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Nanoparticle-Mediated Protein Delivery to Cells

The introduction of specific proteins into cells represents a powerful alternative to gene delivery, which is the current standard method for expression of specific proteins in cells. In NSEC research at RPI, silica nanoparticles have been used as carriers to deliver into the cell cytoplasm a variety of proteins that can influence cellular behavior. The goal is to manipulate and direct cell function in a variety of human tissues, including normal cells, cancer cells, and stem cells. By immobilizing specific proteins, it is possible to influence various components of the cell and dictate cell function and fate, as well as enable detection of specific intracellular mechanisms for further study.

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Microcapsules Containing Suspensions of Carbon Nanotubes

NSEC researchers at UIUC have demonstrated an important step towards the realization of self-healing of passivation cracks in electronic components. Specifically, microcapsules have been synthesized in which single-wall carbon nanotubes (SWNTs) suspended in organic solvents are encapsulated. When ruptured, these SWNT microcapsules release their liquid cores.  Voltage probes are then inserted into this liquid material and scanned over the range of -50 V to +50 V, revealing that the SWNTs migrate to form a conductive pathway across the probe tips.

This research may contribute to the ultimate development of strategies for the automatic self-repair of defects that can occur in electronic components and assemblies as a result of a variety of factors.

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The Paper Battery Company, Inc. Formed to Capitalize on Technology Developed at RPI’s NSEC

NSEC researchers at RPI have nanoengineered a lightweight, ultra-thin paper battery geared toward meeting the demanding design and energy requirements of tomorrow’s electronic and electrical apparatus. Over 90 percent of the device is cellulose that has been infused with aligned carbon nanotubes that act as electrodes and enable electrical conduction. The battery is completely integrated - its components are attached molecularly to each other - and can be printed like paper. It can be rolled, twisted, folded, or cut into any shape with no loss of mechanical integrity or electrical efficiency. Battery sheets can be stacked - like printer paper - to boost the total energy and, significantly, the devices can function either as high-energy batteries or high-power supercapacitors. 

A patent has been filed to protect the technology and The Paper Battery Company, Inc. has been formed to engineer the paper-based super-capacitors and batteries from a common starting sheet of nanocomposite material made in a high volume process. The company’s vision is to develop and market the next generation of storage devices and set a standard for clean, renewable energy.

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“Molecules to the MAX!” Released in 3D English, Japanese and Mandarin

The Molecularium® Project at RPI’s NSEC has produced “Molecules to the MAX!”, a 40-minute 3D Imax adventure that follows Oxy, Hydro, Hydra, Carbón, and other characters as they navigate the colorful world of atoms and molecules in search of life. With help of the Molecularium — a spaceship that can shrink to nanoscale sizes — the group explores the “secret worlds” and molecular structures of everyday objects including a snowflake, chewing gum, a penny, and even a human cell. Audiences experience amazingly small places and incredibly big ideas!

“Molecules to the MAX!” has now been translated into Mandarin and Japanese. The 2D show is currently playing in the New Mexico Museum of Space History in Alamogordo, NM, the McWane Science Center’s IMAX Dome in Birmingham, AL, as well as the National Museum of Natural History in Taichung, Taiwan, all to great acclaim.

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Ultrafast hydration structures

Fundamental understanding of self- assembly in water requires knowledge of water structure at the nanoscale. In an NSEC collaboration, RPI and UIUC researchers employed a combination of experimental and computational methods to reconstruct the behavior of water measured via synchrotron X-rays. As a result, the space- and time-dependent behavior of water at femtosecond timescales and Ångström lengthscales can be simultaneously known for the first time. An understanding of electrostatic interactions in water is important for a wide range of applications, including gene therapy, cancer vaccines, water purification technologies, and the development of antimicrobial agents.

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Colloidal Superdispersants

NSEC researchers have designed, synthesized, and investigated the effects of a cationic comb polymer on the stability of aqueous silica suspensions of varying ionic strength and pH. Both the comb polymer and its homopolymer analog were synthesized and analyzed leading to the discovery that comb polymer-coated silicamicrospheres (green) remain stable over a broad range of solution conditions, whereas pure homopolymer-coated silica microspheres (red) rapidly fall out of suspension.

By using superdispersants, biphasic nanoparticle mixtures have been designed for direct-write assembly of complex 3D nanostructured materials. These novel inks enable 3D printing strategies for rapid prototyping and near net shape fabrication with an order of magnitude improvement in feature resolution.

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Molecular Level Assembly of Novel BioHybrid Materials

Mimicking the principles learned from Nature’s microtubules can lead to the next generation of materials with improved mechanical efficiency and a wide range of functional properties. Dinu, Dordick et al. (Advanced Materials 2009; Small 2009) have shown that biological function can be conferred to carbon nanotubes. Specifically, direct attachment of tubulin dimer cytoskeletal protein (schematically shown in green and violet) onto multi-wall carbon nanotubes (MWNTs, shown as graphite cylinders) form tubulin-MWNT conjugates. The geometries of these conjugates are a function of the tubulin concentration (AFM image of flowerlike geometry). MWNT-templated tubulin undergoes self-assembly with free tubulin in solution to yield functional nano- and mesoscale architectures, including biohybrids of microtubule-encapsulated nanotubes. These could be manipulated in synthetic, non-physiological environments by surface-attached kinesin molecular motors to create new sensors or actuators. (Jonathan S. Dordick (RPI)).

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Self-organization of Water Induced by Electrons

Fundamental understanding of self-assembly in water requires knowledge of water structure and dynamics in the vicinity of solutes, specifically in the hydration shells. In an NSEC collaboration, Wong (UIUC) and Garde (RPI) use a new hybrid experimental-computational method, Linear Response Imaging (LRI), to reconstruct the dynamical behavior of water from a library of dynamical structure factor data measured at 3rd generation synchrotron X-ray sources. The Green’s function of water is extracted from, and subsequently used to reconstruct, the space- and time-dependent behavior of water at femtosecond timescales and Ångstrom lengthscales.

Shown: The hydration structure around an ‘electron-like’ distribution of negative charge moving with a steady state linear velocity: (a) well below the speed of sound in water (v=250 m/s); (b) near the speed of sound (v=1000 m/s). (Gerard C. L. Wong (UIUC), Shekhar Garde (RPI)).

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Imaging the Structure and Flow of Gels in Microchannels

For the first time, Conrad and Lewis (UIUC) have directly imaged the structure and flow behavior of colloidal gels in microchannels using confocal microscopy. Silica particles are first coated with a cationic polyelectrolyte, and then flocculated by the addition of an anionic polyelectrolyte. In the quiescent state, the suspension is an isotropic and homogeneous gel. Under shear flow, the suspension contains dense clusters that yield at intercluster boundaries, resulting in network breakup at high shear rates. These structural changes coincide with a transition from plug-like flow at low pressures to fluid-like flow behavior at high pressures of the gels.

