Preparation and stabilization of silver nanoparticles by a thermo-responsive pentablock terpolymer

Thermo-reversible silver nanoparticles (Ag-NPs) were prepared by the sodium borohydride reduction of silver nitrate (AgNO₃) in the presence of a pentablock terpolymer, poly(N-isopropylacrylamide)-b-poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PNIPAM₁₅₀-PEO₁₃₆-PPO₄₅-PEO₁₃₆-PNIPAM₁₅₀). The pentablock terpolymer-stabilized silver nanoparticles (Pentablock-S-Ag) were characterized by UV-VIS spectroscopy, X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and transmission electron microscopy (TEM). At temperatures below lower critical solution temperature (LCST) of Pentablock-S-Ag solutions, the obtained Ag-NPs are well-dispersed with spherical shape, and their sizes mainly depend upon the molar ratios of pentablock terpolymer to AgNO₃; at temperatures above LCST, the size of Ag-NPs decreases and their aggregates are observed due to the collapse and shrinkage of the thermo-responsive PNIPAM and PPO segments. A reversible dispersion-aggregation process upon recyclically changing temperature is also observed.     

Dispersion of single-walled carbon nanotubes with an extended diazapentacene derivative

The dispersion of SWCNTs by using a novel diazapentacene derivative is reported. The proposed pi-pi interactions between the diazapentacene derivative and SWCNTs leave their inherent properties virtually intact, as observed by several photophysical measurements. This approach is very attractive for manipulation of SWCNTs for electronic applications.     

Dispersion and separation of small-diameter single-walled carbon nanotubes

The dispersion of small-diameter single-walled carbon nanotubes (SWNTs) produced by the CoMoCAT method in tetrahydrofuran (THF) with the use of amine was studied. The absorption, photoluminescence, and Raman spectroscopies showed that the dispersion and centrifugation process leads to an effective separation of metallic SWNTs from semiconducting SWNTs. Since this method is simple and convenient, it is highly applicable to an industrial utilization for widespread applications of SWNTs.     

Dispersion of single-walled carbon nanotubes in alcohol-cholic acid mixtures

A procedure for dispersing single-walled carbon nanotubes (SWNTs) for the preparation of suspensions with high concentrations of individual nanotubes in various solvents was described. The most stable suspensions were obtained from a mixture of ethanol with cholic acid at an acid concentration of 0.018 mol/kg.     

NMR detection of single-walled carbon nanotubes in solution

The detection of nanotube carbons in solution by (13)C NMR is reported. The highly soluble sample was from the functionalization of (13)C-enriched single-walled carbon nanotubes (SWNTs) with diamine-terminated oligomeric poly(ethylene glycol) (PEG(1500N)). The ferromagnetic impurities due to the residual metal catalysts were removed from the sample via repeated magnetic separation. The nanotube carbon signals are broad but partially resolved into two overlapping peaks, which are tentatively assigned to nanotube carbons on semiconducting (upfield) and metallic (downfield) SWNTs. The solid-state NMR signals of the same sample are similarly resolved. Mechanistic and practical implications of the results are discussed.     

Dispersion of single-walled carbon nanotubes of narrow diameter distribution

The dispersibility and bundle defoliation of single-walled carbon nanotubes (SWNTs) of small diameter (<1 nm) have been evaluated on CoMoCAT samples with narrow distribution of diameters. As previously observed by photoluminescence and Raman spectroscopy, the CoMoCAT sample exhibits a uniquely narrow distribution of (n,m) structures that remains unchanged after different dispersion conditions. This narrow distribution allowed us to develop a method for quantifying the dispersability of the samples from their optical absorption spectra in terms of two ratios: the "resonance ratio" and the "normalized width." The former is defined as the quotient of the resonant band area and its nonresonant background. The latter is defined as the ratio of the width of the band at half-height to the peak height on a spectrum that has been normalized at 900 nm, making this an intensive property, rather than varying with the path length. In this study of the CoMoCAT sample, we have used the S22 transition corresponding to the (6,5) nanotube to do these calculations, which is the most abundant species. These two ratios provide a quantitative tool to compare different dispersion parameters (time of sonication, degree of centrifugation, etc.) on the same type of sample. From this comparison, an optimal procedure that maximizes the spectral features was selected; this procedure allowed us to contrast various surfactants at different pH values and concentrations. Several surfactants were as good or even better than the one we have used in previous studies, dodecylbenesulfonic acid sodium salt (NaDDBS). Despite differences in their dispersion abilities, none of the surfactants investigated generated new features in the absorption spectra nor changed the distribution of nanotube types, which confirms that the high selectivity of the CoMoCAT sample is in the original sample rather than caused by selective suspension of specific (n,m) nanotubes.     

