Dispersing phase viscometry and zeta potential in alumina dispersions

The stabilities of alumina dispersions were studied as a function of poly- and low molecular weight electrolyte concentration, using viscometry of the dispersing phase, and zeta potential measurements. The relation of polyelectrolyte adsorption to polymer concentration (at different low molecular weight electrolyte concentrations) was found to depend upon the dimensions of the polymer (which were a priori known to decrease with increasing poly- and low molecular weight electrolyte concentration). The occurrence of flocculation and bridging in the destabilization mechanism of the alumina dispersions was also characterized.     

Cellulose Nanocrystal-Assisted Dispersion of Luminescent Single-Walled Carbon Nanotubes for Layer-by-Layer Assembled Hybrid Thin Films

Highly stable single-walled carbon nanotube (SWNT) dispersions are obtained after ultrasonication in cellulose nanocrystal (CN) aqueous colloidal suspensions. Mild dispersion conditions were applied to preserve the SWNT length in order to facilitate the identification of hybrid objects. This led to a moderate dispersion of 24% of the SWNTs. Under these conditions, atomic force microscopy (AFM) and transmission electron microscopy (TEM) experiments succeeded in demonstrating the formation of hybrid particles in which CNs are aligned along the nanotube axis by a self-assembly process. These SWNT/CN dispersions are used to create multilayered thin films with the layer-by-layer method using polyallylamine hydrochloride as a polyelectrolyte. Homogeneous films from one to eight bilayers are obtained with an average bilayer thickness of 17 nm. The presence of SWNTs in each bilayer is attested to by characteristic Raman signals. It should be noted that these films exhibit a near-infrared luminescence signal due to isolated and well-separated nanotubes. Furthermore, scanning electron microscopy (SEM) suggests that the SWNT network is percolating through the film.     

Highly flexible polyelectrolyte nanotubes

A pressure-filter-template approach was employed to prepare polyelectrolyte nanotubes through layer-by-layer deposition in the alumina template. With the thicker wall, the ordered polymer nanotubes possess a high flexibility. The results demonstrate that the electrostatic interactions of polyelectrolytes play a key role in fabricating water-soluble charged polymer nanotubes. The structure of the polyelectrolyte nanotube was confirmed by SEM, TEM, and UV, respectively.     

Surfactant dispersed multi-walled carbon nanotube/polyetherimide nanocomposite membrane

Carbon nanotube based nanocomposite membranes have been fabricated through solution casting by embedding multi-walled carbon nanotubes (MWCNTs) within polyetherimide (PEI) polymer host matrix. In order to achieve fine dispersion of nanotubes and facilitate strong interfacial adhesion with the polymer matrix, the nanotubes were first treated with surfactants of different charges, namely anionic sodium dodecyl chloride, cationic cetyl trimethyl ammonium chloride and non-ionic Triton X100, prior to the dispersion in the PEI dope solution. Dispersion of MWCNTs in N-methyl-2-pyrrolidone solvent showed that the agglomeration and entanglement of the nanotubes were greatly reduced upon the addition of Triton X100. Scanning electron microscopy and atomic force microscopy examination has evidenced the compatibility of Triton X100 dispersed MWCNTs with the polymer matrix in which a promising dispersion and adhesion has been observed at the MWCNT-PEI interface. The increase in both thermal stability and mechanical strength of the resulting Triton X100 dispersed MWCNT/PEI nanocomposite indicated the improved interaction between MWCNTs and PEI. This study demonstrated the role of Triton X100 in facilitating the synergetic effects of MWCNTs and PEI where the resulting composite membrane is anticipated to have potential application in membrane based gas separation.     

Association between branched poly(ethyleneimine) and sodium dodecyl sulfate in the presence of neutral polymers

In the present paper, the effect of different neutral polymers on the self-assemblies of hyperbranched poly(ethyleneimine) (PEI) and sodium dodecyl sulfate (SDS) has been investigated at different ionization degrees of the polyelectrolyte molecules. The investigated uncharged polymers were poly(ethyleneoxide), poly(vinylpyrrolidone) and dextran samples of different molecular mass. Dynamic light scattering and electrophoretic mobility measurements demonstrate that the high molecular mass PEO or PVP molecules adsorb considerably onto the surface of the PEI/SDS nanoparticles. At appropriate concentrations of PVP or PEO, sterically stabilized colloidal dispersions of the polyelectrolyte/surfactant nanoparticles with hydrophobic core and hydrophilic corona can be prepared. These dispersions have considerable kinetic stability at high ionic strengths where the accelerated coagulation of the PEI/SDS nanoparticles results in precipitation in the absence of the neutral polymers. In contrast, the addition of dextran does not affect considerably the kinetic stability of PEI/SDS mixtures because of its low adsorption affinity towards the surface of the polyelectrolyte/surfactant nanoparticles.     

