A fluorescent chemosensor for Cu2+ ions based on dansyl-functionalized thiol stabilized silver nanoparticles containing 2-aminothiophenyl units as the Cu2+ binding sites is developed. A decrease in fluorescence at 497 nm and an increase in fluorescence at 410 nm with an isoemissive point at 445 nm upon the addition of Cu2+ ions is attributed to the formation of a Cu2+ complex in aqueous acetonitrile based on an energy transfer mechanism. This sensor has excellent sensitivity and selectivity over other metal ions and has a detection limit as low as 5.0 × 10-10 mol L-1.