Herein, in order to obtain Ca2+-sensitive QDs, we selected hexametaphosphate, that is a metal chelating agent, as coating material to synthesize hexametaphosphate-capped CdS QDs. The prepared hexametaphosphate-capped CdS QDs showed a significant fluorescent enhancement with the addition of Ca2+, because of the for mation of calcium polyphosphate shell. F−can react with Ca2+ to generate insoluble calcium fluoride, and the Ca2+ -induced fluorescence enhancement would be inhibited. Therefore, the proposed CdS QDs could be used to effectively detect both Ca2+and F−.