Low-field Nuclear Magnetic Resonance Spectroscopy Research Articles

Low-field, not low quality: 1D simplification, selective detection, and heteronuclear 2D experiments for improving low-field NMR spectroscopy of environmental and biological samples.

Understanding environmental change is challenging and requires molecular-level tools to explain the physicochemical phenomena behind complex processes. Nuclear magnetic resonance (NMR) spectroscopy is a key tool that provides information on both molecular structures and interactions but is underutilized in environmental research because standard "high-field" NMR is financially and physically inaccessible for many and can be overwhelming to those outside of disciplines that routinely use NMR. "Low-field" NMR is an accessible alternative but has reduced sensitivity and increased spectral overlap, which is especially problematic for natural, heterogeneous samples. Therefore, the goal of this study is to investigate and apply innovative experiments that could minimize these challenges and improve low-field NMR analysis of environmental and biological samples. Spectral simplification (JRES, PSYCHE, singlet-only, multiple quantum filters), selective detection (GEMSTONE, DREAMTIME), and heteronuclear (reverse and CH3/CH2/CH-only HSQCs) NMR experiments are tested on samples of increasing complexity (amino acids, spruce resin, and intact water fleas) at-high field (500 MHz) and at low-field (80 MHz). A novel experiment called Doubly Selective HSQC is also introduced, wherein 1H signals are selectively detected based on the 1H and 13C chemical shifts of 1H-13C J-coupled pairs. The most promising approaches identified are the selective techniques (namely for monitoring), and the reverse and CH3-only HSQCs. Findings ultimately demonstrate that low-field NMR holds great potential for biological and environmental research. The multitude of NMR experiments available makes NMR tailorable to nearly any research need, and low-field NMR is therefore anticipated to become a valuable and widely used analytical tool moving forward.

Ultra-stable fully-aromatic microporous polyamide membrane for molecular sieving of nitrogen over volatile organic compound

Microporous polymer membranes are promising candidates for industrial membrane-based gas separation because of their high separation performance. However, their relatively low stability due to the local rearrangement of polymer chains during usage remains a problem. Hence, we propose the construction of a fully aromatic polymer structure in a microporous polymer membrane to enhance membrane stability. Four triptycene-based microporous polyamides were synthesized via the polymerization of 2,6,14-triaminotriptycene with aromatic acyl chloride and/or aliphatic acyl chlorides. Their properties were characterized and compared by using nuclear magnetic resonance (NMR) and Brunauer–Emmett–Teller analyses. The synthesized polyamides were fabricated into composite membranes by employing a solution process; their stability was evaluated for the molecular sieving of nitrogen over volatile organic compounds such as cyclohexane. Low-field NMR and X-ray photoelectron spectroscopy were used to investigate the differences in the properties of membranes with different structures at different times. The results showed that the fully aromatic polyamide membrane made from 2,6,14-triaminotriptycene and aromatic acyl chloride displayed constant rejection (99 %) and nitrogen permeability (approximately 50 Barrer) for the molecular sieving of nitrogen over cyclohexane during 100-d experiments, indicating good stability. This approach paves the way for the industrialization of microporous polymer membranes from a theoretical perspective.

Enhancement Mechanism of Minerals on Lignite-Water Molecule Interactions.

Five coal samples were prepared by deashing Shengli lignite in distinct phases, which consisted of residual ash from spontaneous combustion. The effects of removal and introduction of inherent minerals on the water reabsorption performance of coal samples were systematically investigated in three aspects: pore structure, oxygen-containing functional groups, and lignite materials. Low-field nuclear magnetic resonance spectroscopy was employed to investigate the changes in the water molecular adsorption tendency of coal samples with the variation in the mineral content. The study elucidates that the hygroscopic performance of the coal samples is significantly reduced due to the massive removal of inherent minerals. However, the pore structure of the coal samples after HCl/HF washing becomes more developed, and the oxygen-containing functional groups on the surface are more exposed, leading to an increase in the equilibrium adsorbed moisture content (EMC) of the coal samples. The binding force between coal samples and water molecules is reduced by the removal of the inherent minerals, which weakens the interaction forces between lignite and water molecules. The oxygen-containing functional groups on the surface of lignite interact with the residual ash from spontaneous combustion to enhance the binding force between lignite and surface water molecules, thus leading to the improved tendency of lignite to adsorb water molecules. The formation of intermediate complexes between minerals and oxygen-containing functional groups, in particular, carboxyl functional groups, on the surface of lignite enhances the acting force of polar sites, which improves the interaction of lignite-water molecules.

