Layer-by-layer electrostatic self-assembly of single-wall carbon nanotube polyelectrolytes

We have used anionic and cationic single-wall carbon nanotube polyelectrolytes (SWNT-PEs), prepared by the noncovalent adsorption of ionic naphthalene or pyrene derivatives on nanotube sidewalls, for the layer-by-layer self-assembly to prepare multilayers from carbon nanotubes with polycations, such as poly(diallyldimethylammonium) or poly(allylamine hydrochloride) (PDADMA or PAH, respectively), and polyanions (poly(styrenesulfonate), PSS). This is a general and powerful technique for the fabrication of thin carbon nanotube films of arbitrary composition and architecture and allows also an easy preparation of all-SWNT (SWNT/SWNT) multilayers. The multilayers were characterized with vis-near-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), surface plasmon resonance (SPR) measurements, atomic force microscopy (AFM), and imaging ellipsometry. The charge compensation in multilayers is mainly intrinsic, which shows the electrostatic nature of the self-assembly process. The multilayer growth is linear after the initial layers, and in SWNT/polyelectrolyte films it can be greatly accelerated by increasing the ionic strength in the SWNT solution. However, SWNT/SWNT multilayers are much more inert to the effect of added electrolyte. In SWNT/SWNT multilayers, the adsorption results in the deposition of 1-3 theoretical nanotube monolayers per adsorbed layer, whereas the nominal SWNT layer thickness is 2-3 times higher in SWNT/polyelectrolyte films prepared with added electrolyte. AFM images show that the multilayers contain a random network of nanotube bundles lying on the surface. Flexible polyelectrolytes (e.g., PDADMA, PSS) probably surround the nanotubes and bind them together. On macroscopic scale, the surface roughness of the multilayers depends on the components and increases with the film thickness.     

Layer-by-layer electrostatic self-assembly of anionic and cationic carbon nanotubes

<正>The layer-by-layer(LBL) self assembly of anionic and cationic multi-walled carbon nanotubes(MWNTs) through electrostatic interaction has been carried out to fabricate all-MWNT multilayer films.The alternate uniform assembly of anionic and cationic MWNTs was investigated by UV-vis spectroscopy.Scanning electron microscopy(SEM) images displayed the growth of the MWNT films.     

Layer-by-layer self-assembly of multiwalled carbon nanotube polyelectrolytes prepared by in situ radical polymerization

Anionic and cationic multiwalled carbon nanotube polyelectrolytes, prepared by covalent modification of multiwalled carbon nanotubes (MWCNTs) with poly(acrylic acid) and poly(acrylamide), were used for the layer-by-layer (LBL) self-assembly of MWCNTs on different substrates with polyelectrolytes, such as poly(diallyldimethylammonium chloride) and sodium poly(styrenesulfonate). Thermogravimetric analysis, Raman spectroscopy, and scanning electron microscopy (SEM) were used to demonstrate the modification of MWCNTs. Investigations using Fourier transform infrared spectroscopy, atomic force microscopy, SEM, and ultraviolet-visible spectroscopy proved this method to be practicable for preparing LBL films.     

Observation of negative and positive trions in the electrochemically carrier-doped single-walled carbon nanotubes

Understanding of electronic and optical features of single-walled carbon nanotubes (SWNTs) has been a central issue in science and nanotechnology of carbon nanotubes. We describe the detection of both the positive trion (positively charged exciton) and negative trion (negatively charged exciton) as a three-particle bound state in the SWNTs at room temperature by an in situ photoluminescence spectroelectrochemistry method for an isolated SWNT film cast on an ITO electrode. The electrochemical hole and electron dopings enable us to detect such trions on the SWNTs. The large energy difference between the singlet bright exciton and the negative and positive trions showing a tube diameter dependence is determined by both the exchange splitting energy and the trion binding energy. In contrast to conventional compound semiconductors, on the SWNTs, the negative trion has almost the same binding energy to the positive trion, which is attributed to nearly identical effective masses of the holes and electrons.     

Coating single-walled carbon nanotubes with phospholipids

Single-walled carbon nanotubes (SWNTs), being hydrophobic by nature, aggregate in water to form large bundles. However, isolated SWNTs possess unique physical and chemical properties that are desirable for sensing and biological applications. Conventionally isolated SWNTs can be obtained by wrapping the tubes with biopolymers or surfactants. The binding modes proposed for these solubilization schemes, however, are less than comprehensive. Here we characterize the efficacies of solubilizing SWNTs through various types of phospholipids and other amphiphilic surfactants. Specifically, we demonstrate that lysophospholipids, or single-chained phospholipids offer unprecedented solubility for SWNTs, while double-chained phospholipids are ineffective in rendering SWNTs soluble. Using transmission electron microscopy (TEM) we show that lysophospholipids wrap SWNTs as striations whose size and regularity are affected by the polarity of the lysophospholipids. We further show that wrapping is only observed when SWNTs are in the lipid phase and not the vacuum phase, suggesting that the environment has a pertinent role in the binding process. Our findings shed light on the debate over the binding mechanism of amphiphilic polymers and cylindrical nanostructures and have implications on the design of novel supramolecular complexes and nanodevices.     

