Monitoring oil displacement processes with k‐t accelerated spin echo SPI

Magnetic resonance imaging (MRI) is a robust tool to monitor oil displacement processes in porous media. Conventional MRI measurement times can be lengthy, which hinders monitoring time‐dependent displacements. Knowledge of the oil and water microscopic distribution is important because their pore scale behavior reflects the oil trapping mechanisms. The oil and water pore scale distribution is reflected in the magnetic resonance T₂ signal lifetime distribution. In this work, a pure phase‐encoding MRI technique, spin echo SPI (SE‐SPI), was employed to monitor oil displacement during water flooding and polymer flooding. A k‐t acceleration method, with low‐rank matrix completion, was employed to improve the temporal resolution of the SE‐SPI MRI measurements. Comparison to conventional SE‐SPI T₂ mapping measurements revealed that the k‐t accelerated measurement was more sensitive and provided higher‐quality results. It was demonstrated that the k‐t acceleration decreased the average measurement time from 66.7 to 20.3 min in this work. A perfluorinated oil, containing no ¹H, and H₂O brine were employed to distinguish oil and water phases in model flooding experiments. High‐quality 1D water saturation profiles were acquired from the k‐t accelerated SE‐SPI measurements. Spatially and temporally resolved T₂ distributions were extracted from the profile data. The shift in the ¹H T₂ distribution of water in the pore space to longer lifetimes during water flooding and polymer flooding is consistent with increased water content in the pore space. Copyright © 2015 John Wiley & Sons, Ltd.     

Magnetic resonance imaging of spatially resolved acrylamide photopolymerization

Magnetic resonance imaging was employed to examine spatially and temporally resolved photopolymerization of acrylamide gels. Fast exchange between free and bound water results in single exponential T₂ decay, where 1/T₂ scales linearly with polymer concentration. Measured T₂s are sensitive to the experimental conditions; however, the 1/T₂ relationship to polymer concentration allows a straightforward interpretation of image contrast changes during photopolymerization. The polymer appears to form at a nearly constant rate until the monomer concentration is significantly depleted. Conventional spin‐echo images and quantitative CPMG‐weighted spin‐echo images were acquired. Photopolymerization of a partially masked sample produced a sharp transition (1 mm width) between polymer and monomer regions of the sample. The image intensity is uniform throughout the illuminated region of the sample, indicating uniform polymer formation. Interrupting the illumination quenches polymer formation. Copyright © 2003 John Wiley & Sons, Ltd.     

Electron spin relaxation times and rapid scan EPR imaging of pH‐sensitive amino‐substituted trityl radicals

Carboxy‐substituted trityl (triarylmethyl) radicals are valuable in vivo probes because of their stability, narrow lines, and sensitivity of their spectroscopic properties to oxygen. Amino‐substituted trityl radicals have the potential to monitor pH in vivo, and the suitability for this application depends on spectral properties. Electron spin relaxation times T₁ and T₂ were measured at X‐band for the protonated and deprotonated forms of two amino‐substituted triarylmethyl radicals. Comparison with relaxation times for carboxy‐substituted triarylmethyl radicals shows that T₁ exhibits little dependence on protonation or the nature of the substituent, which makes it useful for measuring O₂ concentration, independent of pH. Insensitivity of T₁ to changes in substituents is consistent with the assignment of the dominant contribution to spin lattice relaxation as a local mode that involves primarily atoms in the carbon and sulfur core. Values of T₂ vary substantially with pH and the nature of the aryl group substituent, reflecting a range of dynamic processes. The narrow spectral widths for the amino‐substituted triarylmethyl radicals facilitate spectral‐spatial rapid scan electron paramagnetic resonance imaging, which was demonstrated with a phantom. The dependence of hyperfine splittings patterns on pH is revealed in spectral slices through the image. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantitative nuclear magnetic resonance imaging of liquids in swelling polymers

