A highly luminescent (quantum yield 21%) mixed chalcogenide silver cluster, Ag₅₆Se₁₃S₁₅ cluster of Ag₂X stoichiometry, protected with a 4-tert-butylbenzyl mercaptan ligand has been synthesized and characterized. Investigation using diverse tools of analysis such as mass spectrometry, elemental analysis, thermogravimetry and X-ray diffraction confirmed this composition. The cluster emits in solution and in the solid state and has been deposited on oxide supports to get red emitting films. The specificity of cluster emission to the mercuric ion Hg(II), among a range of heavy metal ions, was used to develop a sensor, which shows sensitivity down to 1 ppb. A pH paper like visual detector was developed by combining the Hg(II)-sensitive emission of the cluster and the insensitive emission of fluorescein isothiocyanate. The test strip showed visual detection down to 1 ppb in real water samples.