Quiescent Mineralisation for Free-standing Mineral Microfilms with a Hybrid Structure.

HYPOTHESIS

The air-solution interface of supersaturated calcium hydrogen carbonate (Ca(HCO)) represents the highest saturation state due to evaporation/CO-degassing, where calcite crystals are expected to nucleate and grow along the interface. Hence, it should be possible to form a free-standing mineral-only calcium carbonate (CaCO) microfilm at the air-solution interface of Ca(HCO). The air-solution interface of phosphate buffered saline (PBS) could represent a phase boundary to introduce a hybrid microstructure of CaCO and carbonate-rich dicalcium hydroxide phosphate (carbonate-rich hydroxylapatite).

EXPERIMENTS

Supersaturated Ca(HCO) was prepared at high pressure and heated to form CaCO microfilms, which were converted to bone-like microfilms at the air-solution interface of PBS by dissolution-recrystallisation. The microfilms were characterised by scanning electron microscopy, 3D confocal microscopy, atomic force microscopy, Fourier transform infrared spectroscopy, laser Raman microspectroscopy, and X-ray photoelectron spectroscopy. An in situ X-ray diffraction (XRD) system that simulates the aforementioned interfacial techniques was developed to elucidate the microfilms formation mechanisms.

FINDINGS

The CaCO and bone-like microfilms were free-standing, contiguous, and crystalline. The bone-like microfilms exhibited a hybrid structure consisting of a surface layer of remnant calcite and a carbonate-rich hydroxylapatite core of plates. The present work shows that the air-solution interface can be used to introduce hybrid microstructures to mineral microfilms.