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)).

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)).

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)).

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 T_g 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)).

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 95^oC, 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)).

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)).

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)).

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)).

Molecularium™

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. (S. Garde, L. S. Schadler, and R. W. Siegel, Mater. Res. Soc. Bulletin 30, 132-133 (2005)). See www.molecularium.com for more information about the Molecularium™ project.