Generic approach for dispersing single-walled carbon nanotubes: the strength of a weak interaction

A generic noncovalent approach for dispersing high concentrations of individual single-walled carbon nanotubes (SWNT) in organic as well as aqueous solutions of synthetic block copolymers is presented. It is suggested that a weak, long-ranged entropic repulsion among polymer-decorated tubes acts as a barrier that prevents the tubes from approaching the attractive part of the intertube potential. The method opens a new route for utilization of block copolymers as compatibilizers for SWNT, improving the incorporation of de-agglomerated SWNT into target polymeric matrixes.     

Antenna chemistry with metallic single-walled carbon nanotubes

We show that, when subjected to microwave fields, surfactant-stabilized single-walled carbon nanotubes (SWNTs) develop polarization potentials at their extremities that readily drive electrochemical reactions. In the presence of transition metal salts with high oxidation potential (e.g., FeCl3), SWNTs drive reductive condensation to metallic nanoparticles with essentially diffusion-limited kinetics in a laboratory microwave reactor. Using HAuCl4, metallic particles and sheaths deposit regioselectively at the SWNT tips, yielding novel SWNT-metal composite nanostructures. This process is shown to activate exclusively metallic SWNTs; a degree of diameter selectivity is observed using acceptors with different oxidation potentials. The reaction mechanism is shown to involve Fowler-Nordheim field emission in solution, where electric fields concentrate at the SWNT tips (attaining approximately 10(9) V/m) due to the SWNT high aspect ratio (approximately 1000) and gradient compression in the insulating surfactant monolayer. Nanotube antenna chemistry is remarkably simple and should be useful in SWNT separation and fractionation processes, while the unusual nanostructures produced could impact nanomedicine, energy harvesting, and synthetic applications.     

Glassy carbon electrodes modified with DNA-partly-wrapped single-walled carbon nanotubes

DNA-partly-wrapped single-walled carbon nanotubes (DNA-p-SWCNTs) were separated from the mixtures of calf thymus DNA and SWCNTs in solution by differential centrifugation for the first time. Average mass ratios of DNA to SWCNTs for DNA-p-SWCNTs and DNA-fully-wrapped-SWCNTs (DNA-f-SWCNTs) were determined to be 0.8 and 2.0, respectively. It has been found that DNA-p-SWCNTs could form a uniform and porous film on glassy carbon electrodes due to special structure of them, which could facilitate the electron transfer between positively-charged compounds and electrodes, and showed good enrichment capability at low ionic strength.     

Metallic single-walled carbon nanotubes for conductive nanocomposites

This article reports an unambiguous demonstration that bulk-separated metallic single-walled carbon nanotubes offer superior performance (consistently and substantially better than the as-produced nanotube sample) in conductive composites with poly(3-hexylthiophene) and also in transparent conductive coatings based on PEDOT:PSS. The results serve as a validation on the widely held view that the carbon nanotubes are competitive in various technologies currently dominated by conductive inorganic materials (such as indium tin oxide).     

Interaction of single-walled carbon nanotubes with sulfuric acid

An increase in the radiation yield of paramagnetic centers in H₂SO₄ + nanotubes (NTs) solutions was evidence of the sensitizing influence of NTs on the low-temperature radiolysis of sulfuric acid, that is, on excitation energy and charge transfer. Under the conditions selected, the influence of NTs extended to distances of 100–300 nm. The presence of NTs also influenced the interstice nanodiffusion of atomic hydrogen by decreasing kinetic heterogeneity of the vitrified matrix surrounding NTs. No chemical interaction between atomic hydrogen and carbon NTs was observed at 77–120 K. The diffusion of radical-base anions occurred following the vacancy mechanism and was independent of the presence of NTs. Nanotubes did not form a separate phase as sulfuric acid solutions were cooled to 77 K. The transition from the vitreous to supercooled liquid state was observed as irradiated and nonirradiated solutions were heated to 175 K; no phase transitions occurred over the temperature range 180–300 K. For the first time, substantial changes in the electronic spectra of sulfuric acid solutions of NTs with time were observed: an intense additional absorption band at 320 nm appeared in the spectra in several days. This band was supposedly related to the formation of complexes between H₂SO₄ molecules and the surface of NTs.     

