Pulsed Electron Spin Resonance Ex Situ Probe for Tooth Biodosimetry

An electron spin resonance (ESR) probe that includes a static field source and a microwave resonator for the measurement of paramagnetic defects in tooth enamel is presented. Such defects are known to be a good marker for quantifying the amount of ionizing radiation dose absorbed in the tooth. The probe can measure the tooth when it is positioned just above its outer surface, i.e., in ex situ geometry. It is operated in pulsed mode at a frequency of ~6.2 GHz that corresponds to the magnitude of the static magnetic field of its permanent magnet. A detailed design of the probe is provided, together with its specifications in terms of measurement volume and signal-to-noise-ratio for a typical sample. Experimental results that verify its sensitivity and capability to measure gamma-irradiated teeth are provided. The current minimal detected signal by the probe corresponds to a radiation dose of ~4 Gy.     

Parallel coil resonators for time-domain radiofrequency electron paramagnetic resonance imaging of biological objects

Resonators suitable for time-domain electron paramagnetic resonance spectroscopy and imaging at a radiofrequency capable of accommodating experimental animals such as mice are described. Design considerations included B(1) field homogeneity, optimal Q, spectral bandwidth, resonator ring-down, and sensitivity. Typically, a resonator with 25-mm diameter and 25-mm length was constructed by coupling 11 single loops in parallel with a separation of 2.5 mm. To minimize the resonator ring-down time and provide the necessary spectral bandwidth for in vivo imaging experiments, the Q was reduced predominantly by overcoupling. Capacitative coupling was utilized to minimize microphonic effects. The B(1) field in the resonator was mapped both radially and axially and found to be uniform and adequate for imaging studies. Imaging studies with phantom objects containing a narrow-line spin probe as well as in vivo objects administered with the spin probe show the suitability of these resonators for valid reproduction of the spin probe distribution in three dimensions. The fabrication of such resonators is simple and can be scaled up with relative ease to accommodate larger objects as well.     

High-Bandwidth Q-Band EPR Resonators

The emerging technology of ultra-wide-band spectrometers in electron paramagnetic resonance—enabled by recent technological advances—provides the means for new experimental schemes, a broader range of samples, and huge gains in measurement time. Broadband detection does, however, require that the resonator provides sufficient bandwidth and, despite resonator compensation schemes, excitation bandwidth is ultimately limited by resonator bandwidth. Here, we present the design of three resonators for Q-band frequencies (33–36 GHz) with a larger bandwidth than what was reported so far. The new resonators are of a loop-gap type with 4–6 loops and were designed for 1.6 mm sample tubes to achieve higher field homogeneity than in existing resonators for 3 mm samples, a feature that is beneficial for precise spin control. The loop-gap design provides good separation of the B ₁ and E field, enabling robust modes with powder samples as well as with frozen water samples as the resonant behavior is largely independent of the dielectric properties of the samples. Experiments confirm the trends in bandwidth and field strength and the increased B ₁ field homogeneity predicted by the simulations. Variation of the position of the coupling rod allows the adjustment of the quality factor Q and thus the bandwidth over a broad range. The increased bandwidth of the loop-gap resonators was exploited in double electron–electron resonance measurements of a Cu(II)-PyMTA ruler to yield significantly higher modulation depth and thus higher sensitivity.     

Dielectric resonator-based resonant structure for sensitive ESR measurements at high-hydrostatic pressures

We present a newly developed microwave probe head that accommodates a gasketed sapphire anvil cell (SAC) for performing sensitive electron spin resonance (ESR) measurements under high-hydrostatic pressures. The system was designed around commercially available dielectric resonators (DRs) having the dielectric permittivity of approximately 30. The microwave resonant structure operates in a wide-stretched double-stacked geometry and resonates in the lowest cylindrical quasi TE(011) mode around 9.2 GHz. The most vital parts of the probe's microwave heart were made of plastic materials, thus making the resonant structure transparent to magnetic field modulation at 100 kHz. The overall ESR sensitivity of the probe was demonstrated for a small speck of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) positioned in the gasket of the SAC, using water as the pressure-transmitting medium. The system was also used for studying pressure-induced changes in spin-relaxation mechanisms of a quasi-1D-conducting polymer, K(1)C(60). For small samples located in the sample hole of the gasket the probe reveals sensitivity that is only approximately 3 times less than that yielded by regular ESR cavities.     

