Directly Bound Deuterons Increase X‐Nuclei Hyperpolarization using Dynamic Nuclear Polarization

This study aimed to investigate the role of neural adaptation in the perception of Haidinger's brushes, an entoptic phenomenon enabling the perception of polarization by the human visual system. The objective was to consider the rotational speed of the pattern in combination with the polarization level of the input light, to identify normal values in a cohort of healthy individuals. An experimental setup was designed and assembled using two LED sources, one of which was filtered by a rotating linear polarizer with adjustable speed. The relative intensity of the LEDs could be changed to tune the polarization level. A cohort of 37 healthy individuals underwent monocular testing at seven polarization levels. For each value, descending and ascending limit tests were conducted to determine the rotational speed thresholds. The rotational speed threshold decreases linearly with the logarithm of the polarization level. All participants could perceive the pattern down to at least 20% polarization, with a dramatic drop in the number of subjects at 10% and 5% polarization levels. The estimated average polarization threshold was 9.7 ± 1.2%. Lower initial rotational thresholds at maximum contrast corresponded with lower polarization thresholds. The study confirms the crucial role of rotational speed in the perception of Haidinger's brushes, especially at low polarization levels. As a normal pattern perception correlates with macular pigment distribution and density, those findings are essential for the development and calibration of screening devices based on Haidinger's brush perception for the early detection of maculopathy and other macular diseases.

In online communities, polarization refers to the phenomenon in which individuals become more divided and extreme in their opinions due to their exposure to specific content. In this paper, we present a network-based framework for evaluating polarization levels in Online Social Networks (OSNs). Starting from a dataset of comments, our framework creates a network of user interactions and leverages the Louvain algorithm, the Rao’s Quadratic Entropy, and ego networks to assess the polarization level of communities and the most influential users. To test our framework, we leveraged a dataset of tweets about climate change. After performing Extraction, Transformation and Loading activities on the dataset, we evaluated its labels, identified communities, and analyzed their polarization level and that of the most influential users. We also analyzed the ego networks of believers and deniers and the aggressiveness of the corresponding tweets. Our analysis revealed the existence of polarized communities and homophily among the most influential users. It also showed that the type of communication used to disseminate information influences the polarization level of both communities and individual users. These results demonstrate our framework’s ability to support the polarization analysis in OSNs.

A study of recrystallization on high-energy electron irradiated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer with composition 65/35 mol% is reported in this paper. Electron irradiated copolymer P(VDF-TrFE) exhibits a high electromechanical performance that is attractive for many applications. The structure and morphology of the recrystallized samples were determined using DSC, X-ray diffraction and FTIR. The polarization behavior of the recrystallized samples was also studied. The nonpolar phase content in the recrystallized samples was much lower than that in the irradiated samples. For irradiated samples that exhibited the best electromechanical performance, the corresponding recrystallized material had a high polar phase content, correspondingly a high remanent polarization was observed. For samples irradiated with higher doses, which have low polarization levels, after recrystallization the material exhibited a much higher polarization level with a very small remanent polarization, an attractive combination for many electromechanical applications.

Typical magnetic resonance experiments are routinely limited by weak signal responses. In some cases, the low intrinsic sensitivity can be alleviated by the implementation of hyperpolarization technologies. Dissolution-dynamic nuclear polarization offers a means of hyperpolarizing small molecules. Hyperpolarized water is employed in several dynamic nuclear polarization studies, and hence accurate and rapid quantification of the 1H polarization level is of utmost importance. The solid-state nuclear magnetic resonance spectrum of water acquired under dissolution-dynamic nuclear polarization conditions has revealed lineshapes which become asymmetric at high levels of 1H polarization, which is an interesting fundamental problem in itself, but also complicates data interpretation and can prevent correct estimations of polarization levels achieved. In previous studies, attempts to simulate the 1H spectral lineshape of water as a function of the 1H polarization led to significant disagreement with the experimental results. Here we propose and demonstrate that such simulations, and therefore polarization quantification, can be implemented accurately, in particular by taking into account the detector dead time during 1H signal acquisition that can lead to severe spectral distortions. Based on these findings, we employed an echo-based radiofrequency pulse sequence to achieve distortion-free 1H spectra of hyperpolarized water, and adequate simulations of these echo-based spectra were implemented to extract the absolute 1H polarization level from the hyperpolarized water signal only, thus alleviating the need for lengthy and insensitive measurements of thermal equilibrium signals.

