Nanodiamond as a New Hyperpolarizing Agent and Its (13)C MRS

Nanomaterials for hyperpolarized nuclear magnetic resonance and magnetic resonance imaging

Nanomaterials play an important role in the development and application of hyperpolarized materials for magnetic resonance imaging (MRI). In this context they can not only act as hyperpolarized materials which are directly imaged but also play a role as carriers for hyperpolarized gases and catalysts for para-hydrogen induced polarization (PHIP) to generate hyperpolarized substrates for metabolic imaging. Those three application possibilities are discussed, focusing on carbon-based materials for the directly imaged particles. An overview over recent developments in all three fields is given, including the early developments in each field as well as important steps towards applications in MRI, such as making the initially developed methods more biocompatible and first imaging experiments with spatial resolution in either phantoms or in vivo studies. Focusing on the important features nanomaterials need to display to be applicable in the MRI context, a wide range of different approaches to that extent is covered, giving the reader a general idea of different possibilities as well as recent developments in those different fields of hyperpolarized magnetic resonance. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Preclinical PET and MR Evaluation of 89Zr- and 68Ga-Labeled Nanodiamonds in Mice over Different Time Scales

Nanodiamonds (NDs) have high potential as a drug carrier and in combination with nitrogen vacancies (NV centers) for highly sensitive MR-imaging after hyperpolarization. However, little remains known about their physiological properties in vivo. PET imaging allows further evaluation due to its quantitative properties and high sensitivity. Thus, we aimed to create a preclinical platform for PET and MR evaluation of surface-modified NDs by radiolabeling with both short- and long-lived radiotracers. Serum albumin coated NDs, functionalized with PEG groups and the chelator deferoxamine, were labeled either with zirconium-89 or gallium-68. Their biodistribution was assessed in two different mouse strains. PET scans were performed at various time points up to 7 d after i.v. injection. Anatomical correlation was provided by additional MRI in a subset of animals. PET results were validated by ex vivo quantification of the excised organs using a gamma counter. Radiolabeled NDs accumulated rapidly in the liver and spleen with a slight increase over time, while rapid washout from the blood pool was observed. Significant differences between the investigated radionuclides were only observed for the spleen (1 h). In summary, we successfully created a preclinical PET and MR imaging platform for the evaluation of the biodistribution of NDs over different time scales.

Recent Developments of Nanodiamond Quantum Sensors for Biological Applications

Measuring certain quantities at the nanoscale is often limited to strict conditions such as low temperature or vacuum. However, the recently developed nanodiamond (ND) quantum sensing technology shows great promise for ultrasensitive diagnosis and probing subcellular parameters at ambient conditions. Atom defects (i.e., N, Si) within the ND lattice provide stable emissions and sometimes spin-dependent photoluminescence. These unique properties endow ND quantum sensors with the capacity to detect local temperature, magnetic fields, electric fields, or strain. In this review, some of the recent, most exciting developments in the preparation and application of ND sensors to solve current challenges in biology and medicine including ultrasensitive detection of virions and local sensing of pH, radical species, magnetic fields, temperature, and rotational movements, are discussed.

Dynamic Nuclear Polarization of Silicon Carbide Micro- and Nanoparticles

Two dominant crystalline phases of silicon carbide (SiC): α-SiC and β-SiC, differing in size and chemical composition, were investigated regarding their potential for dynamic nuclear polarization (DNP). ²⁹Si nuclei in α-SiC micro- and nanoparticles with sizes ranging from 650 nm to 2.2 μm and minimal oxidation were successfully hyperpolarized without the use of free radicals, while β-SiC samples did not display appreciable degrees of polarization under the same polarization conditions. Long T₁ relaxation times in α-SiC of up to 1600 s (∼27 min) were recorded for the ²⁹Si nuclei after 1 h of polarization at a temperature of 4 K. Interestingly, these promising α-SiC particles allowed for direct hyperpolarization of both ²⁹Si and ¹³C nuclei, resulting in comparably strong signal amplifications. Moreover, the T₁ relaxation time of ¹³C nuclei in 750 nm-sized α-SiC particles was over 33 min, which far exceeds T₁ times of conventional ¹³C DNP probes with values in the order of 1-2 min. The present work demonstrates the feasibility of DNP on SiC micro- and nanoparticles and highlights their potential as hyperpolarized magnetic resonance imaging agents.

