A new methodology has been developed for DNA detection that interfaces optical amplification properties of cationic conjugated polyelectrolytes with highly selective target-induced DNA strand displacement. The probe solution contains a cationic conjugated polyelectrolyte (CCP-1), partly hybridized duplex DNA labeled with a fluorescein at the 5'-terminus, and endonuclease Hae III. Excitation of the CCP-1 leads to efficient energy transfer from CCP-1 to fluorescein. In the presence of a complementary DNA strand to one strand of the probe duplex, a hairpin DNA with the recognition site of endonuclease Hae at the double-stranded stem is released following its cleavage by Hae III to generate short DNA fragment carrying fluorescein. The relatively weak electrostatic interactions between the DNA fragment and CCP-1 lead fluorescein far away from CCP-1 and inefficient energy transfer between them is present. Thus, the DNA can be detected by fluorescence spectra in view of the observed CCP-1 or fluorescein emission changes in aqueous solutions. To avoid utilizing unstable Hae III endonuclease, a new system based on RNA-cleaving DNAzyme was further developed. The protocol offers a convenient approach for homogeneous, selective, and sensitive DNA assay in aqueous solution without using any denaturation steps. Compared with previously reported DNA sensors based on conjugated polyelectrolytes, our new method is highly sequence specific and a single-nucleotide mismatch can be clearly detected in target DNA.