Optimization Methods for the Design of Sensitive Surface ESR Resonators

Electron spin resonance (ESR) is a powerful spectroscopic technique that has many applications in a wide variety of scientific fields, including chemistry, biology, materials science, and physics. One significant drawback of conventional ESR, however, is its relatively low sensitivity compared to other spectroscopic techniques. Arguably, the most dominant element affecting ESR sensitivity is the microwave resonator used to pick up the ESR signal of the spins. Traditionally, ESR mostly employs a limited set of resonator configurations (e.g., rectangular cavity, dielectric, or loop-gap resonator) that are suboptimal with respect to a wide range of samples. In principle, a smart resonator design can be used to optimize spin sensitivity for a given sample’s properties. In this work, we make use of an efficient Genetic Algorithm (GA) approach to numerically solve, analyze, and optimize a unique class of surface microresonators. The GA is based on a method of moments code, customized directly to render the complexity of a particular resonator’s geometries in our search. The main purposes of the algorithm are to routinely generate more sensitive microresonators, optimized for a predefined sample’s dimensions, and to study the functional relations between the devices’ resonance frequency, quality, and filling factors and their topology, in order to reach a rational optimal design. Preliminary results associated with new, unique, and sensitive surface microresonators are shown and analyzed. Such resonators are cheap and easy to produce on a mass scale with an arbitrary surface geometry.     

Development of Very-Low-Temperature Millimeter-Wave Electron-Spin-Resonance Measurement System

We report the development of a millimeter-wave electron-spin-resonance (ESR) measurement system at the University of Fukui using a ³He/⁴He dilution refrigerator to reach temperatures below 1 K. The system operates in the frequency range of 125–130 GHz, with a homodyne detection. A nuclear-magnetic-resonance (NMR) measurement system was also developed in this system as the extension for millimeter-wave ESR/NMR double magnetic-resonance (DoMR) experiments. Several types of Fabry–Pérot-type resonators (FPR) have been developed: A piezo actuator attached to an FPR enables an electric tuning of cavity frequency. A flat mirror of an FPR has been fabricated using a gold thin film aiming for DoMR. ESR signal was measured down to 0.09 K. Results of ESR measurements of an organic radical crystal and phosphorous-doped silicon are presented. The NMR signal from ¹H contained in the resonator is also detected successfully as a test for DoMR.     

Variable Coupling Scheme for High-Frequency Electron Spin Resonance Resonators Using Asymmetric Meshes

The sensitivity of a high-frequency electron spin resonance (ESR) spectrometer depends strongly on the structure used to couple the incident millimeter wave to the sample that generates the ESR signal. Subsequent coupling of the ESR signal to the detection arm of the spectrometer is also a crucial consideration for achieving high spectrometer sensitivity. In previous work, we found that a means for continuously varying the coupling was necessary for attaining high sensitivity reliably and reproducibly. We report here on a novel asymmetric mesh structure that achieves continuously variable coupling by rotating the mesh in its own plane about the millimeter-wave transmission-line optical axis. We quantify the performance of this device with nitroxide spin label spectra in both a lossy aqueous solution and a low-loss solid-state system. These two systems have very different coupling requirements and are representative of the range of coupling achievable with this technique. Lossy systems, in particular, are a demanding test of the achievable sensitivity and allow us to assess the suitability of this approach for applying high-frequency ESR, e.g., to the study of biological systems at physiological conditions. The variable coupling technique reported on here allows us to readily achieve a factor of ca. 7 improvement in the signal-to-noise ratio at 170 GHz and a factor of ca. 5 at 95 GHz over what has previously been reported for lossy samples.     

Determination of Spin Concentrations in ESR Tomography as Applied for the Spatial Distribution of Spin Labels in Human Skin

