Bioinspired Halochromic Crystallization of Calcium Carbonate
Abstract
Colors in biominerals primarily arise from the presence of small organic pigments that are occluded during biomineralization processes; however, the physicochemical origin of these hues has been overlooked. Here, inspired by sea urchins, colored calcite crystals with bio-inspired 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 and subsequently stabilized within the calcium carbonate-hybrid pigments, showing lavender-blue to violet-blue hues upon crystallization. Deprotonated NZ not only leads to various shades of blue but also favors the formation of calcite over vaterite, while having minimal impact on ACC formation. The resulting hybrid pigments exhibit enhanced color stability in response to changes in pH, as demonstrated by textile finishing assays. 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.