Synergistically Enhanced Dispersion of Native Protein–Carbon Nanotube Conjugates by Fluoroalcohols in Aqueous Solution

Carbon nanotubes (CNTs) are anticipated as an important new material for use in nanotechnology applications because of their excellent mechanical and electrical properties. For their development, a highly stable dispersion of debundled CNTs is indispensable. Herein we present a new method to enhance dispersibility of single‐walled carbon nanotubes (SWNTs) with proteins using alcohols as co‐solvents. Addition of fluoroalcohols in solution increased the SWNT dispersion by more than one order of magnitude without protein denaturation. Enhancement of SWNT dispersion through addition of alcohols was attributed to the decreased hydrophobic interaction among SWNTs. This novel approach enables us to produce biofunctional CNTs such as one‐dimensional nanobiosensors and drug carriers that can penetrate cells.     

Structure and stability of arthropodan hemocyanin Limulus polyphemus

In the hemolymph of many arthropodan species, respiratory copper proteins of high molecular weight, termed hemocyanins (Hcs) are dissolved. In this communication, we report on the protein stability of different hemocyanin species (Crustacea and Chelicerata) using fluorescence spectroscopy. Five to seven major electrophoretically separable protein chains (structural subunits) were purified by fast protein liquid chromatography (FPLC) ion exchange chromatography from different hemocyanins with very high sequence homology of the active site regions binding copper ions (CuA and CuB), and especially the relative sequence positions of histidine (His) and tryptophan (Trp) residues of these protein segments are in all cases identical. The conformational stabilities of the native dodecameric aggregates and their isolated structural subunits towards various denaturants (pH and guanidine hydrochloride (Gdn.HCl)) indicate that the quaternary structure is stabilized by hydrophilic and polar forces, whereby both, the oxy- and apo-forms of the protein are considered. These two classes of Crustacea and Chelicerata Hcs have the similar Trp-fluorescence quantum yields, but different values of lambda(max) emission (about 325 and 337 nm, respectively). Differences in the quantum yields are observed of the oxy- and apo-forms, which must be attributed to the fluorescence quenching effect of the two copper ions (CuA and CuB) in the active site. The position of emission maximum indicates tryptophan side chains are situated in a non-polar environment. Denaturation studies of Hcs by Gdn.HCl indicate that the denaturation process consists of two steps: dissociation of the native molecule into its structural subunits and denaturation of the subunits at concentrations >1.5M Gdn.HCl. Two steps of denaturation are also observed after keeping the protein in buffer solutions at different pH values with different pH-stability for holo-oxy and apo-Hc forms.     

3,4,9,10-Perylene Tetracarboxylic Acid Noncovalently Modified Multiwalled Carbon Nanotubes: Synthesis,Characterization, and Application for Electrochemical Determination of 2-Aminonaphthalene

Carbon nanotubes have been intensively studied for their diverse applications but are insoluble in water. In this paper, 3,4,9,10-perylene tetracarboxylic acid noncovalently modified multiwalled carbon nanotubes were prepared by a facile approach and applied successfully for electrochemical determination of 2-aminonaphthalene. Infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, and electrochemical methods were used to characterize the hybridized nanotubes. The results reveal that the hybrids exhibit high dispersibility in water, and a glassy carbon electrode modified by the hybrids displayed a higher electrochemical response toward 2-aminonaphthalene than bare glassy carbon and multiwalled carbon nanotube–glassy carbon electrodes with a linear dynamic range of 15.0–500.0 nM and a detection limit of 4.5 nM. The modified hybrid electrode was successfully applied for the determination of 2-aminonaphthalene in water.     

Novel Copper(II)-Selective Potentiometric Sensor Based on a Folic Acid-Functionalized Carbon Nanotube Material

