The enhanced fluorescence properties & colloid stability of aqueous CdSe/ZnS QDs modified with N-alkylated poly(ethyleneimine)

Abstract

We have synthesized N-alkylated branched poly(ethyleneimine) by alkylation of poly(ethyleneimine) amino groups with the corresponding bromo-alkanes. N-alkylated branched poly(ethyleneimine) could directly insulate the hydrophobic quantum dots to prepare amino-functionalized hydrophilic quantum dots. Herein, an exhaustive research on the transfer photoluminescence efficiency has been performed by analyzing three parameters (the hydrophobic chain length, degree of alkylation, and molar ratio of N-alkylated branched poly(ethyleneimine) to quantum dots). The obtained N-alkylated branched poly(ethyleneimine)-quantum dots were characterized by Fourier-transform infrared spectroscopy, transmission electron microscopy, and dynamic light scattering, which indicated that N-alkylated branched poly(ethyleneimine) have been effectively coated on the quantum dots' surfaces. In comparison with previous poly(ethyleneimine)-quantum dots prepared by direct ligand–exchange reactions, the N-alkylated branched poly(ethyleneimine)-quantum dots had enhanced photoluminescence intensity and colloidal stability in various biologically relevant environmental conditions (e.g., over a wide pH range, temperature, salt concentrations, and PBS buffer solution). This procedure was versatile and has been successfully extended to other hydrophobic quantum dots, such as Ag and Fe₃O₄ quantum dots.