Title

Electrospinning of biopolymers and their optoelectronic applications

Date of Completion

January 2009

Keywords

Chemistry, Polymer|Physics, Optics|Engineering, Materials Science

Degree

Ph.D.

Abstract

Electrospinning has been explored to fabricate high aspect ratio nanofibers with high surface area from various biopolymers, including DNA, Silk, and poly(lactic acid). For fabricating DNA nanofibers, DNA complexed with a cationic surfactant (cetyltrimethylamonium chloride, or CTMA) is used. The DNA-CTMA complex can be electrospun far more easily than DNA alone. For the first time, DNA nanofiber based freestanding electrospun meshes are fabricated. In these nanofibers, both DNA's double helix and its ordered secondary structure with the cationic surfactant are retained. The mechanical stretching of these nanofibers resulted in stress-induced alignment of the DNA strands within the DNA-CTMA fibers. ^ Using a hemicyanine fluorophore, the ability of DNA to specifically bind with small molecules is explored. The encapsulation of the fluorophore in the nanofibers resulted into amplified emission as compared to thin films of identical composition. The origin of enhancement is attributed to both fiber morphology and specific interactions (groove-binding) between the chromophore and DNA. These specific interactions have also resulted in longer lifetimes of the dye in its excited state, uniform distribution of dye within the matrix and improved photostability in comparison to the dye in a poly (methyl methacrylate) [PMMA] fibers (as a control). DNA-CTMA nanofibers are also explored as a host matrix for the energy transfer between multiple fluorophores. The spatial organization between multiple fluorophores is achieved using their different modes of specific interaction between DNA's double helical structure i.e. intercalation and minor groove binding. Such hierarchical organization provided a basis for efficient energy transfer. The simultaneous emission due to energy transfer from these fluorophores is utilized in the fabrication of DNA based white light luminescent nanofibers. The emission color, as well as color temperature, of these nanofibers is tuned by varying the ratio and loading of these fluorophores in DNA nanofibers. ^ Silk based nanofibers are fabricated using complete aqueous electrospinning of a genetically engineered protein polymer i.e. Silk-Elastin-Like Protein (SELP). The electrospinning of a SELP polymer resulted in a uniform, high strength ribbon-like morphology. The change in width of nanoribbons is studied as a function of electrospinning process parameters, such as solution concentration, applied voltage, collecting distance, and rate of spinning. The secondary structure of these nanoribbons is also analyzed by FTIR and WAXD. A facile chemical synthesis of bioactive water-dispersible conducting polymer system is also described. Finally, a novel approach to construct a composite material comprised of aligned electrospun nanofibers onto a flexible substrate consisting of microfilament yarn is developed. These fiber-based materials are easily processable into a variety of morphologies and have promise for applications in molecular electronics, filtration, sensors, and the medical industry. ^