Self-assembly in hybrid organic/inorganic nanostructured materials
Self-assembly is a promising route for potentially designing new nanoparticle-based materials and devices with novel properties. The motivation for this concept is based on the notion that biomimetic principles can be utilized to precisely organize the particles into ordered structures. However, knowledge of the processes that occur during self-assembly is required to fabricate these materials and devices. Because the significant length and time scales of these processes are difficult to gain access to experimentally, a multiscale modeling and simulation approach is useful for investigating materials assembly. This approach can also be used to verify the reliability of different computational approaches and to calculate properties that can be verified experimentally.
Here I present computer simulation results for the self-assembly of nanoscale cubic polyhedral oligomeric silsesquioxane (POSS) molecules functionalized with polymeric tethers. The development of coarse-grained models and interaction potentials, on the basis of insights from quantum mechanical calculations and explicit atom simulations, for studying polymer-tethered POSS assemblies will be discussed. The mesoscale particle simulations predict self-assembly of the molecules into bulk structures similar to those found in coil-coil block copolymer and surfactant systems. The influence of various thermophysical and material parameters on self-assembly, including temperature, concentration, nanoparticle shape, polymer tether composition, and the number of tether substituents, is examined. This work is a collaborative effort with Sharon Glotzer and John Kieffer (University of Michigan), Matthew Neurock (University of Virginia), and Clare McCabe and Peter Cummings (Vanderbilt University).
Structure-function properties of bacterial pore-forming toxin proteins
Recent bioterrorism acts emphasize the need to gain a fundamental understanding of bacterial pathogenicity. Although advances have been made toward elucidating the structure of several anthrax toxins, the molecular mechanism(s) of action are not completely understood. It is currently believed that late stage anthrax infection is caused by the transport of toxic proteins secreted by the bacterium Bacillus anthracis through an ion channel formed by a third protein that is secreted by the organism. This hypothesis can be tested, in part, by precise measurements and theoretical predictions of the ionic current flowing through single anthrax channels. Because the crystallographic structure of the nanopore is not yet available, knowledge of the pore's thermophysical and electrophysiological properties is important for validating and improving existing models of channel structure. This information will aid the development of more effective therapeutic agents against anthrax, and may also provide the aid the sensitive and selective detection of anthtrax toxins using single nanopores.
To test the structural model for the anthrax nanopore, the single channel current-voltage relationship is measured in the presence of monovalent electrolyte solutions and under a wide range of electrolyte concentrations and pH values. Advanced statistical signal processing algorithms are applied to the stochastic fluctuations in the single-channel ionic current time series to identify the number, value and lifetimes of all the conductance states. Insights from these experimental and theoretical measurements on pore structural behavior will be discussed. This work is being performed in collaboration with John Kasianowicz and Vincent Stanford (NIST), Tam Nguyen (SAIC-NCI), and Maria Kurnikova (Carnegie Mellon University).