The variation of nuclear magnetic resonance (NMR) relaxation parameters (T₁, T₂) within a polymer during swelling, limits the absolute accuracy with which liquid concentration profiles can be obtained using NMR imaging. In this article a study of the diffusion of decalin into ultra-high molecular weight polyethylene (UHMWPE) is reported. The study illustrates the use of a method of analysis whereby quantitative solvent profiles can be obtained from data influenced by both T₁ and T₂ contrast effects. A T₁ and T₂ map are obtained at a point in the uptake of liquid where the greatest range in liquid concentration is obtained at a point in the uptake of liquid where the greatest range in liquid concentration is observed. The intensity of signal corresponding to liquid in the polymer is compared to that of pure liquid in a reference sample, and correlations for T₁ and T₂ values versus signal intensity are used to deconvolve relaxation contrast, to yield the true liquid concentration. The technique was used to study the effect of degree of crosslinking of UHMWPE on the swelling kinetics and decalin transport within the polymer. A spin-echo imaging technique was used with a recycle delay approximately equal to the average spin-lattice relaxation time of the liquid, and an echo time approximately half the average spin-spin relaxation time. Under these conditions the relaxation contrast was significant, yet the mass uptake data derived from the concentration profiles obtained, using the method of analysis described, agreed well with gravimetric data. © 1995 John Wiley & Sons, Inc.     

Theoretical study of a simple rotational‐echo double‐resonance NMR for homonuclear spin‐1/2 pairs

We investigate theoretically intriguing aspects of a simple rotational‐echo double‐resonance (REDOR) NMR technique for homonuclear spin‐1/2 pairs undergoing MAS. The simple technique sets Gaussian soft π pulses at every half MAS rotational period in the pulse sequence. The reduction in rotational echo amplitude (the REDOR echo reduction) is observed at the end of the evolution period t_e = (n + 1)T_r, where T_r is a MAS rotational period. The exact average Hamiltonians for the homonuclear REDOR (hm‐REDOR) technique are calculated by dividing the evolution period into four periods. We show theoretically and experimentally that the hm‐REDOR technique produces the REDOR echo reductions for homonuclear spin‐1/2 pairs. In addition, the theoretical results reveal that the REDOR echo reductions are independent of the chemical‐shift difference, δ, under a simple condition of κ = δ/ω_r ≥ 6 and t_e < 10 ? (1/d′), where ω_r is the sample spinning frequency and d′ is the dipolar coupling constant expressed in Hz. We call this simple condition the master condition. This means that the REDOR echo reductions for a homonuclear spin‐1/2 pair can be calculated under the master condition by considering only d′ and ω_r, which is the case for a heteronuclear spin pair. Finally, we demonstrate that four‐phase cycling yields the multiple‐quantum filtered hm‐REDOR experiment, where the appearance of the REDOR echo reductions shows that the echo reductions are definitely attributable to the homonuclear dipolar interaction even if there is a slight unwanted effect from the recovered chemical‐shift anisotropy in these reductions. Copyright © 2015 John Wiley & Sons, Ltd.     

Magnetic resonance imaging characterization of intact smoked cigarettes

The interior of intact, extinguished cigarettes following smoldering and puffing combustion was examined by proton magnetic resonance imaging (MRI). Spin-echo imaging sequences were employed to image substances with high molecular mobility such as water, smoke condensate, and waxy materials native to unburned tobacco. Single-point imaging (SPI) methods were employed to image the more rigid components, such as tobacco cell wall polysaccharides and cellulose acetate fibers inside the filter. The distribution of spin–spin relaxation times (T₂) of the tobacco and filters was measured using a low-field ¹H NMR bench-top spectrometer. One-dimensional profiles and two-dimensional images revealed the distribution of combustion and pyrolysis products deposited on the unburned portion of tobacco and in the filter of the cigarette. Image features as small as 25 μm were resolved. The current results demonstrate the feasibility of employing MRI to study combustion in burning cigarettes and other materials in real time.     

Nuclear Spin‐Noise Spectra of Hyperpolarized Systems

Spin‐noise appeal : Detection of NMR spin‐noise is very appealing when dilute hyperpolarized species are considered. Continuous monitoring of the noise absorption at the Larmor frequency enables determination of T₁ and T₂*, independently of the static magnetic field. An inductively coupled microcoil located inside the NMR tube (see picture) allows acquisition of ¹²⁹Xe spin‐noise spectra without radio‐frequency excitation.

