Overhauser DNP and EPR in a Mobile Setup: Influence of Magnetic Field Inhomogeneity

Power-dependent Overhauser dynamic nuclear polarization (DNP) enhancements and continuous-wave electron paramagnetic resonance (EPR) spectra of nitroxide radicals were measured in the magnetic field of a mobile Halbach-array permanent magnet and compared with results from a commercially available electromagnet. DNP saturation factors for varying microwave power were obtained from both measurement series and used to investigate how the increased magnetic field inhomogeneity present in the Halbach magnet affects the saturation efficiency. An EPR detection system was designed to allow continuous-wave EPR measurements at microwave power up to 20?W. Our results show that despite the lower magnetic field homogeneity, a Halbach-array magnet can be used for EPR and DNP-enhanced nuclear magnetic resonance of high quality providing almost the same performance as a more homogeneous electromagnet.     

DNP-EPR多功能谱仪中微波桥的设计与实现

在自主研制的动态核极化（Dynamic Nuclear Polarization,DNP）分子影像装置的基础上,提出了一种集DNP和电子顺磁共振（Electron Paramagnetic Resonance,EPR）于一体的多功能谱仪,并对其中的关键部件之一--微波桥进行了设计.微波桥的引入,实现了DNP微波发射机的集成化,以及在DNP谱仪基础上的EPR功能扩展.通过结构设计、电路仿真及系统测试,完成了高频谱纯度、高动态范围的微波发射机以及低噪声系数的微波检测系统的设计与制作.并通过DNP增强实验以及连续波EPR实验对微波桥的性能进行了验证.     

Stability and mobility of the endohedral metallofullerene of La@C<Subscript>82</Subscript> in polycarbonate polymer films

An La@C₈₂-doped polymer (bisphenol-A polycarbonate) was prepared. EPR spectra of La@C₈₂ in a solid polymer film and in a polycarbonate solution in o-dichlorobenzene were studied. The EPR spectrum shape of La@C₈₂ was shown to be sensitive to the phase state of the polymer. La@C₈₂ in the film was found to have a high chemical stability below the glass formation temperature, thus permitting one to use the polymer for the storage and study of endohedral metallofullerenes.     

A multi-sample 94 GHz dissolution dynamic-nuclear-polarization system

We describe the design and initial performance results of a multi-sample dissolution dynamic-nuclear-polarization (DNP) polarizer based on a Helium-temperature NMR cryostat for use in a wide-bore NMR magnet with a room-temperature bore. The system is designed to accommodate up to six samples in a revolver-style sample changer that allows changing samples at liquid-Helium temperature and at pressures ranging from ambient pressure down to 1 mbar. The multi-sample setup is motivated by the desire to do repetitive in vivo measurements and to characterize the DNP process by investigating samples of different chemical composition. The system can be loaded with up to six samples simultaneously to reduce sample loading and unloading. Therefore, series of experiments can be carried out faster and more reliably. The DNP probe contains an oversized microwave cavity and includes EPR and NMR capabilities for monitoring the DNP process. In the solid state, DNP enhancements corresponding to ～45% polarization for [1-(13)C]pyruvic acid with a trityl radical have been measured. In the initial liquid-state acquisition experiments described here, the polarization was found to be ～13%, corresponding to an enhancement factor exceeding 16,000 relative to thermal polarization at 9.4 T and ambient temperature.     

Low Field (10 mT) Pulsed Dynamic Nuclear Polarization

EPR irradiation by a train of inverting pulses has potential advantages over continuous-wave EPR irradiation in DNP applications; however, it has previously been used only at high field (5 T). This paper presents the design and testing of an apparatus for performing pulsed DNP experiments at 10 mT with large samples (17 ml). Experimental results using pulsed DNP with an aqueous solution of a narrow-linewidth paramagnetic probe are presented. A maximum DNP enhancement of about -36 with a train of inverting pulses (width 500 ns, repetition time 4 micros) was measured. A preliminary comparison showed that, when the same enhancement value is considered, the pulsed DNP technique requires an average power that is about three times higher than that required with the CW irradiation. However, for in vivo DNP applications it is very important to minimize the average power deposited in the sample. From the experimental results reported in this work, when considering the maximum enhancement, the pulsed technique requires only 2% of the average power necessary with the CW DNP technique. We believe that this reduction in the average power can be important for future DNP studies with large biological samples.     

