Functionalization of water-soluble gold nanoparticles for biological applications

Water-soluble gold nanoparticles represent an appealing scaffold for the preparation of robust and biocompatible bioconjugates. Indeed, many examples of gold nanoparticles-bioconjugates as new materials in several fields as material science, biology and medicine have been reported in the literature. The organic monolayer protecting the metallic core plays a key role in determining the properties of the system as stability, solubility, and specific interactions with biological environment. The present thesis is focused on the functionalization of water-soluble gold nanoparticles in order to develop new tools in diagnostics, drug-delivery and enhanced immuno-sensing. Gold nanoparticles protected by mixtures of ligands of different nature have been taken into consideration in the development of the three main projects of this thesis. The first project is about the synthesis of gold nanoparticles with a gold core of 1.7 nm suited for crystallization, in order to perform diffractometric analysis aimed to solve the structure of larger systems than that already reported and to find other geometries of the gold core. To this aim, gold nanoparticles protected by a monolayer of p-mercaptobenzoic acid have been synthesized, purified and characterized. The choice of an aromatic ligand with a carboxylic group imparts stability to the clusters and plays a strategic role in crystals formation. Crystallization trials under a variety of different conditions and preliminary observations about the stability of the nanoparticles are reported. Up to now suitable crystals for X-ray analysis could not be obtained. The second project is part of an ongoing investigation of the morphological organization of the monolayer protecting gold nanoparticles in order to complete previous studies carried out in our research group. Recent results from our laboratories, obtained by ESR measurements, support the formation of ࢠpatchesࢠdomains in the mixed-monolayer of water-soluble gold nanoparticles when mixtures of perfluoroalkyl- and alkylthiolates are used to form the monolayer. The complexity of these systems may also be increased introducing functional thiolates in the monolayer in a controlled topology. The preliminary results obtained so far should be completed with other investigations using different methodologies and supported by studies also on flat surfaces. Moreover, to understand the ability of the amphiphilic thiols to phase-segregate, we thought to study also micellar aggregates. The final goal is to use this phase-segregated monolayers to create clusters of functional thiols for multivalent recognition. Water-soluble gold nanoparticles coated by amphiphilic thiols of different lipophobicity have been prepared and characterized, and new ligands suited for the studies on micelles and on 2D self-assembled monolayers have been designed and synthesized. The results of Electron Spin Resonance (ESR), Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) studies on these systems are reported and discussed. The third project is aimed to find new synthetic strategies to obtain biocompatible gold nanoparticles presenting multiple bioactive residues for multivalent recognition processes. In particular, a mimetic of the antigen GM3 Ganglioside Lactone with demonstrated antimelanoma reactivity was introduced in the monolayer of water-soluble gold nanoparticles for the development of a biological therapy against cancer. The preparation of nanoparticles of different size and loading of the antigen-mimetic is reported, together with their characterization and the preliminary biological investigations.