Non-Invasive Characterization of the Organic Coating of Biocompatible Quantum Dots Using Nuclear Magnetic Resonance Spectroscopy

Colloidal quantum dots, made of semiconductor cores and surface coated with an organic shell, have generated much interest in areas ranging from spectroscopy to charge and energy transfer interactions to device design, and as probes in biology. Despite the remarkable progress in the growth of these materials, rather limited information about the molecular arrangements of the organic coating is available. Here, several nuclear magnetic resonance (NMR) spectroscopic techniques have been combined to characterize the surface ligand structure(s) on biocompatible CdSe-ZnS quantum dots (QDs). These materials have been prepared via a photoinduced ligand exchange method in which the native hydrophobic coating is substituted, in situ, with a series of polyethylene glycol-modified lipoic acid-based ligands. We first combined diffusion ordered spectroscopy with heteronuclear single-quantum coherence measurements to outline the conditions under which the detected proton signals emanate from only surface-bound ligands and identify changes in the proton shifts between free and QD-bound ligands in the sample. Quantification of the ligand density on different size QDs was implemented by comparing the sharp ¹H signature(s) of lateral groups in the ligands (e.g., the OCH₃ group) to an external standard. We found that both the molecular architecture of the ligand and the surface curvature of the QDs combined play important roles in the surface coverage. Given the non-invasive nature of NMR as an analytical technique, the extracted information about the ligand arrangements on the QD surfaces in hydrophilic media will be highly valuable; it has great implications for the use of QDs in targeting and bioconjugation, cellular imaging, and energy and charge transfer interactions.