Radiation-induced stable radicals in calcium phosphates: Results of multifrequency epr, ednmr, eseem, and endor studies

This article presents the results of a study of radiation-induced defects in various synthetic calcium phosphate (CP) powder materials (hydroxyapatite—HA and octacalcium phosphate—OCP) by electron paramagnetic resonance (EPR) spectroscopy at the X, Q, and W-bands (9, 34, 95 GHz for the microwave frequencies, respectively). Currently, CP materials are widely used in orthopedics and dentistry owing to their high biocompatibility and physico-chemical similarity with human hard tissue. It is shown that in addition to the classical EPR techniques, other experimental approaches such as ELDOR-detected NMR (EDNMR), electron spin echo envelope modulation (ESEEM), and electronnuclear double resonance (ENDOR) can be used to analyze the electron–nuclear interactions of CP powders. We demonstrated that the value and angular dependence of the quadrupole interaction for14 N nuclei of a nitrate radical can be determined by the EDNMR method at room temperature. The ESEEM technique has allowed for a rapid analysis of the nuclear environment and estimation of the structural positions of radiation-induced centers in various crystal matrices. ENDOR spectra can provide information about the distribution of the nitrate radicals in the OCP structure. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Published under the CC BY 4.0 license.Authors would like to thank the Russian Foundation for Basic Research, project no. 18-29-11086. Institute of Solid State Physics, University of Latvia as the Center of Excellence received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01–2016-2017- TeamingPhase2 under grant agreement No. 739508, project CAMART2

HYSCORE Spectroscopy to Resolve Electron–Nuclear Structure of Vanadyl Porphyrins in Asphaltenes from the Athabasca Oil Sands In Situ Conditions

The purpose of this work is to analyze the electron–nuclear interactions of the vanadyl-porphyrin (VP) complexes in oil asphaltenes. Asphaltenes from the Athabasca oil sands were studied by HYperfine Sublevel CORrelation Spectroscopy (HYSCORE) electron paramagnetic resonance (EPR). It makes it possible to resolve and interpret complex hyperfine spectra of intrinsic VP with strong and weak hyperfine interactions between the electron magnetic moment and various nuclear spins (1H, 14N, 51V). The main parameters of spin-Hamiltonian for the VP spin system are determined. The axially symmetric structure of the VP complexes is revealed, and the local nuclear environment of the paramagnetic center is investigated. The results can be used for the study of asphaltene electron–nuclear structure and asphaltene aggregates with the aim of elucidating asphaltenes’ transformation(s) under the influence of external treatment

Analysis of the Osseointegration Process of Dental Implants by Electron Paramagnetic Resonance: An In Vivo Study

This research work presents an analysis of the process of an implant’s osseointegration to the jawbone tissue. The purpose of this work was to describe the processes of assimilation and the biochemical dynamics which occur during dental implantation using implants with different macro-microstructure surfaces at the level of stable free radicals using the electron paramagnetic resonance (EPR) method. The experimental investigation was conducted on seven Vietnamese minipigs over twelve months old and weighing up to 30 kg using implants with various macro-microstructure surfaces (SLA, RBM, and HSTTM) and implantation systems, namely the Adin, Sunran, Biomed, and Osstem systems. The integration of the implant into the bone triggered biochemical processes with the formation of stable free radicals. The EPR method was used to identify the formed paramagnetic species and to study the dynamics of the interaction between the surface of the implant and the bone after one and two months. The concentration of carbonate surface centers increased with the time that the implant was connected to the hard tissue. The “Sunran” and “HSTTM” were established as the most suitable implantation system and surface type, respectively, thanks to the highest rate of osseointegration (assimilation) with the bone (hard) tissue. Thus, the EPR method provides the opportunity to study implantation processes

Incorporation of Manganese (II) in Beta-Tricalcium Phosphate from EPR and ENDOR Measurements for Powders

Powders of β-tricalcium phosphate (β-TCP, Ca3PO4) doped with manganese (Mn2+) are comprehensively analyzed with electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques. The modeling of the spectra permitted to calculate the values of zero-field splitting (B20 = −904 MHz; B40 = −1.41 MHz and B43 = 195.2 MHz) and explain the origin of the low-field hyperfine structures as the allowed spin transitions of fine structure. Three structurally inequivalent positions for Mn2+ in the β-TCP crystal lattice are identified and their g-factors and hyperfine constants are quantified. The obtained results can serve as fundamental background to the study of structurally disordered matrices with high spin (S ≥ 1) impurities which are important for catalytic systems

Incorporation of Manganese (II) in Beta-Tricalcium Phosphate from EPR and ENDOR Measurements for Powders

Powders of β-tricalcium phosphate (β-TCP, Ca3PO4) doped with manganese (Mn2+) are comprehensively analyzed with electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques. The modeling of the spectra permitted to calculate the values of zero-field splitting (B20 = −904 MHz; B40 = −1.41 MHz and B43 = 195.2 MHz) and explain the origin of the low-field hyperfine structures as the allowed spin transitions of fine structure. Three structurally inequivalent positions for Mn2+ in the β-TCP crystal lattice are identified and their g-factors and hyperfine constants are quantified. The obtained results can serve as fundamental background to the study of structurally disordered matrices with high spin (S ≥ 1) impurities which are important for catalytic systems

