Belloni, Carlo (2017) Plasmonic NanoHole Arrays for Label-free Biosensors. [Magistrali biennali]
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The present work focuses on plasmonic properties of nanostructures to be used as optical biosensors. A biosensor is a device able to detect and recognize a specific biological substance, called analyte. The sensitive component of a biosensor is the receptor, which binds to a specific analyte and the interaction is then translated into a measurable and quantifiable signal by another component called transducer. This latter, in this work, is a NanoHole Array (NHA), which has been designed, synthesized, characterized, functionalized and tested. A NHA is a thin metallic film (of noble metals, here Au) of thickness 50-100 nm, patterned with a periodic array of holes (here the hole diameter is around 320 nm), in this case, an hexagonal array of circular holes. The physical property exploited in NHAs is the Surface Plasmon Resonance (SPR), resulting from the coupling of an electromagnetic (EM) field (UV-VIS-NIR) with surface conduction electrons of the metallic nanostructure. In particular, NHAs take advantage of Surface Plasmon Polaritons (SPPs), which are EM waves travelling at the interface between a metal and a dielectric. Moreover, at the resonance, this device exhibit a peculiar optical property called Extraordinary Optical Transmission, in which the light transmitted by the NHA is more than that of a single hole, whose area corresponds to the sum of the nanoholes area. The Fano-like nature of the EOT phenomenon has been investigated. Therefore, NHA transmittance spectrum consists in a sharp band, whose peak position depends on geometrical parameters of the structures (period, radius, thickness) and on the surrounding dielectric environment. Thus, a change of the dielectric environment and hence of the refractive index leads to a change in the resonance condition and hence in a red-shift of the EOT peak, on which the biosensing transduction mechanism is based. Bulk and local sensitivities of the NHA have been experimentally measured and compared to the simulated results (obtained with FEM simulations), with good agreement: Sbulk = 281 nm/RIU (exp.) vs 290 nm/RIU (exp), Slocal = (2.9 +/- 0.1) RIU-1 vs 3.2 RIU-1. Finally, biosensing tests have been performed following a suitable functionalization protocol. A thiols layer is employed as ligand between the gold surface and the receptor, namely, Biotin. Incubations with different analyte concentrations (Streptavidin, between 10-10 M and 10-6 M ) have been performed, obtaining the sensing curve (which follows a Langmuir isotherm) and the corresponding Limit of Detection (LoD) of 4.7·10-8 M.
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