Controlled synthesis and assembly of ultra-small nanoclusters for biomedical applications.

Inorganic nanomaterials have been studied extensively in recent years because of their unique physical and chemical characteristics. Moreover, ultra-small nanoclusters (USNCs), which are less than 3 nm in size, exhibit extraordinary properties different from those of larger-sized nanoparticles. For example, ultra-small iron oxide nanoclusters (NCs) show paramagnetic properties; ultra-small noble metal NCs exhibit bright fluorescence; most USNCs gain in vivo renal clearance capability to guarantee safety. Therefore, USNCs could be promising safe materials for biomedical applications. In this mini-review, we summarize recent advances in the controlled synthesis, assembly, and biomedical applications of USNCs. We also discussed future challenges and perspectives in the development of USNC-based nanomedicine.