Emerging applications of near-infrared fluorescent metal nanoclusters for biological imaging

Fluorescent metal nanoclusters (MNCs) have recently emerged as a novel kind of promising fluorescent probes for biological imaging because of their ultrasmall core size (<2 nm), strong photoluminescence, facile availability and good biocompatibility. In this review, we provide an update on recent advances in the development of near infrared (NIR)-emitting MNCs in terms of synthesis strategies and bioimaging applications. We mainly focus on the utilization of NIR-emitting MNCs (including Au, Ag, Cu and alloy NCs) either as single modal imaging (fluorescence intensity-based imaging, fluorescence lifetime imaging, two-photon imaging) probes or as multimodal imaging (such as NIR fluorescence/X-ray computed tomography/magnetic resonance imaging, NIR fluorescence/photoacoustic imaging/magnetic resonance imaging, NIR fluorescence/single photon emission computed tomography) probes in biological cells and tissues. Finally, we give a brief outlook on the future challenges and prospects of developing NIR-emitting MNCs for bioimaging.