Recent progress in chemical detection with single-walled carbon nanotube networks

Single-walled carbon nanotubes (SWNTs) have had significant impact on the development of gas sensors in the last decade. However, useful applications of SWNTs are limited by the lack of manufacturable routes to device formation. This Highlight article chronicles recent progress in this area and demonstrates the great promise of a new room temperature deposition method for SWNT networks in gas sensing applications. This liquid deposition technique allows the deposition of pre-treated, highly aligned SWNT networks on a wide variety of substrates. A significant advantage of SWNT-network sensors is that fluctuations in the electrical response of individual SWNTs become less important as the size of the network increases. Therefore, device properties can be controlled by the overall density of the network rather than the physical properties of any individual SWNT. At densities where semiconducting pathways dominate, highly sensitive thin-film chemoresistive sensors can be fabricated. Such devices also have higher signal-to-noise ratios and are easier to fabricate than devices based on a single SWNT.     

Chemical vapor depositions of single-walled carbon nanotubes catalyzed by uniform fe(2)o(3) nanoclusters synthesized using diblock copolymer micelles

Inversed micelles formed by polystyrene-block-poly(2-vinyl-pyridine) in toluene loaded with FeCl3 were used to synthesize and deliver discrete Fe2O3 nanoclusters with uniform diameters to flat substrates. Single-walled carbon nanotubes (SWNTs) were grown by chemical vapor deposition using these nanoclusters as the catalysts. Atomic force microscope characterizations revealed that high density SWNT mats were grown on the surface and the diameter of nanotubes was controlled by the diameter of nanoclusters. Electrical measurement revealed that the dense SWNT mats contained both semiconducting and metallic SWNTs and could be used to build thin film transistors.     

Free-standing highly conductive transparent ultrathin single-walled carbon nanotube films

Transparent and conductive single-walled carbon nanotube (SWNT) films are of great importance to a number of applications such as optical and electronic devices. Here, we describe a simple approach for preparing free-standing highly conductive transparent SWNT films with a 20-150 nm thickness by spray coating from surfactant-dispersed aqueous solutions of SWNTs synthesized by an improved floating-catalyst growth method. After the HNO(3) treatment, dipping the SWNT films supporting on glass substrates in water resulted in a quick and nondestructive self-release to form free-standing ultrathin SWNT films on the water surface. The obtained films have sufficiently high transmittance (i.e., 95%), a very low sheet resistance (i.e., ～120 Ω/sq), and a small average surface roughness (i.e., ～3.5 nm for a displayed 10 × 10 μm area). Furthermore, the floating SWNT films on the water surface were easily transferred to any substrates of interest, without intense mechanical and chemical treatments, to preserve their original sizes and network structures. For example, the transferred SWNT films on poly(ethylene terephthalate) films are mechanically flexible, which is a great advantage over conventional indium-tin oxide (ITO) and therefore strongly promise to be "post ITO" for many applications.     

Preparation of horizontally aligned single-walled carbon nanotubes with floating catalyst

A strategy to prepare horizontally aligned single-walled carbon nanotubes(SWNTs) at moderate temperatures(≤600 ℃) were developed.Using ferocene as the catalyst precursor,Fe nanoparticles are formed in the gaseous phase and catalyze the nucleation and growth of SWNTs in situ.Then the resultant SWNTs are deposited onto the substrates downstream and aligned by the surface lattice of the ST-cut single crystal quartz.The preparation of SWNT arrays at moderate temperatures is important for combining the tube growth with device fabrication.     

