Field-cycling NMR Relaxometry Research Articles

Fast Field-Cycling Nuclear Magnetic Resonance Relaxometry of Perfluorosulfonic Acid Ionomers and Their Perfluorosulfonyl Fluoride Precursors Membranes.

The spin-lattice relaxation rates (R1) of fluorine nuclei in perfluorosulfonic acid (PFSA) ionomer membranes and their precursor solid perfluorosulfonyl fluoride (PFSF) were measured by fast field-cycling (FFC) NMR relaxometry. The XRD profiles of PFSA and PFSF are similar and show a characteristic peak, indicating the alignment of main chains. While the SAXS profiles of the PFSA membranes show two peaks, those of the solid PFSF lack the ionomer peak which is characteristic of hydrophilic side chains in the PFSA ionomer membranes. The Larmor frequency dependence of R1 obeys power law and the indices are dependent on the sample and temperature. The indices of the PFSA membranes change from -1/2 to -1 along with the Larmor frequency and temperature dependence decrease, which is consistent with the generalized defect diffusion model. Estimated activation energies are in good agreement with those obtained from dynamical mechanical analysis and dielectric spectroscopy, indicating the segmental motion of the backbones as the common origin of these observations. On the other hand, the index changes to -3/4 in the case of the PFSFs, which has been predicted by the reptation model.

A New Model for Solving Hydrological Connectivity Inside Soils by Fast Field Cycling NMR Relaxometry

In this paper, a new quantitative approach for estimating the structural and functional connectivity inside soil by Fast Field Cycling (FFC) NMR relaxometry is presented, tested by measurements carried out in three samples with different texture characteristics. Measurements by FFC NMR relaxometry have been carried out using water-suspended samples and Proton Larmor frequencies (νL) ranging in the 0.015–35 MHz interval. Two non-degraded soil samples, with different textural characteristics, and a degraded soil collected in a badland area, were analyzed. For a given soil and any applied Proton Larmor frequency, the distribution of the longitudinal relaxation times, T1, (i.e., relaxogram) measured by FFC NMR has been integrated, and the resulting S-shaped curve (i.e., relaxogram integration curve) was represented, for the first time, by Gumbel’s diagram. This new representation of the relaxogram integration curve, transforming the S-shaped curve into a straight line, allowed for distinguishing three linear components, corresponding to three different relaxation time ranges, characterized by three different slopes. Two points, identified by the abrupt slope changes of the relaxogram integration curve plotted in Gumbel’s diagram, are used to identify two characteristic values of relaxation time, T1A and T1B, which define three well-known pore size classes (T1 < T1A micro-pores, T1A < T1 < T1B meso-pores, and T1 > T1B macro-pores). The relaxogram integration curve allowed for calculating the non-exceeding empirical cumulative frequency, F(T1), corresponding to the characteristic T1A and T1B values. The analysis demonstrated that the relaxogram can be used to determine the pore-size ranges of each investigated sample. Finally, using the slope values of the three components of the relaxogram integration curve, a new definition of the Structural Connectivity Index, SCI, and Functional Connectivity Index, FCI, was proposed.

Physicochemical characterization of green sodium oleate-based formulations. Part 3. Molecular and collective dynamics in rodlike and wormlike micelles by proton nuclear magnetic resonance relaxation

HypothesisSodium oleate (NaOL) self-aggregates in water forming rodlike micelles with different length depending on NaOL concentration; when KCl is added wormlike micelles form, which entangle giving rise to a viscoelastic dispersion. It is expected that aggregates with different size and shape exhibit different internal and overall molecular motions and collective dynamics. ExperimentsTwo low viscosity NaOL/water and two viscoelastic NaOL/KCl/water formulations with different NaOL concentration (0.23 and 0.43 M) were investigated by 1H fast field cycling NMR relaxometry over broad temperature and Larmor frequency ranges, after a first screening by 1H and 13C NMR spectroscopy at high frequency. FindingsThe analysis of the collected data indicated that fast conformational isomerization and rotation of NaOL about its long molecular axis and lateral diffusion of NaOL around the axis of the cylindrical aggregates are slightly affected by the aggregate shape and length. On the other hand, fluctuations of the local order director are quite different in the fluid and viscoelastic systems, reflecting the shape and size of the aggregates. Quantitative information was obtained on activation energy for fast internal and overall motions, correlation times and activation energy for lateral diffusion, and coherence length for collective order fluctuations.

High Resolution 31P Field Cycling NMR Reveals Unsuspected Features of Enzyme-Substrate-Cofactor Dynamics.

