Novel fluorescent silver nanoclusters prepared by core/shell polymer nanoparticles as templates were applied to detecting copper (Cu²⁺) ions in aqueous solution based on fluorescence quenching. Changes in fluorescence intensity allow a quick response and sensitive detection of Cu²⁺ ions at concentrations as low as 20.9 nM. Also, excellent selectivity of Cu²⁺ ions was observed in anti-interference experiments against other metal cations. These fluorescent silver nanoclusters embedded with the polymer nanoparticles have the advantage of long-term stability and good water solubility, which are suitable to be combined with microfluidic systems for real-time sensing of Cu²⁺ ions. Detection in real water samples by our method also presented good results in the experiments, showing the potential of silver nanoclusters embedded with polymer nanoparticles as a practical analytical tool.