Mesoporous structure and evolution mechanism of hydroxycarbonate apatite microspheres

Monodisperse mesoporous hydroxycarbonate apatite microspheres (MHAMs) were fabricated by soaking calcium carbonate microspheres (CCMs) in a cetyltrimethylammonium bromide (CTAB)/Na₂HPO₄/cyclohexane/n-butanol emulsion system. After soaking CCMs in the emulsion system at 20 °C, hydroxycarbonate apatite nanocrystals nucleate heterogeneously on the surfaces of CCMs via a dissolution-precipitation reaction. The as-formed nanocrystals aggregate to form mesopores with the pore size of ∼ 3.9 nm and ∼ 9.0 nm. The smaller mesopores are derived from the direct aggregation of the nanocrystals, which do not change obviously with the reaction time. In contrast, the larger mesopores are formed by using CTAB micelles as templates, and their pore size decreases from ∼ 9.0 nm to ∼ 7.4 nm with increasing the reaction time from 6 h to 1 day. After reaction for 3 or 5 days, the larger mesopores disappear because of unstability of the CTAB micelles. If reaction temperature is kept at 50 °C, the conversion rate of CCMs to MHAMs is greater than that at 20 °C, and the corresponding mesoporous structure is unimodal with the pore size of ∼ 3.9 nm. Simulated body fluid immersion tests reveal that MHAMs have a great in vitro bioactivity, which is attributed to the mesoporous structure and B-type CO₃^2- substitution of MHAMs.