Nuclear Relaxation Time Research Articles

Multi-shelled hollow porous carbon nanospheres-based evaporator for highly efficient solar-driven desalination

Carbon-based materials stand out as photothermal materials for interfacial solar evaporation due to their high solar absorptance, chemical stability, adjustable structure, ease of preparation, and low cost compared to other candidate materials. Development of multifunctional carbon-based materials that can endow the fabricated evaporators with reduced energy loss and evaporation enthalpy is highly demanded to achieve extraordinary evaporation rates. Herein, high-solar-absorptivity multi-shelled hollow porous carbon nanospheres are fabricated and incorporated with an insulating hydrogel bottom layer into an integrated solar evaporator. It achieves a fast photothermal response and a remarkably high solar evaporation rate of 2.4 kg m−2 h−1 under one sun. Multiphysics simulation indicates that the porous multi-shelled hollow structure enables sufficient and effective interactions between water and the hydrophilic carbon surfaces, thus producing more intermediate water (IW) and lowering the evaporation enthalpy. The solar-driven water evaporation in the evaporator is probed by in-situ low-field nuclear magnetic resonance relaxation time measurements, based on which conversion of free water (FW) into IW during solar evaporation is proposed. The molecular dynamics simulations reveal that the evolution of FW clusters into IW is facilitated on the carbon surface, thus replenishing the evaporated IW and maintaining continuous high evaporation rates. The demonstrated solar evaporator exemplifies the effectiveness of structural and thermal management in enhancing solar-driven desalination.

Predicting the Stability of Formulations Containing Lyophilized Human Serum Albumin and Sucrose/Trehalose Using Solid-State NMR Spectroscopy.

Stabilization of proteins by disaccharides in lyophilized formulations depends on the interactions between the protein and the disaccharide (system homogeneity) and the sufficiently low mobility of the system. Human serum albumin (HSA) was lyophilized with disaccharides (sucrose and/or trehalose) in different relative concentrations. Solid-state nuclear magnetic resonance (ssNMR) spectroscopy 1H T1 and 1H T1ρ relaxation times were measured to determine the homogeneity of the lyophilized systems on 20-50 and 1-3 nm domains, respectively, with 1H T1 relaxation times also being used to determine the β-relaxation rate. HSA/sucrose systems had longer 1H T1 relaxation times and were slightly more stable than HSA/trehalose systems in almost all cases shown. HSA/sucrose/trehalose systems have 1H T1 relaxation times between the HSA/sucrose and HSA/trehalose systems and did not result in a more stable system compared with binary systems. Inhomogeneity was evident in a sample containing relative concentrations of 10% HSA and 90% trehalose, suggesting trehalose crystallization during lyophilization. Under these stability conditions and with these ssNMR acquisition parameters, a 1H T1 relaxation time below 1.5 s correlated with an unstable sample, regardless of the disaccharide(s) used.

Suppressing effect of shortening vapor cell stem on polarization-induced magnetic gradient relaxation of Xe in NMR co-magnetometers

The Rb polarization-induced magnetic field gradient is one of the crucial sources of Xe spin relaxation in NMR co-magnetometers. The additional volume of the atomic vapor cell stem can enhance the effective magnetic field gradient through the diffusion effect of the optically polarized-Rb atoms, which affects the spin relaxation properties of Xe and degrades the long-term stability of NMR co-magnetometers. Based on the discrete steps random walk diffusion model, the magnetic gradient relaxation properties of Xe diffusing in an asymmetric cell composed of a cubic part and a cylindrical tube (stem) are characterized. Simulation results indicate that a shorter stem can effectively prolong the nuclear spin relaxation time under a constant magnetic field gradient. To verify this conclusion, a cell stem truncating actuator is developed using a high-power CO2 laser to shorten the cell stem, and the magnetic field gradient coil is used to perform the gradient compensation to examine the impact of various stem lengths on the spin relaxation rate. Experiments indicate that under the influence of atomic diffusive motions, shortening the stem length can effectively suppress the polarization-induced magnetic gradient and increase the 129Xe transverse relaxation time from 4.5 s to 21 s. The study posts practical applications for vapor cells with a stem, which is expected to enhance the performance of atomic spin co-magnetometers.

