Portable Nuclear Magnetic Resonance Research Articles

Recent Applications of Unilateral NMR to Objects of Cultural Heritage

Although nuclear magnetic resonance (NMR) is recognized as a powerful tool in many areas of research, among the investigative techniques used in the field of cultural heritage its application is still largely unknown. One of the reasons for this is that artifacts are complex heterogeneous systems whose analysis requires a multi-disciplinary approach. In addition, major drawbacks in the analysis of objects belonging to cultural heritage are their limited quantity, number of samples collected from the artifact, and their immovability. Consequently, a methodological approach where non-destructive, and possibly non-invasive techniques are used, is advisable. In recent years, thanks to the development of portable instruments, there has been an increasing use of the NMR methodology in the cultural heritage field. The use of portable NMR has allowed us to study several materials in the cultural heritage, such as frescoes, stones, wood, paper, and paintings, to address the challenges in monitoring dampness in historical masonries, to evaluate the penetration depth of a hydrophobic treatment into a porous material, and to study of the effect of cleaning procedures on artifacts. In this paper, recent studies illustrating the potential of NMR portable methodologies in this field of research are reported.

Structural integrity and healing efficiency study of micro-capsule based composite materials via 1H NMR relaxometry

In this work we present a novel approach utilizing nuclear magnetic resonance (NMR) relaxometry to assess the structural stability of microcapsules employed as self-healing agents in advanced aerospace composites both in ambient and harsh environmental conditions. We successfully correlate the amount of the encapsulated self-healing agent with the signal intensity and confirm non-destructively the quantity of the encapsulated self-healing agent mass for the first time in the literature using 1H NMR spin–spin relaxation techniques on urea–formaldehyde (UF) microcapsules of different diameters containing an epoxy healing agent. The amount of self-healing agent is shown to increase by reducing the capsule diameter; however, the reduced shell mass renders the capsules more fragile and prone to failure. Most notably, via NMR experiments conducted during thermal cycling simulating flight conditions, we demonstrate that the microcapsule integrity under thermal fatigue varies according to their size. Especially we experimentally verify that the microcapsules with the most sensitive shells are the 147 nm and 133 nm diameter microcapsules, which are the most commonly used in self-healing systems. Finally, we were able to retrieve the same results using a portable NMR spectrometer developed in-house for in situ microcapsule testing, thus demonstrating the potential of NMR relaxometry as a powerful non-destructive evaluation tool for the microcapsule production line.

On Design Challenges of Portable Nuclear Magnetic Resonance System

This article studies the optimal design approach for a portable nuclear magnetic resonance (NMR) system for use in non-destructive flow measurement applications. The mechanical and electromagnetic design procedures were carried out using the Ansys Maxwell finite-element analysis (FEA) software tool. The proposed procedure considered homogeneity and strength constraints while ensuring the desired functionality of the intended device for a given application. A modified particle swarm optimization (MPSO) algorithm was proposed as a reference design framework for optimization stages. The optimally designed NMR tool was prototyped, and its functionality was validated via several case studies. To assess the functionality of the prototyped device, Larmor frequency for hydrogen atom was captured and compared with theoretical results. Furthermore, the functionality and accuracy of the prototyped NMR tool is compared to the off-the-shelf NMR tool. Results demonstrated the feasibility and accuracy of the prototyped NMR tool constrained by factors, such as being lightweight and compact.

Challenges and advances in measuring sap flow in agriculture and agroforestry: A review with focus on nuclear magnetic resonance.

Sap flow measurement is one of the most effective methods for quantifying plant water use.A better understanding of sap flow dynamics can aid in more efficient water and crop management, particularly under unpredictable rainfall patterns and water scarcity resulting from climate change. In addition to detecting infected plants, sap flow measurement helps select plant species that could better cope with hotter and drier conditions. There exist multiple methods to measure sap flow including heat balance, dyes and radiolabeled tracers. Heat sensor-based techniques are the most popular and commercially available to study plant hydraulics, even though most of them are invasive and associated with multiple kinds of errors. Heat-based methods are prone to errors due to misalignment of probes and wounding, despite all the advances in this technology. Among existing methods for measuring sap flow, nuclear magnetic resonance (NMR) is an appropriate non-invasive approach. However, there are challenges associated with applications of NMR to measure sap flow in trees or field crops, such as producing homogeneous magnetic field, bulkiness and poor portable nature of the instruments, and operational complexity. Nonetheless, various advances have been recently made that allow the manufacture of portable NMR tools for measuring sap flow in plants. The basic concept of the portal NMR tool is based on an external magnetic field to measure the sap flow and hence advances in magnet types and magnet arrangements (e.g., C-type, U-type, and Halbach magnets) are critical components of NMR-based sap flow measuring tools. Developing a non-invasive, portable and inexpensive NMR tool that can be easily used under field conditions would significantly improve our ability to monitor vegetation responses to environmental change.

