Fabrication of thin-film electrochemical sensors from single-walled carbon nanotubes by vacuum filtration

We report a simple strategy for fabricating a thin-film filtering membrane electrode (FME) of single-walled carbon nanotubes (SWCNTs) by vacuum filtration. A SWCNT-FME, comprised of SWCNTs and mixed cellulose ester (MCE), is applicable in two detection modes. The face-mode SWCNT-FME directly employs the exposed SWCNT sensing layer and possesses remarkable properties as thin-film electrochemical sensor platforms, including good flexibility, wide potential window, large surface area and excellent analytical performance resembling traditional carbon nanotube-based electrochemical sensors. The back-mode SWCNT-FME has a tunable selectivity produced by combining the separation function of the outer MCE membrane with the high surface area of the inner SWCNT sensing layer. As an example of surface functionalization, an electrodeposition method was proposed for the controlled decoration of a SWCNT-FME with either low-density discrete flower-like gold nanoparticles (GNPs) or continuous GNP films. The resulting SWCNT-GNP composites were proved to possess high surface area and excellent electrocatalytic activity for the oxidation of glucose.     

Chemistry of single-walled carbon nanotubes

In this Account we highlight the experimental evidence in favor of our view that carbon nanotubes should be considered as a new macromolecular form of carbon with unique properties and with great potential for practical applications. We show that carbon nanotubes may take on properties that are normally associated with molecular species, such as solubility in organic solvents, solution-based chemical transformations, chromatography, and spectroscopy. It is already clear that the nascent field of nanotube chemistry will rival that of the fullerenes.     

Formylation of single-walled carbon nanotubes

Formyl or aldehyde groups are transferred to the surface of single-walled carbon nanotubes (SWCNTs) by reaction of reduced carbon nanotubes with N-formylpiperidine. This could open the way for more versatile chemical modification reactions of carbon nanotubes than is currently possible using functionalization methods reported to date. The formylated SWCNTs were characterized by thermogravimetric analysis-mass spectrometry and Raman, UV-vis-NIR and FTIR spectroscopy. The location and distribution of the functional groups was determined by AFM using electrostatic interactions with gold nanoparticles. The formylated SWCNTs were further derivatized with a fluorescent dye and studied using fluorescence spectroscopy.     

Structural characterization of atomically regulated nanocrystals formed within single-walled carbon nanotubes using electron microscopy

The structural chemistry of nanoscale materials encapsulated within single-walled carbon nanotubes (SWNTs) is reviewed. SWNTs form atomically thin channels within a restricted diameter range, and their internal van der Waals surfaces regulate the growth behavior of encapsulated crystals in a precise fashion, leading to atomically regulated growth. The structural properties of these systems are largely dictated by the structural chemistry of the bulk material, although significant deviations from bulk structures are often observed, with lower surface coordinations and substantial lattice distortions.     

Ultrafast hydrogen sensing through hybrids of semiconducting single-walled carbon nanotubes and tin oxide nanocrystals

We report an ultrafast and sensitive hydrogen (H(2)) sensing platform using semiconducting single-walled carbon nanotubes (SWCNTs) decorated with tin oxide (SnO(2)) nanocrystals (NCs). The hybrid SnO(2) NC-SWCNT platform shows a response time of 2-3 seconds to 1% H(2) under room temperature and can fully recover within a few minutes in air.     

Fabrication of vertically aligned single-walled carbon nanotubes in atmospheric pressure non-thermal plasma CVD

A fabrication technique of high-purity vertically aligned single-walled carbon nanotubes (VA-SWCNTs) using atmospheric pressure plasma enhanced chemical vapor deposition is presented. Although densely mono-dispersed Fe-Co catalysts of a few nanometers is primarily responsible for VA-SWCNT growth, carbon precipitation was virtually absent in the thermal CVD regime at 700 °C. On the other hand, high-purity VA-SWCNTs without measurable defects were grown at 4 μm min⁻¹ by applying atmospheric pressure radio-frequency discharge (APRFD) which has been previously developed for this purpose. The results proved that cathodic ion sheath adjacent to the substrates, where a large potential drop exists, also plays an essential role for the controlled growth of SWCNTs, while ion damage to the VA-SWCNTs is inherently avoided due to high collision frequency among molecules in atmospheric pressure. Operation regime of APRFD and tentative reaction mechanisms for VA-SWCNT growth are discussed along with optical emission spectroscopy of near substrate region.     