The spectral–spatial electron spin resonance (ESR) imaging experiment provides information about the spatial and spectral properties of the examined sample, as well as the intensity distribution. Obtaining quantitative statements of the respective sample requires the usage of reference standards such as diphenylpicrylhydrazyl (DPPH) or charcoal. By means of ultraviolet-visible spectroscopy a DPPH sample has been quantified to become the reference DPPH standard for the following experiments. Moreover, charcoal together with DPPH serves as a standard to determine the spatial dimension. An ESR tomographic device has been modified concerning the sample shaping and arranging in a TE₁₀₂ resonator to allow the spatially resolved quantitative determination of spin concentrations of spin probes (S = 1/2) with an acceptable error. Only standard ESR equipment has been used except the additional gradient coils and their computer-aided controls for recording the tomogram. The presented results, i.e., the spatially resolved determination of spin concentrations, seem to be useful to describe, e.g., transport processes in biological objects. As a first application of the developed method, the spatial resolution of the effect of ultraviolet irradiation on the concentration of a spin probe in human skin could be determined. Altogether, the newly introduced method widens the field of applications of ESR spectroscopy and tomography. Authors' address: Werner Herrmann, Institute of Pharmacy, Free University Berlin, Kelchstrasse 31, 12169 Berlin, Germany     

Synthesis of Ge-Sn at high pressure and high temperature in a laser-heated diamond anvil cell

Ge–Sn compound is predicted to be a direct band gap semiconductor with a tunable band gap. However, the bulk synthesis of this material by conventional methods at ambient pressure is unsuccessful due to the poor solubility of Sn in Ge. We report the successful synthesis of Ge–Sn in a laser-heated diamond anvil cell (LHDAC) at ~7.6 GPa &; ~2000 K. In situ Raman spectroscopy of the sample showed, apart from the characteristic Raman modes of Ge TO (Г) and β-Sn TO (Г), two additional Raman modes at ~225 cm^?1 (named Ge–Sn₁) and ~133 cm^?1 (named Ge–Sn₂). When the sample was quenched, the Ge–Sn₁ mode remained stable at ~215 cm^?1, whereas the Ge–Sn₂ mode had diminished in intensity. Comparing the Ge–Sn Raman mode at ~225 cm^?1 with the one observed in thin film studies, we interpret that the observed phonon mode may be formed due to Sn-rich Ge–Sn system. The additional Raman mode seen at ~133 cm^?1 suggested the formation of low symmetry phase under high P–T conditions. The results are compared with Ge–Si binary system.     

Permeability Studies of Redox-Sensitive Nitroxyl Spin Probes Through Lipid Membranes Using an L-Band ESR Spectrometer

Electron spin resonance (ESR) studies were carried out for 2 mM ¹⁴N-labeled deuterated 3-methoxycarbonyl-2,2,5,5-tetramethyl-pyrrolidine-1-oxyl (MC-PROXYL) and 3-carboxy-2,2,5,5,-tetramethyl-pyrrolidin-1-oxyl (carboxy-PROXYL) in pure water and various concentrations of liposomal solution by using an L-band ESR spectrometer. The ESR parameters, such as the line width, hyperfine coupling constant, g-factor, rotational correlation time and partition parameter, were reported for the samples. The changes in the line width were observed for ¹⁴N-labeled deuterated MC-PROXYL and carboxy-PROXYL in liposomal solution. The hyperfine coupling constant was observed for both nitroxyl spin probes. The permeable and impermeable nature of nitroxyl radicals was demonstrated using the ESR L-band spectra. The rotational correlation time increases with increasing concentration of liposome. The partition parameter for ¹⁴N-labeled deuterated MC-PROXYL in liposomal solution increases with increasing concentration of liposome, which reveals that the nitroxyl spin probe permeates into lipid membrane. The lipid peaks were observed for 2 mM ¹⁴N-labeled deuterated MC-PROXYL in 200, 300 and 400 mM liposomal concentration. The lipid peaks were not observed for ¹⁴N-labeled deuterated carboxy-PROXYL. These results indicate the permeable and impermeable nature of ¹⁴N-labeled deuterated nitroxyl spin probe.     

Development of High-Pressure ESR System Using Micro-coil

We have developed the high-pressure electron spin resonance (ESR) system using a micro-coil in the frequency region up to around 2 GHz and potentially 10 GHz. The hybrid-type piston-cylinder pressure cell whose maximum pressure reaches 4 GPa was used. In this study, we obtained ESR spectra at 2.3 GPa successfully, which can never be obtained by the single-layer piston-cylinder pressure cell. The minimum detectable spin number was estimated to be the order of 10¹² spins/G. Moreover, it is shown that the sensitivity can be improved by two orders of magnitude using the field modulation technique. This high-pressure ESR technique is a promising one to achieve the sensitivity and the high pressure simultaneously.     

