A highly sensitive electrochemical sensor for the detection of mercury(ii) ions (Hg2+) was developed by using a three dimensional ordered macroporous polyaniline-platinum (3DOM PANI-Pt) composite film as a sensing platform. 3DOM PANI-Pt was realized by electrochemical co-deposition of Pt nanoparticles and PANI into the sacrificed silica template through the redox reaction involving the monomer aniline (AN) and PtCl6 2-. The G-rich oligonucleotide strand functionalized Au nanoparticle was employed as the Hg2+-mediated structural switch and to enhance the sensitivity. In the absence of Hg2+, the oligonucleotide strand formed an intramolecular duplex where the G-rich sequence was partially caged. In the presence of Hg2+, the release of the G-rich sequence was observed due to the stabilization of T-Hg2+-T, which formed an active G-quadruplex DNAzyme and catalyzed the reduction of H2O2. Under the optimal conditions, the current signal of H2O2 increased with increasing Hg2+ concentration in the dynamic range from 10-13 to 10-6 M, and the detection limit up to 8.7 × 10-14 M was seen. Further, the sensor was successfully utilized for the determination of Hg2+ in an authentic aquatic sample, with an acceptable accuracy compared to the method commercially available. In this paper, we proposed a general and simple strategy because only one oligonucleotide strand was required for both Hg2+ recognition and signal amplification, potentially allowing the detection of other metal ions or trace pollutants in environmental matrices by simply employing various DNA or aptamer probes.