Plasmonic Nanostructures for Biosensing Applications

The aim of this work is the study, the design and the nanofabrication of innovative plasmonic nanostructured materials to develop label-free optical biosensors. Noble metalbased nanostructures have gained interest in the last years due to their extraordinary optical properties, which allow to develop optical biosensors able to detect very low concentrations of specific biomolecules, called analyte, down to the picomolar range. Such biosensors rely on the Surface Plasmon Resonance (SPR) excitation which occurs under specific conditions that depend both on the morphology of the nanostructure and on the adjacent dielectric medium. Therefore, the binding of the biomolecules to metal surfaces is revealed as a change in the SPR condition. Four kinds of nanostructures are investigated in this work: ordered and disordered nanohole array (o-NHA, d-NHA), nanoprism array (NPA) and nanodisk array (NDA). The o-NHA and d-NHA consist of a thin metallic film (50 - 100 nm) patterned with, respectively, a hexagonal and a disordered array of circular holes. The NPA consists of a honeycomb lattice of triangle shaped nanoprisms with edges of about 100 - 200 nm and height of 40 - 80 nm. Finally, the NDA consists of a disordered array of non-interacting disks with 100 - 300 nm diameter and 40 - 80 nm height. The first two support the Extended-SPR whereas the last two, due to their three-dimensional confinement, present Localized-SPR property. Two colloidal techniques are employed for the scalable and cost-effective synthesis of wide areas of nanostructures that allow a fine control of the morphology: NanoSphere Lithography (NSL) and Sparse Colloidal Lithography (SCL). Ordered arrays were nanofabricated by NSL (i.e., NPA and o-NHA) whereas disordered nanostructures were synthesized by the SCL (i.e., NDA and d-NHA). Firstly, the nanostructures are simulated by Finite Element Method (FEM) computations and their performances in revealing small variations of the dielectric medium at the interface is evaluated as a function of their geometrical parameters. Simulated local sensitivities range from 3.1 nm/RIU of the o-NHA up to 13.6 nm/RIU of the NPA. Afterwards, the sensing performances are evaluated experimentally with nanofabricated samples and comparable but slightly smaller sensitivities are obtained. Secondly, a proof-of-concept protocol for the detection assay, that relies on the binding of streptavidin protein to the biotinylated gold surfaces, is exploited to test the nanostructures as biosensors. A 4.4 nM limit of detection is reached with the best performing biosensor (NPA) and picomolar ones are expected for NPA and NDA with a suitable improvement of the functionalization protocol. Finally, complementary single stranded RNA molecules were used, respectively, as bioreceptor and analyte. Revealing short sequences of non-coding RNA, called microRNA, is fundamental for the medical research since these oligonucleotides act as biomarkers for specific diseases, like tumors. Signals of about 13 nm are obtained from the binding of bioreceptor to the nanostructure and from the hybridization of the analyte oligonucleotide at saturation concentrations (∼ 1 μM), indicating that for the moment the developed protocol is quite effective down to the 100 nM range. Of course, for reading the nm or even sub-nM range further optimizations are needed.