Microwave-Free 13C Hyperpolarization of Diamond Particles Enabled by Magic Angle Spinning and NV Centers

Nuclear hyperpolarization is essential for enhancing the sensitivity of Nuclear Magnetic Resonance (NMR) at room temperature. While diamond nitrogen-vacancy (NV) centers are promising candidates for this, their strong spin Hamiltonian anisotropy typically requires complex microwave-driven protocols to achieve polarization transfer in randomly oriented powders. This paper investigates whether magic angle spinning (MAS) can replace microwave irradiation to drive polarization transfer in diamond particles.

Researchers utilized diamond particles with varying 13C isotopic enrichment (1.1% and 20%) and sizes (0.2–2 μm). The experimental setup integrated a laser for optical excitation directly into a standard MAS NMR probe. By spinning the diamond powder at the magic angle (54.74°) relative to the external magnetic field, the team exploited rotation-induced level anticrossings (LACs) within a three-spin system consisting of an NV center, a substitutional nitrogen (P1) center, and a 13C nucleus. They modeled the spin dynamics using a combination of the seven-level NV photophysics model and Landau-Zener transitions at the LACs.

The study reports significant light-induced 13C polarization enhancements—up to 411-fold for natural abundance samples and 280-fold for isotopically enriched samples—without the use of microwaves. The MAS-based approach successfully averages out the orientation-dependent spin transition energies, enabling nearly all NV centers in the powder to contribute to the polarization process. The researchers observed that 13C polarization buildup is mediated by spin diffusion, and they confirmed that the steady-state polarization levels exceed 0.1%, which is nearly an order of magnitude higher than previously reported results for micro- and nanodiamonds at room temperature.

This work establishes a microwave-free, optically driven hyperpolarization platform that is compatible with standard solid-state NMR hardware. By leveraging MAS to overcome the orientation limitations of diamond color centers, this method simplifies the implementation of hyperpolarized diamond for sensitive magnetic resonance applications. It provides a scalable path toward using diamond nanoparticles as polarization agents for external analytes, potentially enabling high-resolution, high-sensitivity NMR at ambient conditions.