Near-infrared II emissive metal clusters: From atom physics to biomedicine

Near-infrared II (NIR-II) imaging at 1100–1700 nm supports deep tissue penetration, low auto-fluorescence interruption, and high imaging resolution due to advances in attenuating photon scattering and absorption. For example, semiconductor quantum dots and carbon nanotubes as pioneer molecular probes were used for hind limbs and cerebrovascular imaging, and organic small molecules and polymers were employed for ultrafast and three-dimensional tumor vasculature imaging. Compared with the reported NIR-II fluorescence probe systems, the atomic-precision metal clusters with unambiguous geometry structure can make the most of the spatial coordination of metal atoms by artificial atom manipulation, thus improving the luminescent quantum yield with tunable emission wavelength. Meanwhile, the water soluble, renal cleanable, and highly stable features make them promising candidates for in vivo imaging. In this review, we aim to summarize the molecular and electronic structure, and optical properties of NIR-II emissive metal clusters. Meanwhile, we highlight their applications in brain, kidney, gastrointestinal imaging and tumor metastasis monitoring. We particularly reviewed recent advances on the biosafety of gold clusters at ultrahigh concentration. Present work will promote the development of NIR-II imaging and make a step towards the clinical application.