In this letter, we demonstrated a practical scheme for the detection of mercury ions in aqueous media at room temperature with high sensitivity and selectivity by using a combination of oligonucleotides, DNA intercalators, and conjugated polymers. This scheme combines the advantages of specific binding interactions between Hg2+ and thymine and optical amplification properties of conjugated polymers. This method is label-free, low cost, and simple to use, and all of the materials are commercially available. It works in a "mix-and-detect" manner. The limit of detection could be improved to 0.27 nM, which is much lower than the maximum level of mercury permitted by the EPA in drinking water. This scheme could also be potentially used as a two-photon sensor for detecting mercury ions in a biological environment where deep penetration is required. A detection limit of as low as similar to 6 nM could be achieved under two-photon excitation.