J.C. Conrad and J.A. Lewis, “Direct Imaging of Colloidal Gels during Microchannel Flow”, Langmuir, 24, 7628-34 (2008).

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The Role of Interfacial Cohesion in Polymer Nanocomposites

A first of its kind combined theory-experiment analysis by Schweizer and Zukoski (UIUC) of adsorbing polymer mediated structural reorganization (depletion aggregation, full dispersion, bridging) of silica nanoparticles in equilibrated miscible polymer nanocomposites of variable chemistry and adsorption affinity has been performed. Quantitative comparison of microscopic theory calculations with small-angle X-ray scattering experiments demonstrate the theoretical approach properly accounts for the effects of adsorbed polymer layers on nanoparticle concentration fluctuations over all length scales for a wide range of volume fractions and interfacial cohesion strengths. A distinctive microphase-separation-like peak in the collective polymer structure factor is predicted. Nanoparticle potential-of-mean-force calculations suggest a criterion for the onset of kinetic gelation. Remarkably small changes in nanoparticle surface or polymer chemistry are shown to result in dramatically different equilibrium and nonequilibrium nanocomposite behavior and properties. This opens up new ways of thinking about the processing of polymer nanocomposites and controlling the state of particle dispersion.

L.B. Hall. B.J. Anderson, C.F. Zukoski and K.S. Schweizer, “Concentration Fluctuations, Local Order and Collective Structure in Polymer Nanocomposites”, Physical Review Letters, in review, 2009.

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IMAX Show Completed – “Molecules to the MAX®”

The second Molecularium® show, “Molecules to the MAX” played at the Giant Screen Cinema Association Trade Expo in Los Angeles and San Diego in March 2009. This 42 minute long large-format (e.g., IMAX) film created at RPI takes you on a ride to NanoSpace with Oxy and her crew to boldly go where only atoms have gone before! Aboard the Molecularium, the most fantastic ship in the universe, you fly through the crystalline structure of a snowflake, explore the metallic maze of a penny, blast through the far reaches of space, escape the tangled polymers of chewing gum, and discover the molecular machinery of a living cell. This animated adventure brings audiences into amazingly small places and fascinates them with incredibly big ideas. Coming soon to IMAX®, IMAX 3D® and other Giant Screen Theatres near you!

Produced by Richard W. Siegel, Linda S. Schadler and Shekhar S. Garde. Please visit www.moleculestothemax.com.

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Local High Schools Adopt Nanotechnology After Attending Rensselaer’s Curriculum Development Summer Institute

During the Summer of 2008 a synergistic effort between the NSEC, the National Center for Learning and Teaching, WSWHE BOCES, and the Workforce Consortium for Emerging Technologies resulted in 5 local high schools introducing new nanotechnology curricula into their classrooms. Each high school contributed $10,000 to the program for teachers’ salaries and supplies. The program started with one week on the Rensselaer campus for an introduction to nanotechnology and familiarization with existing curricula. The high school faculty members spent the following three weeks developing their own curricula, which ranged from a new year-long course combining chemistry and technology to new modules integrated into existing physics and chemistry courses. As a result of this program, hundreds of high school students will be exposed to nanotechnology in the coming year. (Linda S. Schadler (RPI)).

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Structure and Dynamics of Biphasic Colloidal Mixtures

We have investigated the structure and dynamics of biphasic colloidal mixtures composed of coexisting attractive and repulsive microspheres by confocal microscopy. Attractive gels formed in the presence of repulsive microspheres are more spatially homogeneous and, on average, contain less interparticle bonds per particle than their unary counterparts. The repulsive microspheres within these mixtures display heterogeneous dynamics, with some species exhibiting freely diffusive Brownian motion while others are trapped within the gel network during aggregation. This work provides new insights into the enhanced printing behavior of biphasic nanoparticle inks. Shown: Confocal micrograph of the biphasic colloidal mixture.

A. Mohraz, E.R. Weeks, and J.A. Lewis, “Structure and Dynamics of Biphasic Colloidal Mixtures” Phys. Rev. Lett. (in review), 2008.

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Self-Assembly of Decorated Nanoparticles in Polymer Nanocomposites

The self-assembly of nanoparticles into complex superstructures with precise geometrical form is typically controlled through particle shape or directional interparticle interactions. A more subtle issue is the creation of anisotropic structures from isotropically interacting spherical particles. While one-dimensional strings can be formed from such particles at high particle loadings, the formation of extended assemblies (cylinders, sheets) at low particle loadings is typically attributed to particles with directional interactions. We have shown that spherical nanoparticles uniformly grafted with polymer chains dispersed in a homopolymer matrix with the same chemistry as the “brush” (but longer, see figure left), can self-assemble into highly anisotropic sheets even at low particle loadings (see figure right). We attribute this unexpected phenomenon to the “polarization” of the segmental cloud of the grafted polymer layer upon the approach of two nanoparticles, coupled to the tendency of the particles to “phase separate” from the homopolymer. This selfassembly process, which is analogous to the behavior of block copolymers and other surfactants inselective solvents, can have profound implications for applications in which it is well established that a small amount of highly anisotropic filler can lead to significantly improved properties. Shown: Spherical nanoparticles uniformly grafted with polymer chains dispersed in a homopolymer matrix with the same chemistry as the “brush” (but longer, see figure left), can self-assemble into highly anisotropic sheets even at low particle loadings (see figure right).

P. Akcora, S.K. Kumar, Y. Li, B.B. Benciewicz, D. Dukes, L.S. Schadler, D. Acehan and J.F. Douglas, “Anisotropic Self-Assembly of Spherical Nanoparticles in Polymer Nanocomposites”, submitted to Science, 2008.

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Nanotube-Assisted Protein Deactivation

There is currently substantial interest in understanding and achieving remote control over protein function on nanomaterials. We have demonstrated for the first time the remote and specific nanotube-mediated deactivation of proteins using near-infrared irradiation (wavelengths 700-1100 nm). The observed deactivation is mediated by a photochemical reaction involving free radicals generated on irradiation of the carbon nanotubes. Such an event has been used to design polyvalent nanotube-peptide conjugates that target and destroy anthrax toxin and to design optically transparent nanotube coatings that possess “self-cleaning” activity following either near-infrared or visible irradiation. Nanotube-assisted deactivation, therefore, represents a general and facile strategy for the targeted destruction of proteins, pathogens, and cells, with applications ranging from antifouling coatings to proteomics and novel therapeutics.

