In nanoscience and nanotechnology, zinc oxide (ZnO) is gaining much research attention due to direct wide band gap (3.3 eV), large exciton binding energy (60 meV), and deep level defects emissions that cover the whole visible range. ZnO nanorods (NRs) in comparison to normal bio molecules and large surface area to volume ratio, allow them to interact within the cell thus are used as convincing intracellular carriers of photosensitizers. Vertical NRs are wave guiding cavities enhancing the light extraction efficiency from devices and are stable photosensitizing agents with their biophotonic, and biodegradation properties, therefore are appealing candidates for the photodynamic therapy of cancer.

The heterojunction LEDs of ZnO NRs/p-GaN are best choice to take the advantage of GaN ideal blue-light emission and fabricated LEDs explore the potential of white LEDs with superior performance. The main objective of this thesis is not only to fabricate ZnO NRs/p-GaN, or ZnO nanotubes (ZNTs)/p-GaN heterostructures, but also to investigate their optical properties for photodynamic therapy. These LEDs have showed enhanced EL intensity covering the visible band (425–750 nm).

ZnO nanorods are grown on the borosilicate glass capillaries (0.7 μm diameter) and then conjugated with photosensitizer. Such glass capillaries having ZnO nanorods complex with photosensitizer on them are used as pointer for intracellular mediated photochemistry in cells to achieve their necrosis. Mitochondrial staining of melanoma and foreskin fibroblast cells was done by Mitotracker Red with the aim of targeting the specific organelle with the prepared ZnO nanowires (NWs) Femtotip to see ROS production. Cytotoxic effects of nanometallic oxides e.g. ZnO-NRs, MnO2 NRs, and Fe2O3 NPs individually and their ligands with photosensitizers in osteosarcoma (U2OS) cells are also explored. Thus bare and ligands of nanometallic oxides, with particular focus of ZnO nanowires are having significant and convincing cytotoxic effects via the liberation of reactive oxygen species as well as Zn+2 ions in labeled cells, thus can be assigned as anticancer agents for breast cancer, melanoma cancer and osteosarcoma cells.

Zinc oxide (ZnO) is an inorganic compound, owing to wide band gap and large binding energy, and holds promising potential in the fields of semiconducting as well as piezoelectric applications with excellent stability and reliability. In addition, ZnO has a plenteous number of nanoscale structures containing unique physical, chemical, electrical, sensing and optical properties. These properties of nanostructures are being unrevealed extensively since last two decades and have become a prominent field of research in nanoscience and nanotechnology.

More specifically, the present dissertation deals with the low temperature synthesis of ZnO nanostructures (nanorods, nanotubes, nanodisks and nanowalls) on a variety of substrates such as silicon, gallium nitride, zinc foil, silver and aluminum; structural characterization and study of their luminescence properties. In paper 1 we investigated the synthesis mechanism of chemically fashioned ZnO nanotubes and their superior emission capability compared to ZnO nanorods with significant enhancements in ultraviolet and visible regions has been studied. These chemically synthesized ZnO nanotubes are further utilized to fabricate a heterostructure with p-GaN thin film in order to achieve white emission (Paper 2). The aim of Paper 3 is to understand the synthesis of ZnO nanorods and their transition into ZnO nanodisks at 55 °C along with temperature dependent micro-photoluminescence studies. However, the second half of the dissertation is devoted to the fabrication of potentiometric cholesterol biosensors through the conjugation of ZnO nanostructures and graphene nanosheets with a thin film of cholesterol oxidase. Paper 4 contains the fabrication of cholesterol biosensor by the deposition of ZnO nanorods on thin silver wire followed by their functionalization under the physical adsorption method. The specificity, reproducibility and stability of the biosensor have been investigated with good linearity slope curve of ~35 mV/ decade. The purpose of papers 5 and 6 is to enhance the sensitivity of the cholesterol biosensor by using ZnO nanowalls and graphene nanosheets as a matrix where the sensitivity of the slope curve is achieved as ~53 and ~82 mV/ decade, respectively.

Recently in a couple of decades, nanotechnology and nanoscience are becoming wide spread fields of research due to the revolutionary advances in the manufacturing processes which enable the realization of infinitesimally modest nanodevices holding a huge variety of fascinating properties and applications. Besides various functional materials, ZnO has captivated interests for a variety of applications in electronics and optoelectronics owing to its unique characteristics; such as, direct wide band gap, large exciton binding energy, semiconducting, photonic, and piezoelectric properties. A distinguished capability of the ZnO material is the effortless synthesis of nanoscale structures with enormous assortments in their morphological and dimensional aspects. Regardless the significant developments in the fabrication of ZnO based homojunction optoelectronic nanodevices, the stable and reproducible p-type conductivity of ZnO material is still a challenge which is one of the paramount factors of the increasing interest for fabrication of heterojunction of ZnO nanostructures with other mainstream ptype semiconductors, such as Si, GaN, and organic materials.

Herein, ZnO nanorods, nanotubes and nanoflowers have been synthesized by solution-based methodology at low temperature (<100 ˚C) and a thorough study on the applications of ZnO nanostructures as white light emitting diodes (LEDs) has been perceived. At the outset, ZnO nanotubes have been synthesized by the trimming of aqueous chemically grown ZnO nanorods with 100% yield and their comparative optical properties have been explored through photoluminescence study, and a profound enhancement in ultraviolet and visible emission is observed (paper I). ZnO nanotubes are further exploited for its promising application as an optoelectronic device. Pure white light emission is observed from the ZnO nanotubes/p-GaN based LED. To analyze the location of the recombination of electron–hole and current transport mechanisms, the EL characteristics of n-ZnO nanotubes/p-GaN heterostructure LED have been investigated under forward and reverse bias. The origin of distinctly different EL peaks under both configurations has been suggested and the influence of increasing values of temperature on the device characteristics is also studied under fixed applied current, in order to check its performance under harsh conditions and for practical  applications (paper II-III). Moreover, it is observed that ZnO-nanotubes/GaN heterostructure LED has an ability to produce an environmentally benign alternative of traditional lighting sources with high color rendering index (CRI) of 96 (paper IV). On the basis of EL, cathodoluminescence and transmission electron microscopy investigations; a robust correspondence has been established between the formation of radiative surface defect states in the nanotubes and the pure cool white light with appropriate color temperature. In paper V, a miniaturized white LED has been developed using Au/n-ZnO nanorods integrated on a glass pipette (having a sharp cylindrical tip with the diameter of 700 nm) which exhibits a broad EL band emission covering the whole visible spectrum range and a CRI value of 73. Besides one-dimensional ZnO nanostructures (nanorods and nanotubes), three-dimensional ZnO dahlia-flower nanoarchitectures have also been fabricated at room temperature relying on natural oxidation based aqueous chemically synthetic approach (paper VI).  Glycineassisted multi-oriented ZnO nanoflowers with highly large surface area to volume ratio have been synthesized on Zn foil substrate through the self-assembly of thin nano-petals as building blocks and polar surfaces of ZnO have been anticipated to be  stabilized through the adsorption of reactive hydroxyl and amide functions of glycine.