Analysis of stability of nanotube dispersions using surface tension isotherms

In this paper, we present the analyses of surface tension of surfactant-stabilized dispersions of carbon nanotubes. This method allows one to study interactions of carbon nanotubes with surfactants at different levels of nanotube loading when optical methods fall short in quantifying the level of nanotube separation. Sodium dodecyl sulfate was used as a stabilizing agent to uniformly disperse single-walled carbon nanotubes in an aqueous media. We show that surface tension is very sensitive to small changes of nanotube and surfactant concentrations. The experimental data suggest that, at moderate concentrations, surfactant displaces carbon nanotubes from the air-water interface and the nanotubes are mostly moved into the bulk of the liquid. By analyzing the surface tension as a function of surfactant concentration, we obtained the dependence of critical micelle concentration on nanotube loading. We then constructed the adsorption isotherm for dodecyl sulfate on carbon nanotubes and bundles of carbon nanotubes. The results of these experiments enabled us to extend the phase diagram of the produced dispersions to a broader range of surfactant and nanotube concentrations.     

Controlled Dispersion and Purification of Protein–Carbon Nanotube Conjugates Using Guanidine Hydrochloride

For the development of biofunctional carbon nanotubes for biosensors, drug carriers, and nanobiocatalysts, their aggregation and biofouling in aqueous solutions are crucial problems because this behavior leads to a reduction of their excellent optical and electrical properties and nanoscale size effects. This paper presents a new method for enhancing the dispersibility of protein–carbon nanotube conjugates and for exfoliating the protein from the carbon nanotube sidewalls through controlling the concentration of guanidine hydrochloride (Gdn ? HCl) in the solution. In medium concentrations (2–3 M ) of Gdn ? HCl, the dispersibility of protein–carbon nanotube conjugates was found to be substantially increased without denaturation or aggregation of the proteins. At higher concentrations (>6 M ) of Gdn ? HCl, pristine carbon nanotubes were precipitated instantly as a result of dissociation of the protein. These phenomena indicate that Gdn ? HCl functions not only as a dispersion adjuvant for biofunctional protein–carbon nanotube conjugates, but also as a cleaning agent for the purification of biofouled carbon nanotubes. The dissociation concentrations of Gdn ? HCl were higher than the midpoint of protein denaturation, suggesting that protein adsorption on carbon nanotubes is more stable than protein folding toward Gdn ? HCl.     

Chemical functionalization of carbon nanotubes by carboxyl groups on stone-wales defects: a density functional theory study

Chemical functionalization of carbon nanotubes with Stone-Wales (SW) defects by carboxyl (COOH) groups is investigated by density functional calculations. Due to the localized donor states induced by the SW defect, the binding of the COOH group with the defective carbon nanotube is stronger than that with the perfect one. A quasi-tetrahedral bonding configuration of carbon atoms, indicating sp3 hybrid bonding, is formed in the adsorption site. The charge distribution analysis shows that, in comparison with benzoic acid, the localized or delocalized pi states on the nanotube would affect the polarities of chemical bonds of the COOH group without losing the acidity. Furthermore, it is found that the double-adsorption system (two COOH groups are respectively adsorbed on two individual carbon atoms of the SW defect) is more energetically favorable than the monoadsorption one. The adsorption of COOH groups leads to a significant change of the electronic states around the Fermi level, which is advantageous for the electrical conductivity. The functionalization by introducing functional groups on the topological defects provides a pathway for applications of carbon nanotubes in chemical sensors and nanobioelectronics.     

Fabrication of carbon nanotube sheets and their bilirubin adsorption capacity

Bilirubin adsorption on carbon nanotube surfaces has been studied to develop a new adsorbent in the plasma apheresis. Powder-like carbon nanotubes were first examined under various adsorption conditions such as temperatures and initial concentrations of bilirubin solutions. The adsorption capacity was measured from the residual concentrations of bilirubin in the solution after the adsorption process using a visible absorption spectroscopy. We found that multi-walled carbon nanotubes (MWCNTs) exhibit greater adsorption capacity for bilirubin molecules than that of single-walled carbon nanotubes (SWCNTs). To guarantee the safety of the adsorbents, we fabricated carbon nanotube sheets in which leakage of CNTs to the plasma is suppressed. Since SWCNTs are more suitable for robust sheets, a complex sheet consisting of SWCNTs as the scaffolds and MWCNTs as the efficient adsorbents. CNT/polyaniline complex sheets were also fabricated. Bilirubin adsorption capacity of CNTs has been found to be much larger than that of the conventional materials because of their large surface areas and large adsorption capability for polycyclic compound molecules due to their surface structure similar to graphite.     

