Atom Probe Tomography and Calcium Carbonate Biominerals
Biominerals represent an intriguing research area to material scientists, with organisms ranging from single-celled bacteria to complex mammalian systems producing functional materials with complex hierarchical structures, implying the fundamental chemistry of crystallisation is tightly controlled during mineral formation. While several biological pathways have been proposed to achieve this, studying biominerals with traditional micro- and spectroscopic techniques can be difficult, due to their brittleness, porosity and instability under high-energy beams.
While well-established in studies of metals and semiconductors, atom probe tomography (APT) is now being increasingly applied to provide nanoscale compositional analysis of biologically produced materials. Promising advancements have employed APT to reconstruct collagenous fibres in mouse femur bones and map the composition at grain boundaries in dental enamel. Surprisingly, APT remains under-utilized in the analysis of CaCO3 biominerals, produced by marine organisms like sea urchins, crustaceans and microalgae. This is despite prominent questions regarding the nanoscale distribution of organic macromolecules and trace ions like Mg and Na within the biomineral, with the location proposedly linked to their role in several stages of mineral formation, including stabilising an amorphous precursor phase before controlling the nucleation and growth of the final crystalline structure.
We have conducted compositional analysis using APT on the calcite spines and teeth of sea urchins, where Mg ions and organic-related OH3 and COH fragment ions can be distinguished from mineral peaks. We additionally identify ion peaks related to organic pigment molecules present only in purple-coloured spine regions. Furthermore, nanoscale clusters of OH3 and COH ions can be observed within mineral spines, while Mg may be preferentially segregated in certain regions. In the teeth, pronounced compositional differences possibly indicate an organic-rich grain boundary within a Mg-rich calcite phase. Future work will aim to identify the composition of organic-rich regions at grain boundaries, expanding our studies to other biomineral systems like corals, where early APT results suggest prominent organic-rich features may be present. We are also interested in determining whether compositional changes can be detected across amorphous and semi-crystalline intermediate phases, hopefully adopting cryogenic preparation techniques to preserve minerals in their native hydrated state.
The preliminary results highlight the growing ability of APT to identify nanoscale structural features within CaCO3 biominerals. This is an exciting development in biomineralisation research, with the potential to elucidate unknown nanoscale biological and chemical pathways which organisms employ in the design and production of functional mineral structures.
