Introduction to compact NMR: A review of methods

Polarization transfer methods for quantitative analysis of flowing mixtures with benchtop 13C NMR spectroscopy

Benchtop NMR spectroscopy is attractive for process monitoring; however, there are still drawbacks that often hamper its use, namely, the comparatively low spectral resolution in 1H NMR, as well as the low signal intensities and problems with the premagnetization of flowing samples in 13C NMR. We show here that all these problems can be overcome by using 1H-13C polarization transfer methods. Two ternary test mixtures (one with overlapping peaks in the 1H NMR spectrum and one with well-separated peaks, which was used as a reference) were studied with a 1 T benchtop NMR spectrometer using the polarization transfer sequence PENDANT (polarization enhancement that is nurtured during attached nucleus testing). The mixtures were analyzed quantitatively in stationary as well as in flow experiments by PENDANT enhanced 13C NMR experiments, and the results were compared with those from the gravimetric sample preparation and from standard 1H and 13C NMR spectroscopy. Furthermore, as a proxy for a process monitoring application, continuous dilution experiments were carried out, and the composition of the mixture was monitored in a flow setup by 13C NMR benchtop spectroscopy with PENDANT. The results demonstrate the high potential of polarization transfer methods for applications in quantitative process analysis with benchtop NMR instruments, in particular with flowing samples.

Fluorescence-Based Protein Stability Monitoring-A Review

Proteins are large biomolecules with a specific structure that is composed of one or more long amino acid chains. Correct protein structures are directly linked to their correct function, and many environmental factors can have either positive or negative effects on this structure. Thus, there is a clear need for methods enabling the study of proteins, their correct folding, and components affecting protein stability. There is a significant number of label-free methods to study protein stability. In this review, we provide a general overview of these methods, but the main focus is on fluorescence-based low-instrument and -expertise-demand techniques. Different aspects related to thermal shift assays (TSAs), also called differential scanning fluorimetry (DSF) or ThermoFluor, are introduced and compared to isothermal chemical denaturation (ICD). Finally, we discuss the challenges and comparative aspects related to these methods, as well as future opportunities and assay development directions.

Low-field NMR with multilayer Halbach magnet and NMR selective excitation

This study introduces a low-field NMR spectrometer (LF-NMR) featuring a multilayer Halbach magnet supported by a combined mechanical and electrical shimming system. This setup offers improved field homogeneity and sensitivity compared to spectrometers relying on typical Halbach and dipole magnets. The multilayer Halbach magnet was designed and assembled using three nested cylindrical magnets, with an additional inner Halbach layer that can be rotated for mechanical shimming. The coils and shim-kernel of the electrical shimming system were constructed and coated with layers of zirconia, thermal epoxy, and silver-paste resin to facilitate passive heat dissipation and ensure mechanical and thermal stability. Furthermore, the 7-channel shim coils were divided into two parts connected in parallel, resulting in a reduction of joule heating temperatures from 96.2 to 32.6 °C. Without the shimming system, the Halbach magnet exhibits a field inhomogeneity of approximately 140 ppm over the sample volume. The probehead was designed to incorporate a solenoidal mini coil, integrated into a single planar board. This design choice aimed to enhance sensitivity, minimize [Formula: see text] inhomogeneity, and reduce impedance discrepancies, transmission loss, and signal reflections. Consequently, the resulting linewidth of water within a 3 mm length and 2.4 mm inner diameter sample volume was 4.5 Hz. To demonstrate the effectiveness of spectral editing in LF-NMR applications at 29.934 MHz, we selectively excited hydroxyl and/or methyl protons in neat acetic acid using optimal control pulses calculated through the Krotov algorithm.

Low-Field Benchtop NMR to Discover Early-Onset Sepsis: A Proof of Concept

Low-field (LF) benchtop NMR is a new family of instruments available on the market, promising for fast metabolic fingerprinting and targeted quantification of specific metabolites despite a lack of sensitivity and resolution with respect to high-field (HF) instruments. In the present study, we evaluated the possibility to use the urinary metabolic fingerprint generated using a benchtop LF NMR instrument for an early detection of sepsis in preterm newborns, considering a cohort of neonates previously investigated by untargeted metabolomics based on Mass Spectrometry (MS). The classifier obtained behaved similarly to that based on MS, even if different classes of metabolites were taken into account. Indeed, investigating the regions of interest mainly related to the development of sepsis by a HF NMR instrument, we discovered a set of relevant metabolites associated to sepsis. The set included metabolites that were not detected by MS, but that were reported as relevant in other published studies. Moreover, a strong correlation between LF and HF NMR spectra was observed. The high reproducibility of the NMR spectra, the interpretability of the fingerprint in terms of metabolites and the ease of use make LF benchtop NMR instruments promising in discovering early-onset sepsis.

Successful combination of benchtop nuclear magnetic resonance spectroscopy and chemometric tools: A review

Low-field nuclear magnetic resonance (NMR) has three general modalities: spectroscopy, imaging, and relaxometry. In the last twelve years, the modality of spectroscopy, also known as benchtop NMR, compact NMR, or just low-field NMR, has undergone instrumental development due to new permanent magnetic materials and design. As a result, benchtop NMR has emerged as a powerful analytical tool for use in process analytical control (PAC). Nevertheless, the successful application of NMR devices as an analytical tool in several areas is intrinsically linked to its coupling with different chemometric methods. This review focuses on the evolution of benchtop NMR and chemometrics in chemical analysis, including applications in fuels, foods, pharmaceuticals, biochemicals, drugs, metabolomics, and polymers. The review also presents different low-resolution NMR methods for spectrum acquisition and chemometric techniques for calibration, classification, discrimination, data fusion, calibration transfer, multi-block and multi-way.