A. Joshi, P. Supriya, S.S. Bale, H. Yang, T. Borca-Tasciuc, and R.S. Kane, “Nanotube-assisted protein deactivation”. Nat. Nanotechnol. 3, 41-45, 2008.

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Site-Specific Control of Distances between and Position of Gold Nanoparticles using Phosphorothioate Modification

Precise control of the position of and distance between nanomaterials is at the heart of nanoscale science and technology. DNA have been shown to be highly programmable molecules resulting in a number of 2-D and 3-D nanostructures. Despite the huge promise, functionalizing these DNAbased nanostructures with nanomaterials has been a challenge in the field. Published methods to meet the challenge have significant limitations, such as the use of nicks in the DNA (introducing instability), the need to purify monofunctionalized DNA-AuNPs using an agarose gel purification method (making it difficult for large-scale preparation), or the introduction of a relatively long, flexible DNA strand between the DNA template and the AuNPs (making it difficult to position the nanomaterials precisely onto the DNA template). We introduced a simple but precisely controllable method to assemble gold nanoparticles (AuNPs) on DNA by using phosphorothioate modification on DNA as an anchor and a bifunctional linker that can connect a AuNP to DNA as a fastener. The chemical attachment between a AuNP and a bifunctional fastener treated modified DNA has been demonstrated in solution by plasmon peak shift as AuNPs aggregate and disassemble due to the DNA hybridization and denaturation. Distance between AuNPs assembled on DNA could be controlled by simply changing the position of the modification on DNA with identical sequences and it could be observed on surfaces by scanning electron microscopy (SEM) images and statistic analyses. The methodology demonstrated can be applied to using DNA for precise distance and topological control of nanomaterials in one, two, and three dimensions.

J. H. Lee, D. P. Wernette, M. V. Yigit, J. Liu, Z. Wang, Y. Lu, “Site-Specific Control of Distances between Gold Nanoparticles using Phosphorothioate Anchors on DNA and a Short Bifunctional Molecular Fastener”, Angew. Chem. Int. Ed., 46, 9006, 2007.

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"Riding Snowflakes" Reaches Broad Audience

The first Molecularium® show, “Riding Snowflakes” premiered at the Children’s museum of Science and Technology (CMOST) in Troy, NY on February 4th, 2005, opened at the Chabot Space and Science Center in Oakland, CA in the Fall
of 2005 and at the LodeStar Planetarium in Albuquerque, NM in the Spring of 2006. In 2007, this digital dome theater show, which takes the audience on a ride into the world of atoms and molecules and teaches them that everything is made of atoms and molecules, has started reaching a broader audience. Riding Snowflakes has been licensed to 3 major planetarium distributors: Spitz/Evans and Sutherland, SkySkan, and eplanetarium. Rensselaer is currently in the process of finalizing
arrangments with a fourth distributor. It was sold to two school districts in 2006, and in 2007 was sold to Thinktank Science Museum in Birmingham, UK, Shasta County Board of Education (CA), Ft Leavenworth School District (KS), Exploritorium Place (KS) and the Children’s Museum – Gengras Planetarium (CT). Shown: Riding Snowflakes in a digital dome.

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Short Course for High School Teachers Brings
Nanotechnology to 9-12 Classrooms

The NSEC faculty and our high school teacher collaborators taught a 2.5-day course to 30 high school teachers from upstate New York in July of 2007 on nanotechnology. The goal was to provide a background in nanotechnology and to give the teachers some hands-on activities to use in the classroom. The lectures included an overview, and an introduction to nanocomposites, biofunctional materials, health impacts of nanomaterials, fuel cells, and solid state lighting. The laboratories included electron microscopy, atomic force microscopy (including the large scale AFM used by our high school collaborators to teach all levels of high school physics), ferrofluids, synthesis of
nanotubes, and a nanoadventure game. The response from the teachers was overwhelming in favor of continuing this short course. Shown: Participating high school teachers synthesize gold and silver nanoparticles as well as ferrofluids.

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Senior NSEC Faculty Elected to National Academy of Engineering

One of our NSEC’s senior faculty participants, Dr. Charles F. Zukoski, was elected to the National Academy of Engineering. He was cited “for research on the manipulation of particle interactions to alter their suspension properties, and for leadership in education.” His research concentrates on understanding the relationships between surface physical chemistry and the material properties of colloidal suspensions. Particular attention is paid to methods of manipulating interparticle forces to alter particle and suspension properties. Zukoski is the William H. and Janet G. Lycan Professor in the Department of Chemical and Biomolecular Engineering and Vice Chancellor for Research at the University of Illinois at Urbana-Champaign.

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NSEC Investigators Author One of 10 Most Accessed Articles in Macromolecules

NSEC investigators Dr. Chang Y. Ryu and Dr. Brian C. Benicewicz authored one of the ten most-accessed articles in the journal Macromolecules (April-June, 2006). The article’s popularity mirrors the scientific impact of this work. Macromolecules is the most-cited journal in the area of Polymer Science, with 71,840 citations in 2005 - over 41,000 more citations than the nearest ranked journal. It is also ranked third in impact factor out of the 75 journals in the polymer science category. The article, “A Versatile Method To Prepare RAFT Agent Anchored Substrates and the Preparation of PMMA Grafted Nanoparticles” was co-authored by Li, Han, Ryu, and Benicewicz. Researchers discovered a novel strategy to efficiently graft polymers on nanoparticle surfaces using a controlled radical polymerization technique. This strategy is applicable for a wide range of monomers to tailor the surface functionality of nanofillers including nanoparticles.

C. Li (RPI), J. Han (RPI), C.Y. Ryu (RPI), and B.C. Benicewicz (RPI), “A Versatile Method to Prepare RAFT Agent Anchored Substrates and the Preparation of PMMA Grafted Nanoparticles”, Macromolecules  2006, 39, 3175 - 3183.

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Room Temperature Assembly of Germanium Nanoparticle-based Photonic Crystals

Through a rapid and low-cost directed self-assembly process, a nanoparticle-based photonic crystal, a three-dimensionally periodic material with unique and powerful optical properties, was formed (Braun, Siegel). Our photonic crystals exhibited the greatest photonic strength to date of any nanoparticle- based systems, and in addition, we demonstrated, for the first time, that germanium nanoparticles could be directly used to create a photonic crystal.  Reflectance spectroscopy, in conjunction with appropriate theoretical models, was used to determine that the germanium photonic crystal had a refractive index contrast of 2.05, the largest refractive index contrast obtained to date for any nanoparticle-based system.

R.G. Shimmin (UIUC), R. Vajtai (RPI), R.W. Siegel (RPI), P.V. Braun (UIUC), “Room-Temperature Assembly of Germanium Photonic Crystals through Colloidal Crystal Templating”, Chem. Mater., 2007, ASAP Article, DOI: 10.1021/cm062893l. Supported by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award Number DMR-0117792.