     

The effect of cationic groups on the stability of 19F MRI contrast agents in nanoparticles

Fluorine‐19 (¹⁹F)‐based contrast agents are increasingly used for magnetic resonance imaging. Conjugated to polymers, they provide an excellent quantitative imaging tool to detect the movement of the polymeric nanoparticles in vivo as there is no background signal in tissue. One of the challenges is the decline in signal intensity when the conjugated hydrophobic fluorinated functionalities aggregate. Therefore, a new fluorinated monomer was prepared from l ‐arginine that carries a 2,2,2‐trifluoroethyl functional group for imaging. The resulting monomer, 2,2,2‐trifluoroethylamide l ‐arginine methacrylamide (3FArgMA), was copolymerized with poly(ethylene glycol) methyl ether methacrylate (PEGMEMA), [2‐(2,3,4,6‐tetra‐O‐acetyl‐α‐d ‐mannopyranosyloxy)ethyl methacrylate or 1‐O‐methacryloyl‐2,3:4,5‐di‐O‐isopropylidene‐β‐d ‐fructopyranose, respectively, using poly(methyl methacrylate) macro‐reversible addition–fragmentation chain transfer polymerization agent. The resulting block copolymers, which varied in 3FArgMA content, were self‐assembled into micelles of hydrodynamic diameters from 25 to 60 nm. The permanently positively charged arginine functionality on the 3FArgMA displayed repulsive forces against aggregation enabling high spin–spin relaxation times (T₂) in acidic as well as alkaline solutions. However, the longer poly(ethylene glycol) side functionality in PEGMEMA enabled better steric stabilization (T₂~30 ms) while the short fructose side chain was not enough to maintain high T₂ values, in particular when a higher 3FArgMA content was used. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1994–2001     

Behavior of Two Almost Identical Spins during the CPMG Pulse Sequence

Multiple‐spin‐echo experiments have found wide use in nuclear magnetic resonance spectroscopy. In particular, the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence is used to determine transverse relaxation times T₂. Herein it is demonstrated, both theoretically and experimentally, that for a pair of almost identical spins‐1/2 the experimental setup can have a profound effect on the observed spin dynamics. It is shown that, in the case of dipolar relaxation, the measured T₂ values can roughly vary between the limits of identical and unlike spins, just depending on the repetition rate of π pulses with respect to chemical shift separation. Such an effect can, in the extreme narrowing regime, amount to a 50 % difference.     

Ultrafast Two‐Dimensional NMR Relaxometry for Investigating Molecular Processes in Real Time

Nuclear spin–lattice (T₁) and spin–spin (T₂) relaxation times provide versatile information about the dynamics and structure of substances, such as proteins, polymers, porous media, and so forth. Multidimensional experiments increase the information content and resolution of NMR relaxometry, but they also multiply the measurement time. To overcome this issue, we present an efficient strategy for a single‐scan measurement of a 2D T₁–T₂ correlation map. The method shortens the experimental time by one to three orders of magnitude as compared to the conventional method, offering an unprecedented opportunity to study molecular processes in real‐time. We demonstrate that, despite the tremendous speed‐up, the T₁–T₂ correlation maps determined by the single‐scan method are in good agreement with the maps measured by the conventional method. The concept of the single‐scan T₁–T₂ correlation experiment is applicable to a broad range of other multidimensional relaxation and diffusion experiments.     

Gradient echo single scan inversion recovery: application to proton and fluorine relaxation studies

Single scan longitudinal relaxation measurement experiments enable rapid estimation of the spin‐lattice relaxation time (T₁) as the time series of spin relaxation is encoded spatially in the sample at different slices resulting in an order of magnitude saving in time. We consider here a single scan inversion recovery pulse sequence that incorporates a gradient echo sequence. The proposed pulse sequence provides spectra with significantly enhanced signal to noise ratio leading to an accurate estimation of T₁ values. The method is applicable for measuring a range of T₁ values, thus indicating the possibility of routine use of the method for several systems. A comparative study of different single scan methods currently available is presented, and the advantage of the proposed sequence is highlighted. The possibility of the use of the method for the study of cross‐correlation effects for the case of fluorine in a single shot is also demonstrated. Copyright © 2015 John Wiley & Sons, Ltd.     

