© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1017 wileyonlinelibrary.com www.particle-journal.com www.MaterialsViews.com REVIEW Sub-nanometer-sized metal clusters, having dimensions between metal atoms and nanoparticles, have attracted tremendous attention in the recent past due to their unique physical and chemical properties. As properties of such materials depend strongly on size, development of synthetic routes that allows precise tuning of the cluster cores with high monodispersity and purity is an area of intense research. Such materials are also interesting owing to their wide variety of applications. Novel sensing strategies based on these materials are emerging. Owing to their extremely small size, low toxicity, and biocompatibility, they are widely studied for biomedical applications. Primary focus of this review is to provide an account of the recent advances in their applications in areas such as environment, energy, and biology. With further experimental and theoretical advances aimed at understanding their novel properties and solving challenges in their synthesis, an almost unlimited field of applications can be foreseen. 1. Introduction The unique physiochemical properties of soluble/dispersible noble metal and semiconducting nanomaterials have contrib- uted to several areas of research pertaining to energy, envi- ronment, and medicine in the past few decades. [1–6] For noble metals, the excitement has been largely due to plasmonic (metallic) nanoparticles (NPs) of diverse shapes. [3,7,8] In the recent past, a new class of nanoscale materials made of a few to tens of atoms, having size <2 nm, often called nanoclus- ters or quantum clusters (QCs) [9] are receiving large attention due to their unique physical and chemical properties. They are believed to have greater implications to the aforemen- tioned areas. Consequently, precise control of the clusters by developing easy synthetic strategies became an active area of research. These materials fall in the NP-to-atom/molecule tran- sition region and exhibit molecule-like properties owing to the gradual emergence of discrete electronic states. [10] Analogous to the size-dependent bandgap and quantum confinement A. Mathew, Prof. T. Pradeep DST Unit of Nanoscience (DST UNS) and Thematic Unit of Excellence (TUE) Department of Chemistry Indian Institute of Technology Madras Chennai 600036, India E-mail: pradeep@iitm.ac.in DOI: 10.1002/ppsc.201400033 Noble Metal Clusters: Applications in Energy, Environment, and Biology Ammu Mathew and Thalappil Pradeep* effects in semiconductor quantum dots, QCs with sizes comparable to the Fermi wavelength of electrons show interesting properties such as size-dependent fluo- rescence, making them distinctly dif- ferent from their NP counterparts. Unlike semiconductor quantum dots, QCs are less toxic and are smaller in dimension, making them superior candidates than the former in terms of biological applica- tions. This review is an attempt to look at the emerging applications of this fasci- nating branch of materials science. Naming of these materials is still a point of debate and a fully acceptable ter- minology has not appeared so far. As a result, authors use a number of names, which include nanomolecules, nanoclus- ters, NPs, faradaurates, monolayer-pro- tected clusters (MPCs), artificial atoms, super atoms, QCs, etc. MPC has been a more acceptable name in the literature, although this has been used before in the context of plasmonic NPs, since 1995. [11] As clusters being discussed these days are atomically precise, naming them as MPCs seems inappropriate and often makes one confuse them with particles of the past. The name “clusters” is sug- gestive of gas-phase analogues of these materials, which are unprotected and unstable in the condensed phase. The name “nanomolecule” appears to imply that other prefixes such as pico, femto, etc. are possible for molecules, which does not make chemical sense, although we note that macromolecules do exist. “Faradaurate” is not appropriate as the suffix “ate” appears to imply a complex ion. Superatom and artificial atom would have been acceptable if the system was only an aggre- gate of atoms, while the materials are indeed molecules and possess properties of ligands quite distinctly. It is in fact these properties that are being used extensively in many of the appli- cations. Moreover, with the recent addition of the crystal struc- tures of non-superatomic systems such as [Au 23 SR 16 ] - , where SR is SC 6 H 11 , the term “superatoms” fails to describe this class of materials appropriately. It is in this context that a more suitable name, QCs, is used, which suggests distinct optical and electronic properties of the system and also resembles the name, quantum dots which indeed they are, although the former are composed of metals. Molecules are indeed quan- tized and therefore, the prefix “quantum” is, at least, partly redundant. We do note that search for a more appropriate name is continuing. Part. Part. Syst. Charact. 2014, 31, 1017–1053