Shaping NIR-II Emission: The Role of Surface Chemistry and Environment in Gold Nanoclusters

Near-infrared II (NIR-II, 900-1700 nm) fluorescence imaging is transforming biological visualization, offering deeper, sharper, and more reliable detection than visible or NIR-I probes. Reduced scattering and autofluorescence in this window enable real-time imaging of tissues and organs. Gold nanoclusters (AuNCs) are promising NIR-II agents due to their atomically precise structures, biocompatibility, and versatile surface chemistry. However, their modest photoluminescence (PL) in aqueous environments, which is crucial for biomedical applications, remains a key limitation, making brightness enhancement a central challenge. The Au-ligand interface is critical: small changes in ligand structure or binding can strongly affect electronic relaxation. Smart ligand design, including bidentate thiols, electron-rich groups, or N-heterocyclic carbenes, stabilizes excited states and suppresses nonradiative losses. Beyond ligand optimization, strategies such as protein or polymer encapsulation, controlled self-assembly, and layer-by-layer coatings have increased quantum yields to nearly 10% in the 900-1300 nm range, underscoring the role of the metal-ligand environment. The nano(bio)interface also dictates practical performance. In complex milieus, proteins, redox agents, and pH fluctuations can stabilize or quench emission. Antifouling coatings (zwitterionic ligands, PEGylation, or rigid carbene shells) help preserve brightness, while kernel locking, heteroatom doping, and hybrid constructs with dyes or biomolecules extend emission beyond 1200 nm and enable red-shifting via Förster energy transfer (FRET) or bioluminescence energy transfer (BRET).Bright, stable AuNCs thus serve as both imaging agents and theranostic platforms, combining fluorescence with drug delivery, phototherapy, or radioenhancement. Their deep-tissue sensitivity makes them powerful tools for monitoring cancer, cardiovascular disease, and neuroinflammation. Yet environmental sensitivity also raises challenges: stability, biotransformation, and immune activation highlight the need for standardized evaluation of colloidal stability, photostability, and biological interactions. In this Account, we summarize strategies to boost AuNC brightness in water, including ligand design, molecular assembly, protein/polymer encapsulation, and controlled self-assembly, achieving PL quantum yields up to 10%. We also discuss how pH, redox conditions, protein binding, and intracellular aggregation shape NIR-II emission, highlighting key principles for advancing their biomedical use.