Method to quickly estimate T1 value by suppressing spin exchange relaxation and magnetic field gradient relaxation in atomic sensors

The accuracy of atomic sensor is determined by the transverse nuclear spin relaxation time (T2). As the longitudinal nuclear spin relaxation time (T1) is the upper limit of T2, it becomes an important metric for evaluating the merits of alkali vapor cells. However, the conventional method for measuring T1 of pulse delay takes dozens of minutes, which will increase the effect of random errors during the testing process. In order to solve this problem, a method to quickly estimate the value of T1 is proposed by suppressing the influence of spin exchange relaxation and magnetic field gradient relaxation terms on T2. Experimental results indicate that the accuracy is maintained at above 95%, and the testing time is reduced to less than 5%.

Efficient Scavenging of TEMPOL Radical by Ascorbic Acid in Solution and Related Prolongation of 13C and 1H Nuclear Spin Relaxation Times of the Solute

Dynamic nuclear polarization for nuclear magnetic resonance (NMR) spectroscopy and imaging uses free radicals to strongly enhance the NMR signal of a compound under investigation. At the same time, the radicals shorten significantly its nuclear spin relaxation times which reduces the time window available for the experiments. Radical scavenging can overcome this drawback. Our work presents a detailed study of the reduction of the TEMPOL radical by ascorbic acid in solution by high-resolution NMR. Carbon-13 and hydrogen-1 nuclear spin relaxations are confirmed to be restored to their values without TEMPOL. Reaction mechanism, kinetics, and the influence of pD and viscosity are thoroughly discussed. The detailed investigation conducted in this work should help with choosing suitable concentrations in the samples for dynamic nuclear polarization and optimizing the measurement protocols.

Rapid measurement of longitudinal nuclear spin relaxation time based on the in-situ longitudinal magnetometer in Rb–Xe comagnetometers

The longitudinal relaxation time T1 of Xe nuclear spins is an important parameter to determine the performance of Rb–Xe comagnetometers. The industry standard delayed pulse method is time-consuming, and requires multiple π pulses and π/2 pulses. In this work, a direct measurement method for obtaining T1 based on the in-situ longitudinal magnetometer is introduced with the measurement period shortened to 3-5 T1. The proposed method requires only a single π pulse, reducing the measurement time by approximately more than 90%. By solving the Bloch equation, the working principle of the longitudinal magnetometer and the measurement method is demonstrated. The experiment shows that the longitudinal magnetometer achieves a linear working range greater than 60 nT, and the difference in measurement results of T1 through different methods at different temperatures and pump beam power is less than 3%. In addition, the longitudinal magnetometer introduced in this method has broad prospects for measuring the basic physical quantities in comagnetometers.

Structural Evolution of Paramagnetic Lanthanide Compounds in Solution Compared to Time- and Ensemble-Average Structures.

Anisotropy in the magnetic susceptibility strongly influences the paramagnetic shifts seen in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) experiments. A previous study on a series of C3-symmetric prototype MRI contrast agents showed that their magnetic anisotropy was highly sensitive to changes in molecular geometry and concluded that changes in the average angle between the lanthanide-oxygen (Ln-O) bonds and the molecular C3 axis due to solvent interactions had a significant impact on the magnetic anisotropy and, consequently, the paramagnetic shift. However, this study, like many others, was predicated on an idealized C3-symmetric structural model, which may not be representative of the dynamic structure in solution at the single-molecule level. Here, we address this by using ab initio molecular dynamics simulations to simulate how the molecular geometry, in particular the angles between the Ln-O bonds and the pseudo-C3 axis, evolves over time in the solution, mimicking typical experimental conditions. We observe large-amplitude oscillations in the O-Ln-C̃3 angles, and complete active space self-consistent field spin-orbit calculations show that this leads to similarly large oscillations in the pseudocontact (dipolar) paramagnetic NMR shifts. The time-averaged shifts show good agreement with experimental measurements, while the large fluctuations suggest that an idealized structure provides an incomplete description of the solution dynamics. Our observations have significant implications for modeling the electronic and nuclear relaxation times in this and other systems where the magnetic susceptibility is exquisitely sensitive to the molecular structure.

