Mohammad F. Islam

Thermal conductivity and interfacial resistance in single-wall carbon nanotube epoxy composites

We report thermal conductivity measurements of purified single-wall carbon nanotube (SWNT) epoxy composites prepared using suspensions of SWNTs in N-N-Dimethylformamide (DMF) and surfactant stabilized aqueous SWNT suspensions. Thermal conductivity enhancement is observed in both types of composites. DMF-processed composites show an advantage at SWNT volume fractions between ϕ∼0.001 to 0.005. Surfactant processed samples, however, permit greater SWNT loading and exhibit larger overall enhancement (64±9)% at ϕ∼0.1. The enhancement differences are attributed to a ten-fold larger SWNT/solid-composite interfacial thermal resistance in the surfactant-processed composites compared to DMF-processed composites. The interfacial resistance is extracted from the volume fraction dependence of the thermal conductivity data using effective medium theory. [C. W. Nan, G. Liu, Y. Lin, and M. Li, Appl. Phys. Lett. 85, 3549 (2004)].

Distinct structural and mechanical properties of the nuclear lamina in Hutchinson–Gilford progeria syndrome

The nuclear lamina is a network of structural filaments, the A and B type lamins, located at the nuclear envelope and throughout the nucleus. Lamin filaments provide the nucleus with mechanical stability and support many basic activities, including gene regulation. Mutations in LMNA, the gene encoding A type lamins, cause numerous human diseases, including the segmental premature aging disease Hutchinson–Gilford progeria syndrome (HGPS). Here we show that structural and mechanical properties of the lamina are altered in HGPS cells. We demonstrate by live-cell imaging and biochemical analysis that lamins A and C become trapped at the nuclear periphery in HGPS patient cells. Using micropipette aspiration, we show that the lamina in HGPS cells has a significantly reduced ability to rearrange under mechanical stress. Based on polarization microscopy results, we suggest that the lamins are disordered in the healthy nuclei, whereas the lamins in HGPS nuclei form orientationally ordered microdomains. The reduced deformability of the HGPS nuclear lamina possibly could be due to the inability of these orientationally ordered microdomains to dissipate mechanical stress. Surprisingly, intact HGPS cells exhibited a degree of resistance to acute mechanical stress similar to that of cells from healthy individuals. Thus, in contrast to the nuclear fragility seen in lmna null cells, the lamina network in HGPS cells has unique mechanical properties that might contribute to disease phenotypes by affecting responses to mechanical force and misregulation of mechanosensitive gene expression.

Single-walled carbon nanotube aerogel-based elastic conductors.

IO N Materials that can remain electrically conducting under large elastic stretching and bending are needed for fabrication of fl exible displays, stretchable electronic implants, artifi cial mechanoreceptors, electrically actuated elastomers for artifi cial muscles, and loudspeakers. [ 1–8 ] One approach to fabricate stretchable or elastic conductors has been to imprint patches of inorganic thin fi lms with conductive interconnects on a prestrained elastomeric substrate. [ 9–12 ] However, the performance and diversity of such elastic conductors are limited by the degree of initial prestrain, the brittleness of the inorganic fi lms, and the compatibility between the inorganic materials and polymeric substrates. [ 1 ] Another approach developed in recent years is to embed networks of conductive nanoparticles such as graphene or carbon nanotubes (CNTs), primarily by direct dispersion using sonication or high shear mixing, into an elastomeric polymer matrix. [ 13–21 ] Graphene-based elastic conductors are transparent and electrically conductive, but electrical conductivity decreases rapidly with tensile strain larger than ≈ 10%. [ 13 , 14 ] Stretchable conductors have also been fabricated using tens of micrometerto millimeter-long bundles and ropes of single-walled (SWCNTs) and multiwalled (MWCNTs) CNTs. However, with a few exceptions, [ 15–19 ] CNT-based elastic conductors offer modest electrical conductivity, require high concentrations of CNTs, are opaque, and their electrical conductivity decreases signifi cantly when stretched. [ 15–17 , 19 ] Furthermore, graphene and CNTs phase-segregate or agglomerate within elastomers during dispersion, which hinders practical-scale usage of grapheneor CNT-based elastic conductors. To avoid such segregation or agglomeration of CNTs, we report a novel stretchable conductor fabrication method utilizing preformed, highly porous 3D networks of SWCNTs, called SWCNT aerogels, and then integrating elastomeric polymers. SWCNT aerogels are ultralight and electrically conducting created by critical-point drying of aqueous elastic gel of individually dispersed SWCNTs. [ 22–26 ] The SWCNTs are connected to each other within the network via van der Waals interactions at the SWCNT–SWCNT junctions. [ 24 , 25 ] The concentration of SWCNTs within the aerogel can be as low as 6 mg mL − 1 and the rest is void space. [ 26 ] The aerogel shapes and sizes are readily

Carbon Nanotubes Reorganize Actin Structures in Cells and ex Vivo

The ability of globular actin to form filaments and higher-order network structures of the cytoskeleton is essential for cells to maintain their shape and perform essential functions such as force generation, motility, and division. Alterations of actin structures can dramatically change a cells ability to function. We found that purified and dispersed single wall carbon nanotubes (SWCNTs) can induce actin bundling in cells and in purified model actin systems. SWCNTs do not induce acute cell death, but cell proliferation is greatly reduced in SWCNT-treated cells with an increase in actin-related division defects. Actin, normally present in basal stress fibers in control cells, is located in heterogeneous structures throughout the SWCNT-treated cell. These SWCNT-induced changes in actin structures are seen functionally in multinucleated cells and with reduced force generation. Ex vivo, purified actin filaments cross-linked with alpha-actinin and formed isotropic networks, whereas SWCNTs caused purified actin filaments to assemble into bundles. While purified, isolated SWCNTs do not appear acutely toxic, this subcellular reorganization may cause chronic changes to cellular functions.

