Investigations On DNA-Mediated Reversible Self-assembly Of Carbon Nanotubes

The preparation and the application of carbon nanomaterials have not been known for a long time. Except the normal nanomaterials properties which are peculiar surface effect, volume effect, quantum effect and etc, they also own the other superior and particular properties, including mechanical, electrical etc, so carbon nanomaterials have been promising candidates for the extensive potential applications in optics, electronics, magnetics, biosensing, and biomedicine, etc. Among these carbon nanomaterials, single-walled carbon nanotubes (SWNTs) are known as the most promising materials in the 21st century. However, SWNTs naturally aggregate by van der Waals force of attraction and hardly soluble in all sorts of solvents, which has precluded the study of SWNTs in practical application. Therefore, exploitation of effective method for regulating the size and shape of SWNTs and reversible self-assembly of the SWNTs is the largest challenge of their developments. So far, considerable effort has been put forth to develop various surface modification methods, both covalent and noncovalent, to manipulate SWNTs with functioned molecules and guide SWNTs for self-assembly. While a few attempts have been made to reversibly assemble SWNTs, even though this method would offer a convenient way of constructing multidimensional nano-objects for device application. Lack of suitable reversibly-responsive materials having affinity for SWNTs may be the greatest challenge in achieving this aim. This dissertation is focused on the environmental modulation (light, pH) of reversible assembly/disassembly of SWNT conjugates based on DNA. The details are summarized as follows:1. Investigation of affinities between single strand DNA (ssDNA) and SWNTs in water. SWNTs can act collectively as quenchers for the ?uorophores,and quench more than 95%. Single strand DNA could be adsorbed onto the surface of SWNTs throughπ-πstacking interaction. The carboxyfluorescein (FAM)-labeled ssDNA (FAM-DNA) wrapped on the surface of SWNTs and thus the FAM-DNA was quenched using certain amount of SWNTs. The surfactants also would be absorbed on the surface of SWNTs. As competitive interaction of DNA and surfactants between individual SWNTs, adding surfactants to the solution FAM-DNA/SWNTs, the FAM fluorescence dramatically recovered. The affinity of SWNT to oligonucleotide and various surfactants thus can be investigated by the fluorescence recovery of FAM-DNA/SWNTs. 2. DNA-mediated carbon nanotubes self-assembly: photoinduced-reversible modulation of SWNTs self-assembly in aqueous solution was realized by azobenzene-tethered DNA. Single strand DNA was adsorbed onto the surface of SWNTs throughπ-πstacking interaction. When the azobenzene moieties are introduced into an ssDNA to produce tentative pseudo-base-pairs, their different photo-transformations will cause the hybridization of the azobenzene-tethered DNA with its completely matched DNA to be photo-regulated. So, azobenzene-tethered ssDNA complexes separately with the pristine SWNTs, mixing the DNA/SWNT composites and irradiating with visible light, formation of trans-conformation of the azobenzene-tethered ssDNA makes the two DNA single strands hybridize, which will bring the DNA/SWNT composites into a more bundled and aggregated state. On the other hand, by irradiating the duplex strand DNA/SWNTs with UV light, the duplex strand DNA dehybridizes such that the SWNTs bundles dissociate into individual nano-subjects.3. Triplex-DNA mediated pH-reversible self-assembly of carbon nanotubes. Single strand DNA was adsorbed onto the surface of SWNTs throughπ-πstacking interaction. The stability of triplexes largely depends on the pH of the solution. So, tow ssDNA which would form triplex-DNA, separately wraping the pristine SWNTs. Mixing DNA/SWNT composites and changing pH (5.0-8.0) of the solution to control the formation/ disorganize of triplex will induce the aggregation/ dispersion self-assembly of SWNTs. At low pH, formation of triplex makes the DNA/SWNT composites into a more bundled and aggregated state; while at high pH, disorganize of triplex brings the DNA/SWNTs bundles to dissociate into individual nano-subjects.