The cellular toxicity of carbon-based nanomaterials was studied as a function of their aspect ratio and surface chemistry. These structures were multiwalled carbon nanotubes, carbon nanofibers, and carbon nanoparticles. Their toxicity was tested in vitro on lung tumor cells. Our work clearly indicated that these materials are toxic while the hazardous effect is size-dependent. Moreover, cytotoxicity is enhanced when the surface of the particles is functionalized after an acid treatment.

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We have determined the mechanical anisotropy of a single microtubule by simultaneously measuring the Young's and the shear moduli in vitro. This was achieved by elastically deforming the microtubule deposited on a substrate tailored by electron-beam lithography with a tip of an atomic force microscope. The shear modulus is 2 orders of magnitude lower than the Young's, giving rise to a length-dependent flexural rigidity of microtubules. The temperature dependence of the microtubule's bending stiffness in the 5-40 C range shows a strong variation upon cooling coming from the increasing interaction between the protofilaments. DOI: 10.1103/PhysRevLett.89.248101 PACS numbers: 87.16.Ka, 81.07.-b, 81.70.Bt, 87.15.La Microtubules are a filamentous assembly of protein subunits, -and -tubulin. They are hollow cylinders with external and internal diameters of 25 nm and 15 nm, respectively [1]. Along with actin and intermediate filaments, they form the eukaryotic cytoskeleton and participate in defining cell morphology. They also perform various vital functions unique to them: they act as building blocks for cilia and flagella and as tracks along which molecular motors move. These roles are determined by their structure and mechanical properties.The complex dynamics of microtubules (MTs) and other cytoskeletal elements play a key role in cell division, motility, and determination of cell shape. Their elastic properties and interaction with the cell membrane are crucial in understanding cell morphology. Quantifying the mechanical properties of microtubules is also necessary for explaining the elasticity of, for example, sensory hair cells and sperm tails. Influence of various physical conditions such as temperature or chemical agents, for example, drugs, could also be better understood by precisely measuring the mechanical response of an MT, governed by Young's (tensile stiffness) and shear modulus. Conventional studies involving atomic force microscopy (AFM) demand a specific biochemical functionalization of the supporting surface, the sample, and the force-exerting tool, the AFM tip, in order to transmit a stretching load to biomolecules [2]. Although these methods work remarkably well for studying the stretchiness of single molecules (DNA, proteins, etc.), they do not give a complete description of complex systems like MTs, which are a loosely connected assembly of protofilaments. In such biomaterials, not only the properties of the individual components, but also the interactions between them -lateral as well as longitudinal -reflected in the elastic properties, play an important role. Here we report for the first time the elastic moduli of MTs stabilized in vitro, using a new approach for measuring mechanical properties of complex biological systems. This method gives both shear and Young's moduli, and demonstrates the high mechanical anisotropy of MT structure.Previously performed experiments on mechanical properties of microtubules reported in the literature (with the exception of the only result obtained using AF...