EPR study of spin and charge dynamics in slightly doped poly(3-octylthiophene)

An initial virgin plasticine-like poly(3-octylthiophene) (P3OT) and this sample modified by annealing and recrystallization were investigated at X-band (10 GHz) and mainly at D-band (140 GHz) electron paramagnetic resonance (EPR) in a wide temperature region. Paramagnetic centers with anisotropic magnetic-resonance parameters were proved to exist in all polymers, namely, mobile polarons whose concentration and susceptibility depend on the temperature and the polymer treatment. Superslow torsional motion of the polymer chains and layers was studied by the saturation transfer method at D-band EPR. Spin-spin and spin-lattice relaxation times were measured separately by the steady-state saturation method at the same waveband EPR. Intrachain and interchain spin diffusion coefficients and conductivity due to polaron dynamics were calculated. It was shown that the charge transport in P3OT is determined by the strong spin-spin interaction and is stimulated by torsion motion of the polymer chains. The total conductivity of P3OT is determined mainly by dynamics of paramagnetic charge carriers. Magnetic, relaxation and dynamics parameters of P3OT were also shown to change during the polymer treatment.     

Theoretical Aspects of Dynamic Nuclear Polarization in the Solid State: The Influence of High Radical Concentrations on the Solid Effect and Cross Effect Mechanisms

Dynamic nuclear polarization (DNP) is used to enhance signals in NMR and MRI experiments. During these experiments microwave (MW) irradiation mediates transfer of spin polarization from unpaired electrons to their neighboring nuclei. Solid state DNP is typically applied to samples containing high concentrations (i.e. 10–40?mM) of stable radicals that are dissolved in glass forming solvents together with molecules of interest. Three DNP mechanisms can be responsible for enhancing the NMR signals: the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Recently, numerical simulations were performed to describe the SE and CE mechanisms in model systems composed of several nuclei and one or two electrons. It was shown that the presence of core nuclei, close to DNP active electrons, can result in a decrease of the nuclear polarization, due to broadening of the double quantum (DQ) and zero quantum (ZQ) spectra. In this publication we consider samples with high radical concentrations, exhibiting broad inhomogeneous EPR line-shapes and slow electron cross-relaxation rates, where the TM mechanism is not the main source for the signal enhancements. In this case most of the electrons in the sample are not affected by the MW field applied at a discrete frequency. Numerical simulations are performed on spin systems composed of several electrons and nuclei in an effort to examine the role of the DNP inactive electrons. Here we show that these electrons also broaden the DQ and ZQ spectra, but that they hardly cause any loss to the DNP enhanced nuclear polarization due to their spin-lattice relaxation mechanism. Their presence can also prevent some of the polarization losses due to the core nuclei.     

EPR diagnostics of laser materials based on ZnSe crystals doped with transition elements

The electron paramagnetic resonance (EPR) spectra observed in laser materials based on zinc selenide (ZnSe) crystals doped with transition elements have been analyzed and identified. It has been shown that, in addition to working impurities (Cr²⁺, Co²⁺, or Fe²⁺), the diffusion layer exhibits EPR spectra of accompanying impurities due to the diffusion of transition elements (chromium, cobalt, or iron) used in the preparation of active materials for quantum electronics (lasers, switches) operating in the mid-infrared range. EPR diagnostics of these impurities can be used in the development of appropriate regimes for minimizing concentrations of accompanying impurities that adversely affect the performance characteristics of laser materials. It has been found that, during the diffusion of transition metals, ions of the accompanying impurity Mn²⁺, which is characterized by extremely informative EPR spectra, are embedded in the crystal lattice. It has been proposed to use these ions as ideal markers to control, on the electronic level, the crystal structure of the active diffusion layer.     

Polymer stabilized Ni-Ag and Ni-Fe alloy nanoclusters: Structural and magnetic properties

We report here the structural and magnetic behaviors of nickel-silver (Ni-Ag) and nickel-iron (Ni-Fe) nanoclusters stabilized with polymer (polypyrrole). High resolution transmission electron microscopy (HRTEM) indicates Ni-Ag nanoclusters to stabilize in core-shell configuration while that of Ni-Fe nanoclusters in a mixed type of geometry. Structural characterizations by X-ray diffraction (XRD) reveal the possibility of alloying in such bimetallic nanoclusters to some extent even at temperatures much lower than that of bulk alloying. Electron paramagnetic resonance (EPR) spectra clearly reveal two different absorption behaviors: one is ascribed to non-isolated Ni²⁺ clusters surrounded by either silver or iron giving rise to a broad signal, other (very narrow signal) being due to the isolated superparamagnetic Ni²⁺ clusters or bimetallic alloy nanoclusters. Results obtained for Ni-Ag and Ni-Fe nanoclusters have been further compared with the behavior exhibited by pure Ni nanoclusters in polypyrrole host. Temperature dependent studies (at 300 and 77 K) of EPR parameters, e.g. linewidth, g-value, line shape and signal intensity indicating the significant influence of surrounding paramagnetic silver or ferromagnetic iron within polymer host on the EPR spectra have been presented.     