Nuclear Spin Relaxation in Cold Atom-Molecule Collisions.

We explore the quantum dynamics of nuclear spin relaxation in cold collisions of 1Σ+ molecules with structureless atoms in an external magnetic field. To this end, we develop a rigorous coupled-channel methodology, which accounts for rotational and nuclear spin degrees of freedom of 1Σ+ molecules and their interaction with an external magnetic field as well as anisotropic atom-molecule interactions. We apply the methodology to study the collisional relaxation of the nuclear spin sublevels of 13CO molecules immersed in a cold buffer gas of 4He atoms. We find that nuclear spin relaxation in the ground rotational manifold (N = 0) of 13CO occurs extremely slowly due to the absence of direct couplings between the nuclear spin sublevels. The rates of collisional transitions between the rotationally excited (N = 1) nuclear spin states of 13CO are generally much higher due to the direct nuclear spin-rotation coupling between the states. These transitions obey selection rules, which depend on the values of space-fixed projections of rotational and nuclear spin angular momenta (MN and MI) for the initial and final molecular states. For some initial states, we also observe a strong magnetic field dependence, which can be understood by using the first Born approximation. We use our calculated nuclear spin relaxation rates to investigate the thermalization of a single nuclear spin state of 13CO(N = 0) immersed in a cold buffer gas of 4He. The calculated nuclear spin relaxation times (T1 ≃ 1 s at T = 1 K at a He density of 10-14 cm-3) display a steep temperature dependence decreasing rapidly at elevated temperatures due to the increased population of rotationally excited states, which undergo nuclear spin relaxation at a much faster rate. Thus, long relaxation times of N = 0 nuclear spin states in cold collisions with buffer gas atoms can be maintained only at sufficiently low temperatures (kBT ≪ 2Be), where Be is the rotational constant.

Elucidation of Liquid Structures and Transport Properties of Highly Concentrated LiN(SO2F)2/Ethylene Carbonate Electrolytes

The Li+ conduction mechanism in highly concentrated ethylene carbonate (EC) solution of lithium bis(fluorosulfonyl)amide (LiFSA) was studied based on the measurements of thermal properties, density, viscosity, ionic conductivity, self-diffusion coefficients, Raman spectra, and proton nuclear magnetic resonance relaxation time. The compositions of EC and LiFSA were varied from dilute to concentrated states ([EC]/[LiFSA] = 9.2–0.80). For compositions of [EC]/[LiFSA] ≤ 1.5, the free solvents decrease, and contact ion pairs and aggregates become dominant, causing a significant change in the transport properties. Faster diffusion of Li+ compared with solvent (Li+ hopping conduction) was observed when the molar composition of [EC]/[LiFSA] ≤ 1.0 as in the case of sulfolane.

Evaluating Pore Structure of Shale Reservoirs through Pore Space Multifractal Characterization

Characteristics of pore structure are crucial to oil exploration and development in shale reservoirs, but practical evaluation of shale pore structure remains a longstanding challenge. In this study, based on nuclear magnetic resonance transverse relaxation time (T2) spectra under water-saturated and irreducible conditions, total pore space and moveable fluid space were skillfully determined. Meanwhile, multifractal spectra were used to characterize T2 distributions of total pore space and movable fluid space. The results show characteristic parameters of high probability measure areas of multifractal spectra (multifractal parameters) have strong negative correlations with T2 geometric mean value, and average radii of pore and throat increase with a decrease in the multifractal parameters, indicating the shale reservoirs with low values of the multifractal parameters may have good pore structure. Moreover, clay mineral content has strong positive effects on the multifractal parameters. In contrast, (quartz+feldspar) mineral content negatively influences on the multifractal parameters. Finally, new accurate models for permeability estimation in core scale and well-logging scale of shales are both proposed based on the multifractal parameters, which are also applicable for the other reservoirs. Overall, this study is important and timely for evaluating pore structure and predicting permeability in shale reservoirs.

Mechanistic origins of methyl-driven Overhauser DNP.

