A cationically charged conjugated polymer (P2) functionalized with quaternary ammonium salt was newly synthesized via Suzuki cross-coupling polymerization. The functionalized P2 features different fluorescence colors according to its phases (blue emission in solution and green emission as asolid) which is caused by intramolecular and intermolecular exciton migration, respectively. The use of P2 as a novel fluorescent sensing platform is demonstrated for mercury ion detection. The detection of mercury ions is accomplished in two steps: (1) the cationic, blue-emitting P2 absorbs an anionic oligonucleotide, polythymidine (PT) via electrostatic interaction to form a complex with green emission due to aggregation between the two species; (2) the addition of mercury ions to the complex produces a new complex of PT-Hg2+ via more favorable specific interaction, resulting in the isolation of P2 and the consequent recovery of blue fluorescence of P2. It suggests that this detection system has high selectivity and sensitivity down to the approximate to 107 M level, even in mixtures of metal ions. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2393-2400