In this Letter, we demonstrate that PicoGreen can be used in combination with cationic conjugated polymers to develop label-free DNA sensors with enhanced detection efficiency and further improved selectivity. The high selectivity enables detection of single nucleotide mismatch even at room temperature. This method uses all commercially available materials. It is low cost, is simple to use, and works in a "mix-and-detect" manner. The detection sensitivity could be improved by a factor of 19 times through resonance energy transfer using poly [(9,9-di(3,3'-N,M-trimethylanimonium)propylfluorenyl-2,7-diyl)-alt-co-(1 ,4-phenylene)] diiodide salt (PFP) as a light harvesting complex, to take advantage of its collective optical response and optical amplification effects. The further improved selectivity is ascribed to the stronger binding interaction between PicoGreen with dsDNA compared to that between PicoGreen and ssDNA, which results in selective repelling of PicoGreen away from the DNA strands.