Surface modification of colloidal semiconductor nanocrystal quantum dots

摘要

Current quantum dot surface modification strategies rely heavily on ligand exchange that removes the nanocrystal\u27s native ligands originated from its synthesis. This can cause etching and introduce surface defects, affecting the nanocrystal\u27s optical properties. In addition, common ligand exchange method fails to control the degree of functionalization or the number of functional groups introduced per nanocrystal.We describe our work on surface modification of semiconductor nanocrystal quantum dots investigating a new approach that not only bypasses ligand exchange and introduces native active ligands with original optical properties, but also is able to control the degree of surface loading, called \u22valence\u22, in semiconductor nanocrystal quantum dots. We show that surface doped quantum dots capped with chemically-active native ligands can be prepared directly from a mixture of ligands with similar chain lengths. Initial ratio between chemically active and inactive ligands is retained on the nanocrystal surface, allowing to control the extent of surface modification.The extent of surface coverage by a particular functional group will have a large impact on a nanocrystal affinity and permeability to a variety of biological structures. It also affects nanocrystal\u27s ability to localize, penetrate, and transport across specific tissues, cellular and subcellular structures. We show that we are able to control the loading of cholestanone per quantum dot nanocrystal. We observed that samples with higher steroid loading infuse themselves more with the lipid membrane compare to those with no or little steroid.To further investigate the surface ligand packing, structure and reactivity, we apply advanced solution NMR techniques to determine surface ligand organization and chemistry. Two-dimension ROESY studies show that ligands with the same chain length tend to homogeneously distribute themselves onto the nanocrystal\u27s surface however ligands with the different chain length tend to form islands. Furthermore, we demonstrate that surface ligand organization can affect the reactivity of quantum dots. Formation of rafts as a result of packing ligands of a same length, increase the local concentration of reactive terminal group and facilitate the chemical reactivity at the surface of quantum dots.We also synthesize multifunctional multidentate polymeric ligand via ADMET. Varying the total dienes-to-Ru catalyst ratio allows us to control the extent of ADMET, which enables us to achieve an accurate control over polydentate ligand size. We use the synthetic polymer as a linkage for constructing gold-QD heterostructure.We hope that this study can provide a new avenue to understand the organic/inorganic boundary of other and more complex nanoparticle/ligand systems.