Loading quantum dots into thermo-responsive microgels by reversible transfer from organic solvents to water

We describe a new method for the preparation of fluorescent inorganic-nanoparticle composite microgels. Copolymer microgels with functional pendant groups were transferred via dialysis into tetrahydrofuran (THF) solution and mixed with colloidal solutions of semiconductor nanocrystals (quantum dots, QDs). CdSe QDs stabilized with trioctylphosphine oxide (TOPO) became incorporated into the microgels via ligand exchange of pendant imidazole (Im) groups for TOPO. PbS QDs stabilized with oleic acid were incorporated into microgels with pendant -COOH groups. This approach worked equally well with microgels based upon poly(N-isopropylacrylamide) (PNIPAM) and those based upon an acetoacetylethyl methacrylate-N-vinylcaprolactam copolymer (PVCL). These composite hybrid materials were colloidally stable in THF, and maintained their colloidal stability after transfer to water, either via dialysis or by sedimentation-redispersion. In water, the composites exhibited similar thermal responsiveness to the parent microgels, with a small shift to lower temperature in the volume phase transition. This approach allows one to use inorganic nanoparticles synthesized under optimum conditions in organic media at high temperature and to prepare composite microgels directly by mixing the components in a water-miscible organic solvent.