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Chain Conformations and Bound Layer Correlations in Polymer Nanocomposites

A combined experimental and theoretical approach (Kumar, Schweizer) has been employed to address theopen question of chain conformation and adsorption in polymer nanocomposites. Small angle neutron scattering (SANS) on mixtures of polystyrene and nanosilica has unequivocally shown that polymers adopt random coil shapes, whose sizes are independent of molecular weight and nanofiller concentration. Our novel microscopic statistical mechanical theory of polymer nanocomposites also predicts the existence of a thin thermodynamically stable bound layer of polymer surrounding dispersed fillers. The experimental polymer scattering signature of this phenomenon is a peak in the SANS spectrum, whose intensity and location are controlled by nanoparticle size and volume fraction. The neutron data are consistent with these predictions thereby providing the first evidence for the existence of nanoscopic layers that play a critical role in promoting miscibility and good filler dispersion.

S. Sen (RPI), Y. Xie (RPI), S.K. Kumar (RPI), H. Yang (RPI), A. Bansal (GE), D.L. Ho (NIST), L. Hall (UIUC), J.B. Hooper (UIUC) and K.S. Schweizer (UIUC), “Chain Conformations and Bound Layer Correlations in Polymer Nanocomposites”, Physical Review Letters, 98, 128302 (2007). Supported by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award Number DMR-0117792.

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Enzyme-catalyzed Directed Assembly of Organogels

Organogelators with excellent ability to gel a broad range of organic solvents as well as natural oils (olive and vegetable oils) were synthesized (Dordick) using all natural building blocks (sugars, fatty acids, and enzymes). This is an example of exquisitely selective enzyme-catalyzed directed assembly – chemical synthesis of the gelators results in poor gel properties due to the lack of selectivity. With their ability to assemble at the nanoscale, and to be prepared from all natural building blocks (sugars, fatty acids, and enzymes), these gelators may be used to encapsulate pharma-ceutical, food, and cosmetic products and to build 3-D biological scaffolds for tissue engineering.

G. John (CUNY), G. Zhu (RPI), J. Li (USM), and J.S. Dordick (RPI), “Enzymatically Derived Sugar-Containing Self-Assembled Organogels with Nanostructured Morphologies”, Angew. Chem. Int. Ed. Engl.  45, 4772-4775 (2006, Cover Article). Supported by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award Number DMR-0117792.

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Cellulose Nanotube Composites as Flexible Power Sources

Nanocomposites have been developed that have enhanced biocompatibility while still exhibiting important properties associated with nanomaterials (Linhardt, Ajayan). Nanoporous cellulose-heparin composites were prepared as blood compatible membranes for kidney dialysis and as electrospun fibers for woven vascular grafts. Of significant interest in combining biological and materials applications, cellulose-oriented carbon nanotube composites have been prepared, which contain ionic liquids as batteries and supercapacitors and a patent application filed.  These flexible, biocompatible devices are being evaluated in a number of applications such as implantable and wearable power sources for medical assist devices.

S. Murugesan (RPI), S. Mousa (RPI), A. Vijayaraghavan (RPI), P.M. Ajayan (RPI), and R.J. Linhardt (RPI), “Ionic liquid derived blood compatible composite membranes for kidney dialysis”, J. Biomed. Mat. Res. Part B – Appl. Biomater. 79B, 298-304 (2006). G. Viswanathan (RPI), S. Murugesan (RPI), V. Pushparaj (RPI), O. Nalamasu (RPI), P.M. Ajayan (RPI), and R.J. Linhardt (RPI), “Preparation of biopolymer fibers using electrospinning from room temperature ionic liquids”, Biomacromol. 7, 415-418 (2006). Supported by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award Number DMR-0117792.

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Nonlinear Elasticity and Yielding of Glassy Nanoparticle Suspensions

The mechanical response of dense nanoparticle suspensions to applied stress and shear is a problem of fundamental scientific importance in soft materials science and also a key enabling aspect of our directwrite assembly approach to fabricating novel nanomaterials and devices.  We have recently developed a microscopic and predictive statistical mechanical theory of strain softening, yielding and shear thinning of glassy nanoparticle suspensions that describes how deformation modifies the local cage constraints that are the origin of elasticity and slow dynamics in these materials. New experiments have been designed to critically test the theoretical predictions for stress-softening and yielding of polymer-coated glassy barium titanate suspensions of practical experimental interest. Comparisons of theory and experiment for the perturbative yield stress and strain, and also the linear elastic shear modulus, have been made (see Figure). Experimental (blue) and theoretical (red) elastic shear modulus and perturbative yield stress as a function of nanoparticle volume fraction. The inset shows the corresponding results for the yield strain; note the essentially quantitative agreement between theory and experiment. The no adjustable parameter theoretical predictions of a high power law or exponential growth of the shear modulus and yield stress, and monotonic reduction of the yield strain, with increasing nanoparticle volume fraction are in good agreement with the observations. This advance in understanding, in conjunction with analogous theoretical progress for dense nanoparticle gels3, provides fundamental guidance for the design of novel nanoparticle based inks for directed assembly.

V. Kobelev and K.S. Schweizer, Physical Review E71, 2005, 0121401. 2R.B. Rao, V.L. Kobelev, Q. Li, J.A. Lewis and K.S. Schweizer, Langmuir, 2006, in press. V. L. Kobelev and K.S. Schweizer, J. Chemical Physics 123, 2005, 164902.

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Synthesis and Aggregation Behavior of Thermally Responsive Star Polymers

Reversible addition-fragmentation chain transfer (RAFT) has been used to prepare well-defined linear and 4-arm star diblock copolymers consisting of N-N’-dimethylacrylamide (DMA) and Nisopropylacrylamide (NIPAM). Poly(NIPAM) is a thermally responsive polymer displaying a lower critical solution temperature(LCST) ca. 32 °C. Upon traversing the LCST of poly(NIPAM), the polymer becomes hydrophobic. Thus, the polymers were characterized by dynamic and static light scattering as a function of temperature. The linear and star polymers were found to aggregate above the LCST of poly(NIPAM). The aggregation behavior of the polymers was modeled by relationships defined for the area of the aggregate core per tethered chain (Ac) and the stretching ratio bDMA. The model equations predicted the final aggregate molecular weight and hydrodynamic radius reasonably well. Thus, the equations were used to rationally design a new series of 4, 6, and 8 arm star polymers that could form unimolecular core-shell nanoparticles (Xagg = 1).

R.H. Lambeth, S. Ramakrishnan, R. Mueller, J.P. Poziemski, G.S. Miguel, L.J. Markoski, C.F. Zukoski, J.S. Moore, Langmuir, 2006, submitted.

