Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A4: Particle Self Assembly |
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Sponsoring Units: DPOLY Chair: Maria Santore, University of Massachusetts Room: Baltimore Convention Center 308 |
Monday, March 13, 2006 8:00AM - 8:36AM |
A4.00001: DNA-Mediated Colloidal Crystallization, Interactions and Dynamics Invited Speaker: DNA has emerged as a powerful and versatile tool for nanoscale self-assembly. Several researchers have assembled nanoparticles and colloids into a variety of structures using the sequence specific binding properties of DNA. Until recently, however, all of the reported structures were disordered, even in systems where ordered colloidal crystals might be expected. We detail the experimental approach and surface preparation that we used to form the first DNA-mediated colloidal crystals, using polystyrene microspheres. We also report the first direct measurements of such DNA-induced interactions between such micron-sized particles. The interactions measured with our optical tweezer method can be modeled in detail by well-known statistical physics and chemistry, boding well for their further application to directed self-assembly. The microspheres’ reversible adhesion dynamics have an unexpected power-law scaling, which we hypothesize is due to the non-exponential kinetics of DNA hybridization process itself. [Preview Abstract] |
Monday, March 13, 2006 8:36AM - 9:12AM |
A4.00002: Assembly and Gelation of Four-Armed DNA Dendrimers Invited Speaker: The disordered-arrested state of matter is ubiquitous in existing materials like glasses, and is prevalent in many new materials, due to the uncommon mechanical, thermal and electrical properties. The ``bottom-up'' construction of new materials is one of the central aims of nanotechnology. DNA is potentially an optimal choice for the construction of three-dimensional supramolecular assemblies since it can self-assemble into long and fairly rigid helices, based simply on sequence complementarity. We numerically study a model designed to mimic the behavior of recently synthesized single-stranded DNA dendrimers. Complementarity of the base sequences of different strands results in the formation of strong cooperative intermolecular links with a valency controlled by the number of strands. We simulate the bulk behavior of a system containing many 4-armed DNA dendrimers and find that in an extremely narrow temperature range the system forms a large-scale, low-density network via a thermo-reversible gel transition. The sharpness of the crossover the gel state can be controlled by the length of the DNA strands, since longer strands will form cooperative bonds over a narrower temperature range. As a result, the percolation temperature of the network formed by the dendrimers can be made arbitrarily close to the gel transition by tuning the length of DNA strands. This is in contrast with recent model systems designed to understand thermo-reversible gelation. Given that gelation and percolation coincide in irreversible chemical gels, this system provides an excellent model material to bridge the understanding between reversible and chemical gels. [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:48AM |
A4.00003: Directing Colloidal Particle Organization Using Soft Lithography and Polyelectrolyte Assembly Invited Speaker: The use of electrostatic interactions to guide colloidal assembly can be used to achieve an elegant level of control on the ordering of particles at surfaces. We have utilized a combination of self-assembled monolayers and electrostatic layer-by-layer assembled thin films as templates for the directed deposition of colloidal particles. In this work, it is also of interest to manipulate two or more colloid components on a surface using attractive and/or repulsive interactions. This work has since led to an investigation into other interactions that can be used to guide colloid assembly. By extending our understanding of interactions between polyelectrolytes and different forms of functional surfaces, we have been able to guide polyelectrolyte coated colloids to different surface regions based on hydrophobic or hydrogen bonding interactions as well as charge attraction. Recent success has been obtained in our group on the use of DNA acid/base pairs to direct the deposition of polyion functionalized colloids to specific regions of surfaces. In this work, the strong and highly specific hydrogen bonding interactions that take place between poly(phosphonic acids) is being pursued. New developments have included the formation of hydrogen bonded Janus particles -- colloids that have been functionalized with a different polybase on either side of the particle. Finally, we have also begun to use a range of soft lithographic methods, combined with polyelectrolyte multilayer assembly on nanometer scale patterns to direct nanoparticles into patterned arrays. The interactions involved in this work, guiding principles, as well as the interplay between hard boundaries and surface chemistry in template surfaces, will be discussed, and potential applications and new collaborations will be addressed in display, microfluidic and biological applications. [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:24AM |
A4.00004: Symmetry, Equivalence and Self-Assembly Invited Speaker: Molecular self-assembly at equilibrium is central to the formation of many biological structures and the emulation of this process through the creation of synthetic counterparts offers great promise for nanofabrication. The central problems in this field are an understanding of how the symmetry of the interacting particles encodes the geometrical structure of the organized structure and the nature of the thermodynamic transitions involved. Our approach is inspired by the self-assembly of actin, tubulin and icosahedral structures of plant and animal viruses. We observe chain, membrane,`nanotube' and hollow icosahedron structures using `equivalent' particles exhibiting an interplay between directional (dipolar and multi-polar) interactions and short-range (van der Waals) interactions. Specifically, a dipolar potential (continuous rotational symmetry) gives rise to chain formation, while potentials having discrete rotational symmetries (e.g., square quadrupole or triangular ring of dipoles) led to the self-organization of nanotube and icosahedral structures with some resemblance to tubulin and icosahedral viruses. The simulations are compared to theoretical models of molecular self-assembly, especially in the case of dipolar fluids where the corresponding analytic theory of equilibrium polymerization is well developed. These computations give insights into the design elements required for the development of synthetic systems exhibiting this type of organization. [Preview Abstract] |
Monday, March 13, 2006 10:24AM - 11:00AM |
A4.00005: Structured Nanocomposites: Organization of Particles Templated in Self-Assembled PEO-PPO-PEO Mesophases Invited Speaker: The design of materials with tailored properties and function requires control over their structure in the nanometer scale. It is advantageous to many engineering applications that this is carried out using self-assembly processes that occur at reasonable concentrations ($\sim $10$^{20}$ particles/L) and short timescales (ns -- $\mu $s). In this work, we have developed a novel approach using an ordered template to control the nanoscale structure of materials that would not otherwise order in solution. We use close-packed cubic and cylindrical mesophases of a thermoreversible block copolymer (PEO-PPO-PEO) to impart spatial order on dispersed nanoparticles. The thermoreversible nature of the template allows for the dispersion of particles synthesized outside the template. This feature extends the applicability of this templating method to many particle-polymer systems and also permits a systematic evaluation of the impact of design parameters on the structure and mechanical properties of the nanocomposites. The approach is extremely robust and we have successfully templated solutions of silica and gold inorganic nanoparticles as well as a series of proteins, which act as organic nanoparticles in our system. The influence of relative size (particle to template sites), relative concentration, temperature and shear are experimentally determined using small angle neutron scattering (SANS) and rheology. SANS with contrast variation is used to characterize the structure of the polymer mesophase and the templated particles in a nanocomposite independently. SANS experiments also demonstrate that shear can be used to align the nanocomposites into single-crystal macro-domains; the first demonstration of the formation of single-crystal nanoparticle superlattices. The outcome of this work serves as a basis for designing new soft nanocomposite materials. [Preview Abstract] |
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