Engineered nanostructures for metal enhanced fluorescence applications in the near-infrared

Abstract

Recent advancements in fabrication techniques allow construction of nanostructures with well-defined features in nanometres scale. Tiny nanostructures that have features below the resolution of optical diffraction limit can now be made in the laboratory. The specific properties of those nanostructures with specific properties made from variety of materials allow us to study and explore many different properties that have never been observed while they are in bulk. One such phenomenon is localised surface plasmon resonance effect, which is exhibited by certain materials when in nanometric size. Their peculiar interaction with light is in such a way that the optical properties such as reflection and transmission deviate from typical characteristics and change according to the material involved and their shapes. Furthermore, this effect could also enhance the electric field in a specific area of the structure. This thesis is motivated by the attractiveness of the tunability of localised surface plasmon resonance and aims at exploring those properties by fabricating multiple types of nanostructures through a low-cost and versatile technique called nanosphere lithography. By improving the technique and combining with other fabrication techniques (such as oxygen plasma etching and argon ion milling), a large variety of nanostructures with hexagonal lattice like as nanocones, nanopencils, and nanofins arrays have been successfully created. Among them, three main types of nanostructure were selected for detailed study: nanotriangle, nanodisc, and nanohole-disc arrays. The distance between the adjacent nanoparticles were changed in those structures and strong interparticle coupling behaviours were observed as the distance between them becomes shorter. Current portable biosensing devices for in vitro studies are limited by the sensitivity limit of the detector, the poor quality of emitters and the size of the devices. In this thesis, the application of localised surface plasmon resonance for near infrared in vitro biosensing is explored. This is achieved through a mechanism called metal enhanced fluorescence. The techniques take advantage of the high electrical field strength and the resonance condition of the plasmon to enable a fluorophore to achieve brighter emission. The greater the resonance and electrical field are, the greater the emission amplification would be. Such effect makes it highly attractive for near infrared in vitro studies, which benefits from high optical penetration of common biology components such as water and lipids, but suffer from poor emission of existing fluorophores. Thus, enhancement of the emission signals through metal enhance fluorescence mechanism is an attractive route to obtain better signal to noise ratio in medical diagnostic, and improve detectability while at the same time reduce the need of a high sensitivity detector which can be costly and large in size. The three chosen nanostructures, i.e. nanotriangular arrays, nanodisc arrays and nanohole-disc arrays have shown marked enhancement in the emission of attached fluorophores up to 83x, 235x, and 411x respectively, making them highly attractive nanostructures for such application.