Imaging techniques for studying solid dosage formulation: Principles and applications

Classic methods for evaluating the disintegration and dissolution kinetics of solid dosage forms are no longer sufficient to meet the growing demands in the pharmaceutical field. Hence, scientists have turned to imaging techniques and computer technology to develop innovative visualization methods. These methods allow for a visual understanding of the disintegration or dissolution process and offer valuable insights into the drug release kinetics. This article aims to provide an overview of the commonly used imaging techniques and their applications in studying the disintegration or dissolution of solid dosage forms. Therefore, imaging presents a novel and alternative approach to understanding the mechanisms of disintegration and dissolution in the formulation study of solid dosages.

Operando benchtop NMR reveals reaction intermediates and crossover in redox flow batteries

Redox flow batteries (RFBs) provide a promising battery technology for grid-scale energy storage. High-field operando NMR analyses of RFBs have yielded useful insight into their working mechanisms and helped improve battery performance. Nevertheless, the high cost and large footprint of a high-field NMR system limit its implementation by a wider electrochemistry community. Here, we demonstrate an operando NMR study of an anthraquinone/ferrocyanide-based RFB on a low-cost and compact 43 MHz benchtop system. The chemical shifts induced by bulk magnetic susceptibility effects differ remarkably from those obtained in high-field NMR experiments, due to the different orientations of the sample relative to the external magnetic field. We apply Evans method to estimate the concentrations of paramagnetic anthraquinone radical and ferricyanide anions. The degradation of 2,6-dihydroxy-anthraquinone (DHAQ) to 2,6-dihydroxy-anthrone and 2,6-dihydroxy-anthranol has been quantified. We further identified the impurities commonly present in the DHAQ solution to be acetone, methanol and formamide. The crossover of DHAQ and impurity molecules through the sseparation Nafion® membrane was captured and quantified, and a negative correlation between the molecular size and crossover rate was established. We show that a benchtop NMR system has sufficient spectral and temporal resolution and sensitivity for the operando study of RFBs, and anticipate a broad application of operando benchtop NMR methods for studying flow electrochemistry targeted for different applications.

Recent advances in microfluidics-based bioNMR analysis

Nuclear magnetic resonance (NMR) has been used in a variety of fields due to its powerful analytical capability. To facilitate biochemical NMR (bioNMR) analysis for samples with a limited mass, a number of integrated systems have been developed by coupling microfluidics and NMR. However, there are few review papers that summarize the recent advances in the development of microfluidics-based NMR (μNMR) systems. Herein, we review the advancements in μNMR systems built on high-field commercial instruments and low-field compact platforms. Specifically, μNMR platforms with three types of typical microcoils settled in the high-field NMR instruments will be discussed, followed by summarizing compact NMR systems and their applications in biomedical point-of-care testing. Finally, a conclusion and future prospects in the field of μNMR were given.

Semi-Autonomic AI LF-NMR Sensor for Industrial Prediction of Edible Oil Oxidation Status

The evaluation of an oil's oxidation status during industrial production is highly important for monitoring the oil's purity and nutritional value during production, transportation, storage, and cooking. The oil and food industry is seeking a real-time, non-destructive, rapid, robust, and low-cost sensor for nutritional oil's material characterization. Towards this goal, a ¹H LF-NMR relaxation sensor application based on the chemical and structural profiling of non-oxidized and oxidized oils was developed. This study dealt with a relatively large-scale oil oxidation database, which included crude data of a ¹H LF-NMR relaxation curve, and its reconstruction into T₁ and T₂ spectral fingerprints, self-diffusion coefficient D, and conventional standard chemical test results. This study used a convolutional neural network (CNN) that was trained to classify T₂ relaxation curves into three ordinal classes representing three different oil oxidation levels (non-oxidized, partial oxidation, and high level of oxidation). Supervised learning was used on the T₂ signals paired with the ground-truth labels of oxidation values as per conventional chemical lab oxidation tests. The test data results (not used for training) show a high classification accuracy (95%). The proposed AI method integrates a large training set, an LF-NMR sensor, and a machine learning program that meets the requirements of the oil and food industry and can be further developed for other applications.

Quality Consistency of Herbal Products: Chemical Evaluation

The widespread utility of herbal products has been rising considerably worldwide, including both developed and developing countries, leading to the rapid growth of their availability in the United States and globally. This substantial increase in consumption of herbal products has witnessed the emergence of adverse effects upon oral administration of certain of these products, and thus has raised safety concerns. The adverse effects caused by the consumption of certain botanical medicines occur primarily as a result of the poor quality of plant raw materials or the finished products, which inherently may affect safety and/or efficacy. The poor quality of some herbal products can be attributed to a lack of proper quality assurance and quality control. A high demand for herbal products that surpasses production, combined with a desire for maximizing profits, along with a lack of rigorous quality control within some manufacturing facilities have led to the emergence of quality inconsistencies. The underlying causes for this involve the misidentification of plant species, or their substitution, adulteration, or contamination with harmful ingredients. Analytical assessments have revealed there to be frequent and significant compositional variations between marketed herbal products. The inconsistency of the quality of herbal products can be ascribed essentially to the inconsistency of the botanical raw material quality used to manufacture the products. Thus, the quality assurance and the quality control of the botanical raw materials is may contribute significantly to improving the quality and consistency of the quality of the end products. The current chapter focuses on the chemical evaluation of quality and consistency of herbal products, including botanical dietary supplements. Different techniques, instruments, applications, and methods used in identifying, quantifying, and generating chemical fingerprints and chemical profiles of the ingredients of the herbal products will be described. The strengths and weaknesses of the various techniques available will be addressed. Limitations of the other approaches including morphological or microscopic analysis and DNA-based analysis will be presented.