Simulation of adsorption of DNA on carbon nanotubes

We report molecular dynamics simulations of DNA adsorption on a single-walled carbon nanotube (SWNT) in an aqueous environment. We have modeled a DNA segment with 12 base pairs (Dickerson dodecamer) and a (8,8) SWNT in water, with counterions to maintain total charge neutrality. Simulations show that DNA binds to the external surface of an uncharged or positively charged SWNT on a time scale of a few hundred picoseconds. The hydrophobic end groups of DNA are attracted to the hydrophobic SWNT surface of uncharged SWNTs, while the hydrophilic backbone of DNA does not bind to the uncharged SWNT. The binding mode of DNA to charged SWNTs is qualitatively different from uncharged SWNTs. The phosphodiester groups of the DNA backbone are attracted to a positively charged SWNT surface while DNA does not adsorb on negatively charged SWNTs. There is no evidence for canonical double-stranded DNA wrapping around either charged or uncharged SWNTs on the very short time scales of the simulations. The adsorption process appears to have negligible effect on the internal stacking structure of the DNA molecule but significantly affects the A to B form conversion of A-DNA. The adsorption of A-DNA onto an uncharged SWNT inhibits the complete relaxation of A-DNA to B-DNA within the time scale of the simulations. In contrast, binding of the A-DNA onto a positively charged SWNT may promote slightly the A to B conversion.     

Effects of surfactant and boron doping on the BWF feature in the Raman spectrum of single-wall carbon nanotube aqueous dispersions

We examine the Breit-Wigner-Fano (BWF) line shape in the Raman spectra of carbon single-wall nanotubes (SWNTs) dispersed in aqueous suspensions. Bundling and electronic effects are studied by comparing undoped SWNTs (C-SWNTs) to boron-doped nanotubes (B-SWNTs) in a variety of different surfactant solutions. For SWNTs dispersed with nonionic surfactants that are less effective in debundling than ionic surfactants, the Raman spectra retain a large BWF feature. However, we demonstrate that even for SWNTs dispersed as isolated nanotubes by ionic surfactants the BWF feature may be present and that the intensity of the BWF is highly sensitive to the specific surfactant. In particular, surfactants with electron-donating groups tend to enhance the BWF feature. Also, modification of the SWNT electronic properties by boron doping leads to enhanced surfactant dispersion relative to undoped C-SWNTs and also to modification of the BWF feature. These observations are in agreement with reports demonstrating an enhancement of the BWF by bundling but also agree with reports that suggest electron donation can enhance the BWF feature even for isolated SWNTs. Importantly, these results serve to caution against using the lack or presence of a BWF feature as an independent measure of SWNT aggregation in surfactant dispersions.     

Metal-enhanced fluorescence of carbon nanotubes

The photoluminescence (PL) quantum yield of single-walled carbon nanotubes (SWNTs) is relatively low, with various quenching effects by metallic species reported in the literature. Here, we report the first case of metal enhanced fluorescence (MEF) of surfactant-coated carbon nanotubes on nanostructured gold substrates. The photoluminescence quantum yield of SWNTs is observed to be enhanced more than 10-fold. The dependence of fluorescence enhancement on metal-nanotube distance and on the surface plasmon resonance (SPR) of the gold substrate for various SWNT chiralities is measured to reveal the mechanism of enhancement. Surfactant-coated SWNTs in direct contact with metal exhibit strong MEF without quenching, suggesting a small quenching distance for SWNTs on the order of the van der Waals distance, beyond which the intrinsically fast nonradiative decay rate in nanotubes is little enhanced by metal. The metal enhanced fluorescence of SWNTs is attributed to radiative lifetime shortening through resonance coupling of SWNT emission to the reradiating dipolar plasmonic modes in the metal.     

A biomimetic "polysoap" for single-walled carbon nanotube dispersion

As-synthesized single-walled carbon nanotubes (SWNTs) are bundled mixtures of different species. The current challenge in the field of carbon nanotube research lies in the processing and separation of SWNTs, which first require efficient dispersion of individual SWNTs in solvents. We report DNA-mimicking polysoap surfactants that disperse SWNTs in aqueous solutions more effectively than DNA. The polysoaps are synthesized by functionalizing the side chain of poly(styrene-alt-maleic acid) with aminopyrene. The synthetic nature of the polysoap opens a new approach to further optimization of not only SWNT dispersion efficiency but also multi-functional SWNT dispersing surfactant.     