Abstract

A folic acid-functionalized carbon nanotube nanomaterial was prepared by immobilizing folic acid molecules on the carbon nanotubes through covalent bonds. The material was characterized using Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy and scanning electron microscopy. Fourier transform infrared spectroscopy confirmed that folic acid molecules were grafted on the carbon nanotube surfaces through the amide bonds between the carboxylic acid functional groups of the oxidized carbon nanotubes and the amine groups of the folic acid molecules. The folic acid molecules bonded to carbon nanotube surfaces led to appreciable changes in the morphology. By using currently obtained folic acid-functionalized carbon nanotube nanomaterial as electroactive material in a polyvinyl chloride membrane, a potentiometric copper (II)-selective sensor was developed. Membrane optimization studies showed that the composition exhibiting the best potentiometric properties was 4.0% (w/w) folic acid–carbon nanotube, 64.0% (w/w) o-nitrophenyl octylether, and 32.0% (w/w) polyvinyl chloride. The developed sensor displayed a linear response in the copper (II) concentration ranging from 1.0?×?10^–6 to 1.0?×?10^–1 M with a correlation coefficient of 0.9993 and a slope of 29.8?±?0.6?mV/decade of activity. The response time, detection limit, and pH working range were determined to be 4?s, 3.8?×?10^–7 M and 4.0–8.0, respectively. The developed sensor showed highly selective and satisfactory potentiometric response for the determination of copper (II) in a Turkish coin.     

Lipid bilayers covalently anchored to carbon nanotubes

The unique physical and electrical properties of carbon nanotubes make them an exciting material for applications in various fields such as bioelectronics and biosensing. Due to the poor water solubility of carbon nanotubes, functionalization for such applications has been a challenge. Of particular need are functionalization methods for integrating carbon nanotubes with biomolecules and constructing novel hybrid nanostructures for bionanoelectronic applications. We present a novel method for the fabrication of dispersible, biocompatible carbon nanotube-based materials. Multiwalled carbon nanotubes (MWCNTs) are covalently modified with primary amine-bearing phospholipids in a carbodiimide-activated reaction. These modified carbon nanotubes have good dispersibility in nonpolar solvents. Fourier transform infrared (FTIR) spectroscopy shows peaks attributable to the formation of amide bonds between lipids and the nanotube surface. Simple sonication of lipid-modified nanotubes with other lipid molecules leads to the formation of a uniform lipid bilayer coating the nanotubes. These bilayer-coated nanotubes are highly dispersible and stable in aqueous solution. Confocal fluorescence microscopy shows labeled lipids on the surface of bilayer-modified nanotubes. Transmission electron microscopy (TEM) shows the morphology of dispersed bilayer-coated MWCNTs. Fluorescence quenching of lipid-coated MWCNTs confirms the bilayer configuration of the lipids on the nanotube surface, and fluorescence anisotropy measurements show that the bilayer is fluid above the gel-to-liquid transition temperature. The membrane protein α-hemolysin spontaneously inserts into the MWCNT-supported bilayer, confirming the biomimetic membrane structure. These biomimetic nanostructures are a promising platform for the integration of carbon nanotube-based materials with biomolecules.     

Influence of acid treatments of carbon nanotube precursors on Ni/CNT in the synthesis of carbon nanotubes

The Ni/CNT catalyst was fabricated by directly dipping carbon nanotube precursors refluxed in 4 M of nitric acid into Ni electroless plating bath, and used to synthesize new carbon nanotubes. The experimental results indicate that the duration of acid-treatment of carbon nanotubes precursors exerts a great influence on the catalysis of Ni/CNT in the synthesis of carbon nanotubes and hence the structures of the new carbon nanotubes. When the carbon nanotubes precursors were refluxed for 0.5 h in 4 M of nitric acid, bamboo-shaped carbon nanotubes (BSCNT) or Y junction carbon nanotubes in the carbon products were obtained. As the duration of acid-treatment of carbon nanotubes precursors increased to 6 h, the as-prepared Ni/CNT displayed higher activity, and the carbon nanotube products were high pure without any Y junction structure or any separation layers in hollow.     

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.     

Unpredictable dispersion states of MWNTs in HDPE: A comparative and comprehensive study

A review of the literature indicates a wide range of dispersion states of carbon nanotubes (CNT) in high density polyethylene (HDPE), with in some cases, formation of micro-composites with bundle-like aggregation of the nanoparticles and in some others, much finer nanotube dispersion leading to higher performance (nano)composite materials. This contribution emphasizes the diversity of these results by comparing the dispersion state of different types of multiwall carbon nanotubes (MWNT) blended within several grades of HDPE via melt processing. It appears that each combination leads to a different dispersion quality. This study highlights that there is not a universal method to blend CNT and HDPE and that the dispersion state of the nanotubes is not readily predictable. Actually, inherent affinity between CNT and polyethylene proved to be rather great but melt viscosity and processing conditions represent key-parameters to be considered for allowing dissociation and dispersion of the nanoparticles within the polyolefinic matrix.     

