Full-Field Calcium K-Edge X-ray Absorption Near-Edge Structure Spectroscopy on Cortical Bone at the Micron-Scale: Polarization Effects Reveal Mineral Orientation

Quantifying structural changes in organised biomineralized surfaces using synchrotron Polarisation-Induced Contrast X-ray Fluorescence

The quantitative characterization of the structure of biomineral surfaces is needed for guiding regenerative strategies. Current techniques are compromised by a requirement for extensive sample preparation, limited length-scales, or the inability to repeatedly measure the same surface over time and monitor structural changes. We aim to address these deficiencies by developing Calcium (Ca) K-edge Polarisation Induced Contrast X-ray Fluorescence (PIC-XRF) to quantify hydroxyapatite (HAp) crystallite structural arrangements in high and low textured surfaces. Minimally prepared human dental enamel was used as an exemplar to quantify initial surface structures, and the disruption caused by short dietary acid exposures. By measuring surfaces at different rotational angles relative to a polarised focused (2×2 µm) monochromatic X-ray source (at either 4049.2 and 4051.1 eV) it was possible to discriminate the principal and secondary orientations of surface crystallites, along with their texture. It was also possible to quantify the organisation of crystallites in both low (enamel cross-sections) and highly textured (facial enamel) surfaces including the identification of crystallites aligned perpendicular to the surface-a challenge for other synchrotron techniques. Surface modifications following short term acid erosion (affecting <20 µm of the enamel surface depth) were detected as significant shifts in principal crystallite orientation (p < 0.001) and as a marked reduction in surface texture (p < 0.001). Findings suggest preferential dissolution of HAp based on crystallite angular orientation. We demonstrate that PIC-XRF is a powerful tool to quantify biomineral surfaces, with minimal sample preparation that enables monitoring of surface structural changes through repeated measurements. STATEMENT OF SIGNIFICANCE: This study introduces Calcium (Ca) K-edge Polarisation Induced Contrast X-ray Fluorescence (PIC-XRF) as a method for quantifying the structure of biomineral surfaces, addressing limitations in existing techniques that require extensive sample preparation and cannot repeatedly measure the same surface. By using minimally prepared dental enamel, PIC-XRF successfully discriminated between principal and secondary orientations of hydroxyapatite crystallites, including end-on crystallites-a challenge for other synchrotron methods. Additionally, PIC-XRF detected significant structural changes due to short-term acid erosion. This technique's potential to repeatedly and non-invasively analyze biomineral surfaces offers new opportunities for understanding surface dynamics and guiding regenerative treatments.

The hidden structure of human enamel

Enamel is the hardest and most resilient tissue in the human body. Enamel includes morphologically aligned, parallel, ∼50 nm wide, microns-long nanocrystals, bundled either into 5-μm-wide rods or their space-filling interrod. The orientation of enamel crystals, however, is poorly understood. Here we show that the crystalline c-axes are homogenously oriented in interrod crystals across most of the enamel layer thickness. Within each rod crystals are not co-oriented with one another or with the long axis of the rod, as previously assumed: the c-axes of adjacent nanocrystals are most frequently mis-oriented by 1°-30°, and this orientation within each rod gradually changes, with an overall angle spread that is never zero, but varies between 30°-90° within one rod. Molecular dynamics simulations demonstrate that the observed mis-orientations of adjacent crystals induce crack deflection. This toughening mechanism contributes to the unique resilience of enamel, which lasts a lifetime under extreme physical and chemical challenges.

