A highly selective assay method has been developed to detect mercury (II) (Hg2+) ions using cationic conjugated polymer (CCP). The transduction mechanism is based on a Hg2+ promoted reaction. In the absence of Hg2+ ions, the CCP can form the complex with an anionic 1,3-dithiole-2-thione derivative through electrostatic interactions. The fluorescence of CCP is efficiently quenched by 1,3-dithiole-2-thione derivative via an electron transfer process. Upon adding Hg2+ ions, the transformation of 1,3-dithiole-2-thione into 1,3-dithiole-2-one inhibits the quenching, and the fluorescence of CCP is recovered. Distinguishing aspects of this assay include the signal amplification of CCPs and a specific Hg2+ promoted reaction. By triggering the change in the emission intensity of CCP, it is possible to detect Hg2+ ions in aqueous solution.