Mechanisms of the rotational dynamics of C70 in C70-cubane heteromolecular crystals.

Fullerenes and cubane (C(8)H(8)) can be arranged to form heteromolecular crystals that exhibit interesting crystal phases. Experimental measurements indicate a rotor-stator phase for C(60)-cubane crystals in which the C(60) molecules rotate freely whereas cubane molecules are essentially static. A similar phase is found for C(70)-cubane crystals but, due to C(70)'s asymmetry, hindered rotations can be observed in specific crystal phases. Details of the rotational dynamics of the fullerenes in these heteromolecular crystals are difficult to be completely assessed by experiments. To this end, we have performed classical molecular dynamics simulations of C(70)-cubane crystals to investigate the behavior of C(70) fullerenes and cubanes in the face-centered cubic and body-centered tetragonal crystallographic phases. Our simulations show that, in the cubic phase, C(70) molecules are allowed to freely rotate whereas cubanes act as molecular bearings. In the tetragonal phase, the cubane molecules also remain practically fixed and the rotation of C(70) fullerenes becomes hindered. In this phase, C(70) molecules rotate around the fivefold axis, which in turn precesses about the c crystallographic direction of the unit cell. Details regarding the dynamics (e.g., energy barriers, reorientational relaxation processes, and phonon-libration coupling) of the C(70) molecules in both crystal phases are discussed. In general, our results agree with previous experimental findings for C(70)-cubane crystals.

Interaction of graphene oxide with cell culture medium: Evaluating the fetal bovine serum protein corona formation towards in vitro nanotoxicity assessment and nanobiointeractions.

The interaction of single-layer graphene oxide (SLGO) and multi-layered graphene oxide (MLGO) with a cell culture medium (i.e. DMEM) was studied by evaluating fetal bovine serum (FBS) protein corona formation towards in vitro nanotoxicity assessment and nanobiointeractions. SLGO and MLGO exhibited different colloidal behavior in the culture medium, which was visualized by cryogenic transmission electron microscopy in situ analysis. Exploring proteomics and bioinformatics tools, 394 and 290 proteins were identified on the SLGO and MLGO hard corona compositions, respectively. From this amount, 115 proteins were exclusively detected on the SLGO and merely 11 on MLGO. SLGO enriched FBS proteins involved in metabolic processes and signal transduction, while MLGO enriched proteins involved in cellular development/structure, and lipid transport/metabolic processes. Such a distinct corona profile is due to differences on surface chemistry, aggregation behavior and the surface area of GO materials. Hydrophilic interactions were found to play a greater role in protein adsorption by MLGO than SLGO. Our results point out implications for in vitro studies of graphene oxide materials concerning the effective dose delivered to cells and corona bioactivity. Finally, we demonstrated the importance of integrating conventional and modern techniques thoroughly to understand the GO-FBS complexes towards more precise, reliable and advanced in vitro nanotoxicity assessment.

Sign Change of Poisson's Ratio for Carbon Nanotube Sheets.

Most materials shrink laterally like a rubber band when stretched, so their Poisson's ratios are positive. Likewise, most materials contract in all directions when hydrostatically compressed and decrease density when stretched, so they have positive linear compressibilities. We found that the in-plane Poisson's ratio of carbon nanotube sheets (buckypaper) can be tuned from positive to negative by mixing single-walled and multiwalled nanotubes. Density-normalized sheet toughness, strength, and modulus were substantially increased by this mixing. A simple model predicts the sign and magnitude of Poisson's ratio for buckypaper from the relative ease of nanofiber bending and stretch, and explains why the Poisson's ratios of ordinary writing paper are positive and much larger. Theory also explains why the negative in-plane Poisson's ratio is associated with a large positive Poisson's ratio for the sheet thickness, and predicts that hydrostatic compression can produce biaxial sheet expansion. This tunability of Poisson's ratio can be exploited in the design of sheet-derived composites, artificial muscles, gaskets, and chemical and mechanical sensors.

Prediction of ordered phases of encapsulated C60, C70, and C78 inside carbon nanotubes.

We report the first detailed fully atomistic molecular dynamics study of the encapsulation of symmetric (C(60)) and asymmetric fullerenes (C(70) and C(78)) inside single-walled carbon nanotubes of different diameters. Different ordered phases have been found and shown to be tube diameter dependent. Rotational structural disorder significantly affecting the volume fraction of the packing was observed for the molecular arrangements of asymmetric fullerenes. Although these effects make more difficult the existence of ordered phases, our results showed that complex packing arrangements (very similar to the ones obtained for C(60)) are also possible for C(70) and C(78). Comparisons with results from continuum and hard-sphere models, ab initio electronic structure calculations, and simulations of the high-resolution transmission electron microscopy images of the obtained fullerene packing phases are also presented.