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Studies on the Self-assembly Behaviors of Complexes between DNA and Triblock Copolymer Micelles

Author: LiuCong
Tutor: ChenDaoYong
School: Fudan University
Course: Polymer Chemistry and Physics
Keywords: triblock copolymer micelles DNA self-assembly
CLC: O631.3
Type: Master's thesis
Year: 2009
Downloads: 27
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Abstract

In recent years, the intersection between biotechnology and materials science has been the focus of researchers in which there are many novel approaches emerging. This highly interdisciplinary field is closely associated with both the physical and chemical properties of organic and inorganic nanoparticles, as well as to the various aspects of molecular cloning recombinant DNA and protein technology, and immune-ology. Optimized biomolecules, such as nucleic acids, proteins, and supramolecular complexes of these components are utilized in the production of nanostructured and mesoscopic achitechtures from organic and inorganic materials. DNA (Deoxyribonu-cleic acid) is the fundamental hereditary material of living organisms and is widely distributed in the natural world. Recently, the use of DNA as a biomacromolecule has been attracting additional attention of researchers in diverse areas of science. DNA, with its famous double helical structure, being a polyelectrolyte with large number of negative charges and specific complimentary base pairing, has been regarded as an ideal macromolecule for creating new functional materials. Utilization of DNA has become an attractive field in science. The publications covered many applications of DNA as electronic, optical, and biomaterials, as catalyst, and in environmental protection.On the other hand, in recent years, self-assembly, as the most important tool kit of "bottom-up" fabrication of nanomaterials, has been the hot field of research which developed rapidly. After decades of development, the research on self-assembly has obtained obvious achievement and progress. The build-up units of self-assembly include inorganic molecules, small organic molecules, polymers and biological macromolecules and so on. The self-organization of these build-up units in bulk media or at interface, can result in the formation of various zero, one, two and three-dimensional nanostructures. People now could primarily control the morphorlogy of these assemblies and mastered the ways of morphorlogy transition. Also the application of these assemblies has also been explorated. However, the current study of molecular self-assembly is still in the stage of developing, far from maturity. Many areas are yet unknown For instance, the template-free formation of two-dimensional planar structure in solution is still a big problem. The intersection degree of molecular self-assembly and life science is low and exploratoration in this area is still quite limited. Here in our work, the special characteristics of DNA (as a polyelectrolyte with large number of negative charges) and self-assembly method are combined. We studied the electrostatic complexation beween single-stranded DNA and triblock copolymer micelles polystyrene-b-poly(2-vinylpyridine)(quaternized)-b-poly(ethylene oxide) (PS-P2VP-PEO) in aqueous solution. The complexes further self-assembled into free-floating two-dimensional nanofilm. Firstly, triblock copolymer PS-P2VP-PEO was quaternized in common solvent dimethylforrnamide (DMF) and then switched into aqueous solution by dialyzing. In this way, triblock micelles with size of ca.20nm were formed with PEO as the shell, positive-charged P2VP as the crona and short hydrophobic PS block as the loose core. The chain length of single stranded DNA was chosen to be close to the perimeter of triblock micelle. The number ratio of micelles to DNA was 1.13:1. Toroid-like complex was formed with one DNA chain circles one micelle.The toroid-like complex has the circular DNA/P2VP backbone due to the interaction between the short quaternized P2VP blocks and DNA and the grafted PS chains gathered towards the backbone. At the same time, the original mutual exclusion of positive P2VP weakened and hydrophobic aggregation was enhanced after electrostatic complexation with DNA. So the toroid-like complexes could not be stabilized by PEO chains and would self-assemble in short time after formation. Simutaneously, deformation of toroid-like complexes happened driven by hydrophobic aggregation of PS domains. This kind of deformation was synergistic due to the comfinement of other toroids within assemblies, that is, toroid-like complexes would be stretched along the same direction. After a period of time, large number of toroid-like complexes would self-assemble in the above-mentioned way and finally formed two-dimensional planar nanofilm. During the whole process of complexation and self-assembly, no precipitation was observed which demonstrated that the formed two-dimensional nanofilms could stabilize in solution (so called "free-floating").What’s more, the density of P2VP increased in the junctions of two toroid-like complexes (black spots in the TEM images). The regular distribution of P2VP in the junctions constitutes the periodical mesostructures of the resulting two-dimensional nanofilm.We carefully tracked the process of complexation and self-organization with DLS and characterized the morphology of triblock copolymer micelles, toroid-like complexes, assemblies in the middle stage and the final two-dimensional planar nanofilms. We utilized AFM in the verification of TEM results of the nanofilms and employed SAXS to certificate the periodical mesostructure of nanofilms. In the comparative experiment, we carried out the complexation between double-stranded DNA and triblock micelles and no nanofilm structure was observed.This kind of template-free preparation of free-floating two-dimensional planar nanofilms in solution has not been reported in former research. Our work is of great significance for the self-assembly of macromolecules and DNA research.

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