Effect of shape on depth profile Nuclear Magnetic Resonance data of multilayered composite structure

ABSTRACT Polymer matrix composite structures with adhesively bonded rubber layers are being used for various industrial applications. The adhesive layer thickness (used for bonding composite and elastomer rubber layers) has to be uniform to achieve desired bond strength. However, this cannot be controlled in practical applications in industry as no physical measurements are possible for large structures. This paper reports application of single-sided portable Nuclear Magnetic Resonance (NMR) at a frequency of 12.88 MHz for the study of the adhesive layer thickness measurement as well as qualitative analysis of the adhesive layer used for bonding these materials. The effect of the shape of the sample on the proton NMR signal which is related indirectly to elastic properties was investigated. Results obtained on flat and curved glass fibre reinforced polymer (GFRP) composite samples were compared. Studies showed there is a change in single-sided NMR depth profile data (slope of amplitude versus depth curve) due to changes in the shape of the composite structure. A multilayer sample with rubber and propellant bonded adhesively to the composite structure was also tested to study the effect of shape on the profile NMR data.

Discrimination between softwood and hardwood based on hemicellulose content obtained with portable nuclear magnetic resonance

Wood is a hygroscopic material that can reach an equilibrium moisture content when ambient temperature and relative humidity are constant. Moisture affects all properties of wood, as well as its preservative treatment. The hygroscopic behavior of wood can be attributed to the hydroxyl groups of its constituents. Since hemicellulose shows the greatest water affinity, it can be considered the main responsible for the ingress of water into the wood mass. Below the fiber saturation point, wood moisture is only stored in the cell walls. Proton Nuclear Magnetic Resonance (NMR) is a relative method used for the evaluation of moisture content distribution in wood and NMR relaxation is an excellent tool to study the hygroscopic behavior of different woods below the fiber saturation point. This work aimed to test the hypothesis of discriminating among softwoods and hardwoods of different botanical species and identifying further sub-clusters of woods based on the NMR proton spin–spin (T2) and spin–lattice (T1) relaxation times of their cell wall water in the hygroscopic moisture range. Importantly, the study was performed using a portable low-cost NMR instrument with which it is possible to investigate wood samples of any size. The main result of this study was that at RH = 94% the relaxation time T2,2, associated with the cell wall bound water, can be used as a marker to discriminate among softwoods and hardwoods.Graphical abstract

Portable NMR for quantification of breast density in vivo: Proof-of-concept measurements and comparison with quantitative MRI

Mammographic Density (MD) is the degree of radio-opacity of the breast in an X-ray mammogram. It is determined by the Fibroglandular: Adipose tissue ratio. MD has major implications in breast cancer risk and breast cancer chemoprevention. This study aimed to investigate the feasibility of accurate, low-cost quantification of MD in vivo without ionising radiation.We used single-sided portable nuclear magnetic resonance (“Portable NMR”) due to its low cost and the absence of radiation-related safety concerns. Fifteen (N = 15) healthy female volunteers were selected for the study and underwent an imaging routine consisting of 2D X-ray mammography, quantitative breast 3T MRI (Dixon and T1-based 3D compositional breast imaging), and 1D compositional depth profiling of the right breast using Portable NMR. For each participant, all the measurements were made within 3–4 h of each other. MRI-determined tissue water content was used as the MD-equivalent quantity. Portable NMR depth profiles of tissue water were compared with the equivalent depth profiles reconstructed from Dixon and T1-based MR images, which were used as the MD-equivalent reference standard.The agreement between the depth profiles acquired using Portable NMR and the reconstructed reference-standard profiles was variable but overall encouraging. The agreement was somewhat inferior to that seen in breast tissue explant measurements conducted in vitro, where quantitative micro-CT was used as the reference standard. The lower agreement in vivo can be attributed to an uncertainty in the positioning of the Portable NMR sensor on the breast surface and breast compression in Portable NMR measurements.The degree of agreement between Portable NMR and quantitative MRI is encouraging. While the results call for further development of quantitative Portable NMR, they demonstrate the in-principle feasibility of Portable NMR-based quantitative compositional imaging in vivo and show promise for the development of safe and low-cost protocols for quantification of MD suitable for clinical applications.

