Abstract: A new coumarin derivative (MS1) containing an imine moiety and a hydroxyl moiety exhibits an enhanced fluorescence in the presence of Hg2+ ions. Other metal ions Al3+, Ca2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Ni2+, Pb2+, and Zn2+ produced only minor changes in the fluorescence values of MS1. The binding ratio of MS1–Hg2+ complexes was determined from the Job plot to be 1:1. The binding constant (K a) of Hg2+ binding to MS1 was found to be 6.85×103 M−1. The maximum fluorescence enhancement caused by Hg2+ binding to MS1 was observed in the pH range of 6.5–9.0. Confocal fluorescence microscopy imaging using RAW 264.7 cells showed that MS1 could be used as an effective fluorescent probe for detecting Hg2+ in living cells. [Copyright &y& Elsevier]