MALDI-TOF-MS as a Powerful Tool for Elucidating Nanocrystal Synthesis Mechanism

The synthesis and development of colloidal nanocrystals still proceed by trial and error.¹ High-quality semiconductor quantum dots and iron oxide nanocrystals are synthesized in organic solvents using universally applicable ligands such as oleic acid. Thereby, starting from a few compounds, a highly complex reaction pathway is rapidly developing, whose essential aspects are still unknown. Insufficient knowledge of fundamental reaction mechanisms, especially the role of reaction intermediates during the formation of colloidal nanocrystals, prevents the expansion of the application potential of many nanoparticle systems.<br/><br/>Therefore, an essential goal of advanced nanocrystal synthesis is a comprehensive understanding of the reaction mechanisms in order to tailor and simultaneously control the triad of size, shape, and crystal structure. Since targeting this triad is hampered by numerous intertwined variables and severely limits the scope of synthetic results, unraveling the essential reaction mechanisms is of paramount importance.<br/><br/>We show exemplarily on the system - iron oxide - that significant aspects of the fundamental reaction mechanism can be elucidated by using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-ToF-MS). The combination of highly pure iron oleate and advanced mass spectrometry enabled us to gain a deep insight into the composition of iron oleate and elucidate a new cascade of redox reactions responsible for converting the individual complexes into each other already at low temperatures and early synthesis stages. Furthermore, we obtained evidence for the catalytic activity of iron in iron oleate by identifying many new complexes, thus essentially contributing to a more profound knowledge of the reaction conditions.²<br/><br/>This study serves as a template for the investigation of related systems^3,4 and reveals fundamental commonalities that will be exploited to unlock the missing tailored and simultaneous control of the triad of shape, size, and crystal structure.<br/><br/>1. Loiudice, A.; Buonsanti, R. Reaction Intermediates in the Synthesis of Colloidal Nanocrystals. <i>Nat. Synth.</i> <b>2022</b>, 1 (5), 344–351.<br/><br/>2. Feld, A.; Weimer, A.; Kornowski, A.; Winckelmans, N.; Merkl, J. P.; Kloust, H.; Zierold, R.; Schmidtke, C.; Schotten, T.; Riedner, M.; Bals, S.; Weller, H. Chemistry of Shape-Controlled Iron Oxide Nanocrystal Formation. <i>ACS Nano </i><b><i>2</i>019</b>, 13 (1), 152–162.<br/><br/>3. Palencia, C.; Yu, K.; Boldt, K. The Future of Colloidal Semiconductor Magic-Size Clusters. <i>ACS Nano</i> <b>2020</b>, 14 (2), 1227–1235.<br/><br/>4. Pun, A. B.; Mazzotti, S.; Mule, A. S.; Norris, D. J. Understanding Discrete Growth in Semiconductor Nanocrystals: Nanoplatelets and Magic-Sized Clusters. <i>Acc. Chem. Res.</i> <b>2021</b>, 54 (7), 1545–1554.