Interactions between single-walled carbon nanotubes and lysozyme

Dispersions of single-walled and non-associated carbon nanotubes in aqueous lysozyme solution were investigated by analyzing the stabilizing effect of both protein concentration and pH. It was inferred that the medium pH, which significantly modifies the protein net charge and (presumably) conformation, modulates the mutual interactions with carbon nanotubes. At fixed pH, in addition, the formation of protein/nanotube complexes scales with increasing lysozyme concentration. Electrophoretic mobility, dielectric relaxation and circular dichroism were used to determine the above features. According to circular dichroism, lysozyme adsorbed onto nanotubes could essentially retain its native conformation, but the significant amount of free protein does not allow drawing definitive conclusions on this regard. The state of charge and charge distribution around nanotubes was inferred by combining electrophoretic mobility and dielectric relaxation methods. The former gives information on changes in the surface charge density of the complexes, the latter on modifications in the electrical double layer thickness around them. Such results are complementary each other and univocally indicate that some LYS molecules take part to binding. Above a critical protein/nanotube mass ratio, depletion phenomena were observed. They counteract the stabilization mechanism, with subsequent nanotube/nanotube aggregation and phase separation. Protein-based depletion phenomena are similar to formerly reported effects, observed in aqueous surfactant systems containing carbon nanotubes.     

Preparation and Absorption Properties in Near Infrared Wavelength of Carbon Nanotubes/Acrylate Coatings

"Carbon nanotubes can be used as absorbent materials at the near infared range. In this study, carbon nanotube was treated with surfactant to prevent aggregation, and the acrylate coating of the carbon nanotube was prepared with the carbon nanotube solution and the near infrared reflection properties of the coating were studied. A series of factors influencing the reflectivity of carbon nanotubes/acrylate coatings at 930 nm, such as PVC concentration, the kinds and concentration of the surfactant and size range of the carbon nanotube, were studied. Based on these, the coating procedure was optimized and the carbon nanotubes/acrylate coating was prepared with good absorption properties. The reflectivity is under 0.1% of the coatings and its transmission is below 1.0% at near infrared wavelength of 840-980 nm."     

The synthesis of nitrogen-doped carbon nanotubes/gold composites and their application to the detection of thioridazine

Nitrogen-doped carbon nanotubes (N-CNTs)/gold composites were synthesized through a simple self-assembly method. The morphology, composition, and optical properties of the resulted composites were investigated by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, Raman spectra, ultraviolet–visible absorption spectrum, and X-ray photoelectron spectroscopic. This nanocomposite combines the advantages of N-CNTs and gold nanoparticales showing many excellent properties such as good dispersibility in water and satisfactory biocompatibility. Cyclic voltammogram experiment shows that N-CNTs/gold composite has high conductivity. Based on these aspects, N-CNTs/gold-modified electrode was applied to the voltammetric determination of thioridazine hydrochloride (TH) successfully. The linear calibration range for the TH sensor was 12?~?850?μM with a detection limit of 1.3?μM at a signal-to-noise ratio of 3, long-term stability, and good reproducibility.     

Investigation on rheological properties of carbon nanotube nanofluids

The effective viscosity of carbon nanotube nanofluids is strongly dependent on the temperature and concentration. The aggregation behaviour that carbon nanotubes exhibit in solution and the orientation variation of single carbon nanotube make rheological properties of nanofluids more complex. With the increase of shear rate, the degree of dispersion and orientation of carbon nanotubes will be improved. Based on previous studies and the fact mentioned above, a reasonable expression for viscosity of carbon nanotube nanofluids has been given, which is associated with the shear rate and aspect ratios of carbon nanotubes. The expression has been validated comparing with previous experimental data.     

Chemical and biochemical sensing with modified single walled carbon nanotubes

The nano dimensions, graphitic surface chemistry and electronic properties of single walled carbon nanotubes make such a material an ideal candidate for chemical or biochemical sensing. Carbon nanotubes can be nondestructively oxidized along their sidewalls or ends and subsequently covalently functionalized with colloidal particles or polyamine dendrimers via carboxylate chemistry. Proteins adsorb individually, strongly and noncovalently along nanotube lengths. These nanotube-protein conjugates are readily characterized at the molecular level by atomic force microscopy. Several metalloproteins and enzymes have been bound on both the sidewalls and termini of single walled carbon nanotubes. Though coupling can be controlled, to a degree, through variation of tube oxidative pre-activation chemistry, careful control experiments and observations made by atomic force microscopy suggest that immobilization is strong, physical and does not require covalent bonding. Importantly, in terms of possible device applications, protein attachment appears to occur with retention of native biological structure. Nanotube electrodes exhibit useful voltammetric properties with direct electrical communication possible between a redox-active biomolecule and the delocalized pi system of its carbon nanotube support.     