Polymer-Solvent Interactions in Modified Starches Pastes-Electrokinetic, Dynamic Light Scattering, Rheological and Low Field Nuclear Magnetic Resonance Approach

Starch paste is a very complex dispersion that cannot be clearly classified as a solution, colloid or suspension and many factors affects its properties. As these ambiguities constitute a barrier to technological development, the aim of this study was to investigate the interaction of starch macromolecules with water by analysing the results of rheological properties, low field nuclear magnetic resonance (LF NMR), dynamic light scattering (DLS) and ζ potential analyses. Starch pastes with a concentration of 1%, prepared with distilled water and buffered to pH values of 2.5, 7.0 and 9.5 were analysed. It was proved that the pH buffering substantially decreased the values of consistency index but the pH value itself was not significant. LF NMR studies indicated that the dissolution of starch in water resulted in a reduction in spin-lattice as well as spin-spin relaxation times. Moreover, changes in relaxation times followed the patterns observed in rheological studies. Electrokinetic and DLS analyses showed that potential values are primarily influenced by the properties of the starches themselves and, to a lesser extent, by the environmental conditions. The conducted research also showed complementarity and, to some extent, substitutability of the applied research methods as well as exclusion chromatography (a method not used in this work).

Non-Invasive Method to Predict the Composition of Requeijão Cremoso Directly in Commercial Packages Using Time Domain NMR Relaxometry and Chemometrics

Low Field Time-Domain Nuclear Magnetic Resonance (TD-NMR) relaxometry was used to determine moisture, fat, and defatted dry matter contents in “requeijão cremoso” (RC) processed cheese directly in commercial packaged (plastic cups or tubes with approximately 200 g). Forty-five samples of commercial RC types (traditional, light, lactose-free, vegan, and fiber) were analyzed using longitudinal (T₁) and transverse (T₂) relaxation measurements in a wide bore Halbach magnet (0.23 T) with a 100 mm probe. The T₁ and T₂ analyses were performed using CWFP-T₁ (Continuous Wave Free Precession) and CPMG (Carr-Purcell-Meiboom-Gill) single shot pulses. The scores of the principal component analysis (PCA) of CWFP-T₁ and CPMG signals did not show clustering related to the RC types. Optimization by variable selection was carried out with ordered predictors selection (OPS), providing simpler and predictive partial least squares (PLS) calibration models. The best results were obtained with CWFP-T₁ data, with root-mean-square errors of prediction (RMSEP) of 1.38, 4.71, 3.28, and 3.00% for defatted dry mass, fat in the dry and wet matter, and moisture, respectively. Therefore, CWFP-T₁ data modeled with chemometrics can be a fast method to monitor the quality of RC directly in commercial packages.

Recent advances in the application of parahydrogen in catalysis and biochemistry

Nuclear Magnetic Resonance (NMR) spectroscopy and Magnetic Resonance Imaging (MRI) are analytical and diagnostic tools that are essential for a very broad field of applications, ranging from chemical analytics, to non-destructive testing of materials and the investigation of molecular dynamics, to in vivo medical diagnostics and drug research. One of the major challenges in their application to many problems is the inherent low sensitivity of magnetic resonance, which results from the small energy-differences of the nuclear spin-states. At thermal equilibrium at room temperature the normalized population difference of the spin-states, called the Boltzmann polarization, is only on the order of 10-5. Parahydrogen induced polarization (PHIP) is an efficient and cost-effective hyperpolarization method, which has widespread applications in Chemistry, Physics, Biochemistry, Biophysics, and Medical Imaging. PHIP creates its signal-enhancements by means of a reversible (SABRE) or irreversible (classic PHIP) chemical reaction between the parahydrogen, a catalyst, and a substrate. Here, we first give a short overview about parahydrogen-based hyperpolarization techniques and then review the current literature on method developments and applications of various flavors of the PHIP experiment.

Time Domain (TD) Proton NMR Analysis of the Oxidative Safety and Quality of Lipid-Rich Foods

Food safety monitoring is highly important due to the generation of unhealthy components within many food products during harvesting, processing, storage, transportation and cooking. Current technologies for food safety analysis often require sample extraction and the modification of the complex chemical and morphological structures of foods, and are either time consuming, have insufficient component resolution or require costly and complex instrumentation. In addition to the detection of unhealthy chemical toxins and microbes, food safety needs further developments in (a) monitoring the optimal nutritional compositions in many different food categories and (b) minimizing the potential chemical changes of food components into unhealthy products at different stages from food production until digestion. Here, we review an efficient methodology for overcoming the present analytical limitations of monitoring a food's composition, with an emphasis on oxidized food components, such as polyunsaturated fatty acids, in complex structures, including food emulsions, using compact instruments for simple real-time analysis. An intelligent low-field proton NMR as a time domain (TD) NMR relaxation sensor technology for the monitoring of T₂ (spin-spin) and T₁ (spin-lattice) energy relaxation times is reviewed to support decision-making by producers, retailers and consumers in regard to food safety and nutritional value during production, shipping, storage and consumption.