Spectral diffusion and dynamic nuclear polarization: Beyond the high temperature approximation

Review: 194 refs.

The sensitivity of NMR spectroscopy is considerably enhanced by dynamic nuclear polarization (DNP). In DNP polarization is transferred from unpaired electrons of a polarizing agent to nearby proton spins. In solids, this transfer is followed by the transport of hyperpolarization to the bulk via 1 H-1 H spin diffusion. The efficiency of these steps is critical to obtain high sensitivity gains, but the pathways for polarization transfer in the region near the unpaired electron spins are unclear. Here we report a series of seven deuterated and one fluorinated TEKPol biradicals to probe the effect of deprotonation on MAS DNP at 9.4 T. The experimental results are interpreted with numerical simulations, and our findings support that strong hyperfine couplings to nearby protons determine high transfer rates across the spin diffusion barrier to achieve short build-up times and high enhancements. Specifically, 1 H DNP build-up times increase substantially with TEKPol isotopologues that have fewer hydrogen atoms in the phenyl rings, suggesting that these protons play a crucial role transferring the polarization to the bulk. Based on this new understanding, we have designed a new biradical, NaphPol, which yields significantly increased NMR sensitivity, making it the best performing DNP polarizing agent in organic solvents to date.

Impact of non-polar solvents in dynamic nuclear polarization at high magnetic fields.

Dynamic nuclear polarization (DNP) employs paramagnetic species to increase the NMR signal of nuclear spins. In liquids, the efficiency of the effect depends on the strength of the interaction between the electron and nuclear spins and the time scales on which this interaction is modulated by the physical motion of the spin-bearing molecules. An approach to quantitatively predict the contribution of molecular motions to the DNP enhancement using molecular dynamics (MD) simulations is developed and illustrated for the nitroxide radical TEMPOL in liquid toluene. A multi-resolution strategy that combines explicit treatment of the solvent at short distances from the free radical with implicit description at large intermolecular distances is adopted. Novel analytical expressions are obtained to correct for the finite spatial extent of the MD simulations. The atomistic and analytical descriptions are sewn seamlessly together by ensuring that for molecular trajectories that start in the near (explicit) region and end in the distant (implicit) region the analytical dipolar spectral densities reproduce the MD estimates. The spectral densities obtained from the developed approach are used to calculate DNP coupling factors separately for the ring and methyl protons of toluene. The agreement with previously reported experimental DNP data at a magnetic field of 3.4 T is noteworthy and encouraging. Maximum obtainable DNP enhancements at other magnetic fields are predicted.

Coupled electron-nuclear spins represent a promising quantum system, where the optically induced electron spin polarization can be dynamically transferred to nuclear spins via the hyperfine interaction. Most experiments on dynamic nuclear polarization (DNP) are performed at cryogenic temperatures and/or in moderate external magnetic fields, the latter approach being very sensitive to the magnetic field orientation. Here, we demonstrate that the 29Si nuclear spins in SiC can be efficiently polarized at room temperature even in the Earth’s magnetic field. We exploit the asymmetric splitting of the optically detected magnetic resonance (ODMR) lines inherent to half-integer S = 3/2 electron spins, such that the certain transitions involving 29Si nuclei can be clearly separated and selectively addressed using radiofrequency (RF) fields. As a model system, we use the V3 silicon vacancy (VSi) in 6H-SiC, which has the zero-filed splitting parameter comparable with the hyperfine interaction constant. Our theoretical model considers DNP under optical excitation in combination with RF driving and agrees very well with the experimental data. We achieve a conversion efficiency of 14% for quadrupole electron spin polarization to nuclear spin polarization, with the theoretical limit at high RF power being 26% under ambient conditions.

