Self-Assembled Growth and Pore Size Control of the Bubble-Template Porous Carbonated Hydroxyapatite Microsphere

Abstract

Flowerlike porous carbonated hydroxyapatite (CHAp) microspheres have been synthesized by employing different chelating ions: disodium ethylene diamine tetraacetate (Na2EDTA) and trisodium citrate (Na3CA) and constructing a CO2 bubble as a novel template. By adjusting the concentration of the chelating ions or the surface tension of the solution, the pore size of the porous CHAp could be well controlled. With an increase of the concentration of the chelating ions, the pore size decreased by degrees, and at the same concentration, the pore size obtained via citric acid (CA) was obviously much smaller than that obtained via ethylenediaminetetraacetic acid (EDTA). On the other hand, with the increased surface tension, the pore size decreased gradually, and this control relation was confirmed by the Laplace equation, the Kelvin equation, and the Clapyeron equation. Most importantly, according to the important effects of the chelating ions and the CO2 bubble in the self-assembly, the self-assembled process and mechanism were further investigated. First, the bubbles gathered into the stable spherical foams and the calcium ions were orientatedly attached onto the bubble surface by the chelating ions. Next, the nucleation emerged in the plateau borders and the orientated release of calcium ion from the bubble surface caused the orientatedly self-assembled growth of CHAp flakes along the interfaces between the bubbles. Finally, the flakes interconnected into the porous structure and the porous microsphere formed. In addition, a better and further explanation on the pore size control by the concentration of the chelating ions was given according to the self-assembled process and mechanism.