Relationship among gelatinization, retrogradation behavior, and impedance characteristics of potato starch

In this study, the physicochemical properties of potato starch from different varieties were investigated. Furthermore, the relationships among gelatinization, retrogradation behavior, and impedance characteristics of potato starch gels were evaluated by texture analysis, low-field nuclear magnetic resonance spectroscopy, and electrical impedance spectroscopy. The results indicated amylose content was positively correlated with setback viscosity, and negatively correlated with To and ΔH. In addition, impedance values of potato starch gels differed in a frequency-dependent manner. Notably, higher frequencies resulted in low diffusion of ions in prepared gels, which combined with the concentration of mobile ions in free water, led to a gradual decrease in impedance module. Compared with phase values, impedance module showed high correlation with gelatinization parameters (To, Tp, and Tc) and viscosity parameters (peak temperature and setback viscosity), more notably at frequencies below 100 Hz. In this context, the electric current flowed through mobile ions that interacted with bound water attached to the starch molecules at lower voltage frequencies, and were repressed by the formation of an ordered and compact gel network during retrogradation. Collectively, these results indicate that impedance spectroscopy can be potentially used as an efficient and reliable method to predict gelatinization and retrogradation behavior of potato starch.

Synthesis and characterization of cellulose nanocrystal-Fe composite nanoparticles and their digestion behavior in simulated gastric fluid

Cellulose nanocrystals (CNC) exhibit great potential as a food emulsifier or functional material template. Herein, CNC-Fe nanoparticles were successfully prepared via an in situ chemical reduction approach. Zeta potential measurements, low-field nuclear magnetic resonance spectroscopy, and atomic force microscopy showed that Fe(III) ions were adsorbed onto CNC when FeCl3 was added to a CNC dispersion. Micromorphological analysis revealed small (diameter = 10.0 ± 2.4 nm) spherical nanoparticles synthesized on the surface of aggregated CNC after the reduction of the Fe(III) ions. Fourier transform infrared spectroscopy revealed an intense peak at 779 cm−1 in the CNC-Fe nanoparticles, which was attributed to FeO stretching vibrations. X-ray photoelectron spectroscopy indicated that the valence state of Fe in CNC-Fe nanoparticles was predominantly ferrous. The synthesized CNC-Fe nanoparticles demonstrated excellent colloidal stability in a dispersion for 21 d and complete, rapid, and spontaneous dissolution in vitro simulated gastric fluid. Our results highlight the potential use of CNC as a template for loading Fe into nanoparticles for Fe fortification in food.

Wet mechanical grinding regulates the micro-nano interfaces and structure of MXene/PVA composite for enhanced mechanical properties and thermal conductivity

The applications of MXene/polymer-based functional materials have been widely reported, while the understanding of the structural evolution of MXene/polymer composites during combination is still limited. Herein, mechanical grinding is applied for the delamination and dispersion of multilayer MXene particles in PVA aqueous solution, during which they are gradually delaminated into nanosheets. Low-field nuclear magnetic resonance spectroscopy and rheological properties analysis demonstrate that the specific surface area and exposed functional groups of MXene sheets are drastically increased, which greatly contributes to the construction of MXene-PVA interfacial interactions and the enhancement of gel networks. Furthermore, the exfoliated MXene nanosheets are uniformly distributed in the resultant MXene/PVA composite films, which greatly facilitates the stress transfer and the transportation of electrons/phonons, leading to significantly improved mechanical properties and electrical/thermal conductivities. This work provides fruitful insights into the optimized fabrication of MXene/polymer composites.

