Abstract
Rare-earth labeling in biological apatite could provide critical information for the pathologic transition (osteoclastic) and physiologic regeneration (osteogenesis) of bone and teeth because of their characteristic site-sensitive fluorescence in different coordinative conditions of various tissues in many biological processes. However, the rare-earth labeling method for biological apatites, i.e., carbonated-hydroxyapatite, has been rarely found in the literature. In this paper, we report a Pourbaix-diagram guided mineralizing strategy to controllable carbonation and doping of rare-earth ions in the hydroxyapatite (HA) lattice. The carbonation process of hydroxyapatite was achieved by controllable mineralization in hydrothermal condition with K2CO3 as the carbonate source, which results into the pure B-type carbonated hydroxyapatite (CHA) with tunable carbonate substitution degree. All of the as-synthesized materials crystalized into P63/m (No. 176) space group with the lattice parameter of a decreases and c increases with the increasing of carbonate content in the reactants. Structural refinement results revealed that the substitution of planar CO32− is superimposed on one of the faces of PO43− tetrahedral sub-units with a rotation angle of 30° in reference to c-axis. All of the hydrothermally synthesized CHA nanocrystals show hexagonal rod-like morphology with the length of 70–110 nm and diameter of 21–35 nm, and the decreasing length/diameter ratio from 3.61 to 2.96 from low to high carbonated level of the samples. Five rare-earth cations, of Pr3+, Sm3+, Eu3+, Tb3+, and Ho3+, were used as possible probe ions that can be doped into either HA or CHA lattice. The site-preference of Tb3+ doping is the same in the crystallographic site of HA and CHA according to characteristic emission peaks of 5D4–7Fj (j = 3–6) transitions in their photoluminescent spectroscopy. Our work provides a controllable carbonation method for rare-earth labeling hydroxyapatite nanomaterials with potential biologically active implant powders for bone repair and tissue regeneration.
Highlights
IntroductionBone and teeth are the fundamental inorganic infrastructure for most animals as well as humankind, which form in a continuous biologically-controlled mineralization process for hydroxyapatite crystallization with hierarchy structures [1]
We focus on the controllable carbonation of hydroxyapatite of B-type based on the themodynamic data in Pourbaix diagram system
The solubility of HA is the lowest in all kinds of calcium phosphate in the system of Ca2+ -PO4 3− -H2 O at 37 ◦ C [43], the surface of bone is covered by a thin layer of hydrated amorphous calcium phosphate [44], which is ready to transform into calcium-deficient and hydroxyl-deficient carbonated hydroxyapatite on the biomieralization epitaxy on bone surface and in the confined biological medium spaces [45]
Summary
Bone and teeth are the fundamental inorganic infrastructure for most animals as well as humankind, which form in a continuous biologically-controlled mineralization process for hydroxyapatite crystallization with hierarchy structures [1]. Substituted forms with varied contents of carbonate for different bones, rather than pure phase hydroxyapatite with the nominal composition of Ca10 (PO4 ) (OH). The carbonate composition varies greatly from different tissues, such as in enamel (3.5 wt%), dentin (5.6 wt%), and bone (7.4 wt%) [4], which could be ascribed to the different mineralization processes either at the periosteal (outer) surface or embedded in an organic matrix biomineralized with the assistance of collagen [5,6]. The pure hydroxyapatite never founds in any biological systems, with all of the known apatite phase in biology composed of Ca-deficient, cation-substitution for Ca and anions of carbonate or fluoride for PO4 3−
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