The Structural and Optical Studies of Process Modified 1-D Nanostructures for Potential Use in Biosensing Applications

Abstract

Integration of functionalized and modified nanostructures (NSs) in various biomedical applications has ushered significant research interests in recent years. The use of functionalized NSs in medicine and biomedical applications are vast and spans in areas such as diagnostics, drug delivery, therapy, antibiotic creams, and bioimaging, to name a few. The current scenario appeals towards surface modification of NSs, which can respond to the needs of biological problems. The main objective of the present work compiled in this thesis is to establish the effect of surface processing of one-dimensional (1-D) NSs on its structural, optical and electrochemical properties as stand alone and in a given biological media. The surface modifications of 1-D NS is performed by forming composites with metallic nanoparticles (NPs) and by post growth processing in a reduced and an oxidizing environment. Two different families of 1-D nanostructures were studied, one belonged to carbon nanotubes and other to oxide nanostructures. In the first section, a comprehensive study of the nanohybrids formed by multiwalled carbon nanotubes (MWCNTs) and metallic Au and Ag-NPs is presented. Functionalization of both –COOH bond and Au-NPs on the walls of MWCNTs has induced stresses which were observed in the X-ray diffraction patterns. The diffusion of Au-NPs in the MWCNTs was clearly observed in the high resolution TEM images, which affected the D and G Raman bands of the MWCNTs significantly. E. coli attachment has modified the local charge densities of Au-NPs-MWCNTs nanohybrids which resulted in the shift of both G and D Raman bands and increased intensity ratio of two bands. This was also reflected in the blue shift of the surface plasmon modes of the Au nanoparticles attached to MWCNTs. It was also revealed that the concentration of Ag-NPs was very vital for the antibacterial activity enhancement in Ag-NPs-MWCNTs nanohybrids. The minimal inhibitory concentration (0.5 mg/ml MWCNTs and 17.5 mg/ml Ag) of Ag-NPs-MWCNTs conjugate was also determined. The charge transfer kinetics of metallic-NPs-MWCNTs nanohybrids were also characterized by modifying the surface of glassy carbon electrode (GCE) by nanohybrids. Both the potential sweep and impedance spectroscopy demonstrated that the diffusion controlled processes were involved at the surface of modified GCE. In addition, it was revealed that the nature of the processes at the surface of nanohybrids modified GCE were quasi-reversible. The highest rate constant of 0.12 s-1 was determined as the concentration of xii Au-NPs was increased in Au-NPs-MWCNTs modified GCE. Conversely, a decreased rate constant of 0.07 s-1 was observed as the concentration of Ag-NPs on the surface of Ag-NPs- MWCNTs modified GCE increased. This suggested that the Au-NPs incorporation at higher concentration in nanohybrids have facilitated fast charge transfer mechanism and slow for Ag- NPs. Finally, nanohybrids modified GCE employed in E. coli surroundings proved that the nanohybrids were efficient for the simultaneous detection of E. coli. In second section, the effect of surface modifications of 1-D ZnO-NSs grown by the vapor–solid mechanism on its antibacterial activity was highlighted. Two sets of ZnO NSs were modified separately; first by annealing in Ar environment and second in oxygen plasma processing. Annealing in Ar resulted in a compressed lattice, which was due to removal of Zn interstitials and increased O vacancies. Plasma oxidation of the ZnO-NSs caused an expansion in the lattice due to the removal of O vacancies and incorporation of excess O, confirmed by X-ray diffraction patterns. Photoluminescence spectroscopy confirmed the surface modification of ZnO-NS, as substantial variation in intensities of visible band was observed as a result of surface modifications, which were used to quantify the Zn and O defects. The antibacterial activity of the modified ZnO-NSs demonstrated that the surface modifications by Ar annealing limited the antibacterial characteristics of ZnO-NSs against Staphylococcus aureus (S. aureus). It was then proved that the O content at the surface of the ZnO-NSs was crucial to tune the antibacterial activity against both selected gram-negative (E. coli) and gram-positive (S. aureus) bacterial species.