Near-Infrared-Luminescent Gold Nanoclusters Stabilized by End-Grafted Biocompatible Polymer Chains for Bioimaging

Gold nanoclusters (AuNCs) are an emerging class of nanomaterials for bioimaging, featuring high photostability, size-dependent luminescence, large Stokes shifts, long photoluminescence lifetimes, and good biocompatibility. However, when stabilized by small ligands, AuNCs exhibit a limited colloidal stability. Here, we report the synthesis of original "hairy" AuNCs with a diameter under 2 nm and NIR photoluminescence (for both one-photon and two-photon excitation), stabilized by biocompatible end-grafted poly(N-acryloyl morpholine) (PNAM) chains prepared by RAFT polymerization. The AuNCs synthesis was carried out directly in water in the presence of thiol-terminated PNAM-SH chains. Compared with AuNCs stabilized by small ligands, our PNAM-AuNCs exhibit much better colloidal stability (unchanged for more than two months), a 2-fold increase in photoluminescence lifetimes, and significantly enhanced quantum yields. Moreover, their properties could be successfully tuned using PNAM chains with molecular weights ranging from 3000 to 11 000 g·mol–1. These PNAM-AuNCs offer an excellent platform for the development of bioimaging probes without organic dyes.