Investigating the plasmonic behaviour of gold nanostructured arrays with finite element method modelling for optical sensing applications

Abstract

Localised surface plasmons (LSPs) exhibit striking optical behaviour with resonant properties tuned by modification of the host nanostructure’s geometry. This work utilises a novel, plasmonic gold nanorod array surface as a sensing platform for diagnostic applications. Biological species were detected through the sensitivity of plasmons to refractive index changes and the binding kinetics were determined. Investigations were Lysozyme, an immune system enzyme, Proteins A and G from common bacteria and Trastuzumab (Herceptin), a breast cancer drug. Generally, the calculated kinetic coefficients were in good agreement with published values.

To improve sensitivity and uniformity of the chips, the effect of surface parameters, such as diameter, height and spacing were investigated through computational modelling. Finite element method (FEM) software, COMSOL Multiphysics was used to find an optimal setup for improved sensitivity of the surface, with nanorod diameter, 35nm, height, 200nm and spacing of 40nm. The small array spacing may be difficult to fabricate so an array with nanorods of 20x150nm and spacing of 60nm may be more viable.

Simulations showed field enhancements within the nanostructured array, which could be utilised for plasmon-enhanced fluorescence (PEF) to amplify the signal from fluorescent molecules. Integration of chips into a well plate produced an easy to use setup within readily available microplate readers in diagnostic laboratories. Chips were characterised, and then modelled to simulate field enhancement factors for comparison to experimental data. Enhancement factors of 2-5 were in good agreement with most experimental values, however some chips had increased experimental enhancement factors of around 20.

The simulation work completed shows how surface properties, including fabrication defects and array coupling, affect the electromagnetic behaviour of the novel nanorod array surface. While the results help to improving fundamental knowledge of plasmonic interactions, simulated plasmonic spectra and local field enhancements can also be used to guide fabrication for a range of LSP applications. Additionally, the surface has been shown to be a feasible sensing platform for diagnostic applications, with in-situ monitoring through the tabletop LSPR device or through the initial PEF setup.

Date of Award	Dec 2020
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Engineering & Physical Sciences Research Council
Supervisor	Robert Pollard (Supervisor) & Sharon Arbuthnot (Supervisor)