Sensors that can detect mercury with high sensitivity and selectivity are very useful in preventing mercury poisoning as well as understanding its distribution. Most fluorescent sensors for Hg_+ detection need an organic solvent, and have poor sensitivity or selectivity. Based on the fact that Hg_+ can bind in between T-T mismatches in DNA and stabilize the T-T mismatches, while other metals cannot, this sensor has a detection limit of 2.4 nM, which is lower than the EPA limit of Hg_+ in drinking water. The sensor is also very selective and is silent to any other metal ions with up to millimolar concentration levels.