Interaction of SiO2 with single-walled carbon nanotubes

The effects of coating of a single-walled carbon nanotube (SWNT) with a nonbonded layer of silica are investigated via model system employing fully coordinated silica clusters. The geometric and electronic structures of the SWNT@SiO(2) composite system are calculated using periodic density functional (DF) calculations for a range of confining silica coatings. We show that silica can provide a protective bound coating to a single walled nanotube, which, importantly, only weakly perturbs the underlying properties of both components. Detailed analysis of the charge redistribution and changes in electronic structure upon coating the SWNT are performed to support this conclusion. Furthermore, as allowed by our versatile model system, the energetics of rotating a silica "bearing" around a nanotube "spindle" is also calculated to indicate the possibilities for SWNT@SiO(2)-based nanomechanical devices.     

Conformational selectivity of peptides for single-walled carbon nanotubes

Carbon nanotubes show promising prospects for applications ranging from molecular electronics to ultrasensitive biosensors. An important aspect to understanding carbon nanotube properties is their interactions with biomolecules such as peptides and proteins, as these interactions are important in our understanding of nanotube interactions with the environment, their use in cellular systems, as well as their interface with biological materials for medical and diagnostic applications. Here we report the sequence and conformational requirements of peptides for high-affinity binding to single-walled carbon nanotubes (SWNTs). A new motif, X(1)THX(2)X(3)PWTX(4), where X(1) is G or H, X(2) is H or D or null, X(3) is null or R, and X4 is null or K, was identified from two classes of phage-displayed peptide libraries. The high affinity binding of the motif to SWNTs required constrained conformations which were achieved through either the extension of the amino acid sequence (e.g., LLADTTHHRPWT) or the addition of a constrained disulfide bond (e.g., CGHPWTKC). This motif shows specific high-affinity to the currently studied SWNTs, compared to previously reported peptides. The conformations of the identified peptides in complex with SWNTs were also characterized with a variety of biophysical methodologies including CD, fluorescence, NMR spectroscopy, and molecular modeling.     

Fluorimetric characterization of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) are a family of structurally related artificial nanomaterials with unusual properties and many potential applications. Most SWCNTs can emit spectrally narrow near-IR fluorescence at wavelengths that are characteristic of their precise diameter and chiral angle. Near-IR fluorimetry therefore offers a powerful approach for identifying the structural species present in SWCNT samples. Such characterization is increasingly important for nanotube production, study, separation, and applications. General-purpose and specialized instruments suitable for SWCNT fluorimetric analysis are described, and methods for interpreting fluorimetric data to deduce the presence and relative abundances of different SWCNT species are presented. Fluorescence methods are highly effective for detecting SWCNTs in challenging samples such as complex environmental or biological specimens because of the methods’ high sensitivity and selectivity and the near absence of interfering background emission at near-IR wavelengths. Current limitations and future prospects for fluorimetric characterization of SWCNTs are discussed.     

Long super-bundles of single-walled carbon nanotubes

200 nm-thick super bundles showing a novel polygonization and densely aligned arrangement are found in long single-walled carbon nanotube (SWNT) strands prepared by the vertical floating catalytic method.     

Protein-assisted solubilization of single-walled carbon nanotubes

We report a simple method that uses proteins to solubilize single-walled carbon nanotubes (SWNTs) in water. Characterization by a variety of complementary techniques including UV-Vis spectroscopy, Raman spectroscopy, and atomic force microscopy confirmed the dispersion at the individual nanotube level. A variety of proteins differing in size and structure were used to generate individual nanotube solutions by this noncovalent functionalization procedure. Protein-mediated solubilization of nanotubes in water may be important for biomedical applications. This method of solubilization may also find use in approaches for controlling the assembly of nanostructures, and the wide variety of functional groups present on the adsorbed proteins may be used as orthogonal reactive handles for the functionalization of carbon nanotubes.     

Discrete dispersion of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) have been effectively wetted and dispersed in saturated sodium hydroxide (NaOH) alcohol-water solutions with little surface damage or shortening of the tubes; the treated material was dissolvable as individual tubes in many common organic solvents.     

Infrared spectroscopy of single-walled carbon nanotubes

By using the spectral moments method, we calculate the infrared spectra of chiral and achiral single-walled carbon nanotubes (SWCNTs) of different diameters and lengths. We show that the number of the infrared modes, their frequencies, and intensities depend on the length and chirality of the nanotubes. Furthermore, the dependence of the infrared spectrum as a function of the size of the SWCNT bundle is analyzed. These predictions are useful to interpret the experimental infrared spectra of SWCNTs.     

Stability of supershort single-walled carbon nanotubes

How short can single-walled carbon nanotubes (SWNTs) be? How stable are such supershort SWNTs (ss-SWNTs)? This work is the first to address these questions. On the basis of binding energy (E(B)), standard heats of formation , and strain energy (E(S)), we found that SWNTs with only one benzene ring in the axial direction, which we refer to as supershort SWNTs (ss-SWNTs), can be thermodynamically stable. On the basis of the data of E(B), , and E(S), the relative stabilities of ss-SWNTs, fullerenes, polycyclic aromatic hydrocarbons, and butadiyne are discussed. This study has laid a theoretical foundation for the possible synthesis of ss-SWNTs.