This paper describes a new multiplexed label-free biosensor. The detection technology is based on nanostructured gold-polymer surfaces. These surfaces support surface plasmon resonance modes that can be probed by a miniaturized optical setup. The optical characterization of the sensing chip shows the sensitivity and the limit-of-detection to refractive index changes. Moreover, by studying the progressive adhesion of molecular monolayers of polyelectrolytes, the decay of the plasmonic mode electric field above the surface has been reconstructed. A multiplexed label-free biosensing device is then described and characterized in terms of sensitivity, lateral resolution, and sensitivity to a model biological assay. The sensitivity in imaging mode of the device is of the order of $10−6$ refractive index units, while the measured lateral resolution is 6.25 μm within a field of view of several tenths of $mm2$, making the instrument unique in terms of multiplexing capability. Finally, the proof-of-concept application of the technology as a point-of-care diagnostic tool for an inflammatory marker is demonstrated.

Nanostructured surfaces have proven to be effective in controlling the electric field distribution and triggering a series of interesting physical effects. In particular, ordered metallic lattices with a typical size of the same order of magnitude of the wavelength of the incident radiation exhibit extraordinary transmission and reflection properties and represent a sensitive tool to exploit surface plasmon resonance for sensing applications. We investigated, either by experimental structural and optical measurements or by modeling and calculations, samples consisting of a two-dimensional array of polymeric pillars embedded in a gold film. In particular, we analyzed the dependence of the plasmonic resonance on the pillar size. We showed that a peculiar interplay among localized modes and propagating surface plasmon polaritons exists for some selected conditions and affects the spectral distribution, lifetime, and field configuration of the plasmonic excitations.

The recent development and technological improvement in dealing with plasmonic metasurfaces has triggered a series of interesting applications related to sensing challenges. Fluorescence has been one of the most studied tools within such a context. With this in mind, we used some well characterized structures supporting plasmonic resonances to study their influence on the emission efficiency of a fluorophore. An extended optical analysis and a complementary investigation through finite-difference time-domain (FDTD) simulations have been combined to understand the coupling mechanism between the excitation of plasmonic modes and the fluorescence absorption and emission processes. The results provide evidence of the spectral shape dependence of fluorescence on the plasmonic field distribution together with a further relationship connected with the enhancement of its signal. It has made evident that the spectral region characterized by the largest relative enhancement closely corresponds to the strongest signatures of the plasmonic modes, as described by both the optical measurements and the FDTD findings.