Electron Paramagnetic Resonance Line Research Articles

Antioxidative and Anti-Inflammatory Phytochemicals and Related Stable Paramagnetic Species in Different Parts of Dragon Fruit.

In this study, we investigated the antioxidant and anti-inflammatory phytochemicals and paramagnetic species in dragon fruit using high-performance liquid chromatography (HPLC) and electron paramagnetic resonance (EPR). HPLC analysis demonstrated that dragon fruit is enriched with bioactive phytochemicals, with significant variations between each part of the fruit. Anthocyanins namely, cyanidin 3-glucoside, delphinidin 3-glucoside, and pelargonidin 3-glucoside were detected in the dragon fruit peel and fresh red pulp. Epicatechin gallate, epigallocatechin, caffeine, and gallic acid were found in the dragon fruit seed. Additionally, 25–100 mg × L−1 of dragon fruit pulp and peel extracts containing enrichment of cyanidin 3-glucoside were found to inhibit the production of reactive oxygen species (ROS), reactive nitrogen species (RNS), inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2) in cell-based studies without exerted cytotoxicity. EPR primarily detected two paramagnetic species in the red samples. These two different radical species were assigned as stable radicals and Mn2+ (paramagnetic species) based on the g-values and hyperfine components. In addition, the broad EPR line width of the white peel can be correlated to a unique moiety in dragon fruit. Our EPR and HPLC results provide new insight regarding the phytochemicals and related stable intermediates found in various parts of dragon fruit. Thus, we suggest here that there is the potential to use dragon fruit peel, which contains anthocyanins, as a natural active pharmaceutical ingredient.

Monte Carlo study of the spin-spin interactions between radicals used for dynamic nuclear polarization

The spectrum of the electron spin-spin interactions largely determines which mechanism is responsible for the growth of the nuclear spin polarization in dynamic nuclear polarization (DNP). When electron spin-spin interactions are weak and their spectrum is narrow, the solid effect (SE) dominates the process. When they are stronger, the cross effect (CE) and thermal mixing (TM) come into play. Then a narrow spectrum favours the CE—that is an exchange of electron Zeeman energy with the nuclear spins—and a broad spectrum also TM—that is an exchange of electron spin-spin interaction energy with the nuclear spins. Moreover, the spectrum of the electron spin-spin interactions critically determines the rate of spectral diffusion of electron spin polarization across the electron spin resonance (ESR) line, and the associated conversion of electron Zeeman energy into electron spin-spin interaction energy. This way electron spin-spin interactions indirectly influence the DNP process. The present work describes Monte Carlo simulations of the spectrum of these interactions for approximately spherical radicals in glasses and analytical approximations of the simulation results. As an example application expressions for the relative strengths of the energy flows due to the CE and TM are derived.

Influence of temperature on the magnetic properties of Mn3O4 nanowires

Single crystalline Mn3O4 nanowires have been synthesized with tetragonal hausmannite structure using a solvothermal method. The structural and morphological evolution of Mn3O4 nanowires have been characterized using powder X-ray diffraction, transmission electron microscopy, and electron resonance spectroscopy. The nanowires were grown uniformly along the (200) direction with a diameter of 5–10 nm range. A relatively broad and intense electron spin resonance (ESR) signal was observed at room temperature, with the g ≈ 2.0. As the synthesis temperature increases from 150 to 250 °C, a decrease in ESR signal intensity and line widths were observed. Mn3O4 displayed a positive Curie-Weiss temperature, θ, which decreases with the increase of synthesis temperature.

Electron Spin Resonance in Heterostructures with Ring Molecules of GeSi Quantum Dots

Heterostructures with annular groups of GeSi quantum dots grown on Si(001) substrates with GeSi nanodisks embedded beneath the surface are investigated by the electron spin resonance technique. It is demonstrated that electrons in annular groups can be localized at the apexes of quantum dots in the annular groups. Narrowing of the electron spin resonance line of the annular electron states localized at the nanodisks is observed. The width of this line decreases when the magnetic field deviates from the structure growth direction, which confirms experimentally the earlier prediction that the spin relaxation time T2 should increase in this case.