Modular High-Intensity Monochromatic In Situ Illumination Set-Up for Investigating ESR Photoactive Centers in Semiconductors

A versatile, modular in situ high-intensity monochromatic illumination set-up installed on a standard Q-band ESR spectrometer equipped with a cryostat and probe head for measurements at cryogenic temperatures, which can be easily assembled from commercially available optical components is presented. Using as monochromatic light sources pig-tailed laser diodes (LDs) or fiber-coupled light-emitting diodes (LEDs), a high efficiency of the light transfer (more than 95%) through an optical guide inserted in the sample holder is achieved in the sample area of the microwave cavity. With various LEDs and LDs, one can perform ESR in situ illumination experiments from UV to far-IR, in both cw and pulse mode. Its operation is illustrated with an experiment revealing the presence of certain ESR silent defects in oxygen-doped floating-zone ultrapure Si samples irradiated at room temperature with high-energy–high-fluence electron beams and pulse annealed up to 300 °C. New information is obtained by comparing the ESR spectra recorded at T = 120 K, without and with 1.06 µm across-the-gap in situ illumination.     

Impregnation of Polycarbonate by Paramagnetic Probe 2,2,6,6-Tetramethyl-4-Hydroxy-Piperidine-1-Oxyl (TEMPOL) in Supercritical CO<Subscript>2</Subscript>

The aim of the research was to test the advantages of spin probe electron paramagnetic resonance approach in studying polymers impregnation with organic molecules in supercritical CO₂ (scCO₂) The impregnation of bisphenol A polycarbonate with the spin probe TEMPOL was carried out at 307–343 K and 11.6–35 MPa. The mean and local concentrations of the spin probe in the polymer were evaluated. An increase in temperature and pressure resulted in a more even distribution of the dopant in the polymer matrix. It was observed that, at 307 K and 19.6 MPa, the spin probe was located only near the surface of the sample. Local mobility of the spin probe molecules was found to be similar in polycarbonate films impregnated in scCO₂ and cast from dichloroethane solution. It was shown that changes in the structure of the surface and bulk of the polymer detected by the atomic force and optical polarization microscopy are not directly related with the distribution of the dopant molecules and their average content in the polymer.     

Recent Advances in High-Frequency Electron Spin Resonance Detection Using a Microcantilever

Our recent developments in highly sensitive high-frequency electron spin resonance (ESR) using a microcantilever are reviewed. ESR signals of a Co Tutton salt microcrystal (<1 μg) have been detected at low temperature at frequencies up to 315 GHz under a static magnetic field using a microcantilever and a modulation technique. The achieved sensitivity is about 10⁹ spins/G at 4.5 K. Moreover, we have shown that similar ESR detection using a microcantilever is possible up to 130 GHz under a pulsed magnetic field without using a modulation technique. The achieved sensitivity is about 10¹¹ spins/G at 1.7 K. These results suggest that the ESR detection using a microcantilever is promising for applications to high-resolution and high-sensitivity terahertz ESR.     

Multifrequency ESR Characterization of Paramagnetic Point Defects in Semiconducting Cubic BN Crystals

Low-frequency (X-band) electron spin resonance (ESR) investigations on commercially available large-grained cubic boron nitride (cBN) superabrasive powders of various coloration, combined with high-frequency (W-band) ESR measurements on oriented submillimeter-size single crystallites selected from the same powder samples, resulted in a clear identification of several types of paramagnetic point defects. The resulting spin Hamiltonian parameters describing the ESR spectra observed in the 3–293 K temperature range and the photosensitivity of the paramagnetic defects observed in amber-colored cBN samples are reported. It is shown that the nature of the paramagnetic centers depends on the color of the investigated samples and that, in many cases, uncontrolled impurities seem to be involved in their structure.     

ESR and TL studies of irradiated Anatolian laurel leaf (Laurus nobilis L.)

Laurel leaf (Laurus nobilis L.) samples that originated from Turkey were analyzed by electron spin resonance (ESR) and thermoluminescence (TL) techniques before and after γ-irradiation. Unirradiated (control) laurel leaf samples exhibit a weak ESR singlet centered at g=2.0020. Besides this central signal were two weak satellite signals situated about 3 mT left and right to it in radiation-induced spectra. The dose–response curve of the radiation-induced ESR signal at g=2.0187 (the left satellite signal) was found to be described well by a power function. Variation of the left satellite ESR signal intensity of irradiated samples at room temperature with time in a long term showed that cellulosic free radicals responsible for the ESR spectrum of laurel leaves were not stable but detectable even after 100 days. Annealing studies at four different temperatures were used to determine the kinetic behavior and activation energy of the radiation-induced cellulosic free radicals responsible from the left satellite signal (g=2.0187) in laurel leaves. TL measurements of the polymineral dust isolated from the laurel leaf samples allowed distinguishing between irradiated and unirradiated samples.     

