A fluorimetric/colorimetric mercury sensor based on Au@SiO₂ core/shell nanoparticles has been developed and demonstrated. The porphyrin derivative (2) was attached to Au@SiO₂ core/shell nanoparticles by covalent bonds and showed a red color and strong fluorescent properties. In the absence of specific metal ions, the porphyrin-functionalized Au@SiO₂ nanoparticles (1) exhibited strong fluorescence emission and were red in color. The addition of Hg²⁺ ion to 1 resulted in a color change from red to green within 10 s and a weak display of fluorescence. Conversely, no significant changes in fluorescence emission or color were observed in the parallel experiments with Li⁺, Na⁺, Ca²⁺, Cu²⁺, Cd²⁺, Co²⁺, Mn²⁺, Cd²⁺, Ag⁺ and Pb²⁺. Regarding the reversibility of 1, the fluorescence and color of 1 in the presence of Hg²⁺ ion were found to be almost reversible when 1 was treated with EDTA solution. Furthermore, a study of the effect of pH on 1 with bound Hg²⁺ ion indicated that the fluorescence intensity and color change of 1 was almost constant between pH 4 and 10. This sensor has excellent selectivity and sensitivity over other metal ions and has detection limits below the maximum contamination level of 1.2 ppb for Hg²⁺ ion in drinking water, as defined by the U. S. Environmental Protection Agency (EPA). The results obtained not only allow a practical sensing application for the Hg²⁺ ion but also serve as a guide for the design of fluorimetric/colorimetric sensors for other targets.