Bioinspired Halochromic Crystallization of Calcium Carbonate

Colors in biominerals mainly arise from the presence of small organic pigments occluded during biomineralization processes, however the physicochemical origin of hues has been overlooked. Here, inspired by sea urchins, colored calcite crystals with biomimetic nanostructure were synthesized using the natural red pigment naphthazarin (NZ) during the formation of amorphous calcium carbonate (ACC), the biological precursor described in vivo. A halochromic ACC crystallization pathway is revealed, where the speciation of NZ is driven by pH variations during crystallization and subsequently stabilized within the calcium carbonate-hybrid pigments, showing lavender-blue to violet-blue hues upon crystallization. Deprotonated NZ not only leads to the various shades of blue, but also controls polymorph selection, with minimal impact on ACC formation. The resulting bio-inspired hybrid pigments exhibit enhanced color stability regarding temperature and pH as evidenced by textile finishing assays for the latter. Overall, this study brings insights into the processes behind the coloration of biomineralized organisms and explores the development of bio-inspired hybrid pigments for coloring applications.