Solution pH and ligand lability modify the optical bandgap of CdSe quantum dots (QDs) in water due to the ability of water to act as a non-inert solvent at the QD surface. Water-soluble QDs are desirable for photocatalytic and biosensing applications, and are commonly prepared through capping with strong-binding thiolate ligands such as 3-mercaptopropionic acid and dihydrolipoic acid. Thiolate capping ligands, however, i) scavenge holes from photoexcited QDs and thereby competitively inhibit hole transfer to reactants during photocatalysis, and ii) form disulfide bonds upon oxidation and dissociate from the QD surface, leaving the inorganic core exposed to the solution environment. To avoid the use of thiolate capping ligands, we use either a phosphonoalkanoic acid or HCl to strip the QD of its native oleate ligands and effect a phase transfer from hexanes into basic water, resulting in effectively “ligand-less” QDs. We use a combination of ground state optical spectroscopies, nuclear magnetic resonance spectroscopy, and x-ray photoelectron spectroscopy to characterize the interaction of hydroxide ions with the QD surface as a function of pH. As the pH of the solution increases, the optical bandgap of these “ligand-less” QDs shifts to lower energy, which we attribute to an increase in negative charge density at the QD surface as the pH increases through both covalent and electrostatic interactions of hydroxide ions with the QD surface.

Dynamic ligand exchange and surface charge density modulate the optical properties of CdSe quantum dots in water as a function of pH

2019 Spring National Meeting

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