Vaterite/Hydroxyapatite Core-Shell Microspheres: Dissolution Kinetics and Mechanism

The synthesis and characterization of spherical vaterite/hydroxyapatite core-shell microspheres with an overall radius of 1.9 +/- 1 mu m is reported. The particle synthesis conditions were optimized by exposing preformed, well-characterized porous, spherulitic vaterite particles to solutions of K2HPO4, KH2PO4, or H3PO4 with concentrations in the range 0.001 to 2 M for varying periods of time (0 to 24 h). Characterization employed XRD, SEM, and optical microscopy. The dissolution of single diffusionally independent core-shell particles was studied via optical microscopy, and the particle area was monitored as a function of time over the duration of single particle dissolution into pure water. The size-time profiles showed two distinct regions assigned respectively to the dissolution of, first, the hydroxyapatite shell, and second, to the more soluble porous vaterite core. Quantitative analysis showed that both core and shell dissolved under "thermodynamic" control with the rate-determining step being the diffusion away from the particle of dissolved solid at a rate reflecting the magnitude of the concentration of dissolved species at the surface of the dissolving solid pinned by the solubility of either hydroxyapatite or vaterite so that the shell dissolved more slowly than the core. Insight into the specific dissolution characteristics of vaterite/hydroxyapatite has significant implications for applicability in a biomedical context. Further, the demonstrated method for extracting mechanistic insights and specific variations in the dissolution rates of different core-shell particle compositions represents a methodology that can be broadly applied to core-shell microparticles.