Experimental-computational study of shear interactions within double-walled carbon nanotube bundles.

The mechanical behavior of carbon nanotube (CNT)-based fibers and nanocomposites depends intimately on the shear interactions between adjacent tubes. We have applied an experimental-computational approach to investigate the shear interactions between adjacent CNTs within individual double-walled nanotube (DWNT) bundles. The force required to pull out an inner bundle of DWNTs from an outer shell of DWNTs was measured using in situ scanning electron microscopy methods. The normalized force per CNT-CNT interaction (1.7 ± 1.0 nN) was found to be considerably higher than molecular mechanics (MM)-based predictions for bare CNTs (0.3 nN). This MM result is similar to the force that results from exposure of newly formed CNT surfaces, indicating that the observed pullout force arises from factors beyond what arise from potential energy effects associated with bare CNTs. Through further theoretical considerations we show that the experimentally measured pullout force may include small contributions from carbonyl functional groups terminating the free ends of the CNTs, corrugation of the CNT-CNT interactions, and polygonization of the nanotubes due to their mutual interactions. In addition, surface functional groups, such as hydroxyl groups, that may exist between the nanotubes are found to play an unimportant role. All of these potential energy effects account for less than half of the ~1.7 nN force. However, partially pulled-out inner bundles are found not to pull back into the outer shell after the outer shell is broken, suggesting that dissipation is responsible for more than half of the pullout force. The sum of force contributions from potential energy and dissipation effects are found to agree with the experimental pullout force within the experimental error.

Tunable biomolecular interaction and fluorescence quenching ability of graphene oxide: application to "turn-on" DNA sensing in biological media.

As a platform for "turn-on" DNA sensing, the level of oxidation of graphene oxide strongly affects its fluorescence quenching ability and binding interactions to single-stranded oligodeoxyribonucleotides (ssODNs), leading to a broad range of sensitivity. Fine-tuning the level of oxidation of graphene oxide yields a DNA-detection platform that is highly sensitive in serum and biological media.

Exfoliation and reassembly of cobalt oxide nanosheets into a reversible lithium-ion battery cathode.

An exfoliation-reassembly-activation (ERA) approach to lithium-ion battery cathode fabrication is introduced, demonstrating that inactive HCoO(2) powder can be converted into a reversible Li(1-x) H(x) CoO(2) thin-film cathode. This strategy circumvents the inherent difficulties often associated with the powder processing of the layered solids typically employed as cathode materials. The delamination of HCoO(2) via a combination of chemical and mechanical exfoliation generates a highly processable aqueous dispersion of [CoO(2) ](-) nanosheets that is critical to the ERA approach. Following vacuum-assisted self-assembly to yield a thin-film cathode and ion exchange to activate this material, the generated cathodes exhibit excellent cyclability and discharge capacities approaching that of low-temperature-prepared LiCoO(2) (~83 mAh g(-1) ), with this good electrochemical performance attributable to the high degree of order in the reassembled cathode.

Graphene oxide, highly reduced graphene oxide, and graphene: versatile building blocks for carbon-based materials.

Isolated graphene, a nanometer-thick two-dimensional analog of fullerenes and carbon nanotubes, has recently sparked great excitement in the scientific community given its excellent mechanical and electronic properties. Particularly attractive is the availability of bulk quantities of graphene as both colloidal dispersions and powders, which enables the facile fabrication of many carbon-based materials. The fact that such large amounts of graphene are most easily produced via the reduction of graphene oxide--oxygenated graphene sheets covered with epoxy, hydroxyl, and carboxyl groups--offers tremendous opportunities for access to functionalized graphene-based materials. Both graphene oxide and graphene can be processed into a wide variety of novel materials with distinctly different morphological features, where the carbonaceous nanosheets can serve as either the sole component, as in papers and thin films, or as fillers in polymer and/or inorganic nanocomposites. This Review summarizes techniques for preparing such advanced materials via stable graphene oxide, highly reduced graphene oxide, and graphene dispersions in aqueous and organic media. The excellent mechanical and electronic properties of the resulting materials are highlighted with a forward outlook on their applications.

Successful stabilization of graphene oxide in electrolyte solutions: enhancement of biofunctionalization and cellular uptake.

Aqueous dispersions of graphene oxide are inherently unstable in the presence of electrolytes, which screen the electrostatic surface charge on these nanosheets and induce irreversible aggregation. Two complementary strategies, utilizing either electrostatic or steric stabilization, have been developed to enhance the stability of graphene oxide in electrolyte solutions, allowing it to stay dispersed in cell culture media and serum. The electrostatic stabilization approach entails further oxidation of graphene oxide to low C/O ratio (~1.1) and increases ionic tolerance of these nanosheets. The steric stabilization technique employs an amphiphilic block copolymer that serves as a noncovalently bound surfactant to minimize the aggregate-inducing nanosheet-nanosheet interactions. Both strategies can stabilize graphene oxide nanosheets with large dimensions (>300 nm) in biological media, allowing for an enhancement of >250% in the bioconjugation efficiency of streptavidin in comparison to untreated nanosheets. Notably, both strategies allow the stabilized nanosheets to be readily taken up by cells, demonstrating their excellent performance as potential drug-delivery vehicles.