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Tailoring the Glass Transition Temperature of Polymer Nanocomposites

It is well known that an overlayer polymer will dewet a chemically identical brush layer, as long as Mgraft << Mmatrix, and when the brush grafting density, σ, follows σ2Nmatrix>>1 (here M denotes molecular weight, and N the chain length). Similar behavior has been postulated for polymer brushes on curved surfaces. In addition, since the glass transition of a planar thin polymer film is known to be affected by surface interactions, we hypothesized that dispersion of “hairy” nanoparticles into a polymer matrix will allow us to readily tune the thermomechanical properties of the resulting nanocomposite. Using silica nanoparticles with 110,000 molecular weight grafted polymer (graft density of .27 chains/nm2, we were able to control the wettability of the matrix by controlling its molecular weight and thus were able to tailor the glass transition temperature of polystyrene. Matrix polymers that wetted the nanoparticles showed an increase in glass transition temperature, and those that were nonwetting showed a decrease in glass transition temperature. This is the first study showing a direct correlation between wetting and changes in glass transition temperature and provides a new avenue for controlling polymer Tg.

A. Bansal, H. Yang, C. Li, B.C. Benicewicz, S.K. Kumar, L.S. Schadler, “Wetting behavior of polymers on surfaces of nanoparticles grafted with polymer brushes,” J. Polymer Science, Part B. Polymer Physics, 2006, in press.

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Colorimetric Sensing Based on Aptamer-directed Assembly of Nanoparticles

Simple and fast colorimetric sensing of a number of molecules such as cocaine and toxins is important for a number of applications such as homeland security, clinical testing and environmental monitoring. By taking advantage of recent advances in aptamer biology and nanotechnology, the NSEC researchers Juewen Liu and Yi Lu have developed a general method of sensing a number of molecules such as cocaine. Aptamers are DNA or RNA that can bind specifically to a target molecule. When two or more gold nanopartilces are linked by double stranded DNA “spiked” with the green DNA cocaine aptamers, they show strong blue color. If there is cocaine present, the cocaine aptamer binds cocaine and changes its conformation, resulting in release of the gold nanoparticles and thus distinct red color.

J. Liu and Y. Lu, Angew. Chem. Int. Ed 45, 2006, 90-94. Highlight Article: News and Views in Nature - M. Famulok and G. Mayer, "Aptamers in Nanoland," Nature 439, 2006, 666- 669.

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Nanotube-based Membranes with Tunable Compression-controlled Porosities

Ajayan and Dordick have developed filters with reversibly tunable pore sizes as the membrane is mechanically compressed and released. During the compression and release of the membrane, the pore size decreases monotonically and recovers reversibly, offering the possibility of using a single membrane that can be set to operate at different pore sizes by simply changing the compression level on the membrane. This adds a new dimension to recent demonstrations of nanotube membrane based filters, and allows for the development of a filter which can be conveniently preset at different pore sizes. Using these compliant pore-tunable nanotube membranes it was possible to efficiently separate a series of proteins with molecular weights ranging from 14-540 kD (ca. 5-20 nm). Mixtures of proteins were also separated in sequence by dynamically changing the compression applied to the membrane. In addition, the membranes, under set levels of compression, were used to entrap active enzymes, yet allowing for smaller substrates and products to pass through. In this fashion, the membranes act as valves for selective molecular transport and reaction. The unique and large stress modulated permeability observed here for the membranes is due to the remarkable structural and mechanical properties of the aligned nanotube architecture and can not be conceived with any other known material. A schematic of the tunable CNM filter at different compression levels (a – c) is shown at the right. The porous PTFE films with pore size of 0.2 μm are used to support the CNM (3.5 mm diameter, 0.3 mm thickness). The piston is mounted to a nut and can move back and forth to compress/release the CNM. (a) Without compression, both big and small particles can pass through the filter; (b) at certain compression big particles are retained due to reduced inter nanotube distance/pore size; (c) all particles are retained at high compression.

X. Li, G.u Zhu, J.S. Dordick, P.M. Ajayan Nature, 2006, submitted.

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Protein-driven Assembly of Single-wall Carbon Nanotubes at 2-D Interfaces

We have discovered that single-wall carbon nanotubes (SWNTs) can be directed to aqueous-organic interfaces with the aid of surfactants. This phenomenon can be used to transport adsorbed proteins from a bulk aqueous phase to an interface, thereby enabling the unique properties of nanomaterials to be exploited at an interface. In particular, SWNTs, by virtue of their high curvature, provide a significantly greater enhancement in the stability of adsorbed proteins in harsh environments, than microscale or macroscopic supports. As a result, the nanotube-mediated interfacial assembly of enzymes significantly enhances enzyme stability at the interface and increases the rate of interfacial biotransformations by over three orders of magnitude relative to that for native enzyme in the bulk aqueous phase. Furthermore, we have demonstrated that this concept can be extended to other nanomaterials, thereby providing a general strategy for performing highly efficient biphasic reactions. The ability to assemble SWNTs at interfaces may provide a route to organize these nanomaterials into 2-D architectures – another active area of research. The figure shows soybean peroxidase (SBP) adsorbed onto single-wall carbon nanotubes (SWNTs). The left image shows the SBP-SWNT conjugates suspended in aqueous buffer (left vial) and then at the interface upon addition of surfactant and hexane (right vial). The graph depicts specific reaction rates for SBP adsorbed onto various supports – SWNTs (circles), 20 nm hydrophobic silica nanoparticles (triangles), and nanoporous glass beads (squares) – at the interface. The error bars indicate the standard deviation of triplicate measurements.

P. Asuri, S. Karajanagi, J.S. Dordick, and R. Kane, “Directed Assembly of Carbon Nanotubes at Liquid-Liquid Interfaces: Nanoscale Conveyors for Interfacial Biocatalysis”, J. Am.Chem. Soc. 128, 2006, 1046-1047.

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Virtual Microscope is Recognized in Science

The Virtual Microscope partnership with a NASA funded project brings the power of a scanning electron microscope into a high school classroom for free. Its Java software allows students to move around samples such as a computer chip and an opal and to focus, change magnification and change contrast. The NSEC has contributed a library of five samples and provided some of the beta testing of this virtual instrument. This year the program has focused on improving the web tools for teachers and students, specifically annotation tools and the web site has recorded 2500 downloads per month in recent months. This achievement was recently highlighted in Science magazine. Visit http://virtual.itg.uiuc.edu/ for a demonstration.

Science 308 (5719), 2005, 173.