Structural, paramagnetic centres and luminescence investigations of the UV radiation-emitting BaZrO3:Gd3+ perovskite ceramic prepared via sol-gel route

Perovskite-type ceramic materials have been widely studied due to their potential use in such applications as electrical, thermal, optical, and immobilizing host materials in nuclear waste. In this work, Gd3+-doped BaZrO3 perovskite samples were prepared by sol-gel method and characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), electron paramagnetic resonance (EPR) spectroscopy, and photoluminescence (PL) analysis. Our XRD analysis confirmed the formation of the sol-gel products, and the SEM data implicated the prepared porous compound as the cause of the gases that evolved during the preparation. Upon excitation at 275 nm, the ceramic material displayed UVB emission at 315 nm (6P7/2 → 8S7/2). The EPR technique was used to study the undoped and gadolinium-doped perovskite BaZrO3 ceramics. The lines in the undoped system were caused by two defect centers. Center I displayed an isotropic line with a g-factor of 1.973 and a linewidth of 48 Gauss, and the center was assigned to an F+ center. Center II exhibited an axially symmetric g-tensor with principal values g|| = 1.895 and g⊥ = 1.978. This center was identified as the Zr3+ ion. The single dominant EPR line with an effective g-value of 1.976 that was observed in the Gd-doped system was attributed to the Gd3+ ion experiencing a weak cubic symmetry crystal field.

Magnetic interactions in the tripod kagome antiferromagnet Mg2Gd3Sb3O14 probed by static magnetometry and high-field ESR spectroscopy

We report an experimental study of the static magnetization $M(H,T)$ and high-field electron spin resonance (ESR) of polycrystalline \MgGd, a representative member of the newly discovered class of the so-called tripod-kagome antiferromagnets where the isotropic Gd$^{3+}$ spins ($S = 7/2$) form a two-dimensional kagome spin-frustrated lattice. It follows from the analysis of the low-$T$ $M(H)$-curves that the Gd$^{3+}$ spins are coupled by a small isotropic antiferromagnetic (AFM) exchange interaction $|J| \approx$ 0.3\,K. The $M(H,T)$-dependences measured down to 0.5\,K evidence a long-range AFM order at $T_{\text{N}} = 1.7$\,K and its rapid suppression at higher fields $\geq 4$\,T. ESR spectra measured in fields up to 15\,T are analyzed considering possible effects of demagnetizing fields, single-ion anisotropy and spin-spin correlations. While the demagnetization effects due to a large sample magnetization in high fields and its shape anisotropy become relevant at low temperatures, the broadening of the ESR line commencing already at $T\lesssim 30$\,K may indicate the onset of the spin-spin correlations far above the ordering temperature due to the geometrical spin frustration in this compound.

Dynamic Nuclear Polarization with Vanadium(IV) Metal Centers

Summary Dynamic nuclear polarization (DNP) harnesses the large polarization of electron spins to dramatically increase nuclear magnetic resonance (NMR) sensitivity. This study expands the scope of DNP beyond its traditional focus on hyperpolarizing the solvent network using exogenous polarizing agents (PAs). We introduce 1H DNP with endogenous V4+ centers positioned in a set of vanadyl complexes with tunable V4+-1H distances. We traced the polarization transfer from V4+ to 1H spins, specifically differentiating between direct V4+-1Hs polarization transfer and the 1H spin-diffusion-mediated bulk solvent 1H polarization buildup and illuminated the effect of the V4+-1H distance on these processes. These results deepen our understanding of polarization pathways and expand the catalog of PAs to broad-line transition metals. This study establishes crucial first steps toward employing strategically positioned endogenous paramagnetic metal centers for DNP and the conceptual framework of hyperfine DNP spectroscopy that merges both spatial and chemical diagnosis of target nuclear spins.

