AmericanChemicalSociety.com

1. Project Overview:

<>Nanoscale materials are generating revolutionary breakthroughs in energy storage and transport materials such as high capacity lithium ion batteries and organic photovoltaic devices. These materials achieve their enhanced performance through increased ability to withstand mechanical stresses and increased interfacial surface area. However, these structures are generally sparse arrays of nanowires with a fill factor per unit volume of roughly 10-15%, which greatly limits the theoretical capacity per unit surface area of electrode. The ideal nano-architecture to maximize the theoretical capacity is unknown. Furthermore, the experimental strategies to synthesize and characterize a range of nanostructures are currently limited.  By creating a flexible platform to systematically study the importance of nano-architecture on electrode capacity, we aim to elucidate the optimum nanostructure for energy storage and transport applications. Our approach utilizes the unique ability of nature to self-assemble an amazing variety of three-dimensional nanostructures from a single protein biotemplate, clathrin. We have used this self-assembling protein to create three-dimensional porous nanostructures with spherical and pyramidal geometries. These protein nanostructures are then site-specifically functionalized using non-covalent peptide-protein interactions. Through careful design of the peptide sequence, we have demonstrated scaffold functionalization to enable three different templating reactions: the synthesis of titanium dioxide, cobalt oxide, and gold nanoparticles, all at room temperature and pressure.

2. Scientific Progress:

2-1. Purification and Self-Assembly of Clathrin Protein Nanostructures.

We have developed protocols to isolate significant quantities of clathrin from a natural source, bovine brain, since clathrin is not synthetically available. Using this material, we are able to form three-dimensional clathrin nanostructures with distinct geometries by dialyzing clathrin into buffers of varying ionic strength and pH. Transmission electron microscopy (TEM) analysis shows that higher salt concentrations (50-100 mM) generate spherical cages while lower salt concentrations (~2 mM) promote formation of pyramidal shapes. Preliminary dynamic light scattering results suggest that assembly is a dynamic process where clathrin molecules come together and disassemble over time before reaching a final assembled structure.

2-2. Development of the TEThER Peptide Strategy.

We have designed peptides to mimic nature's strategy of using binding partners to customize protein function. We call our strategy Template Engineering Through Epitope Recognition (TEThER). TEThER peptides are bi-functional and serve to functionalize clathrin molecules for interaction with inorganic materials. An amino acid sequence known to bind to a specific site on clathrin is fused upstream of sequences that have been shown to interact with one of three materials: titanium dioxide, cobalt oxide, and gold. By using these peptides, we are able to engineer distinct site-specific functions into the clathrin assemblies without chemical or genetic alterations.

2-3. Synthesis and Characterization of Titanium Dioxide Nanoparticles.

Spherical clathrin cage assemblies were covalently crosslinked with paraformaldehyde before incubation with the titanium dioxide-nucleating TEThER peptide and addition of titanium (IV) bis(ammonium lactato) dihydroxide (TiBALDH). High resolution TEM, diffraction pattern analysis, and energy dispersive spectroscopy (EDS) show that we have generated single crystalline nanoparticles of anatase titanium dioxide with an average diameter of 160 nm at ambient conditions. Anatase, the form of titanium dioxide favored for use in dye-sensitized solar cells, is typically generated in high temperature processes that result in nanoparticles <10 nm. It is possible that these larger particles of anatase will have useful transport and reactive properties distinct from the smaller particles.

2-4. Synthesis and Characterization of Cobalt Oxide Nanoparticles.

By replacing the titanium dioxide-nucleating TEThER peptide with one that nucleates cobalt oxide and adding cobalt chloride to the functionalized clathrin cages in the presence of a reducing agent (sodium borohydride), we synthesized cobalt oxide nanoparticles. These particles are amorphous and have an average diameter of 55 nm. The TEThER peptides exhibit distinct function; using the titanium dioxide-nucleating sequence in these cobalt oxide conditions did not result in nanoparticle formation.

2-5. Synthesis and Characterization of Gold Nanoparticles.

We have also demonstrated growth of noble metal nanoparticles by incubating the clathrin cages with a TEThER peptide that nucleates gold then adding chloroauric acid and letting the reaction occur in the dark. TEM and diffraction pattern analysis showed that nanoparticles generated by this route are spherical and polycrystalline and have an average diameter of 20 nm. Interestingly, clathrin cages without gold-nucleating TEThER peptide also resulted in growth of gold nanoparticles in these conditions, but the particles have irregular morphology and an average diameter of 200 nm, suggesting that the TEThER peptide may be playing the role of a capping agent in this case.

3. Trainee Stipend and Conference Travel Support:

Stipend support of 50% was provided to one postdoctoral scholar, Todd Ostomel, Ph.D., for five months. In addition, one Ph.D. graduate student, Alia Schoen, participated in the project and was supported through a university fellowship. Through support of ACS-PRF funds, Alia Schoen attended the AVS Conference, Materials Research Society Spring Meeting, and the American Chemical Society National Meeting to present her research results. This resulted in publication of two peer-reviewed symposium proceedings.