Flavin-derived self-organization and chirality separation of single-walled carbon nanotubes

Date of Completion

January 2008

Keywords

Chemistry, Organic|Chemistry, Physical|Engineering, Materials Science

Degree

Ph.D.

Abstract

Formed by rolling up a two-dimensional sheet of one or more layer of graphite, graphene, carbon nanotubes (SWNTs) are the marvel materials of modern materials science. They are phenomenally strong and stiff, and have the unusual property of being excellent conductors of heat along the tube's axis, but good thermal insulators across it. But it is their electrical characteristics that excite the most interest. Especially, single-walled carbon nanotube (SWNTs), formed by one layer of cylindrical graphene, has better physical properties over multi-walled carbon nanotubes (MWNTs) having over two layer of graphene. Depending on the precise way they are rolled up, which is defined by ( n,m) vector, SWNTs can be made into either metals or semiconductors. So far, SWNTs can generally only be fabricated in batches that vary widely, both in the diameter of the individual tubes and in the orientation of their graphene lattice relative to the tube axis, the property known as chirality. Separating out these various conformations is a challenging, but one that must be solved if nanotubes are ever to fulfill their electrifying potential in devices. ^ This thesis presents that flavin-based helical self-assembly can impart multi degrees of SWNTs separation (i.e., metallicity, diameter, chirality, and handedness). As opening chapters for carbon nanotube and flavin derivative, Chapter 1 provide the introduction of carbon nanotubes, especially single-walled tubes, and the current state-of-the-art nanotube separation. Also, Chapter 1 presents a variety of naturally-occurring flavin derivatives, their redox behavior, and their biological utilization as cofactors for various proteins. ^ Motivated by chemoluminescence of flavin mononucleotide (FMN, phosphorylated form of Vitamin B2) with bacterial luciferase, Chapter 2 discuss about the synthesis and covalent attachment of flavin mononucleotide (FMN, phosphorylated form of Vitamin B2) analogue to oxidized SWNTs. Along with nine step synthesis of synthetic FMN, this study provided two findings: (i) isoalloxazine ring of FMN has strong interaction with the sidewalls of nanotubes and (ii) covalently-attached isoalloxazine ring onto nanotube can be either extended or collapsed by surfactant (long-term) and redox agents or sonication (short-term) from the nanotube surface. ^ The aforementioned two findings lead to utilize strong π-π interaction of biologically-relevant FMN for nanotube dispersing agents. Chapter 3 discuss that FMN imparts effective dispersion of SWNTs via helical self-assembly of flavin moiety. This isoalloxazine self-assembly onto nanotube in aqueous media is held by (i) quadruple hydrogen-bonding along the adjacent uracil moiety of isoalloxazine rings, and (ii) concentric π-π interaction between isoalloxazine ring toward the underlying nanotube sidewall, and (iii) phosphate group of FMN imparting anionic dispersion and individualization of nanotube in water. In addition, precipitation-less replacement of FMN with sodium dodecyl benzene sulfate (SDBS) enable us to probe the relative binding constant of helical FMN self-organization on SWNTs. The significantly higher affinity of the FMN assembly for (8,6) nanotube results in an 85% chirality enrichment from a nanotube sample with broad diameter distribution. ^ Chapter 4 revisits the surfactant amine-assisted semiconducting ( sem-) SWNTs separation in THF media. For this, we synthesized an asymmetric diacetylenic surfactant amine(57ECA), in which adequate chemical anisotropy was generated along its tail to probe the molecular dynamics in the presence and absence of nanotubes via NMR. This surfactant can simulate the similar sem-SWNTs separation, like ODA. This study suggests that the surfactant amine head is firmly immobilized onto the nanotube surface together with acidic water, while the aliphatic tail progressively gains larger mobility as it gets farther from the SWNT. The spectroscopic results indicate that the sem-enriched sample is populated mainly from small nanotube bundles containing three SWNTs. Molecular simulations in conjunction with previously determined HNO3/H2SO4 oxidation depths for met- and sem-SWNTs indicate that the strong pinning of the amine surfactants on the sem-enriched SWNTs bundles is a result of a well-ordered arrangement of amine salts separated with a monomolecular layer of H2O molecules. Such continuous 2D arrangement of hydrated amine salts can be attained from a specific type and diameter nanotubes, which provides the necessary surfactant stability for effective THF dispersion and subsequent enrichment. ^ In Chapter 5, this thesis and its prospectus of flavin-derived carbon nanotube applications are concluded. ^

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