Optimisation of Gold Nanorod Array for Sensing Applications

Localised surface plasmon resonance (LSPR) based sensors are typically considered less sensitive than surface plasmon resonance (SPR) based devices. Here we present a device which utilizes a gold nanorod array to amplify the LSPR signal and sensitivity for biosensing applications. The device detects protein binding interactions and ensures selectivity to specific binding events by utilizing protein binding pairs. Antibody, anti-Lysozyme was immobilized on the surface to detect immune system enzyme, Lysozyme in solution. The current setup was capable of detecting Lysozyme at nanomolar concentrations (31.25nM). However, the current setup may not be optimized for maximum sensor signal. To determine the optimal setup required, the surface was modelled using finite element method software, COMSOL Multiphysics. In the model, the effect of surface parameters on the sensor signal was investigated. We found that the greatest magnitude of LSPR signal occurred for nanorod arrays with diameter of 25nm, height of 200nm and centre-to-centre spacing of 60nm. The electric field distribution was also calculated and the variation throughout the array was determined. It was found to be strongest in x-direction, at the top and middle of the nanorod at distances of 1nm and 4nm from the surface respectively. If immobilized proteins could be positioned at these distances and position on nanorod, binding interactions would experience greater fields and hence produce a stronger plasmonic response. In addition, protein binding occurring between nanorods at heights above the nanorod midpoint would also experience significantly stronger fields due to enhanced coupling. The results obtained show how the current setup is functional, but could be further modified for optimal sensitivity.