Re-Examination of the Polymer Encapsulation of Quantum Dots for Biological Applications

Abstract

The application of semiconductor core–shell quantum dots in biology requires the dispersion of these nanoparticles in water. A widely used phase transfer strategy is their encapsulation with amphiphilic polymers, a method that is believed to preserve the original ligand shell of quantum dots. In this contribution, we test this belief for the first time using a common encapsulation protocol based on poly(styrene-co-maleic anhydride) and ethanolamine and demonstrate that the original quantum dot ligand shell is in fact altered during the polymer encapsulation. In particular, our results show that primary amines, which are typically added to increase the hydrophilicity of the encapsulating polymer, displace the original ligands as a necessary step in the phase transfer process. We demonstrate that the surface of quantum dots synthesized using phosphinic acid as a ligand is readily modified by ethanolamine and hence shows high phase transfer efficiency, whereas quantum dots synthesized using oleic acid ligands did not show reactivity toward the primary amine and failed to transfer in the same conditions. Instead, a thiol, mercaptoethanol, was employed to displace the original oleic acid ligands and promote efficient poly(styrene-co-maleic anhydride) encapsulation. This work also provides practical guidelines for how the quantum dot community can better design and implement phase transfer protocols for a given composition, in order to maximize the efficiency and durability of the resulting water-dispersible nanoparticles for biological applications.