Xenon NMR Research Articles

Exploring the structure of type V deep eutectic solvents by xenon NMR spectroscopy.

Hydrophobic non-ionic (type V) deep eutectic solvents (DESs) have recently emerged as a new class of sustainable materials that have shown unique properties in several applications. In this study, type V DESs thymol : camphor, menthol : thymol and eutectic mixtures (EMs) based on menthol : carboxylic acids with variable chain length, are experimentally investigated using xenon NMR spectroscopy, with the aim to clarify the peculiar nanostructure of these materials. The results, obtained from the analysis of the 129Xe chemical shifts and of the longitudinal relaxation times, reveal a correlation between the deviation from ideality of the DESs and their structure free volume. Furthermore, the effect of varying the composition of DESs and EMs on the liquid structure is also studied.

Exploring cavities in Type II porous liquids with xenon

The porous topology of Type II Porous Liquids (PLs), prepared using Noria-OEt as an empty molecular host dissolved in 15-crown-5 solvent, is investigated with 129Xe NMR Spectroscopy. Xenon NMR parameters, such as chemical shifts and T1 relaxation times, measured in PLs solutions prepared at various Noria-OEt concentrations, are related to the intrinsic microporous structure of the material. The experimental data are interpreted on the basis of a multiple-site model for xenon atoms within the multiple PLs cavities. Diffusion NMR experiments show a similar mobility of xenon in PLs compared with the pure solvent.

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.

Advanced textural characterization of biogenic silica by nitrogen physisorption, positron annihilation lifetime spectroscopy and hyperpolarized 129Xe NMR spectroscopy

This work addresses the advanced textural characterization of biogenic silica obtained from different Si-accumulating biomasses and agricultural waste products like rice, oat and spelt husk as well as horsetail. These materials exhibit a non-uniform, disordered pore structure in a broad size range deviating from well-ordered “artificial” species of porous materials. For the first time in literature, this contribution determines and validates mainly the mesoporosity by means of nitrogen physisorption, positron annihilation lifetime spectroscopy (PALS) and hyperpolarized 129 Xenon NMR (HP- 129 Xe NMR). Nitrogen physisorption and HP 129 Xe NMR both were able to describe the pore structure in the mesopore range. In addition, the specific surface area and the mesopore volume were determined by nitrogen physisorption. PALS complemented the results by an assessment of microporosity in biogenic silica. It was shown that biogenic silica contains micropores (1.5 nm–1.9 nm), mesopores in a broad range and indications of macroporosity. In addition, the results led to the assumption that different plant origins and plant species evoke different textural properties and inter-connections between the pores in biogenic silica. This contribution demonstrates that only a combination of all techniques is viable to characterize the non-uniform and complex pore structure of biogenic silica of different origin. • The environmental and economical impact of extracting silica from biomasses wastes. • Three different techniques (positron annihilation lifetime spectroscopy, 129 Xe-NMR, and N 2 adsorption) have been utilized. • The study helped to get deeper insights on the inner porosity of the naturally-build heterogeneous porosity of biomasses. • Closed micropores and broad mesopores with interconnectivity have been detected.

Twist-bend nematic phase of the liquid crystal dimer CB7CB: orientational order and conical angle determined by 129Xe and 2H NMR spectroscopy

The liquid crystal dimer 1ʺ,7ʺ-bis(4-cyanobiphenyl-4ʹ-yl) heptane (CB7CB) has been shown to possess a nematic–nematic phase transition at 376 K. The phase below this temperature has been identified as a globally uniaxial twist-bend nematic phase, NTB. Within the temperature range 376–388 K, a classic uniaxial nematic phase, N, appears. The NTB phase has a helical structure and the liquid crystal director, n, is tilted with respect to the helix axis; these are formed into a conglomerate of degenerate domains having opposite handedness. In a magnetic field, the helix axis orients along the field. In the present studies, the properties of CB7CB are investigated using 129Xe NMR spectroscopy of dissolved xenon and 2H NMR of CB7CB-d4 and the probe 4-octyl-4ʹ-cyanobiphenyl-d2 also dissolved in CB7CB. In a uniaxial liquid crystalline environment, the 129Xe shielding tensor is cylindrically symmetric due to the deformation of the electron cloud by anisotropic forces associated with the director. The anisotropic part of the shielding tensor depends upon the orientational order parameter of the liquid crystal with respect to the applied magnetic field and the conical (aka tilt) angle of the director. The temperature dependence of the orientational order parameter and of the conical angle has been determined independently from 129Xe and previous 2H NMR experiments. In the NTB phase, the averaged Saupe ordering matrix contains three off-diagonal elements. The temperature dependence of one of these, resulting from the phase chirality, has been determined from the 2H quadrupolar splittings.

