A single thiourea-appended 1,8-naphthalimide chemosensor for three heavy metal ions: Fe3+, Pb2+, and Hg2+

• A new off-on multifunctional sensor for Fe3+, Pb2+, and Hg2+ is reported.
• The detection is quickly responsive, highly selective and sensitive.
• The sensor can be applied in real sample analysis and living cell imaging.
• A new sensing mechanism is proposed and verified.

A 1,8-naphthalimide-thiourea conjugate was developed for highly selective and sensitive colorimetric detection of Fe3+ and Pb2+ in MeCN/H2O (99/1, v/v) and fluorescent recognition of Hg2+ in MeCN/H2O (85/15, v/v). The linear concentration ranges for Fe3+, Pb2+, and Hg2+ were 0–150, 0–80, and 0–90 μM with the detection limits of 6.86 μM, 5.09 μM, and 82.1 nM, respectively. The sensor responded to Fe3+, Pb2+, and Hg2+ rapidly and a large number of coexisting ions such as Na+, K+, Ca2+, Mg2+, Fe2+, Cu2+, Zn2+, Cr3+, Ni2+, Mn2+, Co2+, Cd2+, and F− showed almost no obvious interference with the detection. Sensing mechanism of the sensor to Fe3+, Pb2+, and Hg2+ was investigated through job's plot experiment, reversible experiment, LC–MS and 1H NMR analysis. Moreover, the off-on sensor was successfully employed to monitor Hg2+ in pond and tap water as well as in living cells.