Electron and energy transfer mechanisms to switch the luminescence of semiconductor quantum dots

Chemosensors, able to signal the presence of target analytes with luminescence enhancements, can be designed around electron and energy transfer processes. Specifically, the association of a target analyte with a complementary receptor can be transduced into a significant change in the emission intensity of an organic fluorophore on the basis of these processes. The photophysical properties of semiconductor quantum dots, however, are significantly more attractive than those of organic dyes. As a result, the identification of strategies to adapt similar transduction mechanisms to quantum dots can lead to the emergence of luminescent chemosensors with improved performances. In this context, we developed two strategies based on electron and/or energy transfer to probe pH and signal receptor -substrate complementarities with luminescent quantum dots. In particular, we coated preformed CdSe -ZnS core -shell quantum dots with pH-sensitive [1,3] oxazines, in one instance, and 4,4'-bipyridinium dications, in the other. Both species exchange electrons and/or energy with the nanoparticles upon excitation, and hence quench their emission. However, their absorption wavelengths, redox potentials and distance from the nanoparticles can be designed to vary in response to pH changes or the presence of complementary receptors. These modifications alter the quenching ability of the organic ligands and culminate into significant enhancements in the luminescence of the inorganic nanoparticles. Thus, our transduction mechanisms can successfully be exploited to signal the presence of target analytes with luminescence changes, and can ultimately lead to the development of innovative luminescent chemosensors based on the unique photophysical properties of semiconductor quantum dots.