate the ability to monitor enzymatic activity through changes in the SWCNT fluorescent signal. These findings pave the way for a rational design of functional SWCNTs, which can be used for optical sensing of enzymatic activity in the near-infrared spectral range.

Carbon nanotube biocompatibility in plants is determined by their surface chemistry

BackgroundAgriculture faces significant global challenges including climate change and an increasing food demand due to a growing population. Addressing these challenges will require the adoption of transformative innovations into biotechnology practice, such as nanotechnology. Recently, nanomaterials have emerged as unmatched tools for their use as biosensors, or as biomolecule delivery vehicles. Despite their increasingly prolific use, plant-nanomaterial interactions remain poorly characterized, drawing into question the breadth of their utility and their broader environmental compatibility.ResultsHerein, we characterize the response of Arabidopsis thaliana to single walled carbon nanotube (SWNT) exposure with two different surface chemistries commonly used for biosensing and nucleic acid delivery: oligonucleotide adsorbed-pristine SWNTs, and polyethyleneimine-SWNTs loaded with plasmid DNA (PEI-SWNTs), both introduced by leaf infiltration. We observed that pristine SWNTs elicit a mild stress response almost undistinguishable from the infiltration process, indicating that these nanomaterials are well-tolerated by the plant. However, PEI-SWNTs induce a much larger transcriptional reprogramming that involves stress, immunity, and senescence responses. PEI-SWNT-induced transcriptional profile is very similar to that of mutant plants displaying a constitutive immune response or treated with stress-priming agrochemicals. We selected molecular markers from our transcriptomic analysis and identified PEI as the main cause of this adverse reaction. We show that PEI-SWNT response is concentration-dependent and, when persistent over time, leads to cell death. We probed a panel of PEI variant-functionalized SWNTs across two plant species and identified biocompatible SWNT surface functionalizations.ConclusionsWhile SWNTs themselves are well tolerated by plants, SWNTs surface-functionalized with positively charged polymers become toxic and produce cell death. We use molecular markers to identify more biocompatible SWNT formulations. Our results highlight the importance of nanoparticle surface chemistry on their biocompatibility and will facilitate the use of functionalized nanomaterials for agricultural improvement.Graphical

Aqueous two-phase extraction of single-walled carbon nanotubes using nonionic surfactant Tween 60

<p indent="0mm">Due to its excellent electronic and optical properties, single-walled carbon nanotubes (SWNTs) have attracted extensive attention. Since the property of SWNTs is determined by their structures, applications like high-performance devices require SWNTs with identical structures and uniform properties. Therefore, the sorting of SWNTs is of great significance. Aqueous two-phase (ATP) extraction is an emerging method that can be utilized to realize the structure sorting of SWNTs. ATP extraction is a repeatable and reliable technique that does not require state-of-the-art equipment. It is efficient and easy to scale up. In this article, by adjusting the concentration of sodium deoxycholate (DOC) and nonionic surfactant Tween 60, the diameter sorting of SWNTs and the separation of high purity (6,5) tubes were achieved. The sorting can be carried out at a higher DOC concentration, which could improve the competitive coating between DOC and Tween 60 on the surface of SWNTs, and further improve the sorting efficiency of the co-surfactant system. The comparison of the separation results between the Tween 60-DOC surfactant system and the SDS-DOC surfactant system shows that the introduction of nonionic surfactant Tween 60 is beneficial to the separation of higher purity (6,5) tubes.

Enhancing the electrocatalytic activity of Fe phthalocyanines for the oxygen reduction reaction by the presence of axial ligands: Pyridine-functionalized single-walled carbon nanotubes

We have examined the electrocatalytic activity of iron phthalocyanine (FePc) and perchlorinated iron phthalocyanine 16(Cl)FePc for the oxygen reduction reaction (ORR) in alkaline medium with the two molecules either adsorbed on the external surface of single-wall carbon nanotubes (SWCNT) or covalently anchored via an axial pyridine ligand on pyridine-functionalized single-wall carbon nanotubes (py-SWCNT). Regardless of the particular phthalocyanine type, the ORR activity is higher when the substrate is py-SWCNT rather than SWCNT. The Tafel slopes for ORR are very similar for the two Fe macrocyclic complexes attached to SWCNTs in the two different configurations, suggesting a common rate-determining step for the ORR for all four catalysts. It is also observed that, for both the SWCNT and py-SWCNT supports, the ORR activity is higher for 16(Cl)FePc than for FePc. This is attributed to the electron-withdrawing effect of the peripheral and non-peripheral chlorine atoms in the macrocyclic ligand. While FeN4 macrocycles are known to be located on the strong binding side of a volcano correlation including several MN4 species, the chlorine substituents decrease the binding energy of O2 on the central Fe cation, thereby moving up the macrocycle catalyst towards the apex of the volcano correlation. Both the carbon surface and macrocyclic ligand effects were optimized with 16(Cl)FePc attached to py-SWCNT, which is more active than both FePc on SWCNT and FePc on py-SWCNT. Here we show that both the axial ligand and the electron withdrawing groups (-Cl) have a combined collaborative effect in increasing the catalytic activity for ORR of Fe-phthalocyanines confined on the external walls of single-wall carbon nanotubes.