35Cl NQR studies of 1‐chloro‐2,4‐dinitrobenzene and 1,2‐dichloro‐3‐nitrobenzene as a function of pressure and temperature

The temperature and pressure dependences of ³⁵Cl nuclear quadrupole resonance (NQR) frequency and spin–lattice relaxation time (T₁) were investigated for 1‐chloro‐2,4‐dinitrobenzene and 1,2‐dichloro‐3‐nitrobenzene. T₁ was measured in the temperature range 77–300 K. Furthermore, the NQR frequency (ν) and T₁ for these compounds were measured as a function of pressure up to 5.1 kbar at 300 K. Relaxation was found to be due to the torsional motion of the molecule and the reorientation motion of the nitro group. By analysing the temperature dependence of T₁, the activation energy for the reorientation motion of the nitro group was obtained. The temperature dependence of the average torsional lifetimes of the molecules and the transition probabilities W₁ and W₂ for the Δm = ±1 and Δm = ±2 transitions, were also obtained. Both compounds showed a non‐linear variation of NQR frequency with pressure. The pressure coefficients were observed to be positive. A thermodynamic analysis of the data was carried out to determine the constant‐volume temperature coefficients of the NQR frequency. The spin–lattice relaxation time T₁ for both the compounds was found to be weakly dependent on pressure, showing that the relaxation is mainly due to the torsional motions. Copyright © 2002 John Wiley & Sons, Ltd.     

Magneto‐Plasmonic Janus Vesicles for Magnetic Field‐Enhanced Photoacoustic and Magnetic Resonance Imaging of Tumors

Magneto‐plasmonic Janus vesicles (JVs) integrated with gold nanoparticles (AuNPs) and magnetic NPs (MNPs) were prepared asymmetrically in the membrane for in vivo cancer imaging. The hybrid JVs were produced by coassembling a mixture of hydrophobic MNPs, free amphiphilic block copolymers (BCPs), and AuNPs tethered with amphiphilic BCPs. Depending on the size and content of NPs, the JVs acquired spherical or hemispherical shapes. Among them, hemispherical JVs containing 50 nm AuNPs and 15 nm MNPs showed a strong absorption in the near‐infrared (NIR) window and enhanced the transverse relaxation (T₂) contrast effect, as a result of the ordering and dense packing of AuNPs and MNPs in the membrane. The magneto‐plasmonic JVs were used as drug delivery vehicles, from which the release of a payload can be triggered by NIR light and the release rate can be modulated by a magnetic field. Moreover, the JVs were applied as imaging agents for in vivo bimodal photoacoustic (PA) and magnetic resonance (MR) imaging of tumors by intravenous injection. With an external magnetic field, the accumulation of the JVs in tumors was significantly increased, leading to a signal enhancement of approximately 2–3 times in the PA and MR imaging, compared with control groups without a magnetic field.     

Long‐Lived Spin States for Low‐Field Hyperpolarized Gas MRI

Parahydrogen induced polarization was employed to prepare a relatively long‐lived correlated nuclear spin state between methylene and methyl protons in propane gas. Conventionally, such states are converted into a strong NMR signal enhancement by transferring the reaction product to a high magnetic field in an adiabatic longitudinal transport after dissociation engenders net alignment (ALTADENA) experiment. However, the relaxation time T₁ of ～0.6 s of the resulting hyperpolarized propane is too short for potential biomedical applications. The presented alternative approach employs low‐field MRI to preserve the initial correlated state with a much longer decay time T_LLSS=(4.7±0.5) s. While the direct detection at low‐magnetic fields (e.g. 0.0475 T) is challenging, we demonstrate here that spin‐lock induced crossing (SLIC) at this low magnetic field transforms the long‐lived correlated state into an observable nuclear magnetization suitable for MRI with sub‐millimeter and sub‐second spatial and temporal resolution, respectively. Propane is a non‐toxic gas, and therefore, these results potentially enable low‐cost high‐resolution high‐speed MRI of gases for functional imaging of lungs and other applications.     