Parameter Visualization of Benchtop Nuclear Magnetic Resonance Spectra toward Food Process Monitoring

Low-cost and user-friendly benchtop low-field nuclear magnetic resonance (NMR) spectrometers are typically used to monitor food processes in the food industry. Because of excessive spectral overlap, it is difficult to characterize food mixtures using low-field NMR spectroscopy. In addition, for standard compounds, low-field benchtop NMR data are typically unavailable compared to high-field NMR data, which have been accumulated and are reusable in public databases. This work focused on NMR parameter visualization of the chemical structure and mobility of mixtures and the use of high-field NMR data to analyze benchtop NMR data to characterize food process samples. We developed a tool to easily process benchtop NMR data and obtain chemical shifts and T2 relaxation times of peaks, as well as transform high-field NMR data into low-field NMR data. Line broadening and time–frequency analysis methods were adopted for data processing. This tool can visualize NMR parameters to characterize changes in the components and mobilities of food process samples using benchtop NMR data. In addition, assignment errors were smaller when the spectra of standard compounds were identified by transferring the high-field NMR data to low-field NMR data rather than directly using experimentally obtained low-field NMR spectra.

Effect of pearling on composition, microstructure, water migration and cooking quality of highland barley (Hordeum vulgare var. Coeleste Linnaeus)

Highland barley (HB) has become popular due to its nutritional benefits, and pearling as a necessary process that could broaden its applications. The influence of pearling on the composition, microstructure, water migration and cooking characteristics of HB was investigated. With different degrees of pearling (DOPs), the levels of nutritious components except for starch and β-glucan, decreased, and the β-glucan content gradually increased and reached a plateau when the DOP was over 15%. Pearling can significantly shorten the cooking time by removing different tissues, and removal of the pericarp layer had significant influence by reducing the cooking time by 17 min (i.e., HB with DOP 5%). From the result of Peleg model fitting and low-field nuclear magnetic resonance spectroscopy, the saturated water absorption of HB did not differ for a DOP 5% and above, and cooking mainly increased the amount of moderately bound water and added small amount of bound water.

Characterization of chitosan/zein composite film combined with tea polyphenol and its application on postharvest quality improvement of mushroom (Lyophyllum decastes Sing.)

In this study, chitosan/zein composite film combined with tea polyphenol was fabricated, and the effect of tea polyphenol on the structure and physical properties of chitosan/zein film was investigated. Low-field nuclear magnetic resonance spectroscopy indicated that the distribution and content of water molecules in chitosan/zein film were changed with the addition of tea polyphenol. The results of fluorescence measurement and secondary structure analysis proved the enhanced physical and structural properties of tea polyphenol composite film. And the film showed favorable stretching resistance and flexibility. Furthermore, the effect of composite film on quality and physiological indexes of mushroom (Lyophyllum decastes Sing.) stored at 4 °C for 15 d were systematically evaluated. The results indicated that the tea polyphenol composite film showed the effectiveness of maintaining color, texture as well as integrity of cell membrane evidenced by relatively lower membrane permeability and malondialdehyde content of mushrooms. Besides, weight loss and respiration rate were efficiently retarded. In comparison with control and chitosan/zein film, the tea polyphenol composite film inhibited the activity of polyphenol oxidase and peroxidase, while enhanced the activity of antioxidant enzymes (superoxide dismutase and catalase). In a word, the chitosan/zein composite film incorporated with tea polyphenol showed the potential as an effective technology for maintaining the postharvest quality of mushroom (Lyophyllum decastes Sing.).

Synergistic interactions between konjac glucomannan and welan gum mixtures

This research presents the physicochemical property of the mixtures induced by blending konjac glucomannan (KGM) and welan gum (WL). The effects of the ratio and concentration on the rheological properties of the KGM and WL mixtures, such as shear rate and strain (large-amplitude oscillatory shear, frequency, temperature, time, stress relaxation, and creep), were studied. The combination analysis of Fourier transform infrared spectroscopy, scanning electron microscopy, thermodynamic differential scanning calorimetry, water mobility, and distribution property analyses through low-field nuclear magnetic resonance spectroscopy were used to determine the authenticity of gels. Finally, a model of the interaction force was constructed to elucidate the interaction mechanism when heating. The result showed that the mixtures of KGM and WL were pseudogel. The synergy effect was positively correlated with concentration, which was the strongest when the KGM (g/kg): WL (g/kg) ratio was 2:8. Abnormal temperature sensitivity (sudden drop of modulus around 30 °C) was observed at the ratio, and the mixture owned good temperature reversibility in 3 cycles. This work expanded the application scope of KGM and WL and provides a method for the determination of gels.

