Gold nanostars : synthesis, stabilization and applications as surface-enhanced Raman scattering tags

This PhD thesis focuses on the preparation and stabilization of star-shaped gold nanoparticles (nanostars), and their use in various applications based on surface-enhanced Raman scattering (SERS) spectroscopy. A general introduction is presented in Chapter 1 where basic concepts about the synthesis of gold nanoparticles, surface functionalization and optical properties of metal nanoparticles, as well as the SERS phenomenon are introduced. Chapter 2 describes the stabilization of gold nanostars using a combination of an aromatic dithiol and a surfactant. Further gold nanostar-seeded growth leads to a series of exotic nanostructures containing an internal gap in which the aromatic thiol is trapped. Due to the presence of hot-spots at these gaps, the new structures are excellent SERS-encoded probes for sensing and imaging applications. Gold nanostars can be also stabilized using a mixture of a thiolated polyethylene glycol (PEG-SH) and smaller thiol molecules such as dodecanethiol (DDT) or SERS-active thiols. Functionalization with PEG and DDT is achieved through a solvent-exchange method, not only for gold nanostars but also for other nanoparticles including silver and gold spheres and gold nanorods. In Chapter 3, this general phase-transfer method for obtaining hydrophobic plasmonic nanoparticles with high stability is detailed. The self-assembly of these nanoparticles leads to SERS-active substrates, that can be compared according to their SERS efficiency. Chapter 4 describes the preparation of SERS-encoded gold nanostars using the same strategy, yet employing Raman-active thiols in this case to induce the phase-transfer. These SERS-active nanostars are applied as SERS-tags for cell differentiation. Chapter 5 deals with the optical response from PEG-stabilized gold nanostars assembled onto colloidal substrates. More specifically, polystyrene beads were loaded with gold nanostars and SERS-labelling was also applied for in vivo cell imaging. In summary, this PhD thesis is expected to contribute to the design of different plasmonic nanomaterials with tailored surface functionality, high colloidal stability, biocompatibility and a huge potential for SERS applications. Star-shaped gold nanoparticles are the most prominent candidates in this direction. In particular, SERS-nanoprobes show great promise for biomedical applications and the results presented here could have enormous implications towards the engineering of new SERS-based multifunctional nanoplatforms