Thomas Brijith, Jardón-Álvarez Daniel, Carmieli Raanan, van Tol Johan, Leskes Michal
Department of Molecular Chemistry & Materials Science, Weizmann Institute of Science, Rehovot 76100, Israel.
Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel.
J Phys Chem C Nanomater Interfaces. 2023 Feb 27;127(9):4759-4772. doi: 10.1021/acs.jpcc.2c08849. eCollection 2023 Mar 9.
In dynamic nuclear polarization nuclear magnetic resonance (DNP-NMR) experiments, the large Boltzmann polarization of unpaired electrons is transferred to surrounding nuclei, leading to a significant increase in the sensitivity of the NMR signal. In order to obtain large polarization gains in the bulk of inorganic samples, paramagnetic metal ions are introduced as minor dopants acting as polarizing agents. While this approach has been shown to be very efficient in crystalline inorganic oxides, significantly lower enhancements have been reported when applying this approach to oxide glasses. In order to rationalize the origin of the difference in the efficiency of DNP in amorphous and crystalline inorganic matrices, we performed a detailed comparison in terms of their magnetic resonance properties. To diminish differences in the DNP performance arising from distinct nuclear interactions, glass and crystal systems of similar compositions were chosen, LiOCaO·2SiO and LiCaSiO, respectively. Using Gd(III) as polarizing agent, DNP provided signal enhancements in the range of 100 for the crystalline sample, while only up to around factor 5 in the glass, for both Li and Si nuclei. We find that the drop in enhancement in glasses can be attributed to three main factors: shorter nuclear and electron relaxation times as well as the dielectric properties of glass and crystal. The amorphous nature of the glass sample is responsible for a high dielectric loss, leading to efficient microwave absorption and consequently lower effective microwave power and an increase in sample temperature which leads to further reduction of the electron relaxation time. These results help rationalize the observed sensitivity enhancements and provide guidance in identifying materials that could benefit from the DNP approach.
在动态核极化核磁共振（DNP-NMR）实验中，未成对电子的大玻尔兹曼极化被转移到周围的原子核上，导致NMR信号的灵敏度显著提高。为了在大量无机样品中获得大的极化增益，引入顺磁性金属离子作为充当极化剂的微量掺杂剂。虽然这种方法在结晶无机氧化物中已被证明非常有效，但将这种方法应用于氧化物玻璃时，据报道增强效果明显较低。为了阐明非晶态和晶态无机基质中DNP效率差异的起源，我们根据它们的磁共振特性进行了详细比较。为了减少因不同核相互作用而产生的DNP性能差异，分别选择了组成相似的玻璃和晶体系统LiOCaO·2SiO和LiCaSiO。使用Gd(III)作为极化剂，对于结晶样品，DNP使Li和Si原子核的信号增强在100左右，而在玻璃中仅高达约5倍。我们发现玻璃中增强效果的下降可归因于三个主要因素：较短的核和电子弛豫时间以及玻璃和晶体的介电特性。玻璃样品的非晶态性质导致高介电损耗，从而导致有效的微波吸收，进而降低有效微波功率并使样品温度升高，这导致电子弛豫时间进一步缩短。这些结果有助于阐明观察到的灵敏度增强现象，并为识别可从DNP方法中受益的材料提供指导。
