Competitive Anionic Exchange of Thiolate Ligands onto Aqueous Phosphonate-Capped Quantum Dots

This study examines the relative binding strength of a series of thiolate ligands to the surfaces of zincblende CdSe/ZnS core/shell quantum dots (QDs) passivated by phosphonate ligands under basic conditions in water by means of isothermal titration calorimetry (ITC). A labile hydrophilic intermediate, 2-aminoethylphosphonic acid (AEP), was introduced in place of native hydrophobic carboxylate ligands to produce water-soluble QDs and provide a universal starting point for competitive anionic exchange reactions. We introduce monothiols of different chain lengths, and a dithiol analogue (dihydrolipoic acid), to probe the effects of chain length and denticity on binding strength. The introduction of each of the ligands produces an exothermic response. An increase in chain length among the monothiols led to an increase in the equilibrium exchange constant, Kex, indicating longer chain lengths lead to stronger binding. However, the shorter chain mercaptopropionic acid showed greater total exothermicity, which appears to indicate a greater total number of accessible binding sites. The introduction of a dithiol ligand produces a larger Kex than the monodentate thiols, consistent with greater binding strength and stability inferred from previous observations, and ligand density comparable to the shorter chain mercaptopropionic acid at saturation. This work should aid in the development of stable and versatile water-soluble QD colloids for bioimaging and sensing applications.

Automated summary

This study examines the relative binding strength of thiolate ligands to the surfaces of zincblende CdSe/ZnS core/shell quantum dots passivated by phosphonate ligands. The results provide insights into the effects of chain length and denticity on binding strength, which are important for developing stable and versatile water-soluble QD colloids.