The dynamic interactions of enzymes and substrates underpins catalysis, yet few techniques can interrogate the dynamics of protein-bound ligands. Here we describe the use of field cycling NMR relaxometry to measure the dynamics of enzyme-bound substrates and cofactors in catalytically competent complexes of GMP reductase. These studies reveal new binding modes unanticipated by x-ray crystal structures and reaction-specific dynamic networks. Importantly, this work demonstrates that distal interactions not usually considered part of the reaction coordinate can play an active role in catalysis. The commercialization of shuttling apparatus will make field cycling relaxometry more accessible and expand its use to additional nuclei, promising more intriguing findings to come.

Influence of Sulfur-Curing Conditions on the Dynamics and Crosslinking of Rubber Networks: A Time-Domain NMR Study.

The characterization of the structural and dynamic properties of rubber networks is of fundamental importance in rubber science and technology to design materials with optimized mechanical properties. In this work, natural and isoprene rubber networks obtained by curing at three different temperatures (140, 150, and 170 °C) and three different sulfur contents (1, 2, and 3 phr) in the presence of a 3 phr accelerator were studied using a combination of low-field time-domain NMR (TD-NMR) techniques, including 1H multiple-quantum experiments for the measurement of residual dipolar couplings (Dres), the application of the Carr–Purcell–Meiboom–Gill pulse sequence for the measurement of the transverse magnetization decay and the extraction of 1H T2 relaxation times, and the use of field cycling NMR relaxometry for the determination of T1 relaxation times. The microscopic properties determined by TD-NMR experiments were discussed in comparison with the macroscopic properties obtained using equilibrium swelling, moving die rheometer, and calorimetric techniques. The obtained correlations between NMR observables, crosslink density values, maximum torque values, and glass transition temperatures provided insights into the effects of the vulcanization temperature and accelerator/sulfur ratio on the structure of the polymer networks, as well as on the effects of crosslinking on the segmental dynamics of elastomers. Dres and T2 were found to show linear correlations with the crosslink density determined by equilibrium swelling, while T1 depends on the local dynamics of polymer segments related to the glass transition, which is also affected by chemical modifications of the polymer chains occurring during vulcanization.

Use of magic sandwich echo and fast field cycling NMR relaxometry on honey adulteration with corn syrup

Adulteration is defined as the intentional addition of a material that is not a part of the nature. In this study, a non-conventional time domain nuclear magnetic resonance (TD-NMR) pulse sequence: magic sandwich echo (MSE) was used to detect the adulteration of honey by glucose syrup (GS) and high fructose corn syrup (HFCS) accompanied with T1 and T2 relaxation times. Also, fast field cycling NMR (FFC-NMR) relaxometry and multivariate analysis were performed to investigate the adulteration. Higher maltose in GS and changing glucose to water ratio of HFCS gave high correlation with the crystal content values. In HFCS adulteration, two separate populations of protons having different T2 values were detected and T1 times were also used to determine GS adulteration. Addition of GS increased T1 while addition of HFCS increased T2 , significantly. The results showed that it is possible to differentiate the unadulterated and adulterated honey samples by using TD-NMR relaxation times and crystal content values obtained by the MSE sequence. By FFC-NMR relaxometry, not only GS addition but also the amount of GS was examined. The multivariate analysis technique of principal component analysis was able to distinguish the types of adulterants. © 2021 Society of Chemical Industry.

Glassy and Polymer Dynamics of Elastomers by 1H-Field-Cycling NMR Relaxometry: Effects of Fillers.

1H spin–lattice relaxation rate (R1) dispersions were acquired by field-cycling (FC) NMR relaxometry between 0.01 and 35 MHz over a wide temperature range on polyisoprene rubber (IR), either unfilled or filled with different amounts of carbon black, silica, or a combination of both, and sulfur cured. By exploiting the frequency–temperature superposition principle and constructing master curves for the total FC NMR susceptibility, χ″(ω) = ωR1(ω), the correlation times for glassy dynamics, τs, were determined. Moreover, the contribution of polymer dynamics, χpol″(ω), to χ″(ω) was singled out by subtracting the contribution of glassy dynamics, χglass″(ω), well represented by the Cole–Davidson spectral density. Glassy dynamics resulted moderately modified by the presence of fillers, τs values determined for the filled rubbers being slightly different from those of the unfilled one. Polymer dynamics was affected by the presence of fillers in the Rouse regime. A change in the frequency dependence of χpol″(ω) at low frequencies was observed for all filled rubbers, more pronounced for those reinforced with silica, which suggests that the presence of the filler particles can affect chain conformations, resulting in a different Rouse mode distribution, and/or interchain interactions modulated by translational motions.

