Isothermal Titration Calorimetry Resolves Sequential Ligand Exchange and Association Reactions in Treatment of Oleate-Capped CdSe Quantum Dots with Alkylphosphonic Acid
Megan Y. Gee, Yi Shen, Andrew B. Greytak
Abstract
This article examines a prototypical nanocrystal surface reaction, the replacement of carboxylate ligands with alkylphosphonate on CdSe quantum dots (QDs), in an anhydrous solvent using isothermal titration calorimetry (ITC). A highly repeatable exothermic signature is detected associated with quantitative displacement of oleate from the QDs as oleic acid. Furthermore, on the basis of the magnitude and time dependence of the heat responses to octadecylphosphonic acid (ODPA) addition over the course of the reaction, an additional chemical process could be unambiguously resolved: the association of ODPA to a separate set of sites on the QD surface. The anionic ligand exchange predominates at first, proceeding with equilibrium constant Kexch ≥ 100 and an exothermic average ΔH ≈ −6.7 kJ mol–1, while the association reaction is also exothermic and proceeds with Ka > 103 M–1. Changes in synthetic and purification conditions led to significant changes in the equivalencies (sites per QD) for these two surface reactions that are clearly revealed by ITC despite no apparent changes in electronic absorption spectra or the QD size. These results demonstrate the capabilities of ITC as a technique for studying QD surface reactions in a manner that is complementary to NMR and optical spectroscopic methods. Finally, we introduce a timetrace analysis to extract additional information from ITC signals in the case of simultaneous equilibria with differing reaction kinetics. We anticipate that the direct thermal measurement provided by ITC will help to resolve the detailed thermodynamics of a variety of QD reactions and enable improved descriptions of QD surfaces.