In vivo detection of cucurbit[6]uril, a hyperpolarized xenon contrast agent for a xenon magnetic resonance imaging biosensor

Hyperpolarized Xenon-129 Chemical Exchange Saturation Transfer (HyperCEST) Molecular Imaging: Achievements and Future Challenges

Molecular magnetic resonance imaging (MRI) is an emerging field that is set to revolutionize our perspective of disease diagnosis, treatment efficacy monitoring, and precision medicine in full concordance with personalized medicine. A wide range of hyperpolarized (HP) 129Xe biosensors have been recently developed, demonstrating their potential applications in molecular settings, and achieving notable success within in vitro studies. The favorable nuclear magnetic resonance properties of 129Xe, coupled with its non-toxic nature, high solubility in biological tissues, and capacity to dissolve in blood and diffuse across membranes, highlight its superior role for applications in molecular MRI settings. The incorporation of reporters that combine signal enhancement from both hyperpolarized 129Xe and chemical exchange saturation transfer holds the potential to address the primary limitation of low sensitivity observed in conventional MRI. This review provides a summary of the various applications of HP 129Xe biosensors developed over the last decade, specifically highlighting their use in MRI. Moreover, this paper addresses the evolution of in vivo applications of HP 129Xe, discussing its potential transition into clinical settings.

R3-Noria-methanesulfonate: A Molecular Cage with Superior Hyperpolarized Xenon-129 MRI Contrast

Hyperpolarized (HP) xenon-129 (129Xe) magnetic resonance imaging (MRI) has the potential to be used as a molecular imaging modality. For this purpose, numerous supramolecular cages have been developed and evaluated in the past. Herein, we report a novel and unique macrocycle that can be successfully utilized for xenon MRI, the resorcinarene trimer methanesulfonate (R3-Noria-MeSO3H). This molecule is capable of two different contrast mechanisms for xenon-MRI, resulting from an increase in the effective spin-spin relaxation and hyperpolarized chemical exchange saturation transfer (HyperCEST). We have demonstrated a superior negative contrast caused by R3-Noria-MeSO3H on HP 129Xe MRI at 3.0 T as well as HyperCEST imaging of the studied macrocycle. Additionally, we have found that the complex aggregation behaviors of R3-Noria-methanesulfonate and its impact on xenon-129 relaxivity are an area for future study.

Cucurbit[6]uril Hyperpolarized Chemical Exchange Saturation Transfer Pulse Sequence Parameter Optimization and Detectability Limit Assessment at 3.0T

Molecular imaging is the future of personalized medicine; however, it requires effective contrast agents. Hyperpolarized chemical exchange saturation transfer (HyperCEST) can boost the signal of Hyperpolarized 129 Xe MRI and render it a molecular imaging modality of high efficiency. Cucurbit[6]uril (CB6) has been successfully employed in vivo as a contrast agent for HyperCEST MRI, however its performance in a clinical MRI scanner has yet to be optimized. In this study, MRI pulse sequence parameter optimization was first performed in CB6 solutions in phosphate-buffered saline (PBS), and subsequently in whole sterile citrated bovine blood. The performance of four different depolarization pulse shapes (sinusoidal, 3-lobe sinc (3LS), rectangular (block), and hyperbolic secant (hypsec) was optimized. The detectability limits of CB6 in a clinical 3.0T MRI scanner was assessed using the optimized pulse sequences. The 3LS depolarization pulses performed best, and demonstrated 24 % depletion in a 25 μM solution of CB6 in PBS. It performed similarly in blood. The CB6 detectability limit was found to be 100 μM in citrated bovine blood with a correspondent HyperCEST depletion of 30 % ±9 %. For the first time, the HP 129 Xe HyperCEST effect was observed in red blood cells (RBC) and had a similar strength as HyperCEST in plasma.

Compartment-specific 129Xe HyperCEST z spectroscopy and chemical shift imaging of cucurbit[6]uril in spontaneously breathing rats

129Xe hyperpolarized gas chemical exchange saturation transfer (HyperCEST) MRI has been suggested as molecular imaging modality but translation to in vivo imaging has been slow, likely due to difficulties of synthesizing suitable molecules. Cucurbit[6]uril-either in readily available non-functionalized or potentially in functionalized form-may, combined with 129Xe HyperCEST MRI, prove useful as a switchable 129Xe MR contrast agent but the likely differential properties of contrast generation in individual chemical compartments as well as the influence of 129Xe signal drifts encountered in vivo on HyperCEST MRI are unknown. Here, HyperCEST z spectroscopy and chemical shift imaging with compartment-specific analysis are performed in a total of 10 rats using cucurbit[6]uril injected i.v. and under a protocol employing spontaneous respiration. Differences in intensity of the HyperCEST effect between chemical compartments and anatomical regions are investigated. Strategies to mitigate influence of signal instabilities associated with drifts in physiological parameters are developed. It is shown that presence of cucurbit[6]uril can be readily detected under spontaneous 129Xe inhalation mostly in aqueous tissues further away from the lung. Differences of effect intensity in individual regions and compartments must be considered in HyperCEST data interpretation. In particular, there seems to be almost no effect in lipids. 129Xe HyperCEST MR measurements utilizing spontaneous respiration protocols and extended measurement times are feasible. HyperCEST MRI of non-functionalized cucurbit[6]uril may create contrast between anatomical structures in vivo.

Improving HyperCEST performance by favorable xenon exchange conditions in liposomal nanocarriers

MRI reporters that combine signal enhancement from saturation transfer with hyperpolarized ¹²⁹ Xe show nanomolar detection sensitivity for in vitro studies. However, they need further improvement for accelerated CEST build-up that sufficiently dominates the intrinsic loss of hyperpolarization under in vivo conditions. This study introduces liposomes with a HyperCEST-active lipopeptide to enhance the efficiency of a well known Xe host, CrA-ma, with medium Xe exchange kinetics in aqueous environment, by two orders of magnitude. The depolarization time for constant saturation power but increasing saturation time is used as a comparative measure to rank different nanocarrier formulations. A variable cage load illustrates that the available CEST sites should be well distributed throughout the nanocarriers to avoid inefficiency from back exchange. For a liposome loading with only 2 mol% CrA-lipopeptide, the higher exchange kinetics allowed us to work even with 17-fold lower saturation power than for CrA-ma itself to achieve significant image contrast with ¹²⁹ Xe. Overall, this study illustrates the wide parameter space that is now available when incorporating CrA-labelled lipopeptides into liposomal carriers.

