Strategies of both self-assembly and metal ion replacement were adopted in the development of new metal ion sensors for Zn2+ and Hg2+. Ligand BPBA, phenylene-bridged bis (pyrro1-2-ylmethyl-eneamine), could self-assemble to form a molecular square in the presence of Zn2+, which showed strong emission in solution. The fluorescent emission of formed BPBA-Zn2+ dropped with the addition of Hg2+. BPBA could be a good Zn2+ sensor candidate and BPBA-Zn2+ could be a good Hg2+ sensor candidate based on the mechanisms of the chelation-enhanced fluorescence effect and the replacement of central metal ion induced chelation-enhanced fluorescence quenching effect, respectively.