Over the last two decades magic angle spinning dynamic nuclear polarization (MAS DNP) has revolutionized NMR for materials characterization, tackling its main limitation of intrinsically low sensitivity. Progress in theoretical understanding, instrumentation, and sample formulation expanded the range of materials applications and research questions that can benefit from MAS DNP. Currently the most common approach for hyperpolarization under MAS consists in impregnating the sample of interest with a solution containing nitroxide radicals, which upon microwave irradiation serve as exogenous polarizing agents. On the other hand, in metal ion based (MI)-DNP, inorganic materials are doped with paramagnetic metal centres, which then can be used as endogenous polarizing agents. In this work we give an overview of the electron paramagnetic resonance (EPR) concepts required to characterize the metal ions and discuss the expected changes in the NMR response due to the presence of paramagnetic species. We highlight which properties of the electron spins are beneficial for applications as polarizing agents in DNP and how to recognize them, both from the EPR and NMR data. A theoretical description of the main DNP mechanisms is given, employing a quantum mechanical formalism, and these concepts are used to explain the spin dynamics observed in the DNP experiment. In addition, we highlight the main differences between MI-DNP and the more common approaches in MAS DNP, which use organic radicals as exogenous polarizing source. Finally, we review some applications of metal ions as polarizing agents in general and then focus particularly on research questions in materials science that can benefit from MI-DNP.

在过去的二十年里，魔角旋转动态核极化 (MAS DNP) 彻底改变了材料表征的 NMR，解决了其固有的低灵敏度的主要限制。理论理解、仪器和样品配方方面的进展扩大了可以从 MAS DNP 中受益的材料应用和研究问题的范围。目前，MAS 下超极化的最常见方法包括用含有硝基氧自由基的溶液浸渍感兴趣的样品，硝基氧自由基在微波照射下充当外源极化剂。另一方面，在金属离子基（MI）-DNP中，无机材料掺杂有顺磁性金属中心，然后可以用作内源偏振剂。在这项工作中，我们概述了表征金属离子所需的电子顺磁共振 (EPR) 概念，并讨论了由于顺磁性物质的存在而导致的 NMR 响应的预期变化。我们重点介绍了电子自旋的哪些特性有利于 DNP 中作为极化剂的应用，以及如何从 EPR 和 NMR 数据中识别它们。采用量子力学形式给出了主要 DNP 机制的理论描述，这些概念用于解释 DNP 实验中观察到的自旋动力学。此外，我们强调了 MI-DNP 与 MAS DNP 中更常见的方法之间的主要区别，后者使用有机自由基作为外源极化源。最后，我们回顾了金属离子作为偏振剂的一些应用，然后特别关注可以受益于 MI-DNP 的材料科学研究问题。