To better expound the sensing mechanism, HRMS, + 4H2O], m/z: 778.16609), further confirming the generation of NPDM-Al 3+ whole with 1:2 stoichiometry, which is consisted with the Job's plot analysis. What's more, the 1 H NMR titration was also recorded in DMSO -d6 to better understand the detailed interaction of NPDM with Al 3+ . As provided in Fig. , with the addition of 2.0 equivalent of Al 3+ , the proton H (phenolic-OH) at δ 12.26 ppm downfield shifted to 12.48 whereas the proton H (free -NH2) at δ 7.40 ppm disappeared, meanwhile, a new peak (-NH) was formed at δ 9.38 ppm, which is caused by interaction between -NH2 and Al 3+ . In addition, the imine proton (8.57 ppm, HC=N) slightly upfield shifted to δ 8.51 ppm. Thus, the protonation state of the PhOH enables the ESIPT process remain unchanged in NPDM-Al 3+ adduct. The above findings supported that the proposed mechanism of NPDM for Al 3+ can be illustrated in Scheme 1.