Self-Assembly of Plasmonic and/or Luminescent Nanostructures.

The aim of this PhD thesis is the fabrication, through bottom-up approaches, of hybrid plasmonic and/or luminescent molecular-nanoparticle nanostructures that are well suitable for the synthesis of functional nanostructures showing structural control (in terms of ability to direct the formation of large assemblies in solution and in the solid-state), peculiar and appealing properties, e.g. optical, electronic or catalytic properties, in the perspective of their applications in different fields of nanotechnology. In addition, the combination of different optical properties belonging to the different molecular components represents an advanced method to manufacture hybrid assemblies, useful for improved optical applications. Gold nanoparticles (Au NPs) exhibit important chemical, electronic and optical properties, due to their size, shape, and electronic structures. Au NPs containing no more than 30-40 atoms are only luminescent because they can be regarded as large molecules with discrete energy levels. While nano-sized Au NPs only show the surface plasmon resonance because the energy level spacing becomes smaller as the number of gold atoms increases. Therefore, the formation of a band may result in the loss of luminescence. Hence, it appears that gold nanoparticles can alternatively be luminescent or plasmonic and this represents a severe constraint for their use as an optical material. To overcome such a limitation, and achieve the above-mentioned purpose, the present study aims at the self-assembly of plasmonic Au NPs with multi-functional emissive building blocks. For these reasons, we have chosen porphyrin molecules and europium complexes as emissive building blocks. In this context, it was synthesized a novel bi-functional porphyrin molecule having two triazine moieties, in two opposite positions of the porphyrin ring, that works as bridges between different gold nanoparticles. Indeed, the molecular structure of these new bi-functional porphyrin molecules allows the formation of an extensive and covalently linked Au NP network, characterized by plasmonic and emissive properties. Effectively, this functional architecture shows a strong surface plasmon, due to the Au nanoparticles, and a strong luminescence signal coming from porphyrin molecules, thus, behaving like an artificial organized plasmonic and fluorescent network. In addition, we experimentally investigated the luminescence quenching for europium complexes bound to Au nanostructures. We have studied this effect for two very similar europium complexes, one of which can covalently assemble on gold nanoparticle surfaces while the other cannot do it. We found that the Au nanostructures covalently surrounded by the Eu complex remained plasmonic and luminescent, while a total emission quenching was observed for the Eu complex not suited to covalent interact with the Au nanostructures. This behaviour was rationalized in terms of through bond vs. through-space interactions between the Eu complex and the Au nanoparticles. Finally, plasmonic Au_ZnO core-shell nanoparticles were prepared by “one-pot” synthesis in which a [Zinc Citrate]- complex acts as ZnO precursor, reducing agent for Au3+ and capping anion for the obtained Au nanoparticles. Gold nanostructures absorb visible light and show localized surface plasmon resonance bands in the visible region. Semiconducting ZnO nanostructures are excellent for ultraviolet detection thanks to their wide bandgap, large free exciton binding energy and high electron mobility. Therefore, the coupling of gold and ZnO nanostructures represents the best-suited way to enhance photodetection. The photocatalytic activity of such core-shell nanostructures was investigated through the photodegradation of a standard methylene blue (MB) solution, according to ISO 10678:2010. Sun light was used as an irradiation source, and it was observed a fast and efficient MB decomposition. To sum up, the most important achievement of this thesis is to have demonstrated that the covalent assembly of suitable molecules on gold nanoparticles surfaces allows the synthesis of architectures showing peculiar and unique properties appealing for future technologies. Noteworthy, some of these hybrid nanomaterials both show a strong surface plasmon and a strong luminescence signal.