Fine-tuning basic inorganic building blocks

Scientists struggle to create even close approximations of the many different structures built from an extremely common mineral building block: calcium carbonate. NCCR Bio-Inspired Materials researchers at Lausanne’s Federal Institute of Technology (EPFL) have demonstrated that the properties of the carbonate are directly influenced by the presence of water, which may be the key to understanding and ultimately imitating this desirable material.

Calcium carbonate (CaCO₃) is an extremely widespread compound in nature. It can be found in geological formations where significant amounts of carbon dioxide are present, such as in corals, skeletons, or egg and snail shells. Nature produces CaCO₃-based materials that display remarkable mechanical and optical properties. It achieves this by controlling the structure, orientation, shape, and arrangement of these carbonate crystals. Scientists have devoted substantial efforts to trying to produce biomimetic versions of these natural materials, but have failed to achieve the level of control required to create crystals with well-defined structures and morphologies.

The NCCR researchers have chosen to focus on amorphous CaCO₃ (ACC), the precursor of both natural and synthetic carbonate crystals. ACC serves as a storage for calcium ions, which are released to build up crystals when needed. Yet, the parameters that lead to the formation of CaCO₃ crystals and that influence their structure and properties are not completely understood. Conditions such as the pH, temperature, and drying method can all impact the formation pathways, structure, and stability of the precursor. The stability of ACC was shown to be directly linked to the amount of water it contains. How this amount can be tuned, however, remains a mystery.

The EPFL scientists chose to fabricate ACC particles within small drops that dry quickly, thereby enabling the quenching of particle formation at different times. To avoid using any organic solvents that are traditionally employed to quench the formation of ACC, the droplets were produced in a microfluidic sprayer. It was found that the longer ACC particles take to form, the more water they contain. The amount of water in ACC also influences the size of the crystals that form: with more water present, the grains of CaCO₃ crystals that form are larger when their particles are subjected to elevated temperatures. This know-how should simplify the processing of ACC into biomimetic materials with tunable structures, and with properties suited to specific applications.

“Our findings are probably not limited to the formation and transformation of calcium carbonate,” adds NCCR Principal Investigator Professor Esther Amstad. “They could by applied to the formation of many other materials that form via precursors such as calcium phosphate, calcium oxalate, or calcium sulfate.”

“We should be able to develop new processes that allow us to assemble ceramic-based composites with a tighter control over their structure, and hence, their properties. And we should certainly be able to design materials that have a far stronger filiation with their natural counterparts.”

References: Du, H.; Steinacher, M.; Borca, C.; Huthwelker, T.; Murello, A.; Stellacci, F.; Amstad, E. Amorphous CaCO3: Influence of the formation time on its degree of hydration and stability, J. Am. Chem. Soc., 2018, 140, 14289   

Du, H.; Amstad, E. Water: How does it influence the CaCO₃ formation?, Angew. Chem. Int. Ed., 2020, 59, 1798.