To demonstrate its potential biological applications, RATU was employed to fluorimetrically detect Hg2+ in living cells. No fluorescent signal could be observed in the untreated cells and the cells only treated with RATU (or Hg2+) (Fig. 14j–l). However, the cells incubated with RATU and Hg2+ emitted notable red fluorescence (Fig. 14m–r). Breadthwise, from Fig. 14m–o (or Fig. 14p–r), the intracellular fluorescence intensity increased upon raising the concentration of RATU. Lengthways, the intracellular fluorescence of Fig. 14p–r was stronger than that of Fig. 14m–o respectively owing to the higher concentration of Hg2+. The normal shape of the cells in bright field images (Fig. 14a–i) confirmed that the cells were viable during the imaging experiments. These results indicated that RATU could be used for detecting Hg2+ in biological samples.