New Perspectives for Parahydrogen-Induced Polarization in Liquid Phase Heterogeneous Hydrogenation: An Aqueous Phase and ALTADENA Study

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.

Heterogeneous Catalysis and Parahydrogen-Induced Polarization

Parahydrogen-induced polarization with heterogeneous catalysts (HET-PHIP) has been a subject of extensive research in the last decade since its first observation in 2007. While NMR signal enhancements obtained with such catalysts are currently below those achieved with transition metal complexes in homogeneous hydrogenations in solution, this relatively new field demonstrates major prospects for a broad range of advanced fundamental and practical applications, from providing catalyst-free hyperpolarized fluids for biomedical magnetic resonance imaging (MRI) to exploring mechanisms of industrially important heterogeneous catalytic processes. This review covers the evolution of the heterogeneous catalysts used for PHIP observation, from metal complexes immobilized on solid supports to bulk metals and single-atom catalysts and discusses the general visions for maximizing the obtained NMR signal enhancements using HET-PHIP. Various practical applications of HET-PHIP, both for catalytic studies and for potential production of hyperpolarized contrast agents for MRI, are described.

PHIP hyperpolarized [1-13C]pyruvate and [1-13C]acetate esters via PH-INEPT polarization transfer monitored by 13C NMR and MRI

Parahydrogen-induced polarization of ¹³C nuclei by side-arm hydrogenation (PHIP-SAH) for [1-¹³C]acetate and [1-¹³C]pyruvate esters with application of PH-INEPT-type pulse sequences for ¹H to ¹³C polarization transfer is reported, and its efficiency is compared with that of polarization transfer based on magnetic field cycling (MFC). The pulse-sequence transfer approach may have its merits in some applications because the entire hyperpolarization procedure is implemented directly in an NMR or MRI instrument, whereas MFC requires a controlled field variation at low magnetic fields. Optimization of the PH-INEPT-type transfer sequences resulted in ¹³C polarization values of 0.66 ± 0.04% and 0.19 ± 0.02% for allyl [1-¹³C]pyruvate and ethyl [1-¹³C]acetate, respectively, which is lower than the corresponding polarization levels obtained with MFC for ¹H to ¹³C polarization transfer (3.95 ± 0.05% and 0.65 ± 0.05% for allyl [1-¹³C]pyruvate and ethyl [1-¹³C]acetate, respectively). Nevertheless, a significant ¹³C NMR signal enhancement with respect to thermal polarization allowed us to perform ¹³C MR imaging of both biologically relevant hyperpolarized molecules which can be used to produce useful contrast agents for the in vivo imaging applications.

Hydrogenative-PHIP polarized metabolites for biological studies

ParaHydrogen induced polarization (PHIP) is an efficient and cost-effective hyperpolarization method, but its application to biological investigations has been hampered, so far, due to chemical challenges. PHIP is obtained by means of the addition of hydrogen, enriched in the para-spin isomer, to an unsaturated substrate. Both hydrogen atoms must be transferred to the same substrate, in a pairwise manner, by a suitable hydrogenation catalyst; therefore, a de-hydrogenated precursor of the target molecule is necessary. This has strongly limited the number of parahydrogen polarized substrates. The non-hydrogenative approach brilliantly circumvents this central issue, but has not been translated to in-vivo yet. Recent advancements in hydrogenative PHIP (h-PHIP) considerably widened the possibility to hyperpolarize metabolites and, in this review, we will focus on substrates that have been obtained by means of this method and used in vivo. Attention will also be paid to the requirements that must be met and on the issues that have still to be tackled to obtain further improvements and to push PHIP substrates in biological applications.