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Preclinical MRI Using Hyperpolarized 129Xe

Although critical for development of novel therapies, understanding altered lung function in disease models is challenging because the transport and diffusion of gases over short distances, on which proper function relies, is not readily visualized. In this review we summarize progress introducing hyperpolarized ¹²⁹Xe imaging as a method to follow these processes in vivo. The work is organized in sections highlighting methods to observe the gas replacement effects of breathing (Gas Dynamics during the Breathing Cycle) and gas diffusion throughout the parenchymal airspaces (3). We then describe the spectral signatures indicative of gas dissolution and uptake (4), and how these features can be used to follow the gas as it enters the tissue and capillary bed, is taken up by hemoglobin in the red blood cells (5), re-enters the gas phase prior to exhalation (6), or is carried via the vasculature to other organs and body structures (7). We conclude with a discussion of practical imaging and spectroscopy techniques that deliver quantifiable metrics despite the small size, rapid motion and decay of signal and coherence characteristic of the magnetically inhomogeneous lung in preclinical models (8).

Rapid analytical CEST spectroscopy of competitive host-guest interactions using spatial parallelization with a combined approach of variable flip angle, keyhole and averaging (CAVKA)

A serious limitation of high resolution ¹²⁹Xe chemical exchange saturation transfer (CEST) NMR spectroscopy for comparing competitive host-guest interactions from different samples is the long acquisition time due to step-wise encoding of the chemical shift dimension. A method of optimized use of ¹²⁹Xe spin magnetization to enable the accelerated and simultaneous acquisition of CEST spectra from multiple samples or regions in a setup is described. The method is applied to investigate the host-guest system of commercially available cucurbit[7]uril (CB7) and xenon with competing guests: cis-1,4-bis(aminomethyl)cyclohexane, cadaverine, and putrescine. Interactions with the different guests prove that the observed CEST signal is from a CB6 impurity and that CB7 itself does not produce a CEST signal. Instead, rapid interactions between xenon and CB7 manifest in the spectrum as a broad saturation response that could be suppressed by cis-1,4-bis(aminomethyl)cyclohexane. This guest prevents interactions at the CB7 portals. The suggested method represents a type of spectroscopic imaging that is capable of capturing the exchange kinetics information of systems that otherwise suffer from shortened T₂ times and yields multiple spectra for comparing exchange conditions with a reduction of >95% in acquisition time. The spectral quality is sufficient to perform quantitative analysis and quantifications relative to a CB6 standard as well as relative to a known blocker concentration (putrescine) that both reveal an unexpectedly high CB6 impurity of ca. 8%.

Enzyme assays with supramolecular chemosensors - the label-free approach

Enzyme activity measurements are essential for many research areas, e.g., for the identification of inhibitors in drug discovery, in bioengineering of enzyme mutants for biotechnological applications, or in bioanalytical chemistry as parts of biosensors. In particular in high-throughput screening (HTS), sensitive optical detection is most preferred and numerous absorption and fluorescence spectroscopy-based enzyme assays have been developed, which most frequently require time-consuming fluorescent labelling that may interfere with biological recognition. The use of supramolecular chemosensors, which can specifically signal analytes with fluorescence-based read-out methods, affords an attractive and label-free alternative to more established enzyme assays. We provide herein a comprehensive review that summarizes the current state-of-the-art of supramolecular enzyme assays ranging from early examples with covalent chemosensors to the most recent applications of supramolecular tandem enzyme assays, which utilize common and often commercially available combinations of macrocyclic host molecules (e.g. cyclodextrins, calixarenes, and cucurbiturils) and fluorescent dyes as self-assembled reporter pairs for assaying enzyme activity.

In vivo hyperCEST imaging: Experimental considerations for a reliable contrast

PURPOSE

HyperCEST contrast relies on the reduction of the solvent signal after selective saturation of the solute magnetization. The scope of this work is to outline the experimental conditions needed to obtain a reliable hyperCEST contrast in vivo, where the "solvent" signal (ie, the dissolved-phase signal) may change over time due to the increase in xenon (Xe) accumulation into tissue.

METHODS

Hyperpolarized ¹²⁹ Xe was delivered to mice at a constant volume and rate using a mechanical ventilator, which triggered the saturation, excitation, and acquisition of the MR signal during the exhale phase of the breath cycle-either every breath or every 2, 3, or 4 breaths. Serial Z-spectra and hyperCEST images were acquired before and after a bolus injection of cucurbit[6]uril to assess possible signal fluctuations and instabilities.

RESULTS

The intensity of the dissolved-phase Xe signal was observed to first increase immediately after the beginning of the hyperpolarized gas inhalation and NMR acquisition, and then decrease before reaching a steady-state condition. Once a steady-state dissolved-phase magnetization was established, a reliable hyperCEST contrast, exceeding 40% signal reduction, was observed.

CONCLUSION

A reliable hyperCEST contrast can only be obtained after establishing a steady-state dissolved phase ¹²⁹ Xe magnetization. Under stable physiological conditions, a steady-state dissolved-phase Xe magnetization is only achieved after a series of Xe inhalations and RF excitations, and it requires synchronization of the breathing rate with the MR acquisition.