Water-soluble and optically pH-sensitive single-walled carbon nanotubes from surface modification

There is great interest in using single-walled carbon nanotubes (SWNTs) as nanoscale probes and sensors in biological electronics and optical devices because the electronic and optical properties of SWNTs are extremely sensitive to the surrounding environments. A well-controlled modification of SWNT surfaces may provide unique interfaces that are sensitive to the biological variables such as pH, glucose, various ions and proteins. In this paper, we report a facile chemical routine to prepare water-soluble SWNTs that still retain their van Hove singularities after acid oxidative treatment. The aqueous solutions (0.03-0.15 mg/mL) are stable for more than a month. The solubility in water for as-treated SWNTs with surfaces modified by carboxylate groups provides us with a unique opportunity to reveal the relationship of the SWNT electronic and optical properties with pH. Here we present the first observation that after surface modification with carboxylate groups, the optical absorption of the first interband transition of as-treated water-soluble semiconducting SWNTs reversibly responds to the pH change in aqueous solutions. Our results indicate that surface modification of SWNTs is a promising way for preparing chemically selective SWNT interfaces, which may open new exciting opportunities for various applications.     

Growth of Single-Walled Carbon Nanotubes with Controlled Structure: Floating Carbide Solid Catalysts

Single-walled carbon nanotube (SWNT) horizontal arrays with specific chirality can be enriched using solid carbide catalysts on substrates. However, scale-up production by continuous loading of the solid catalysts onto the substrates is challenging. Described here is the preparation of a floating carbide solid catalyst (FSC) for the controlled growth of SWNTs. The FSC, titanium carbide (TiC) nanoparticle, was directly obtained in the carrier gas phase by decomposition and carbonization of the titanocene dichloride precursor at high temperature. By using the TiC nanoparticle FSC, both SWNT horizontal arrays and randomly distributed networks can be obtained. The chirality of the as-grown SWNTs were thermodynamically controlled to have fourfold symmetry. Further optimization of growth condition resulted in an abundance of (16,8) tubes with about a 74 % content. This FSC chemical vapor deposition (FSCCVD) method has potential for realizing mass growth of SWNTs with controlled structures.     

Lipid-carbon nanotube self-assembly in aqueous solution

One major drawback associated with single-walled carbon nanotubes (SWNTs) in the liquid phase is their hydrophobicity-induced aggregation, which prevents utilization of the unique physical and chemical properties of single SWNTs. Recently it has been found that lysophospholipids, or single-tailed phospholipids, can readily form supramolecular complexes with SWNTs and the resultant SWNT solubility is superior to that provided by nucleic acids, proteins, and surfactants such as sodium dodecyl sulfate. Using transmission electron microscopy, lysophospholipids were observed forming striations on SWNTs in a vacuum. Although the morphology of the striations seemingly favors the hemimicellular model, serious doubts remain about the arrangement of individual lipids within the striations. Here we present an in silico study of the binding of zwitterionic lysophosphatidylcholine to an SWNT. We present compelling evidence that the binding of lipid surfactants to cylindrical nanostructures in the liquid phase does not obey any of the three popular models in the literature. Understanding the binding of lipid amphiphiles to SWNTs facilitates the bottom-up design of novel nanostructures for supramolecular chemistry and nanotechnology and fuels new field studies of nanotoxicity and nanomedicine.     

Real‐Time Nitrophenol Detection Using Single‐Walled Carbon Nanotube Based Devices

Single‐walled carbon nanotube (SWNT) based devices have been developed for the real‐time detection of nitrophenols in aqueous solution. SWNTs are assembled to electrodes using AC dielectrophoresis technique. The SWNT devices exhibit not only high sensitivity to nitrophenol compounds, but also good reusability. Charge transfer between nitro group and SWNTs, and the metal‐nanotube interface modification are hypothesized to be the possible origins of conductance change. These results indicated that the SWNT devices can be utilized as a simple, low cost, sensitive, and reusable platform for real‐time detection of nitrophenol compounds.     

