A metal ion sensor is provided. The metal ion sensor includes a nanoparticle core doped with a first luminescent material and a shell enclosing the nanoparticle core. The shell includes a second luminescent material and binding sites of outer metal ions. The first luminescent material and the second luminescent material emit light with mutually different colors when excited by an excitation source. The luminescence intensity of the light emitted from the second luminescent material varies in accordance with the binding amount of the outer metal ions, such that the emission color of the combined luminescence of the first luminescent material and the second luminescent material is changed.