ACS PRF | ACS
All e-Annual Reports
43408-G7
Growth of Inorganic Nanoparticles in Patterned Arrays Using Biological Templates
The ability to fabricate assemblies comprising small (1-10 nm) nanoparticles into arrays of predetermined spacing and arrangement, while of fundamental interest and importance, is not readily accomplished. Evidence from larger nanoparticles suggests that array assembly offers a new avenue to obtain novel catalytic properties. However, it is unknown how the spatial characteristics of arrays consisting of particles small enough to exhibit nanosize effects will influence their catalytic features.
Nature has perfected nanostructures that are precise and highly organized, and our current strategy exploits a self-assembling biological system as templates to direct the organization of inorganic nanoparticle arrays. In this work, we are investigating the assembly of 1-5 nm particles into different lattice configurations using the streptavidin protein system as a template, with a vision for enabling future studies in nanoscale catalysis. Streptavidin is known to self-assemble at the air-water and liquid-solid interface.
We aimed to manipulate protein array configuration on a solid substrate. In doing so, we established protocols in which we can reliably deposit a lipid bilayer on mica using Langmuir-Blodgett deposition, and subsequently grow a single monolayer of highly ordered domains after incubation with this substrate. We can then track macroscopic crystal growth and molecular assembly with fluorescence microscopy and atomic force microscopy, respectively.
We have demonstrated that in general, the results for the liquid-solid interface are similar and consistent with those at the air-water interface, with a few notable differences. Analogous to crystallization at the air-water interface, both forms of streptavidin yield H-like domains with lattice parameters that have C222 symmetry at pH 7. At pH 4, the native, truncated form of streptavidin yields needle-like domains consisting of molecules arranged in P1 symmetry. Unlike crystalline domains grown at the air-water interface, however, the lattice parameters of this P1 crystal are unique had not yet been reported. The presence of a solid substrate does not appear to dramatically alter streptavidin's two-dimensional crystallization behavior, suggesting that local intermolecular interactions between proteins are more significant than interactions between the interface and protein. Our results also demonstrate that screening the electrostatic repulsion between protein molecules by modulating ionic strength will increase growth rate while decreasing crystalline domain size and macroscopic defects.
Ligand-inhibition and fluorescence recovery after photobleaching were then used to elucidate the concentration-dependent mechanism for the divergent crystal forms. We have measured the diffusion coefficient of molecules in P1-forming conditions to be approximately twice that of molecules in C222-forming concentrations, which is consistent with proteins which are bound to the surface through one and two ligands, respectively. The differential flexibility associated with the binding state is therefore likely to alter the subtle protein interactions involved in crystallization.
Finally, we have shown that these domains are indeed functional by attaching biotinylated gold nanoparticles to the crystals. The ability to modulate molecular configuration, crystalline defects, and domain size on a functional array supports the potential application of this system toward the assembly of catalytic materials.
This work has had a significant impact on the careers of the PI and participating students. PRF funding has supported the research of two graduate students and one undergraduate student, one of whom is an underrepresented group in science and engineering. It has also resulted in one M.S. thesis and an undergraduate senior honors thesis. These students made significant progress in establishing the experimental system and protocols that will be the foundation of the PI's future investigations in the area of biologically-templated hybrid materials. This grant has also supported the publication of two manuscripts and a presentation at a national conference.