Electrochemical and conductivity measurements of single-wall carbon nanotube network electrodes

The electrochemical response of two-dimensional networks of pristine single-wall carbon nanotubes (SWNTs) has been investigated. SWNTs were grown by catalyzed chemical vapor deposition on an insulating SiO2 substrate, and then electrically contacted by lithographically defined Au electrodes. Subsequent insulation of the contact electrodes enabled the electrochemical properties of the SWNT network to be isolated and directly studied for the first time. The electrochemical activity of the SWNT network was found to be strongly dependent on the applied potential. For the same SWNT electrode, the limiting current for the oxidation of 5 mM Fe(phen)32+ was found to be much greater than expected based on the signal for the reduction of 5 mM Ru(NH3)63+. Simultaneous conductance and electrochemical measurements demonstrated decreasing conductance as the potential was scanned negative (versus Ag/AgCl) with the minimum conductance at around the reduction potential for Ru(NH3)63+. These results are consistent with the presence of both metallic and semiconducting SWNTs in the SWNT network electrode. Moreover, these results show that through appropriate choice of mediator and electrode potential, metallic SWNTs can be electrochemically addressed independently of semiconducting SWNTs.     

Electronically selective chemical functionalization of carbon nanotubes: correlation between Raman spectral and electrical responses

Single-walled carbon nanotubes (SWNTs) demonstrate remarkable electronic and mechanical properties useful in developing areas such as nanoelectromechanical systems and flexible electronics. However, the highly inhomogeneous electronic distribution arising from different diameters and chirality in any given as-synthesized SWNT samples imposes severe limitations. Recently demonstrated selective chemical functionalization methods may provide a simple scalable means of eliminating metallic tubes from SWNT transistors and electronic devices. Here, we report on combined electron transport and Raman studies on the reaction of 4-bromobenzene diazonium tetrafluoroborate directly with single and networks of SWNT transistors. First, Raman studies are carried out on isolated individual SWNTs grown on SiO2/Si substrates by chemical vapor deposition with and without metal contacts. Metallic tubes are found to have, on average, higher reactivity toward diazonium reagents. However, a considerable degradation of electrical properties of semiconducting tubes occurs if the reaction is carried out to the point where the conductivity of metallic tubes is significantly suppressed. Insights from single-tube studies are then applied to elucidate the electrical and the Raman responses of SWNT random network transistors of different channel lengths to chemical functionalization.     

Synthesis of single- and double-walled carbon nanotube forests on conducting metal foils

Here, we report a highly efficient growth of single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) on conducting metal foils. We found that foils made of Ni-based alloys with Cr or Fe serve as excellent substrates for SWNT (DWNT) synthesis. In significant contrast, a CNT grown on Ni, Fe foils contains a significant ratio of MWNTs. This result opens up an economical route for the mass production of SWNT (DWNT) forests and also enables the straightforward integration of CNTs into nanoelectronic devices, such as field emission displays.     

RBM band shift-evidenced dispersion mechanism of single-wall carbon nanotube bundles with NaDDBS

The dispersion process of single-wall carbon nanotube (SWNT) by using sodium dodecylbenzene sulfonate (NaDDBS) was studied by means of surface tension measurements, ultraviolet-visible (UV-Vis) spectroscopy, scanning electron microscopy (SEM), and transmission electron spectroscopy (TEM). The critical micelle concentration (CMC) and the concentration where the surface tension begins to drop increase by the presence of SWNT. The isotherm of NaDDBS amount adsorbed on SWNT shows the plateau region at 0.2-6 mM and the saturated region above 40 mM. The external surface of SWNT bundle is fully covered with adsorbed NaDDBS at the plateau region, showing that SWNTs can be dispersed with the bundle form. On the other hand, SWNTs are dispersed in individual tubes at the saturated region, where the adsorption amount corresponds to coating of individual tube surfaces with NaDDBS. This dispersion state was confirmed by SEM and TEM observations. The effect of the dispersion state of SWNTs on radial breathing mode in Raman spectrum gave inherent peak shifts, being the in situ evidences on the step-wise dispersion mechanism of the SWNT bundle to the individual tubes.     