Pilot Quality-Assurance Study of a Third-Generation Batch-Mode Clinical-Scale Automated Xenon-129 Hyperpolarizer

We present a pilot quality assurance (QA) study of a clinical-scale, automated, third-generation (GEN-3) ¹²⁹Xe hyperpolarizer employing batch-mode spin-exchange optical pumping (SEOP) with high-Xe densities (50% natural abundance Xe and 50% N₂ in ~2.6 atm total pressure sourced from Nova Gas Technologies) and rapid temperature ramping enabled by an aluminum heating jacket surrounding the 0.5 L SEOP cell. ¹²⁹Xe hyperpolarization was performed over the course of 700 gas loading cycles of the SEOP cell, simulating long-term hyperpolarized contrast agent production in a clinical lung imaging setting. High levels of ¹²⁹Xe polarization (avg. %P_Xe = 51.0% with standard deviation σ_PXe = 3.0%) were recorded with fast ¹²⁹Xe polarization build-up time constants (avg. T_b = 25.1 min with standard deviation σ_Tb = 3.1 min) across the first 500 SEOP cell refills, using moderate temperatures of 75 °C. These results demonstrate a more than 2-fold increase in build-up rate relative to previously demonstrated results in a comparable QA study on a second-generation (GEN-2) ¹²⁹Xe hyperpolarizer device, with only a minor reduction in maximum achievable %P_Xe and with greater consistency over a larger number of SEOP cell refill processes at a similar polarization lifetime duration (avg. T₁ = 82.4 min, standard deviation σ_T1 = 10.8 min). Additionally, the effects of varying SEOP jacket temperatures, distribution of Rb metal, and preparation and operation of the fluid path are quantified in the context of device installation, performance optimization and maintenance to consistently produce high ¹²⁹Xe polarization values, build-up rates (T_b as low as 6 min) and lifetimes over the course of a typical high-throughput ¹²⁹Xe polarization SEOP cell life cycle. The results presented further demonstrate the significant potential for hyperpolarized ¹²⁹Xe contrast agent in imaging and bio-sensing applications on a clinical scale.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

Comprehensive Quantitative and Calibration-Free Evaluation of Hyperpolarized Xenon-Host Interaction by Multiparametric NMR

The probing of microscopic environments by hyperpolarized xenon NMR has spurred investigations in supramolecular chemistry as well as important biosensing and molecular imaging applications. While xenon exchange with host structures at micromolar concentrations and below can be readily detected, a quantitative analysis is limited, requiring complementary experimentation by different methodologies and thus lacking completeness and compromising the validity and comparability of numerical results. Here, a new NMR measurement and data analysis approach is introduced for the comprehensive characterization of the host-xenon binding dynamics. The application of chemical exchange saturation transfer of hyperpolarized ¹²⁹Xe under parametric modulation of the saturation RF amplitude and xenon gas saturation of the solution enables a delineation of exchange mechanisms and, through modeling, a numerical estimation of the various reaction rate constants (and thus magnetization exchange rate constants), the xenon affinity, and the total host molecule concentration. Only the numerical xenon solubility is additionally required for input, a quantity that has a low impact on the measurement uncertainty and is derivable from metrological data collections. Signal calibration by a reference material may thus be avoided, qualifying the method as calibration-free. For demonstration a xenon exchange with the host cucurbit[6]uril at low concentration is investigated, with the numerical results being validated by standard quantitative NMR data obtained at high concentration. The readiness to evaluate xenon exchange for the one sample at hand and in a single experimental attempt by the proposed method may allow comprehensive quantitative studies in supramolecular chemistry, biomacromolecular structure and dynamics, and sensing.

Cryptophane-xenon complexes for 129Xe MRI applications

The use of magnetic resonance imaging (MRI) and spectroscopy (MRS) in the clinical setting enables the acquisition of valuable anatomical information in a rapid, non-invasive fashion. However, MRI applications for identifying disease-related biomarkers are limited due to low sensitivity at clinical magnetic field strengths. The development of hyperpolarized (hp) ¹²⁹Xe MRI/MRS techniques as complements to traditional ¹H-based imaging has been a burgeoning area of research over the past two decades. Pioneering experiments have shown that hp ¹²⁹Xe can be encapsulated within host molecules to generate ultrasensitive biosensors. In particular, xenon has high affinity for cryptophanes, which are small organic cages that can be functionalized with affinity tags, fluorophores, solubilizing groups, and other moieties to identify biomedically relevant analytes. Cryptophane sensors designed for proteins, metal ions, nucleic acids, pH, and temperature have achieved nanomolar-to-femtomolar limits of detection via a combination of ¹²⁹Xe hyperpolarization and chemical exchange saturation transfer (CEST) techniques. This review aims to summarize the development of cryptophane biosensors for ¹²⁹Xe MRI applications, while highlighting innovative biosensor designs and the consequent enhancements in detection sensitivity, which will be invaluable in expanding the scope of ¹²⁹Xe MRI.

This review aims to summarize the development of cryptophane biosensors for ¹²⁹Xe MRI applications, while highlighting innovative biosensor designs and the consequent enhancements in detection sensitivity, which will be invaluable in expanding the scope of ¹²⁹Xe MRI.

Enabling Clinical Technologies for Hyperpolarized 129 Xenon Magnetic Resonance Imaging and Spectroscopy

Hyperpolarization is a technique that can increase nuclear spin polarization with the corresponding gains in nuclear magnetic resonance (NMR) signals by 4-8 orders of magnitude. When this process is applied to biologically relevant samples, the hyperpolarized molecules can be used as exogenous magnetic resonance imaging (MRI) contrast agents. A technique called spin-exchange optical pumping (SEOP) can be applied to hyperpolarize noble gases such as 129 Xe. Techniques based on hyperpolarized 129 Xe are poised to revolutionize clinical lung imaging, offering a non-ionizing, high-contrast alternative to computed tomography (CT) imaging and conventional proton MRI. Moreover, CT and conventional proton MRI report on lung tissue structure but provide little functional information. On the other hand, when a subject breathes hyperpolarized 129 Xe gas, functional lung images reporting on lung ventilation, perfusion and diffusion with 3D readout can be obtained in seconds. In this Review, the physics of SEOP is discussed and the different production modalities are explained in the context of their clinical application. We also briefly compare SEOP to other hyperpolarization methods and conclude this paper with the outlook for biomedical applications of hyperpolarized 129 Xe to lung imaging and beyond.

Magnetic Resonance Detection of Gas Microbubbles via HyperCEST: A Path Toward Dual Modality Contrast Agent

Gas microbubbles are an established clinical ultrasound contrast agent. They could also become a powerful magnetic resonance (MR) intravascular contrast agent, but their low susceptibility-induced contrast requires high circulating concentrations or the addition of exogenous paramagnetic nanoparticles for MR detection. In order to detect clinical in vivo concentrations of raw microbubbles via MR, an alternative detection scheme must be used. HyperCEST is an NMR technique capable of indirectly detecting signals from very dilute molecules (concentrations well below the NMR detection threshold) that exchange hyperpolarized 129 Xe. Here, we use quantitative hyperCEST to show that microbubbles are very efficient hyperCEST agents. They can accommodate and saturate millions of 129 Xe atoms at a time, allowing for their indirect detection at concentrations as low as 10 femtomolar. The increased MR sensitivity to microbubbles achieved via hyperCEST can bridge the gap for microbubbles to become a dual modality contrast agent.