Development of a magnetic measurement device for thin ribbon samples

This paper presents a magnetic measurement device for thin ribbon samples, which are produced by rapid cooling technique. This device enables us to measure magnetic properties easily by only inserting a ribbon sample into a sample holder. The sample holder was made by bakelite to fix any width sample. A long solenoid coil was used to generate a uniform magnetic field and the sample holder was placed at the mid part of the solenoid. The magnetic field strength was measured using a shunt resistor and the magnetic flux density and magnetization in sample ribbons were evaluated by using search coils. The accuracy of measurement was verified with an amorphous metal ribbon sample. Next, we have measured magnetic properties of some magnetic shape memory alloys, which have different compositions. The measured results are compared and we clarified the effect of Sm contents on the magnetic properties.     

Rotational Correlation Time Studies on Nitroxyl Radicals Using 300 MHz ESR Spectrometer in High Viscous Liquid

The mobility studies on ¹⁴N-labeled TEMPONE, TEMPO, carbamoyl-PROXYL, carboxy-PROXYL in high viscous liquid were carried out on a 300 MHz electron spin resonance (ESR) spectrometer. The ESR parameters, such as a line width, signal intensity ratio, g-factor, hyperfine coupling constant and correlation time, were determined. The line width broadening increases twofold in high viscous samples of ¹⁴N-labeled carbamoyl-PROXYL and carboxy-PROXYL, but this line broadening is negligibly small in the high viscous sample (85% glycerol) of ¹⁴N-labeled TEMPO. The correlation time also increases (~30 times) in the high viscous sample (85% glycerol) of ¹⁴N-labeled carbamoyl-PROXYL and carboxy-PROXYL, but there is no considerable increase in the high viscous sample of ¹⁴N-labeled TEMPO. TEMPONE has the narrowest line width and is also highly sensitive to viscosity. The correlation time increases (~13 times) in the high viscous sample (85% glycerol) of ¹⁴N-labeled TEMPONE. Therefore, this study reveals that the ¹⁴N-labeled TEMPONE radical is the most suitable spin probe for in vivo studies in high viscous biological fluids.     

Photolithographic production of glass sample holders for improved sensitivity and resolution in ESR microscopy

Electron spin resonance microscopy (ESRM) is an imaging method aiming at the observation of stable free radicals in small samples with a spatial resolution of about 1 micrometer. One of the challenges associated with the useof ESRM in conjunction with small biological samples (e.g., single cells) is containing these samples in a manner that will minimize the effect on the quality factor of the resonator but yet enable easy handling and simultaneous optical and ESR observation. Here we present a new type of flat samples that provide an adequate answer to this challenge. The samples are made of thin glass coverslips, manufactured by photolithography techniques. Details of the manufacturing process as well as the expected improvements in sensitivity and resolution are provided.     

Phase transformation and quantum confinement effect in CdSe/Se multilayer thin films prepared by physical vapour deposition

CdSe/Se multilayer (ML) thin films with different thickness ratios of Se and CdSe sublayers were prepared by using a thermal evaporation method. Prepared samples were annealed at temperature 300 K. From X-ray diffraction (XRD) studies, samples prepared at room temperature showed a (100) plane of CdSe with wurtzite structure, whereas the annealed samples confirmed the cubic structure. Stress created in ML systems was calculated from XRD data and found that it increases with decreasing particle size. The energy band gap value of a CdSe/Se ML thin film is shifted to a value higher than that of the bulk CdSe (1.74 eV) semiconductor. This is due to decrease in the crystallite size smaller than the Bohr exciton diameter of CdSe (11.2 nm). Crystallite sizes (≈5 nm) were calculated from UV–VIS data with the predictions of an effective mass approximation model. The photoluminescence peak of the ML samples is split into two bands having nearest values due to the emissions from spin–orbit split-up of the excited energy state.