Decorating carbon nanotubes with co-precipitated magnetite nanocrystals

The superior properties of carbon nanotubes (CNTs) are best manifest in bulk materials when the CNTs are organized in tandem and embedded in a continuous matrix. Decorating the CNTs with magnetic nanoparticles (MNPs) facilitates their expedient organization with a magnetic field. One of the most convenient methods for their decoration is to first treat the CNTs with nitric or sulfuric acid, or a mixture of the two, and then co-precipitate MNPs in situ. Here, six variations of this protocol are compared to identify their influence on the decoration of multi-walled CNTs (MWNTs). Acid-treated MWNTs are scrutinized using XPS, and the decorated MWNTs are examined using X-ray diffraction, transmission electron microscopy, Fourier transformer infrared spectroscopy and vibrating sample magnetometry. The results show that (1) treatment with nitric acid provides the highest (~ 100%) attachment of MNPs to the MWNT walls, (2) sulfuric acid best preserves the MWNTs with only ~ 8% weight loss, and (3) after acid-treatment, the MWNTs must be washed and filtered prior to co-precipitation to prevent the consumption of up to 70% of the iron through side reactions that yield non-magnetic phases.     

Torsional behavior of single-walled carbon nanotubes

Atomistic simulations were performed to investigate the deformation behavior of single-walled carbon nano-tubes (SWCNTs) under torsional loading. The evolutions of the potential energy and stresses were presented. Radial distribution functions (RDFs) were calculated to analyze structural evolution during torsional deformation. The results show that during torsion, the tensile stress component along the tube axis is most significant and other stress components are almost negligible. The tensile stress stretched the C–C bonds until they reached the bond length of 0.18 nm. The torsional strength of the SWCNTs is about 30% of the tensile strength. Buckling took place at a few degrees of torsional angle and propagated along the tube as the torsional angle increased, and collapse of the tube wall followed buckling. These structural evolutions can be well described with the RDFs. Two new peaks appeared at 0.21 nm and 0.18 nm in the RDFs, corresponding to the minimum spacing between the atoms in the collapsed layers, and the maximum bond length that can be reached in stretching before rupture.     

Electron-induced cutting of single-walled carbon nanotubes

Electron beam irradiation with moderate fluences of approximately 10¹⁶-10¹⁷ electrons per cm² is used for controllable, bulk-scale cutting of single-walled carbon nanotubes (SWCNTs). The effectiveness of high energy electron irradiation in cutting SWCNTs is dependent on the nature of the sidewall. While pristine nanotubes are very stable under irradiation conditions, ozonated SWCNTs combined with a moderate fluence of electrons resulted in bulk-scale cutting of nanotubes. The length distribution of the cut SWCNTs could be controlled by adjusting the irradiation fluence. The average length of the cut nanotubes was 65 nm with 85% of the nanotubes shorter than 100 nm.     

Photophysics of individual single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are cylindrical graphitic molecules that have remained at the forefront of nanomaterials research since 1991, largely due to their exceptional and unusual mechanical, electrical, and optical properties. The motivation for understanding how nanotubes interact with light (i.e., SWNT photophysics) is both fundamental and applied. Individual nanotubes may someday be used as superior near-infrared fluorophores, biological tags and sensors, and components for ultrahigh-speed optical communications systems. Establishing an understanding of basic nanotube photophysics is intrinsically significant and should enable the rapid development of such innovations. Unlike conventional molecules, carbon nanotubes are synthesized as heterogeneous samples, composed of molecules with different diameters, chiralities, and lengths. Because a nanotube can be either metallic or semiconducting depending on its particular molecular structure, SWNT samples are also mixtures of conductors and semiconductors. Early progress in understanding the optical characteristics of SWNTs was limited because nanotubes aggregate when synthesized, causing a mixing of the energy states of different nanotube structures. Recently, significant improvements in sample preparation have made it possible to isolate individual nanotubes, enabling many advances in characterizing their optical properties. In this Account, single-molecule confocal microscopy and spectroscopy were implemented to study the fluorescence from individual nanotubes. Single-molecule measurements naturally circumvent the difficulties associated with SWNT sample inhomogeneities. Intrinsic SWNT photoluminescence has a simple narrow Lorentzian line shape and a polarization dependence, as expected for a one-dimensional system. Although the local environment heavily influences the optical transition wavelength and intensity, single nanotubes are exceptionally photostable. In fact, they have the unique characteristic that their single molecule fluorescence intensity remains constant over time; SWNTs do not "blink" or photobleach under ambient conditions. In addition, transient absorption spectroscopy was used to examine the relaxation dynamics of photoexcited nanotubes and to elucidate the nature of the SWNT excited state. For metallic SWNTs, very fast initial recovery times (300-500 fs) corresponded to excited-state relaxation. For semiconducting SWNTs, an additional slower decay component was observed (50-100 ps) that corresponded to electron-hole recombination. As the excitation intensity was increased, multiple electron-hole pairs were generated in the SWNT; however, these e-h pairs annihilated each other completely in under 3 ps. Studying the dynamics of this annihilation process revealed the lifetimes for one, two, and three e-h pairs, which further confirmed that the photoexcitation of SWNTs produces not free electrons but rather one-dimensional bound electron-hole pairs (i.e., excitons). In summary, nanotube photophysics is a rapidly developing area of nanomaterials research. Individual SWNTs exhibit robust and unexpectedly unwavering single-molecule fluorescence in the near-infrared, show fast relaxation dynamics, and generate excitons as their optical excited states. These fundamental discoveries should enable the development of novel devices based on the impressive photophysical properties of carbon nanotubes, especially in areas like biological imaging. Many facets of nanotube photophysics still need to be better understood, but SWNTs have already proven to be an excellent starting material for future nanophotonics applications.     