Catalytic hydrogenation with parahydrogen: a bridge from homogeneous to heterogeneous catalysis

One of the essential themes in modern catalysis is that of bridging the gap between its homogeneous and heterogeneous counterparts to combine their individual advantages and overcome shortcomings. One more incentive can now be added to the list, namely the ability of transition metal complexes to provide strong nuclear magnetic resonance (NMR) signal enhancement upon their use in homogeneous hydrogenations of unsaturated compounds with parahydrogen in solution. The addition of both H atoms of a parahydrogen molecule to the same substrate, a prerequisite for such effects, is implemented naturally with metal complexes that operate via the formation of a dihydride intermediate, but not with most heterogeneous catalysts. Despite that, it has been demonstrated in recent years that various types of heterogeneous catalysts are able to perform the required pairwise H2 addition at least to some extent. This has opened a major gateway for developing highly sensitive and informative tools for mechanistic studies of heterogeneous hydrogenations and other processes involving H2. Besides, production of catalyst-free fluids with NMR signals enhanced by 3-4 orders of magnitude is essential for modern applications of magnetic resonance imaging (MRI), including biomedical research and practice. The ongoing efforts to design heterogeneous catalysts which can implement the homogeneous (pairwise) hydrogenation mechanism are reported.

Heterogeneous hydrogenation of phenylalkynes with parahydrogen: hyperpolarization, reaction selectivity, and kinetics

Parahydrogen-induced polarization (PHIP) is a powerful technique for studying hydrogenation reactions in both gas and liquid phases.

Parahydrogen-Based Hyperpolarization for Biomedicine

Magnetic resonance (MR) is one of the most versatile and useful physical effects used for human imaging, chemical analysis, and the elucidation of molecular structures. However, its full potential is rarely used, because only a small fraction of the nuclear spin ensemble is polarized, that is, aligned with the applied static magnetic field. Hyperpolarization methods seek other means to increase the polarization and thus the MR signal. A unique source of pure spin order is the entangled singlet spin state of dihydrogen, parahydrogen (pH2 ), which is inherently stable and long-lived. When brought into contact with another molecule, this "spin order on demand" allows the MR signal to be enhanced by several orders of magnitude. Considerable progress has been made in the past decade in the area of pH2 -based hyperpolarization techniques for biomedical applications. It is the goal of this Review to provide a selective overview of these developments, covering the areas of spin physics, catalysis, instrumentation, preparation of the contrast agents, and applications.

Hyperpolarized NMR Spectroscopy: d-DNP, PHIP, and SABRE Techniques

The intensity of NMR signals can be enhanced by several orders of magnitude by using various techniques for the hyperpolarization of different molecules. Such approaches can overcome the main sensitivity challenges facing modern NMR/magnetic resonance imaging (MRI) techniques, whilst hyperpolarized fluids can also be used in a variety of applications in material science and biomedicine. This Focus Review considers the fundamentals of the preparation of hyperpolarized liquids and gases by using dissolution dynamic nuclear polarization (d-DNP) and parahydrogen-based techniques, such as signal amplification by reversible exchange (SABRE) and parahydrogen-induced polarization (PHIP), in both heterogeneous and homogeneous processes. The various new aspects in the formation and utilization of hyperpolarized fluids, along with the possibility of observing NMR signal enhancement, are described.

Aqueous, Heterogeneous Parahydrogen-Induced 15N Polarization

The successful transfer of parahydrogen-induced polarization to ¹⁵N spins using heterogeneous catalysts in aqueous solutions was demonstrated. Hydrogenation of a synthesized unsaturated ¹⁵N-labeled precursor (neurine) with parahydrogen (p-H₂) over Rh/TiO₂ heterogeneous catalysts yielded a hyperpolarized structural analog of choline. As a result, ¹⁵N polarization enhancements of over two orders of magnitude were achieved for the ¹⁵N-ethyl trimethyl ammonium ion product in deuterated water at elevated temperatures. Enhanced ¹⁵N NMR spectra were successfully acquired at 9.4 T and 0.05 T. Importantly, long hyperpolarization lifetimes were observed at 9.4 T, with a ¹⁵N T₁ of ~6 min for the product molecules, and the T₁ of the deuterated form exceeded 8 min. Taken together, these results show that this approach for generating hyperpolarized species with extended lifetimes in aqueous, biologically compatible solutions is promising for various biomedical applications.

