Synthetically programmable nanoparticle assembly using DNA

The primary goal of this dissertation was to explore the potential for using DNA in controlling the organization and interactions of colloidal particles on the nanometer length scale.; Accordingly, this dissertation presents the first example of using synthetic oligonucleotides and their sequence-specific interactions for rationally organizing gold nanoparticles into assemblies that canvas macroscopic distances. This method involved the modification of gold particles with alkylthiol-capped oligonucleotides. These DNA/nanoparticle hybrids were shown to be capable of hybridizing with complementary polynucleotides in solution. It was further demonstrated that "linking" polynucleotides can be used to assemble the particles into macroscopic assemblies. The oligonucleotide connectors used in this strategy provide control over interparticle distance, particle periodicity, and the thermal stability of the resulting bioinorganic materials. Proof-of-concept examples involving homoparticle and heteroparticle systems are provided.; Importantly, it was shown that the optical properties of the DNA-modified nanoparticles were significantly affected by hybridization with linker strands and other gold particles. This effect has been attributed to the electromagnetic interactions of the particles and results in a striking red to blue color change in solution. The physical origins of these optical effects have been studied and determined to be related to a decrease in interparticle distance and aggregate size that accompany hybridization. Significantly, these observations have led to the development of a highly selective colorimetric detection method for DNA, where target oligonucleotides trigger the assembly of the nanoparticles into network structures. These novel materials exhibit exceptionally sharp melting transitions, which translates into ultrahigh detection selectivity. This unusual melting behavior was determined to be a result of using probes with distance dependent optical properties in the context of a network structure with multiple hybridization links between the probes.; Finally, the DNA-based methodology was extended to transparent, solid substrates for the formation of gold nanoparticle multi-layers and for developing ultrasensitive DNA diagnostic methods. Importantly, this DNA-based assembly strategy is not limited to gold and could easily be extended to a wide range of other metal, semiconductor, or insulator nanoparticle based materials, giving entry into an entire new class of nanostructured materials.