We reported a core–shell structured nanocomposite for Hg(II) sensing and removal, using superparamag-netic Fe3O4nanodots as the core and silica molecular sieve MCM-41 as the shell. A rhodamine derivedemission “Off-On” probe was grafted into the tunnels of MCM-41. The resulting nanocomposite was fullycharacterized and studied with electron microscopy images, XRD curves, thermogravimetric analysisand N2adsorption/desorption isotherms, confirming the core–shell structure and the successful probeloading. The Hg(II) sensing and removal performance of this nanocomposite was then explored in detail.Experimental data suggested that the emission from the probe increased with the increasing Hg(II) con-centration, showing emission “Off-On” effect and linear response towards Hg(II) ion. Fast response, goodselectivity and low detection limit were also obtained, owing to the highly ordered tunnels from MCM-41.In addition, good recycling performance was achieved by this nanocomposite.