Molecular dynamics simulations vs field-cycling NMR relaxometry: Structural relaxation mechanisms in the glass-former glycerol revisited.

We combine field-cycling (FC) relaxometry and molecular dynamics (MD) simulations to study the rotational and translational dynamics associated with the glassy slowdown of glycerol. The 1H NMR spin-lattice relaxation rates R1(ω) probed in the FC measurements for different isotope-labelled compounds are computed from the MD trajectories for broad frequency and temperature ranges. We find high correspondence between experiment and simulation. Concerning the rotational motion, we observe that the aliphatic and hydroxyl groups show similar correlation times but different stretching parameters, while the overall reorientation associated with the structural relaxation remains largely isotropic. Additional analysis of the simulation results reveals that transitions between different molecular configurations are slow on the time scale of the structural relaxation at least at sufficiently high temperatures, indicating that glycerol rotates at a rigid entity, but the reorientation is slower for elongated than for compact conformers. The translational contribution to R1(ω) is well described by the force-free hard sphere model. At sufficiently low frequencies, universal square-root laws provide access to the molecular diffusion coefficients. In both experiment and simulation, the time scales of the rotational and translational motions show an unusually large separation, which is at variance with the Stokes-Einstein-Debye relation. To further explore this effect, we investigate the structure and dynamics on various length scales in the simulations. We observe that a prepeak in the static structure factor S(q), which is related to a local segregation of aliphatic and hydroxyl groups, is accompanied by a peak in the correlation times τ(q) from coherent scattering functions.

Translational and rotational diffusion of three glass forming alcohols by 1H field cycling NMR relaxometry

1H field-cycling NMR relaxometry was applied to investigate the dynamics of the three glass forming alcohols 2-phenylbutan-1-ol (BEP), 2-(trifluoromethyl)phenetyl alcohol (2TFMP), and 4-(trifluoromethyl)phenetyl alcohol (4TFMP), all having a phenyl ring as substituent. 1H longitudinal relaxation rates, R1, were measured at Larmor frequencies (ν) from 0.01 to 35 MHz in the liquid phase of the three alcohols between 213 and 313 K. Data analysis was performed using master curves built on the basis of the frequency temperature superposition principle exploiting the NMR susceptibility representation. Longitudinal relaxation was considered to arise from two dynamic processes, i.e. translational diffusion and molecular rotations. For the first process a force-free hard-sphere model was used, whereas the phenomenological Davidson-Cole function was employed to model the second motional process. The analysis allowed translational and rotational correlation times to be determined over a wide time scale (10−11 - 10−3 s). The ratio between the two correlation times indicated the formation of hydrogen bonded networks for all alcohols, while their trends with temperature suggested that BEP forms stronger hydrogen bonds. The resulting self-diffusion coefficients were in agreement with the values independently determined from the slope of R1vs ν1/2 at low frequencies.

Differential scanning calorimetry and NMR study of water confined in a mesoporous bioactive glass

The emergence of a new generation of nanostructured materials opened a wide range of new potential applications for Bioglasses such as DNA vaccination, cellular treatment or drug delivery with a well-controlled loading and release of (bio)active molecules. In this study we compare structural, dynamic and thermal properties of water confined in two CaO–SiO2–P2O5 mesoporous bioactive glasses (MBGs). One of the MBGs is prepared by standard sol-gel methods and the other by microfluidic procedures, therefore both MBGs have the same molar compositions (92% SiO2, 6% CaO, and 2% P2O5) but different textural properties (surface areas, porous volume and grain architecture). These materials are intended to interact with body fluids which are essentially composed of water, it is therefore crucial to understand the water-MBG interaction. With this objective, we apply a complementary approach based on 1H field-cycling NMR relaxometry, 1H magic-angle spinning NMR spectroscopy and differential scanning calorimetry (DSC). With low-field relaxometry we observe that adsorbed water has the same behavior in the mesoporous channels of both systems, but that the escaping of water molecules towards the bulk is slightly faster in the microfluidic material. By combining 1H MAS NMR and DSC to study a series of MBG samples at variable temperature and water content, we observe that for low temperature and low water content, pooling of water molecules is less pronounced in the case of the microfluidic MBG. All our results therefore point towards a microcapsule organization of the microfluidic MBG that provides better water access to the full porous volume.

