Thermal changes in young and mature bone nanostructure probed with Ca 2p excitations

Abstract

Thermal dependence of mineralized bone structure is examined both experimentally by measuring the near edge x-ray absorption fine structure (NEXAFS) and theoretically by applying the 3DSL model to clarify relationship between the local electronic and atomic structure and hierarchical organization of skeleton. The high energy resolution NEXAFS spectra are acquired near the Ca 2p edges in native bone heated from RT up to 450 °C and hydroxyapatite (Ca10(PO4)6(OH)2, OH-Ap) to understand the interplay of short-, long- and super-range order parameters of bone matter. Our focus is on the thermal changes of spectral distribution of oscillator strength for Ca 2p1/2,3/2 → 3d transition in bone and OH-Ap. The investigations have confirmed the assignment of the OH-Ap-to-bone spectral changes to the predicted hierarchy effect on electronic and atomic structure of mineralized bone. At RT the OH-Ap-to-bone red shift of the transition energy is found equal ≈ 0.2 eV and ≈ 0.1 eV for mature and young bone respectively. We stated that the shift behaves irregular and its magnitude varies from 0.1 eV up to 0.3 eV when the heating temperature grows. Two mechanisms associated with the thermal-induced dehydration of the inter-nanocrystallite spaces and with the subsequent atomic restructuring of the nanocrystallite interface in mineralized phase are revealed. We have detected that the OH-Ap-to-bone red shifts of the Ca 2p → 3d transition in young bone are smaller than those of in mature bone. The origin of the age differences is discussed.