A "Smart" ¹²⁸Xe NMR Biosensor for pH-Dependent Cell Labeling.

Here we present a "smart" xenon-129 NMR biosensor that undergoes a peptide conformational change and labels cells in acidic environments. To a cryptophane host molecule with high Xe affinity, we conjugated a 30mer EALA-repeat peptide that is α-helical at pH 5.5 and disordered at pH 7.5. The (129)Xe NMR chemical shift at room temperature was strongly pH-dependent (Δδ = 3.4 ppm): δ = 64.2 ppm at pH 7.5 vs δ = 67.6 ppm at pH 5.5, where Trp(peptide)-cryptophane interactions were evidenced by Trp fluorescence quenching. Using hyper-CEST NMR, we probed peptidocryptophane detection limits at low-picomolar (10(-11) M) concentration, which compares favorably to other NMR pH reporters at 10(-2)-10(-3) M. Finally, in biosensor-HeLa cell solutions, peptide-cell membrane insertion at pH 5.5 generated a 13.4 ppm downfield cryptophane-(129)Xe NMR chemical shift relative to pH 7.5 studies. This highlights new uses for (129)Xe as an ultrasensitive probe of peptide structure and function, along with potential applications for pH-dependent cell labeling in cancer diagnosis and treatment.

Xenon NMR of liquid crystals confined to cylindrical nanocavities: a simulation study.

Applications of liquid crystals (LCs), such as smart windows and the ubiquitous display devices, are based on controlling the orientational and translational order in a small volume of LC medium. Hence, understanding the effects of confinement to the liquid crystal phase behaviour is essential. The NMR shielding of (129)Xe atoms dissolved in LCs constitutes a very sensitive probe to the details of LC environment. Linking the experimental results to microscopic phenomena calls for molecular simulations. In this work, the NMR shielding of atomic (129)Xe dissolved in a uniaxial thermotropic LC confined to nanosized cylindrical cavities is computed from coarse-grained (CG) isobaric Monte Carlo (MC) simulations with a quantum-chemically (QC) pre-parameterised pairwise-additive model for the Xe nuclear shielding tensor. We report the results for the (129)Xe nuclear shielding and its connection to the structure and order of the LC appropriate to two different cavity sizes, as well as a comparison to the results of bulk (non-confined) simulations. We find that the confinement changes the LC phase structure dramatically and gives rise to the coexistence of varying degrees of LC order, which is reflected in the Xe shielding. Furthermore, we qualitatively reproduce the behaviour of the mean (129)Xe chemical shift with respect to temperature for atomic Xe dissolved in LC confined to controlled-pore glass materials. In the small-radius cavity the nematic - paranematic phase transition is revealed only by the anisotropic component of the (129)Xe nuclear shielding. In the larger cavity, the nematic - paranematic - isotropic transition is clearly seen in the Xe shielding. The simulated (129)Xe NMR shielding is insensitive to the smectic-A - nematic transition, since in the smectic-A phase, the Xe atoms largely occupy the imperfect layer structure near the cavity walls. The direct contribution of the cavity wall to (129)Xe nuclear shielding is dependent on the cavity size but independent of temperature. Our results show that the combination of CG simulations and a QC pre-parameterised (129)Xe NMR shielding allows efficient studies of the phase behaviour and structure of complex systems containing thousands of molecules, and brings us closer to the simulation of NMR experiments.

Predicting Porous Molecular Crystals and Clathrates

The last decade has seen dramatic improvements in the theories and computer algorithms underlying computational Crystal Structure Predictions [1]. It is now possible to reliably obtain the most likely crystal structures of at least simple molecules starting from nothing more than a drawing of the molecule. We can now go even further and look for rare and exotic kinds of crystals such as porous molecular crystals, clathrates and inclusion compounds among our predictions and calculate their physical properties [2], paving the way for the "science of hypothetical materials". In our poster, we present results on the prediction of fluorophenol xenon clathrates. We have performed crystal structure predictions by global lattice energy searches on o- and m-fluorophenol. The predicted structures have then been analyzed for porosity and their likelihood of being clathrates. From the several thousands of predicted structures, we select a few likely candidates according to an empirical rule based on the guest to host volume ratio [3]. Results from solid state xenon-129 NMR indicate that we have successfully determined the crystal structures of both o- and m-fluorophenol xenon clathrates and we suggest that Crystal Structure Prediction in combination with xenon-129 NMR is a powerful method for determining the structures of clathrates in general.

