Design, Synthesis and Characterization of Novel Nanomaterials

Abstract

The present thesis entitled “Design, Synthesis and Characterization of Novel Nanomaterials” is divided into five chapters, staring with a general introduction. The remaining chapters focus on four different areas/projects that I have worked on. Chapter 1: Introduction to nanomaterials This chapter reviews the basic concepts of nanomaterials and their fabrication methods. Nanomaterials are defined as materials whose dimensions (at least one) are below 100 nm. One of the most exciting aspects of nanomaterials is that their properties may differ significantly from those of the corresponding bulk materials. Nanomaterials fabrication methods can be broadly classified according to whether the assembly follows either i) the bottom-up approach or ii) the top-down approach. These methods have been discussed with various examples including the self-assembly of proteins, peptides and small molecules. In the top-down approach synthetic procedures for Graphene Oxide and its application are discussed. All characterization techniques that are used for characterizing the nanomaterials are also described briefly. Chapter 2 Section A: Self-assembly of 1-Hydroxy benzotriazole (HOBT) in water The studies presented in Chapter 2 identifies HOBT as the smallest non-peptide building block that spontaneously self-assembles into hollow micro tubular structures upon evaporation of water. The tubes form under ambient conditions by rolling over of crystalline sheets of HOBT. The packing of HOBT in the tubes seem to be predominantly driven by intermolecular π-stacking interactions between the aromatic rings of HOBT. These structural and packing patterns are similar to those found in nanotubes formed by the self-assembly of peptides and other larger molecules. The cavities of these thermolabile microtubes act as molds for casting gold nanoparticles for the synthesis of gold microrods with monodisperse dimensions. The non-reacting inner surfaces of the cavities have been used to uniquely synthesize R6G-functionalized gold microrods. With these features, HOBT is an important novel non-peptide building block for accessing micro and nanometric materials for their applications in medicine, biology and molecular biotechnology. Section B: Controlling the orientation of self-assembly of HOBT microtubes The studies presented in this chapter address the self-assembly of HOBT into microtubular structures in different solvents of varying polarities (H2O and DCM:MeOH) to understand the role of solvent volatility and its direction on the orientation of the HOBT microtubes. HOBT self-assembles from DCM:MeOH mixtures in its bipolar canonical form and is coordinated with its water of hydration, similar to its crystals obtained from water. FTIR and TGA data shows that MeOH is also integrated with the microtubes. We observe for the first time that the orientation of microtubular self-assembly is controlled in the direction of evaporation of the solvent. We demonstrate further this feature by controlling the orientation of HOBT self-assembly in exclusively vertical direction through controlled vertical evaporation of the solvent mixture DCM:MeOH (9:1). Additionally, the unique transition between vertical and horizontal orientations for self-assembled HOBT microtubes is achieved by simple change of solvation between aqueous and organic solvents. These results reveal a dynamic relationship between the rate of evaporation of solvent and the rates of formation of different self-assembled morphologies. The rate of evaporation of the solvent primarily governs the rate of formation of the tubes, rather than their orientations in three dimensions. Chapter 3: Chemical origins of debris in Graphene Oxide (GO) This chapter is focused on the investigation of the carbonyl rich fragments arising from GO and provides an understanding of its formation. The fragments are expelled from GO due to an uncontrolled nucleophile driven reaction in aqueous medium leaving the holes on the sheet. These fragments are carbonyl rich small (5 ± 2 nm) nonaromatic molecules that form as by-products of oxidative chemical reactions that occur at the sp3 clusters on the basal surface of GO sheets when they are treated with nucleophilic bases under aqueous conditions. The structure and size of the debris, and hence that of the hole, depend on the size of the sp3 cluster on the sheet. These debris fall out of the GO sheet surface, leading to formation of nanometer sized holes. Formation of debris and hence the holes can be avoided by using anhydrous polar solvents. This work sheds new light on the fundamental structure of GO and the prevention of debris from it during redox reactions enabling better control over functionalization of the GO surface. Chapter 4: Measurement of mechanical properties of polypeptide fragment from Insulin like growth factor binding protein nanotubes by the Peak Force QNM method This chapter describes the discovery of Polypeptide fragment from an IGFBP-2. This fragment self-assembles spontaneously and reversibly into nanotubular structures under oxidizing conditions. These nanotubes were characterized by using Transmission electron microscopy. Notably as compared to the monomer, an increase in intrinsic fluorescence upon self-assembly. The thermal stability of these nanotubes is realized form the fluorescence studies. Peak Force Quantitative Nanomechanical Mapping method of AFM was used to measure the Young’s modulus of the nanotubes. These nanotubes were found to have Young’s modulus value of ~10 Gpa, which is comparable to those of bones presumably due to intermolecular disulphide bonds. These nanotubes will have potential applications in tissue engineering. Chapter 5: Probing the pathways of n→π* interaction in peptides This chapter deals with the theoretical study of n→π* interaction in designed peptidomimetics. The n→π* interaction involves the delocalization of the lone pair of the donor group into the antibonding orbital (π*) of a carbonyl group. However despite beeing extensively studied there exists a debate over the validation of these n→π* interaction which is reminiscent to Bürgi and Dunitz trajectory. This chapter present our findings that peptidomimetics containing the 5,6-dihydro-4H-1,3-oxazine (Oxa) and 5,6-dihydro-4H-1,3-thiazine (Thi) functional groups at the C-terminus of Pro selectively stabilizes the cis conformer by reverse n→πi-1* interaction. These systems have been used to study the n→πi1* interaction using Natural Bond Orbital (NBO) method. Our study reveals that the energetically most favorable trajectory of a nucleophile for a favorable n→π* interaction presumably to facilitate the overlap between the lonepair of the nucleophile and the antibonding orbital of the carbonyl group. The geometrical requirements for the optimum n→π* interaction depends on the relative orientations of the orbitals that are involved. This study has implications for more accurately identifying long distant n→π* interaction.