Bulk (13)C polarization can be strongly enhanced in diamond at room temperature based on the optical pumping of nitrogen-vacancy color centers. This effect was confirmed by irradiating single crystals at a ~50 mT field promoting anticrossings between electronic excited-state levels, followed by shuttling of the sample into an NMR setup and by subsequent (13)C detection. A nuclear polarization of ~0.5%--equivalent to the (13)C polarization achievable by thermal polarization at room temperature at fields of ~2000 T--was measured, and its bulk nature determined based on line shape and relaxation measurements. Positive and negative enhanced polarizations were obtained, with a generally complex but predictable dependence on the magnetic field during optical pumping. Owing to its simplicity, this (13)C room temperature polarizing strategy provides a promising new addition to existing nuclear hyperpolarization techniques.

Polarizing nuclear spins is of fundamental importance in biology, chemistry and physics. Methods for hyperpolarizing 13C nuclei from free electrons in bulk usually demand operation at cryogenic temperatures. Room temperature approaches targeting diamonds with nitrogen-vacancy centres could alleviate this need; however, hitherto proposed strategies lack generality as they demand stringent conditions on the strength and/or alignment of the magnetic field. We report here an approach for achieving efficient electron-13C spin-alignment transfers, compatible with a broad range of magnetic field strengths and field orientations with respect to the diamond crystal. This versatility results from combining coherent microwave- and incoherent laser-induced transitions between selected energy states of the coupled electron–nuclear spin manifold. 13C-detected nuclear magnetic resonance experiments demonstrate that this hyperpolarization can be transferred via first-shell or via distant 13Cs throughout the nuclear bulk ensemble. This method opens new perspectives for applications of diamond nitrogen-vacancy centres in nuclear magnetic resonance, and in quantum information processing.

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Dynamic nuclear polarization (DNP) is a versatile option to improve the sensitivity of NMR and MRI. This versatility has elicited interest for overcoming potential limitations of these techniques, including the achievement of solid-state polarization enhancement at ambient conditions, and the maximization of (13) C signal lifetimes for performing in vivo MRI scans. This study explores whether diamond's (13) C behavior in nano- and micro-particles could be used to achieve these ends. The characteristics of diamond's DNP enhancement were analyzed for different magnetic fields, grain sizes, and sample environments ranging from cryogenic to ambient temperatures, in both solution and solid-state experiments. It was found that (13) C NMR signals could be boosted by orders of magnitude in either low- or room-temperature solid-state DNP experiments by utilizing naturally occurring paramagnetic P1 substitutional nitrogen defects. We attribute this behavior to the unusually long electronic/nuclear spin-lattice relaxation times characteristic of diamond, coupled with a time-independent cross-effect-like polarization transfer mechanism facilitated by a matching of the nitrogen-related hyperfine coupling and the (13) C Zeeman splitting. The efficiency of this solid-state polarization process, however, is harder to exploit in dissolution DNP-enhanced MRI contexts. The prospects for utilizing polarized diamond approaching nanoscale dimensions for both solid and solution applications are briefly discussed.

Dynamic nuclear polarization (DNP) followed by sudden sample dissolution, is a topic of active investigation owing to the method's unique prospects for the delivery of NMR spectra and images with unprecedented sensitivity. This experiment achieves hyperpolarization by the combined effects of electron-nuclear irradiation and cryogenic operation; the exploitation of these states occurs following a sudden melting and flushing of the resulting pellet from its original environment into a conventional, liquid-state setting. This melting and flushing usually demands using the equivalent of a few milliliters of hot solvent, a procedure which although well suited for in vivo studies leads to an excessive sample volume when considering typical applications within analytical settings. The present study explores a way of reducing the ensuing dilution of the hyperpolarized analytes, by employing a combination of immiscible liquids for performing the melting and flushing. It is shown that suitable combinations of immiscible solvents -both in terms of their heat capacities and densities-allow one to melt the targeted cryogenic pellet and dissolve the hyperpolarized analytes in a fraction of the solvent hitherto required. By tailoring the resulting volume to the needs of a conventional 5mm NMR probe, a substantial sensitivity enhancement can be added to the hyperpolarization process. An extra benefit may arise from using radicals that preferentially dissolve in the immiscible organic phase, by way of a lengthening of the relaxation time of the investigated analytes. Examples of these principles are given, and further potential extensions of this approach are discussed.