Towards Implementation of Nanodiamonds with Nitrogen-Vacancy Defects as Hyperpolarized MRI Contrast Agents

The unique properties of negatively-charged Nitrogen-Vacancy (NV^-) defect in diamond, make nanodiamonds with NV^- defects attractive for various applications. One of the important properties of NV^- centers is the possibility to efficiently initialize them in the m_s = 0 state at room temperature up to a very high degree (&gt;90%) by illumination with green light. Using one of the dynamic nuclear spin polarization (DNP) techniques the polarization of NV^- centers can be transferred to surrounding nuclear spins, increasing the polarization of nuclear spins beyond thermal equilibrium. The advantage of nanodiamonds is their high surface to volume ratio. This provides an opportunity for efficient transfer of the nuclear polarization from inside of the diamond particle to external target nuclear spins. Therefore, it is expected that nanodiamonds with NV^- centers can be efficiently used as polarized magnetic resonance imaging (MRI) contrast agents.¹<br/>One of the most important factors for the success of realization of NDs as hyperpolarized MRI agents is the possibility to achieve the properties of diamonds material that would fulfil the requirement for the efficient polarization transfer from NV^- centers to nuclear spins inside and outside of diamond particles. Those properties include high NV^- concentration, good spin properties of NV^- centers and nuclear spins, and tailored ¹³C composition. The desirable nanodiamonds properties can be achieved by improving synthesis and treatment techniques, with the help of careful characterization.<br/>We show that simultaneous electron irradiation and annealing, with adjusted irradiation dose, provide a high conversion efficiency of substitutional nitrogen (P1 center) to NV^- defects in diamond powder². The presence of P1 defects, which constitute a noise source in the diamond lattice, is undesirable. Therefore, high conversion efficiency of P1 centers to NV^- defects, without important creation of other irradiation-induced defects, is also beneficial for spin properties both, of NV centers and ¹³C nuclear spins.<br/>Having tailored ¹³C concentration, it is possible to increase the hyperpolarized signal³. Using the high pressure high temperature (HPHT) growth method, we synthesized diamond particles (20 nm to 2 µm) with different ¹³C content (5-30 %).⁴ To transfer the nuclear polarization from nuclear spins located inside of diamond particles to nuclear spins outside diamond, we also synthesized “core-shell” particles, where nanodiamonds with a natural (1.1%) abundance of ¹³C doped with NV centers are covered with a diamond layer with ¹³C content close to 100%.⁵ The synthesized material was analyzed with Scanning Electron Microscopy as well as Electronic Paramagnetic Resonance (EPR), Nuclear Magnetic Resonance (NMR) and Optically Detected Magnetic Resonance spectroscopy.<br/>As a first step towards the realization of DNP techniques in nanodiamonds, we used micro-sized diamond material with improved properties to demonstrate the implementation of nuclear hyperpolarization, using a combined EPR-NMR system.<br/><br/>References:<br/>1) A. Ajoy, K. Liu, R. Nazaryan, X. Lv, P. R. Zangara, B. Safvati, G. Wang, D. Arnold, G. Li, A. Lin, et al., Scien. Advan. 4 (2018).<br/>2) Y. Mindarava, R. Blinder, C. Laube, W. Knolle, B. Abel, C. Jentgens, J. Isoya, J. Scheuer, J. Lang, I. Schwartz, B. Naydenov, F. Jelezko, Carbon 170 (2020).<br/>3) A. J. Parker, K. Jeong, C. E. Avalos, B. J. M. Hausmann, C. C. Vassiliou, A. Pines, J. P. King, Phys. Rev. B 100 (2019).<br/>4) Y. Mindarava, R. Blinder, Y. Liu, J. Scheuer, J. Lang, V. Agafonov, V. A. Davydov, C. Laube, W. Knolle, B. Abel, B. Naydenov, F. Jelezko, Carbon 165 (2020).<br/>5) Y. Mindarava, R. Blinder, V. A. Davydov, M. Zaghrioui, V. N. Agafonov, C. Autret, P. Balasubramanian, R. Gonzalez, F. Jelezko, submitted.