Towards Endogeneous DNP-NMR in Semiconducting Nanoparticles

Many physical and chemical properties of nanomaterials are dominated by the properties of their surfaces. The importance of surfaces stems from the large surface-to-volume ratio of nanomaterials. These materials are synthesized and processed using ligand molecules, which bind and alter the properties of nanomaterials surfaces. Understanding the role of ligands in the rational design of functional nanomaterials is one of the main challenges in this field. The structure and properties characterization of these nanomaterials requires techniques such as TEM, FTIR UV-Vis-IR, and more. Information acquired from these techniques is invaluable, yet it is limited when attempting to probe a ligand-inorganic interface from a collective of nanostructured materials.<br/>Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is a powerful tool for probing local structures at the atomic scale. The inherently low sensitivity of NMR could be overcome with dynamic nuclear polarization (DNP). Typically, organic radicals are added as polarizing agents (PA) to the sample in the exogenous approach. Polarization transfer from electron spin to the nuclear spin occurs upon microwave irradiation, thus enhancing the NMR signal. However, when dealing with ligand-coated nanomaterials, the presence of radicals can dramatically affect the interface under investigation and interfere with its characterization.<br/>This work demonstrates the endogenous DNP approach in nanoparticles for the first time. Where the PAs are transition metal ions with unpaired electrons incorporated in the inorganic lattice. Mn-doped CdS nanoparticles were synthesized using the standard hot-injections technique and characterized using TEM, EPR, and ICP-MS. In addition to a significant DNP enhancement for ¹¹³Cd nuclei, surface and core moieties were observed. This approach potentially paves the way for advanced characterization of the ligand-inorganic interface for surfactant-coated nanoparticles.