Exploring the Properties of the V_B^- Defect in hBN: Optical Spin Polarization, Rabi Oscillations, and Coherent Nuclei Modulation

Optically active point defects in semiconductors have received great attention in the field of solid-state quantum technologies. Hexagonal boron nitride, with an ultra-wide band gap E_g = 6 eV, containing a negatively charged boron vacancy (V_B^-) with unique spin, optical, and coherent properties presents a new two-dimensional platform for the implementation of quantum technologies. This work establishes the value of V_B^ - spin polarization under optical pumping with {\lambda}ext = 532 nm laser using high-frequency ({\nu}mw = 94 GHz) electron paramagnetic resonance (EPR) spectroscopy. In optimal conditions polarization was found to be P = 38.4 %. Our study reveals that Rabi oscillations induced on polarized spin states persist for up to 30-40 microseconds, which is nearly two orders of magnitude longer than what was previously reported. Analysis of the coherent electron-nuclear interaction through the observed electron spin echo envelope modulation (ESEEM) made it possible to detect signals from remote nitrogen and boron nuclei, and to establish a corresponding quadrupole coupling constant Cq = 180 kHz related to nuclear quadrupole moment of 14N. These results have fundamental importance for understanding spin properties of boron vacancy

Creation of Negatively Charged Boron Vacancies in Hexagonal Boron Nitride Crystal by Electron Irradiation and Mechanism of Inhomogeneous Broadening of Boron Vacancy-Related Spin Resonance Lines

Optically addressable high-spin states (S ≥ 1) of defects in semiconductors are the basis for the development of solid-state quantum technologies. Recently, one such defect has been found in hexagonal boron nitride (hBN) and identified as a negatively charged boron vacancy (VB−). To explore and utilize the properties of this defect, one needs to design a robust way for its creation in an hBN crystal. We investigate the possibility of creating VB− centers in an hBN single crystal by means of irradiation with a high-energy (E = 2 MeV) electron flux. Optical excitation of the irradiated sample induces fluorescence in the near-infrared range together with the electron spin resonance (ESR) spectrum of the triplet centers with a zero-field splitting value of D = 3.6 GHz, manifesting an optically induced population inversion of the ground state spin sublevels. These observations are the signatures of the VB− centers and demonstrate that electron irradiation can be reliably used to create these centers in hBN. Exploration of the VB− spin resonance line shape allowed us to establish the source of the line broadening, which occurs due to the slight deviation in orientation of the two-dimensional B-N atomic plains being exactly parallel relative to each other. The results of the analysis of the broadening mechanism can be used for the crystalline quality control of the 2D materials, using the VB− spin embedded in the hBN as a probe

Study of Tricalcium Phosphate Ceramics Doped with Gadolinium Ions with Various EPR Techniques

Tricalcium phosphate (TCP)-based materials, such as β-Ca3(PO4)2 doped with rare earth ions (RE), have shown applications as biomaterials, lighting emitting materials, scintillating materials, in vivo imaging probes, and thermoluminescent dosimeters. Their properties are found to be dependent on the distribution of RE3+ on Ca2+ sites that can be controlled by pulsed electron paramagnetic resonance (EPR) and electron spin echo envelop modulation (ESEEM) experiments. The main spectroscopic parameters (spin Hamiltonian values) of Gd3+ and nitrogen impurity centers are quantitatively determined (g-factor, the fine structure parameters D and E, the hyperfine constants A) as well as dynamic characteristics: spin–lattice T1 and spin–spin T2 relaxation times. Based on the analysis of the EPR datasets, the interatomic distance between Gd3+ and 31P was estimated in the dipole–dipole approximation. Two structurally nonequivalent Gd3+ positions in the β-TCP structure have been identified. The obtained valuable results demonstrate applicability of modern EPR techniques to characterize Gd-TCP systems despite the powder structure of the material and high electron spin S = 7/2 of Gd3+ ions

Peculiarities of charge compensation in lithium-doped hydroxyapatite

Hydroxyapatite (HA) remains one of the most popular materials for various biomedical applications and its fields of application have been expanding. Lithium (Li+) is a promising candidate for modifying the biological behavior of HA. Li+ is present in trace amounts in the human body as an alkaline and bioelectric material. At the same time, the introduction of Li+ into the HA structure required charge balance compensation due to the difference in oxidation degree, and the scheme of this compensation is still an open question. In the present work, the results of the theoretical and experimental study of the Li+-doped HA synthesis are presented. According to X-ray diffraction data, Fourier transform infrared spectroscopy as well as the combination of electron paramagnetic resonance methods, the introduction of Li+ in the amount up to 0.05 mol% resulted in the preservation of the HA structure. Density functional theory calculations show that Li+ preferentially incorporates into the Ca (1) position with a small geometry perturbation. The less probable positioning in the Ca (2) position leads to a drastic perturbation of the anion channel