Rapid and Sensitive Identification and Discrimination of Bound/Unbound Ligands on Colloidal Nanocrystals via Direct Analysis in Real-Time Mass Spectrometry.

Direct analysis in real-time mass spectrometry (DART-MS) has been applied to the characterization of colloidal nanocrystal surface ligands. The nanocrystals (NCs) in colloidal suspension were purified and deposited onto a solid substrate, and the solvent was allowed to evaporate. Ligand desorption was thermally stimulated using a temperature ramp from 30 °C up to 530 °C, and the desorbed ligands were introduced into a DART-MS instrument where metastable He atoms provide energy for ionization and fragmentation through the reaction with ambient vapors including O2 and H2O. The method allows the identification of ligand species with various functional groups, even in complex, mixed-ligand samples. Bound and unbound molecules can be distinguished based on the desorption temperature. In ideal cases, the desorption profile for a given molecule can be analyzed according to methods adapted from thermal desorption spectroscopy (TDS) to estimate desorption activation energy for NC-bound ligands. Results are presented and discussed for different nanocrystal and ligand types. The method is a promising complement to the range of existing tools for NC ligand analysis.