A portable NMR platform with arbitrary phase control and temperature compensation.

In this paper, we present a custom-designed nuclear magnetic resonance (NMR) platform based on a broadband complementary metal-oxide-semiconductor (CMOS) NMR-on-a-chip transceiver and a synchronous reference signal generator, which features arbitrary phase control of the excitation pulse in combination with phase-coherent detection at a non-zero intermediate frequency (IF). Moreover, the presented direct digital synthesis (DDS)-based frequency generator enables a digital temperature compensation scheme similar to classical fieldlocking without the need for additional hardware. NMR spectroscopy and relaxometry measurements verify the functionality of the proposed frequency reference and temperature compensation scheme as well as the overall state-of-the-art performance of the presented system.

Decay of a Roman age pine wood studied by micro magnetic resonance imaging, diffusion nuclear magnetic resonance and portable nuclear magnetic resonance

Wood is a hygroscopic biodegradable porous material widely used by men in the past to create artworks. Its total preservation over time is quite rare and one of the best preservation modalities is waterlogging. Observing the anatomy of waterlogged archaeological wood could also be complicated because of its bacterial degradation. However, the characterization of wood morphology and conservation state is a fundamental step before starting any restoration intervention as it allows to extract information about past climatic conditions and human activities. In this work, a micro-invasive approach based on the combined use of high-resolution magnetic resonance imaging (MRI) and diffusion-nuclear magnetic resonance (NMR) was tested both on a modern and an ancient pine wood sample. Furthermore, a completely non-invasive analysis was performed by using portable NMR. This multi-analytical NMR approach allowed to highlight the effect of decay on the wood microstructure, through alterations in the pores size, tortuosity, and images contrast of the ancient pine compared to the modern one. This work pointed out the different but complementary multi-parametric information that can be obtained by using NMR and tested the potential of high-field MRI and low-field portable NMR in the detection of wood diagnostic features.

Sensing mammographic density using single-sided portable Nuclear Magnetic Resonance

This research paper presents a quantitative approach to sensing mammographic density (MD) using single-sided portable Nuclear Magnetic Resonance (NMR). It focuses on three main techniques: spin–lattice relaxation (recovery) time (T1), spin–spin relaxation (decay) time (T2), and Diffusion (D) techniques by testing whether or not the aforementioned techniques are in agreement with the gold standard and with each other when used for scanning breast tissue specimens with a variety of mammographic densities (MDs). The high mammographic density (HMD), intermediate MD, and low mammographic density (LMD) regions of each slice were identified according to the mammogram images. Subsequently, the grayscale values for these regions were quantified. One region was measured from the first sample while the remaining ones were measured from the second sample. The same areas were then exposed to portable NMR, and the sequences used as following: the stimulated echo sequence for diffusion (D), the Carr-Purcell-Meiboom-Gill (CPMG) sequence for T2, and saturation recovery sequence for T1. The correlations between the grayscale values and NMR techniques were strongly correlated. The Pearson correlation coefficient, R, of T1 (%) versus grayscale value, D (%) versus grayscale value, and T2 (%) versus grayscale value, was 0.91, 0.91, and 0.93, respectively. Furthermore, the relative water content of the breast slices based on T1, T2, and diffusion (D) measurements were strongly in agreement with each other. The Pearson correlation coefficient, R, of D (%) versus T1 (%), D (%) versus T2 (%), and T1 (%) versus T2 (%), was 0.984, 0.966, and 0.9868, respectively. The three pulse sequences can be employed in a portable NMR device to deliver continuous quantitative measurements of MD in breast tissue samples. As a result, the method demonstrated to be acceptable for determining the distribution of MDs among breast tissue samples without the need for additional qualitative analysis.

