Tuneable and robust long range surface plasmon resonance for biosensing applications

• A simple procedure to tune the penetration depth of long-range SPR sensors is shown.
• The Teflon AF delamination of long-range SPR sensors is straightforwardly overcome.
• The optical quality of the SPR-supporting gold is correlated to its roughness.
• A nanometer-thin layer of plasma-polymerized allylamine improves the gold quality.
• A figure of merit 20-fold better compared to regular SPR sensors is achieved.

A multilayered biosensing architecture based on long range surface plasmons (LRSPs) is reported. LRSPs originate from the coupling of surface plasmons on the opposite sides of a thin metal film embedded in a symmetrical refractive index environment. With respect to regular SPs, LRSPs are characterized by extended electromagnetic field profiles and lower losses, making them of high interest in biosensing, especially for large biological entities. LRSPs-supporting layer structures are typically prepared by using fluoropolymers with refractive indices close to that of water. Unfortunately, fluoropolymers have low surface energies which can translate into poor adhesion to substrates and sub-optimal properties of coatings with surface plasmon resonance-active metal layers such as gold. In this work, a multilayered fluoropolymer structure with tuneable average refractive index is described and used to adjust the penetration depth of LRSP from the sensor surface. The proposed methodology also provides a simple solution to increase the adhesion of LRSP-supporting structures to glass substrates. Towards taking full advantage of long range surface plasmon resonance sensors, a novel approach based on the plasma-polymerization of allylamine is also described to improve the quality of gold layers on fluoropolymers such as Teflon AF. Through these advancements, long range surface plasmon resonance sensors were fabricated with figures of merit as high as 466 RIU−1. The remarkable performance of these sensors combined with their high stability is expected to foster applications of LRSPR in biosensing.