Quantifying motional dynamics in nuclear magnetic resonance logging

The motional dynamics of nuclear magnetic resonance (NMR) logging tools can significantly influence the measurement performance of such tools. NMR logging is used for geophysical evaluation in geological environments, primarily quantifying formation porosity and fluid volumes, as well as providing a qualitative estimation of permeability. NMR logging tools are conveyed via two main mechanisms; wireline logging and logging while drilling (LWD). We conduct detailed simulations to quantify the impact of tool motion on NMR measurements during logging. This involves conducting electromagnetic simulations which quantify the magnetic fields generated by a logging tool, and subsequently introducing motion profiles within the relevant spin dynamic calculations. This enables tool motional dynamics to be imposed on the signal acquisition. Several movement profiles are considered: linear axial movement to replicate wireline logging tool motion, as well as axial harmonic and lateral harmonic movement to simulate the shocks and vibrations experienced during logging while drilling. Lateral motion is observed to cause a greater degree of signal attenuation relative to axial motion due to the cylindrical shape of the excited volume. The magnitude of motion (e.g. the velocity of linear motion or the amplitude of harmonic motion) is demonstrated to increase the severity of signal attenuation, as expected. However, the frequency of harmonic motion demonstrates a more complex effect on the measured signal. The harmonic interaction between the motion frequency and measurement frequency (determined by the echo spacing) can cause wave interference which results in enhanced or diminished signal attenuation. Finally, we demonstrate that reducing both the magnetic field gradient as well as the echo spacing reduce the degree of signal attenuation observed during measurement. The results presented in this work demonstrate how the optimisation of key design parameters can be used to control the sensitivity of NMR logging tools towards motion.

Applicable and cost-efficient microplastic analysis by quantitative 1 H-NMR spectroscopy using benchtop NMR and NoD methods

In continuation of our work on the proof-of-concept that quantitative NMR spectroscopy may be a valuable tool in microplastic (MP) analysis and quantification, we present here investigations using low-field NMR spectrometers and nondeuterated solvents for the analysis of solutions of MP particles in suitable solvents. The use of low-field NMR spectrometers (benchtop NMR) that are considerably more cost-effective in terms of purchase and operating costs compared with high-field NMR spectrometers and the use of nondeuterated solvents (NoD method) leads to an applicable and cost-efficient method for mass-based MP analysis. For benchtop 80-MHz NMR, limits of detection for polyvinylchloride (PVC), polyethylene terephthalate (PET), and polystyrene (PS) are in the same range as if a high-field 500-MHz NMR spectrometer was used for quantification (500 MHz: PET 1 μg/ml, PVC 42 μg/ml, and PS 9 μg/ml; 80 MHz: PET 4 μg/ml, PVC 19 μg/ml, and PS 21 μg/ml) for polymers being dissolved in deuterated solvents. The same is true for the corresponding limits of quantification. Moreover, it is shown for the first time that quantitative determination of the mass concentration of PET, PVC, and PS is also possible using NoD methods by evaluating the integrals of polymer-specific signals relative to an internal or external standard. Detection limits for NoD methods are in a similar range as if deuterated solvents were used (PET 2 μg/ml, PVC 39 μg/ml, and PS 8 μg/ml) using a high-field 500-MHz spectrometer or the 80-MHz spectrometer (PET 5 μg/ml).

RotoMate: An open-source, 3D printed autosampler for use with benchtop nuclear magnetic resonance spectrometers

Benchtop nuclear magnetic resonance (NMR) spectrometers are versatile analytic instruments with low acquisition and operation cost. However, in the basic version, samples must be manually measured one after the other. We herein describe the open-source autosampler RotoMate that allows the automated operation of such instruments. The hardware is easily assembled from 3D-printed and inexpensive off-the-shelf parts, and is controlled by an Arduino Uno. A software package interlinks the operation of the autosampler with the software of the NMR spectrometer and the software for the processing of the spectra. Experiments for up to 30 samples can be inserted into an interactive sample list. The autosampler automatically inserts and ejects the samples, initiates measurements on the spectrometer according to parameters specified in the sample list, and interacts with a common NMR software in the processing and visualization of the obtained spectroscopic raw data. If an internal standard is present, conversions and yields of chemical reactions are automatically calculated, enabling e.g. the monitoring of reactions. The device was fitted to a Magritek Spinsolve instrument and can interact with a free academic version of ACD NMR software to process the spectra, but can likely be adapted to similar instruments and spectroscopy software packages.

Recent Applications of Benchtop Nuclear Magnetic Resonance Spectroscopy

Benchtop nuclear magnetic resonance (NMR) spectroscopy uses small permanent magnets to generate magnetic fields and therefore offers the advantages of operational simplicity and reasonable cost, presenting a viable alternative to high-field NMR spectroscopy. In particular, the use of benchtop NMR spectroscopy for rapid in-field analysis, e.g., for quality control or forensic science purposes, has attracted considerable attention. As benchtop NMR spectrometers are sufficiently compact to be operated in a fume hood, they can be efficiently used for real-time reaction and process monitoring. This review introduces the recent applications of benchtop NMR spectroscopy in diverse fields, including food science, pharmaceuticals, process and reaction monitoring, metabolomics, and polymer materials.

