DNA Functionalization Of Non-gold Nanomaterials And The New Roles Of Small Molecules In Self-assembly

DNA-based functionalization and programmable self-assembly of nanomaterials is an important research direction in DNA nanotechnology. Single-walled carbon nanotubes (SWNTs) possess unique electronic and optical properties, which make them very attractive for applications in materials science and next generation nanoelectronics. It has been reported that DNA wrapping on SWNTs can largely improve their water solubility, based on which length and chirality-based separations of SWNTs have been successfully realized in literature. This enables an effective strategy for DNA functionalization of SWNTs to achieve a new type of building block for DNA-programmable self-assembly. Platinum has been well-demonstrated for its catalytic activity in various laboratory and industrial processes. To explore the potential use of platinum nanoparticles in self-assembly based material fabrication, it is critical to realize a stable and reliable DNA decoration of platinum nanoparticles (PtNPs) by a defined number of DNA molecules. Besides the use of DNA for controllable nanoparticle assembly, a logic control on the assembly-disassembly (aggregation/dispersion) behavior of nanoparticles based on small molecular and ionic interactions is the key for the development of novel chemical logic devices as well as multi-responsive sensing devices, and the building of strongly coupled plasmonic nanoassemblies. The work in this thesis will be focused on the above aspects.In the first work, we developed a new strategy to obtain DNA functionalized SWNTs with largely maintained hybridization activity of the grafted DNA sequences. We took advantage of a literature method to disperse SWNTs in an aqueous solution by DNA wrapping under sonication, based on the non-covalent π-π interactions between DNA bases and the electron-conjugated SWNT surface. By introducing a double stranded DNA insert between the SWNT and a specially grafted DNA sequence (by hybridizing with the DNA strand wrapped around a SWNT), we were able to achieve highly hybridizable DNA-conjugated SWNTs for self-assembly purposes. Our experiments showed that the resulting DNA-SWNT conjugates exhibited fast hybridization kinetics compared to the case when the hybridizing sequences were embedded within the wrapping strand. As a step forward, we achieved reversible aggregation-dispersion control over the DNA conjugated SWNTs based on sequence-specific DNA hybridization and a strand-displacement strategy to disrupt DNA basepairing. With the DNA functionalized SWNTs as a templating material, linear arrays of gold nanoparticles were assembled on SWNTs through DNA sequence-specific bonding between the gold nanoparticles and SWNTs. As well, a pH-responsive DNA-SWNT hybrid hydrogel was prepared by taking an i-motif half-sequence as the DNA graft on SWNTs, which would provide more structural and functional versatilities to DNA-based hydrogels. Besides, a preliminary DNA sensing assay was also realized with a DNA target serving as a molecular linker that brought two parts of SWNTs into aggregates. This strategy might be further elaborated in the future to achieve a much improved sensitivity based on the unique physical properties of SWNTs.The next pursuit of this thesis was to realize the preparation and gel electrophoretic isolation of valence-controllable DNA-modified platinum nanoparticles. We started our efforts by synthesizing PtNPs with a reasonably narrow size distribution, good solution stability and relatively dense surface charge, three key factors favoring their DNA decorations and electrophoretic isolations. It was then found that the PtNPs bearing different numbers of DNA ligands could migrate as discrete bands during gel electrophoresis. Thanks to the excellent valence controllability of the gel-purified PtNP-DNA conjugates as well as the metal-thiol bonding compatibility between gold and platinum nanoparticles, Pt-Au hetero-nanostructures with valence-dictated compositional and structural control were successfully obtained. Our work, for the first time, realized the gel electrophoretic isolation of DNA functionalized non-gold metal nanoparticles bearing a discrete number of DNA ligands. The "molecule-like" PtNP-DNA hybrids bearing accurately defined bonding valences are appealing for applications in DNA-programmable nanoparticle assembly toward sensing, catalysis and various other applications.In a following chapter, we demonstrated that the electrostatic and chemical (complexing and gold-thiol bonding) interactions existing in a gold nanoparticle/Zn2+/dithiothreitol ternary chemical system is "programmable" and can be utilized to regulate the aggregation and dispersion of gold nanoparticles via XOR and INHIBIT logic. Because the color transitions of gold nanoparticles from red to purple-blue were especially sensitive, we could observe the visible logic outputs by naked eyes. In addition to the XOR and INHIBIT regulations, logic AND could be realized by replacing the negatively charged gold nanoparticles with cationic dyes. Benefiting from the exactly same inputs for the three chemical logic gates, a "half-subtractor" and a "half-adder" were built by integrating the XOR logic with INHIBIT or AND. We believe this simple chemical logic system will be valuable for a further investigation toward smart molecular/nanosystems, novel sensing platforms and new controls in nanoparticle assembly.Discrete AuNP assemblies with strongly coupled plasmonic optical properties were obtained by making use of the complexing interactions between silver ions and BSPP ligands, which could be isolated and purified by agarose gel electrophoresis. Transmission electron microscopic analysis evidenced a very high accuracy of the assembled products. The AuNP-based discrete nanostructures could be functionalized by DNA molecules to achieve more complicated nanoparticle superstructures by making use of DNA’s programmability in self-assembly. This work will have potential applications in plasmon resonance coupling, surface-enhanced Raman scattering and nano-catalysis.