Dispersing carbon nanotubes using surfactants

We review the recent advances in dispersing single-wall carbon nanotubes (SWNTs) using amphiphilic surfactants in aqueous solutions. Three aspects are discussed. (1) On the organization of surfactant molecules with SWNTs, new insights at the microscopic level arise from electron microscopy and detailed computer simulation studies. (2) Quantitative measurements, such as molecular interactions between functional groups and SWNTs, the coverage of surfactant on SWNTs in solution, the charge state of the SWNT/surfactant complex, and the degree of dispersion are critical for better understanding dispersion mechanisms and for the further development of dispersion strategies. (3) The thermodynamic driving forces and the role of metastability in the structure of surfactant dispersed SWNT suspensions are analyzed. An outlook on practical and fundamental issues is also presented.     

Electrochemical templating of metal nanoparticles and nanowires on single-walled carbon nanotube networks

The use of single-walled carbon nanotube (SWNT) networks as templates for the electrodeposition of metal (Ag and Pt) nanostructures is described. Pristine SWNTs, grown on insulating SiO2 surfaces using catalyzed chemical vapor deposition, served as the working electrode. In the simplest case, electrical contact was made by depositing a gold strip on the SWNT substrate (device 1). Deposition of Ag and Pt over extensive periods (30 s) resulted in a high density of particles on the SWNTs, with almost contiguous nanowire formation from the Au/SWNT boundary moving to isolated nanoparticles at further distances from the contact. For direct electrochemical studies of Ag and Pt nucleation, the assembly was coated in a resist layer and a small window opened up to expose only the electrically connected SWNTs to solution (device 2). In this case, the electrochemical signature in voltammetric and amperometric studies of metal deposition was due solely to processes at the SWNTs. Coupled with high-resolution microscopy measurements (atomic force microscopy and field emission scanning electron microscopy), this approach provided detail on the nucleation and growth mechanisms of Ag and Pt on SWNTs under electrochemical control. In particular, Ag growth was found to be rapid and progressive with an increasing nanoparticle density with time, whereas Pt deposition was characterized by lower nucleation densities and slower growth rates with a tendency for larger particles to be produced over long times.     

Electorchemical Studies of Cytochrome c on Electodes Modified by Single—Wall Carbon Naotubes

Single-wall carbon nanotubes(SWNTs) modified gold electrodes were prepared by using two different methods.The electrochemical behavior of cytochrome c on the modified gold electrodes was investigated.The first kind of SWNT-modified electrode (noted as SWNT/Au electrode)was prepared by the adsorption of carboxylterminated SWNTs from DMF dispersion on the gold electrode.The oxidatively processed SWNT tips were covalently modified by coupling with amines (AET) to form amide linkage.Via Au-S chemical bonding,the self-assembled monolayer of thiol-unctionalized nanotubes on gold surface was fabricated so as to prepare the others SWNT-modified electrode (noted as SWNT/AET/Au electrode).It was shown from cyclic voltammetry cxperiments that cytochrome c exhibited direct electrochemical responses on the both electrodes, but only the current of controlled diffusion existed on the SWNT/Au electrode while both the currents of controlled diffusion and adsorption of cytochrome c occurred on the SWNT/AET/Au electrode.Photoelastic Modulation Infared Reflection Absorpthion Spectroscopy (PEM-IRRAS) and Quartz Crystal Microbalance (QCM) were employed to verify the adsorption of SWNTs on the gold electrodes.The results proved that SWNTs could enhance the direct electron transfer proecss between the electrodes and redox proteins.     

Crystal plane dependent growth of aligned single-walled carbon nanotubes on sapphire

On single-crystal substrates, such as sapphire (alpha-Al 2O 3) and quartz (SiO 2), single-walled carbon nanotubes (SWNTs) align along specific crystallographic axes of the crystal, indicating that the SWNT growth is influenced by the crystal surface. Here, we show that not only the orientation, but also the diameter and chirality of SWNTs are affected by the crystal plane of the sapphire substrate. The aligned SWNTs grown on the A- and R-planes of sapphire have narrower diameter distributions than randomly oriented tubes produced on the C-plane sapphire and amorphous SiO 2. Photoluminescence measurements reveal a striking difference between the aligned SWNTs: near-zigzag tubes are observed on the A-plane and near-armchair tubes on the R-plane. This study shows the route for the diameter and chirality control of SWNTs by surface atomic arrangements of a single-crystal substrate.