Role of the paramagnetic donor-like defects in the high n-type conductivity of the hydrogenated ZnO microparticles

The magnetic and electronic properties of the hydrogenated highly conductive zinc oxide (ZnO) microparticles were investigated by electron paramagnetic resonance (EPR) and contactless microwave (MW) conductivity techniques in the wide temperature range. The EPR spectra simulation allowed us to resolve four overlapping EPR signals in ZnO microparticles. The Lorentzian EPR line with isotropic g-factor 1.9623(5) was related to the singly ionized oxygen vacancy. Another Lorentzian line with g|| = 1.9581(5), g⊥ = 1.9562(5) was attributed to the zinc interstitial shallow donor center, while EPR signal with g|| = 1.9567(5), g⊥ = 1.9556(5) and Gaussian lineshape was assigned to the hydrogen interstitial shallow effective-mass-like donor. The EPR signal with g|| = 1.9538(5), g⊥ = 1.9556(5) and Lorentzian lineshape was tentatively attributed to the shallow donor center. The charge transport properties in ZnO microparticles have been investigated by the contactless MW conductivity technique at T = 5–296 K. Two conduction mechanisms, including ionization of electrons from the shallow donors to the conduction band and hopping conduction process, have been distinguished. The hopping conduction process follows Mott’s variable-range hopping T−1/4 law at T = 10–100 K. The evaluated values of the average hopping distance (15.86 Å), and hopping energy (1.822 meV at 40 K) enable us to estimate the donor concentration in the investigated ZnO microparticles as ~ 1018 cm−3.

1H Thermal Mixing Dynamic Nuclear Polarization with BDPA as Polarizing Agents.

Dynamic Nuclear Polarization (DNP) is a sensitivity enhancing technique for Nuclear Magnetic Resonance. A recent discovery of Overhauser Effect (OE) DNP in insulating systems under cryogenic conditions using 1,3-bisdiphenylene-2-phenylallyl (BDPA) as the polarizing agent (PA) has caught attention due to its promising DNP performance at a high magnetic field and under fast magic angle spinning conditions. However, the mechanism of OE in insulating-solids/BDPA is unclear. We present an alternative explanation that the dominant underlying DNP mechanism of BDPA is Thermal Mixing (TM). This is ascertained with the discovery that TM effect is enhanced by multi-electron spin coupling, which is corroborated by an asymmetric electron paramagnetic resonance line shape signifying the coexistence of clustered and isolated BDPA species, and by hyperpolarized electron spin populations giving rise to an electron spin polarization gradient which are characteristic signatures of TM DNP. Finally, quantum mechanical simulations using spatially asymmetrically coupled three electron spins and a nuclear spin demonstrate that triple-flip DNP, with hyperfine fluctuations turned off, can yield the 1H DNP profile as observed with BDPA. Clarifying the DNP mechanism is critical to develop design principles for optimizing the PA for achieving optimal DNP efficiency.

EPR and magnetization studies of the manganites La0.7-xEuxSr0.3MnO3 (x = 0.4, 0.5, 0.6, 0.7) and La0.3Nd0.4Sr0.3MnO3 at different temperatures: Conductivity due to hopping of small polarons

Four Eu-Sr manganites, La0.7-xEuxSr0.3MnO3; x = 0.4, 0.5, 0.6, 0.7, and one Nd-Sr manganite Nd0.4La0.3Sr0.3MnO3, were studied by EPR (electron paramagnetic resonance) at 9.6 GHz at temperatures in the range 175 – 400 K. These studies were further supplemented by magnetization measurements. The positions and widths of the EPR lines of all the Eu-Sr manganite samples, containing Eu3+ ions, changed with temperature in a similar manner. The EPR data showed that these Eu-Sr manganites are ferromagnetic, whose magnetic transition temperature from the ferromagnetic to the paramagnetic state (TC) decreased gradually as the Eu content (x) increased, specifically from TC ~ 111 K (x = 0.5) to TC ~ 57 K (x = 0.7). Furthermore, in these samples the FMR (ferromagnetic resonance) lines appeared significantly above the respective TC, specifically, at 290 K (x = 0.4), 280 K (x = 0.5), 250 K (x = 0.6), together with the EPR lines. As for the sample with x = 0.7 no FMR lines were observed in the temperature range investigated here. The temperature dependence of the EPR linewidth is found to be linear in the various La0.7-xEuxSr0.3MnO3 samples, caused by the presence of conductivity due to small-polaron hopping The peak-to-peak first-derivative EPR linewidths, ΔBpp, of these Eu-doped samples fitted well above the temperature, at which the minimum of ΔBpp occurs, to the expression: ΔBpp (T) = ΔBpp,min + ATexp(-Ea/kBT), with the values of the activation energies being Ea = 0.20 eV, 0.17 eV, 0.11 eV, 0.09 eV for x = 0.4, 0.5, 0.6, 0.7, respectively. On the other hand, for the Nd-doped sample, the ΔBpp in Nd0.4La0.3Sr0.3MnO3 decreased monotonically with temperature above TC in accordance with the Curie-Weiss law as ΔBpp (T) = ΔBpp,0 + C/(T-TC). The magnetization measurements were exploited to determine TC for the various samples. The results agreed with those determined by the EPR measurements presented here. In addition, the magnetization data was analyzed to obtain the values of the critical exponents in the phase transition from the paramagnetic to the ferromagnetic state. The critical exponents, as determined for the La0.7-xEuxSr0.3MnO3 sample with x = 0.5, are β = 0.24 ± 0.02, γ = 1.10 ± 0.5, δ = 5.58.

