박상황 (POSTECH)

신승구 (POSTECH)

임성준 (DGIST)

서종철 (POSTECH)

박상황 (POSTECH)

노동연 (POSTECH)

이규동 (DGIST)

신승구 (POSTECH)

임성준 (DGIST)

서종철 (POSTECH)

Surface ligands on quantum dots (QDs) play a crucial role in modulating their chemical properties and enabling diverse functional applications. Understanding type and quantity of ligands is essential to study surface chemistry. Traditional characterization tools like UV/Vis absorption spectroscopy and transmission electron microscopy (TEM) are lack in providing direct stoichiometric insights. In matrix-assisted laser desorption/ionization (MALDI), ionization typically occurs when the matrix absorbs UV light and forms radical cations, which subsequently ionize analyte molecules via electron transfer. However, in systems like QDs that strongly absorb UV light themselves, an alternative mechanism becomes relevant: photoexcited QDs can transfer electrons to a neutral matrix with high electron affinity, leading to the formation of QD ions. This suggests that, for UV-absorbing analytes such as quantum dots, the electron affinity of the matrix plays a crucial role in achieving efficient ionization. In this study, we present a fluorinated porphyrin (TPFP) with high electron affinity as an effective matrix for MALDI-MS, demonstrating particular suitability for the gentle ionization of quantum dots. Compared to traditional electron-transfer matrices, TPFP's high electron affinity enables efficient electron withdrawal from photoexcited CdSe quantum dots, resulting in elevated ion yields at reduced laser powers with minimal ligand loss. This approach facilitates the quantitative assessment of ligand coverage and the monitoring of ligand exchange reactions, especially for quantum dots capped with carboxylate and thiolate ligands. Overall, this work positions MALDI-MS with TPFP as a robust platform for probing both the core and surface chemistries of QDs, expanding the toolbox for nanomaterial characterization.