Synthesis and Functionalization of Copper-Doped Indium Phosphate Quantum Dots with Ratiometric Fluorescence

The doped quantum dots (QDs) that feature dual emission in one nanocrystal and long fluorescence lifetime have gained great interest in biosensing and bioimaging applications. In this work, we report the synthesis and functionalization of copper ion-doped indium phosphate (Cu:InP) QDs that exhibit simultaneous green InP emission and red Cu2+ emission. The oil-soluble QDs were synthesized by adsorbing Cu2+ onto InP core, followed by growing zinc selenide (ZnSe) shell via the successive ion layer adsorption reaction. During the synthesis, fluorescence ratiometry (and resultant multiple fluorescence colors) can be generated by changing either the dopant amount or the shell-growth time, but they act in a different manner: increasing the Cu2+ amount results in quenched InP emission and oppositely improved Cu2+ emission; the increase of shell-growth time leads to continuously improved Cu2+ emission relative to constant InP emission. Further, functionalization of the oil-soluble Cu:InP QDs with dihydrolipoic acid-polyethylene glycol (DHLA-PEG) via ligand exchange produces the water-soluble and biocompatible dual-emission QDs. The PEGylated Cu:InP QDs present desirable charge neutrality and excellent thermal stability and photostability, thereby holding high potential in a diversity of biomedical applications.