129Xe ultra-fast Z spectroscopy enables micromolar detection of biosensors on a 1 T benchtop spectrometer

The availability of a benchtop nuclear magnetic resonance (NMR) spectrometer, of low cost and easily transportable, can allow detection of low quantities of biosensors, provided that hyperpolarized species are used. Here we show that the micromolar threshold can easily be reached by employing laser-polarized xenon and cage molecules reversibly hosting it. Indirect detection of caged xenon is made via chemical exchange, using ultra-fast Z spectroscopy based on spatio-temporal encoding. On this non-dedicated low-field spectrometer, several ideas are proposed to improve the signal.

Photosensitive MRI biosensor for BCRP-Targeted uptake and light-induced inhibition of tumor cells

Riboflavin and its derivatives are the most important coenzymes in vivo metabolism, and are closely related to life activities. In this paper, the first photolysis ¹²⁹Xe biosensor was developed by combining cryptophane-A with riboflavin moiety, which showed photosensitivity recorded by hyperpolarized ¹²⁹Xe NMR/MRI technology with an obvious chemical shift change of 5.3 ppm in aqueous solution. Cellular fluorescence imaging confirmed that the biosensor could be enriched in MCF-7 cells, and MTT assays confirmed that the cytotoxicity was enhanced after irradiation. Findings suggested that the biosensor has a potential application in tumor targeting and the inhibition of tumor cell proliferation after photodegradation.

Cyclodextrin-Based Contrast Agents for Medical Imaging

Cyclodextrins (CDs) are naturally occurring cyclic oligosaccharides consisting of multiple glucose subunits. CDs are widely used in host–guest chemistry and biochemistry due to their structural advantages, biocompatibility, and ability to form inclusion complexes. Recently, CDs have become of high interest in the field of medical imaging as a potential scaffold for the development of a large variety of the contrast agents suitable for magnetic resonance imaging, ultrasound imaging, photoacoustic imaging, positron emission tomography, single photon emission computed tomography, and computed tomography. The aim of this review is to summarize and highlight the achievements in the field of cyclodextrin-based contrast agents for medical imaging.

Decacationic Pillar[5]arene: A New Scaffold for the Development of 129Xe MRI Imaging Agents

A decacationic water-soluble pillar[5]arene possessing a nonsolvated hydrophobic core has been designed and synthesized. This supramolecular host is capable of binding xenon, as evidenced by hyperCEST depletion experiments. Fluorescence-based studies also demonstrate that xenon binds into the cavity of the pillararene with an association constant of 4.6 × 103 M-1. These data indicate that the water-soluble pillararene is a potential scaffold for building contrast agents that can be detected by xenon-129 magnetic resonance imaging.

Molecular Sensing with Host Systems for Hyperpolarized 129Xe

Hyperpolarized noble gases have been used early on in applications for sensitivity enhanced NMR. 129Xe has been explored for various applications because it can be used beyond the gas-driven examination of void spaces. Its solubility in aqueous solutions and its affinity for hydrophobic binding pockets allows "functionalization" through combination with host structures that bind one or multiple gas atoms. Moreover, the transient nature of gas binding in such hosts allows the combination with another signal enhancement technique, namely chemical exchange saturation transfer (CEST). Different systems have been investigated for implementing various types of so-called Xe biosensors where the gas binds to a targeted host to address molecular markers or to sense biophysical parameters. This review summarizes developments in biosensor design and synthesis for achieving molecular sensing with NMR at unprecedented sensitivity. Aspects regarding Xe exchange kinetics and chemical engineering of various classes of hosts for an efficient build-up of the CEST effect will also be discussed as well as the cavity design of host molecules to identify a pool of bound Xe. The concept is presented in the broader context of reporter design with insights from other modalities that are helpful for advancing the field of Xe biosensors.

129Xe NMR-Protein Sensor Reveals Cellular Ribose Concentration

Dysregulation of cellular ribose uptake can be indicative of metabolic abnormalities or tumorigenesis. However, analytical methods are currently limited for quantifying ribose concentration in complex biological samples. Here, we utilize the highly specific recognition of ribose by ribose-binding protein (RBP) to develop a single-protein ribose sensor detectable via a sensitive NMR technique known as hyperpolarized 129Xe chemical exchange saturation transfer (hyper-CEST). We demonstrate that RBP, with a tunable ribose-binding site and further engineered to bind xenon, enables the quantitation of ribose over a wide concentration range (nM to mM). Ribose binding induces the RBP "closed" conformation, which slows Xe exchange to a rate detectable by hyper-CEST. Such detection is remarkably specific for ribose, with the minimal background signal from endogenous sugars of similar size and structure, for example, glucose or ribose-6-phosphate. Ribose concentration was measured for mammalian cell lysate and serum, which led to estimates of low-mM ribose in a HeLa cell line. This highlights the potential for using genetically encoded periplasmic binding proteins such as RBP to measure metabolites in different biological fluids, tissues, and physiologic states.

