A simple colorimetric method for detection of Hg2 + in water was proposed. Au nanoparticles and 2,6-pyridinedicarboxylic acid were used for sensing Hg2 +. Sensing mechanisms were demonstrated by TEM and UV–Visible measurements. It showed the solution color changes from red to blue upon addition of Hg2 +. The method selectively detected Hg2 + among seventeen different metal ions.