Plasmonic nanostructures for metal-enhanced applications

  • Milan Adelt

Student thesis: Doctoral Thesis

Abstract

Surface plasmon resonance is an optical phenomenon which has attracted a broad research interest due to the ability to harvest and localize incident light in the nanoscale and to enhance electromagnetic fields in the vicinity of certain metallic nanostructures. These enhanced electromagnetic fields have the ability to improve photophysical and molecular properties of molecular species of interest, giving rise to “metal-enhanced” techniques with improved performance. Techniques and processes involving fluorescence or chemical reactions are often hindered by low quantum yields and insufficient photostability or low reaction yields, respectively. By implementing metallic nanostructures with the structure and plasmonic properties adapted to characteristics of involved molecules and experimental conditions, these limitations can be overcome. In this thesis, the fabrication, characterization, application of various plasmonic nanostructures and metallic substrates with characteristics suitable for metal-enhanced fluorescence (MEF) or plasmonic photocatalysis are described. In the first part, an emphasis is placed on systematically investigating an anisotropic coating of the ends of gold nanorods (GNRs) with silica as well as the influence of growth conditions on the formation of silica shells through electron microscopy and absorption spectroscopy. Such modification could allow a selective positioning of molecules of interest near the ends of GNRs where their strong longitudinal plasmon resonance can be exploited. By exploiting unique surface characteristics of GNRs, three nanostructures with various coating morphologies were produced: fully encapsulated core-shell GNRs, GNRs with only one end coated with silica, and dumbbell-like GNRs (dGNRs) with both ends coated. The study was performed at conditions, where controlling the solubility and micellization of the surfactant cetyltrimethylammonium bromide (CTAB) is possible, which can be tuned to affect the morphology and stability of resulting silica shells. A combination with an appropriate aspect ratio of GNRs led to a significant improvement of the growth yield of dGNRs. Thus, a protocol for a high-yield synthesis of dGNRs was developed, with a maximum yield exceeding 90%. This study advanced the understanding of the anisotropic coating process on GNRs which is relevant for GNR-based nanostructures for fluorescence/scattering amplifiers or plasmon-enhanced catalysis. In the second part, aluminium foil was tested as an affordable substrate for MEF in the UV-Vis region of spectrum using bovine serum albumin (BSA) and BSA-encapsulated gold nanoclusters (BSA-AuNCs). Fluorescence of BSA and BSA-AuNCs was evaluated by analysing fluorescence emissions of amino acid tryptophan and AuNCs. Steady-state and time-resolved fluorescence spectroscopy of BSA and BSA-AuNCs immobilized and dried on the two sides (shiny and matt) of the aluminium foil revealed that the matt side supported a moderate fluorescence enhancement of both tryptophan and AuNCs emissions, while the fluorescence emission intensity on the shiny side decreased, suggesting quenching of fluorescence. It is believed that the observed enhancement on the matt side originated due to the nanoscale corrugation which supported generation of locally enhanced electromagnetic fields responsible for an improved excitation efficiency of fluorophores, increased radiative decay rate of tryptophan in BSA as well as improved Förster resonance energy transfer (FRET) from tryptophan to AuNCs in BSA-AuNCs. The observed emission enhancement suggests the matt side could be used as a simple substrate for MEF in the UV and blue region of visible spectrum. In the third part, plasmonic substrates with periodic arrays of three-dimensional gold nanobowls (AuNB) of various diameters and optical properties suitable for MEF in the UV and visible spectrum were fabricated. AuNBs were rigorously analysed both experimentally and numerically to determine their physical and optical properties. The substrates exhibited significant reflectance minima from UV to 500 nm, indicating strong absorption in this region, which is particularly valuable for MEF with the limited number of substrates active in the UV. Numerical simulations were employed to probe size- and excitation-dependent spatial distribution of electric fields of AuNBs in the UV and visible spectrum given by their specific geometry. The effect of fine structural features such the wall thickness between NBs and the thickness of Au coating on the electric field distribution was investigated as well. The results suggest that the wall thickness influences the electric field intensity at rims of NBs and the coating thickness influences the strength and the size of electric fields induced inside NBs. It is hoped that this work provides insight into designing and optimizing AuNB substrates for fluorescence applications with optical properties tailored to photophysical properties of incorporated fluorophores. Finally, the performance of AuNBs for MEF in the UV and visible spectrum was investigated using BSA-AuNCs, which is believed to be the first MEF study on AuNBs performed in the UV with a protein and fluorescent AuNCs as emitters. Fluorescence emission was investigated on two groups of small- and large-diameter AuNBs under various excitation wavelengths and angles. It was found that the fluorescence emission of BSA-AuNCs exhibited a structure-specific angular dependence on both types of AuNB arrays. Large-diameter AuNB supported a consistent broadband enhancement of fluorescence emission BSA-AuNCs for excitation wavelengths in the UV and visible regions of spectrum, which was attributed to the broadband excitation rate enhancement mediated by the geometry of nanobowls and a structure-dependent coupling of BSA-AuNCs with AuNB arrays. This led to the highest observed fluorescence enhancement of 34.5-fold of the AuNC emission on large-diameter AuNBs. Small-diameter AuNB arrays induced less consistent angular dependence of the fluorescence emission of BSA-AuNCs. This was interpreted to arise from different angle-dependent optical properties of small-diameter AuNBs composed of arrays of nanocaps which do not support mechanisms typical for large-diameter bowl-like architectures. It is hoped that by combining findings gained by structural and fluorescence characterizations, implications for further use of AuNBs in fluorescence applications in UV and visible regions of spectrum could be better understood.
Date of Award20 Jan 2023
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council)
SupervisorYu Chen (Supervisor) & David Birch (Supervisor)

Cite this

'