Non-Invasive Assessment of PVA-Borax Hydrogel Effectiveness in Removing Metal Corrosion Products on Stones by Portable NMR.

The cleaning of buildings, statues, and artworks composed of stone materials from metal corrosion is an important topic in the cultural heritage field. In this work the cleaning effectiveness of a PVA-PEO-borax hydrogel in removing metal corrosion products from different porosity stones has been assessed by using a multidisciplinary and non-destructive approach based on relaxation times measurement by single-sided portable Nuclear Magnetic Resonance (NMR), Scanning Electron Microscopy—Energy Dispersive Spectroscopy (SEM-EDS), and Raman Spectroscopy. To this end, samples of two lithotypes, Travertine and Carrara marble, have been soiled by triggering acidic corrosion of some copper coins in contact with the stone surface. Then, a PVA-PEO-borax hydrogel was used to clean the stone surface. NMR data were collected in untreated, soiled with corrosion products, and hydrogel-cleaned samples. Raman spectroscopy was performed on PVA-PEO-borax hydrogel before and after cleaning of metal corrosion. Furthermore, the characterization of the dirty gel was obtained by SEM-EDS. The combination of NMR, SEM-EDS and Raman results suggests that the mechanism behind the hydrogel cleaning action is to trap heavy metal corrosion products, such as Cu2+ between adjacent boron ions cross-linked with PVA. Moreover, the PVA-PEO-borax hydrogel cleaning effectiveness depends on the stone porosity, being better in Carrara marble compared to Travertine.

Characterization of Structural Bone Properties through Portable Single-Sided NMR Devices: State of the Art and Future Perspectives.

Nuclear Magnetic Resonance (NMR) is a well-suited methodology to study bone composition and structural properties. This is because the NMR parameters, such as the relaxation time, are sensitive to the chemical and physical environment of the 1H nuclei. Although magnetic resonance imaging (MRI) allows bone structure assessment in vivo, its cost limits the suitability of conventional MRI for routine bone screening. With difficulty accessing clinically suitable exams, the diagnosis of bone diseases, such as osteoporosis, and the associated fracture risk estimation is based on the assessment of bone mineral density (BMD), obtained by the dual-energy X-ray absorptiometry (DXA). However, integrating the information about the structure of the bone with the bone mineral density has been shown to improve fracture risk estimation related to osteoporosis. Portable NMR, based on low-field single-sided NMR devices, is a promising and appealing approach to assess NMR properties of biological tissues with the aim of medical applications. Since these scanners detect the signal from a sensitive volume external to the magnet, they can be used to perform NMR measurement without the need to fit a sample inside a bore of a magnet, allowing, in principle, in vivo application. Techniques based on NMR single-sided devices have the potential to provide a high impact on the clinical routine because of low purchasing and running costs and low maintenance of such scanners. In this review, the development of new methodologies to investigate structural properties of trabecular bone exploiting single-sided NMR devices is reviewed, and current limitations and future perspectives are discussed.

Single-Sided Portable NMR Investigation to Assess and Monitor Cleaning Action of PVA-Borax Hydrogel in Travertine and Lecce Stone.

In this work, we investigated the potential of PVA-borax hydrogel for cleaning limestones and the dependence of the cleaning on the porosity of the rock and on the action time of the hydrogel treatment. Towards this goal, we used a nuclear magnetic resonance (NMR) spectrometer, developed for non-invasive and non-destructive applications on cultural heritage. T2-NMR parameters were quantified on different samples of Lecce stone and Travertine cut perpendicular (Pe) and parallel (Pa) to the bedding planes under different experimental conditions: untreated samples, treated with Paraloid B72 and cleaned with PVA-PEO-borax hydrogel applied for 4 min and 2 h. The T2 results suggest that the effectiveness of the cleaning strongly depended on the porosity of the stones. In Lecce stone, the hydrogel seemed to eliminate both the paramagnetic impurities (in equal measure with 4 min and 2 h treatment) and Paraloid B72. In Travertine Pe, characterized by a smaller pore size compared to Lecce stone, no significant effects were found regarding both the cleaning and the treatment with Paraloid B72. In Travertine Pa, characterized by a larger pore size than the other two samples, the hydrogel seemed to clean the paramagnetic agents (it worked better if applied for a longer time) but it did not appear to have any effect on Paraloid B72 removal.

