Biomaterials built by biology: Mechanism and applications of hyperbranched fractal plasmonic structures

PART 1: NON-TECHNICAL SUMMARY The behavior of common metals changes dramatically as the size of the metal decreases. The color and electrical properties of metals like gold and silver change dramatically as they get smaller. People have used the color changes and electrical properties of ultrasmall metal clusters to make medical devices, environmental sensors, and electronic components. However, it is very difficult to control matter at very small size regimes. We recently showed how tiny protein fragments can actually cause metal clusters to assemble into larger wire-like structures. This is a key step to building larger structures that people can use. Our goal in this research is to use protein fragments to create functional devices. We will incubate these short protein fragments with small metallic clusters. Our preliminary data showed that the protein acts as a guide and can cause the metallic clusters to self-assemble into remarkably complex structures of an intermediate size (bigger than the source clusters but smaller than bulk metal). That is, we are using the protein guides to assemble tiny clusters of metals into wires. After doing controlled experiments to understand the mechanism, we will determine the limits of this technique, i.e., we will determine how big we can make the wires. We will then use the resulting wires to make a sensor that can quickly diagnose and discriminate between different respiratory viruses. Finally, we will use the color changes resulting from the sizes changes to train the next-generation of materials scientists. PART 2: TECHNICAL SUMMARY Hierarchical plasmonic biomaterials are interesting because they are useful. These biomaterials are built from small nanoparticles that combine into larger micron-sized structures. As a result of their unique structure, hierarchical plasmonic biomaterials have tunable optical, electronic, and structural properties. These properties can then be harnessed for medical devices, environmental sensors, electronic components, etc. Thus, the long-term goal of this research is to develop theory and applications related to biologically-constructed hierarchical nanoparticles. More specifically, our preliminary work uses diffusion-limited aggregation in the presence of defined peptide sequences to build hierarchical silver biomaterials with a fractal snowflake-like design (see figures below). These fractal systems were constructed from ~30 nm silver nanospheres incubated with certain short peptides (5 – 20 residues). We now seek funds to conduct additional studies to derive a coherent mechanistic understanding of how the peptide length, amino acid sequence, and nanoparticle surface so dramatically impacts biomaterial morphology. We hypothesize that the hierarchical assembly is driven by bridging interactions between the nanoparticle surface the peptide scaffold. To test this hypothesis, Objective #1 will study the structure/activity relationship of the peptide length and charge: We will monitor nanoparticle morphology as a function of peptide design. Objective #2 will perturb the nanoparticle surface chemistry to elucidate the role that surface ligands have on particle assembly into hierarchical structures. Objective #3 will build even larger biomaterials by combining multiple self-assemblies face-to-face—this work will lead to customized wires and lattices based on DNA-guided assembly. Objective #4 will deploy these insights for a practical sensor that discriminates between respiratory diseases via a simple color change. We also include objectives that use the colorimetric changes inherent to plasmonic materials to recruit and educate a new generation of materials scientists. Thus, the project outcomes are new knowledge and new materials to create hierarchical plasmonic biomaterials that benefit society as well as newly trained materials scientists. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.