Rotational correlation times, diffusion coefficients and quadrupolar peaks of the protic ionic liquid ethylammonium nitrate by means of 1H fast field cycling NMR relaxometry

The protic ionic liquid (PIL) ethylammonium nitrate (EAN) is characterized by hydrogen bonding between the NH bonds of the cations and the oxygen atoms of the anions. The three-dimensional H-bond network of EAN very much resembles that of water. Although this first ionic liquid is known for more than 100 years, there is still lack of information about the rotational and translational dynamics, and how both are related to each other. For that purpose, we measured the 1H nuclear magnetic relaxation dispersion curves (NMRD) of EAN by means of Fast Field Cycling (FFC) NMR relaxometry. We were able to analyze the measured 1H spin-lattice relaxation rates R1 covering a large temperature range between 248 and 333 K simultaneously. Applying the well-known Bloembergen-Purcell-Pound (BPP) approach, we decomposed the total relaxation rates into intramolecular and intermolecular contributions, describing the rotational and translational dynamics of the ethylammonium cation. Here, we report rotational correlation times, τR, and translational diffusion coefficients, DT, both derived from the total 1H spin-lattice relaxation rates as a function of temperature and frequency. Self-diffusion coefficients, DT, were also determined from the total relaxation rates in the low frequency range, wherein only the intermolecular relaxation contribution is frequency dependent. Then, DT results from the slope of a linear fit of the spin-lattice relaxation rate R1 as a function of the square root of frequency, ν. The diffusion coefficients obtained from both methods are in good agreement with self-diffusion coefficients measured by pulsed-field-gradient (PFG) NMR. The rotational correlation times, τR, derived from the intramolecular 1H relaxation contribution were compared to correlation times obtained from high-field NMR deuteron relaxation experiments, dielectric spectroscopy (DS) and femto-second-infrared (fs-IR) spectroscopy. For EAN, we also observed a local enhancement of the 1H spin-lattice relaxation rates R1 at high frequencies. This so-called quadrupole relaxation enhancement (QRE) results from the interference of the Zeeman transition energy of the 1H spins and the energy difference of two levels of the quadrupole 14N nuclei. QRE is usually known for solids, but rarely for the liquid state as observed here for EAN. Overall, we show that FFC relaxometry provides access to rotational correlation times, translational diffusion coefficients and quadrupole relaxation enhancement at the same time.

Glassy and Polymer Dynamics of Elastomers by 1H Field-Cycling NMR Relaxometry: Effects of Cross-Linking.

1H spin lattice relaxation rate (R1) dispersions were acquired by field-cycling (FC) NMR relaxometry between 0.01 and 35 MHz over a wide temperature range on polyisoprene (IR), polybutadiene (BR), and poly(styrene-co-butadiene) (SBR) rubbers, obtained by vulcanization under different conditions, and on the corresponding uncured elastomers. By exploiting the frequency–temperature superposition principle, χ″(ωτs) master curves were constructed by shifting the total FC NMR susceptibility, χ″(ω) = ωR1(ω), curves along the frequency axis by the correlation times for glassy dynamics, τs. Longer τs values and, correspondingly, higher glass transition temperatures were determined for the sulfur-cured elastomers with respect to the uncured ones, which increased by increasing the cross-link density, whereas no significant changes were found for fragility. The contribution of polymer dynamics, χpol″(ω), to χ″(ω) was singled out by subtracting the contribution of glassy dynamics, χglass″(ω), well represented using a Cole–Davidson spectral density. For all elastomers, χpol″(ω) was found to represent a small fraction, on the order of 0.05–0.14, of the total χ″(ω), which did not show a significant dependence on cross-link density. In the investigated temperature and frequency ranges, polymer dynamics was found to encompass regimes I (Rouse dynamics) and II (constrained Rouse dynamics) of the tube reptation model for the uncured elastomers and only regime I for the vulcanized ones. This is clear evidence that chemical cross-links impose constraints on chain dynamics on a larger space and time scale than free Rouse modes.