Improved Configuration of Halbach Magnets with a Homogeneous Magnetic Field for Portable NMR Device

Halbach array magnets are widely used in portable nuclear magnetic resonance (NMR) devices that the homogeneity of the magnetic field generated by the array affects the imaging quality. In this paper, we propose some improvements to the construction of the Halbach magnets to enhance magnetic field uniformity. Using a Halbach array model comprising 16 magnets, all the calculations are based on 3D finite element method (FEM) analysis and optimized using the particle swarm optimization (PSO) algorithm. Comparisons of the results are shown to support the observations that the optimized and improved constructions can generate a more homogeneous magnetic field.

Circadian Variation of Root Water Status in Three Herbaceous Species Assessed by Portable NMR.

Roots are at the core of plant water dynamics. Nonetheless, root morphology and functioning are not easily assessable without destructive approaches. Nuclear Magnetic Resonance (NMR), and particularly low-field NMR (LF-NMR), is an interesting noninvasive method to study water in plants, as measurements can be performed outdoors and independent of sample size. However, as far as we know, there are no reported studies dealing with the water dynamics in plant roots using LF-NMR. Thus, the aim of this study is to assess the feasibility of using LF-NMR to characterize root water status and water dynamics non-invasively. To achieve this goal, a proof-of-concept study was designed using well-controlled environmental conditions. NMR and ecophysiological measurements were performed continuously over one week on three herbaceous species grown in rhizotrons. The NMR parameters measured were either the total signal or the transverse relaxation time T2. We observed circadian variations of the total NMR signal in roots and in soil and of the root slow relaxing T2 value. These results were consistent with ecophysiological measurements, especially with the variation of fluxes between daytime and nighttime. This study assessed the feasibility of using LF-NMR to evaluate root water status in herbaceous species.

Nuclear magnetic resonance immunoassay of tetanus antibodies based on the displacement of magnetic nanoparticles.

A nuclear magnetic resonance (NMR) immunoassay based on the application of carbon-coated iron nanoparticles conjugated with recognition molecules was designed. The principle of the assay is that ELISA plates are coated with a capture element, and then an analyte is added and detected by conjugating the magnetic nanoparticles with recognition molecules. Afterwards, the elution solution (0.1-M sodium hydroxide) is added to displace the magnetic nanoparticles from the well surfaces into the solution. The detached magnetic nanoparticles reduce transverse relaxation time (T2) values of protons from the surrounding solution. A portable NMR relaxometer is used to measure the T2. Magnetic nanoparticles conjugated with streptavidin, monoclonal antibodies, and protein G were applied for the detection of biotinylated albumin, prostate-specific antigen, and IgG specific to tetanus toxoid (TT). The limit of detection of anti-TT IgG was 0.08-0.12 mIU/mL. The reproducibility of the assay was within the acceptable range (CV < 7.4%). The key novelty of the immunoassay is that the displacement of the nanoparticles from the solid support by the elution solution allows the advantages of the solid phase assay to be combined with the sensitive detection of the T2 changes in a volume of liquid.

Zero- to Ultralow-Field NMR Spectroscopy of Small Biomolecules.

Nuclear magnetic resonance (NMR) spectroscopy is a well-established analytical technique used to study chemicals and their transformations. However, high-field NMR spectroscopy necessitates advanced infrastructure, and even cryogen-free benchtop NMR spectrometers cannot be readily assembled from commercially available components. We demonstrate construction of a portable zero-field NMR spectrometer employing a commercially available magnetometer and investigate its applications in analytical chemistry. In particular, J-spectra of small representative biomolecules [13C]-formic acid, [1-13C]-glycine, [2,3-13C]-fumarate, and [1-13C]-d-glucose were acquired, and an approach relying on the presence of a transverse magnetic field during the detection was investigated for relaxometry purposes. We found that the water relaxation time strongly depends on the concentration of dissolved d-glucose in the range of 1-10 mM suggesting opportunities for indirect assessment of glucose concentration in aqueous solutions. Extending analytical capabilities of zero-field NMR to aqueous solutions of simple biomolecules (amino acids, sugars, and metabolites) and relaxation studies of aqueous solutions of glucose highlights the analytical potential of noninvasive and portable ZULF NMR sensors for applications outside of research laboratories.