Paramagnetic Organocobalt Capsule Revealing Xenon Host-Guest Chemistry

We investigated Xe binding in a previously reported paramagnetic metal-organic tetrahedral capsule, [Co4L6]4-, where L2- = 4,4'-bis[(2-pyridinylmethylene)amino][1,1'-biphenyl]-2,2'-disulfonate. The Xe-inclusion complex, [XeCo4L6]4-, was confirmed by 1H NMR spectroscopy to be the dominant species in aqueous solution saturated with Xe gas. The measured Xe dissociation rate in [XeCo4L6]4-, koff = 4.45(5) × 102 s-1, was at least 40 times greater than that in the analogous [XeFe4L6]4- complex, highlighting the capability of metal-ligand interactions to tune the capsule size and guest permeability. The rapid exchange of 129Xe nuclei in [XeCo4L6]4- produced significant hyperpolarized 129Xe chemical exchange saturation transfer (hyper-CEST) NMR signal at 298 K, detected at a concentration of [XeCo4L6]4- as low as 100 pM, with presaturation at -89 ppm, which was referenced to solvated 129Xe in H2O. The saturation offset was highly temperature-dependent with a slope of -0.41(3) ppm/K, which is attributed to hyperfine interactions between the encapsulated 129Xe nucleus and electron spins on the four CoII centers. As such, [XeCo4L6]4- represents the first example of a paramagnetic hyper-CEST (paraHYPERCEST) sensor. Remarkably, the hyper-CEST 129Xe NMR resonance for [XeCo4L6]4- (δ = -89 ppm) was shifted 105 ppm upfield from the diamagnetic analogue [XeFe4L6]4- (δ = +16 ppm). The Xe inclusion complex was further characterized in the crystal structure of (C(NH2)3)4[Xe0.7Co4L6]·75 H2O (1). Hydrogen bonding between capsule-linker sulfonate groups and exogenous guanidinium cations, (C(NH2)3)+, stabilized capsule-capsule interactions in the solid state and also assisted in trapping a Xe atom (∼42 Å3) in the large (135 Å3) cavity of 1. Magnetic susceptibility measurements confirmed the presence of four noninteracting, magnetically anisotropic high-spin CoII centers in 1. Furthermore, [Co4L6]4- was found to be stable toward aggregation and oxidation, and the CEST performance of [XeCo4L6]4- was unaffected by biological macromolecules in H2O. These results recommend metal-organic capsules for fundamental investigations of Xe host-guest chemistry as well as applications with highly sensitive 129Xe-based sensors.

XeUS: A second-generation automated open-source batch-mode clinical-scale hyperpolarizer

We present a second-generation open-source automated batch-mode ¹²⁹Xe hyperpolarizer (XeUS GEN-2), designed for clinical-scale hyperpolarized (HP) ¹²⁹Xe production via spin-exchange optical pumping (SEOP) in the regimes of high Xe density (0.66-2.5 atm partial pressure) and resonant photon flux (~170 W, Δλ = 0.154 nm FWHM), without the need for cryo-collection typically employed by continuous-flow hyperpolarizers. An Arduino micro-controller was used for hyperpolarizer operation. Processing open-source software was employed to program a custom graphical user interface (GUI), capable of remote automation. The Arduino Integrated Development Environment (IDE) was used to design a variety of customized automation sequences such as temperature ramping, NMR signal acquisition, and SEOP cell refilling for increased reliability. A polycarbonate 3D-printed oven equipped with a thermo-electric cooler/heater provides thermal stability for SEOP for both binary (Xe/N₂) and ternary (⁴He-containing) SEOP cell gas mixtures. Quantitative studies of the ¹²⁹Xe hyperpolarization process demonstrate that near-unity polarization can be achieved in a 0.5 L SEOP cell. For example, %P_Xe of 93.2 ± 2.9% is achieved at 0.66 atm Xe pressure with polarization build-up rate constant γ_SEOP = 0.040 ± 0.005 min^-1, giving a max dose equivalent ≈ 0.11 L/h 100% hyperpolarized, 100% enriched ¹²⁹Xe; %P_Xe of 72.6 ± 1.4% is achieved at 1.75 atm Xe pressure with γ_SEOP of 0.041 ± 0.001 min^-1, yielding a corresponding max dose equivalent of 0.27 L/h. Quality assurance studies on this device have demonstrated the potential to refill SEOP cells hundreds of times without significant losses in performance, with average %P_Xe = 71.7%, (standard deviation σ_P = 1.52%) and mean polarization lifetime T₁ = 90.5 min, (standard deviation σ_T = 10.3 min) over the first ~200 gas mixture refills, with sufficient performance maintained across a further ~700 refills. These findings highlight numerous technological developments and have significant translational relevance for efficient production of gaseous HP ¹²⁹Xe contrast agents for use in clinical imaging and bio-sensing techniques.

Paramagnetic Shifts and Guest Exchange Kinetics in ConFe4-n Metal-Organic Capsules

We investigate the magnetic resonance properties and exchange kinetics of guest molecules in a series of hetero-bimetallic capsules, [Co_nFe_4-nL₆]^4- (n = 1-3), where L^2- = 4,4'-bis[(2-pyridinylmethylene)amino]-[1,1'-biphenyl]-2,2'-disulfonate. H bond networks between capsule sulfonates and guanidinium cations promote the crystallization of [Co_nFe_4-nL₆]^4-. The following four isostructural crystals are reported: two guest-free forms, (C(NH₂)₃)₄[Co_1.8Fe_2.2L₆]·69H₂O (1) and (C(NH₂)₃)₄[Co_2.7Fe_1.3L₆]·73H₂O (2), and two Xe- and CFCl₃-encapsulated forms, (C(NH₂)₃)₄[(Xe)_0.8Co_1.8Fe_2.2L₆]·69H₂O (3) and (C(NH₂)₃)₄[(CFCl₃)Co_2.0Fe_2.0L₆]·73H₂O (4), respectively. Structural analyses reveal that Xe induces negligible structural changes in 3, while the angles between neighboring phenyl groups expand by ca. 3° to accommodate the much larger guest, CFCl₃, in 4. These guest-encapsulated [Co_nFe_4-nL₆]^4- molecules reveal ¹²⁹Xe and ¹⁹F chemical shift changes of ca. -22 and -10 ppm at 298 K, respectively, per substitution of low-spin Fe^II by high-spin Co^II. Likewise, the temperature dependence of the ¹²⁹Xe and ¹⁹F NMR resonances increases by 0.1 and 0.06 ppm/K, respectively, with each additional paramagnetic Co^II center. The optimal temperature for hyperpolarized (hp) ¹²⁹Xe chemical exchange saturation transfer (hyper-CEST) with [Co_nFe_4-nL₆]^4- capsules was found to be inversely proportional to the number of Co^II centers, n, which is consistent with the Xe chemical exchange accelerating as the portals expand. The systematic study was facilitated by the tunability of the [M₄L₆]^4- capsules, further highlighting these metal-organic systems for developing responsive sensors with highly shifted ¹²⁹Xe resonances.

