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

We use 1 H and 31 P 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/phosponic acid exchange interaction and the ratio between the Cd surface excess and the ligand density indicate that phosponic acids bind as hydrogen phosphonates to the CdSe surface.