Nanodiamond-enabled biomedical imaging

Biomedical imaging allows in vivo studies of organisms, providing valuable information of biological processes at both cellular and tissue levels. Nanodiamonds have recently emerged as a new type of probe for fluorescence imaging and contrast agent for magnetic resonance and photoacoustic imaging. Composed of sp3-carbon atoms, diamond is chemically inert and inherently biocompatible. Uniquely, its matrix can host a variety of optically and magnetically active defects suited for bioimaging applications. Since the first production of fluorescent nanodiamonds in 2005, a large number of experiments have demonstrated that fluorescent nanodiamonds are useful as photostable markers and nanoscale sensors in living cells and organisms. In this review, we focus our discussion on the recent advancements of nanodiamond-enabled biomedical imaging for preclinical applications.

Phase-Encoded Hyperpolarized Nanodiamond for Magnetic Resonance Imaging

Surface-functionalized nanomaterials are of interest as theranostic agents that detect disease and track biological processes using hyperpolarized magnetic resonance imaging (MRI). Candidate materials are sparse however, requiring spinful nuclei with long spin-lattice relaxation (T1) and spin-dephasing times (T2), together with a reservoir of electrons to impart hyperpolarization. Here, we demonstrate the versatility of the nanodiamond material system for hyperpolarized 13C MRI, making use of its intrinsic paramagnetic defect centers, hours-long nuclear T1 times, and T2 times suitable for spatially resolving millimeter-scale structures. Combining these properties, we enable a new imaging modality, unique to nanoparticles, that exploits the phase-contrast between spins encoded with a hyperpolarization that is aligned, or anti-aligned with the external magnetic field. The use of phase-encoded hyperpolarization allows nanodiamonds to be tagged and distinguished in an MRI based on their spin-orientation alone, and could permit the action of specific bio-functionalized complexes to be directly compared and imaged.

Direct hyperpolarization of micro- and nanodiamonds for bioimaging applications - Considerations on particle size, functionalization and polarization loss

Due to the inherently long relaxation time of ¹³C spins in diamond, the nuclear polarization enhancement obtained with dynamic nuclear polarization can be preserved for a time on the order of about one hour, opening up an opportunity to use diamonds as a new class of long-lived contrast agents. The present communication explores the feasibility of using ¹³C spins in directly hyperpolarized diamonds for MR imaging including considerations for potential in vivo applications.

Nanodiamond-enhanced MRI via in situ hyperpolarization

Nanodiamonds are of interest as nontoxic substrates for targeted drug delivery and as highly biostable fluorescent markers for cellular tracking. Beyond optical techniques, however, options for noninvasive imaging of nanodiamonds in vivo are severely limited. Here, we demonstrate that the Overhauser effect, a proton–electron polarization transfer technique, can enable high-contrast magnetic resonance imaging (MRI) of nanodiamonds in water at room temperature and ultra-low magnetic field. The technique transfers spin polarization from paramagnetic impurities at nanodiamond surfaces to ¹H spins in the surrounding water solution, creating MRI contrast on-demand. We examine the conditions required for maximum enhancement as well as the ultimate sensitivity of the technique. The ability to perform continuous in situ hyperpolarization via the Overhauser mechanism, in combination with the excellent in vivo stability of nanodiamond, raises the possibility of performing noninvasive in vivo tracking of nanodiamond over indefinitely long periods of time.

Hyperpolarized magnetic resonance imaging can enhance imaging contrast by orders of magnitude, but applications are limited by the thermal relaxation of hyperpolarized states. Here, Waddington et al. demonstrate the on-demand hyperpolarization of hydrogen spins through the Overhauser effect with nanodiamonds.