Xenon NMR of phase biaxiality in liquid crystals

Biaxial thermotropic nematic liquid crystals would be of great importance in liquid crystal display technology. Less than a decade ago, such liquid crystals were suggested. The biaxiality of the phases was confirmed using (2)H NMR spectroscopy of deuterated probe molecules. The spectra were collected from a sample rotating around an axis perpendicular to the external magnetic field, resulting in a two-dimensional powder pattern. We have proposed an alternate technique that is based on the second order quadrupole shift detectable in (131)Xe NMR spectra of dissolved xenon. The method has many advantages, such as the NMR spectra are taken from a static sample and the (131)Xe quadrupole coupling tensor is extremely sensitive to the symmetry of the phase. In the present study, we report results obtained on a 600-MHz NMR spectrometer. Together with the data of our earlier study, they confirm that the asymmetry parameter of the (131)Xe quadrupole coupling tensor in the nematic phase of a ferroelectric liquid crystal is 0.85 and in the smectic A phase ca 0.62, indicating significant phase biaxiality.

Rational design of hyperpolarized xenon NMR molecular sensor for the selective and sensitive determination of zinc ions

Although Zn(2+) ions are involved in large numbers of physiopathological processes, non-invasive detection of Zn(2+) ions in opaque biological samples remains a huge challenge. Here, we developed a novel zinc-responsive hyperpolarized (HP) (129)Xe-based NMR molecular sensor. This HP (129)Xe-based NMR molecular sensor was synthesized by attaching 2-(diphenylphosphino) benzenamine as ligand for zinc ions to the xenon-binding supramolecular cage, cryptophane. The (129)Xe NMR spectroscopy of such molecular sensor was shifted up to 6.4 ppm in the presence of Zn(2+) ions, which was nearly four times larger than that of the reported similar sensor. The application of the sensor would benefit low concentration detection by using indirect NMR/MRI method. The response exhibited high sensitivity and selectivity as discriminated from other six potentially competing metal ions. The application of this sensor in the analysis of zinc ions in the rat serum samples was also evaluated. The strategy is generally applicable in developing sensitive and selective sensors for quantitative determination of zinc ions.

Nanoemulsion Contrast Agents with Sub-picomolar Sensitivity for Xenon NMR

A new type of contrast agent for Xe NMR based on surfactant-stabilized perfluorocarbon-in-water nanoemulsions has been produced. The contrast agent uses dissolved hyperpolarized xenon gas as a nonperturbing reporting medium, as xenon freely exchanges between aqueous solution and the perfluorocarbon interior of the droplets, which are spectroscopically distinguishable and allow for chemical exchange saturation transfer (CEST) detection of the agent. Nanoemulsions with droplet diameters between 160 and 310 nm were produced and characterized using hyperpolarized (129)Xe combined with CEST detection. Saturation parameters were varied and data were modeled numerically to determine the xenon exchange dynamics of the system. Nanoemulsion droplets were detected at concentrations as low as 100 fM, corresponding to <1 μL of perfluorocarbon per liter of solution. The straightforward, inexpensive production of these agents will facilitate future development toward molecular imaging and chemical sensing applications.

ZSM-5/SBA-15 microporous/mesoporous composites prepared by a microwave-assisted zeolitisation of Al-SBA-15 mesoporous solids

ZSM-5/SBA-15 composites with high textural properties were synthesised by a microwave-assisted zeolitisation of Al-SBA-15 in the presence of tetrapropylammonium bromide. The effect of the hydrothermal treatment temperature and of ammonium fluoride, often used as a catalyst, was studied. Resulting materials were characterised by XRD, SEM, N2 sorption, HR-TEM, hyperpolarised Xe NMR, pyridine sorption as well as by their stability toward boiling water. It was shown that the hydrothermal synthesis has to be carried out at 140°C in order to better preserve the structure of the mesoporous precursor. Composite materials could be obtained only from a fluoride-free synthesis at short crystallisation times, t<5h. Appearance of 2–5nm nanoparticles in some of the corresponding samples, has been related to the increase in the total acidity. Inclusion of zeolitic material in the remaining Al-SBA-15 structure was demonstrated by hyperpolarised xenon NMR studies.