Fast field cycling NMR relaxometry as a tool to monitor Parmigiano Reggiano cheese ripening

It is widely recognized that the longer the ageing, the more valuable Parmigiano Reggiano (PR) cheese becomes, due to the improvement of its sensorial and nutritional properties. Up to now, the evaluation of PR properties has been performed on samples mainly aged up to 40 months. For this reason, this study was aimed at collecting information about the chemical-physical characteristics of PR cheeses after ageing at 24, 48 and 84 months. The basic analyses on water amount, protein content and volatile organic compounds (VOC) revealed that PR ageing is associated to the decomposition of the organic components into smaller units. This drives water molecules closer to the aforementioned units, thereby leading to a reduction of water activity. Moreover, it appears, from VOC investigation, that either the total amount of long chain fatty acids (LCFA) or the sole 9Z-octadecenoic acid considerably increased with PR aging, thereby making these molecular systems good fingerprint to monitor PR ripening. Fast field cycling (FFC) NMR relaxometry revealed different components of the molecular dynamics in the complex PR cheese samples, which were attributed to water trapped in casein micelles, water bound to polar groups in organic components, proteins, and fats. In particular, the reduction of the correlation times associated to the organic components was explained by the molecular size decrement following degradation processes. The ageing-independent component of the motion was assigned to water molecules bound to polar groups in organic moieties. Finally, the ageing-independent contribution to the overall relaxation rate was assigned to water molecules in casein micelles. This study revealed that FFC NMR relaxometry can be a promising technique to monitor the chemical physical changes during Parmigiano Reggiano cheese ripening.

Use of 1H-NMR spectroscopy, diffusometry and relaxometry for the characterization of thermally-induced degradation of motor oils

We explore the capability of 1H NMR spectroscopy, relaxation and diffusion for the characterization of thermally-induced degradation of fresh motor oils. Temperature dependent measurements of diffusion show a weak sensitivity to thermal degradation. In contrast, both the transverse relaxation at 1H Larmor frequencies of 20 MHz and 400 MHz, as well as the longitudinal relaxation at 60 kHz showed to be sensitive to thermally-induced effects at measured temperatures within the range −10 °C to 30 °C. Results show that the highest sensitivity significance can be reached by fast field-cycling NMR relaxometry measurements at low magnetic fields.

Enzyme-Substrate-Cofactor Dynamical Networks Revealed by High-Resolution Field Cycling Relaxometry.

The remarkable power and specificity of enzyme catalysis rely on the dynamic alignment of the enzyme, substrates, and cofactors, yet the role of dynamics has usually been approached from the perspective of the protein. We have been using an underappreciated NMR technique, subtesla high-resolution field cycling 31P NMR relaxometry, to investigate the dynamics of the enzyme-bound substrates and cofactor on guanosine-5'-monophosphate reductase (GMPR). GMPR forms two dead end, yet catalytically competent, complexes that mimic distinct steps in the catalytic cycle: E·IMP·NADP+ undergoes a partial hydride transfer reaction, while E·GMP·NADP+ undergoes a partial deamination reaction. A different cofactor conformation is required for each partial reaction. Here we report the effects of mutations designed to perturb cofactor conformation and ammonia binding with the goal of identifying the structural features that contribute to the distinct dynamic signatures of the hydride transfer and deamination complexes. These experiments suggest that Asp129 is a central cog in a dynamic network required for both hydride transfer and deamination. In contrast, Lys77 modulates the conformation and mobility of substrates and cofactors in a reaction-specific manner. Thr105 and Tyr318 are part of a deamination-specific dynamic network that includes the 2'-OH of GMP. These residues have comparatively little effect on the dynamic properties of the hydride transfer complex. These results further illustrate the potential of high-resolution field cycling NMR relaxometry for the investigation of ligand dynamics. In addition, exchange experiments indicate that NH3/NH4+ has a high affinity for the deamination complex but a low affinity for the hydride transfer complex, suggesting that the movement of ammonia may gate the cofactor conformational change. Collectively, these experiments reinforce the view that the enzyme, substrates, and cofactor are linked in intricate, reaction-specific, dynamic networks and demonstrate that distal portions of the substrates and cofactors are critical features in these networks.

Dielectric relaxation and proton field-cycling NMR relaxometry study of dimethyl sulfoxide/glycerol mixtures down to glass-forming temperatures.