High NV density in a pink CVD diamond grown with N2O addition

Growing high-purity diamond containing dense negatively charged nitrogen-vacancy (NV−) centre ensembles is desirable for the development of sensitive quantum sensors that explore the coherent manipulation of the spin states of this atomic-scale defect. By using N2O as a dopant, we demonstrate that millimetre-thick single crystals can be grown by Chemical Vapour Deposition (CVD) with substitutional nitrogen concentrations as high as 26 ppm. With a high-energy electron irradiation treatment and in-situ annealing, up to 20% of this nitrogen can be successfully converted into NV− centres leading to densities of almost 5 ppm and to a crystal displaying pink colouration and appealing optical properties. The longitudinal relaxation T1 time in such a highly doped diamond is measured to be around 3.5 ms at 300 K while inhomogeneous dephasing time T2∗ is estimated to 600 ns. Despite the high NV density, Electron Spin Resonance lines are resolved with clear hyperfine splitting induced by the nuclear spin of nitrogen. The dependence of T1 on the orientation of the magnetic field suggests that relaxation is dominated by NV-NV dipole interactions when no magnetic field is applied. Such engineered crystals constitute a promising platform for developing future quantum sensing applications.

13C Dynamic Nuclear Polarization using SA-BDPA at 6.7 T and 1.1 K: Coexistence of Pure Thermal Mixing and Well-Resolved Solid Effect.

SA-BDPA is a water-soluble, narrow-line width radical previously used for dynamic nuclear polarization (DNP) signal enhancement in solid-state magic angle spinning NMR spectroscopy. Here, we report the first study using SA-BDPA under dissolution DNP conditions (6.7 T and 1.15 K). Longitudinal-detected (LOD)-electron spin resonance (ESR) and 13C DNP measurements were performed on samples containing 8.4 M [13C]urea dissolved in 50:50 water:glycerol (v/v) doped with either 60 or 120 mM SA-BDPA. Two distinct DNP mechanisms, both "pure" thermal mixing and a well-resolved solid effect could clearly be identified. The radical's ESR line width (30-40 MHz), broadened predominantly by dipolar coupling, excluded any contribution from the cross effect. Microwave frequency modulation increased the enhancement by DNP at the lower radical concentration but not at the higher radical concentration. These results are compared to data acquired with trityl radical AH111501, highlighting the unusual 13C DNP properties of SA-BDPA.

Weak Antiferromagnet Iron Borate FeBO3. Classical Object for Magnetism and the State of the Art

The simple lattice and magnetic structure, the high Neel temperature, the narrow antiferromagnetic resonance line of FeBO3, and the narrow electron paramagnetic resonance line of its isostructural diamagnetic analogs MBO3:Fe3+ (M = Ga, In, Sc, Lu) make iron borate unique for investigations and applications. Iron borate is a model crystal for numerous experimental and theoretical studies, including spin crossovers and metallization at megabar pressures and many-electron effects in optics and X-ray spectroscopy. The recent works dealing with the investigation of the properties of FeBO3 are reviewed.

