The chemical sensing for the convenient detection of mercuric ion (II) (Hg(2+)) have been widely explored with the use of various sensing materials and techniques. It still remains a challenge to achieve ultrasensitive but simple, rapid, and inexpensive detection to metal ions. Here we report a surface-enhanced Raman scattering (SERS) chip for the femtomolar (fM) detection of Hg(2+) by employing silver-coated gold nanoparticles (Au@Ag NPs) together with an organic ligand. 4,4'-Dipyridyl (Dpy) can control the aggregation of Au@Ag NPs via its dual interacting sites to Ag nanoshells to generate strong Raman hot spots and SERS readouts. However, the presence of Hg(2+) can inhibit the aggregation of Au@Ag NPs by the coordination with Dpy, and as a result the SERS signals of Dpy are quenched. On the basis of these findings, a SERS chip has been fabricated by the assembly of Au@Ag NPs on a piece of silicon wafer and the further modification with Dpy. The exchange of Dpy from the chip into the aqueous Hg(2+) droplet results in the quenching of Raman signals of Dpy, responding to 10 fM Hg(2+) that is about 6 orders of magnitude lower than the limit defined by the U.S. Environmental Protection Agency in drinkable water. Each test using the SERS chip only needs a droplet of 20 μL sample and is accomplished within ∼4 min. The SERS chip has also been applied to the quantification of Hg(2+) in milk, juice, and lake water.