Mercury ion is of great threat to the human kind due to its high toxicity in live systems. Consequently, its detection at nanolevel is of current interest. The rhodamine derivative is one of the rarest examples of fluorescent chemosensors for Hg2+, wherein a phthalaldehydic acid moiety which shows antibacterial activity, is responsible for specific binding. By using fluorescence correlation spectroscopic (FCS) study of the probe, it has been possible to detect Hg2+ in solution at a level as low as 5 pM. This sensing, applicable to fluorescence lifetime imaging of live cells, provides a novel microscopic technique. (C) 2017 Elsevier B.V. All rights reserved.