Purification of Quantum Dots by Gel Permeation Chromatography and the Effect of Excess Ligands on Shell Growth and Ligand Exchange

This article describes the use of gel permeation chromatography (GPC) as a means to separate natively capped colloidal CdSe and CdSe/Cd_<i>x</i>Zn_1–<i>x</i>S quantum dots (QDs) from small-molecule impurities in hydrophobic solvents. A range of analysis techniques, including ¹H NMR, diffusion-ordered NMR analysis (DOSY), and thermogravimetric analysis (TGA) have been used to compare the nature and quantities of ligands adsorbed on the QDs after GPC and after alternative purification methods. We show that the GPC purified samples display lower ligand-to-QD ratio (135 oleate substituents per nanocrystal for CdSe QDs with lowest-energy absorption peak at 534 nm) than what we can achieve by the multiple precipitation/redissolution method, and the GPC purified samples are stable at both room temperature and high temperature (180–200 °C for CdSe QDs). The achievement of an efficient and highly reproducible method for the preparation of clean QD samples allowed us to test whether impurities that reside in samples prepared by standard purification methods have a significant effect on further surface modification reactions. We found that the reactivity of CdSe QDs toward precursors for CdS shell growth was profoundly affected by the presence of excess ligands in impure QD samples prepared by multiple precipitations and that the removal of excess ligands and impurities significantly improved the speed and reliability by which water-soluble CdSe/Cd_<i>x</i>Zn_1–<i>x</i>S QDs could be prepared by ligand exchange with cysteine. GPC purification provides a preparative-scale, consistent, size-based purification of QDs without perturbing the solvent environment and as such could serve as the basis for advanced syntheses and enable detailed measurements of QD surface chemical properties