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ABB Corporation Begins the Commercialization Process

Based on a five-year effort of field grading materials, the NSEC has transferred field grading technology to ABB. This transfer of technology included biannual visits to RPI by ABB scientists and a three-week visit of an NSEC postdoctoral associate to ABB. The results of the precommercial trial created questions and in 2005/2006, those questions were addressed. As a result, ABB has started a gateway process for moving the material towards commercial use. If successful, this technology will have a significant impact on the lifetime of several components used in high voltage transmission and will also improve the processing methods currently in use. A set of joint patents with RPI and ABB are pending, and a licensing agreement between them was signed in the Fall of 2005.

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Structure and Properties of Nanoparticle Gels

The phase behavior, structure, and viscoelastic properties of high volume fraction nanoparticle-polymer suspensions have been systematically studied through combined experiment and theory in both the equilibrium fluid and nonequilibrium gel states for the first time. Depletion attraction driven gelation induces nanoparticle structural reorganization over many length scales, including the formation of dense, percolated mesoscale clusters. A novel microscopic statistical mechanical theory has been developed and shown to be in good agreement with experiment for both equilibrium collective nanoparticle structure over all length scales and the location of the gel boundary. The theory predicts power law dependences of the gel elastic modulus on attraction strength (polymer concentration), spatial range (polymer size), and nanoparticle volume fraction, which has been experimentally verified in a model system comprised of hard-sphere nanoparticles suspended in a non-adsorbing polymer solution.

Top Paper Award - A. Shah, Y. L. Chen, K. S. Schweizer, C. F. Zukoski, J. Phys. Condensed Matter 15, 4751 (2003).

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Direct Writing in Three Dimensions

Concentrated nanoparticle gels were developed for direct writing of complex 3-D structures, including space-filling solids and structures with high aspect ratio walls or spanning (unsupported) elements. These nanostructured inks were engineered to exhibit a well-controlled viscoelastic response necessary for flow during deposition, and subsequent shape retention of the as-deposited features even when they span gaps in the underlying layer(s). The high nanoparticle volume fraction minimizes drying-induced shrinkage enabling the assembly of 3-D nanostructured ceramics.

Q. Li and J. A. Lewis, Adv. Mat. 15 (19), 1639-43 (2003); Invited Cover Article - J. A. Lewis and G. M. Gratson, Materials Today 32-39 (July/August, 2004).


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Interfacially Tailored Fillers for Polymer Nanocomposites

Monodisperse polymer layers with spatially controlled chemistry were grown on the surfaces of nanoparticles at low and high graft density. These interfacial regions possessed higher molecular weight and lower polydispersity (< 1.2) than previously achieved by other synthetic approaches. This critical advance opens up new avenues for both tuning and studying interfacial-driven effects on polymer nanocomposite properties, as well as creating supramolecularly organized nanoparticle fillers with tailored optical and electronic behavior.


Cover Article - C. Li, B. C. Benicewicz, J. Polym. Sci.: Part A: Polym. Chem. 43, in press; C. Li, B. C. Benicewicz, Macromolecules, in review (2005).


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Nanoscale Curvature Influences Protein Structure and Function

The first in-depth analysis of the effect of nanomaterial size on protein structure and function has been performed, wherein the size and associated curvature of silica nanoparticles and single-wall carbon nanotubes (SWNTs) were found to strongly control the structure and resulting catalytic activity of several enzymes. In the case of lysozyme, as the silica particle size drops from 100 to 4 nm, the enzyme retains an increasing fraction of native alpha-helix content concomitant with an increasing native activity. In a related study performed under the denaturing conditions of organic solvents or at 95oC, near native enzyme activity is retained with soybean peroxidase bound to SWNTs; stabilization is not achieved on flat surfaces (e.g., highly oriented pyrolytic graphite (HOPG)). A novel hypothesis has thus been advanced, wherein the unique curvature of nanomaterials that are on a size scale similar to biological molecules, strongly stabilizes protein structure and function, particularly under denaturing conditions. This curvature appears to disfavor lateral protein-protein interactions, which often cause microaggregation and deactivation.

A. A. Vertegel, R. W. Siegel, and J. S. Dordick, Langmuir 20, 6800-6807 (2004). Cover Article; P. Asuri, S. Karajanagi, R. S. Kane, and J. S. Dordick, Nat. Biotechnol ., submitted (2005).

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DNAzyme-Catalyzed Assembly and Sensing

Genetic control of stimuli-response assembly and disassembly of nanoparticles and carbon nanotubes at ambient conditions has been demonstrated using analyte-specific DNAzymes. For chemical sensing, DNAzyme-nanoparticle hybrids have been transformed into simple, highly sensitive and selective colorimetric sensors for a broad range of analytes including metal ions and organic molecules, with tunable detection range. For selected assembly, the activity and turnover of catalytic DNA bound to MWNTs. This has application in sensor development and in the generation of responsive materials.

Yi Lu, J. Am. Chem. Soc. 125, 6642-6643 (2003).

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Enzyme-Nanomaterial-Polymer Composites as Antifouling Surfaces

The molecular level interactions that govern the structure, function, and stability of proteins on the surface of nanoscale materials is being elucidated through experimental and computational strategies. This information has been used to assemble functional nanobiocomposites. In one example, enzyme-nanomaterial-polymer composites have been prepared and are endowed with surface active properties that completely resist protein fouling (Dordick, Kane). This is important as the initial attachment of proteins to surfaces represents the key event in the binding of microorganisms, which ultimately lead to the formation of intractable biofilms. The enzyme-nanomaterial-polymer composites were prepared with the proteolytic enzyme subtilisin (commonly found in laundry detergents) and trypsin, both conjugated to single-walled carbon nanotubes and embedded within a poly(methyl methacrylate) polymer. Challenging the composite with a blood serum protein resulted in the complete resistance of the polymer surface to the protein. For polymer without the enzyme-nanotube conjugate, high protein binding was observed. Microbial and biological molecule adhesion to surfaces is a problem that impacts industrial, medical, and security applications. Retarding the binding of these agents onto surfaces will extend the lifetime of a material, eliminate toxic compounds (e.g., chemical and biological threats), prevent spread of disease, and enable medical and implantable materials to be more benign.

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As we enter the 21st century, a truly grand challenge facing the materials research community is to educate and excite he general public about our scientific world, and help create a science literate world. In response to this challenge we created the Molecularium® (Schadler, Garde, Siegel). The Molecularium™ is a digital dome theater (like a planetarium), but instead of taking the audience on a ride into the stars, the Molecularium® ship takes the audience on a ride into the world of atoms and molecules. This fantastical ship can shrink to molecular sizes and move as fast as the speed of light. To achieve scientific accuracy we have merged advanced scientific computation with state-of-the-art digital animation, providing scientifically correct molecular animation. The first Molecularium® show, “Riding Snow Flakes,” premiered at the Children’s Museum of Science Technology in Troy, NY on February 4th, 2005. Soon the Houston Museum of Science will be a beta test the show. In this musical comedy aimed at K-5, Oxy, Hydro, and Hydra navigate the ship through the clouds, where they travel in a snowflake, watch it melt, and feel and observe the wind blowing past them. Musical segments emphasize the main message that everything is made of atoms and molecules. Children also learn about the three states of matter (solid, liquid, and gas) and take a ride along a polymer molecule. These concepts are in agreement with typical state science based outcomes for K-5 students, but are taught such that all ages will learn. The Molecularium® show is accompanied by both classroom preparation materials for teachers and hands-on activities for the students. We are also actively assessing the learning of both school groups and the general public as they participate in the Molecularium® program. Preliminary assessment shows significant learning benefits.

