Vaskar Sardhalia, Claudio Dos Reis Ferreira, Guillaume P Laurent, Mohamed Selmane, Anne Vallée, Mathieu Frégnaux, Xiaoyan Li, Marta de Frutos, Andrew Fitch, Clemens N Z Schmitt, Shahrouz Amini, Zhaoyong Zou, Iryna Polishchuk, Boaz Pokroy, Peter Fratzl, Thierry Azaïs, Nadine Nassif, Marie Albéric
Colors in biominerals often arise from the occlusion of low-molecular-weight organic pigments within calcium carbonate (CaCO3) mineral phases during biomineralization processes; however, the physicochemical origin of these colors has been overlooked. Here, inspired by the growth of colored sea urchin spines that occurs through amorphous CaCO3 (ACC) precursors, we show an in-vitro halochromic modulation of ACC crystallization. In this mechanism, the deprotonation of a natural red pigment naphthazarin (NZ) is driven by pH variations during mineral formation and subsequently stabilized within CaCO3- hybrid pigments. The latter show lavender-blue to violet-blue hues upon crystallization. Deprotonated NZ has a minimal impact on ACC formation and crystallization, which occurs through local dissolution and reprecipitation mechanisms; however, it limits the formation of vaterite, and calcite distorted nanodomains. Deprotonated NZ nano-inclusions that interact with calcite through water are thus formed. Overall, this study brings insights into the processes behind the coloration of biomineralized organisms.