Chain-End Grafting of RAFT Polyelectrolyte Macroligands to Stabilize Near-Infrared-Emitting Gold Nanoclusters

With their photoluminescence properties, gold nanoclusters (AuNCs, d<2 nm) show great promise in various fields, including sensing, imaging and photosensitization. However, important challenges remain both regarding their colloidal stability and surface functionalization. Here, we investigate the synthesis of AuNCs with end-grafted polymer chains bearing multiple functional lateral groups. Water-soluble polymer chains are synthesized by RAFT controlled radical polymerization to present positive or negative lateral charges, and a thiol end-group to promote grafting to gold. These polyelectrolyte chains are obtained by copolymerization of N-acryloylmorpholine with a comonomer, either cationic (acryloylamino propyltrimethylammonium chloride, AAPTA) or anionic (2-acrylamido-2-methyl-1-propanesulfonate, AMPS), at different charge densities. The AuNCs are synthesized in situ in the presence of the macromolecular ligands, directly in water at room temperature. The cationic copolymers successfully lead to highly stable, NIR-emitting AuNCs fully characterized in terms of size (HR-TEM, ¹H DOSY NMR) and spectrophysical properties. The anionic copolymers yield instead a mixture of AuNCs and larger gold nanoparticles. This study offers an important insight into the influence of electrostatic charges and functional groups of the polymer ligand on the stabilization and optical physical properties of the AuNCs. In addition, it paves the way for the preparation of a large range of functionalized AuNCs.