A general chemical shift decomposition method for hyperpolarized 13C metabolite magnetic resonance imaging

Metabolic imaging with hyperpolarized carbon‐13 allows sequential steps of metabolism to be detected in vivo. Potential applications in cancer, brain, muscular, myocardial, and hepatic metabolism suggest that clinical applications could be readily developed. A primary concern in imaging hyperpolarized nuclei is the irreversible decay of the enhanced magnetization back to thermal equilibrium. Multiple methods for rapid imaging of hyperpolarized substrates and their products have been proposed with a multi‐point Dixon method distinguishing itself as a robust protocol for imaging [1‐¹³C]pyruvate. We describe here a generalized chemical shift decomposition method that incorporates a single‐shot spiral imaging sequence plus a spectroscopic sequence to retain as much spin polarization as possible while allowing detection of metabolites that have a wide range of chemical shift values. The new method is demonstrated for hyperpolarized [1‐¹³C]pyruvate, [1‐¹³C]acetoacetate, and [2‐¹³C]dihydroxyacetone. Copyright © 2016 John Wiley & Sons, Ltd.     

Red‐ and Green‐Emitting Iridium(III) Complexes for a Dual Barometric and Temperature‐Sensitive Paint

A new dual luminescent sensitive paint for barometric pressure and temperature (T) is presented. The green‐emitting iridium(III) complex [Ir(ppy)₂(carbac)] (ppy=2‐phenylpyridine; carbac=1‐(9H‐carbazol‐9‐yl)‐5,5‐dimethylhexane‐2,4‐dione) was applied as a novel probe for T along with the red‐emitting complex [Ir(btpy)₃], (btpy=2‐(benzo[b]thiophene‐2‐yl)pyridine) which functions as a barometric (in fact oxygen‐sensitive) probe. Both iridium complexes were dissolved in different polymer materials to achieve optimal responses. The probe [Ir(ppy)₂(carbac)] was dispersed in gas‐blocking poly(acrylonitrile) microparticles in order to suppress any quenching of its luminescence by oxygen. The barometric probe [Ir(btpy)₃], in turn, was incorporated in a cellulose acetate butyrate film which exhibits good permeability for oxygen. The effects of temperature on the response of the oxygen probe can be corrected by simultaneous optical determination of T, as the poly(acrylonitrile) microparticles containing the temperature indicator are incorporated into the film. The phosphorescent signals of the probes for T and barometric pressure, respectively, can be separated by optical filters due to the ≈75 nm difference in their emission maxima. The dual sensor is applicable to luminescence lifetime imaging of T and barometric pressure. It is the first luminescent dual sensor material for barometric pressure/T based exclusively on the use of Ir^III complexes in combination with luminescence lifetime imaging.     

Nitroxide spin probe study of probe size,hydrogen bonding and polymer matrix rigidity effects on poly(acrylic acid)/poly(ethylene oxide) complexes

An electron spin resonance (ESR) spin probe study was performed on 1 : 1 by weight poly(acrylic acid) (PAA)/poly(ethylene oxide) (PEO) complex over the 100–450 K temperature range with a series of tetramethylpiperidyloxy‐based spin probes. Measurements of the parameters T_5mT, Ta and Td demonstrated the effects of probe size and the strength of hydrogen bonding. The probes in the series Tempone, Tempo, Tempol and Tamine (respectively 4‐oxo‐, unsubstituted, 4‐hydroxy‐ and 4‐amino‐2,2,6,6,‐tetramethylpiperidine ‐1‐oxyl) displayed noticeable increases in the hydrogen‐bonding effect, as indicated by Ta and Td. These increases correlated with increasing hydrogen bond acceptor strength. On the other hand, as the probe size became larger, T_5mT gradually increased due to the free volume decrease. These effects were analyzed using the established theoretical relationship of T_5mT to probe volume expressed by f. Meanwhile, in order to investigate the effect of polymer matrix rigidity, a similar study was performed with a nitroxide spin probe, 2,2,6,6‐tetramethyl‐1‐piperidine‐1‐oxyl (Tempo), on PAA/PEO complexes of different weight compositions. The quantitative fast motion fraction in the composite ESR spectrum was calculated. The influence of changes in the composition of PAA on the molecular mobility was characterized by changes of the spectral parameters and τ_c. The molecular mobility was shown to diminish with increasing content of PAA in PAA/PEO blends duo to the restriction of the polymer matrix rigidity increase. Copyright © 2003 John Wiley & Sons, Ltd.     

