超极化核磁共振方法的原理和应用

doi:10.3866/PKU.WHXB201906018

摘要/Abstract

摘要：

核磁共振(NMR)技术凭借其高空间分辨率，宽时间响应尺度和非侵入检测等特点，在化学分析和医疗诊断中发挥着重要的作用。但是原子核的低极化使现阶段NMR技术的灵敏度较低。超极化技术是一类可以有效提高NMR灵敏度的方法。其通过物理或化学过程把原子核自旋态推向一个偏离热力学平衡的状态，使NMR信号强度得到几个数量级的提升，极大地改善了灵敏度。多种超极化技术已经在各个领域崭露头角。本文用较为形象的描述对几种常见的超极化技术包括：动态核极化、光泵、光核极化、化学诱导动态核极化、仲氢诱导极化。从其精巧的原理和广泛的应用进行介绍，有助于人们对超极化技术的认知。

关键词: 动态核极化, 光泵, 光核极化, 化学诱导动态核极化, 仲氢诱导极化, 极化转移, 超精细耦合

Abstract:

Nuclear magnetic resonance (NMR) is an effective and widely adapted technique that can be used for medical diagnosis and chemical analysis. However, its application has been limited by low sensitivity originating from the extremely low polarization of nuclear spins that follow a typical Boltzmann distribution. In principal, it is possible to break this Boltzmann distribution using different physical or chemical mechanisms to generate hyperpolarization and increase NMR sensitivity by several orders of magnitude. The crucial point of hyperpolarization is to transfer the polarization from highly polarized systems to nuclear spins. For example, the unpaired electrons in organic radicals or other systems exhibit much higher polarization than that of nuclear spins (~660 times higher than ¹H) under the same magnetic field. The high polarization of electrons at thermal equilibrium can be transferred to nuclear spins via microwave irradiation and hyperfine coupling. This hyperpolarization method is called dynamic nuclear polarization (DNP) and has been successfully and widely applied for the evaluation of the protein structure and the examination of nanomaterial surface chemistry. Electron spins can also be hyperpolarized using circularly polarized light (CPL) or nonpolarized light in some systems, and this polarization can be transferred to nuclear spins as well. These hyperpolarization methods are referred to as optical pumping (OP) and optical nuclear polarization (ONP), respectively. A common application of OP is the production of hyperpolarized noble gases, including hyperpolarized xenon-129, which can be used in magnetic resonance imaging of lungs or evaluation of porous structures. For ONP, the nitrogen-vacancy center in diamond is the most promising system that has demonstrated the ability to track the precession of a single spin. In addition, electrons can be polarized by certain chemical reactions as used in chemically induced dynamic nuclear polarization (CIDNP). CIDNP can be used to study the active sites of proteins and identify low-concentration intermediates that are formed during chemical processes. In addition to electrons, hydrogen molecules with unique spin state, i.e., parahydrogen, can be converted to hyperpolarized NMR signals via hydrogen addition reactions, which is known as parahydrogen induced polarization (PHIP). PHIP was originally used to understand the mechanisms of hydrogenation processes, but has recently become a promising hyperpolarization technique via the protocols of signal amplification by reversible exchange (SABRE). Herein, the basic mechanisms and potential applications of DNP, OP, CIDNP, and PHIP techniques are reviewed. These emerging hyperpolarization techniques have the potential to push the limits of NMR beyond current conceptions.

Key words: Dynamics nuclear polarization, Optical pumping, Optical nuclear polarization, Chemically induced dynamic nuclear polarization, Para-hydrogen induced polarization, Polarization transfer, Hyperfine coupling

MSC2000:

O646.8

庞振峰,管晗曦,高李娜,曹伟成,尹竟琳,孔学谦. 超极化核磁共振方法的原理和应用[J]. 物理化学学报, 2020, 36(4): 1906018.

Zhenfeng Pang,Hanxi Guan,Lina Gao,Weicheng Cao,Jinglin Yin,Xueqian Kong. Fundamentals and Applications of NMR Hyperpolarization Techniques[J]. Acta Physico-Chimica Sinica, 2020, 36(4): 1906018.

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Mechanism	Overhauser effect	Solid effect	Cross effect	Thermal mixing
Sample type	Liquid; alkali metals organic	Dielectric samples in the solid	Dielectric samples in the solid	Dielectric samples in the solid state
Conditions	ω_eτ < 1	δ, Δ < ω_n	δ < ω_n < Δ	δ, Δ > ω_n
		Low concentration of radicals. No dipolar coupling between radicals.	Dipolar coupling between two types of radicals. Frequency difference between two radicals equals to the nuclear frequency	High concentration of radicals. Strong coupling between radicals.
Dependence	ω_eτ < 1	~ B₀^-2	~ B₀^-1	δ
Microwave frequency for maximal DNP enhancement	ω = ω_e	ω = ω_e ± ω_n	Depends on EPR line shape	Depends on EPR line shape ω ≈ ω_e ± δ

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