We have developed a new highly selective and sensitive technique for the detection of Hg2+ using DNA-functionalized gold nanoparticles (Au NPs) and OliGreen. This system is the first that allows the detection of Hg2+ based on the release of DNA molecules, induced by conformational changes on An NP surfaces, and its sensitivity is highly dependent upon surface DNA density. When Hg2+ ions interact with the thymidine units of the DNA molecules bound to the Au NPs through Au-S bonds, the conformations of these DNA derivatives change from linear to hairpin structures, causing the release of some of the DNA molecules from the surface of the An NPs into the bulk solution to react with OliGreen. The fluorescence of OliGreen-DNA complexes increased with increasing concentration of Hg2+, and Hg2+ could be detected at concentrations as low as 25 nM. A linear correlation existed between the fluorescence intensity and the concentration of Hg2+ over the range 0.05-2.5 mu M (R-2 = 0.98). This simple and cost-effective probe was applied to determine the spiked Hg2+ in the pond samples; the recoveries (96-102%) suggested low matrix interference and high sensitivity.