Metal nanoclusters: New fluorescent probes for sensors and bioimaging

Fluorescent metal nanoclusters (NCs) as a new class of fluorophores have attracted more and more attention due to their unique electronic structures and the subsequent unusual physical and chemical properties. The size of metal NCs approaches the Fermi wavelength of electrons, between metal atoms and nanoparticles, resulting in molecule-like properties including discrete energy levels, size-dependent fluorescence, good photostability and biocompatibility. These excellent properties make them ideal fluorescent probes for biological application. Up to now, significant efforts have been devoted to the synthesis, property and application studies of gold and silver NCs. Recently, a growing number of studies on copper and other metal clusters have also been reported. In this review article, we focus on summarizing recent advances in controllable synthesis strategies, chemical and optical properties, and sensing and imaging applications of metal NCs (mainly including Au, Ag, Cu, etc.). Finally, we conclude with a look at the future challenges and prospects of the future development of metal NCs.