Separation of Anti-Phase Signals Due to Parahydrogen Induced Polarization via 2D Nutation NMR Spectroscopy

The present work introduces a novel method for the selective detection of ¹ H NMR anti-phase signals caused by the pairwise incorporation of parahydrogen into olefins on noble-metal-containing catalysts. Via a two-dimensional (2D) nutation NMR experiment, the anti-phase signals of hyperpolarized ¹ H nuclei are separated due to their double nutation frequency compared to that of thermally polarized ¹ H nuclei. For demonstrating this approach, parahydrogen induced polarization (PHIP) was achieved via the hydrogenation of propene with parahydrogen on platinum-containing silica and investigated by in situ ¹ H MAS NMR spectroscopy under continuous-flow conditions, that is, the hydrogenation reaction was performed inside the magnet of the NMR spectrometer. The 2D nutation NMR experiment described in the present work is useful for the separation of overlapping anti-phase and in-phase signals due to hyperpolarized and thermally polarized ¹ H nuclei, respectively, which is important for research in the field of heterogeneous catalysis.

Pairwise hydrogen addition in the selective semihydrogenation of alkynes on silica-supported Cu catalysts

Mechanistic insight into the semihydrogenation of 1-butyne and 2-butyne on Cu nanoparticles supported on partially dehydroxylated silica (Cu/SiO_2-700) was obtained using parahydrogen.

Mechanistic insight into the semihydrogenation of 1-butyne and 2-butyne on Cu nanoparticles supported on partially dehydroxylated silica (Cu/SiO_2-700) was obtained using parahydrogen. Hydrogenation of 1-butyne over Cu/SiO_2-700 yielded 1-butene with ≥97% selectivity. The surface modification of this catalyst with tricyclohexylphosphine (PCy₃) increased the selectivity to 1-butene up to nearly 100%, although at the expense of reduced catalytic activity. Similar trends were observed in the hydrogenation of 2-butyne, where Cu/SiO_2-700 provided a selectivity to 2-butene in the range of 72–100% depending on the reaction conditions, while the catalyst modified with PCy₃ again demonstrated nearly 100% selectivity. Parahydrogen-induced polarization effects observed in hydrogenation reactions catalyzed by copper-based catalysts demonstrate the viability of pairwise hydrogen addition over these catalysts. Contribution of pairwise hydrogen addition to 1-butyne was estimated to be at least 0.2–0.6% for unmodified Cu/SiO_2-700 and ≥2.7% for Cu/SiO_2-700 modified with PCy₃, highlighting the effect of surface modification with the tricyclohexylphosphine ligand.

Production of Pure Aqueous 13 C-Hyperpolarized Acetate by Heterogeneous Parahydrogen-Induced Polarization

A supported metal catalyst was designed, characterized, and tested for aqueous phase heterogeneous hydrogenation of vinyl acetate with parahydrogen to produce 13 C-hyperpolarized ethyl acetate for potential biomedical applications. The Rh/TiO2 catalyst with a metal loading of 23.2 wt % produced strongly hyperpolarized 13 C-enriched ethyl acetate-1-13 C detected at 9.4 T. An approximately 14-fold 13 C signal enhancement was detected using circa 50 % parahydrogen gas without taking into account relaxation losses before and after polarization transfer by magnetic field cycling from nascent parahydrogen-derived protons to 13 C nuclei. This first observation of 13 C PHIP-hyperpolarized products over a supported metal catalyst in an aqueous medium opens up new possibilities for production of catalyst-free aqueous solutions of nontoxic hyperpolarized contrast agents for a wide range of biomolecules amenable to the parahydrogen induced polarization by side arm hydrogenation (PHIP-SAH) approach.

