Abstract
Gold nanoparticle (AuNP) molecular tags yield a significant signal enhancement in SPR-based immunoassays. While the structure functions of the AuNP tags have been exhaustively investigated, the roles of the evanescent field intensity and distribution remains to be elucidated. Long range surface plasmon resonance (LRSPR) possesses significantly larger fields and longer penetration depth than conventional SPR (cSPR). We report here on the optimization of AuNP tag LRSPR signal enhancement immunoassays using both theoretical simulations built using the finite element method and experimental measurements. A model sandwich immunoassay for the carcinoembryonic antigen (CEA) was then used to demonstrate the feasibility of AuNP tag enhanced LRSPR. The experimental measurements were in excellent agreement with the theoretical model and up to 50-fold improvement in sensitivity was obtained when using 50 nm AuNP tags in comparison to label-free LRSPR. However, the benefit of AuNP tag signal amplification approaches was found to be greater for cSPR than for LRSPR.