Effect of modifiers on bending recovery of fully degradable brush wire monofilament

In this paper, the effects of three modifiers, such as PEG, ATBC, and DOP, on the modification of PBAT/PLA-based fully degradable brush monofilaments were studied. The microstructure and the mechanical properties of the PBAT/PLA brush wire before and after modification were characterized by Fourier-transform infrared spectroscopy, scanning electron microscope, and universal material testing machine. The effect of modifiers on bending recovery was discussed in detail and was analyzed combined with the results of differential scanning calorimetry and low-field nuclear magnetic resonance spectroscopy. The results showed that with an increase in the content of modifiers, the tensile strength and the impact strength of the PBAT/PLA brush wire would decrease, and the bending recovery rate would increase. The effect of PEG and ATBC on bending recovery is weaker than that of DOP, and the bending recovery rate of the PBAT/PLA brush wire modified by DOP is close to 80%. With an increase in the content of modifiers, the melting enthalpy and transverse relaxation time T2 of PBAT/PLA would increase. This research has guiding significance for the preparation of fully degradable brush wires.

Rapid Identification of Adulteration in Edible Vegetable Oils Based on Low-Field Nuclear Magnetic Resonance Relaxation Fingerprints.

Most current approaches applied for the essential identification of adulteration in edible vegetable oils are of limited practical benefit because they require long analysis times, professional training, and costly instrumentation. The present work addresses this issue by developing a novel simple, accurate, and rapid identification approach based on the magnetic resonance relaxation fingerprints obtained from low-field nuclear magnetic resonance spectroscopy measurements of edible vegetable oils. The relaxation fingerprints obtained for six types of edible vegetable oil, including flaxseed oil, olive oil, soybean oil, corn oil, peanut oil, and sunflower oil, are demonstrated to have sufficiently unique characteristics to enable the identification of the individual types of oil in a sample. By using principal component analysis, three characteristic regions in the fingerprints were screened out to create a novel three-dimensional characteristic coordination system for oil discrimination and adulteration identification. Univariate analysis and partial least squares regression were used to successfully quantify the oil adulteration in adulterated binary oil samples, indicating the great potential of the present approach on both identification and quantification of edible oil adulteration.

Polysaccharide-based hydrogels crosslink density equation: A rheological and LF-NMR study of polymer-polymer interactions

A simple relation between pendant groups of polymers in hydrogels is introduced to determine the crosslink density of (complex) hydrogel systems (mixtures of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) modified nanocellulose, alginate, scleroglucan and Laponite in addition of crosslinking agents). Furthermore, the rheological properties and their great potential connection to design complex hydrogel systems with desired properties have been thoroughly investigated. Hydrogel structures governing internal friction and flow resistance were described by the predominant effect of ionic, hydrogen, and electrostatic interactions. The relationship between rheological properties and polymer-polymer interactions in the hydrogel network is explained and expressed in a new mathematical model for determining the crosslink density of (crosslinked) hydrogels based on single or mixture of polymer systems. In the end, the combined used of rheology and low field nuclear magnetic resonance spectroscopy (LF-NMR) for the characterization of hydrogel networks is developed.

Molecular insights into the loss of phytosterols during the neutralisation of corn oil

Herein, the physical migration and chemical conversion of phytosterols during the alkali neutralisation process of corn oil were investigated with molecular insights. A significant decrease in the phytosterol content was attributed to the migration from oil to the soapstock, along with slight chemical conversions between free phytosterols and phytosteryl esters in the heterogeneous system. Moreover, the migration of phytosterols was positively correlated with the water content in a model system through low-field nuclear magnetic resonance spectroscopy, which was attributed to the hydrogen bonding between free phytosterols and water. Compared to sodium oleate and sodium linoleate, sodium palmitate caused a higher loss of phytosterols due to its low solubility. X-ray diffraction analysis confirmed the formation of a bilayer structure of the sodium salt of fatty acids, suggesting the aggregation of the sodium salt of fatty acids and phytosterols at neutralisation temperatures. Small-angle X-ray scattering analysis suggested that the concentration of the NaOH solution affected the structure of sodium oleate and the area of the polar head base, which was associated with the loss of free phytosterols. The study presents an important fundamental scientific basis for the retention of phytosterols toward the development of more healthy and nutritious edible oils.