31P-Solid-State NMR Characterization and Catalytic Hydrogenation Tests of Novel heterogenized Iridium-Catalysts

The synthesis of novel robust and stable iridium-based immobilized catalysts on silica-polymer hybrid materials (Si-PB-Ir) is described. These catalysts are characterized by a combination of 1D 31P CP-MAS and 2D 31P-1H HETCOR and J-resolved multinuclear solid state NMR experiments. Different binding situations such as singly and multiply coordinated phosphines are identified. Density functional theory (DFT) calculations are performed to corroborate the interpretation of the experimental NMR data, in order to propose a structural model of the heterogenized catalysts. Finally, the catalytic activity of the Si-PB-Ir catalysts is investigated for the hydrogenation of styrene employing para-enriched hydrogen gas.

Toward Production of Pure 13C Hyperpolarized Metabolites Using Heterogeneous Parahydrogen-Induced Polarization of Ethyl[1-13C]acetate

Here, we report the production of ¹³C-hyperpolarized ethyl acetate via heterogeneously catalyzed pairwise addition of parahydrogen to vinyl acetate over TiO₂-supported rhodium nanoparticles, followed by magnetic field cycling. Importantly, the hyperpolarization is demonstrated even at the natural abundance of ¹³C isotope (ca. 1.1%) along with the easiest separation of the catalyst from the hyperpolarized liquid.

Surface ligand-directed pair-wise hydrogenation for heterogeneous phase hyperpolarization

para-Hydrogen induced polarization is a technique of magnetic resonance hyperpolarization utilizing hydrogen's para-spin state for generating signal intensities at magnitudes far greater than state-of-the-art magnets. Platinum nanoparticle-catalysts with cysteine-capping are presented. The measured polarization is the highest reported to date in water, paving pathways for generating medical imaging contrast agents.

Production of Catalyst-Free Hyperpolarised Ethanol Aqueous Solution via Heterogeneous Hydrogenation with Parahydrogen

An experimental approach for the production of catalyst-free hyperpolarised ethanol solution in water via heterogeneous hydrogenation of vinyl acetate with parahydrogen and the subsequent hydrolysis of ethyl acetate was demonstrated. For an efficient hydrogenation, liquid vinyl acetate was transferred to the gas phase by parahydrogen bubbling and almost completely converted to ethyl acetate with Rh/TiO2 catalyst. Subsequent dissolution of ethyl acetate gas in water containing OH(-) ions led to the formation of catalyst- and organic solvent-free hyperpolarised ethanol and sodium acetate. These results represent the first demonstration of catalyst- and organic solvent-free hyperpolarised ethanol production achieved by heterogeneous hydrogenation of vinyl acetate vapour with parahydrogen and the subsequent ethyl acetate hydrolysis.

Single-atom gold catalysis in the context of developments in parahydrogen-induced polarization

A highly isolated monoatomic gold catalyst, with single gold atoms dispersed on multiwalled carbon nanotubes (MWCNTs), has been synthesized, characterized, and tested in heterogeneous hydrogenation of 1,3-butadiene and 1-butyne with parahydrogen to maximize the polarization level and the contribution of the pairwise hydrogen addition route. The Au/MWCNTs catalyst was found to be active and efficient in pairwise hydrogen addition and the estimated contributions from the pairwise hydrogen addition route are at least an order of magnitude higher than those for supported metal nanoparticle catalysts. Therefore, the use of the highly isolated monoatomic catalysts is very promising for production of hyperpolarized fluids that can be used for the significant enhancement of NMR signals. A mechanism of 1,3-butadiene hydrogenation with parahydrogen over the highly isolated monoatomic Au/MWCNTs catalyst is also proposed.

