Bright Type II Quantum Dots

It is a paradigm that the exciton splitting characteristics of type II quantum dots (QDs) result in low quantum yield materials. However, reported herein is the synthesis of ZnSe/CdS/ZnS type II QDs with quantum yields as high as 61%. Most interesting is the fact that the enhancement in brightness is due to an increase in the radiative rate, rather than the suppression of surface trap-related nonradiative processes, which was found to be minimal. These QDs have been water-solubilized using two different methods and retain a substantial fraction of their brightness. The water-soluble type II QDs were conjugated to a dye to examine their potential as fluorescence resonance energy transfer (FRET) donors. A significant FRET efficiency of 61 ± 3% was measured using time-correlated single photon counting, which demonstrates the potential for these materials to be used in FRET-based chemical and biological sensing applications.