Ligand Counting and Size Determination of Quantum Dots Using Electronegative Matrix-Assisted Laser Desorption Ionization Mass Spectrometry

Surface-capping ligands on quantum dots (QDs) modify the physical and chemical properties of QDs to enable diverse applications. Accurate qualitative and quantitative analyses of these ligands are therefore essential for the advancement of QD research; however, conventional methods (e.g., UV-vis, TEM) seldom provide direct stoichiometric insight. MS can offer a solution, but traditional MALDI induces an excessive ligand loss. Here, we employed a highly electronegative, fluorinated porphyrin (TPFP) as a soft MALDI matrix for CdSe QDs to preserve the ligands and yield high ion signals at reduced laser power. Comparative studies with electron-transfer matrixes (DCTB, 9-nitroanthracene, dithranol, and pentacene) indicate that the highly electronegative TPFP extracts electrons from photoexcited QDs, lowering ionization thresholds and preventing ligand detachment. Using TPFP-MALDI-MS, we determined QD core sizes and counted the number of surface ligands in carboxylate- and thiolate-capped QDs, and for thiolate-capped QDs in particular, we further evaluated ligand numbers and ligand densities with various core sizes. This approach provides a reliable strategy to determine QD core sizes, to count ligand numbers, and to estimate the surface coverage with improved resolution. These findings underscore the importance of electron affinity of a matrix for soft ionization and expand MS-based tools for probing QD core and surface chemistry, paving the way for more reliable characterization of nanocrystalline systems.