Asymmetric Calcium Carbonate Morphologies Regulated by Poly(γ-glutamic acid)

Asymmetry is pervasive in natural mineralized structures, ranging from tip growth in plants to one-ended elongation of biominerals, such as sea-urchin spicules and molluscan shell pillars. In contrast, solution-phase biomimetic mineralization typically yields apolar and symmetric morphologies, and the controlled emergence of asymmetry remains limited. Here, using poly(γ-glutamic acid) (PGA) as a model polyelectrolyte, we demonstrate that an ordered regulation of calcium carbonate morphologies from symmetric dumbbell-like structures to spindle-shaped forms can be achieved, accompanied by morphological symmetry breaking. At low PGA concentrations, elongated but largely symmetric particles are obtained. At intermediate concentrations, symmetric necking structures emerge that act as morphological precursors to asymmetry. Upon further increasing PGA concentration, fore-aft asymmetry develops through morphological splitting, after which one end persistently dominates and sharpens, yielding stable single-ended conical morphologies in solution. FTIR and Raman spectroscopy results indicate that PGA continuously adsorbs onto the crystal surface during the nucleation and growth of CaCO₃ and participates in the morphological evolution, thereby playing a key role in regulating the crystal morphology.