Conformation analysis of nucleic acids and proteins adsorbed on single-shell carbon nanotubes

This paper presents a concise review of the experimental and calculated data reported in the literature on the noncovalent interactions of DNA and proteins with the nonfunctionalized carbon nanotubes. Our Raman scattering and electron microscopy data on carbon nanotubes and SEIRA spectral data on changes in the conformational state of the main biological polymers (DNA, Poly, BSA, and RNase) in reactions with single-shell carbon nanotubes allowed us to define the character of noncovalent interactions in the tube biomolecule system. An analysis of the data showed that reactions of DNA with nanotubes lead to the binding on the surface of the nanotube and form stable complexes with van der Waals interactions, in which stacking plays the major role and which changes the hydrogen bonds in the biological molecule with structure rearrangements. Albumin and RNase are presumably adsorbed at the conventional binding sites of these proteins on the nanotube with participation of hydrophobic interaction and π stacking, as indicated by structure rearrangements in proteins.     

WC/CNT纳米复合材料制备及其对甲醇氧化的电催化性能

采用表面修饰技术将碳纳米管(CNT)表面羧基化, 通过羧基将钨离子基团修饰到碳纳米管的外表面, 再通过原位还原碳化技术, 将钨离子基团还原成碳化钨(WC)纳米微粒, 制备出WC/CNT纳米复合材料. 采用FTIR、XRD、SEM、HRTEM和N2吸附等分析测试手段对样品的形貌、晶相组成和微观结构特征进行了表征. FTIR和N2吸附结果表明, 硝化后, 在碳纳米管表面羧基化的同时比表面积增加; XRD结果表明, WC/CNT样品由碳纳米管、WC以及非化学计量比的氧化钨组成; SEM和HRTEM结果表明, WC纳米颗粒均匀地分散于碳纳米管的外表面,并与碳纳米管构成了复合材料. 采用循环伏安法测试了样品在碱性条件下对甲醇氧化的电催化性能, 结果表明, 复合材料对甲醇氧化的电催性能明显强于WC 和碳纳米管, 并在实验结果的基础上探讨了复合材料催化性能提高的原因.     

Overcoming the insolubility of carbon nanotubes through high degrees of sidewall functionalization

The use of carbon nanotubes in materials applications has been slowed due to nanotube insolubility and their incompatibility with polymers. We recently developed two protocols to overcome the insoluble nature of carbon nanotubes by affixing large amounts of addends to the nanotube sidewalls. Both processes involve reactions with aryl diazonium species. First, solvent-free functionalization techniques remove the need for any solvent during the functionalization step. This delivers functionalized carbon nanotubes with increased solubility in organic solvents and processibility in polymeric blends. Additionally, the solvent-free functionalization process can be done on large scales, thereby paving the way for use in bulk applications such as in structural materials development. The second methodology involves the functionalization of carbon nanotubes that are first dispersed as individual tubes in surfactants within aqueous media. The functionalization then ensues to afford heavily functionalized nanotubes that do not re-rope. They remain as individuals in organic solvents giving enormous increases in solubility. This protocol yields the highest degree of functionalization we have obtained thus far-up to one in nine carbon atoms on the nanotube has an organic addend. The proper characterization and solubility determinations on nanotubes are critical; therefore, this topic is discussed in detail.     

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.     