Dispersion and orientation of single-walled carbon nanotubes in a chromonic liquid crystal

A post-synthesis alignment of individual single-walled carbon nanotubes (SWCNTs) is desirable for translating their unique anisotropic properties to a macroscopic scale. Here, we demonstrate excellent dispersion, orientation and concomitant-polarised photoluminescence of SWCNTs in a nematic chromonic liquid crystal. The methods to obtain stable suspension are described, and order parameters of the liquid crystal matrix and of the nanotubes are measured independently.     

Dispersion of multi-wall carbon nanotubes by an ionic liquid-based polyether in aqueous solution

Multi-wall carbon nanotubes (MWCNTs) can be effectively dispersed by an ionic liquid-based polyether, poly(1-glycidyl-3-methylimidazolium chloride) (PGMIC) in aqueous solution. The amount of dispersed MWCNTs increases with the increasing of PGMIC concentration, and then decreases. Reaggregation of MWCNTs is observed when PGMIC exceeded the optimal concentration, which may be due to the conformational change of PGMIC molecules around MWCNT. The ultrasonic dispersion method is better than stirring method in the PGMIC solution. Furthermore, the acidic solution is convenient to prepare stable MWCNTs suspensions. Through the characterizations of ultraviolet–visible–near infrared, thermogravimetric analysis and Fourier transform infrared, it can be concluded that electrostatic repulsions, hydrophobic effect, n–π, and cation–π interactions played important roles in the dispersion of MWCNTs.     

Temperature induced restoration of fluorescence from oxidised single-walled carbon nanotubes in aqueous sodium dodecylsulfate solution

Fluorescence intensity from single walled carbon nanotubes in aqueous sodium dodecylsulfate solution is shown to be strongly temperature dependent in the range 5-60 degrees C. Emission peaks corresponding to particular nanotube species which have been quenched due to oxidation by aerated water show sharp transitions at distinct temperatures as the sample is heated. The temperature at which the transition occurs is found to be species dependent and has been interpreted as being proportional to the valence band edge potential of the nanotube. The results are explained quantitatively using the Nernst equation to measure the increase in reduction potential of the solution as the temperature is raised. The removal of dissolved O(2) and CO(2) is thought to be a significant driving force in the reversal of the redox reaction which causes oxidation of the nanotubes.     

Light-controlled single-walled carbon nanotube dispersions in aqueous solution

We have succeeded in dispersing single-walled carbon nanotubes (SWNTs) into an aqueous solution of poly(ethylene glycol)-terminated malachite green derivative (PEG-MG) through simple sonication. It was found that UV exposure caused reaggregation of these predispersed SWNTs in the same aqueous medium, as adsorbed PEG-MG photochromic chains could be effectively photocleavaged from the nanotube surface. The observed light-controlled dispersion and reaggragation of SWNTs in the aqueous solution should facilitate the development of SWNT dispersions with a controllable dispersity for potential applications.     

Aggregation behavior of single-walled carbon nanotubes in dilute aqueous suspension

The aggregation behavior of colloidal single-walled carbon nanotubes (SWNT) in dilute aqueous suspensions was investigated using a novel light scattering measurement technique. The aggregation of SWNT in three suspensions was examined: (1) nanotubes after acid treatment; (2) as-received nanotubes stabilized by a nonionic surfactant; and (3) acid-treated nanotubes with nonionic surfactant. Continuous light scattering measurements of the SWNT suspensions (probing the 38-436 nm length scale) made over two weeks showed that the nanotubes in each sample formed networks with fractal-like structures. The as-received nanotubes were stable over the measurement period, while the acid-treated nanotube suspension showed greater dispersion variability over time, yielding looser structures at large length scales and more compact structures at smaller length scales. The addition of surfactant to the acid-treated suspension significantly enhanced nanotube dispersion.     