Persistence length control of the polyelectrolyte layer-by-layer self-assembly on carbon nanotubes

We have studied layer-by-layer polyelectrolyte self-assembly on pristine individual single-wall carbon nanotubes as a function of solution ionic strength. We report the existence of an ionic strength threshold for the deposition, below which the majority of nanotubes remain uncoated. Once the ionic strength reaches the threshold value, the majority of the individual nanotubes become coated with polyelectrolytes. Our results indicate that the self-assembly process likely involves wrapping of polymer chains around nanotubes and that the polymer chain's ability to bend in order to accommodate the nanotube curvature is one of the critical parameters controlling layer-by-layer electrostatic self-assembly on these one-dimensional templates.     

Aqueous dispersions of single-wall and multiwall carbon nanotubes with designed amphiphilic polycations

Poor solubility of single-walled and multiwalled carbon nanotubes (NTs) in water and organic solvents presents a considerable challenge for their purification and applications. Macromolecules can be convenient solubilizing agents for NTs and a structural element of composite materials for them. Several block copolymers with different chemical functionalities of the side groups were tested for the preparation of aqueous NT dispersions. Poly(N-cetyl-4-vinylpyridinium bromide-co-N-ethyl-4-vinylpyridinium bromide-co-4-vinylpyridine) was found to form exceptionally stable NT dispersions. It is suggested that the efficiency of macromolecular dispersion agents for NT solubilization correlates with the topological and electronic similarity of polymer-NT and NT-NT interactions in the nanotube bundles. Raman spectroscopy and atomic force and transmission electron microcopies data indicate that the polycations are wrapped around NTs forming a uniform coating 1.0-1.5 nm thick. The ability to wind around the NT originates in the hydrophobic attraction of the polymer backbone to the graphene surface and topological matching. Tetraalkylammonium functional groups in the side chains of the macromolecule create a cloud of positive charge around NTs, which makes them hydrophilic. The prepared dispersions could facilitate the processing of the nanotubes into composites with high nanotube loading for electronic materials and sensing. Positive charge on their surface is particularly important for biological and biomedical applications because it strengthens interactions with negatively charged cell membranes. A high degree of spontaneous bundle separation afforded by the polymer coating can also be beneficial for NT sorting.     

Template Synthesis of Nano-structured Materials for Electrochemical Applications

As an important preparation method of nano-structured materials, template synthesis^[1] attracted great interests in recent years. Different kinds of template such as anodic porous alumina, polymer and nano-channel glass templates have been widely studied. Compared with other templates, the size of holes in the porous alumina template can be easily controlled by properly adjusting the condition of anodization. In addition, Nano-structured material prepared from the template also provides an ideal system^[2] to investigate the effects of size of materials toward electrode's performance. In this paper, various nano-structured materials such as spinel LiMn₂O₄ and carbon nanotubes by using porous alumina template have been prepared and characterized.     

Layer-by-Layer electrostatic self-assembly of polyelectrolyte nanoshells on individual carbon nanotube templates

Carbon nanotubes have been featured prominently in the nanotechnology research for some time, yet robust strategies for noncovalent chemical modification of the nanotube surface are still missing. Such strategies are essential for the creation of functional device architectures. Here, we present a new general procedure for carbon nanotube modification based on polyelectrolyte layer-by-layer assembly. We have built multilayer structures around individual carbon nanotube bridges by first modifying the nanotube surface with a pyrene derivative followed by layer-by-layer deposition of polyelectrolyte macroions on the nanotube. Transmission electron microscopy and scanning confocal fluorescence microscopy images confirm the formation of nanometer-thick amorphous polymer nanoshells around the nanotubes. These multilayer polyelectrolyte shells on individual carbon nanotubes introduce nearly unlimited opportunities for the incorporation of various functionalities into nanotube devices, which, in turn, opens up the possibility of building more complex multicomponent structures.     