The Overhauser effect in the dynamic nuclear polarization (DNP) of non-conducting solids has drawn much attention due to the potential for efficient high-field DNP as well as a general interest in the underlying principles that enable the Overhauser effect in small molecules. We recently reported the observation of 1H and 2H Overhauser effects in H3C- or D3C-functionalized Blatter radical analogs, which we presumed to be caused by methyl rotation. In this work, we look at the mechanism for methyl-driven Overhauser DNP in greater detail, considering methyl librations and tunneling in addition to classical rotation. We predict the temperature dependence of these mechanisms using density functional theory and spin dynamics simulations. Comparisons with results from ultralow-temperature magic angle spinning-DNP experiments revealed that cross-relaxation at temperatures above 60K originates from both libration and rotation, while librations dominate at lower temperatures. Due to the zero-point vibrational nature of these motions, they are not quenched by very low temperatures, and methyl-driven Overhauser DNP is expected to increase in efficiency down to 0K, predominantly due to increases in nuclear relaxation times.

Synthesis of Manganese Zinc Ferrite Nanoparticles in Medical-Grade Silicone for MRI Applications.

The aim of this project is to fabricate hydrogen-rich silicone doped with magnetic nanoparticles for use as a temperature change indicator in magnetic resonance imaging-guided (MRIg) thermal ablations. To avoid clustering, the particles of mixed MnZn ferrite were synthesized directly in a medical-grade silicone polymer solution. The particles were characterized by transmission electron microscopy, powder X-ray diffraction, soft X-ray absorption spectroscopy, vibrating sample magnetometry, temperature-dependent nuclear magnetic resonance relaxometry (20 °C to 60 °C, at 3.0 T), and magnetic resonance imaging (at 3.0 T). Synthesized nanoparticles were the size of 4.4 nm ± 2.1 nm and exhibited superparamagnetic behavior. Bulk silicone material showed a good shape stability within the study's temperature range. Embedded nanoparticles did not influence spin-lattice relaxation, but they shorten the longer component of spin-spin nuclear relaxation times of silicone's protons. However, these protons exhibited an extremely high r2* relaxivity (above 1200 L s-1 mmol-1) due to the presence of particles, with a moderate decrease in the magnetization with temperature. With an increased temperature decrease of r2*, this ferro-silicone can be potentially used as a temperature indicator in high-temperature MRIg ablations (40 °C to 60 °C).

Adsorbate/adsorbent interactions in microporous zeolites: mechanistic insights from NMR relaxation and DFT calculations

Nuclear magnetic resonance (NMR) relaxation is an effective and non-invasive technique for probing guest-host interactions in porous materials. In particular, the ratio of longitudinal-to-transverse nuclear spin relaxation time constants T1/T2 has been demonstrated as a robust indicator of adsorbate/adsorbent interactions in mesoporous media. However, the use of NMR relaxation times in microporous materials to probe interactions and dynamics remains relatively unexplored. Herein, we investigate and describe the effect of the aluminium content in microporous HZSM-5 zeolites on the NMR relaxation times of a range of common liquid probe molecules. In particular, we discuss the NMR relaxation time behavior of liquids with hydrophilic (water, methanol) and hydrophobic (toluene, methyl cyclohexane) properties adsorbed over HZSM-5 samples with varying silica-to-alumina ratios (SAR = SiO2/Al2O3). Our results demonstrate that highly polar molecules show high sensitivity to aluminium content (i.e., surface acidity), with T1/T2 ratios increasing significantly for higher acidity zeolites. Conversely, for molecules with low polarity, the T1/T2 ratio as a function of SAR remains approximately constant, and in the zeolites with low SAR is much lower compared to that of water and methanol. Density functional theory (DFT) calculations are employed to contrast the surface interaction mechanisms of water and toluene within model zeolite structures of varying SAR, and provide molecular level insights into the observed trends in NMR relaxation behavior.

The Effect of Disorder on Endogenous MAS-DNP: Study of Silicate Glasses and Crystals.

