Fluorescence signal amplification by cation exchange in ionic nanocrystals.

Lighting-up time: A cation-exchange reaction releases thousands of divalent cations from nonfluorescent CdSe ionic crystals and triggers fluorescence from thousands of originally nonfluorescent Fluo-4 fluorophores to obtain a large fluorescence amplification (see picture) and a low detection limit in bioassays. The technique is fast, simple, with a large dye-to-reporter labeling ratio, and flexible in selection of nanocrystals and fluorophores.