Magnetic Particle Spectroscopy with One-Stage Lock-In Implementation for Magnetic Bioassays with Improved Sensitivities

In recent years, magnetic particle spectroscopy (MPS) has become a highly sensitive and versatile sensing technique for quantitative bioassays. It relies on the dynamic magnetic responses of magnetic nanoparticles (MNPs) for the detection of target analytes in the liquid phase. There are many research studies reporting the application of MPS for detecting a variety of analytes including viruses, toxins, nucleic acids, and so forth. Herein, we report a modified version of the MPS platform with the addition of a one-stage lock-in design to remove the feedthrough signals induced by external driving magnetic fields, thus capturing only MNP responses for improved system sensitivity. This one-stage lock-in MPS system is able to detect as low as 781 ng multi-core Nanomag50 iron oxide MNPs (micromod Partikeltechnologie GmbH) and 78 ng single-core SHB30 iron oxide MNPs (Ocean NanoTech). We first demonstrated the performance of this MPS system for bioassay-related applications. Using the SARS-CoV-2 spike protein as a model, we have achieved a detection limit of 125 nM (equal to 5 pmole) for detecting spike protein molecules in the liquid phase. In addition, using a streptavidin-biotin binding system as a proof-of-concept, we show that these single-core SHB30 MNPs can be used for Brownian relaxation-based bioassays while the multi-core Nanomag50 cannot be used. The effects of MNP amount on the concentration-dependent response profiles for detecting streptavidin were also investigated. Results show that by using a lower concentration/ amount of MNPs, concentration-response curves shift to a lower concentration/amount of target analytes. This lower concentration-response indicates the possibility of improved bioassay sensitivities by using lower amounts of MNPs.

Reimagining magnetic resonance instrumentation using open maker tools and hardware as protocol

Over the course of its history, the field of nuclear magnetic resonance spectroscopy has been characterized by alternating periods of intensive instrumentation development and rapid expansion into new chemical application areas. NMR is now both a mainstay of routine analysis for laboratories at all levels of education and research. On the other hand, new instrumentation and methodological advances promise expanded functionality in the future. At the core of this success is a community fundamentally dedicated to sharing ideas and collaborative advancements, as exemplified by the extensive remixing and repurposing of pulse sequences. Recent progress in modularity, automation, and 3D printing have reignited the tinkering spirit and demonstrate great promise to mature into a maker space that will enable similarly facile sharing of new applications and broader access to magnetic resonance.

A portable, submersible, MR sensor - The Proteus magnet

This paper details the design, fabrication, and testing of a new portable magnet, generically termed the Proteus magnet, that can undertake a wide range of MR measurements. The Proteus magnet is intended for ¹H measurements of liquids and is fully functional when submersed in the sample of interest. The Proteus magnet is fabricated from a pair of low-cost, commercial, NdFeB disk magnets, axially polarized, with their North and South poles aligned. The two N52 NdFeB magnets - 31.75 mm diameter and 6.35 mm thickness were separated by 10 mm. The gap between the magnets is sufficient for a RF shield and transverse rectangular solenoid RF probe. The sensor was evaluated through a series of measurements including bulk CPMG, saturation recovery T₁, self-diffusion, T₁ - T₂, and D - T₂.

Overhauser dynamic nuclear polarization (ODNP)-enhanced two-dimensional proton NMR spectroscopy at low magnetic fields

The majority of low-field Overhauser dynamic nuclear polarization (ODNP) experiments reported so far have been 1D NMR experiments to study molecular dynamics and in particular hydration dynamics. In this work, we demonstrate the application of ODNP-enhanced 2D J-resolved (JRES) spectroscopy to improve spectral resolution beyond the limit imposed by the line broadening introduced by the paramagnetic polarizing agent. Using this approach, we are able to separate the overlapping multiplets of ethyl crotonate into a second dimension and clearly identify each chemical site individually. Crucial to these experiments is interleaved spectral referencing, a method introduced to compensate for temperature-induced field drifts over the course of the NMR acquisition. This method does not require additional hardware such as a field-frequency lock, which is especially challenging when designing compact systems.

Low-field benchtop NMR spectroscopy: status and prospects in natural product analysis†

INTRODUCTION

Since a couple of years, low-field (LF) nuclear magnetic resonance (NMR) spectrometers (40-100 MHz) have re-entered the market. They are used for various purposes including analyses of natural products. Similar to high-field instruments (300-1200 MHz), modern LF instruments can measure multiple nuclei and record two-dimensional (2D) NMR spectra.

OBJECTIVE

To review the commercial availability as well as applications, advantages, limitations, and prospects of LF-NMR spectrometers for the purpose of natural products analysis.

METHOD

Commercial LF instruments were compared. A literature search was performed for articles using and discussing modern LF-NMR. Next, the articles relevant to natural products were read and summarised.

RESULTS

Seventy articles were reviewed. Most appeared in 2018 and 2019. Low costs and ease of operation are most often mentioned as reasons for using LF-NMR.

CONCLUSION

As the spectral resolution of LF instruments is limited, they are not used for structure elucidation of new natural products but rather applied for quality control (QC), forensics, food and health research, process control and teaching. Chemometric data handling is valuable. LF-NMR is a rapidly developing niche and new instruments keep being introduced.