Mixtures of glycerol and dimethyl sulfoxide (DMSO) are studied by dielectric spectroscopy (DS) and by 1H field-cycling (FC) NMR relaxometry in the entire concentration range and down to glass-forming temperatures (170-323 K). Molecular dynamics is accessed for 0 < xDMSO ≤ 0.64, at higher concentration phase separation occurs. The FC technique provides the frequency dependence of the spin-lattice relaxation rate which is transformed to the susceptibility representation and thus allows comparing NMR and DS results. The DS spectra virtually do not change with xDMSO and T, only the relaxation times become shorter. This is in contrast to the non-associated mixture toluene/quinaldine for which strong spectral changes occur. The FC relaxation spectra of glycerol in solution with DMSO or (deuterated) DMSO-d6 display a bimodal structure with a high-frequency part reflecting rotational and a low-frequency part reflecting translational dynamics. Regarding the rotational contribution in the glycerol/DMSO-d6 mixtures, no spectral change with xDMSO and T is observed. Yet, the non-deuterated mixture reveals a broader relaxation spectrum. Time constants τrot(T) probed by the two techniques complement each, a range 10-11 s < τ < 10 s is covered. The glass transition temperature Tg(xDMSO) is determined, yielding Tg = 149.5 ± 1 K of pure DMSO by extrapolation. Analysing the low-frequency FC NMR spectra allows to determine the diffusion coefficient Dtrans. Its logarithm shows a linear xDMSO-dependence as does lg τrot. The ratio Dtrans/Drot is independent of xDMSO and its low value indicates large separation of translation and rotation. The corresponding unphysically small hydrodynamic radius indicates strong failure of Stokes-Einstein-Debye relation. Such anomaly is taken as characteristics of a 3d hydrogen-bonded network. We conclude, although DMSO is an aprotic liquid the molecule is continuously incorporated in the hydrogen network of glycerol. Both molecules display common dynamics, i.e., no decoupling of the component dynamics is found in contrast to quinaldine/toluene with a similar Tg difference of its components.

Quantification of manganous ions in wine by NMR relaxometry

Proton relaxation in model and real wines is investigated for the first time by fast field cycling NMR relaxometry. The relaxation mechanism unambiguously originates form proton interaction with paramagnetic ions naturally present in wines. Profiles of a white Chardonnay wine from Burgundy, a red Medoc, and model wines are well reproduced by Solomon-Bloembergen-Morgan equations. Relaxation is primarily governed by interactions with Mn2+. A straightforward model-independent quantification of the manganese ion concentration (down to few tens of μg/L) is proposed.

Comparing different processing methods in apple slice drying. Part 2 solid-state Fast Field Cycling 1H-NMR relaxation properties, shrinkage and changes in volatile compounds

The objective of this paper was to study the effects of three different drying methods: microwave (MW), hot air (HA) and a combination of both (HY), on the 1H NMR relaxation properties, shrinkage and volatile compounds of Golden Delicious apple slices. Fast field cycling NMR relaxometry revealed that the HA samples dried at different temperatures (65 and 75 °C) appeared to contain less restrained water as compared to the sample obtained by MW heating at the same temperatures. In fact, the longitudinal relaxation times (T1) of the water molecules in the HA dried slices were shorter than those measured for the MW dried samples, thereby revealing that in the MW slices, water molecules behave more like as a pseudo-ice layer. For this reason, shrinkage was much more accentuated in apples dried by the HA and hybrid methods, compared to the MW procedure. Likewise, HA drying had a bigger effect than MW and HY on the aromatic profile of the apple slices, causing a significant decrease in esters while ketones increased.

Probing Dynamics and Wettability of Water and Oil in Conventional and Unconventional Sandstone Rock Cores by Field-Cycling NMR Relaxometry

Fast-field-cycling (FFC) NMR is a technique that can probe the longitudinal nuclear magnetic relaxation dispersion (NMRD) profiles by varying the magnetic field to a large extent. This allows probi...

Trapping of Gd(III) Ions by Keplerate Polyanionic Nanocapsules in Water: A 1H Fast Field Cycling NMR Relaxometry Study

Polyoxomolybdates of the type [{MoVIMoVI5O21(H2O)6}12{MoV2O4(L)}30](12+30n)–, also referred to as keplerates (Kps), are polyanionic nanocapsules bearing 20 {Mo9O9} pores connected to a central cavity by 20 channels. In solution, they can entrap cations in different sites of their porous structure (in the internal cavity, in the pores, or on the capsule surface) in equilibrium with cations free in solution. The porosity of Kps was previously exploited to prepare nanostructured potential contrast agents through the self-assembly of KpOAc or KpHPO4 (i.e., Kp with L = acetate or hydrogen phosphate) macroanions and Gd(III) cations in water, accompanied by stabilization with F-127 Pluronics. The strongly enhanced water proton relaxivity observed at 20 MHz was postulated to derive from the trapping of Gd(III) aqua ions by the nanocapsules. In the present work, 1H fast field cycling NMR relaxometry allowed this hypothesis to be verified on aqueous suspensions containing either KpOAc or KpHPO4 and Gd(III) in diffe...