Metal-dielectric nanostructures - Synthesis and characterization for SERS analysis

Surface-enhanced Raman scattering (SERS) spectroscopy can lead to the fabrication of high-sensitivity label-free sensors for detection purposes in many fields, such as biomedical diagnostics and therapy monitoring, but also agricultural and environmental monitoring. The electromagnetic enhancement plays an important role in the SERS effect; the resonant excitation of Localized Surface Plasmons (LSPs) of noble metal nanoparticles (NPs) – in the regions close to their surface – is the main reason of the scattered light enhancement. Such effect is strongly dependent on the type, size, shape, inter-particle distance and aggregation state of the NPs. Recently, the fabrication of tuneable plasmonic nanostructures constituted by Ag NPs on flexible elastomeric polydimethylsiloxane (PDMS) matrices has been reported. These kind of substrates can potentially fulfil the need of flexible and wearable devices, able to constantly monitor people’s health, by checking their physiological and biochemical parameters. The aim of this work is to investigate fast and low-cost methods for the synthesis of stable SERS active substrates, based on Ag-NPs assembled on PDMS membranes. Two main approaches have been followed. As first attempt, Ag-NPs have been sputtered on PDMS membranes by varying the synthesis parameters (current, deposition time, distance from target). To increase the stability of the NPs on the substrate the PDMS was functionalised before the sputtering deposition. (3- mercaptopropyl)trimethoxysilane (MPTMS) and (3-Aminopropyl)triethoxysilane (APTES) were used to promote the adhesion between sputtered-Ag and PDMS. The second strategy was, instead, based on the static-incubation of PDMS samples in a AgNO3 solution at different conditions. Synthesis parameters (monomer to curing ratio, thickness of the substrate, incubation temperature, cross-linking degree, precursor concentration, presence of ethanol in the solution) have been deeply analysed one-by-one to find the best conditions for the synthesis. Different characterizations techniques have been carried out to have a deep insight into the Raman response of all the synthetized substrates: UV-Vis transmittance analysis and Raman mapping of mercaptobenzoic acid (MBA), a Field Emission Scanning Electron Microscopy (FESEM) characterization, along with an image analysis on the FESEM micrographs and a stability characterization in Tris-EDTA buffer and water. In conclusion, an assay for the detection of miRNAs cancer biomarkers both by SERS and ELISA was carried out on the best sample. To this aim, a set of synthesis conditions was chosen for each method of synthesis and the assays performed on the obtained samples (PDMS, APTES-PDMS, MPTMS-PDMS and curing PDMS). Finally, the best conditions of synthesis in the particular case of static incubation have also been used for the development of a all-PDMS multi-chamber microfluidic chip for fast and low cost detection purposes, but still with high sensitivity and tunability. The bioassay was carried out also in the case of the chip, in order to achieve the final application and in the view of an effective implementation.