In this Letter we present a new probe, 2-amino-3-hydroxy-2-(hydroxymethyl) propyl 2-(7- hydroxy-2-oxo-2H-chromen-4-yl) acetate (PMR), which can reversibly detect mercuric ions (Hg2+) in HEPES buffer under physiological conditions. Possible interference with other analytes was examined. PMR displays a highly selective decrease of its fluorescence at 460nm when it reacts with Hg2+. Interestingly, the probe can also be used as a ...