Pilot multi-site quality assurance study of batch-mode clinical-scale automated xenon-129 hyperpolarizers

We present a pilot quality assurance (QA) study of spin-exchange optical pumping (SEOP) performed on two nearly identical second-generation (GEN-2) automated batch-mode clinical-scale ¹²⁹Xe hyperpolarizers, each utilizing a convective forced air oven, high-power (~170 W) continuous pump laser irradiation, and xenon-rich gas mixtures (~1.30 atm partial pressure). In one study, the repeatability of SEOP in a 1000 Torr Xe/900 Torr N₂/100 Torr ⁴He (2000 Torr total pressure) gas mixture is evaluated over the course of ~700 gas loading cycles, with negligible decrease in performance during the first ~200 cycles, and with high ¹²⁹Xe polarization levels (avg. %P_Xe = 71.7% with standard deviation σ_PXe = 1.5%), build-up rates (avg. γ_SEOP = 0.019 min^-1 with standard deviation σ_γ = 0.003 min^-1) and polarization lifetimes (avg. T₁ = 90.5 min with standard deviation σ_T = 10.3 min) reported at moderate oven temperature of ~70 °C. Although the SEOP cell in this study exhibited a detectable performance decrease after 400 cycles, the cell continued to produce potentially useable HP ¹²⁹Xe with %P_Xe = 42.3 ± 0.6% even after nearly 700 refill cycles. The possibility of "regenerating" "dormant" (i.e., not used for an extended period of time) SEOP cells using repeated temperature cycling methods to recover %P_Xe is also demonstrated. The quality and consistency of results show significant promise for translation to clinical-scale production of hyperpolarized ¹²⁹Xe contrast agents for imaging and bio-sensing applications.

Functionalized Lipopeptide Micelles as Highly Efficient NMR Depolarization Seed Points for Targeted Cell Labelling in Xenon MRI

Improving diagnostic imaging and therapy by targeted compound delivery to pathological areas and across biological barriers is of urgent need. A lipopeptide, P-CrA-A2, composed of a highly cationic peptide sequence (A2), an N-terminally attached palmitoyl chain (P) and cryptophane molecule (CrA) for preferred uptake into blood-brain barrier (BBB) capillary endothelial cells, was generated. CrA allows reversible binding of Xe for NMR detection with hyperpolarized nuclei. The lipopeptide forms size-optimized micelles with a diameter of about 11 nm at low micromolar concentration. Their high local CrA payload has a strong and switchable impact on the bulk magnetization through Hyper-CEST detection. Covalent fixation of CrA does not impede micelle formation and does not hamper its host functionality but simplifies Xe access to hosts for inducing saturation transfer. Xe Hyper-CEST magnetic resonance imaging (MRI) allows for distinguishing BBB endothelial cells from control aortic endothelial cells, and the small micelle volume with a sevenfold improved CrA-loading density compared to liposomal carriers allows preferred cell labelling with a minimally invasive volume (≈16 000-fold more efficient than ¹⁹ F cell labelling). Thus, these nanoscopic particles combine selectivity for human brain capillary endothelial cells with great sensitivity of Xe Hyper-CEST MRI and might be a potential MRI tool in brain diagnostics.

Multiplexed 129Xe HyperCEST MRI Detection of Genetically Reconstituted Bacterial Protein Nanoparticles in Human Cancer Cells

Gas vesicle nanoparticles (GVs) are gas-containing protein assemblies expressed in bacteria and archaea. Recently, GVs have gained considerable attention for biotechnological applications as genetically encodable contrast agents for MRI and ultrasonography. However, at present, the practical use of GVs is hampered by a lack of robust methodology for their induction into mammalian cells. Here, we demonstrate the genetic reconstitution of protein nanoparticles with characteristic bicone structures similar to natural GVs in a human breast cancer cell line KPL-4 and genetic control of their size and shape through expression of reduced sets of humanized gas vesicle genes cloned into Tol2 transposon vectors, referencing the natural gas vesicle gene clusters of the cyanobacteria planktothrix rubescens/agardhii. We then report the utility of these nanoparticles as multiplexed, sensitive, and genetically encoded contrast agents for hyperpolarized xenon chemical exchange saturation transfer (HyperCEST) MRI.

High Xe density, high photon flux, stopped-flow spin-exchange optical pumping: Simulations versus experiments

Spin-exchange optical pumping (SEOP) can enhance the NMR sensitivity of noble gases by up to five orders of magnitude at Tesla-strength magnetic fields. SEOP-generated hyperpolarised (HP) ¹²⁹Xe is a promising contrast agent for lung imaging but an ongoing barrier to widespread clinical usage has been economical production of sufficient quantities with high ¹²⁹Xe polarisation. Here, the 'standard model' of SEOP, which was previously used in the optimisation of continuous-flow ¹²⁹Xe polarisers, is modified for validation against two Xe-rich stopped-flow SEOP datasets. We use this model to examine ways to increase HP Xe production efficiency in stopped-flow ¹²⁹Xe polarisers and provide further insight into the underlying physics of Xe-rich stopped-flow SEOP at high laser fluxes.

Probing Reversible Guest Binding with Hyperpolarized 129Xe-NMR: Characteristics and Applications for Cucurbit[n]urils

Cucurbit[n]urils (CB[n]s) are a family of macrocyclic host molecules that find various applications in drug delivery, molecular switching, and dye displacement assays. The CB[n]s with n = 5-7 have also been studied with 129Xe-NMR. They bind the noble gas with a large range of exchange rates. Starting with insights from conventional direct detection of bound Xe, this review summarizes recent achievements with chemical exchange saturation transfer (CEST) detection of efficiently exchanging Xe in various CB[n]-based supramolecular systems. Unprecedented sensitivity has been reached by combining the CEST method with hyperpolarized Xe, the production of which is also briefly described. Applications such as displacement assays for enzyme activity detection and rotaxanes as emerging types of Xe biosensors are likewise discussed in the context of biomedical applications and pinpoint future directions for translating this field to preclinical studies.

