Transferring colloidal quantum dots (QDs) synthesized in nonpolar organic solvents to water or alcohols makes them more suitable for biomedical applications. In previous studies, polymeric imidazole ligands have been successful in stabilizing QDs in water with relatively high quantum yields and low nonspecific binding in in vitro and in vivo biological contexts, but the nature of imidazole coordination of the QD surface in these cases has been elusive. Here, to further study imidazole binding, QDs initially bearing hydrophobic surface coordination were quantitatively exchanged with a set of imidazole-bearing small-molecule ligands to facilitate colloidal dispersions in methanol. Native oleate-capped zincblende CdSe/ZnS core/shell QDs were used as a common starting point to investigate a series of ligand exchanges with small-molecule imidazole ligands: L-histidine methyl ester dihydrochloride (L-HME), L-histidine ethyl ester dihydrochloride (L-HEE), and L-histidine monohydrochloride (L-H). As a point of comparison, we also studied the exchange with a non-imidazole amino acid, D-phenylalanine methyl ester hydrochloride (D-PME) to establish that ligands unambiguously bind to the QD surface through the imidazole group. Gel permeation chromatography was successfully applied to purify the QDs before and after ligand exchange to ensure the efficient removal of excess ligands. Inductively coupled plasma-optical electron spectroscopy and 1H-NMR indicated a combination of three distinct ligand exchanges that occur simultaneously: Z-type dissociation of Zn(oleate)2, L-type association of the L-HME ligand, and X-type anionic exchange of the oleate ligands with Cl-, all of which contribute to an overall ligand exchange and dispersion in methanol. Variable-Temperature 1H-NMR measurements of the L-HME ligand and L-HME capped QDs confirm that the L-HME ligands are bound to the QD surface and promote colloidal stability in methanol. Imidazole ligand exchange was further confirmed by dynamic light scattering and zeta potential measurements.

Understanding the surface passivation of quantum dots by small-molecule imidazole ligands: Exchange on oleate-capped cdse/zns core/shell quantum dots

ACS Spring 2022

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