Some EPR spin probe and spin label studies of polymer systems

The application of EPR spin probe and spin label technique for solving the two problems of polymer physical chemistry is considered. The first problem is the determination of conformational state and chain sizes in amorphous solid polymers. This determination is based on the analysis of the intramolecular dipolar broadening of EPR spectra of spin labelled macromolecules in glassy solvents or in the bulk of unlabeled polymers at 77 K. The second problem is the determination of molecular mobility and structure of two polymer colloid systems: (a) the complex of colloidal silica and synthetic polycation macromolecule and (b) polymer-surfactant micellar organized systme.     

Polyacetylene doped with iron trichloride : Improved conductivity and stability

Trans polyacetylene (CH)_x has been doped with FeCl₃ solutions in Nitromethane. In the “in-situ” EPR experiment we have observed the evolution of the EPR spectra with time during the doping process. That enabled us to gain information about the rate of dopant diffusion in solid material. Both EPR and conductivity measurements show that the doped material is reasonably stable even in the presence of air. Raman spectroscopic studies show the usual features observed in doped trans polyacetylene.     

A Dynamic Nuclear Polarization spectrometer at 95 GHz/144 MHz with EPR and NMR excitation and detection capabilities

A spectrometer specifically designed for systematic studies of the spin dynamics underlying Dynamic Nuclear Polarization (DNP) in solids at low temperatures is described. The spectrometer functions as a fully operational NMR spectrometer (144 MHz) and pulse EPR spectrometer (95 GHz) with a microwave (MW) power of up to 300 mW at the sample position, generating a MW B(1) field as high as 800 KHz. The combined NMR/EPR probe comprises of an open-structure horn-reflector configuration that functions as a low Q EPR cavity and an RF coil that can accommodate a 30-50 μl sample tube. The performance of the spectrometer is demonstrated through some basic pulsed EPR experiments, such as echo-detected EPR, saturation recovery and nutation measurements, that enable quantification of the actual intensity of MW irradiation at the position of the sample. In addition, DNP enhanced NMR signals of samples containing TEMPO and trityl are followed as a function of the MW frequency. Buildup curves of the nuclear polarization are recorded as a function of the microwave irradiation time period at different temperatures and for different MW powers.     

A field-sweep/field-lock system for superconducting magnets--Application to high-field EPR

We describe a field-lock/field-sweep system for the use in superconducting magnets. The system is based on a commercially available field mapping unit and a custom designed broad-band 1H NMR probe. The NMR signal of a small water sample is used in a feedback loop to set and control the magnetic field to high accuracy. The current instrumental configuration allows field sweeps of +/-0.4 T and a resolution of up to 10(-5) T (0.1 G) and the performance of the system is demonstrated in a high-field electron paramagnetic resonance (EPR) application. The system should also be of utility in other experiments requiring precise and reproducible sweeps of the magnetic field such as DNP, ENDOR or PELDOR.     

Low-Field, Time-Resolved Dynamic Nuclear Polarization with Field Cycling and High-Resolution NMR Detection

Dynamic nuclear polarization (DNP) effects in aqueous solutions of stable 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) radicals were studied in a pulsed mode of pumping the electron paramagnetic resonance (EPR) transitions. Our fast field cycling setup allowed us to perform the EPR pumping at low magnetic fields and to detect the enhanced nuclear magnetic resonance signals at 7 T with high spectral resolution. Pumping was performed at two different frequencies, 300 MHz and 1.4 GHz, corresponding to magnetic fields around 10 and 48.6 mT, respectively. For both frequencies, the DNP enhancements were close to the limiting theoretical values of ?110 (¹⁴N TEMPOL) and ?165 (¹⁵N TEMPOL). Our pulsed experiments exploit coherent motion of the electronic spins in the radio-frequency field as seen by an oscillatory component in the dependence of the DNP effect on the radio-frequency pulse duration. The DNP enhancement was studied in detail as a function of the pulse length, duty cycle, delay between the pulses, and applied power. The analysis of the results shows that pulsed DNP experiments provide an opportunity to achieve enhancements of about ?110 with relatively low applied power as compared to the standard continuous-wave DNP experiments. An adequate theoretical approach to the problem under study is suggested.     

An Electron Paramagnetic Resonance Study of the Orbital Ordering Compound MgTi<Subscript>2</Subscript>O<Subscript>4</Subscript>

We report the electron paramagnetic resonance (EPR) studies of MgTi₂O₄ in the 300–140 K range. Above the transition temperature T _t (~258 K), the EPR results indicate that MgTi₂O₄ is paramagnetic. The parameters of the EPR spectra show an anomalous change at T _t. The clear EPR lines can be observed in temperature between T _t and 220 K. Besides that the EPR intensity, g value, and EPR linewidth increase with decreasing temperature; in temperature range below 220 K, no clear EPR line can be detected. The EPR spectra results demonstrate that magnetic spin-singlet state and the orbital density wave of MgTi₂O₄ system are formed gradually with decreasing temperature at low temperature range.     

