Chemistry at the ligand-nanoparticle interface: how coordination geometry, lability and photocatalytic crosslinking of ligands affects the properties of aqueous quantum dots
by Takeuchi, Hiroko; Gotz, Marion G.; Omogo, Benard O.; Goldfogel, Matthew J.; Warren, Julie M.; Fennel, Brandon D.; Heyes, Colin D.
Using nanoparticles in biol. imaging applications requires them to be made water sol., colloidally stable, maintain a small size and to have control over the nanoparticle-biomol. interaction. Often, improving some of these properties causes worsening of others. Polymer ligands are used to improve long-term colloidal stability but at the expense of large size, while small coordinating ligands maintains a small size at the expense of stability. Using bidentate rather than monodentate coordinating ligands does improve colloidal stability, but how this coordination geometry affects the interaction of the nanoparticle with biomols. has not been as well studied. Here, we modified a near IR dye to serve as a highly-sensitive reporter for how thiolated target mols. interact with nanoparticle surfaces consisting of monodentate or bidentate coordinated ligands. Specifically, we fit the adsorption profiles to the Hill equation and the parameters are used to provide a microscopic picture of how coordination geometry affects ligand d. and lability, which underlies the interaction. Surprisingly, bidentate ligands are worse at inhibiting non-specific adsorption of thiolated mols. to QD surfaces at low target:QD ratios, becoming better as the ratio increases - but only if the nanoparticle surface area is large enough to overcome steric effects. We also present the synthesis of a photocrosslinkable ligand that uses the photocatalytic activity of the quantum dot itself to improve the crosslinking process and, in turn, the long-term aq. colloidal stability of those same quantum dots with only a moderate increase in size. These results are used provide a better understanding of the fundamental chem. at the nanoparticle-ligand interface that is expected to lead to improved nanomaterials, specifically for biol. applications.