Demonstration of heterogeneous parahydrogen induced polarization using hyperpolarized agent migration from dissolved Rh(I) complex to gas phase

Parahydrogen-induced polarization (PHIP) was used to demonstrate the concept that highly polarized, catalyst-free fluids can be obtained in a catalysis-free regime using a chemical reaction with molecular addition of parahydrogen to a water-soluble Rh(I) complex carrying a payload of compound with unsaturated (C=C) bonds. Hydrogenation of norbornadiene leads to formation of norbornene, which is eliminated from the Rh(I) complex and, therefore, leaves the aqueous phase and becomes a gaseous hyperpolarized molecule. The Rh(I) metal complex resides in the original liquid phase, while the product of hydrogen addition is found exclusively in the gaseous phase based on the affinity. Hyperpolarized norbornene ¹H NMR signals observed in situ were enhanced by a factor of approximately 10 000 at a static field of 47.5 mT. High-resolution ¹H NMR at a field of 9.4 T was used for ex situ detection of hyperpolarized norbornene in the gaseous phase, where a signal enhancement factor of approximately 160 was observed. This concept of stoichiometric as opposed to purely catalytic use of PHIP-available complexes with an unsaturated payload precursor molecule can be extended to other contrast agents for both homogeneous and heterogeneous PHIP. The Rh(I) complex was employed in aqueous medium suitable for production of hyperpolarized contrast agents for biomedical use. Detection of PHIP hyperpolarized gas by low-field NMR is demonstrated here for the first time.

NMR of hyperpolarised probes

Increasing the sensitivity of NMR experiments is an ongoing field of research to help realise the exquisite molecular specificity of this technique. Hyperpolarisation of various nuclei is a powerful approach that enables the use of NMR for molecular and cellular imaging. Substantial progress has been achieved over recent years in terms of both tracer preparation and detection schemes. This review summarises recent developments in probe design and optimised signal encoding, and promising results in sensitive disease detection and efficient therapeutic monitoring. The different methods have great potential to provide molecular specificity not available by other diagnostic modalities.

Parahydrogen-induced polarization in heterogeneous catalytic processes

Parahydrogen-induced polarization of nuclear spins provides enhancements of NMR signals for various nuclei of up to four to five orders of magnitude in magnetic fields of modern NMR spectrometers and even higher enhancements in low and ultra-low magnetic fields. It is based on the use of parahydrogen in catalytic hydrogenation reactions which, upon pairwise addition of the two H atoms of parahydrogen, can strongly enhance the NMR signals of reaction intermediates and products in solution. A recent advance in this field is the demonstration that PHIP can be observed not only in homogeneous hydrogenations but also in heterogeneous catalytic reactions. The use of heterogeneous catalysts for generating PHIP provides a number of significant advantages over the homogeneous processes, including the possibility to produce hyperpolarized gases, better control over the hydrogenation process, and the ease of separation of hyperpolarized fluids from the catalyst. The latter advantage is of paramount importance in light of the recent tendency toward utilization of hyperpolarized substances in in vivo spectroscopic and imaging applications of NMR. In addition, PHIP demonstrates the potential to become a useful tool for studying mechanisms of heterogeneous catalytic processes and for in situ studies of operating catalytic reactors. Here, the known examples of PHIP observations in heterogeneous reactions over immobilized transition metal complexes, supported metals, and some other types of heterogeneous catalysts are discussed and the applications of the technique for hypersensitive NMR imaging studies are presented.

New investigations of technical rhodium and iridium catalysts in homogeneous phase employing para-hydrogen induced polarization

It is shown that the para-hydrogen induced polarization (PHIP) phenomenon in homogenous solution containing the substrate styrene is also observable employing simple inorganic systems of the form MCl(3)·xH(2)O (M=Rh, Ir) as catalyst. Such observation confirms that already very simple metal complexes enable the creation of PHIP signal enhancement in solution. This opens up new pathways to increase the sensitivity of NMR and MRT by PHIP enhancement using cost-effective catalysts and will be essential for further mechanistic studies of simple transition metal systems.