Water-Dispersible Gold Nanoclusters: Synthesis Strategies, Optical Properties, and Biological Applications.

Atomically precise gold nanoclusters (Au NCs) are an emerging class of quantum-sized nanomaterials. The intrinsic discrete electronic energy levels endowed them with fascinating electronical and optical properties. They have been widely applied in the field of optoelectronics, photovoltaics, catalysis, bio-sensing, bio-imaging and therapeutics. Nevertheless, most Au NCs are synthesized in organic solvents and are insoluble in aqueous solution, which restricted their biological applications. In this review, we focused on the recent progress in the preparation of water-soluble Au NCs and their biological applications. We first reviewed their synthetic methods, including direct synthesis from hydrophilic templates and indirect phase transfer of hydrophobic Au NCs. We then discussed their photophysical properties such as emission enhancement and their influence factors. We next summarized their latest applications in the fields of biosensing, biolabeling, and bioimaging. Finally, we outlined the challenges and potentials for the future development of these Au NCs.