For traditional experimental methods, the dynamic formation and transformation of amorphous calcium carbonate (ACC) are hard to capture due to its metastability and ease of transforming rapidly to the more stable phase. The emergence of microfluidic technology provides an effective approach on that issue, thus attracting widespread researchers' interest in crystallization research. In this study, based on laminar microfluidics, we demonstrate a microfluidic approach toward the study of the formation and transformation of ACC. Through the control of the velocity and concentration of CaCl 2 and Na 2 CO 3 , the crystallization process was observed on a chip under the microscope. We show by in situ confocal micro Raman spectroscopy and theoretical calculation based on fluid dynamics simulation that the phase transformation pathways are different under the different supersaturation levels in laminar microfluidics. ACC does not always appear in the crystallization process; when supersaturation above the critical supersaturation of ACC occurred, the crystalline CaCO 3 phases appeared after the metastable intermediates ACC through the transformation mechanism of dissolution−recrystallization, while the crystalline CaCO 3 phases formed without the intermediates ACC when supersaturation below the critical supersaturation of ACC occurred. Our work provides an approach for the investigation of metastable intermediates and support for one-step and multistep nucleation mechanisms.
As
a precursor phase, amorphous calcium carbonate (ACC) plays a key role
in the formation of CaCO3 biominerals, but the detailed
information about the structure evolution during the crystallization
process of ACC is limited. Herein, based on the confined environment
in microfluidics, we have demonstrated a strategy to investigate the
crystallization processes of ACC. The characteristics of a confined
environment in microfluidics were analyzed through COMSOL Multiphysics
simulations. By mixing equimolar CaCl2 and Na2CO3 aqueous solutions directly, the reactive crystallization
of CaCO3 was monitored on chip by online microscope observation
and micro Raman spectroscopy scanning. Combined with offline scanning
electron microscopy characterization, we showed that branched aggregate
ACC(I) precipitated first once mixing reactant solutions, then the
more ordered and whiskerlike ACC(II) was an unexpected result. These
whiskerlike ACC(II) either gradually transformed to spherical structure
nanocrystalline, vaterite was then formed from it through spherulitic
growth mechanism, or the initial rhombohedral crystallographic calcite
was formed from whiskerlike ACC(II), then the complete rhombohedral
calcite crystals formed through the dissolution/recrystallization
of ACC(II). Our results showed an intuition-based way to directly
observe the structure evolution of ACC crystallization in the confined
environment; it could be of inspiration for the understanding of biomineralization
processes.
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