Overhauser dynamic nuclear polarization (ODNP)-enhanced two-dimensional proton NMR spectroscopy at low magnetic fields

The majority of low-field Overhauser dynamic nuclear polarization (ODNP) experiments reported so far have been 1D NMR experiments to study molecular dynamics and in particular hydration dynamics. In this work, we demonstrate the application of ODNP-enhanced 2D J-resolved (JRES) spectroscopy to improve spectral resolution beyond the limit imposed by the line broadening introduced by the paramagnetic polarizing agent. Using this approach, we are able to separate the overlapping multiplets of ethyl crotonate into a second dimension and clearly identify each chemical site individually. Crucial to these experiments is interleaved spectral referencing, a method introduced to compensate for temperature-induced field drifts over the course of the NMR acquisition. This method does not require additional hardware such as a field-frequency lock, which is especially challenging when designing compact systems.

Progress in miniaturization and low-field nuclear magnetic resonance

In this paper, we review the latest developments in miniaturization of NMR systems with an emphasis on low-field NMR. We briefly cover the topics of magnet and coil miniaturization, elaborating on the advantages and disadvantages of miniaturized coils for different applications. The main part of the article is dedicated to progress in NMR electronics. Here, we touch upon software-defined radios as an emerging gadget for NMR before we provide a detailed discussion of NMR-on-a-chip transceivers as the ultimate solution in terms of miniaturization of NMR electronics. In addition to discussing the miniaturization capabilities of the NMR-on-a-chip approach, we also investigate the potential use of NMR-on-a-chip devices for an improved NMR system performance. Here, we also discuss the possibility of combining the NMR-on-a-chip approach with EPR-on-a-chip spectrometers to form compact DNP-on-a-chip systems that can provide a significant sensitivity boost, especially for low-field NMR systems.

Lead-207 NMR spectroscopy at 1.4 T: Application of benchtop instrumentation to a challenging I = ½ nucleus

The practicality of obtaining liquid- and solid-state ²⁰⁷ Pb nuclear magnetic resonance (NMR) spectra with a low permanent-field magnet is investigated. Obtaining ²⁰⁷ Pb NMR spectra of salts in solution is shown to be viable for samples as dilute as 0.05 M. The concentration dependence of the ²⁰⁷ Pb chemical shifts for lead nitrate was investigated; the results are comparable with those obtained with high-field instruments. Likewise, the isotope effect of substituting D₂ O for H₂ O as the solvent was investigated and found to be comparable with those reported previously. Obtaining solid-state ²⁰⁷ Pb NMR spectra is challenging, but we demonstrate the ability to obtain such spectra for three unique solid samples. An axially symmetric ²⁰⁷ Pb powder pattern for lead nitrate and the powder pattern expected for lead chloride reveal linewidths dominated by shielding anisotropy, while ²⁰⁷ Pb-^35/37 Cl J-coupling dominates in the methylammonium lead chloride perovskite material. Finally, recent innovations and the future potential of the instruments are considered.

NMR spectroscopy goes mobile: Using NMR as process analytical technology at the fume hood

Nuclear magnetic resonance (NMR) is potentially a very powerful process analytical technology (PAT) tool as it gives an atomic resolution picture of the reaction mixture without the need for chromatography. NMR is well suited for interrogating transient intermediates, providing kinetic information via NMR active nuclei, and most importantly provides universally quantitative information for all species in solution. This contrasts with commonly used PAT instruments, such as Raman or Flow-infrared (IR), which requires a separate calibration curve for every component of the reaction mixture. To date, the large footprint of high-field (≥400 MHz) NMR spectrometers and the immobility of superconducting magnets, coupled with strict requirements for the architecture for the room it is to be installed, have been a major obstacle to using this technology right next to fume hoods where chemists perform synthetic work. Here, we describe the use of a small, lightweight 60 MHz Benchtop NMR system (Nanalysis Pro-60) located on a mobile platform, that was used to monitor both small and intermediate scale Grignard formation and coupling reactions. We also show how low field NMR can provide a deceptively simple yes/no answer (for a system that would otherwise require laborious off-line testing) in the enrichment of one component versus another in a kilogram scale distillation. Benchtop NMR was also used to derive molecule specific information from Flow-IR, a technology found in most manufacturing sites, and compare the ease at which the concentrations of the reaction mixtures can be derived by NMR versus IR.

Magnetic-Nanosensor-Based Virus and Pathogen Detection Strategies before and during COVID-19

The novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes coronavirus disease 2019 (COVID-19), is a threat to the global healthcare system and economic security. As of July 2020, no specific drugs or vaccines are yet available for COVID-19; a fast and accurate diagnosis for SARS-CoV-2 is essential in slowing the spread of COVID-19 and for efficient implementation of control and containment strategies. Magnetic nanosensing is an emerging topic representing the frontiers of current biosensing and magnetic areas. The past decade has seen rapid growth in applying magnetic tools for biological and biomedical applications. Recent advances in magnetic nanomaterials and nanotechnologies have transformed current diagnostic methods to nanoscale and pushed the detection limit to early-stage disease diagnosis. Herein, this review covers the literature of magnetic nanosensors for virus and pathogen detection before COVID-19. We review popular magnetic nanosensing techniques including magnetoresistance, magnetic particle spectroscopy, and nuclear magnetic resonance. Magnetic point-of-care diagnostic kits are also reviewed aiming at developing plug-and-play diagnostics to manage the SARS-CoV-2 outbreak as well as preventing future epidemics. In addition, other platforms that use magnetic nanomaterials as auxiliary tools for enhanced pathogen and virus detection are also covered. The goal of this review is to inform the researchers of diagnostic and surveillance platforms for SARS-CoV-2 and their performances.