Structural characterization of single-walled carbon nanotube bundles by experiment and molecular simulation

A procedure, combining molecular simulation, Raman spectroscopy, and standard nitrogen adsorption, is developed for structural characterization of single-walled carbon nanotube (SWNT) samples. Grand canonical Monte Carlo simulations of nitrogen adsorption are performed on the external and internal adsorption sites of homogeneous arrays of SWNTs of diameters previously determined by Raman spectroscopy of the sample. The results show the importance of the peripheral grooves of a nanotube bundle at low relative pressure and the insensitivity of nanotube diameter toward adsorption on the external surface of the bundle at higher pressures. Simulations also reveal that samples containing thin nanotubes have less internal adsorption capacity that saturates at lower pressure than those comprising large diameter nanotubes. The fraction of open-ended nanotubes in a sample can be estimated by scaling the simulated internal adsorption inside nanotubes to obtain a near perfect fit between simulated and experimental isotherms. This procedure allows extrapolation of adsorption properties to conditions in which all nanotubes in the sample are open-ended.     

Amine basicity (pKb) controls the analyte binding energy on single walled carbon nanotube electronic sensor arrays

A wide range of analytes adsorb irreversibly to the surfaces of single walled carbon nanotube electronic networks typically used as sensors or thin-film transistors, although to date, the mechanism is not understood. Using thionyl chloride as a model electron-withdrawing adsorbate, we show that reversible adsorption sites can be created on the nanotube array via noncovalent functionalization with amine-terminated molecules of pKa < 8.8. A nanotube network comprising single, largely unbundled nanotubes, near the electronic percolation threshold is required for the effective conversion to a reversibly binding array. By examining 11 types of amine-containing molecules, we show that analyte adsorption is largely affected by the basicity (pKb) of surface groups. The binding energy of the analyte is apparently reduced by its adsorption on the surface chemical groups instead of directly on the SWNT array itself. This mediated adsorption mechanism is supported by X-ray photoelectron spectroscopy (XPS) and molecular potential calculations. Reversible detection with no active regeneration at the parts-per-trillion level is demonstrated for the first time by creating a higher adsorption site density with a polymer amine, such as polyethyleneimine (PEI). Last, we demonstrate that this transition to reversibility upon surface functionalization is a general phenomenon.     

Alternate layer-by-layer adsorption of single- and double-walled carbon nanotubes wrapped by functionalized beta-1,3-glucan polysaccharides

A great deal of attention has been focused on exploiting novel methods to fabricate thin carbonaceous capsules from multiple components for advanced materials. A layer-by-layer (LbL) method is therefore being introduced to synthesize thin and multi-carbon nanotube (CNT)-based hollow capsules from CNT complexes with cationic or anionic complementarily functionalized beta-1,3-glucans as building-blocks. These ionic beta-1,3-glucans wrap around single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) to form water-soluble complexes with ionic groups on their exterior surface. Alternate self-assembly of these CNT complexes on the silica particles is demonstrated in solution by electrostatic interactions. The LbL adsorption processes were carefully monitored by zeta-potential measurements, frequency shifts of a quartz crystal microbalance (QCM), and electron micrographs. Silica particles were then dissolved away by HF acid to obtain CNT-based hollow capsules composed of SWNTs and DWNTs. We believe that these novel surface adsorption methods are useful for potential design of CNT-based advanced functional materials.     

Adsorption of gases in carbon nanotubes: are defect interstitial sites important?

Molecular simulations are used to shed light on an ongoing controversy over where gases adsorb on single walled carbon nanotube bundles. We have performed simulations using models of carbon nanotube bundles composed of tubes of all the same diameter (homogeneous) and tubes of different diameters (heterogeneous). Simulation data are compared with experimental data in an effort to identify the best model for describing experimental data. Adsorption isotherms, isosteric heats of adsorption, and specific surface areas have been computed for Ar, CH 4, and Xe on closed, open, and partially opened homogeneous and heterogeneous nanotube bundles. Experimental data from nanotubes prepared from two different methods, electric arc and HiPco, were examined. Experimental adsorption isotherms and isosteric heats for nanotubes prepared by the electric arc method are in best agreement with simulations for heterogeneous bundles of closed nanotubes. Models including adsorption in defect interstitial channels are required to achieve good agreement with experiments. Experimental isosteric heats and specific surface areas on HiPco nanotubes are best described by a model consisting of heterogeneous bundles with approximately 11% of the nanotubes opened.     

