Quick-and-easy preparation and purification of quantum dot–loaded liposomes

Quantum dots are very attractive as fluorescent markers because of their excellent optical properties. For this reason, they have also been used to label liposomes by means of encapsulation, though their feasibility as liposome labels is often hampered by the presence of unencapsulated quantum dots. Until now, laborious gradient ultracentrifugation or less efficient size exclusion chromatography has been the methods of choice to remove unencapsulated quantum dots. Of these two strategies, size exclusion chromatography is most commonly used, despite the known poor separation. Consequently, this prompts for a choice between purification methods yielding high-purity quantum dot-loaded liposomes but low yields or vice versa. Herein, we present a novel high-yield and high-purity methodology to remove unencapsulated quantum dots in a quick and efficient manner based on electrostatic binding of quantum dots to ion-exchange beads. This was accomplished either by means of short column chromatography or via a simple pull-down approach. The purification efficiency was easily assessed via analytical gel electrophoresis, and by copper-mediated quenching of quantum dot fluorescence, it was established that the quantum dots were not adhered to the liposomes but encapsulated inside these. Furthermore, the recovery degree of quantum dot-loaded liposomes after ion-exchange purification was found to be excellent compared with size exclusion chromatography. Lastly, a method is presented to quantify the number of quantum dots encapsulated in the liposomes by the combined efforts of particle counting and inductively coupled plasma mass spectrometry.