Influence of Oxidation Degree of Graphene Oxide on Its Nuclear Relaxivity and Contrast in MRI

Graphene oxide (GO) serves as a versatile platform for various applications, with the oxygen content of GO playing an important role in governing its properties. In the present study, different GO types covering a wide range of oxidation degree were prepared using our newly developed two-step method involving ball milling of graphite followed by its oxidation to GO. In addition to the variations in their physicochemical properties, the different GO types exhibited differences in proton relaxivity due to their paramagnetic nature. Nuclear magnetic resonance spectroscopy studies showed that the degree of oxidation of GO perturbs its nuclear relaxation properties and, together with intercalated Mn2+ ions, provides large contrast variation in magnetic resonance imaging (MRI). The study for the first time reveals that the surface chemistry of GO affects its relaxivity and opens up new avenues for developing tunable GO-based contrast agents in magnetic resonance imaging for diagnostics and therapies.

Spectroscopic Techniques for Monitoring Thermal Treatments in Fish and Other Seafood: A Review of Recent Developments and Applications

Cooking is an important processing method, that has been used since ancient times in order to both ensure microbiological safety and give desired organoleptic properties to the cooked food. Fish and other seafood products are highly sensitive to thermal treatments and the application of severe heat can result in negative consequences on sensory and nutritional parameters, as well as other quality attributes of the thermally processed products. To avoid such undesired effects and to extend the shelf life of these perishable products, both the heat processing methods and the assessment techniques used to monitor the process should be optimized. In this review paper, the most common cooking methods and some innovative ones will first be presented with a brief discussion of their impact on seafood quality. The main methods used for monitoring heat treatments will then be reviewed with a special focus on spectroscopic techniques, which are known to be rapid and non-destructive methods compared to traditional approaches. Finally, viewpoints of the current challenges will be discussed and possible directions for future applications and research will be suggested. The literature presented in this review clearly demonstrates the potential of spectroscopic techniques, coupled with chemometric tools, for online monitoring of heat-induced changes resulting from the application of thermal treatments of seafood. The use of fluorescence hyperspectral imaging is especially promising, as the technique combines the merits of both fluorescence spectroscopy (high sensitivity and selectivity) and hyperspectral imaging (spatial dimension). With further research and investigation, the few current limitations of monitoring thermal treatments by spectroscopy can be addressed, thus enabling the use of spectroscopic techniques as a routine tool in the seafood industry.

Solid-phase nuclear magnetic resonance immunoassay for the prostate-specific antigen by using protein-coated magnetic nanoparticles

A solid phase NMR-based sandwich immunoassay for the prostate-specific antigen (PSA) is presented. Carbon-encapsulated iron nanoparticles were functionalized with bovine serum albumin, coupled to monoclonal antibodies, and then used as magnetic labels. A nitrocellulose membrane with 8-μm pores was coated with capture antibodies and subsequently incubated with a serum sample and a suspension of the nanoconjugate. Test strips were placed in a portable homemade NMR relaxometer. Magnetic nanoparticles attached to nitrocellulose decrease the T2 relaxation time of the water protons located inside the pores of the membrane. Thus, T2 is inversely proportional to the concentration of the antigen (PSA) in the sample. The assay can be performed within 4 h. The detection limit is 0.44 ng mL^-1. Kallikrein 2, human chorionic gonadotropin, and α-fetoprotein do not interfere. Graphical abstractSchematic representation of NMR relaxometry-based sandwich dot blot immunoassay of a prostate-specific antigen (PSA). Magnetic nanoparticles bound to immunosorbent decrease the transverse relaxation times (T2) of the water protons located within the pores of the membrane. RF coil: radiofrequency coil.

Benchtop NMR for the monitoring of bioprocesses

This mini-review highlights the potential of benchtop nuclear magnetic resonance (NMR) for the monitoring of bioprocesses. It describes recent perspectives opened by the reduced size of devices in relaxometry, magnetic resonance imaging and NMR spectroscopy. In particular, the recent emergence of the benchtop NMR spectroscopy gives access to many applications thanks to the implementation of advanced experiments.

Application of green analytical chemistry to a green chemistry process: Magnetic resonance and Raman spectroscopic process monitoring of continuous ethanolic fermentation

Compact ¹ H NMR and Raman spectrometers were used for real-time process monitoring of alcoholic fermentation in a continuous flow reactor. Yeast cells catalyzing the sucrose conversion were immobilized in alginate beads floating in the reactor. The spectrometers proved to be robust and could be easily attached to the reaction apparatus. As environmentally friendly analysis methods, ¹ H NMR and Raman spectroscopy were selected to match the resource- and energy-saving process. Analyses took only a few seconds to minutes compared to chromatographic procedures and were, therefore, suitable for real-time control realized as a feedback loop. Both compact spectrometers were successfully implemented online. Raman spectroscopy allowed for faster spectral acquisition and higher quantitative precision, NMR yielded more resolved signals thus higher specificity. By using the software Matlab for automated data loading and processing, relevant parameters such as the ethanol, glycerol, and sugar content could be easily obtained. The subsequent multivariate data analysis using partial linear least-squares regression type 2 enabled the quantitative monitoring of all reactants within a single model in real time.