Single wall carbon nanotubes enter cells by endocytosis and not membrane penetration

BackgroundCarbon nanotubes are increasingly being tested for use in cellular applications. Determining the mode of entry is essential to control and regulate specific interactions with cells, to understand toxicological effects of nanotubes, and to develop nanotube-based cellular technologies. We investigated cellular uptake of Pluronic copolymer-stabilized, purified ~145 nm long single wall carbon nanotubes (SWCNTs) through a series of complementary cellular, cell-mimetic, and in vitro model membrane experiments.ResultsSWCNTs localized within fluorescently labeled endosomes, and confocal Raman spectroscopy showed a dramatic reduction in SWCNT uptake into cells at 4°C compared with 37°C. These data suggest energy-dependent endocytosis, as shown previously. We also examined the possibility for non-specific physical penetration of SWCNTs through the plasma membrane. Electrochemical impedance spectroscopy and Langmuir monolayer film balance measurements showed that Pluronic-stabilized SWCNTs associated with membranes but did not possess sufficient insertion energy to penetrate through the membrane. SWCNTs associated with vesicles made from plasma membranes but did not rupture the vesicles.ConclusionsThese measurements, combined, demonstrate that Pluronic-stabilized SWCNTs only enter cells via energy-dependent endocytosis, and association of SWCNTs to membrane likely increases uptake.

Electronic devices based on purified carbon nanotubes grown by high-pressure decomposition of carbon monoxide

The excellent properties of transistors, wires and sensors made from single-walled carbon nanotubes (SWNTs) make them promising candidates for use in advanced nanoelectronic systems1. Gas-phase growth procedures such as the high-pressure decomposition of carbon monoxide (HiPCO) method2,3 yield large quantities of small-diameter semiconducting SWNTs, which are ideal for use in nanoelectronic circuits. As-grown HiPCO material, however, commonly contains a large fraction of carbonaceous impurities that degrade the properties of SWNT devices4. Here we demonstrate a purification, deposition and fabrication process that yields devices consisting of metallic and semiconducting nanotubes with electronic characteristics vastly superior to those of circuits made from raw HiPCO. Source–drain current measurements on the circuits as a function of temperature and backgate voltage are used to quantify the energy gap of semiconducting nanotubes in a field-effect transistor geometry. This work demonstrates significant progress towards the goal of producing complex integrated circuits from bulk-grown SWNT material.

Ultracompressible, High Rate Supercapacitors from Graphene-Coated Carbon Nanotube Aerogels

Emerging applications for electrochemical energy storage require devices that not only possess high power and energy, but also are capable of withstanding mechanical deformation without degradation of performance. To this end, we have constructed electric double layer capacitors (EDLCs), also referred to as supercapacitors, using thick, ultracompressible graphene-coated carbon nanotube aerogels as electrodes. These electrodes showed a high capacitance in both aqueous and room-temperature ionic liquid (RTIL) electrolytes, achieving between 60 and100 F/g, respectively, with the performance stable over hundreds of charge/discharge cycles and at high rates exceeding 1 V/s. This performance was retained fully under 90% compression of the systems, allowing us to construct cells with high volumetric capacitances of ∼5-18 F/cm(3) in aqueous and RTIL electrolytes, respectively, which are 50-100 times higher than comparable compressible EDLCs (∼0.1 F/cm(3)). Further, the volumetric capacitances approach values reported for compressible pseudocapacitors (∼15-30 F/cm(3)) but without the degraded lifetime and reversibility that typically plague compressible pseudocapacitors. The electrodes demonstrated largely strain-invariant ion transport with no change in capacitance and high-rate performance even at 90% compressive strain. This material serves as an excellent platform for exploring the possibility for use of extremely compressible EDLCs with negligible degradation in capacitance in applications such as electric vehicles and wearable electronics.

Normal modes and density of states of disordered colloidal solids.

Measuring Motion Within a solid, atoms vibrate about their mean position in a series of frequencies known as the normal modes, which relate to the thermal and mechanical transport properties of the material. D. Kaya et al. (p. 656) used video microscopy to observe the motion of colloidal crystals made from microgel particles. The colloidal particles varied slightly in their properties, allowing the behavior of disordered materials to be probed. Long-wavelength plane-wave modes were observed, characteristic of perfect crystals, and a conventional elastic behavior, modified by short-wavelength features, was also observed, in spite of the disorder of the colloidal crystals. The analysis method will allow studies on the effects of different types of disorder on the structure of the normal modes and the elasticity in a range of material systems. The motion of colloidal gel particles is used to determine the mechanical and thermal properties of a disordered system. The normal modes and the density of states (DOS) of any material provide a basis for understanding its thermal and mechanical transport properties. In perfect crystals, normal modes are plane waves, but they can be complex in disordered systems. We have experimentally measured normal modes and the DOS in a disordered colloidal crystal. The DOS shows Debye-like behavior at low energies and an excess of modes, or Boson peak, at higher energies. The normal modes take the form of plane waves hybridized with localized short wavelength features in the Debye regime but lose both longitudinal and transverse plane-wave character at a common energy near the Boson peak.

Fluctuations and Rheology in Active Bacterial Suspensions

We probe nonequilibrium properties of an active bacterial bath through measurements of correlations of passive tracer particles and the response function of a driven, optically trapped tracer. These measurements demonstrate violation of the fluctuation-dissipation theorem and enable us to extract the power spectrum of the active stress fluctuations. In some cases, we observe 1/sqrt[omega] scaling in the noise spectrum which we show can be derived from a theoretical model incorporating coupled stress, orientation, and concentration fluctuations of the bacteria.

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.