Stable sequestration of single-walled carbon nanotubes in self-assembled aqueous nanopores

We demonstrate the ability to stably sequester individual single-walled carbon nanotubes (SWNTs) within self-contained nanometer-scale aqueous volumes arrayed in an organic continuum. Large areal densities of 4 × 10(9) cm(-2) are readily achieved. SWNTs are incorporated into a surfactant mesophase which forms 2.3 nm diameter water channels by lyotropic self-assembly. Near-infrared fluorescence spectroscopy demonstrates that the SWNTs exist as well-dispersed tubes that are stable over several months and through multiple cycles of heating and cooling. Absence of physical distortion of the mesophase suggests that the SWNTs are stabilized by adsorbed surfactants that do not extend considerably from the surface. Our findings have important implications for templated assembly of carbon nanotubes using soft mesophases and the development of functional nanocomposites.     

Polymeric surfactants with tricyclic rigidity from natural rosin: Synthesis and dispersion of single-walled carbon nanotubes in aqueous solution

Polymeric surfactants from natural rosin containing a unique tricyclic rigid structure were prepared and their structures were characterized by IR, ¹H NMR and GPC. Their surface activities including hydrophile—lipophile balance values (HLB), emulsification properties, foaming properties, critical micelle concentration (CMC) and the minimum surface tension (γCMC) were evaluated. With the increase in ratio of hydrophobic to hydrophilic in polymeric surfactant, the CMC values of the surfactants decreased, and the emulsification and foaming properties of these surfactants increased, and the HLB values of them decreased. This type of polymeric surfactants was used to disperse single-walled carbon nanotubes (SWCNTs) in water, and their dispersal capacities were comparatively determined by UV-visible absorption spectroscopy. Thermogravimetric analysis (TGA) was used to quantify the amount of surfactant attached onto the nanotubes. More intuitive image of their dispersion states were obtained by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The results showed that this type of polymeric surfactants had good dispersion capacity and dispersion stability to SWCNTs in water through strong hydrophobic attraction and weak van der Waals interactions.     

Singling out the electrochemistry of individual single-walled carbon nanotubes in solution

Bandgap fluorescence spectroscopy of aqueous, micelle-like suspensions of SWNTs has given access to the electronic energies of individual semiconducting SWNTs, while substantially lower is the success achieved in the determination of the redox properties of SWNTs as individual entities. Here we report an extensive voltammetric and vis-NIR spectroelectrochemical investigation of true solutions of unfunctionalized SWNTs and determine the standard electrochemical potentials of reduction and oxidation as a function of the tube diameter of a large number of semiconducting SWNTs. We also establish the Fermi energy and the exciton binding energy for individual tubes in solution. The linear correlation found between the potentials and the optical transition energies is quantified in two simple equations that allow one to calculate the redox potentials of SWNTs that are insufficiently abundant or absent in the samples.     

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.     

Bioelectrochemical single-walled carbon nanotubes

Metalloproteins and enzymes can be immobilized on SWNTs of different surface chemistry. The combination of high surface area, robust immobilization and inherent nanotube electrochemical properties is of promising application in bioelectrochemistry.     

Stabilization and functionalization of single-walled carbon nanotubes with polyvinylpyrrolidone copolymers for applications in aqueous media

In spite of their useful electronic and mechanical properties, the utility of single-walled carbon nanotubes (SWNT) is limited by their inherent insolubility in aqueous environments and the difficulty in functionalizing the material while retaining opto-electric properties. In this article, the synthesis of polyvinylpyrrolidone (PVP) copolymers for wrapping SWNTs is described. Studies include the effect of polymer composition and charge on SWNT stability in aqueous solution and the variation of the stability of wrapped SWNTs during high-dilution conditions using dialysis studies. Finally, the PVP-copolymer strategy is used to create polymer wrappers with functional handles for the incorporation of useful chemical functionality. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 337–343     

Dispersion of pristine single-walled carbon nanotubes in water by a thiolated organosilane: application in supramolecular nanoassemblies

We report a novel technique to disperse pristine single-walled carbon nanotubes in water by ultrasonication in the presence of a thiolated organosilane and subsequent ultracentrifugation. As revealed by the sharp features in the visible range of the absorption spectrum and by transmission electron microscopy, the collected supernatant fraction was composed of small bundles of nanotubes coated by a thin layer of organosilane molecules. We hypothesized and demonstrated that the organosilane adsorbed onto the nanotubes through the thiol group leaving the silane group extruding out. The pristine properties of the nanotubes and the versatile chemistry available for sol-gel materials make the reported dispersed nanotubes an excellent scaffold for the realization of supramolecular nanoassemblies suitable for different applications. As examples, we decorated them with silica nanobeads, by using a water-in-oil nanoemulsion system, and with gold nanoparticles, by a previous derivatization with a second layer of thiolated organosilane providing exposed thiol groups.     

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.