In dynamic nuclear polarization nuclear magnetic resonance (DNP-NMR) experiments, the large Boltzmann polarization of unpaired electrons is transferred to surrounding nuclei, leading to a significant increase in the sensitivity of the NMR signal. In order to obtain large polarization gains in the bulk of inorganic samples, paramagnetic metal ions are introduced as minor dopants acting as polarizing agents. While this approach has been shown to be very efficient in crystalline inorganic oxides, significantly lower enhancements have been reported when applying this approach to oxide glasses. In order to rationalize the origin of the difference in the efficiency of DNP in amorphous and crystalline inorganic matrices, we performed a detailed comparison in terms of their magnetic resonance properties. To diminish differences in the DNP performance arising from distinct nuclear interactions, glass and crystal systems of similar compositions were chosen, Li2OCaO·2SiO2 and Li2CaSiO4, respectively. Using Gd(III) as polarizing agent, DNP provided signal enhancements in the range of 100 for the crystalline sample, while only up to around factor 5 in the glass, for both 6Li and 29Si nuclei. We find that the drop in enhancement in glasses can be attributed to three main factors: shorter nuclear and electron relaxation times as well as the dielectric properties of glass and crystal. The amorphous nature of the glass sample is responsible for a high dielectric loss, leading to efficient microwave absorption and consequently lower effective microwave power and an increase in sample temperature which leads to further reduction of the electron relaxation time. These results help rationalize the observed sensitivity enhancements and provide guidance in identifying materials that could benefit from the DNP approach.

The mechanism of using NMR for cancer detection.

Today, a most powerful technique to detect cancer development in patient is the use of the MRI (Magnetic Resonance imaging) technology. Thus, it is important to understand how NMR measurements can differentiate normal cells from pre-neoplastic cells and cancer cells. In this talk, we will first review the history of the development of the NMR technique for cancer detection. Then, we will discuss the physical mechanism that allows the NMR measurement to distinguish the normal cells, the pre-cancer cells and cancer cells. One key point is that, the contrast of MRI image was found to be based on difference in nuclear relaxation times (T1 and T2) rather than based on the concentration difference of cellular water. At present, there is still an active interest to understand the mechanisms behind relaxation time changes of water protons during cancer development. In this talk, I will review several possible physical mechanisms that could explain the relaxation time changes between normal cells and cancer cells.

Functionalized Controlled Porous Glasses for Producing Radical-Free Hyperpolarized Liquids by Overhauser DNP

Overhauser dynamic nuclear polarization (ODNP) can be used as a tool for NMR signal enhancement and happens on very short time scales. Therefore, ODNP is well suited for the measurement of fast-flowing samples, even in compact magnets, which is beneficial for the real-time monitoring of chemical reactions or processes. ODNP requires the presence of unpaired electrons in the sample, which is usually accomplished by the addition of stable radicals. However, radicals affect the nuclear relaxation times and can hamper the NMR detection. This is circumvented by immobilizing radicals in a packed bed allowing for the measurement of radical-free samples when using ex situ DNP techniques (DNP build-up and NMR detection happen at different places) and flow-induced separation of the hyperpolarized liquid from the radicals. Therefore, the synthesis of robust and chemically inert immobilized radical matrices is mandatory. In the present work, this is accomplished by immobilizing the radical glycidyloxy-tetramethylpiperidinyloxyl with a polyethyleneimine (PEI) linker on the surface of controlled porous glasses (CPG). Both the porosity of the CPGs and also the size of the PEI-linker were varied, resulting in a set of distinct radical matrices for continuous-flow ODNP. The study shows that CPGs with PEI-linkers provide robust, inert and efficient ODNP matrices.

Measurement of longitudinal nuclear spin relaxation time in NMR gyroscope by real-time monitoring

In the atomic cell of a nuclear magnetic resonance gyroscope, a transverse bias field is applied to enable the in situ alkali magnetometer to sense the variations in the longitudinal magnetic field. During a single relaxation process of the noble gas, the longitudinal relaxation time is obtained by monitoring and fitting the signal of the magnetometer in real-time. The relaxation times measured using our method are nearly identical to those obtained using the conventional delayed pulse method, but in contrast, our method saves ∼90% of the time. By using the new method, fast and accurate batch testing for a large number of atomic cells can be achieved to optimize the manufacturing of the cells, which is helpful in speeding up the development of gyroscopes.

