We describe the application of a recently discovered family of materials called quantum clusters, which are sub-nanometer particles composed of a few atoms with well-defined molecular formulae, exhibiting intense absorption and emission in the visible region in metal ion sensing, taking Ag 25 as an example. The changes in the optical properties of the cluster, in both absorption and emission upon exposure to various metal ions in aqueous medium are explored. The cluster can detect Hg 2+ down to ppb levels. It can also detect 5d block ions (Pt 2+, Au 3+ and Hg 2+) down to ppm limits. Hg 2+ interacts with the metal core as well as the functional groups of the capping agents and the interaction is concentration-dependent. To understand the mechanism behind this type of specific interaction, we have used spectroscopic and microscopic techniques such as UV-vis spectroscopy, luminescence spectroscopy, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). Specific reasons responsible for the interaction of Hg 2+ have been proposed. © 2011 Elsevier B.V.