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Unified Relationship between Polymer Nanocomposite and Thin Film Thermomechanical Behavior

Recent computer simulations have suggested that the thermomechanical behavior of polymer nanocomposites should be akin to polymer thin films. For the first time, this hypothesis has been experimentally verified by studying the Tg and viscoelastic behavior of silica/polystyrene (PS) nanocomposites comprised of fillers with either bare surfaces or grafted PS layer, which behave like "free" or wetting surfaces, respectively. These data strongly suggest that there is a unified relationship between the polymer nanocomposite and thin film behavior associated with an effective confinement distance.

A. Bansal, H. Yang, C. Li, K. Cho, B. C. Benicewicz, S. K. Kumar, L. S. Schadler, Nature Materials, in review; A. Bansal, H. Yang, C. Li, B. C. Benicewicz, S. K. Kumar, L. S. Schadler, Physical Review Letters, submitted (2005).

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Biomimetic Templating

The organization of Cd2+ ions within the interhelical pores between DNA strands yielded DNA-membrane complexes, which upon subsequent reaction with H2S formed CdS nanorods of controllable widths and crystallographic orientation. This biomimetic strategy, inspired by processes such as bone formation, represents an unprecedented level of control over the crystallization of nanoparticles. These have potential in the highly selective templating of biological and nonbiological materials into precise geometries and function.

H. Liang, T. Angelini, J. Ho, P. V. Braun, and G. C. L. Wong, J. Am. Chem. Soc. 125, 11786-11787 (2003).

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Understanding Protein-Nanomaterial Interactions

The molecular level interactions that govern the structure, function, and stability of proteins on the surface of nanoscale materials is being elucidated through experimental and computational strategies. This information has been used to assemble functional nanobiocomposites. This has resulted in a simple route to active and water-soluble SWNT-protein conjugates in a single step. These solubilized nanotube-protein complexes may have application in the selective assembly of nanotubes for biological, electronic, and materials applications.

S. S. Karajanagi, A. A. Vertegel, R. S. Kane, and J. S. (2004), Langmuir 20, 11594-11599 (2004); L. Yang, J. S. Dordick, and S. Garde, Biophys. J . 87, 812-821 (2004).

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Molecularium®: "Riding Snowflakes" is a 23-minute digital dome theater movie, which is a magical, musical adventure into the world of molecules. The show teaches viewers that "everything is made of atoms and molecules" and about the 3 states of matter, solids, liquids, and gas. It opened on February 4, 2005 at the Children's Museum of Science and Technology in Troy, NY to a K-99 audience. The museum has had sold out shows ever since. The show is now being distributed around the U.S. and worldwide. Beta test sites have also been established to gather assessment data.

Visit www.molecularium.com for more information about the Molecularium® project.

S. Garde, L. S. Schadler, and R. W. Siegel, Mater. Res. Soc. Bulletin 30, 132-133 (2005).

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DzymeTech Startup Based on NSEC Technology

A startup company called DzymeTech has recently begun the steps required to commercialize technology for highly sensitive and selective colorimetric sensors for a broad range of molecules including Pb2+ and adenosine. This technology was developed in professor Yi Lu 's laboratory at UIUC. The sensors make it possible for these small and mobile detectors to provide real-time detection in a variety of environments including households, factories, and for homeland security.

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Nanocomposite Science and Technology

This resource book contains the background on this emerging technology, the underlying science and the future, particularly from the perspective of applications. Topics include nanocomposites based on inorganic materials (metals and ceramics) and their applications, polymer-based and polymer-filled nanocomposites with an emphasis on interface engineering for optimum performance, naturally occurring systems of nanocomposites and their lessons for engineering, and a final chapter on nanocomposite modeling by P. Keblinski. The first printing sold out and Wiley-VCH printed a second run in spring 2005. In addition, they have asked for a second edition, which will be written in 2006.

Nanocomposite Science and Technology, P.M. Ajayan, P. Braun, and L.S. Schadler, Wiley-VCH Verlag, Weinham, 2003.

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Nanoparticle Control of Polymer Supermolecular Morphology

A collaborative effort between L.S. Schadler, R.W. Siegel, Y. Akpalu (RPI), ABB, and Albany International focuses on using nanoparticles to control the supermolecular morphology of semicrystalline polymers and their properties. Figure 25 shows the effect of 20 nm diameter TiO2 nanoparticles dried or coated with N-(2-aminoethyl)3-aminopropyl-trimethoxysilane (AEAPS) on low-density polyethylene (LDPE). There is no change in unit cell dimension, degree of crystallinity, average lamellar thickness, or average spherulite size. The supermolecular structure, however, is impacted. Neat LDPE and the dried sample exhibit a well-defined, impinging, banded spherulite structure. The nanoparticles are embedded between the lamellae. In great contrast, no well-developed banded spherulites are observed in the AEPS sample, in which nanoparticles segregate to interspherulitic regions. This supermolecular structure is critical in controlling electrical breakdown strength.

The effect of nanoparticles on polyethylene terephthalate (PET) is dramatically different. The kinetics of crystallization, lamellar thickness, degree of crystallinity and supermolecular structure are all altered upon the addition of nanoparticles (Figure 26). Nanoparticle concentrations of 2 wt% and 3 wt% suppress crystallization kinetics while higher loadings accelerate crystal formation. This behavior sharply contradicts reported observations on microscale fillers. This altered morphology led to a significant improvement in PET wear properties.

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Nanoscale Biomineralization and Templating

A wide range of biomineralization and templating methods exist for organizing inorganic materials at a wide range of length-scales (Wong). In our recent NSEC work in collaboration with P. V. Braun, we show that crystallographic control of the inorganic nanostructures is possible using synthetic biomolecular templates comprised of anionic DNA and cationic membranes, which self-assemble into a multi-lamellar structure where a periodic one dimensional (1-D) lattice of parallel DNA chains is confined between stacked two dimensional (2-D) lipid sheets (see Figure 27). We have organized Cd2+ ions within the interhelical pores between DNA strands, and subsequently reacted them with H2S to form CdS nanorods of controllable widths and
crystallographic orientation.

