Au-Ag-core-shell nanoparticles have been synthesized using synthetic fluorescent dip eptide beta-Ala-Trp (beta-Ala is beta-alanine; Trp is L-tryptophan) in water at pH 6.94 and at room temperature. The synthesis of the Au-Ag core-shell nanomaterial does not involve any external reducing and stabilizing agents, and the constituents of dipeptide beta-alanine and L-tryptophan are naturally occurring. Therefore, the synthesis procedure is ecofriendly. Moreover, the shell thickness has also been controlled, and the optical property of the core-shell nanomaterial varies with the shell thickness. The core-shell nanomaterial exhibits a fascinating fluorescenceiproperty. This fluorescent Au@Ag core-shell nanoparticle can detect toxic Hg-II ions ultrasensitively (with a lower limit of detection of 9 nM) even in presence of Zn-II, Cd-II, and other bivalent metal ions (Ca-II, Mg-II, Ni-II, Mn-II, Ba-II, Sr-II, Pb-II, and Fe-II). Au-Ag core-shell nanomaterials can also be reused for sensing Hg-II ions.