Dissolution behavior of vaterite spherulite in solutions containing phosphate ions

Vaterite is a crystalline polymorph of calcium carbonate that exhibits low stability in comparison with the other crystalline polymorphs of calcium carbonate. It often assumes a spherical shape. The physical properties of vaterite are utilized in advanced biomaterials such as drug delivery systems (DDSs). The potential application of vaterite in DDSs demands a comprehensive understanding of its dissolution rate as a function of PO₄concentration. Using in situ optical studies, we analyzed the dissolution behavior of vaterite in a simulated PO₄-containing biogenic environment. The varying physicality (i.e., the pseudo-physiological conditions) of vaterite were investigated using high-resolution cryogenic transmission electron microscopy and small-angle X-ray scattering. In addition, we measured the PO₄/Ca ratio in each part of vaterite spherulites using field-emission scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy and inductively coupled plasma atomic emission spectrometry. We categorized the vaterite spherulites into three parts® the shell, mantle, and core parts® on the basis of their dissolution behavior and PO₄response. The PO₄response to the dissolution dynamics increases toward the outer part, although the adsorption rate decreases in the presence of PO₄and PO₄fluorescence materials. The composition of the shell part was nearly identical to that of ideal vaterite, whereas the inner part exhibited a high C/Ca ratio and a poorly crystalline phase referred to as vaterite-like material. The dissolution rate of each part was 0.231.58nm/ s (shell), 0.633.19nm/ s (mantle), and 1.385.71 nm/s (core). The vaterite and vaterite-like materials were further identified according to their composition and particle size distribution.