Ligand exchange of colloidal semiconductor nanocrystals is widely utilized to modify the surface chemistry of quantum dots (QDs) to enhance their solubility and stability and, importantly, manipulate their electrical and optical properties. Recently, quantitative and qualitative measurements have been made in order to obtain a better understanding of ligand exchange reactions on QDs. We present the implementation of isothermal titration calorimetry (ITC) in order to quantitatively determine the thermodynamic properties of the ligand exchange reaction from oleate to n-dodecylphosphonic acid (DDPA) on the surface of 3.1 nm cadmium selenide (CdSe) nanocrystals. ITC is a direct measurement of the heat of reaction of the ligand exchange as a dynamic process, as opposed to most other investigations that have been conducted to date which infer the thermodynamics of the reaction by post-exchange studies after equilibrium has been established. By monitoring the heat generated as DDPA was titrated into oleate-capped CdSe QDs, we found this ligand exchange reaction to be an exothermic process.  We report the number of oleate ligands displaced by DDPA to be about 65 per QD, and the enthalpy was determined to be -4.837 kJ mol<sup>-1</sup>. Additionally, we calculate the Gibb’s free energy to be -56.8 kJ mol<sup>-1</sup> indicating this is a spontaneous displacement of surface ligands. These results are in agreement with previous qualitative studies and provide quantitative thermodynamic insight to this ligand exchange reaction. This study illustrates the use of ITC as a method to quantify in real time ligand exchange reactions to provide a detailed understanding of the modification of the surface chemistry of QDs.

Quantifying the thermodynamics of the surface modification of oleate-capped CdSe quantum dots with dodecylphosphonic acid via isothermal titration calorimetry

253rd National Meeting (2017)

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