In this study, An efficient OFF–ON fluorescent compound integrated with nitrobenzofurazan and phenolphthalein (NBFP) has been prepared for the detection of metals ions. NBFP was characterized with 1H NMR, 13C NMR, COSY-NMR, FTIR, and Mass spectrometry. The fluorescent response towards metal ions displayed that NBFP is selective for Hg (II) and Al (III), with a detection limit of 14.7 µM and 16.3 µM for Hg(II) and Al(III), respectively. Job's plot for NBFP + Hg (II) complex, and NBFP + Al (III) complex confirmed the 1:2 ligand–metal stoichiometry. Binding constant (Ksv) from Stern-Volmer equation was found to be 1.60 × 102 M−1 for NBFP + Hg (II) complex and 9.25 × 102 M−1 for NBFP + Al (III) complex respectively, indicating a strong interaction between ligand and selective metal ions in the presence of other ions. The quantum yield of NBFP was found to be ΦL = 0.028, exhibiting a weak and wide emission band. Cytotoxicity of NBFP has been evaluated before application in living cell fluorescent imaging for the detection of metal ions. The result showed that NBFP exhibits good biocompatibility in living cells and displayed a good response for the detection of Al (III) and Hg (II) ions in living cells using the confocal microscope.