Radiation-Induced Stable Radicals in Calcium Phosphates: Results of Multifrequency EPR, EDNMR, ESEEM, and ENDOR Studies

Fadis F Murzakhanov,Andris Antuzevics,Natalia V Petrakova,Vladimir S Komlev,Margarita A Goldberg,Sergei B Orlinskii,Marat R Gafurov,Georgy V Mamin,Alexander Yu Fedotov,Boris V Yavkin,Peter O Grishin

doi:10.3390/app11167727

Abstract

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 electron-nuclear 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 for 14N 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.

Highlights

There are no paramagnetic centers in nominally pure samples (HA and OCP); no electron paramagnetic resonance (EPR) signals in calcium phosphate (CP) materials can be observed
An X-ray or γ-irradiation procedure performed on the samples leads to the formation of radiation centers that can be successfully detected by EPR (Figure 1)
The electron spin echo envelope modulation (ESEEM) spectrum contains a complete set of signals from all magnetic nuclei (1 H, νz ≈ 14.2 MHz), which indicates the position of the nitrate radical directly in the crystal lattice of the OCP sample

Summary

Sample Synthesis

HA powders were synthesized by precipitation from aqueous solutions according to the reaction (Equation (1)), using reactants of analytical grade (Labtech, Russia) and deionized water: 10Ca(NO3 )2 + 6(NH4 ) HPO4 + 8NH4 OH →Ca10 (PO4 ) (OH)2 + 20NH4 NO3 + 6H2 O (1). A total of 10 g of DCPD powder was immersed in 1000 mL of 1.5 M sodium acetate aqueous solution, with a pH value of 8.8 ± 0.2. The powder was kept in the solution for 24 h, with a constant stirring rate at 35 ◦ C, and thoroughly washed in distilled water and dried overnight at 37 ◦ C. The corresponding preparation of the jawbone sample for EPR measurements was carried out by separating it from the implant and sawing it into blocks. For X-ray and γ irradiation, the powder samples were packaged in plastic containers and plastic bags, and irradiated in air at room temperature

Experimental Setup

EPR Spectroscopy

Conclusions