Effects of temperature rising inhibitor and shrinkage reducing admixture on pore structure of early-age cement paste

High-strength and high-performance concrete (HSHPC) is frequently used in practical engineering, but HSHPC tends to crack at an early age because of the large temperature drop (temperature shrinkage) and autogenous contraction. Therefore, a temperature rising inhibitor (TRI) and a shrinkage reducing admixture (SRA) are adopted to improve the properties of HSHPC. The macroproperties of concrete are closely linked to its pore structure. In this paper, the effects of the water-to-cement ratio (w/c) and TRI and SRA contents on the pore size distribution (PSD) and porosity of early-age cement paste were tested by low-field nuclear magnetic resonance spectroscopy (LF-NMR). The test results showed that the PSDs of samples generally contain two peaks. The most probable pore diameter (MPPD) increases with the w/c and reduces with the TRI content and age. However, the MPPD changes only slightly with SRA content. For the same SRA or TRI content, the MPPD changes in samples containing both SRA and TRI are similar to those of samples with TRI or SRA only. However, the combined use of SRA and TRI has an apparent influence on the PSDs of samples. The porosity becomes large with the w/c and SRA content but declines with TRI content. The porosities of samples with both SRA and TRI addition are significantly lower than those of samples without the SRA and/or TRI at 28 days. A modified two-peaks model, which is adapted from the Shimomura and Maekawa model, is developed for cement pastes with the TRI and/or SRA. The curves of the modified two-peaks model coincide with the experimental data.

Compact low-field NMR spectroscopy and chemometrics applied to the analysis of edible oils

Compact nuclear magnetic resonance (NMR) spectroscopy combined with chemometric tools opens new perspectives for NMR use. This work compares the potential of 43, 60 and 400 MHz NMR spectroscopy for quality control of edible oils. Partial least squares regression (PLSR) and support vector regression (SVR) models built on the three NMR devices had equivalent performances for fatty acids and iodine value, and the models built with the low field spectra were equivalent to the high field. Moreover, performances for calibration indicated that most of the models built with medium/or high-resolution fields presented reproducibility values lower than the minimum accepted by the American Oil Chemists' Society (AOCS). Compared to classical methods, this new approach allows the application of medium resolution devices as a sample screening tool in analytical laboratories since it allows the spectrum obtention in a few seconds, without the need for sample preparation or the use of deuterated solvents.

Impact of crosslinking on organic solvent nanofiltration performance in polydimethylsiloxane composite membrane: Probed by in-situ low-field nuclear magnetic resonance spectroscopy

Polydimethylsiloxane (PDMS) has been widely used in gas separation, pervaporation, and organic solvent nanofiltration. The crosslinking and penetration behavior of PDMS has been of great importance for fabricating composite membranes. In this study, an in-situ low-field nuclear magnetic resonance (LF-NMR) spectroscopy was employed to probe the crosslink density of PDMS during the crosslinking process. The relationship between crosslink density and nanostructure of PDMS selective layer was systematically investigated. It was found when the PDMS network had low crosslink density (υe < 1.32 × 10-3 mol cm-3), the un-crosslinked PDMS chains could easily infiltrate into the pores of the substrate, leading to a defective active layer. Therefore, the rejection for Evans blue from organic solvents was below 3.9%. With the increasing crosslink density to 1.57 × 10-3 mol cm-3, the integrity of the PDMS selective layer was remarkably reinforced with a rejection of 99.0%. Moreover, the as-prepared membrane exhibited good performance for concentrating trace vitamin B12 from different solvents. The swelling property was responsive monitored with T2 relaxation time using LF-NMR.