Polarization induced contrast X-ray fluorescence at submicrometer resolution reveals nanometer apatite crystal orientations across entire tooth sections

For biomedical research, successful imaging of calcified microstructures often relies on absorption differences between features, or on employing dies with selective affinity to areas of interest. When texture is concerned, e.g. for crystal orientation studies, polarization induced contrast is of particular interest. This requires sufficient interaction of the incoming radiation with the volume of interest in the sample to produce orientation-based contrast. Here we demonstrate polarization induced contrast at the calcium K-edge using submicron sized monochromatic synchrotron X-ray beams. We exploit the orientation dependent subtle absorption differences of hydroxyl-apatite crystals in teeth, with respect to the polarization field of the beam. Interaction occurs with the fully mineralized samples, such that differences in density do not contribute to the contrast. Our results show how polarization induced contrast X-ray fluorescence mapping at specific energies of the calcium K-edge reveals the micrometer and submicrometer crystal arrangements in human tooth tissues. This facilitates combining both high spatial resolution and large fields of view, achieved in relatively short acquisition times in reflection geometry. In enamel we observe the varying crystal orientations of the micron sized prisms exposed on our prepared surface. We easily reproduce crystal orientation maps, typically observed in polished thin sections. We even reveal maps of submicrometer mineralization fronts in spherulites in intertubular dentine. This Ca K-edge polarization sensitive method (XRF-PIC) does not require thin samples for transmission nor extensive sample preparation. It can be used on both fresh, moist samples as well as fossilized samples where the information of interests lies in the crystal orientations and where the crystalline domains extend several micrometers beneath the exposed surface.

Ex vivo detection of calcium phosphate and calcium carbonate in rat blood serum

The total calcium (tCa) in blood serum comprises free Ca²⁺ ions (fCa), protein-bound calcium (prCa), and complexed calcium by small anions (cCa). The cCa fraction, in addition to fCa, has been indicated to have some physiological activity. However, there is little evidence for the structure of its constituents. Here we report an ex vivo detection of the cCa constituents by synchrotron X-ray absorption near-edge structure spectroscopy. We collected the data directly on rat blood serum and, by making use of the reference samples, derived a spectrum that exhibits the features of cCa constituents. Among the features are those of the complexes of calcium phosphate and calcium carbonate. The detected complexes in the cCa fraction are mainly Ca(η²-HPO₄)(H₂O)₄ and Ca(η¹-HCO₃)(H₂O)₅⁺, in which HPO₄^2- and HCO₃^- serve as bidentate and unidentate ligands, respectively. The remained H₂O molecules on the coordination sphere of Ca²⁺ enable these complexes to behave partially like aquated Ca²⁺ ions in protein-binding. Besides, as the dominant part of prCa, albumin-bound calcium (albCa) exhibits a spectrum that closely resembles that of fCa, indicating weak interactions between the protein carboxyl groups and calcium. The weak-bound cCa and albCa, along with fCa and the relevant anions, compose a local chemical system that could play a role in maintaining the calcium level in blood.

Local electronic structure and nanolevel hierarchical organization of bone tissue: theory and NEXAFS study

Theoretical and experimental investigations of native bone are carried out to understand relationships between its hierarchical organization and local electronic and atomic structure of the mineralized phase. The 3D superlattice model of a coplanar assembly of the hydroxyapatite (HAP) nanocrystallites separated by the hydrated nanolayers is introduced to account the interplay of short-, long- and super-range order parameters in bone tissue. The model is applied to (i) predict and rationalize the HAP-to-bone spectral changes in the electronic structure and (ii) describe the mechanisms ensuring the link of the hierarchical organization with the electronic structure of the mineralized phase in bone. To check the predictions the near-edge x-ray absorption fine structure (NEXAFS) at the Ca 2p, P 2p and O 1s thresholds is measured for native bone and compared with NEXAFS for reference compounds. The NEXAFS analysis has demonstrated the essential hierarchy induced HAP-to-bone red shifts of the Ca and P 2p-to-valence transitions. The lowest O 1s excitation line at 532.2 eV in bone is assigned with superposition of core transitions in the hydroxide OH^-(H₂O) _m anions, Ca²⁺(H₂O) _n cations, the carboxyl groups inside the collagen and [PO₄]^2- and [PO₄]^- anions with unsaturated P-O bonds.