Coaxial carbon nanotubes as shielded nanowires

We report transverse polarizabilities of coaxial carbon nanotubes using first principles density functional theory. These results demonstrate a shielding of the inner nanotube from electric fields by the outer nanotube. This study has implications for nanoelectronics, specifically for the possibility of using coaxial nanotubes as nanoelectrical wires. Shielding is predicted to be on the order of 95% by high-level polarizability calculations. This shielding occurs regardless of whether the outer nanotube is metallic or semiconducting. In addition, a series of calculations on coaxial nanotubes where the inner nanotube is not centered show that the shielding still occurs with approximately the same magnitude. These calculations therefore indicate that it would be possible to use a coaxial carbon nanotube as a shielded nanowire.     

Direct measurements of interactions between polypeptides and carbon nanotubes

The interactions of various polypeptides with individual carbon nanotubes (CNTs), both multiwall (MW) and single wall (SW), were investigated by atomic force microscopy (AFM). While adhesion forces arising from electrostatic attraction interactions between the protonated amine groups of polylysine and carboxylic groups on the acid-oxidized multi-wall carbon nanotubes (Ox-MWCNTs) dominate the interaction at a low pH, weaker adhesion forces via the hydrogen bonding between the neutral -NH2 groups of polylysine and -COO- groups of the Ox-MWCNTs were detected at a high pH. The adhesion force was further found to increase with the oxidation time for Ox-MWCNTs and to be negligible for oxidized single-wall carbon nanotubes (Ox-SWCNTs) because carboxylate groups were only attached onto the nanotube tips in the latter whereas onto both the nanotube tips and sidewall in the former. Furthermore, it was demonstrated that proteins containing aromatic moieties, such as polytryptophan, showed a stronger adhesion force with Ox-MWCNTs than that of polylysine because of the additional pi-pi stacking interaction between the polytryptophan chains and CNTs.     

Monte Carlo simulations of hydrogen adsorption in alkali-doped single-walled carbon nanotubes

Monte Carlo simulations and Widom's test particle insertion method have been used to calculate the solubility coefficients (S) and the adsorption equilibrium constants (K) in single-walled (10,10) armchair carbon nanotubes including single nanotubes, and nanotube bundles with various configurations with and without alkali dopants. The hydrogen adsorption isotherms at room temperature were predicted by following the Langmuir adsorption model using the calculated constants S and K. The simulation results were in good agreement with experimental data as well as the grand canonical Monte Carlo simulation results reported in the literature. The simulations of nanotube bundle configurations suggest that the gravimetric hydrogen adsorption increases with internanotube gap size. It may be attributed to favorable hydrogen-nanotube interactions outside the nanotubes. The effect of alkali doping on hydrogen adsorption was studied by incorporating K+ or Li+ ions into nanotube arrays using a Monte Carlo simulation. The results on hydrogen adsorption isotherms indicate hydrogen adsorption of 3.95 wt% for K-doping, and 4.21 wt% for Li-doping, in reasonable agreement with the experimental results obtained at 100 atm and room temperature.     

甘氨酸在纳米碳管中的吸附及性质的分子模拟

Molecular Mechanics and Molecular dynamics have been performed to study the adsorption and the diffusion,and optimize the configuration and the energy of glycine molecules in carbon nanotubes. The results of the simulation indicate that the configuration of glycine has been changed,and those varieties will bring on the changes of the biological properties via molecular biology. Carbon nanotube shows relatively strong sorption for glycine molecules,and carbon nanotubes and glycine molecules will produce relatively strong interaction of π-π electrons. The motions between glycine molecules and carbon nanotubes will keep very synergistic status in order to make the system remaining in the state of optimal energy among the simulation.     

