Halochromic modulation of amorphous calcium carbonate crystallization driven by pH-responsive bioinspired pigments

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.