Impact of storage time and temperature of salad heads on the quality of fresh-cut Cichorium endivia

The impact of temperature and storage time of escarole salad heads on the quality of processed, fresh-cut salad was investigated. Based on temperatures recorded in a fresh-cut processing plant, salad heads were stored for four different storage times (0, 5, 9, and 12 d) at four temperatures (4, 7, 10, and 12 °C) before being processed. A range of quality criteria (technical yield, global visual aspect, pink cut surfaces, mechanical texture, aerobic microflora, respiration rate, atmosphere composition in fresh-cut salad pouches, nuclear magnetic resonance transverse relaxation time) were measured on processing day, after 7 d storage at 4 °C of the fresh-cut salad packaged in pouches, and 1 d after pouch opening. The results are presented for salads grown in southeast France and early-season harvest, validated by experimental repetitions with a late-season harvest and salads from another geographical origin. Storage of salad heads caused deterioration of all quality attributes except for total aerobic bacteria. The global visual aspect was the most sensitive to changes in storage conditions of salad heads (significant reduction in quality for 5 d over 7 °C). In contrast, mechanical texture (maximum load for the shear test) was only significantly different for the fresh-cut salad prepared from salad heads stored for 9 d at 12 °C. For all quality criteria, fresh-cut salads processed from salad heads stored for 5 d at 4 and 7 °C were not significantly different from those of non-stored salad heads. For storage time of 5 d or less, a temperature of 7 °C is likely as good as 4 °C for escarole salad-head storage intended for fresh-cut processing.

Physicochemical and Morphological Study of the Saccharomyces cerevisiae Cell-Based Microcapsules with Novel Cold-Pressed Oil Blends

Vegetable oils rich in polyunsaturated fatty acids are a valuable component of the human diet. Properly composed oil blends are characterized by a 5:1 ratio of ω6/ω3 fatty acids, which is favorable from a nutritional point of view. Unfortunately, their composition makes them difficult to use in food production, as they are susceptible to oxidation and are often characterized by a strong smell. Encapsulation in yeast cells is a possible solution to these problems. This paper is a report on the use of native and autolyzed yeast in the encapsulation of oils. The fatty acid profile, encapsulation efficiency, morphology of the capsules obtained, and thermal behavior were assessed. Fourier transform infrared analysis and low-field nuclear magnetic resonance relaxation time measurements were also performed. The process of yeast autolysis changed the structure of the yeast cell membranes and improved the loading capacity. Lower encapsulation yield was recorded for capsules made from native yeast; the autolysis process significantly increased the value of this parameter. It was observed that NY-based YBMCs are characterized by a high degree of aggregation, which may adversely affect their stability. The average size of the AY capsules for each of the three oil blends was two times smaller than the NY-based capsules. The encapsulation of oils in yeast cells, especially those subjected to the autolysis process, ensured better oxidative stability, as determined by DSC, compared to fresh blends of vegetable oils. From LF NMR analysis of the relaxation times, it was shown that the encapsulation process affects both spin-lattice T1 and spin-spin T2* relaxation times. The T1 time values of the YBMCs decreased relative to the yeast empty cells, and the T2* time was significantly extended. On the basis of the obtained results, it has been proven that highly unsaturated oils can be used as an ingredient in the preparation of functional food via protection through yeast cell encapsulation.

Prediction of 1H Singlet Relaxation via Intermolecular Dipolar Couplings Using the Molecular Dynamics Method.

Dissolution dynamic nuclear polarization has been applied in various fields, including chemistry, biology, and medical science. To expand the scope of these applications, the nuclear singlet state, which is decoherence-free against dipolar relaxation between spin pairs, has been studied experimentally, theoretically, and numerically. The singlet state composed of proton spins is used in several applications, such as enhanced polarization preservation, molecular tagging to probe slow dynamic processes, and detection of ligand-protein complexes. In this study, we predict the lifetimes of the nuclear spin states composed of proton spin pairs using the molecular dynamics method and quantum chemistry simulations. We consider intramolecular dipolar, intermolecular dipolar between solvent and solute, chemical shift anisotropy, and spin-rotation interactions. In particular, the relaxation rate of intermolecular dipolar interactions is calculated using the molecular dynamics method for various solvents. The calculated values and the experimental values are of the same order of magnitude. Our program would provide insight into the molecular design of several NMR applications and would be helpful in predicting the nuclear spin relaxation time of synthetic molecules in advance.