Plasmonic nanorod metamaterials for biosensing

Abstract

Label-free plasmonic biosensors rely either on surface plasmon polaritons or on localized surface plasmons on continuous or nanostructured noble-metal surfaces to detect molecular-binding events^1,2,3,4. Despite undisputed advantages, including spectral tunability³, strong enhancement of the local electric field^5,6 and much better adaptability to modern nanobiotechnology architectures⁷, localized plasmons demonstrate orders of magnitude lower sensitivity compared with their guided counterparts³. Here, we demonstrate an improvement in biosensing technology using a plasmonic metamaterial that is capable of supporting a guided mode in a porous nanorod layer. Benefiting from a substantial overlap between the probing field and the active biological substance incorporated between the nanorods and a strong plasmon-mediated energy confinement inside the layer, this metamaterial provides an enhanced sensitivity to refractive-index variations of the medium between the rods (more than 30,000 nm per refractive-index unit). We demonstrate the feasibility of our approach using a standard streptavidin–biotin affinity model and record considerable improvement in the detection limit of small analytes compared with conventional label-free plasmonic devices.