We have developed a chip-based scanometric method for the detection of mercuric ion (Hg²⁺). This method takes advantage of the cooperative binding and catalytic properties of DNA-functionalized gold nanoparticles and the selective binding of a thymine−thymine mismatch for Hg²⁺. The limit of detection of this assay in buffer and environmentally relevant samples (lake water) is 10 nM (2 ppb) Hg²⁺, which is the U.S. Environmental Protection Agency (EPA) limit of [Hg²⁺] for drinkable water and 1 order of magnitude lower than previous colorimetric assays. This assay is capable of discriminating Hg²⁺ from 15 other environmentally relevant metal ions. The method is attractive for potential point-of-use applications due to its high throughput, convenient readout, and portability.