Recent advances in the synthesis, characterization, and biomedical applications of ultrasmall thiolated silver nanoclusters

With ultrasmall particle sizes of similar to 1 nm, thiolate-protected silver nanoclusters (or thiolated Ag NCs) have recently emerged as an attractive frontier of nanoparticle research because of their unique molecular-like properties, such as well-defined molecular structures, HOMO-LUMO transitions, quantized charging, and strong luminescence. Such intriguing physicochemical properties have made thiolated Ag NCs a new class of promising theranostic agents for a wide spectrum of biomedical applications, such as bioimaging, antimicrobial agents, and disease diagnostics and therapy. In turn, the promising applications of thiolated Ag NCs have also fuelled the cluster community to develop more efficient strategies to synthesize high-quality Ag NCs with well-defined size, structure, and surface. In this review article, we first survey recent advances in developing efficient synthetic strategies for thiolated Ag NCs, highlighting the underlying chemistry that makes the delicate control of their sizes and surfaces possible. In the second section, we discuss recent advances in characterization techniques for ultrasmall thiolated Ag NCs, including their physical, chemical, and biological properties. The emerging characterization techniques are central to the development of cluster chemistry. In the last section, we highlight some examples demonstrating the vast possibilities of thiolated Ag NCs for biomedical applications. We conclude this review article by pointing out some challenging issues related to thiolated Ag NCs, and hopefully these can encourage more concerted efforts on their study from the research communities of cluster chemistry, noble metal chemistry, biology, biomedicine, etc.