Recent Developments in Nanosensors for Imaging Applications in Biological Systems

Sensors are key tools for monitoring the dynamic changes of biomolecules and biofunctions that encode valuable information that helps us understand underlying biological processes of fundamental importance. Because of their distinctive size-dependent physicochemical properties, materials with nanometer scales have recently emerged as promising candidates for biological sensing applications by offering unique insights into real-time changes of key physiological parameters. This review focuses on recent advances in imaging-based nanosensor developments and applications categorized by their signal transduction mechanisms, namely, fluorescence, plasmonics, MRI, and photoacoustics. We further discuss the synergy created by multimodal nanosensors in which sensor components work based on two or more signal transduction mechanisms.

Reaction Monitoring Using SABRE-Hyperpolarized Benchtop (1 T) NMR Spectroscopy

The conversion of [IrCl(COD)(IMes)] (COD = cis,cis-1,5-cyclooctadiene, IMes = 1,3-bis(2,4,6-trimethyl-phenyl)imidazole-2-ylidene) in the presence of an excess of para-hydrogen (p-H₂) and a substrate (4-aminopyridine (4-AP) or 4-methylpyridine (4-MP)) into [Ir(H)₂(IMes)(substrate)₃]Cl is monitored by ¹H NMR spectroscopy using a benchtop (1 T) spectrometer in conjunction with the p-H₂-based hyperpolarization technique signal amplification by reversible exchange (SABRE). A series of single-shot ¹H NMR measurements are used to monitor the chemical changes that take place in solution through the lifetime of the hyperpolarized response. Non-hyperpolarized high-field ¹H NMR control measurements were also undertaken to confirm that the observed time-dependent changes relate directly to the underlying chemical evolution. The formation of [Ir(H)₂(IMes)(substrate)₃]Cl is further linked to the hydrogen isotope exchange (HIE) reaction, which leads to the incorporation of deuterium into the ortho positions of 4-AP, where the source of deuterium is the solvent, methanol-d₄. Comparable reaction monitoring results are achieved at both high-field (9.4 T) and low-field (1 T). It is notable that the low sensitivity of the benchtop (1 T) NMR enables the use of protio solvents, which when used here allows the effects of catalyst formation and substrate deuteration to be separated. Collectively, these methods illustrate how low-cost low-field NMR measurements provide unique insight into a complex catalytic process through a combination of hyperpolarization and relaxation data.

Development of an Immunosorbent for Solid-Phase NMR-Based Assay

The conditions for constructing an immunosorbent reagent for solid-phase NMR analysis were optimized. For this purpose, we increased the area of ​​the sensitized portion of the membrane to fit the relaxometer coil size and added the agent sorption buffer. This provided the penetration of the anti-ligand molecules into the membrane thickness and their uniform distribution.

Low-field 1H-NMR spectroscopy for compositional analysis of multicomponent polymer systems

The accurate characterization of relative composition in multicomponent polymer systems such as statistical copolymers, block copolymers, and polymer blends is critical to understanding and predicting their behavior. Typically, polymer compositional analysis is performed using 1H Nuclear Magnetic Resonance (NMR) Spectroscopy which provides quantitative chemical group concentrations without prior calibration. This utility has led 1H NMR spectroscopy to become a routine method for the molecular characterization of polymers. Unfortunately, due to cost constraints, NMR spectroscopy is rarely used for routine materials verification such as quality control in industrial settings that commonly lack on-site advanced instrumentation facilities. Recently, low-field or so-called benchtop NMR spectrometers have been introduced commercially as a less expensive alternative to higher field, and costlier, NMR spectrometers. Here, we examine the capability of a low-field 1H NMR spectrometer (60 MHz) for the compositional analysis of select block copolymers and polymer blends by direct comparison with results obtained using a 400 MHz NMR spectrometer. In the analysis of high 1,4-content polyisoprene we find quantitative agreement between the 400 and 60 MHz spectrometers. Furthermore, quantitative agreement is demonstrated for compositional analysis of commercially available poly(styrene-b-isoprene-b-styrene) (SIS) and poly(styrene-b-butadiene-b-styrene) (SBS) triblock copolymers and polymer blends of polystyrene/polyisoprene (PS/PI) and polystyrene/poly(methyl methacrylate) (PS/PMMA) that also serve as proxies for statistical and block copolymer analysis. Overall, we find low-field 1H NMR spectroscopy to be an accessible, powerful and useful tool for polymer characterization.

Progress in low-field benchtop NMR spectroscopy in chemical and biochemical analysis

The employment of spectroscopically-resolved NMR techniques as analytical probes have previously been both prohibitively expensive and logistically challenging in view of the large sizes of high-field facilities. However, with recent advances in the miniaturisation of magnetic resonance technology, low-field, cryogen-free "benchtop" NMR instruments are seeing wider use. Indeed, these miniaturised spectrometers are utilised in areas ranging from food and agricultural analyses, through to human biofluid assays and disease monitoring. Therefore, it is both intrinsically timely and important to highlight current applications of this analytical strategy, and also provide an outlook for the future, where this approach may be applied to a wider range of analytical problems, both qualitatively and quantitatively.