Electron paramagnetic resonance study of type-II silicon clathrate with low sodium guest concentration

An electron paramagnetic resonance (EPR) study of type-II silicon (Si) clathrate with low sodium (Na) guest concentration provides insights into the properties of this cagelike Si allotrope and into Na as an $n$-type dopant. An EPR line arising from unterminated Si bonds in a disordered Si phase is identified in the system. The presence of this amorphouslike component helps explain the thermally activated, hopping-related conductivity that has long been observed in the system but which has been poorly understood. This study also identifies ``superhyperfine'' lines surrounding each Na hyperfine line due to interactions of the donor electron with a $^{29}\mathrm{Si}$ nucleus on the cage and confirms the identification of hyperfine lines due to interactions of two neighboring Na nuclei. Analysis of the strength of these interactions shows that the electron wave function associated with the Na donor extends onto surrounding cages consistent with identification of Na as a shallow donor, and provides quantitative evidence of Na clustering.

Application of spherical harmonics for DEER data analysis in systems with a conformational distribution

Double electron–electron resonance (DEER) and other pulse electron paramagnetic resonance (EPR) techniques are valuable tools for determining distances between paramagnetic centres. DEER theory is well developed for a scenario where relative orientations of paramagnetic centres do not affect the DEER data. In particular, such theory enables a number of approaches for extracting distance distributions. However, in a more general case, when orientation selection effects become substantial, the analytical theory of DEER is less well developed, therefore quite commonly researchers rely on a comparison of some model-based simulations with experimental data. This work elaborates the theory of DEER with orientation selection effects, focusing on a scenario of a moderate conformational disorder, leading to an orientation distribution in a pair of paramagnetic centres. The analytical treatment based on expansions into spherical harmonics, provides important insights into the structure of DEER data. As follows from this treatment, DEER spectra with orientation selection can be represented as a linear combination of modified Pake pattern (MPP) components. The conformational disorder has a filtering effect on the weights of MPP components, specifically by significantly suppressing MPP components of higher degrees. The developed theory provides a pathway for model-based simulations of DEER data where orientation distribution is defined by analytical functions with parameters. The theory based on spherical harmonics expansions was also applied to develop an iterative processing algorithm based on Tikhonov regularization, which disentangles the distance and orientation information in a model-free manner. As an input, this procedure takes several DEER datasets measured at various positions of an EPR line, and outputs a distance distribution and orientation distribution information encoded in a set of coefficients related to the weights of MPP components. The model-based and model-free approaches based on the developed theory were validated for a nitroxide biradical and a spin-labelled protein.

Structural Dynamics and Topology of the Inactive Form of S21 Holin in a Lipid Bilayer Using Continuous-Wave Electron Paramagnetic Resonance Spectroscopy.

The bacteriophage infection cycle plays a crucial role in recycling the world's biomass. Bacteriophages devise various cell lysis systems to strictly control the length of the infection cycle for an efficient phage life cycle. Phages evolved with lysis protein systems, which can control and fine-tune the length of this infection cycle depending on the host and growing environment. Among these lysis proteins, holin controls the first and rate-limiting step of host cell lysis by permeabilizing the inner membrane at an allele-specific time and concentration hence known as the simplest molecular clock. Pinholin S21 is the holin from phage Φ21, which defines the cell lysis time through a predefined ratio of active pinholin and antipinholin (inactive form of pinholin). Active pinholin and antipinholin fine-tune the lysis timing through structural dynamics and conformational changes. Previously we reported the structural dynamics and topology of active pinholin S2168. Currently, there is no detailed structural study of the antipinholin using biophysical techniques. In this study, the structural dynamics and topology of antipinholin S2168IRS in DMPC proteoliposomes is investigated using electron paramagnetic resonance (EPR) spectroscopic techniques. Continuous-wave (CW) EPR line shape analysis experiments of 35 different R1 side chains of S2168IRS indicated restricted mobility of the transmembrane domains (TMDs), which were predicted to be inside the lipid bilayer when compared to the N- and C-termini R1 side chains. In addition, the R1 accessibility test performed on 24 residues using the CW-EPR power saturation experiment indicated that TMD1 and TMD2 of S2168IRS were incorporated into the lipid bilayer where N- and C-termini were located outside of the lipid bilayer. Based on this study, a tentative model of S2168IRS is proposed where both TMDs remain incorporated into the lipid bilayer and N- and C-termini are located outside of the lipid bilayer. This work will pave the way for the further studies of other holins using biophysical techniques and will give structural insights into these biological clocks in molecular detail.

Ultralong Spin Lifetime in Light Alkali Atom Doped Graphene.