Effectiveness of ultrasound‐assisted immersion thawing on the thawing rate and physicochemical properties of chicken breast muscle

This study evaluated the effect of air thawing (AT), water thawing (WT), and ultrasound-assisted immersion thawing (UT) at different power levels (200, 300, 400, and 500 W) on the thawing rate, physicochemical properties, and protein structure of chicken breast muscle (pectoralis), and the weight of each sample was approximately 106 ± 3g. UT shortened the total thawing time and decreased the cutting force with increasing ultrasound power. Additionally, UT at 300 W (UT-300) remarkably reduced the thawing and cooking losses of the samples compared to AT, WT, and other UT powers (p<0.05). Low-field nuclear magnetic resonance spectroscopy showed that the T21 and T22 of the UT-300 samples were shorter than those of the AT and WT samples (p<0.05), which revealed that UT-300 reduced the mobility and losses of both immobilized and free water. Moreover, UT-300 remarkably reduced the damage to the myofibrillar protein (MP) structure. Overall, with appropriate ultrasonic power, the thawing rate increased and changes in the MP structure were reduced. PRACTICAL APPLICATION: This study found that compared to that of AT samples, the thawing time of the UT-300 samples markedly decreased by 57%. In addition, UT-300 could reduce the damage to the myofibrillar protein structure, which was very beneficial for further processing of frozen foods.

The Effect of α-, β- and γ-Cyclodextrin on Wheat Dough and Bread Properties.

Cyclodextrins (CDs) are cyclic oligosaccharides that have found widespread application in numerous fields. CDs have revealed a number of various health benefits, making them potentially useful food supplements and nutraceuticals. In this study, the impact of α-, β-, and γ-CD at different concentrations (up to 8% of the flour weight) on the wheat dough and bread properties were investigated. The impact on dough properties was assessed by alveograph analysis, and it was found that especially β-CD affected the viscoelastic properties. This behavior correlates well with a direct interaction of the CDs with the proteins of the gluten network. The impact on bread volume and bread staling was also assessed. The bread volume was in general not significantly affected by the addition of up to 4% CD, except for 4% α-CD, which slightly increased the bread volume. Larger concentrations of CDs lead to decreasing bread volumes. Bread staling was investigated by texture analysis and low field nuclear magnetic resonance spectroscopy (LF-NMR) measurements, and no effect of the addition of CDs on the staling was observed. Up to 4% CD can, therefore, be added to wheat bread with only minor effects on the dough and bread properties.

Low-field nuclear magnetic resonance detection of changes in the water distribution in citric acid biosludge during dewatering

Activated sludge treatment of citric acid wastewater creates large quantities of residual biological sludge that is difficult to dewater and dispose of. In order to deepen the understanding of the sludge dewatering process, low-field nuclear magnetic resonance (NMR) spectroscopy was used to study changes in internal, vicinal, interstitial, and free water contents and binding degree during citric acid sludge dewatering. The effects of polyaluminum chloride (PAC), PAC and calcium oxide (PAC + CaO), and cationic polyacrylamide (CPAM) treatment and altering the doses of these coagulants were evaluated. Obvious changes were detected in the distribution of water between interstitial and free forms during sludge dewatering. The transverse relaxation time (T2) and water content obtained by low-field NMR spectroscopy indicated that CPAM played an important role in the sludge cake formation stage because of its flocculation. In this process, the vicinal, interstitial, and free water contents of the sludge decreased by 0.581 g/g-dry matter (DM), 6.695 g/g-DM, and 51.867 g/g-DM, respectively, under vacuum filtration (t = 2 min), that are much higher than other conditioning agents. In the sludge cake shrinkage stage with vacuum filtration for 8 min, PAC + CaO pretreatment gave a higher dewatering rate (10.783 g/g-DM) than CPAM (7.098 g/g-DM) because of the role of the skeleton structure. Overall, the treatment effects of these three conditioners were ranked in the order CPAM > PAC + CaO > PAC. Low-field NMR spectroscopy proved to be a convenient and reliable method to investigate changes in the water distribution in sludge during the treatment process.