Potential‐induced Raman behavior of individual (R)‐di‐2‐naphthylprolinol molecules on a Ag‐modified Ag electrode

The applicability of surface‐enhanced Raman spectroscopy is demonstrated to probe the adsorption behavior of individual molecules on a Ag electrode. High‐quality SERS spectra of (R)‐di‐2‐naphthylprolinol (DNP) were obtained from ultradilute solutions (10⁻¹² M ) on the Ag‐nanoparticle‐modified Ag electrode, which is attributed to the high electromagnetic (EM) effect of the SERS‐active system as well as to the strong adsorption and interaction of DNP molecules with Ag. The stable SERS spectra present remarkable potential dependence, which gives evidence for the behavior of individual DNP molecules on the Ag surface. Based on statistical analysis for the probability of DNP molecules located in ‘hot spots’, we propose an SERS mechanism for individual molecules in the electrode system, in combination with the hot‐spot model and orientation of the probe molecules. Copyright © 2010 John Wiley & Sons, Ltd.     

EPR investigations of mesoporous silica doped with metal transitions ions

Electron paramagnetic resonance (EPR) experiments were performed on mesoporous silica powders in which (1,4,8,11-tetraazacyclotetradecane) cyclam groups were incorporated. These functionalised groups allow an easy binding with copper and nickel ions. Comparative studies are carried out on samples functionalised by cyclam groups located either inside the pores or in the walls of the mesoporous structures. Copper and nickel EPR parameters, including g-tensors and hyperfine components are determined and relevant electronic, magnetic and structural information are obtained. The EPR spectra intensities and line-widths are investigated on the temperature range [4 K,300 K] to clarify the relative dispersion or agglomeration of the doping ions in the matrices as well as their possible thermally activated mobility and ions pairing. As a support of the experimental EPR investigations, numerical simulations of the geometry of metallic ion environments and their electronic properties are carried out and discussed. The possibility of dynamic Jahn Teller (JT) effect in the temperature range [200, 60 K] is discussed for the nickel doped matrices where the low temperature quenched JT configuration is thought to favour the formation of Ni³⁺ pairs.     

EPR spectroscopy of protein microcrystals oriented in a liquid crystalline polymer medium

Correlation of the g-tensor of a paramagnetic active center of a protein with its structure provides a unique experimental information on the electronic structure of the metal site. To address this problem, we made solid films containing metalloprotein (Desulfovibrio gigas cytochrome c(3)) microcrystals. The microcrystals in a liquid crystalline polymer medium (water/hydroxypropylcellulose) were partially aligned by a shear flow. A strong orientation effect of the metalloprotein was observed by EPR spectroscopy and polarizing optical microscopy. The EPR spectra of partially oriented samples were simulated, allowing for molecular orientation distribution function determination. The observed effect results in enhanced sensitivity and resolution of the EPR spectra and provides a new approach towards the correlation of spectroscopic data, obtained by EPR or some other technique, with the three-dimensional structure of a protein or a model compound.     

Electron paramagnetic resonance and optical absorption studies of manganese ions doped in polyvinyl(alcohol) complexed with polyethylene glycol polymer films

Electron paramagnetic resonance (EPR), optical absorption, and infrared spectral studies have been carried out on Mn²⁺ ions doped in poly(vinyl alcohol) (PVA) complexed with polyethylene glycol (PEG) films prepared by solution cast technique. The EPR spectra of 0.25?mol% Mn²⁺ ions doped polymer complex (PVA+PEG) at room temperature exhibit sextet hyperfine structure (hfs), centered at g????1.99. The spin?CHamiltonian parameter values indicate that the ground state of Mn²⁺ ion is d⁵ and the site symmetry around Mn²⁺ ions in tetragonally distorted octahedral site. The spin concentration participating in the resonance is measured as a function of temperature and it is observed that it obeys Boltzmann??s law. The paramagnetic susceptibility (??) is calculated from the EPR data at various temperatures (93?C333?K) and it obeys the Curie?CWeiss law. The optical absorption spectra exhibits two bands which are assigned to ⁶A_1g (S)??⁴A_1g (G) or ⁴E_g (G) and ⁶A_1g (S)??⁴T_2g (G) transitions. The infrared spectrum exhibits few bands due to the presence of O?CH, C?CH, and C=C groups.