Rapid synthesis of highly luminescent and stable Au20 nanoclusters for active tumor-targeted imaging in vitro and in vivo

Rapid synthesis of protein-stabilized Au20 nanoclusters (Au20NCs) with high fluorescence quantum yield (QY) up to ∼15% is successfully achieved by manipulating the reaction kinetics. The as-obtained Au20NCs, identified by mass spectrometry, have an average size of 2.6 nm, with strong fluorescence emission at 620 nm (2.00 eV) upon excitation at either 370 nm (3.35 eV) or 470 nm (2.64 eV). The advantages of the as-obtained Au20NCs, including small sizes, high fluorescence QY, excellent photostability, non-toxicity, and good stability in biological media, make them ideal candidates as good luminescent probes for optical imaging in vitro and in vivo. Our results demonstrate that the uptake of Au20NCs by both cancer cells and tumor-bearing nude mice can be improved by receptor-mediated internalization, compared with that by passive targeting. Because of their selective accumulation at the tumor sites, the Au20NC probes can be used as potential indicators for cancer diagnosis. This work not only provides a new understanding of the rapid synthesis of highly luminescent Au20NCs but also demonstrates that the functionalized-Au20NCs are excellent probes for active tumor-targeted imaging in vitro and in vivo.