Today’s great challenges of energy and informational technologies are addressed with a singular compound, Li- and Na-doped few-layer graphene. All that is impossible for graphite (homogeneous and high-level Na doping) and unstable for single-layer graphene works very well for this structure. The transformation of the Raman G line to a Fano line shape and the emergence of strong, metallic-like electron spin resonance (ESR) modes attest the high level of graphene doping in liquid ammonia for both kinds of alkali atoms. The spin-relaxation time in our materials, deduced from the ESR line width, is 6–8 ns, which is comparable to the longest values found in spin-transport experiments on ultrahigh-mobility graphene flakes. This could qualify our material as a promising candidate in spintronics devices. On the other hand, the successful sodium doping, this being a highly abundant metal, could be an encouraging alternative to lithium batteries.

Trivalent gadolinium doped SrZrO3 perovskite ceramic: Sol-gel synthesis, paramagnetic centres, and luminescence studies

Trivalent gadolinium (Gd3+) doped SrZrO3 perovskite ceramic has been prepared and their X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), electron paramagnetic resonance (EPR), and photoluminescence (PL) spectra are recorded. The existence of divalent and tetravalent cations in SrZrO3 along with a trivalent dopant ion provides an opportunity to study the effect of charge compensators on the system properties. The cubic phase is evident from XRD diffraction patterns of the product. The SEM analysis suggests the formation of voids and pores in the ceramics caused by gases those liberated during the process of sintering. The EPR technique has been used to investigate the un-doped and Gd3+-doped SrZrO3 ceramics. The un-doped SrZrO3 shows an axially symmetric EPR line with g∥ = 1.94 and g⊥ = 1.99, which is attributed to the Zr3+ ion. The Gd3+-doped SrZrO3 shows dominant EPR lines with effective g-values at 1.93, 2.04, and 2.35. Weak lines are observed at 2.72, 3.64, 6.08, and 15.5. At higher dopant concentrations, a broad line with geff ~2.1 is seen, together with low field lines. In the low field region of the EPR spectrum, lines appear because of immediate environment distortions. Under the excitation of 273 nm, the emission spectrum shows band 6P7/2 → 8S7/2 (313 nm). The emission at 313 nm suggests that the prepared sample can be used as a narrow band UV radiation source. Emission and EPR analysis of the sample confirm that Gd3+ is distributed in both Sr2+ and Zr4+ sites in the SrZrO3 matrix.

Long‐Standing Optically Excited Magnetic States in ZnO Nanoparticles

The spectral dependence of the light‐induced magnetic states generated in zinc oxide nanoparticles by illumination during an electron paramagnetic resonance (EPR) experiment is studied for three incident light wavelengths (451, 405, and 370 nm). The minimal wavelength excitation is found to be in the visible violet region, very close to 405 nm for all of the light‐induced EPR lines. This points to a close relationship between their originating centers, despite the difference in their dynamics previously revealed by saturation recovery measurements. The study of the light‐induced signals decay with time (after illumination removal) and with temperature (under constant illumination) confirms the existence of two groups of lines characterized by different dynamics. The quite unusual persistence of these magnetic states after excitation removal opens promising possibilities for encoding information within semiconducting nanoparticles.

Crossover from a Solid Effect to Thermal Mixing 1H Dynamic Nuclear Polarization with Trityl-OX063.

Trityl-OX063 is a narrow-line, water-soluble, and biocompatible polarizing agent, widely used for dynamic nuclear polarization (DNP) amplified NMR of 13C, but not of the abundant 1H nuclear spin, for which the ineffective solid effect (SE) is expected to be operational. Surprisingly, we observed a crossover from SE to thermal mixing (TM) DNP of 1H with increasing Trityl-OX063 concentration at 7 T. We experimentally ascertained diagnostic signatures of TM-DNP that have only been theoretically predicted: (i) an electron paramagnetic resonance (EPR) spectrum that maintains an asymmetrically broadened EPR line from strong e-e couplings and (ii) hyperpolarization, i.e., cooling of select electron-spin populations, manifested in a characteristic pump-probe electron double-resonance spectrum under DNP conditions. Low microwave power requirements, high polarization transfer rates, and efficient DNP at high magnetic fields are the key benefits of TM-DNP.