90th Commemorative Account: Fluorescent Metal Nano-Clusters as Next Generation Fluorescent Probes for Cell Imaging and Drug Delivery Kankan Bhattacharyya* and Saptarshi Mukherjee* Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhopal 462 066, Madhya Pradesh, India E-mail: kankan.bhattacharyya@gmail.com, saptarshi@iiserb.ac.in Received: November 17, 2017; Accepted: December 9, 2017; Web Released: December 26, 2017 Kankan Bhattacharyya Professor Kankan Bhattacharyya worked at Indian Association for the Cultivation of Science (IACS) as Professor and ex-Director since 1987–2016. After retirement from IACS, he has joined IISER Bhopal in December 2016. He is a Senior Editor of The Journal of Physical Chemistry and a member of the Editorial Advisory Board of Bulletin of the Chemical Society of Japan and Chemistry Letters. Saptarshi Mukherjee Dr. Saptarshi Mukherjee did his PhD under the supervision of Professor Kankan Bhattacharyya at IACS, Kolkata and carried out his post-doctoral research with Professor H. Peter Lu at Bowling Green State University, Ohio, USA. He joined the Department of Chemistry, IISER Bhopal in December 2008. His research interests include luminescent metal nanoclusters, protein unfolding and refolding using ultrafast and single molecule spectroscopy. He is presently a Professor in the Department of Chemistry, IISER Bhopal. He has received the INSA Young Scientist Medal in Chemical Sciences and is also a founding member of the Indian National Young Academy of Science. Abstract Fluorescent metal nano-clusters with size-dependent proper- ties have emerged as the next generation fluorophores with ver- satile applications. In this article, we give a brief overview on three fluorescent metal nano-clusters, (gold, silver and copper). Because of their non-toxicity and solubility in water they are highly suitable for biological systems and in particular, live cell imaging. We show that they may be used for distinguishing cancer and non-cancer cells and selective killing of cancer cells. We also discuss theireffect on enzyme catalysis. 1. Introduction Metal nano-clusters refer to 12 nanometer sized particles containing a few (<40) metal atoms. Their properties are vastly different from those oflarger size nanoparticles with thousands of metal atoms. 16 Coinage metals like Au, Ag and Cu have excellent electrical conductivities because of the presence of a single unpaired electron in a s-orbital as a free electron in the conduction band. Chemically, they are more stable(less reac- tive) than the alkali metals. The size of these nano-clusters (³1 nm) is smaller than the wavelength of the excitation light (³400700 nm) and is comparable to the Fermi wavelength of the conducting electrons. In a nano-cluster, due to the small size, the continuous metallic band is replaced by discrete electronic states and this makes them strongly fluorescent, especially for the noble metal nano-clusters. 310 In contrast to quantum dots (CdS etc.), the fluorescent noble metal nano- clusters are soluble in water, possess high photo-stability and are nontoxic. 1117 Owing to these properties, the nano-clusters are becoming increasingly popular as next generation fluores- cent probes and are widely applied in chemistry, physics, biology and materials science. 1326 The development of effi- cient chemical motifs for selective marking ofionic/neutral analytes of biologicalimportance has been an active area of research. 21,22 The concept of “nanodrugs” has also been a topic of burgeoning research interests owing to their potential to be used in treating life threatening diseases. 23,24 Thus, if specific locales withinabio-molecule can be specifically mapped and Bull. Chem. Soc. Jpn. 2018, 91, 447–454 | doi:10.1246/bcsj.20170377 © 2018 The Chemical Society of Japan | 447