Air separation by single wall carbon nanotubes: thermodynamics and adsorptive selectivity

Separation of a nitrogen-oxygen mixture (air) by single wall carbon nanotubes has been studied using grand canonical Monte Carlo simulations at a range of nanotube diameters, temperatures, and pressures. It is demonstrated that depending on these operating parameters, the extent of adsorptive selectivity can vary significantly. Detailed calculations are also presented for the adsorption isotherms, energies, and isosteric heats of pure nitrogen, oxygen, and their mixture at 100 K in a carbon nanotube of 12.53-A diameter. In single-component simulations, it is found that near saturation loading nitrogen forms only an annular layer close to the nanotube wall, while smaller-sized oxygen also occupies the region near the center of the nanotube. In mixture adsorption, the energetically favored nitrogen is preferentially adsorbed at low loadings. However, at high loadings oxygen replaces nitrogen due to the dominant entropic effects, and therefore a high selectivity towards oxygen is observed close to the saturation loading. The effect of the entropic change on mixture adsorption is evident from the calculated isosteric heats of adsorption. The mixture isotherms obtained from simulations are found to be in good agreement with the predictions based only on the pure component simulation results.     

Carbon nanotubes as solid-phase extraction sorbents prior to atomic spectrometric determination of metal species: A review

New materials have significant impact on the development of new methods and instrumentation for chemical analysis. From the discovery of carbon nanotubes in 1991, single and multi-walled carbon nanotubes – due to their high adsorption and desorption capacities – have been employed as sorption substrates in solid-phase extraction for the preconcentration of metal species from diverse matrices. Looking for successive improvements in sensitivity and selectivity, in the past few years, carbon nanotubes have been utilized as sorbents for solid phase extraction in three different ways: like as-grown, oxidized and functionalized nanotubes. In the present paper, an overview of the recent trends in the use of carbon nanotubes for solid phase extraction of metal species in environmental, biological and food samples is presented. The determination procedures involved the adsorption of metals on the nanotube surface, their quantitative desorption and subsequent measurement by means of atomic spectrometric techniques such as flame atomic absorption spectrometry, electrothermal atomic absorption spectrometry or inductively coupled plasma atomic emission spectrometry/mass spectrometry, among others. Synthesis, purification and types of carbon nanotubes, as well as the diverse chemical and physical strategies for their functionalization are described. Based on 140 references, the performance and general properties of the applications of solid phase extraction based on carbon nanotubes for metal species atomic spectrometric determination are discussed.     

碳纳米管内N2吸附行为及等量吸附热的GCMC分子模拟研究

碳纳米管及石墨烯具有高比表面积、高化学稳定性以及高耐蚀性等优点,被认为是一种理想的吸附材料。分子模拟技术的发展和应用丰富了人们对吸附机理研究的方式,而简单气体吸附体系的吸附机理研究对吸附理论的发展有着重要的推动作用。本文以单壁碳纳米管（SWCNT）-N₂吸附体系为研究对象,首先通过透射扫描电镜和氮气吸/脱附测试对所选用碳纳米管的微观孔形貌及吸/脱附等温线进行了表征,然后根据对应孔径参数采用巨正则蒙特卡罗方法对该体系的吸附过程进行了分子模拟,并详细研究了碳纳米管孔径和温度对该体系吸附行为的影响。结果显示,SWCNT孔径越小,吸附能力则越强;孔半径为0.746nm的SWCNT的吸附体系发生凝聚相变的临界温度为66K。通过对等量吸附热进行计算发现,孔半径0.746、1.15、1.56和1.83 nm的SWCNT-N₂吸附体系对应的初始固-液等量吸附热分别为10.9、9.2、8.6和8.4 kJ/mol。67.5K时,孔半径1.56和1.83 nm的吸附体系的等量吸附热有热峰出现。     

Structural and reactivity properties of finite length cap-ended single-wall carbon nanotubes

The reaction of C2 with growing single-wall carbon nanotubes of different chiralities is investigated using density functional theory. It is found that the energy of the frontier orbitals for (5,5) and (6,6) armchair carbon nanotubes exhibits periodic behavior with an increasing number of carbon atoms in the nanotube. Such periodic behavior induces oscillations in the reaction energy released by adsorption of C2 to the nanotube open edge. In contrast, the energy of the frontier orbitals of the (6,5) chiral tube remains constant as the number of C atoms increases, and the same stability is observed in the adsorption energy. It is suggested that this may be one of the reasons for the low percent of armchair single-wall carbon nanotubes found in the experimental synthesis.