Interactions and dispersion stability of aluminum oxide colloidal particles in electroless nickel solutions in the presence of comb polyelectrolytes

The effect of comb polyelectrolytes on the dispersion stability of colloidal alumina particles in DI water and commercial electroless nickel (EN) solutions was investigated. Adsorption of polyelectrolytes and major EN components onto colloidal alumina was assessed by TGA, chemical analysis, and zeta potential measurements. Zeta potential measurements were made during titrations of comb-polyelectrolyte-stabilized dispersions with EN solutions to full ionic strength for the first time. The compilation of titration curves made with varying amounts of comb polyelectrolytes provides high resolution and novel insight into the particle/surfactant/EN systems. Continuous decrease in particle/EN components surface interactions with the increase in comb polyelectrolyte coverage is observed. Laser diffraction measurements reveal steric stabilization of nano- and submicronmeter alumina dispersions in both DI water and EN solutions with >7 wt% and >2 wt% comb polyelectrolyte, respectively.     

Template synthesized molecularly imprinted polymer nanotube membranes for chemical separations

In this report, we describe the synthesis of a molecularly imprinted polymer (MIP) nanotube membrane, using a porous anodic alumina oxide (AAO) membrane by surface-initiated atom transfer radical polymerization (ATRP). The use of a MIP nanotube membrane in chemical separations gives the advantage of high affinity and selectivity. Furthermore, because the molecular imprinting technique can be applied to different kinds of target molecules, ranging from small organic molecules to peptides and proteins, such MIP nanotube membranes will considerably broaden the application of nanotube membranes in chemical separations and sensors. This report also shows that the ATRP route is an efficient procedure for the preparation of molecularly imprinted polymers. Furthermore, the ATRP route works well in its formation of MIP nanotubes within a porous AAO membrane. The controllable nature of ATRP allows the growth of a MIP nanotube with uniform pores and adjustable thickness. Thus, using the same route, it is possible to tailor the synthesis of MIP nanotube membranes with either thicker MIP nanotubes for capacity improvement or thinner nanotubes for efficiency improvement.     

Debundling of single-walled nanotubes by dilution: observation of large populations of individual nanotubes in amide solvent dispersions

Large-scale debundling of single-walled nanotubes has been demonstrated by dilution of nanotube dispersions in the solvent N-methyl-2-pyrrolidone (NMP). At high concentrations some very large (approximately 100 s of micrometers) nanotube aggregates exist that can be removed by mild centrifugation. By measurement of the absorbance before and after centrifugation as a function of concentration the relative aggregate and dispersed nanotube concentrations can be monitored. No aggregates are observed below CNT approximately 0.02 mg/mL, suggesting that this can be considered the nanotube dispersion limit in NMP. After centrifugation, the dispersions are stable against sedimentation and further aggregation for a period of weeks at least. Atomic force microscopy (AFM) studies on deposited films reveal that the bundle diameter distribution decreases dramatically as concentration is decreased. Detailed data analysis suggests the presence of an equilibrium bundle number density and that the dispersions self-arrange themselves to always remain close to the dilute/semidilute boundary. A population of individual nanotubes is always observed that increases with decreasing concentration until almost 70% of all dispersed objects are individual nanotubes at a concentration of 0.004 mg/mL. The number density of individual nanotubes peaks at a concentration of approximately 10(-2) mg/mL. Both the mass fraction and the partial concentration of individual nanotubes can also be measured and behave in similar fashion. Comparison of the number density and partial concentration also of individual nanotubes reveals that the individual nanotubes have average molar masses of approximately 700,000 g/mol. The presence of individual nanotubes in NMP dispersion was confirmed by photoluminescence spectroscopy. Concentration dependence of the photoluminescence intensity confirms that the AFM measurements reflect the diameter distributions in situ. In addition, Raman spectroscopy confirms the presence of large quantities of individual nanotubes in the deposited films. Finally, the nature of the solvent properties required for dispersion are discussed.     

