Plasmonic sensors on 2D ordered structures

Colloidal lithography with polystyrene spheres allows for the fabrication of hybrid polymer/metal two-dimensional ordered surfaces. They consist of a hexagonal lattice of plasma-polymerized acrylic acid (ppAA) or poly(methyl methacrylate) (PMMA) pillars embedded in an optically thick gold film deposited on a glass substrate. Such a kind of nanostructured system has been shown to support either propagating Surface Plasmon Polaritons or “Mie-like” localized resonances, and appears to be particularly interesting for sensing applications. Tuning the structural parameters, a strong interaction among delocalized and localized plasmonic modes can be obtained together with a good coupling with light. This opens the way towards an optical biomolecular sensor system in which a modification/adhesion on the free nanostructured surface can be easily detected by a simple, near normal reflectance measurement performed from the substrate side, despite the relatively large gold thickness. The simple configuration allows for a surface plasmon resonance (SPR) imaging configuration and enables the real-time multiplexed detection of several analytes. The sensing performance of the surfaces (sensitivity to refractive index change and to the adhesion of molecular monolayers) has been tested using standard spectroscopic techniques. The electromagnetic field’s spatial distribution within the nanostructures and its intensity enhancement have been numerically calculated by finite difference time domain (FDTD) simulations. The results, including the calculated reflectance spectra, are in good agreement with the experimental data.