Nanoparticle-Based Contrast Agents for 129Xe HyperCEST NMR and MRI Applications

Spin hyperpolarization techniques have enabled important advancements in preclinical and clinical MRI applications to overcome the intrinsic low sensitivity of nuclear magnetic resonance. Functionalized xenon biosensors represent one of these approaches. They combine two amplification strategies, namely, spin exchange optical pumping (SEOP) and chemical exchange saturation transfer (CEST). The latter one requires host structures that reversibly bind the hyperpolarized noble gas. Different nanoparticle approaches have been implemented and have enabled molecular MRI with 129Xe at unprecedented sensitivity. This review gives an overview of the Xe biosensor concept, particularly how different nanoparticles address various critical aspects of gas binding and exchange, spectral dispersion for multiplexing, and targeted reporter delivery. As this concept is emerging into preclinical applications, comprehensive sensor design will be indispensable in translating the outstanding sensitivity potential into biomedical molecular imaging applications.

Rotaxane Probes for the Detection of Hydrogen Peroxide by 129 Xe HyperCEST NMR Spectroscopy

The development of sensitive and chemically selective MRI contrast agents is imperative for the early detection and diagnosis of many diseases. Conventional responsive contrast agents used in 1 H MRI are impaired by the high abundance of protons in the body. 129 Xe hyperCEST NMR/MRI comprises a highly sensitive complement to traditional 1 H MRI because of its ability to report specific chemical environments. To date, the scope of responsive 129 Xe NMR contrast agents lacks breadth in the specific detection of small molecules, which are often important markers of disease. Herein, we report the synthesis and characterization of a rotaxane-based 129 Xe hyperCEST NMR contrast agent that can be turned on in response to H2 O2 , which is upregulated in several disease states. Added H2 O2 was detected by 129 Xe hyperCEST NMR spectroscopy in the low micromolar range, as well as H2 O2 produced by HEK 293T cells activated with tumor necrosis factor.

Metabolic and Molecular Imaging with Hyperpolarised Tracers

Since reaching the clinic, magnetic resonance imaging (MRI) has become an irreplaceable radiological tool because of the macroscopic information it provides across almost all organs and soft tissues within the human body, all without the need for ionising radiation. The sensitivity of MR, however, is too low to take full advantage of the rich chemical information contained in the MR signal. Hyperpolarisation techniques have recently emerged as methods to overcome the sensitivity limitations by enhancing the MR signal by many orders of magnitude compared to the thermal equilibrium, enabling a new class of metabolic and molecular X-nuclei based MR tracers capable of reporting on metabolic processes at the cellular level. These hyperpolarised (HP) tracers have the potential to elucidate the complex metabolic processes of many organs and pathologies, with studies so far focusing on the fields of oncology and cardiology. This review presents an overview of hyperpolarisation techniques that appear most promising for clinical use today, such as dissolution dynamic nuclear polarisation (d-DNP), parahydrogen-induced hyperpolarisation (PHIP), Brute force hyperpolarisation and spin-exchange optical pumping (SEOP), before discussing methods for tracer detection, emerging metabolic tracers and applications and progress in preclinical and clinical application.

Protein Nanostructures Produce Self-Adjusting Hyperpolarized Magnetic Resonance Imaging Contrast through Physical Gas Partitioning

Signal amplification strategies are critical for overcoming the intrinsically poor sensitivity of nuclear magnetic resonance (NMR) reporters in noninvasive molecular detection. A mechanism widely used for signal enhancement is chemical exchange saturation transfer (CEST) of nuclei between a dilute sensing pool and an abundant detection pool. However, the dependence of CEST amplification on the relative size of these spin pools confounds quantitative molecular detection with a larger detection pool typically making saturation transfer less efficient. Here we show that a recently discovered class of genetically encoded nanoscale reporters for ¹²⁹Xe magnetic resonance overcomes this fundamental limitation through an elastic binding capacity for NMR-active nuclei. This approach pairs high signal amplification from hyperpolarized spins with ideal, self-adjusting saturation transfer behavior as the overall spin ensemble changes in size. These reporters are based on gas vesicles, i.e., microbe-derived, gas-filled protein nanostructures. We show that the xenon fraction that partitions into gas vesicles follows the ideal gas law, allowing the signal transfer under hyperpolarized xenon chemical exchange saturation transfer (Hyper-CEST) imaging to scale linearly with the total xenon ensemble. This conceptually distinct elastic response allows the production of quantitative signal contrast that is robust to variability in the concentration of xenon, enabling virtually unlimited improvement in absolute contrast with increased xenon delivery, and establishing a unique principle of operation for contrast agent development in emerging biochemical and in vivo applications of hyperpolarized NMR and magnetic resonance imaging.

High Exchange Rate Complexes of 129 Xe with Water-Soluble Pillar[5]arenes for Adjustable Magnetization Transfer MRI

Macrocyclic host structures for generating transiently bound ¹²⁹ Xe have been used in various ultra-sensitive NMR and MRI applications for molecular sensing of biochemical analytes. They are based on hyperpolarized nuclei chemical exchange saturation transfer (Hyper-CEST). Here, we tested a set of water-soluble pillar[5]arenes with different counterions in order to compare their potential contrast agent abilities with that of cryptophane-A (CrA), the most widely used host for such purposes. The exchange of Xe with such compounds was found to be sensitive to the type of ions present in solution and can be used for switchable magnetization transfer (MT) contrast that arises from off-resonant pre-saturation. We demonstrate that the adjustable MT magnitude depends on the interplay of saturation parameters and found that the optimum MT contrast surpasses the CrA CEST performance at moderate saturation power. Since modification of such water-soluble pillar[5]arenes is straightforward, these compounds can be considered a promising platform for designing various sensors that may complement the field of Xe HyperCEST-based biosensing MRI.

