Towards label-free biosensors based on localized surface plasmon resonance

Medical diagnostics is in constant search of new tools and devices able to provide in short time, accurate and versatile tests performed on patients. Nanotechnology has contributed largely in developing biosensors of smaller size at a lower cost by using a minimal amount of sample. Biosensors aim to monitor and diagnosticate “in situ” the patient status and the diseases caused by alteration of the body metabolism by, for example, the detection of gene mutations, alteration of gene expression or alteration of proteins. The aim of this work is the development of biosensors that satisfy the requirements which are critical for applications. A biosensor must be i) easy to use, ii) economically convenient, and therefore preferentially label free, iii) highly sensitive, iv) reversible, v) and suitable for Point of Care Testing, that is to be used ”in situ” on the patient. We have focused on biosensors based on the optical properties of nanostructured metals as Au or Ag, in particular by using on Localized Surface Plasmon Resonance (LSPR) spectroscopy. Nanostructured metals under irradiation of electromagnetic wave (as light) exhibit intense absorption bands as results of the localized electronic charges of the metal surface coming into resonance with the incident energy. According to the Mie’s theory, the LSPR absorption band feature changes when the refractive index of the media surrounding the metal nanostructures is varied. Of particular interest for our purpose are the possible changes of the LSPR band features taking place under molecular interactions occurring at the nanostructures surfaces: the shift of LSPR bands is the “transducer” of molecular interactions. These changes can be easily detected by conventional UV-Vis spectroscopy, in transmittance mode. While a large number of studies have been carried out on monodisperse nanoparticles suspended in solution, gold nanoparticles (NPs) deposited on a transparent surface open the possibility to fabricate biosensor based on multiplex array platforms. Nonetheless, one of the major problems in using these plasmonic materials for biosensing purpose is related to the stability of the metal NPs in different solvents and in particular in aqueous solutions. In this study we demonstrate i) the possibility to achieve highly stable NPs by simple thermal evaporation of Au on a substrate commercially available, the Fluorine Tin Oxide (FTO) (Chapter 2); ii) a reproducible variation of the LSPR bands under formation of organic selfassembled monolayers (SAMs), iii) reversible changes in the features of the LSPR bands, (Chapter 3), iv) a specific and reproducible LSPR band changes under molecular interactions occurring at NPs surfaces, as DNA hybridization (Chapter 4). This work demonstrates that the plasmonic material based on Au NPs deposited on FTO surfaces represents a convenient platform for biosensors because of i) inexpensive fabrication, ii) stability of this material in various solvent, including water, of, iii) the easy way to detect the molecular interaction, and iv) the good sensitivity to molecular interactions.