Dynamic Nuclear Polarization in Solid-State NMR Spectroscopy

Dynamic nuclear polarization (DNP) has transformed solid-state nuclear magnetic resonance (NMR) by dramatically enhancing signal intensity through the transfer of spin polarisation from unpaired electrons to nuclear spins. In practice, a sample is doped with a polarising agent—typically a stable free radical or paramagnetic metal complex—and irradiated with microwaves at cryogenic temperatures and high magnetic fields. The primary mechanisms for polarisation transfer include the cross effect and the solid effect, which differ in their dependence on electron–nuclear and electron–electron interactions. Once nuclei close to the radical become hyperpolarised, spin diffusion relays this enhanced polarisation through the bulk of the solid, enabling rapid acquisition of multidimensional and surface-selective spectra. Magic-angle spinning (MAS) is often employed to average anisotropic interactions, while bespoke microwave sources and low-temperature probes optimise DNP build-up times and sensitivity gains. This synergy of polarising agents, microwave technology and MAS has unlocked new insights into biomaterials, catalysts, inorganic frameworks and battery materials by allowing observation of low-gamma nuclei and surface species that were previously inaccessible by conventional solid-state NMR.

Research from Nature Portfolio

Recent studies have demonstrated the power of DNP in probing the interfaces of calcium-based nanomaterials. By combining DNP with solid-state NMR of the low-sensitivity isotope calcium-43, researchers have distinguished surface and core species in hydroxyapatite nanoparticles at natural isotopic abundance. The approach delivered two-dimensional correlation spectra in significantly reduced experimental times, yielding unprecedented molecular-level insight into calcium environments that underpin bone and tooth biomineralisation. This work showcases how DNP can extend the reach of NMR to challenging nuclei and complex interfaces, opening new avenues for characterising hybrid biomaterials and nanosystems.

Research from all publishers

Investigations into biradical design have provided finer control over DNP efficiency under magic-angle spinning. A series of deuterated and fluorinated biradicals were synthesised to dissect the role of proximal protons in polarisation transfer. Experimental data combined with numerical simulations confirmed that strong hyperfine couplings to phenyl-ring protons govern rapid build-up and high enhancements. Guided by these findings, a novel biradical, NaphPol, was developed to deliver record sensitivity gains in organic solvents, illustrating how judicious chemical modification can fine-tune DNP performance.

Advances in metal-ion based DNP have demonstrated direct polarisation of low-abundance nuclei throughout bulk inorganic materials. By doping an electrode compound with paramagnetic Fe(III) centres, researchers achieved uniform enhancement of oxygen-17 signals without reliance on long-range spin diffusion. This strategy lifts the traditional requirement for efficient spin diffusion, enabling high-quality NMR spectra of otherwise intractable isotopes in battery electrode materials. Such endogenous polarisation agents broaden the scope of DNP to systems where conventional radicals cannot penetrate or where uniform bulk enhancement is essential.

Endogenous DNP has also been applied to composite battery electrodes, where substitutional Fe(III) ions serve dual roles as both polarisation sources and functional dopants. Lithium nuclei in Li₄Ti₅O₁₂ exhibited enhancements of up to 180, while the DNP activity could be monitored and reactivated through electrochemical cycling. This integration of polarisation and electrochemical functionality points to a new paradigm for in situ studies of energy storage materials, facilitating atomic-scale characterisation of active electrode processes under realistic operating conditions.

Dynamic Nuclear Polarization in Solid-State NMR Spectroscopy publication trend

The graph below shows the total number of articles in dynamic nuclear polarization in solid-state nmr spectroscopy across all publications each year (not limited to Nature Index journals).

Technical terms

Dynamic nuclear polarization: A method for enhancing NMR signals by transferring polarisation from electron spins to nuclear spins via microwave irradiation.

Magic-angle spinning: A technique in solid-state NMR where the sample is spun at 54.74° to the magnetic field to average anisotropic interactions.

Polarising agent: A molecule or metal complex with unpaired electrons used to deliver electron spin polarisation to nearby nuclei.

Cross effect: A DNP mechanism in which polarisation transfer occurs through three-spin interactions involving two electrons and one nucleus.

Spin diffusion: A process where hyperpolarisation spreads among nuclear spins through mutual dipolar couplings.

Hyperfine coupling: The interaction between electron and nuclear magnetic moments that facilitates polarisation transfer in DNP.

References

1. Interfacial Ca2+ environments in nanocrystalline apatites revealed by dynamic nuclear polarization enhanced 43Ca NMR spectroscopy. Nature Communications (2017).
2. Enabling Natural Abundance 17O Solid-State NMR by Direct Polarization from Paramagnetic Metal Ions. The Journal of Physical Chemistry Letters (2020).
3. Endogenous Dynamic Nuclear Polarization for Sensitivity Enhancement in Solid-State NMR of Electrode Materials. The Journal of Physical Chemistry C (2020).