An Investigation of Photo‐ and Pressure‐Induced Effects in a Pair of Isostructural Two‐Dimensional Spin‐Crossover Framework Materials

Two new isostructural iron(II) spin‐crossover (SCO) framework (SCOF) materials of the type [Fe(dpms)₂(NCX)₂] (dpms=4,4′‐dipyridylmethyl sulfide; X=S ( SCOF‐6(S) ), X=Se ( SCOF‐6(Se) )) have been synthesized. The 2D framework materials consist of undulating and interpenetrated rhomboid (4,4) nets. SCOF‐6(S) displays an incomplete SCO transition with only approximately 30 % conversion of high‐spin (HS) to low‐spin iron(II) sites over the temperature range 300–4 K (T_1/2=75 K). In contrast, the NCSe^? analogue, SCOF‐6(Se) , displays a complete SCO transition (T_1/2=135 K). Photomagnetic characterizations reveal quantitative light‐ induced excited spin‐state trapping (LIESST) of metastable HS iron(II) sites at 10 K. The temperature at which the photoinduced stored information is erased is 58 and 50 K for SCOF‐6(S) and SCOF‐6(Se) , respectively. Variable‐pressure magnetic measurements were performed on SCOF‐6(S) , revealing that with increasing pressure both the T_1/2 value and the extent of spin conversion are increased; with pressures exceeding 5.2 kbar a complete thermal transition is achieved. This study confirms that kinetic trapping effects are responsible for hindering a complete thermally induced spin transition in SCOF‐6(S) at ambient pressure due to an interplay between close T_1/2 and T(LIESST) values.     

Slow relaxation of longitudinal multispin orders in weakly and strongly coupled two‐spin systems

Longitudinal multispin order (LOMO) corresponds to a nonequilibrium population distribution in spin systems that exhibit scalar (J), dipolar, or quadrupolar coupling. We investigated the relaxation of longitudinal two‐spin order (2‐LOMO) in systems that had either weakly or strongly J‐coupled spins. Our results indicated longer relaxation times for the 2‐LOMO state compared with the corresponding longitudinal single‐spin state (1‐LOMO). Accessing nuclear spin states that have relaxation times longer than T₁, without the use of external contrast agents, is potentially useful for in vivo imaging and also for studying systems using dynamically hyperpolarized nuclear spins where longer life times are sought to increase the time available to study (bio)chemical events. Copyright © 2012 John Wiley & Sons, Ltd.     

Confined Crystallization of Spin‐Crossover Nanoparticles in Block‐Copolymer Micelles

Nanoparticles of the spin‐crossover coordination polymer [FeL(bipy)]_n were synthesized by confined crystallization within the core of polystyrene‐block‐poly(4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer micelles. The 4VP units in the micellar core act as coordination sites for the Fe complex. In the bulk material, the spin‐crossover nanoparticles in the core are well isolated from each other allowing thermal treatment without disintegration of their structure. During annealing above the glass transition temperature of the PS block, the transition temperature is shifted gradually to higher temperatures from the as‐synthesized product (T_1/2↓=163 K and T_1/2↑=170 K) to the annealed product (T_1/2↓=203 K and T_1/2↑=217 K) along with an increase in hysteresis width from 6 K to 14 K. Thus, the spin‐crossover properties can be shifted towards the properties of the related bulk material. The stability of the nanocomposite allows further processing, such as electrospinning from solution.