A rhodamine-based optical sensor Rh-3S has been developed for selective detection of Hg2+ aqueous solution as well as in living cells. UV-vis and fluorescence spectroscopic studies reveal that Rh-35 shows marked sensitivity and selectivity to Hg2+ with a chelation-induced "off-on" response in acetonitrile (ACN)/MOPS buffer (10 mM, pH 73, v/v, 1:1). The Rh-3S sensor binds Hg2+ in a 1:1 stoichiometry with an apparent binding constant 3.71 x 10(6)M(-1) (log K = 6.57) and displays a distinct change in color and fluorescence upon the alteration of free Fief levels in solution with a reversible response and little interference with other biological relevant metal ions. Live cell confocal imaging studies demonstrate that the sensor is also capable of imaging the presence of He ions as well as its dynamic changes in live cells. Published by Elsevier B.V.