US Department of Energy, Ames Laboratory, Ames, Iowa 50011, USA.
Phys Chem Chem Phys. 2013 Apr 21;15(15):5553-62. doi: 10.1039/c3cp00039g.
We systematically studied the enhancement factor (per scan) and the sensitivity enhancement (per unit time) in (13)C and (29)Si cross-polarization magic angle spinning (CP-MAS) NMR boosted by dynamic nuclear polarization (DNP) of functionalized mesoporous silica nanoparticles (MSNs). Specifically, we separated contributions due to: (i) microwave irradiation, (ii) quenching by paramagnetic effects, (iii) the presence of frozen solvent, (iv) the temperature, as well as changes in (v) relaxation and (vi) cross-polarization behaviour. No line-broadening effects were observed for MSNs when lowering the temperature from 300 to 100 K. Notwithstanding a significant signal reduction due to quenching by TOTAPOL radicals, DNP-CP-MAS at 100 K provided global sensitivity enhancements of 23 and 45 for (13)C and (29)Si, respectively, relative to standard CP-MAS measurements at room temperature. The effects of DNP were also ascertained by comparing with state-of-the-art two-dimensional heteronuclear (1)H{(13)C} and (29)Si{(1)H} correlation spectra, using, respectively, indirect detection or Carr-Purcell-Meiboom-Gill (CPMG) refocusing to boost signal acquisition. This study highlights opportunities for further improvements through the development of high-field DNP, better polarizing agents, and improved capabilities for low-temperature MAS.
我们系统地研究了功能化介孔硅纳米粒子（MSNs）的动态核极化（DNP）增强的（13）C 和（29）Si 交叉极化魔角旋转（CP-MAS）NMR 的增强因子（每次扫描）和灵敏度增强（每单位时间）。具体来说，我们将贡献归因于：（i）微波辐照，（ii）顺磁效应的猝灭，（iii）冻结溶剂的存在，（iv）温度，以及（v）弛豫和（vi）交叉极化行为的变化。当将温度从 300 降低到 100 K 时，MSNs 没有观察到线展宽效应。尽管由于 TOTAPOL 自由基的猝灭导致信号显著减少，但在 100 K 下进行的 DNP-CP-MAS 相对于室温下的标准 CP-MAS 测量，分别为（13）C 和（29）Si 提供了 23 和 45 的全局灵敏度增强。通过比较使用间接检测或 Carr-Purcell-Meiboom-Gill（CPMG）重聚焦分别增强信号采集的最新二维异核（1）H{（13）C}和（29）Si{（1）H}相关谱，也确定了 DNP 的效果。本研究通过开发高场 DNP、更好的极化剂和提高低温 MAS 的能力，突出了进一步改进的机会。