To investigate the experimental technique and the high magnetic field effect in the dynamic nuclear polarization (DNP) experiments, X-band 1H-DNP and high-power subterahertz wave irradiation experiments have been performed on bisdiphenylene-phenylallyl-doped toluene solution at room temperature. We have established a sample-shuttle X-band DNP experimental system, and investigated the external magnetic field, the microwave power and 1H T 1 −1 dependences of the Overhauser enhancement factor (e). A maximal signal enhancement of approximately −19 has been observed. As for the subterahertz wave experiments, we have constructed a nuclear magnetic resonance and electron spin resonance measurement system for the experiments under the magnetic field of about 10.7 T, with a high-power light source gyrotron CW-I (TE22,8 mode, f ∼ 299.23 GHz). when the high-power subterahertz wave of a gyrotron was irradiated on the toluene solution in order to confirm the evidence of DNP, the sample has evaporated due to the electromagnetic wave heating.

An apparatus for pulsed ESR and DNP experiments using optically excited triplet states down to liquid helium temperatures

Liquid state DNP for water accessibility measurements on spin-labeled membrane proteins at physiological temperatures

The coherent transfer of electron spin polarization to nuclei by means of a microwave pulse sequence is a promising new approach to enhancing the sensitivity of solid-state nuclear magnetic resonance (NMR). The development of pulse sequences for dynamic nuclear polarization (DNP) of bulk nuclei is far from complete, as is the understanding of what makes a good DNP sequence. In this context, we introduce a new sequence, termed Two-Pulse Phase Modulation (TPPM) DNP. We provide a general theoretical description for electron-proton polarization transfer by periodic DNP pulse sequences and find it in excellent agreement with numerical simulations. In experiments at 1.2T, TPPM DNP generates a higher gain in sensitivity than existing sequences XiX (X-inverse-X) and TOP (Time-Optimized Pulsed) DNP but does so at relatively high nutation frequencies. In contrast, we find that the XiX sequence performs very well at nutation frequencies as low as 7MHz. A combination of theoretical analysis and experimental investigation makes clear that fast electron-proton polarization transfer, due to a well-preserved dipolar coupling in the effective Hamiltonian, correlates with a short build-up time of the dynamic nuclear polarization of the bulk. Experiments further show that the performances of XiX and TOP DNP are affected differently by the concentration of the polarizing agent. These results constitute important reference points for the development of new and better DNP sequences.

Nuclear magnetic resonance (NMR) techniques are extensively used in many areas of basic and clinical research, as well as in diagnostic medicine. However, NMR signals are intrinsically weak, and this imposes substantial constraints on the amounts and concentrations of materials that can be detected. The signals are weak because of the low energies characteristic of NMR and the resulting very low (typically 0.0001-0.01%) polarization of the nuclear spins. Here, we show that exposure to very low temperatures and high magnetic fields, in conjunction with nanoparticle-mediated relaxation enhancement, can be used to generate extremely high nuclear polarization levels on a realistic timescale; with copper nanoparticles at 15 mK and 14 T, (13)C polarization grew towards its equilibrium level of 23% with an estimated half-time of about 60 hours. This contrasts with a (13)C half-time of at least one year in the presence of aluminium nanoparticles. Cupric oxide nanoparticles were also effective relaxation agents. Our findings lead us to suspect that the relaxation may be mediated, at least in part, by the remarkable magnetic properties that some nanoparticle preparations can display. This methodology offers prospects for achieving polarization levels of 10-50% or more for many nuclear species, with a wide range of potential applications in structural biology and medicine.