To explore the sensing mechanism of RATU for Hg2+, Job’s plot experiments were performed (Fig. 8). A maximal increment of the fluorescence intensity between RATU and RATU-Hg2+ appeared at 0.67 (Hg2+ molar fraction). According to n = x/(1 − x), in which x is the molar fraction of Hg2+ corresponding to the maximal fluorescence change and n is the binding stoichiometry, 0.67 indicates the formation of a 1:2 complex between RATU and Hg2+.