Online Monitoring of Intelligent Polymers for Drug Release with Hyperpolarized Xenon

Welcome to the guest zone: By combining hyperpolarized xenon and simple low-field NMR devices it is possible to obtain more control over hydrogels that show potential as drug delivery systems. An alternative way of polymer swelling-degree determination is demonstrated with real-time NMR analysis. An ideal region for solvent uptake can be defined in which the absorbed solvent molecules are completely confined in the nano-porous network of the hydrogel.

Pore Structure and Interconnectivity of CdS Aerogels and Xerogels by Hyperpolarized Xenon NMR

Metal chalcogenide aerogels and xerogels are unique materials that combine semiconducting nanostructures with porosity and are thus of interest for photocatalysis and sensing. To assess the feasibility for applications dependent on molecular transport, the pore structure and interconnectivity in CdS aerogels and xerogels were probed by a combination of conventional techniques and hyperpolarized (HP) 129Xe NMR. HP 129Xe NMR of the aerogels was consistent with two distinct types of accessible pores (adsorption sites), and 2-D EXSY NMR data suggest that these are connected. In contrast, a single resonance of low intensity with a temperature dependence consistent with the dissolution of Xe within a polymer matrix, that is, transport facilitated at high temperatures due to chain motion, is observed in xerogels. 13C NMR and thermal gravimetric analysis data reveal the presence of residual organics in both xerogels and aerogels, but a relatively higher proportion in the former, due, in part, to the surfactant used in their preparation. These data, combined with the absence of any meaningful adsorption isotherm for N2 at 77 K (where limited thermal chain motion precludes gas adsorption) are consistent with a dynamic blocking of the xerogel pores. The results reveal the important role of residual organics and drying regimens on the accessibility of pores in metal chalcogenide gel architectures.

Hyperpolarized xenon NMR and MRI signal amplification by gas extraction

A method is reported for enhancing the sensitivity of NMR of dissolved xenon by detecting the signal after extraction to the gas phase. We demonstrate hyperpolarized xenon signal amplification by gas extraction (Hyper-SAGE) in both NMR spectra and magnetic resonance images with time-of-flight information. Hyper-SAGE takes advantage of a change in physical phase to increase the density of polarized gas in the detection coil. At equilibrium, the concentration of gas-phase xenon is approximately 10 times higher than that of the dissolved-phase gas. After extraction the xenon density can be further increased by several orders of magnitude by compression and/or liquefaction. Additionally, being a remote detection technique, the Hyper-SAGE effect is further enhanced in situations where the sample of interest would occupy only a small proportion of the traditional NMR receiver. Coupled with targeted xenon biosensors, Hyper-SAGE offers another path to highly sensitive molecular imaging of specific cell markers by detection of exhaled xenon gas.

Probing polymer colloids by 129Xe NMR

Model aqueous dispersions of polystyrene, poly(methyl methacrylate), poly( n-butyl acrylate) and a statistical copolymer poly( n-butyl acrylate- co-methyl methacrylate) were studied using xenon NMR spectroscopy. The 129Xe NMR spectra of these various latexes reveal qualitative and quantitative differences in the number of peaks and in their line widths and chemical shifts. Above the glass transition temperature, exchange between xenon sorbed in the particle core and free xenon outside the particles is fast on the 129Xe spectral time-scale and a single 129Xe signal is observed. At temperatures below the glass transition temperature, the exchange between sorbed and free xenon is slow on the 129Xe spectral time-scale and two 129Xe NMR signals can be observed. If the signal of sorbed 129Xe is observed, its chemical shift, line width and integral relative to the integral of free 129Xe can be used for the characterization of the particle core. The line width of free 129Xe provides the residence time of xenon outside the particles and can be used to determine the rate constant characterizing the kinetics of penetration of xenon in the particles. This rate constant emerges as promising parameter for the characterization of the polymer particle surface.

Nanochannels of Two Distinct Cross-Sections in a Porous Al-Based Coordination Polymer