Design and characterization of three-dimensional carbon nanotube foams

We demonstrate a new method that makes use of colloidal silica templates to fabricate porous three-dimensional architectures of carbon nanotubes (CNTs). CNTs were grown on monolayered and multilayered structures of colloidal silica using chemical vapor deposition. Porous CNT membranes and three-dimensional carbon nanotube foams were obtained by treating these silica-CNTs structures with HF. The membranes and foams of CNT so obtained were chemically and mechanically stable and were characterized by using scanning electron microscopy and energy dispersive spectroscopy.     

Aligned arrays of nanotubes and segmented nanotubes on substrates fabricated by electrodeposition onto nanorods

We describe an electrochemical-based approach to create vertically aligned nanotube arrays on substrates. Initially, nanoporous anodic alumina films are used as templates to electrodeposit nanorods, and then the alumina templates are removed and nanotube arrays are electrodeposited using the nanorod arrays as templates. We have used this approach to fabricate gold nanotube arrays using nickel nanorods as templates. By anodizing the ends of the nickel nanorods before gold electrodeposition, no deposition occurs at the ends of the rods, resulting in open-ended nanotubes. In addition, we have used layered nickel-gold nanorods as templates to create gold nanostructure arrays with alternating segments of filled and empty nanotubes. This approach is versatile and may be used to electrodeposit a wide range of nanotube materials with good control over the nanotube dimensions.     

Synthesis of Eu2O3 nanotube arrays through a facile sol-gel template approach

Eu2O3 nanotubes have been successfully fabricated by an improved sol-gel template method within the nanochannels of porous anodic alumina templates. The morphology, structure, and composition of the nanotubes were characterized by means of X-ray diffraction techniques, scanning electron microscope, transmission electron microscopy, and selected-area electron diffraction. The results show that the Eu2O3 nanotubes are polycrystalline with a cubic structure. The outer diameter of nanotubes is 50-80 nm, and the thickness of the tube wall is about 5 nm. The mechanism of nanotube formation was discussed.     

Water-dissolvable sodium sulfate nanowires as a versatile template for the fabrication of polyelectrolyte- and metal-based nanotubes

This study presents the synthesis of water-dissolvable sodium sulfate nanowires, where Na(2)SO(4) nanowires were produced by an easy reflux process in an organic solvent, N,N-dimethylformamide (DMF) and formed from the coexistence of AgNO(3), SnCl(2), dodecylsodium sulfate (SDS), and cetyltrimethylammonium bromide (CTAB). Na(2)SO(4) nanowires were derived from SDS, and the morphology control of the Na(2)SO(4) nanowires was established by the cooperative effects of Sn and NO(3)(-), while CTAB served as the template and led to homogeneous nanowires with a smooth surface. Since the as-synthesized sodium sulfate nanowires are readily dissolved in water, these nanowires can be treated as soft templates for the fabrication of nanotubes by removing the Na(2)SO(4) core. This process is therefore significantly better than other reported methodologies to remove the templates under harsh condition. We have demonstrated the preparation of biocompatible polyelectrolyte (PE) nanotubes using a layer-by-layer (LbL) method on the Na(2)SO(4) nanowires and the formation of Au nanotubes by the self-assembly of Au nanoparticles. In both nanotube synthesis processes, PEI (polyethylenimine), PAA (poly(acrylic acid)), and Au nanoparticles served as the building blocks on the Na(2)SO(4) templates, which were then rinsed with water to remove the core templates. This unique water-dissolvable template is anticipated to bring about versatile and flexible downstream applications.     

UV-light mediated decomposition of a polyester for enrichment and release of semiconducting carbon nanotubes

Purification of single-walled carbon nanotubes using conjugated polymers to selectively disperse either semiconducting or metallic nanotubes is effective and has received significant attention. However, the interaction between the conjugated polymer and the nanotube surface is very strong, making it difficult to remove the adsorbed polymer. Here, we report a poly(carbazole-co-terephthalate) polymer that is not only selective for semiconducting carbon nanotubes but can also be largely removed from the nanotube surface via irradiation with UV light. Irradiation of the polymer-nanotube dispersion causes degradation of ester linkages in the polymer backbone, effectively cutting the polymer into fragments that no longer bind strongly to the nanotube surface. Characterization of the electronic nature of the samples was carried out via the combination of absorption, Raman, and fluorescence spectroscopy. In addition, thermogravimetric analysis allowed determination of the amount of polymer left on the nanotube surface after irradiation and indicated that a large proportion of the polymer is removed. The reported methodology opens new possibilities for purification of semiconducting single-walled carbon nanotubes and their isolation from the polymeric dispersant.     

