Study of paramagnetic defect centers in as-grown and annealed TiO2 anatase and rutile nanoparticles by a variable-temperature X-band and high-frequency (236 GHz) EPR

Detailed EPR investigations on as-grown and annealed TiO₂ nanoparticles in the anatase and rutile phases were carried out at X-band (9.6 GHz) at 77, 120-300 K and at 236 GHz at 292 K. The analysis of EPR data for as-grown and annealed anatase and rutile samples revealed the presence of several paramagnetic centers: Ti³⁺, O^-, adsorbed oxygen (O₂^-) and oxygen vacancies. On the other hand, in as-grown rutile samples, there were observed EPR lines due to adsorbed oxygen (O₂^-) and the Fe³⁺ ions in both Ti⁴⁺ substitutional positions, with and without coupling to an oxygen vacancy in the near neighborhood. Anatase nanoparticles were completely converted to rutile phase when annealed at 1000° C, exhibiting EPR spectra similar to those exhibited by the as-grown rutile nanoparticles. The high-frequency (236 GHz) EPR data on anatase and rutile samples, recorded in the region about g = 2.0 exhibit resolved EPR lines, due to O^- and O₂^- ions enabling determination of their g-values with higher precision, as well as observation of hyperfine sextets due to Mn²⁺ and Mn⁴⁺ ions in anatase.

Development and validation of an ex vivo electron paramagnetic resonance fingernail biodosimetric method

There is an imperative need to develop methods that can rapidly and accurately determine individual exposure to radiation for screening (triage) populations and guiding medical treatment in an emergency response to a large-scale radiological/nuclear event. To this end, a number of methods that rely on dose-dependent chemical and/or physical alterations in biomaterials or biological responses are in various stages of development. One such method, ex vivo electron paramagnetic resonance (EPR) nail dosimetry using human nail clippings, is a physical biodosimetry technique that takes advantage of a stable radiation-induced signal (RIS) in the keratin matrix of fingernails and toenails. This dosimetry method has the advantages of ubiquitous availability of the dosimetric material, easy and non-invasive sampling, and the potential for immediate and rapid dose assessment. The major challenge for ex vivo EPR nail dosimetry is the overlap of mechanically induced signals and the RIS. The difficulties of analysing the mixed EPR spectra of a clipped irradiated nail were addressed in the work described here. The following key factors lead to successful spectral analysis and dose assessment in ex vivo EPR nail dosimetry: (1) obtaining a thorough understanding of the chemical nature, the decay behaviour, and the microwave power dependence of the EPR signals, as well as the influence of variation in temperature, humidity, water content, and O₂ level; (2) control of the variability among individual samples to achieve consistent shape and kinetics of the EPR spectra; (3) use of correlations between the multiple spectral components; and (4) use of optimised modelling and fitting of the EPR spectra to improve the accuracy and precision of the dose estimates derived from the nail spectra. In the work described here, two large clipped nail datasets were used to test the procedures and the spectral fitting model of the results obtained with it. A 15-donor nail set with 90 nail samples from 15 donors was used to validate the sample handling and spectral analysis methods that have been developed but without the interference of a native background signal. Good consistency has been obtained between the actual RIS and the estimated RIS computed from spectral analysis. In addition to the success in RIS estimation, a linear dose response has also been achieved for all individuals in this study, where the radiation dose ranges from 0 to 6 Gy. A second 16-donor nail set with 96 nail samples was used to test the spectral fitting model where the background signal was included during the fitting of the clipped nail spectra data. Although the dose response for the estimated and actual RIS calculated in both donor nail sets was similar, there was an increased variability in the RIS values that was likely due to the variability in the background signal between donors. Although the current methods of sample handling and spectral analysis show good potential for estimating the RIS in the EPR spectra of nail clippings, there is a remaining degree of variability in the RIS estimate that needs to be addressed; this should be achieved by identifying and accounting for demographic sources of variability in the background nail signal and the composition of the nail matrix.

A multifrequency EPR study of Fe2+ and Mn2+ ions in a ZnSiF(6).6H2O single crystal at liquid-helium temperatures

A liquid-helium temperature study of Fe2+ and Mn2+ ions has been carried out on a single crystal of Fe2+-doped ZnSiF(6).6H2O at 5-35K at 170, 222.4 and 333.2G Hz. The spectra are found to be an overlap of two magnetically inequivalent Fe2+ ions, as well as that of an Mn2+ ion. From the simulation of the EPR line positions for the Fe2+ (d6, S=2) ion the spin-Hamiltonian parameters were estimated for the two inequivalent Fe2+ ions at the various temperatures. From the relative intensities of lines the absolute sign of the fine-structure parameters have been estimated. In addition, the fine-structure and hyperfine-structure spin-Hamiltonian parameters for the Mn2+ ion, present as impurity at interstitial sites, were estimated from the hyperfine allowed and forbidden line positions. The particular virtues of such a single-crystal study vs. that on powders are noted.

A 236-GHz Fe EPR STUDY OF NANO-PARTICLES OF THE FERRO-MAGNETIC ROOM-TEMPERATURE SEMICONDUCTOR Sn(1-x)Fe(x)O(2)(x=0.005)

High frequency (236 GHz) electron paramagnetic resonance (EPR) studies of Fe(3+) ions at 255 K are reported in a Sn(1-x)Fe(x)O(2) powder with x = 0.005 which is a ferromagnetic semiconductor at room temperature. The observed EPR spectrum can be simulated reasonably well as overlap of spectra due to four magnetically inequivalent high-spin (HS) Fe(3+) ions (S = 5/2). The spectrum intensity is calculated, using the overlap I(BL) + (I(HS1)+I(HS2)+I(HS3)+I(HS4))×e(-0.00001×B), where B is the magnetic field intensity in Gauss, I represents the intensity of an EPR line (HS1, HS2, HS3, HS4), and BL stands for the base line. (The exponential factor, as found by fitting to the experimental spectrum, is related to the Boltzmann population distribution of energy levels at 255 K, which is the temperature of the sample in the spectrometer.) These high-frequency EPR results are significantly different from those at X-band. The large values of the zero-field splitting parameter (D) observed here for the four centers at the high frequency of 236 GHz are beyond the capability of X-band, which can only record spectra of ions only with much smaller D values than those reported here.