Atomically Precise Sulfur Atom Doping Amplifies Shortwave Infrared Photoluminescence of Metal Nanoclusters for In Vivo Bioimaging.

Ultrasmall metal nanoclusters (MNCs) are promising renal-clearable probes for diverse biomedical applications and clinical translations. However, the low photoluminescence (PL) efficiency of hydrophilic MNCs, particularly in the shortwave infrared (SWIR) region, presents a major constraint on their bioimaging performance. Herein, we report a unique strategy to amplify MNCs' SWIR PL via precise sulfur doping. Through the optimized synthesis of Au13-16Ag12-9(MHA)18 NCs (MHA = 6-mercaptohexanoic acid) and subsequent sulfide treatment, two sulfur atoms are controllably doped into the NCs. Combined spectroscopic and theoretical studies revealed that the sulfur atoms occupy the trigonal Ag3 facets of the icosahedral kernel, which induces kernel contraction, enhances stability, and suppresses nonradiative decay rate (∼250-fold) in the NCs. The MNCs therefore enable high-contrast SWIR PL imaging of lymphatic vessels and inflamed lymph nodes. This study offers a new approach to improve SWIR PL and establishes new structure-property relationships in MNCs, which will advance the development of cluster probes for versatile bioapplications.