Influence of Dy3+ and Tb3+ doping on 13C dynamic nuclear polarization

Dynamic nuclear polarization (DNP) is a technique that uses a microwave-driven transfer of high spin alignment from electrons to nuclear spins. This is most effective at low temperature and high magnetic field, and with the invention of the dissolution method, the amplified nuclear magnetic resonance (NMR) signals in the frozen state in DNP can be harnessed in the liquid-state at physiologically acceptable temperature for in vitro and in vivo metabolic studies. A current optimization practice in dissolution DNP is to dope the sample with trace amounts of lanthanides such as Gd3+ or Ho3+, which further improves the polarization. While Gd3+ and Ho3+ have been optimized for use in dissolution DNP, other lanthanides have not been exhaustively studied for use in C13 DNP applications. In this work, two additional lanthanides with relatively high magnetic moments, Dy3+ and Tb3+, were extensively optimized and tested as doping additives for C13 DNP at 3.35 T and 1.2 K. We have found that both of these lanthanides are also beneficial additives, to a varying degree, for C13 DNP. The optimal concentrations of Dy3+ (1.5 mM) and Tb3+ (0.25 mM) for C13 DNP were found to be less than that of Gd3+ (2 mM). W-band electron paramagnetic resonance shows that these enhancements due to Dy3+ and Tb3+ doping are accompanied by shortening of electron T1 of trityl OX063 free radical. Furthermore, when dissolution was employed, Tb3+-doped samples were found to have similar liquid-state C13 NMR signal enhancements compared to samples doped with Gd3+, and both Tb3+ and Dy3+ had a negligible liquid-state nuclear T1 shortening effect which contrasts with the significant reduction in T1 when using Gd3+. Our results show that Dy3+ doping and Tb3+ doping have a beneficial impact on C13 DNP both in the solid and liquid states, and that Tb3+ in particular could be used as a potential alternative to Gd3+ in C13 dissolution DNP experiments.

Hyperpolarized Nanodiamond Surfaces

The widespread use of nanodiamond as a biomedical platform for drug-delivery, imaging, and subcellular tracking applications stems from its nontoxicity and unique quantum mechanical properties. Here, we extend this functionality to the domain of magnetic resonance, by demonstrating that the intrinsic electron spins on the nanodiamond surface can be used to hyperpolarize adsorbed liquid compounds at low fields and room temperature. By combining relaxation measurements with hyperpolarization, spins on the surface of the nanodiamond can be distinguished from those in the bulk liquid. These results are likely of use in signaling the controlled release of pharmaceutical payloads.

BDPA-Doped Polystyrene Beads as Polarization Agents for DNP-NMR

The aromatic free radical BDPA (α,γ-bisdiphenylene-β-phenylallyl), which has been widely used as a polarizing agent for Dynamic Nuclear Polarization (DNP) of hydrophobic analytes, has been incorporated into nanometer-scale polystyrene latex beads. We have shown that the resulting BDPA-doped beads can be used to hyperpolarize (13)C and (7)Li nuclei in aqueous environments, without the need for a glassing cosolvent. DNP enhancement factors of between 20 and 100 were achieved with overall BDPA concentrations of 2 mM or less. These Highly-Effective Polymer/Radical Beads (HYPR-beads) have potential use as an inexpensive polarizing agent for water-soluble analytes, and also have applications as model nanoparticles in DNP studies.

Hyperpolarized nanodiamond with long spin-relaxation times

The use of hyperpolarized agents in magnetic resonance, such as ¹³C-labelled compounds, enables powerful new imaging and detection modalities that stem from a 10,000-fold boost in signal. A major challenge for the future of the hyperpolarization technique is the inherently short spin-relaxation times, typically <60 s for ¹³C liquid-state compounds, which limit the time that the signal remains boosted. Here we demonstrate that 1.1% natural abundance ¹³C spins in synthetic nanodiamond can be hyperpolarized at cryogenic and room temperature without the use of free radicals, and, owing to their solid-state environment, exhibit relaxation times exceeding 1 h. Combined with the already established applications of nanodiamonds in the life sciences as inexpensive fluorescent markers and non-cytotoxic substrates for gene and drug delivery, these results extend the theranostic capabilities of nanoscale diamonds into the domain of hyperpolarized magnetic resonance.

Hyperpolarized carbon nuclei are promising contrast agents for magnetic resonance imaging, but typically possess relaxation times below one minute. Here, the authors demonstrate cryogenic and room temperature hyperpolarization of ¹³C in synthetic nanodiamonds with relaxation times exceeding one hour.