A Supramolecular Approach for the Facile Solubilization and Separation of Covalently Functionalized Single‐Walled Carbon Nanotubes

Through a combination of an electronic‐type selective diazonium‐based attachment of a Hamilton receptor unit onto the carbon nanotube framework and a supramolecular recognition approach of a cyanuric acid derivative, we herein introduce a highly promising strategy for the tuning of carbon nanotube solubility and, directly related to that, a solution‐based easy and straightforward separation of covalently functionalized carbon nanotube derivatives with respect to their unfunctionalized counterparts. The supramolecular complexation of the cyanuric acid derivative provides the driving force for the dramatically increased dispersibility and for the long‐time stability of the individualized single‐walled carbon nanotube derivatives in chloroform. The selective covalent functionalization of metallic carbon nanotubes can easily be analyzed with the aid of scanning Raman microscopy techniques. The functional derivatives have furthermore been characterized by UV/Vis‐NIR and fluorescence spectroscopy as well as by mass spectrometric coupled thermogravimetric analysis. The investigation of the supramolecular complexation is based on an in‐depth UV/Vis‐NIR analysis and atomic force microscopy investigations.     

Surface modification of multiwalled carbon nanotubes via nitroxide‐mediated radical polymerization

The nitroxide‐mediated radical polymerization of styrene was carried out on the surfaces of multiwalled carbon nanotubes (MWNTs) initiated by an MWNT‐supported initiator multiwalled carbon nanotube–2″,2″,6″,6″‐tetramethylpiperidinyloxy (MWNT–Tempo). The content of polystyrene grafted from the surface was controlled by changes in the polymerization conditions, such as the reaction times or the ratios of monomers to initiators. The obtained polystyrene‐grafted multiwalled carbon nanotubes (MWNT–PSs) were further used to initiate the polymerization of 4‐vinylpyridine to get polystyrene‐b‐poly(4‐vinylpyridine)‐grafted multiwalled carbon nanotubes (MWNT–PS‐b‐P4VPs). In contrast to unmodified MWNTs, MWNT–PSs had relatively good dispersibility in various organic solvents, such as tetrahydrofuran, CHCL₃, and o‐dichlorobenzene. The structures and properties of MWNT–PSs and MWNT–PS‐b‐P4VPs were characterized and studied with several methods, including thermogravimetric analysis, Fourier transform infrared, ultraviolet–visible, and transmission electron microscopy. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4656–4667, 2006     

Modification of multiwall carbon nanotubes by grafting from controlled polymerization of styrene: Effect of the characteristics of the nanotubes

Linear polystyrene chains were grown from the convex surface of two commercially available multiwall carbon nanotubes (MWCNTs) with similar diameter but different lengths. The MWCNTs were supplied from Bayer Material Science® (purity >95%, external diameter = 13–16 nm, length = 1–10 μm, denoted MWCNT_BMS⁹⁵) and FutureCarbon GmbH (purity >99%, external diameter = 15 nm, length = 5–50 μm, denoted MWCNT_FC⁹⁹). The MWCNTs were oxidized with nitric acid, consecutively reacted with thionyl chloride, glycol or poly(ethylene glycol), 2‐bromo‐2‐methylpropionyl bromide and finally with styrene under atom transfer radical polymerization (ATRP) conditions. The content of polystyrene grafted from the surface of the MWCNTs can be controlled by adjusting the molecular weight of the poly(ethylene glycol), the initiator concentration and the monomer to carbon nanotube weight ratio. Under comparable experimental conditions, a higher amount of polystyrene is grafted from the MWCNT_BMS⁹⁵ than from MWCNT_FC⁹⁹. The difference in dimensions and the state of aggregation of the carbon nanotubes influence the grafting from polymerization reactions, where relative shorter and tightly aggregated carbon nanotubes promote higher polymerizations yields than longer and less aggregated carbon nanotubes. The increase of the viscosity of the carbon nanotube dispersion decreases the polymer grafting content. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1035–1046, 2010     

