Probing the Localized Surface Plasmon Field of a Gold Nanoparticle-Based Fibre Optic Biosensor

Gold nanoparticles (GNP) have been used in a variety of localized surface plasmon resonance (LSPR)-based optical sensor systems and in a variety of forms, such as colloidal suspensions, immobilized GNP on flat surfaces or optical fibres. A key parameter affecting the sensitivity of these systems is the effective depth of penetration of the surface plasmons. This study aims to determine the plasmon penetration depth in the case of an immobilized GNP-based LSPR optical biosensor. The optical biosensor used for experimentation is a U-bend fibre optic probe of 200-μm core diameter and 1.5-mm bend diameter on which GNP is immobilized. Formation of multilayered nanostructures on the immobilized GNP was used to investigate the field of the localized surface plasmons. Two multilayered nanostructures were explored in this study, viz. a polyelectrolyte multilayer formed by layer-by-layer (LBL) deposition of oppositely charged polyelectrolytes and an immunoglobulin G (IgG) multilayer formed through sequential immobilization of two mutually specific antibodies. Measurement of LSPR absorbance change with deposition of each analyte layer was used to determine the plasmon penetration depth (dP) of the LSPR biosensor. Probing the plasmon field with an IgG multilayer gave rise to at least twofold higher dP compared to dP obtained from the polyelectrolyte multilayer. The effect of GNP size was also studied, and GNP of three diameters, viz. 18, 36 and 45 nm, were used. The 36-nm-diameter GNP exhibited the highest dP. The outcomes of this study may provide leads for optimization of LSPR-based sensors for various biosensing applications.