单壁碳纳米管分离研究

根据导电性能不同,单壁碳纳米管可以分为金属型和半导体型。目前所有方法制备的单壁碳纳米管,其产物为金属型和半导体型单壁碳纳米管的混合物,且很难将它们分开,这极大地阻碍了单壁碳纳米管在很多领域的应用。本文介绍了单壁碳纳米管的结构与导电性能的关系,着重综述了最新金属型和半导体型单壁碳纳米管的分离方法。     

Chemical modification of single-walled carbon nanotubes with sulfur-containing functionalities

Photolysis of cyclic disulfides with single-walled carbon nanotubes led to a sidewall modification of the surface to introduce sulfur-containing functional groups, confirming by means of Raman, XPS, FT-IR, mass and UV–vis–NIR measurements. Subsequent treatment of sulfur-containing substituents modified SWNTs with gold nanoparticles gave an attachment of gold on the surface of SWNTs through thioalkylthiol linkage.     

Modification of fluorinated single-walled carbon nanotubes with aminosilane molecules

Single-walled carbon nanotubes (SWCNTs) modified with amino groups were prepared via chemical addition of fluorine on the carbon nanotube surface by plasma treatment. The amino termination makes possible to realize hybrid nanostructures made out of SWCNTs and alkoxy-silane (3′-(aminopropyl)tri-ethoxysilane (APTES)) molecules. The functionalization of the SWCNTs was evidenced by transmission electron microscopy, infrared spectroscopy and thermogravimetric measurements. It was found that the application of a dc electric field enhances the assembly of APTES modified SWCNTs into ordered films with rectifying diode behavior.     

Adsorption of hydrogen on single-walled carbon nanotubes with defects

We present molecular dynamics (MD) simulations and density functional theory (DFT) calculations of hydrogen adsorption on single-walled carbon nanotubes (SWCNT) with various kinds of defects. The nature of defects, which is characterized here by the number of carbon atoms present in a ring on the surface of nanotube, plays a significant role in determining the hydrogen adsorption capacity of the SWCNT. Nanotubes containing the Stone–Wales defect with 5 and 8-member rings were found to have the largest hydrogen adsorption ability that increases further with the number of rings with such defects. Whereas, the presence of defects with 5, 3-5-8-member rings and the Stone–Wales defect with 5 and 7-member rings decreases the adsorption ability of the defective SWCNT significantly with respect to defect-free nanotubes. Our results indicate that the huge discrepancies in hydrogen storage capacities of SWCNT reported in the literature could be attributed to the nature of defects present in nanotubes. DFT calculations also reveal that the adsorption energy depends not only on the nature and number of defects present on the surface of nanotube but also on the equilibrium structure of rings.     

Selective interaction of single-walled carbon nanotubes with conducting dendrimer

We report on the transport properties of a system composed of single-wall carbon nanotubes (SWNTs) noncovalently linked to a new electrically conducting dendrimer poly(amidoamine) modified with a substituted naphthalenediimide (PAMAMC). SEM images show how the adsorption of the conducting dendrimer on SWNTs leads to the unroping of the bundles. The adsorption of PAMAMC molecules on SWNTs has been also investigated by electrical transport measurements. The electrical conductance of SWNTs drastically increases upon adsorption of conducting dendrimer. UV–Vis spectroscopy indicates that there was a modification in the electronic structure of the dendrimer as consequence of nanotube introduction while the appearance of new bands on the Raman spectra may suggest that metallic nanotubes are selectively functionalized.     

Chemical modification of single-walled carbon nanotubes with peroxytrifluoroacetic acid

A new and simple method for chemical modification of single-walled carbon nanotubes (SWNTs) is presented. Purified SWNTs ropes prepared by CVD growth were reacted with peroxytrifluoroacetic acid (PTFAA) under ultrasonication. Samples before and after treatment were characterized using Raman, FTIR, UV/Vis/NIR, XPS, and AFM. Data from these experiments conclusively showed that, in addition to oxygen-based functional groups, trifluoroacetic groups were covalently attached to the SWNTs. Moreover, these modified SWNTs were shortened into ca. 300 nm in length in the same step of functionalization, resulting in exfoliation of nanotube ropes to yield small bundles and individual nanotubes. The resultant SWNTs were easily dispersed in polar solvents such as dimethylformamide, water and ethanol. The PTFAA treatment described herein should be useful to tailor SWNTs’ chemical and physical properties and to broaden their chemical processibility and reactivity.