Synthesis and Characterization of PEGylated Luminescent Gold Nanoclusters Doped with Silver and Other Metals

Doping of fluorescent noble metal nanoclusters is being pursued to manipulate the structure of such materials along with improving physicochemical characteristics such as long-term stability and photoluminescence quantum yield. Here, we synthesize metal-doped and alloyed ultrasmall gold nanoclusters (AuNCs) directly in water using a facile one-step coreduction reaction with bidentate dithiolane PEGylated ligands that terminate in different functional groups including a methoxy, carboxy, amine, and azide. Two primary types of cluster materials were the focus of synthesis and characterization: first, a series of doped/alloyed Ag-doped AuNCs, where the ratio of Au:Ag was varied across a wide range including 99:1, 98:2, 90:10, 80:20, 50:50, 20:80, 10:90, and 2:98 along with pure AuNC and AgNC controls; second, doped Au:D NCs, where D included Pt, Cu, Zn, and Cd. Physical characterization of the modified AuNCs included TEM analysis of size, XPS/EDX analysis of dopant content, and a detailed analysis of photophysical properties including absorption and photoluminescence profiles, quantum yields over time, photoluminescence lifetimes, and examination of energy levels for selected materials. The addition of just a few Ag dopant atoms per AuNC yielded significant enhancement in quantum yield along with improving long-term photostability especially in comparison to materials with a very high Ag content. Preliminary cell imaging applications of the Ag-doped AuNCs were also investigated. Facilitated cellular uptake by mammalian cells via endocytosis following modification with cell penetrating peptides was confirmed by colabeling with specific cellular markers. Long-term intracellular photostability and lack of aggregation were confirmed with microinjection studies, and cytoviability assays showed the doped clusters to be minimally toxic.