Tuning the mechanical properties of graphene oxide paper and its associated polymer nanocomposites by controlling cooperative intersheet hydrogen bonding.

The mechanical properties of pristine graphene oxide paper and paper-like films of polyvinyl alcohol (PVA)-graphene oxide nanocomposite are investigated in a joint experimental-theoretical and computational study. In combination, these studies reveal a delicate relationship between the stiffness of these papers and the water content in their lamellar structures. ReaxFF-based molecular dynamics (MD) simulations elucidate the role of water molecules in modifying the mechanical properties of both pristine and nanocomposite graphene oxide papers, as bridge-forming water molecules between adjacent layers in the paper structure enhance stress transfer by means of a cooperative hydrogen-bonding network. For graphene oxide paper at an optimal concentration of ~5 wt % water, the degree of cooperative hydrogen bonding within the network comprising adjacent nanosheets and water molecules was found to optimally enhance the modulus of the paper without saturating the gallery space. Introducing PVA chains into the gallery space further enhances the cooperativity of this hydrogen-bonding network, in a manner similar to that found in natural biomaterials, resulting in increased stiffness of the composite. No optimal water concentration could be found for the PVA-graphene oxide nanocomposite papers, as dehydration of these structures continually enhances stiffness until a final water content of ~7 wt % (additional water cannot be removed from the system even after 12 h of annealing).

Chemically active reduced graphene oxide with tunable C/O ratios.

Organic dispersions of graphene oxide can be thermally reduced in polar organic solvents under reflux conditions to afford electrically conductive, chemically active reduced graphene oxide (CARGO) with tunable C/O ratios, dependent on the boiling point of the solvent. The reductions are achieved after only 1 h of reflux, and the corresponding C/O ratios do not change upon further thermal treatment. Hydroxyl and carboxyl groups can be removed when the reflux is carried out above 155 °C, while epoxides are removable only when the temperature is higher than 200 °C. The increasing hydrophobic nature of CARGO, as its C/O ratio increases, improves the dispersibility of the nanosheets in a polystyrene matrix, in contrast to the aggregates formed with CARGO having lower C/O ratios. The excellent processability of the obtained CARGO dispersions is demonstrated via free-standing CARGO papers that exhibit tunable electrical conductivity/chemical activity and can be used as lithium-ion battery anodes with enhanced Coulombic efficiency.

Bio-inspired borate cross-linking in ultra-stiff graphene oxide thin films.

Adjacent graphene oxide nanosheets in a thin-film structure have been covalently cross-linked in a fashion similar to the cell walls of higher-order plants. The resulting ultra-stiff structure exhibits a maximum storage modulus of 127 GPa that can be tuned by varying borate concentration.

Luminescent infinite coordination polymer materials from metal-terpyridine ligation.

A new class of infinite coordination polymers (CP) was synthesized using a tetrahedral tetrakis[4-(4'-phenyl-2,2':6',2''-terpyridine)phenyl]methane ligand as an organic node to direct the three-dimensional growth of the network and M(II) (M = Zn, Fe, Ni, and Ru) ions as inorganic linkers, an approach that is the opposite of the metal-as-a-node strategy used in the construction of metal-organic frameworks (MOFs). The unusual rod-like morphology of the resulting microporous materials can be tuned via solvents and reaction conditions. The covalent entrapment of a [Ru(tpy)(2)](2+) moiety in the skeleton of the 3D-network enables the Ru-CP to exhibit room-temperature luminescence.

Evolution of order during vacuum-assisted self-assembly of graphene oxide paper and associated polymer nanocomposites.

Three mechanisms are proposed for the assembly of ordered, layered structures of graphene oxide, formed via the vacuum-assisted self-assembly of a dispersion of the two-dimensional nanosheets. These possible mechanisms for ordering at the filter-solution interface range from regular brick-and-mortar-like growth to complete disordered aggregation and compression. Through a series of experiments (thermal gravimetric analysis, UV-vis spectroscopy, and X-ray diffraction) a semi-ordered accumulation mechanism is identified as being dominant during paper fabrication. Additionally, a higher length-scale ordered structure (lamellae) is identified through the examination of water-swelled samples, indicating that further refinements are required to capture the complete formation mechanism. Identification of this mechanism and the resulting higher-order structure it produces provide insight into possibilities for creation of ordered graphene oxide-polymer nanocomposites, as well as the postfabrication modification of single-component graphene oxide papers.

Evolution of size and shape in the colloidal crystallization of gold nanoparticles.

The addition of dodecanethiol to a solution of oleylamine-stabilized gold nanoparticles in chloroform leads to aggregation of nanoparticles and formation of colloidal crystals. Based on results from dynamic light scattering and scanning electron microscopy we identify three different growth mechanisms: direct nanoparticle aggregation, cluster aggregation, and heterogeneous aggregation. These mechanisms produce amorphous, single-crystalline, polycrystalline, and core-shell type clusters. In the latter, gold nanoparticles encapsulate an impurity nucleus. All crystalline structures exhibit fcc or icosahedral packing and are terminated by (100) and (111) planes, which leads to truncated tetrahedral, octahedral, and icosahedral shapes. Importantly, most clusters in this system grow by aggregation of 60-80 nm structurally nonrigid clusters that form in the first 60 s of the experiment. The aggregation mechanism is discussed in terms of classical and other nucleation theories.