Molecular dynamics simulations on the effects of diameter and chirality on hydrogen adsorption in single walled carbon nanotubes

We present systematic molecular dynamics simulation studies of hydrogen storage in single walled carbon nanotubes of various diameters and chiralities using a recently developed curvature-dependent force field. Our main objective is to address the following fundamental issues: 1. For a given H2 loading and nanotube type, what is the H2 distribution in the nanotube bundle? 2. For a given nanotube type, what is the maximal loading (H2 coverage)? 3. What is the diameter range and chirality for which H2 adsorption is most energetically favorable? Our simulation results suggest strong dependence of H2 adsorption energies on the nanotube diameter but less dependence on the chirality. Substantial lattice expansion upon H2 adsorption was found. The average adsorption energy increases with the lowering of nanotube diameter (higher curvature) and decreases with higher H2 loading. The calculated H2 vibrational power spectra and radial distribution functions indicate a strong attractive interaction between H2 and nanotube walls. The calculated diffusion coefficients are much higher than what has been reported for H2 in microporous materials such as zeolites, indicating that diffusivity does not present a problem for hydrogen storage in carbon nanotubes.     

Tryptophan exposure and accessibility in the chitooligosaccharide-specific phloem exudate lectin from pumpkin (Cucurbita maxima). A fluorescence study

The exposure and accessibility of the tryptophan residues in the chitooligosaccharide-specific pumpkin (Cucurbita maxima) phloem exudate lectin (PPL) have been investigated by fluorescence spectroscopy. The emission λ_max of native PPL, seen at 338 nm was red-shifted to 348 nm upon denaturation by 6 M Gdn.HCl in the presence of 10 mM β-mercaptoethanol, indicating near complete exposure of the tryptophan residues to the aqueous medium, whereas a blue-shift to 335 nm was observed in the presence of saturating concentrations of chitotriose, suggesting that ligand binding leads to a decrease in the solvent exposure of the tryptophan residues. The extent of quenching was maximum with the neutral molecule, acrylamide whereas the ionic species, iodide and Cs⁺ led to significantly lower quenching, which could be attributed to the presence of charged amino acid residues in close proximity to some of the tryptophan residues. The Stern–Volmer plot for acrylamide was linear for native PPL and upon ligand binding, but became upward curving upon denaturation, indicating that the quenching occurs via a combination of static and dynamic mechanisms. In time-resolved fluorescence experiments, the decay curves could be best fit to biexponential patterns, for native protein, in the presence of ligand and upon denaturation. In each case both lifetimes systematically decreased with increasing acrylamide concentrations, indicating that quenching occurs predominantly via a dynamic process.     

两亲分子对碳纳米管的分散稳定作用

综述了近年来国内外对碳纳米管在两亲分子水溶液中的分散作用研究, 从表面活性剂、聚合物和生物大分子三方面, 分别阐述了用非成键法对碳纳米管进行分散的不同机理. 离子型表面活性剂或聚电解质主要靠亲水基团之间的静电斥力阻止碳纳米管之间的聚集, 而非离子型表面活性剂或大分子则主要靠亲水基团所产生的空间位阻使分散体系保持稳定.     

Understanding surfactant aided aqueous dispersion of multi-walled carbon nanotubes

Dispersions of multi-walled carbon nanotubes (MWNTs) assisted by surfactant adsorption were prepared for a number of ionic and non-ionic surfactants including sodium 4-dodecylbenzenesulfonate (NaDDBS), hexadecyl(trimethyl)azanium bromide (CTAB), sodium dodecane-1-sulfonate (SDS), Pluronic? F68, Pluronic? F127, and Triton? X-100 to examine the effects of nanotube diameter, surfactant concentration, and pH on nanotube dispersability. Nanotube diameter was found to be an important role in surfactant adsorption rendering single-walled carbon nanotube studies as unreliable in predicting MWNT dispersive behavior. Similar to other reports, increasing surfactant concentrations resulted in a solubility plateau. Quantification of nanotube solubility at these plateaus demonstrated that CTAB is the best surfactant for MWNTs at neutral pH conditions. Deviations from neutral pH demonstrated negligible influence on non-ionic surfactant adsorption. In contrast, both cationic and anionic surfactants were found to be poor dispersing aids for highly acidic solutions while, CTAB remained a good surfactant under strongly basic conditions. These pH dependent results were explained in the context of nanotube surface ionization and Debye length variation.