2, Florida State University, Tallahassee, Florida, United States
In FRET-based systems, a specific quencher is normally able to quench the fluorescence only from those fluorophore donors that have significant overlap of their emission spectra with the absorption spectrum of the quencher when the donor and quencher are brought into proximity. In our work, we construct a non-toxic FRET system choosing NIR-emitting CuInS2/ZnS colloidal quantum dots (QDs) as donor while water-soluble IRDye QC-1 dark quencher (quenching range from ~500 to 800 nm) as acceptor. Highly luminescent CuInS2/ZnS QDs (photoluminescence quantum yields ~55%) are synthesized by successive ionic layer adsorption and reaction, which are further transferred into water via cap exchange of the native ligands for amine-functionalized His-PIMA-PEG/NH2 polymer. The water-soluble CuInS2/ZnS QDs possess excellent pH resistibility over a broad range (3-13) and long-term colloidal stability more than one year (ambient condition at 4 °C) accompanied by slight decrease in photoluminescence quantum yields (from 22% to 15%). These amine-terminated QDs can be readily coupled with NHS ester group of the IRDye QC-1 dark dye. The polymer coated QD-dark quencher pair shows efficient FRET evidenced by significant decrease in both photoluminescence intensity and lifetime of the donor via tuning the number of dark quencher per QD.