Dispersion of multi-wall carbon nanotubes in polyethylenimine: A new alternative for preparing electrochemical sensors

In this work we report on the analytical performance of glassy carbon electrodes modified with a dispersion of multi-wall carbon nanotubes in polyethylenimine (GCE/(PEI/CNT)). The resulting electrodes show an excellent electrocatalytic activity toward different bioanalytes like ascorbic acid, dopamine, 3,4-dihydroxyphenylacetic acid (dopac) and hydrogen peroxide. An important decrease in the overvoltages for the oxidation of ascorbic acid (505 mV) and hydrogen peroxide (350 mV) and for the reduction of hydrogen peroxide (450 mV), as well as a dramatic improvement in the reversibility of the electrochemical behavior of dopamine and dopac is obtained. The currents are higher than those obtained with other dispersant agents like Nafion, concentrated acids or chitosan, evidencing the high efficiency of the dispersion in PEI. The GCE/(PEI/CNT) demonstrated to be highly reproducible, with 3.0% RSD for the sensitivity of hydrogen peroxide for 10 electrodes prepared with five different dispersions. Differences in sensitivity of 10.0% were obtained for hydrogen peroxide with electrodes prepared using the same dispersion even after 14 days preparation. The CNT/PEI layer immobilized on glassy carbon electrodes has been also used as a platform for building supramolecular architectures based on the self-assembling of polyelectrolytes without any pretreatment of the electrode surface, oxidation or derivatization of the carbon nanotubes, just taking advantages of the polycationic nature of the polymer used for dispersing the nanotubes. The self-assembling of glucose oxidase has allowed us to obtain a supramolecular multistructure for glucose biosensing, with detection limits of 11 μM (0.02 g/L). Such an excellent performance of GCE/(PEI/CNT) toward hydrogen peroxide and the effectiveness of the use of CNT/PEI as a platform for obtaining supramolecular multistructures, represents a very good alternative for developing other enzymatic biosensors.     

Poly(lactic acid) composites with poly(lactic acid)‐modified carbon nanotubes

This work reports the study of the effect of chemical functionalization of carbon nanotubes on their dispersion in poly(lactic acid). The nanotubes were functionalized by the 1,3‐dipolar cycloaddition reaction, generating pyrrolidine groups at the nanotube surface. Further reaction of the pyrrolidine groups with poly(lactic acid) was studied in solution and in the polymer melt. The former involved refluxing the nanotubes in a dimethylformamide/polymer solution; the latter was carried out by direct melt mixing in a microcompounder. The carbon nanotubes collected after each process were characterized by thermogravimetry and by X‐ray photoelectron spectroscopy, showing evidence of polymer bonded to the nanotube surface only when the reaction was carried out in the polymer melt. The composites with polymer modified nanotubes present smaller average agglomerate area and a narrower agglomerate area distribution. In addition, they show improved tensile properties at low CNT concentration and present lower electrical resistivity. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3740–3750     

Comparative study of the electrochemical behavior and analytical applications of (bio)sensing platforms based on the use of multi-walled carbon nanotubes dispersed in different polymers

This review present a critical comparison of the electrochemical behavior and analytical performance of glassy carbon electrodes (GCE) modified with carbon nanotubes (CNTs) dispersed in different polymers: polyethylenimine (PEI), PEI functionalized with dopamine (PEI-Do), polyhistidine (Polyhis), polylysine (Polylys), glucose oxidase (GOx) and double stranded calf-thymus DNA (dsDNA). The comparison is focused on the analysis of the influence of the sonication time, solvent, polymer/CNT ratio, and nature of the polymer on the efficiency of the dispersions and on the electrochemical behavior of the resulting modified electrodes. The results allow to conclude that an adequate selection of the polymers makes possible not only an efficient dispersion of CNTs but also, and even more important, the building of successful analytical platforms for the detection of different bioanalytes like NADH, glucose, DNA and dopamine.