Cryptophane Nanoscale Assemblies Expand 129Xe NMR Biosensing

Cryptophane-based biosensors are promising agents for the ultrasensitive detection of biomedically relevant targets via 129Xe NMR. Dynamic light scattering revealed that cryptophanes form water-soluble aggregates tens to hundreds of nanometers in size. Acridine orange fluorescence quenching assays allowed quantitation of the aggregation state, with critical concentrations ranging from 200 nM to 600 nM, depending on the cryptophane species in solution. The addition of excess carbonic anhydrase (CA) protein target to a benzenesulfonamide-functionalized cryptophane biosensor (C8B) led to C8B disaggregation and produced the expected 1:1 C8B-CA complex. C8B showed higher affinity at 298 K for the cytoplasmic isozyme CAII than the extracellular CAXII isozyme, which is a biomarker of cancer. Using hyper-CEST NMR, we explored the role of stoichiometry in detecting these two isozymes. Under CA-saturating conditions, we observed that isozyme CAII produces a larger 129Xe NMR chemical shift change (δ = 5.9 ppm, relative to free biosensor) than CAXII (δ = 2.7 ppm), which indicates the strong potential for isozyme-specific detection. However, stoichiometry-dependent chemical shift data indicated that biosensor disaggregation contributes to the observed 129Xe NMR chemical shift change that is normally assigned to biosensor-target binding. Finally, we determined that monomeric cryptophane solutions improve hyper-CEST saturation contrast, which enables ultrasensitive detection of biosensor-protein complexes. These insights into cryptophane-solution behavior support further development of xenon biosensors, but will require reinterpretation of the data previously obtained for many water-soluble cryptophanes.

Brain Imaging Using Hyperpolarized 129Xe Magnetic Resonance Imaging

Hyperpolarized (HP) ¹²⁹Xe magnetic resonance imaging (MRI) is a novel iteration of traditional MRI that relies on detecting the spins of ¹H. Since ¹²⁹Xe is a gaseous signal source, it can be used for lung imaging. Additionally, ¹²⁹Xe dissolves in the blood stream and can therefore be detectable in the brain parenchyma and vasculature. In this work, we provide detailed information on the protocols that we have developed to image ¹²⁹Xe within the brains of both rodents and human subjects.

Cyclodextrin-Based Pseudorotaxanes: Easily Conjugatable Scaffolds for Synthesizing Hyperpolarized Xenon-129 Magnetic Resonance Imaging Agents

Hyperpolarized (HP) xenon-129 (Xe) magnetic resonance (MR) imaging has the potential to detect biological analytes with high sensitivity and high resolution when coupled with xenon-encapsulating molecular probes. Despite the development of numerous HP Xe probes, one of the challenges that has hampered the translation of these agents from in vitro demonstration to in vivo testing is the difficulty in synthesizing the Xe-encapsulating cage molecule. In this study, we demonstrate that a pseudorotaxane, based on a γ-cyclodextrin macrocycle, is easily synthesized in one step and is detectable using HyperCEST-enhanced ¹²⁹Xe MR spectroscopy.

Quantitative biosensor detection by chemically exchanging hyperpolarized 129Xe

Quantitative modeling and evaluation of biosensor detection by hyperpolarized ¹²⁹Xe chemical exchange saturation transfer (Hyper-CEST).

A cryptophane-based "turn-on" 129Xe NMR biosensor for monitoring calmodulin

We present the first cryptophane-based "turn-on" 129Xe NMR biosensor, employing a peptide-functionalized cryptophane to monitor the activation of calmodulin (CaM) protein in solution. In the absence of CaM binding, interaction between the peptide and cryptophane completely suppresses the hyperpolarized 129Xe-cryptophane NMR signal. Biosensor binding to Ca2+-activated CaM produces the expected 129Xe-cryptophane NMR signal.

Preparation of biogenic gas vesicle nanostructures for use as contrast agents for ultrasound and MRI

Gas vesicles (GVs) are a unique class of gas-filled protein nanostructures that are detectable at subnanomolar concentrations and whose physical properties allow them to serve as highly sensitive imaging agents for ultrasound and MRI. Here we provide a protocol for isolating GVs from native and heterologous host organisms, functionalizing these nanostructures with moieties for targeting and fluorescence, characterizing their biophysical properties and imaging them using ultrasound and MRI. GVs can be isolated from natural cyanobacterial and haloarchaeal host organisms or from Escherichia coli expressing a heterologous GV gene cluster and purified using buoyancy-assisted techniques. They can then be modified by replacing surface-bound proteins with engineered, heterologously expressed variants or through chemical conjugation, resulting in altered mechanical, surface and targeting properties. Pressurized absorbance spectroscopy is used to characterize their mechanical properties, whereas dynamic light scattering (DLS)and transmission electron microscopy (TEM) are used to determine nanoparticle size and morphology, respectively. GVs can then be imaged with ultrasound in vitro and in vivo using pulse sequences optimized for their detection versus background. They can also be imaged with hyperpolarized xenon MRI using chemical exchange saturation transfer between GV-bound and dissolved xenon-a technique currently implemented in vitro. Taking 3-8 d to prepare, these genetically encodable nanostructures enable multimodal, noninvasive biological imaging with high sensitivity and potential for molecular targeting.

Continuous flow production of concentrated hyperpolarized xenon gas from a dilute xenon gas mixture by buffer gas condensation

We present a new method for the continuous flow production of concentrated hyperpolarized xenon-129 (HP ¹²⁹Xe) gas from a dilute xenon (Xe) gas mixture with high nuclear spin polarization. A low vapor pressure (i.e., high boiling-point) gas was introduced as an alternative to molecular nitrogen (N₂), which is the conventional quenching gas for generating HP ¹²⁹Xe via Rb-Xe spin-exchange optical-pumping (SEOP). In contrast to the generally used method of extraction by freezing Xe after the SEOP process, the quenching gas separated as a liquid at moderately low temperature so that Xe was maintained in its gaseous state, allowing the continuous delivery of highly polarized concentrated Xe gas. We selected isobutene as the candidate quenching gas and our method was demonstrated experimentally while comparing its performance with N₂. Isobutene could be liquefied and removed from the Xe gas mixture using a cold trap, and the concentrated HP ¹²⁹Xe gas exhibited a significantly enhanced nuclear magnetic resonance (NMR) signal. Although the system requires further optimization depending on the intended purpose, our approach presented here could provide a simple means for performing NMR or magnetic resonance imaging (MRI) measurements continuously using HP ¹²⁹Xe with improved sensitivity.

129Xe NMR-based sensors: biological applications and recent methods

Molecular systems that target analytes of interest and host spin-hyperpolarized xenon lead to powerful ¹²⁹Xe NMR-based sensors.