Acta Physico-Chimica Sinica ›› 2020, Vol. 36 ›› Issue (4): 1906018.doi: 10.3866/PKU.WHXB201906018

Special Issue: Solid-State Nuclear Magnetic Resonance

• Review • Previous Articles     Next Articles

Fundamentals and Applications of NMR Hyperpolarization Techniques

Zhenfeng Pang,Hanxi Guan,Lina Gao,Weicheng Cao,Jinglin Yin,Xueqian Kong*()   

  • Received:2019-06-04 Accepted:2019-07-09 Published:2020-03-12
  • Contact: Xueqian Kong E-mail:kxq@zju.edu.cn
  • Supported by:
    the National Key Research and Development Program of China(YFA0203600);the Zhejiang Provincial National Nature Science Foundation, China(R19B050003);the Zhejiang University K. P. Chao's High Technology Development Foundation, China(2018RC009)

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