Synthesis, characterization and properties of water-soluble gold nanoparticles with tunable core size

Robust, water-soluble gold clusters protected by monolayers of ligands containing a short alkyl chain (C7) close to the gold surface and a triethylene glycol monomethylether unit (TEG) to impart solubility in water and other polar solvents were prepared and characterized. Thiol 7 (N1-{2-[2-(2-methoxyethoxy) ethoxy] ethyl}-8-sulfanyloctanamide) constitutes a good and versatile capping agent for the preparation of these nanoparticles. By tuning the Au/thiol ratio and sodium borohydride addition rate, nanoparticles with different core diameters ranging from 1.5 to 4.2 nm, as determined by TEM analysis, could be obtained. The size distribution of the gold cores appears to become broader as the Au/thiol ratio used to prepare the nanoparticles increases. Characterization of these nanoclusters also by NMR, UV-Vis and FTIR spectroscopies is reported. Solubility properties have been studied in a large variety of solvents and different solubility behaviors were observed for nanoparticles of different sizes. Exchange reactions were carried out successfully with small (1.9 nm) and large nanoparticles (4.2 nm) using dodecanethiol as the entering thiol. This demonstrates that these materials can be used for the preparation of nanoclusters with different functional groups soluble in polar solvents including water. The synthetic procedure described represents a facile route to tailoring the size and solubility properties of Au nanoparticles.