Hydrothermal synthesis of Mg-substituted tricalcium phosphate nanocrystals
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Research Letter
Hydrothermal synthesis of Mg-substituted tricalcium phosphate nanocrystals Wei Cui, Shaogang Wang, Rui Yang, and Xing Zhang, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning 110016, China; School of Materials Sciences and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China Address all correspondence to Xing Zhang at [email protected] (Received 8 June 2019; accepted 31 July 2019)
Abstract In this study, Mg-substituted tricalcium phosphate (Mg-TCP) nanoparticles were synthesized by hydrothermal reactions of Mg-calcite mesocrystals from echinoderm skeletons. Following the biomineralization of echinoderms, Mg-calcite powder was synthesized via the solid-state transition of Mg-amorphous calcium carbonate prepared by a wet-chemical precipitation method, which can also be used to fabricate Mg-TCP. We illustrated that Mg-calcite with a certain level of Mg substitution led to the formation of Mg-TCP through the ion-exchange reactions in the hydrothermal system. Therefore, this study provides a new pathway for the synthesis of Mg-TCP nanoparticles.
Introduction The skeletons of echinoderms mainly consisting of Mg-calcite biominerals showed hierarchical structures and excellent mechanical property.[1,2] Echinoderm skeletons such as sea urchin (SU) spines[1,3] and teeth[4,5] behave like single crystals. Recent studies revealed that these echinoderm skeletons were composed of well-oriented nanoparticles, called mesocrystals,[1,6] like those existed in other biominerals[7,8] and metal alloys.[9–11] Mesocrystals form from the ordered array of amorphous precursor particles through a nonclassical nucleation pathway.[12–15] Natural calcium carbonate skeletons, like coral, cuttlebones, and SU spines, have been hydrothermally reacted to form β-tricalcium phosphate [β-TCP, Ca3(PO4)2] or hydroxyapatite [HA, Ca5(PO4)3OH] as bone grafts, considering their hierarchical open-cell structures suitable for tissue ingrowth. Compared to HA, β-TCP is biodegradable, which can be fully replaced by newly formed bone.[2,16,17] This is likely due to the relatively higher Ksp of β-TCP (10−28.9) than that of HA (10−58.4).[18] However, β-TCP is difficult to be fabricated in aqueous solutions at low temperature due to the preferential formation of HA under the same condition. Therefore, β-TCP powders were usually prepared by calcination of precursor materials or solid-state reactions at high temperature.[19,20] Previous studies[2,21,22] showed that SU spines consisting of Mg‒calcite can be converted to Mg-substituted tricalcium phosphate (Mg-TCP) by a hydrothermal reaction. On the other hand, skeletons including coral, cuttlebone, and seashells consisting of aragonite or calcite without Mg2+ ion substitution were converted to HA.[21–23] Thus, Mg2+ ions played an important role in the hydrothermal reaction of Mg-calcite to Mg-TCP
(not HA). However, the underlying mechanisms for the effects of Mg on the formation of Mg-TCP remain unclear. In this study, natural calcite skelet
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