Plasmonic Nanostructures For Spectroscopy Detection and Label-Free Biosensing

Metallic nanostructures exhibit rich optical and electrical properties: for instance, when the light interacts with a metal nanoparticle (NP), its conduction electrons can be driven by the incident electric field in collective oscillations, known as localized surface plasmon resonances (LSPRs). These resonances give rise to a drastic alteration of the incident radiation pattern and to striking effects, such as the subwavelength localization of electromagnetic (EM) energy, the formation of high intensity hot spots at the NP surface, or the directional scattering of light out of the structure. For a specific nanoparticle, the wavelength of the LSPR is very sensitive to a variety of factors, as the size, shape, interparticle spacing, and dielectric environment. LSPRs can also couple to the EM fields emitted by molecules, atoms, or quantum dots placed in the vicinity of the NP, leading in turn to a strong modification of the radiative and nonradiative properties of the emitter. For all these fascinating properties, they are currently used as benchmark for biomedical applications, including detection, labeling, cell tagging and sorting, imaging enhancers, and as therapeutic agents. In order to generate any meaningful or conclusive information for clinical diagnostics, the simultaneous detection of several targets is often needed. Therefore a platform capable of performing multiplexed biological detection is an indispensable tool for accurate clinical diagnostics. In this regard, many small nanoparticles working as specific sensing elements can build up an overall sensor for parallel analyte detection. The research activities performed during my PhD have been oriented to explore new solutions for developing multiplexed sensing platforms for protein detection. Both labelled and label-free techniques have been investigated, and different fabrication processes and functionalization methods are presented. The results showed in this thesis demonstrate the great potential of plasmonic metal nanoparticles and nanostructures for the detection of protein antigens and other biological species, including effective immobilisation of target analytes. More specifically, I have investigated the effect of Au-based nanoCones array, fabricated by means of direct nanoimprint technique over large area, on protein capturing and on the enhancement in optical signal. The presence of Au nanoCones array leads to an enhancement of the electric field on the apex of the cone, enabling the detection of molecules. Such kind of device has the potential to be employed as a disposal multiplex nanoarray for bioactivities. In this thesis work the proof-of-principle of a new label-free technique for fast detection of proteins binding is also shown. The proposed method employs functionalized gold plasmonic nanoparticles randomly deposited on a microfluidic device. The detection of analytes is based on the measurements of the Rayleigh scattering intensity change of functionalized gold nanorods by means of dark-field microscopy. Promising outcomes showed that this technique could become a valid alternative to the conventional surface plasmon resonance spectroscopy. As a further development, it may allow multiplexed protein analysis towards high-throughput screening of biomolecular interactions, point-of-care applications and resource-limited settings.