Synthesis and characterization of metal nanostructures and their employment as optical sensors for heavy metal ions detection in water

In this thesis, the importance of the detection of heavy metal ions in water and the most common analytical techniques, focusing on optical methods will be analysed. Moreover, the theoretical foundations behind the optical properties of noble metal nanomaterials (nanoparticles, NPs and fluorescent nanoclusters, NCs) will be explained. In addition, the synthetic strategies of both nanostructures and their employment in the detection of heavy metal ions in water will be compared. Finally, in the experimental part carried out during the PhD period, the syntheses and the optical features of four different silver and gold nanostructures will be exposed. In this last section, the syntheses of four different nanostructures will be explained. Silver nanoparticles stabilized by sodium 3-mercapto-1-propanesulphonate represent the first case of study, this system is characterized by nanoparticles with a mean diameter of about 3 nm and a surface plasmon resonance (SPR) band centred at λ = 400 nm. These silver nanostructures are able to detect Co(II), Ni(II) and Hg(II) through the shift of SPR band. Cobalt and nickel ions shift the energy peak to longer wavelengths followed of a band broadening. The high-resolution transmission electron microscopy (HRTEM) images show a clear aggregation mechanism induced by the presence of these ions. On the contrary, the presence of Hg(II) ions shifts the SPR band to shorter wavelengths. The analyses conducted on this system have highlighted that this optical behaviour refers to a direct interaction of Ag atoms on the NPs surface with Hg(II) ions, as a consequence the Ag/Hg amalgam formation occurs. Silver nanoclusters stabilized with polymethacrylic acid (PMAA) represent the second system that will be considered. These nanostructures have a mean diameter of about 1.5 nm and show an emission band in the orange-red range of the visible (660 nm). The colloidal solution is responsive to Pb(II) ions, in fact, the emission intensity increases in their presence. This is due to co-action of photoinduced electron transfer (PET) and chelation-enhanced fluorescence (CHEF) effects. The PET mechanism rules the PL emission of the bare nanosystem, but once in presence of Pb(II) ions, the chelation action of carboxyl groups onto lead contaminants closes the non-radiative de-excitation pathways in favour of the radiative ones and as a consequence the emission intensity is enhanced. The PL emission is a function of Pb(II) ion concentration with a linear range from 0 to 1 µM, and a LOD of 60 nM, very close to the guideline suggested by World Health Organization of 50 nM. A first prototype of a solid-state device has been developed, employing as host material, poly(ethylene glycol) diacrylate (PEGDA), which embeds as guest material the silver nanoclusters with PMAA. This prototype is still able to detect the presence lead(II) ions, leaving unchanged the interaction mechanism between AgNCs-PMAA and Pb(II). This hybrid material is one of the few cases present in the literature, in which noble metal nanoclusters were encapsulated in a solid matrix for environmental pollution detection. Hydrophilic gold nanoclusters stabilized with two different ligands (mercaptohexanoic acid and lipoic acid modified with zwitterionc group) and hydrophobic gold doped with silver nanoclusters shielded with mercaptobenzanoic acid represent the third and the fourth system that will be considered. The gold nano-system shows an emission spectrum in the IR range, while that gold and silver has an emission at the edge between visible and IR regions. Preliminary tests for both nano-systems in presence of metal ions are carried out, revealing good selectivity for Cd(II) and Zn(II) in the cases of AuNCs and AuAgNCs, respectively.