Binding of Phosphonic Acids to CdSe Quantum Dots: A Solution NMR Study

We use H-1 and P-31 solution nuclear magnetic resonance spectroscopy to analyze the binding of phosphonic acid ligands to wurtzite CdSe quantum dots (CdSe QDs). CdSe QDs synthesized with phosphonic acids as a surfactant have a ligand shell composed of phosphonic acid and phosphonic acid anhydride moieties. Titrations of as-synthesized QDs with excess oleic acid do not induce desorption of phosphonic species, whereas titration of oleic acid exchanged QDs with excess phosphonic acid shows that the latter quantitatively replaces the oleic acid with a 1:1 stoichiometry. Both the stoichiometry of the oleic acid/phosphonic acid exchange interaction and the ratio between the Cd surface excess and the ligand density indicate that phosphonic acids bind as hydrogen phosphonates to the CdSe surface.