A new aluminum naphthalenedicarboxylate Al(OH)(1,4-NDC) x 2 H2O compound has been synthesized. The crystal structure exhibits a three-dimensional framework composed of infinite chains of corner-sharing octahedral Al(OH)2O4 with 1,4-naphthanedicarboxylate ligands forming two types of channels with squared-shape cross-section. The large channels present a cross-section of 7.7 x 7.7 A(2), while the small channels are about 3.0 x 3.0 A(2). When water molecules are removed, no structural transformation occurs, generating a robust structure with permanent porosity and remarkable thermal stability. 2D (1)H-(13)C heteronuclear correlation NMR measurements, together with the application of Lee-Goldburg homonuclear decoupling, were applied, for the first time, to porous coordination polymers revealing the spatial relationships between the (1)H and (13)C spin-active nuclei of the framework. To demonstrate the open pore structure and the easy accessibility of the nanochannels to the gas phase, highly sensitive hyperpolarized (HP) xenon NMR, under extreme xenon dilution, has been applied. Xenon can diffuse selectively into the large nanochannels, while the small ones show no substantial uptake of xenon due to severe restrictions along the channels that prevent the diffusion. Two-dimensional exchange experiments showed the exchange time to be as short as 15 ms. Through variable-temperature HP (129)Xe NMR experiments we were able to achieve an unprecedented description of the large nanochannel space and surface, a physisorption energy of 10 kJ mol(-1), and the chemical shift value of xenon probing the internal surfaces. The large pore channels are straight, parallel, and independent, allowing one-dimensional anisotropic diffusion of gases and vapors. Their walls are composed of the naphthalene moieties that create an unique environment for selective sorption. These results prompted us to measure the storage capacity toward methanol, acetone, benzene, and carbon dioxide. The selective adsorption of methanol and acetone vs that of water, together with the permanent porosity and high thermal stability, makes this compound a suitable matrix for separation and purification.

Direct Observation of Atoms Entering and Exiting l-Alanyl-l-valine Nanotubes by Hyperpolarized Xenon-129 NMR

The kinetics of gas --> channel and channel --> gas exchange of Xe in self-assembled L-alanyl-L-valine (AV) nanotubes was facilitated by continuous flow hyperpolarized xenon-129 two-dimensional exchange NMR spectroscopy. Cross-peaks due to gas atoms entering or exiting were observed at Xe pressures of 92, 1320, and 3300 mbar, corresponding to Xe fractional occupancies ranging from 0.047 to 0.64. At each pressure, the rate of desorption from the channels was determined by fitting the mixing time dependence of the cross-peak and diagonal-peak signals to a magnetization exchange model, assuming steady-state Langmuir adsorption under hyperpolarized gas flow conditions. The observed rate constant for desorption of Xe from AV nanotubes decreased from 4.5 s-1 to 2.0 s-1 over the occupancy range studied, a finding that is consistent with a decrease in the diffusivity in the channels.

A Cryptophane Biosensor for the Detection of Specific Nucleotide Targets through Xenon NMR Spectroscopy

A Cryptophane Biosensor for the Detection of Specific Nucleotide Targets through Xenon NMR Spectroscopy

2D Multinuclear NMR, Hyperpolarized Xenon and Gas Storage in Organosilica Nanochannels with Crystalline Order in the Walls

The combination of 2D 1H-13C and 1H-29Si solid state NMR, hyperpolarized 129Xe NMR, synchrotron X-ray diffraction, together with adsorption measurements of vapors and gases for environmental and energetic relevance, was used to investigate the structure and the properties of periodic mesoporous hybrid p-phenylenesilica endowed with crystalline order in the walls. The interplay of 1H, 13C, and 29Si in the 2D heteronuclear correlation NMR measurements, together with the application of Lee-Goldburg homonuclear decoupling, revealed the spatial relationships (<5 angstroms) among various spin-active nuclei of the framework. Indeed, the through-space correlations in the 2D experiments evidenced, for the first time, the interfaces of the matrix walls with guest molecules confined in the nanochannels. Organic-inorganic and organic-organic heterogeneous interfaces between the matrix and the guests were identified. The open-pore structure and the easy accessibility of the nanochannels to the gas phase have been demonstrated by highly sensitive hyperpolarized (HP) xenon NMR, under extreme xenon dilution. Two-dimensional exchange experiments showed the exchange time to be as short as 2 ms. Through variable-temperature HP 129Xe NMR experiments we were able to achieve an unprecedented description of the nanochannel space and surface, a physisorption energy of 13.9 kJ mol-1, and the chemical shift value of xenon probing the internal surfaces. These results prompted us to measure the high storage capacity of the matrix towards benzene, hexafluorobenzene, ethanol, and carbon dioxide. Both host-guest, CH...pi, and OH...pi interactions contribute to the stabilization of the aromatic guests (benzene and hexafluorobenzene) on the extended surfaces. The full carbon dioxide loading in the channels could be detected by synchrotron radiation X-ray diffraction experiments. The selective adsorption of carbon dioxide (ca. 90 wt %) vs that of oxygen and hydrogen, together with the permanent porosity, high thermal stability, and high degree of order, makes this a suitable matrix for purifying hydrogen in clean-energy generation.