Interaction Studies of Localized Surface Plasmon Resonance Immunosensor Based on Gold Nanoparticles

Gold nanoparticles (AuNPs) have been widely used for various applications because of their unique properties. In this study, theoretical and experimental studies have been carried out to study the signal characteristics of the AuNPs-based localized surface plasmon resonance (LSPR) immunosensor. The finite-difference time domain (FDTD) method has been chosen to obtain information regarding optical properties and electric field profiles of AuNPs with different structures and configurations. The simulation results show that the distance between particles greatly affects the extinction and scattering spectra. In addition, there was a significant enhancement in the electric field due to the presence of dielectric material on the surface of AuNPs. In the case of dimer nanoparticles, the enhancement of the electric field is up to 20 times higher than that of single AuNPs. Experimentally, the enhancement of the electric field causes the detection range of the sensor to be limited. From the different concentrations of bovine serum albumin (BSA) investigated, the sensor is only able to detect BSA from 1 ng/mL to
$10 \mu \text{g}$
/mL. We encountered the Hook effect in this experiment which was characterized by a decrease in the LSPR signal at a concentration of
$100 \mu \text{g}$
/mL. A thicker dielectric layer can enhance the electric field around the nanoparticles and as a result, interference from changes in refractive index (RI) originating from other materials around the nanoparticles becomes difficult to avoid.