The strong electrostatic interactions align the templated CdS (002) polar planes parallel to the negatively charged sugar-phosphate DNA backbone, which indicates that molecular details of the DNA molecule are imprinted onto the inorganic crystal structure. The resultant nanorods have (002) planes tilted by 60° with respect to the rod axis, in contrast to all known II-IV semiconductor nanorods. This biomimetic strategy, inspired by processes such as bone formation, represents an unprecedented level of control over the crystallization of nanoparticles. The work was published as a JACS Communication, and has been subsequently featured in Chemical and Engineering News in October 2003, and again in December 2003 as a ‘2003 Chemistry Highlight’ (see Figure 28).

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Biocatalytic Synthesis of Sugar-Based Gelators

Low molecular weight, biodegradable, and biocompatible trehalose esters have been
synthesized (Dordick) using lipase catalysis in acetone along with a suitable ester donor (see Figure 29). The resulting perfectly symmetrical structures are highly uniform trehalose 6,6’- diesters with ester sizes ranging from acetate to stearate (C2-C18). When placed in suitable solvents, such as alkanes, alcohols, acetone, etc., the sugar diesters assemble into highly uniform nanofibers (d ~ 50 nm). Interestingly, chemical synthesis of trehalose diesters results in a mixture of isomeric products, which do not form uniform nanofibers and have poor gelation qualities. Thus, the extraordinary selectivity of enzymatic catalysis results in highly uniform and functional
gelator materials. We envision applications of these multiscale porous materials as scaffolds for tissue engineering and in selectively permeable membranes that are highly hydrophilic and resistant to biofouling.

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Carbon Nanotube Based Gas Sensor Based on MWNT Arrays

Figures 30 and 31 show recent collaborative work in the NSEC (Ajayan, Koratkar) at RPI. The idea of gas sensing here is based on an aligned multiwalled carbon nanotube (MWNT) array electrode. Gases break down at specific voltages, but conventional breakdown sensors have bulky architectures since very high voltages are needed for the breakdown of most common gases. Here the nanotubes concentrate the electric field at their tiny tips and hence bring down (several fold, for example, from ~1000 volts for planar metal electrodes to ~100 volts for nanotube electrode, for a set electrode separation) the value of the applied voltage needed for breakdown. The use of nanotube electrodes could ultimately lead to the fabrication of small portable breakdown gas sensing devices.

A. Modi et al., Nature 424, 171-174, 2003.

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Self-Cleaning Enzyme-Nanotube- Polymer Composite

One interesting discovery from the NSEC project (Dordick) involves the formation of self-cleaning (and potentially self-healing) coatings. The figure depicts such a material,
where enzyme attached to nanotubes (carbon or other) is placed in a simple polymer (e.g., PMMA) and then this material acts as a coating or film endowed with biocatalytic
activities. Foreign objects (e.g., bacterial cells) that come in contact with the coating are destroyed by the action of specific enzymes in the coating. This material may prove useful as a coating to resist and destroy pathogens that come in contact with a surface coated with the bionanocomposite (e.g., textiles and fabrics, machinery, etc.).

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Nanotube-Based Molecular Junctions

The figure shows recent collaborative work of Ajayan with colleagues in Germany, Mexico, the U.K., and Belgium. The idea of connecting individual seamless nanotubes by welding was proposed for the first time. Electron irradiation and heat were used to form the welded junctions of single-walled carbon nanotubes, as shown in the figure. These first-time experiments demonstrate that single-walled nanotubes can be welded together, suggesting controlled fabrication of molecular circuits and three dimensional nanotube networks.

M. Terrones, F. Banhart, N. Grobert, J. C. Charlier, H. Terrones and P. M. Ajayan, Physical Review Letters 89, 75505, 2002.

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Carbon Nanotube Osmotic Membranes

Using molecular dynamics simulations to study the osmotic flow of water through a carbon nanotube membrane (main panel), we (Garde) have discovered that during the process of squeezing out of water the nanotube membrane moves in molecular size jumps, and exhibits large resistance for the removal of the last molecular water layer. Furthermore, the flow rate for short and long nanotubes is very high, independent of tube length, and can be described quantitatively by a random walk model. These large-scale simulations demonstrate the possibility of using nanotube arrays as novel nanofluidic devices. Future studies will explore whether the incorporation of “switches”, based on functionalizing tube ends or electrostatic charging, will allow for the control of selective flow of specific species across nanotube membranes.

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Structure and Depletion Forces in Polymer Nanocomposites

Molecular dynamics simulation (Keblinski) and statistical mechanical theory (Schweizer) have been employed to study pair correlations, thermodynamics, and depletion forces between two nanoparticles as a function of particle size, polymer degree of polymerization and concentration, and local chain stiffness under athermal (purely repulsive) conditions. This knowledge provides a starting point for manipulation of polymer-mediated interactions between nanoparticles, and control of nanoparticle dispersion. The figure shows depletion forces for particles and polymers of comparable sizes as a function of nanoparticle separation for dense concentrated polymer solutions and melts. At melt-like density, strongly oscillatory forces are found with a large repulsive barrier corresponding to a segment thick layer of polymer separating the nanoparticles. The excellent agreement between theory and simulation demonstrates the accuracy of the former, and provides a foundation for treating many-particle problems.

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Hydrophobically-Linked Hybrid Nanounits

The schematic sketch illustrates the hydrophobic attachment of alkyl-thiol-capped gold nanoclusters onto acetone-activated MWNTs through molecular interdigitation. The nanoclusters are also interlinked to each other through a similar mechanism. This was a first-time demonstration of building hybridmolecular building blocks using 0-D and 1-D units. Such hyrophobically linked units and structures are compatible with many biological systems and will be important for creating biomolecular nanostructures and devices.

A.V. Ellis, K. Vijayamohanan, R. Goswami, N. Chakrapani, L. S. Ramanathan, P. M. Ajayan, and G. Ramanath, Nano Letters 3, 279-282, 2003.

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Templateless Room-temperature Assembly of Nanowired Networks

The figure schematically illustrates the self-assembly of nanowired networks from gold nanoclusters at room temperature through processing in biphasic liquid mixtures (Ramanath). The diameter of the nanowires can be controlled precisely by merely tailoring the size of the nanocluster building blocks. The extent of coalescence and networking can be controlled by appropriate choice of liquid components as shown by the two example transmission electron micrographs. Ongoing work is focusing on leveraging this technique for assembling nanowired networks of different materials in aligned forms.

A.V. Ellis, K. Vijayamohanan, R. Goswami, N. Chakrapani, L. S. Ramanathan, P. M. Ajayan, and G. Ramanath, Nano Letters 3, 279-282, 2003.

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