A sensitive on‐chip probe–based portable nuclear magnetic resonance for detecting low parasitaemia plasmodium falciparum in human blood

AbstractMalaria elimination programmes are dependent on a rapid, cost‐effective and portable diagnostic tool to detect and quantify malaria parasites in human blood. Herein, we report a cost‐effective and portable nuclear magnetic resonance (pNMR) for malaria diagnosis. In this present study, we have developed and characterized a sensitive and low‐cost novel on‐chip NMR probe. This NMR probe has improved mismatching problem between radiofrequency (RF) coil and transmission line that results in enhancement in NMR sensitivity. Developed pNMR represents a rapid, cost‐effective and highly sensitive method to detect very low parasitaemia plasmodium falciparum (0.0001%) in infected human blood in comparison with other available malaria diagnosis techniques. The experimental results show that very low parasitic load can be detected rapidly by the change in the transverse relaxation rate (T2) of NMR, suggesting a good sensitivity. A good linear relationship with a correlation coefficient of 0.94 between the change in transverse relaxation rate (ΔT2) and the different parasitaemia (%) has been observed, indicating that pNMR is a potential tool for real‐time, reusable and high‐sensitivity analysis of malaria parasite in human blood.

Ultrasensitive and fast detection of pathogens using Europium-containing polystyrene nanospheres in a homemade portable NMR diagnostic system

The use of nuclear magnetic resonance (NMR) platforms to detect biological samples via magnetic nanoparticles has shown great importance in clinical applications. In numerous studies, superparamagnetic nanoparticles, including ferromagnetic nanoparticles, have been adopted to improve the performance of magnetic nanoparticles. Here we developed a simple and novel europium (Eu)-containing polystyrene nanosphere (PS NS)-based NMR system for rapid pathogen detection in clinical samples. Using this proposed portable NMR diagnostic system, we found that during pathogen detection, NPs with lower susceptibility (e.g., Eu-containing PS NSs) produced an unexpected detection effect compared with iron oxide NPs. Our Eu-containing PS NS-based NMR diagnostic system showed higher detection sensitivity, broader detection range, and higher stability than conventional iron oxide NP-based NMR platforms do. Using an inexpensive microcontact-printed Ag microcoil-based NMR probe, we demonstrated unprecedented detection sensitivity and speed with minimal sample preparation for pathogen analyses. The sample concentration of 0.2 ng/mL of hepatitis B virus surface antigen (HBsAg) and 10 CFU/mL of Clostridium difficile can be attained by the NMR diagnostic system within 5 s, and concentration-detection ranges spanned over three orders of magnitude. With advantages of low-cost NMR probe, a portable instrument, and convenient operation, our proposed NMR platform may provide a promising point-of-care diagnostic system in clinical applications.

A Portable NMR Spectrometer With a Probe Head Combining RF and DC Capabilities to Generate Pulsed-Field Gradients

Portable nuclear magnetic resonance (NMR) spectrometers are used in various scientific fields. A vast number of applications are suitable for low-field permanent magnets. However, most of these systems lack the ability of pulsed-field gradients as these require cumbersome additional hardware. We herein report an NMR probe head that incorporates a radio frequency (RF)-coil equipped with pulsed-field gradient capabilities in a single center-tapped solenoid. We developed a circuitry allowing for the application of direct currents without disturbing the RF reception capabilities. Furthermore, we present a System-on-Chip-based NMR spectrometer. Combined with the NMR probe head, it represents a versatile low-cost system capable of field gradient generation. The probe head is characterized in terms of scatter parameters, quality factor, and transversal field homogeneity. We exhibit simulation data of the field gradient and present the required external circuitry. Experimental results show further the dephasing capabilities of the gradient and give insights into the spatial shape. Finally, the system is applied for the mapping of J-couplings in 1-butanol via zero-quantum coherences. A theoretical analysis of the pulse sequence is given for a homonuclear two-spin system. The experimental results demonstrate the potential use of this setup.