Spontaneous and Directed Symmetry Breaking in the Formation of Chiral Nanocrystals

A possible route to biological homochirality is through chiral amplification of a randomly perturbed racemic mixture of chiral molecules or catalyst. Some models have suggested that such amplification occurred due to asymmetric discrimination of chiral biotic\prebiotic molecules when they adsorbed onto crystalline surfaces. While chiral amplification has been demonstrated on surfaces of both chiral and achiral crystals, the mechanism that would produce an enantiomeric imbalance in the chiral surfaces has not been addressed. Here we report strong chiral amplification in the colloidal synthesis of intrinsically chiral lanthanide nanocrystals, quantitatively measured via the strong circularly polarized luminescence of the lanthanide ions within the nanocrystals. The amplification involves spontaneous symmetry breaking into either left- or right-handed nanocrystals below a critical temperature. Furthermore, chiral tartaric acid molecules in the solution act as an external “chiral field”, sensitively directing the amplified nanocrystal handedness through a discontinuous transition between right- and left-handed excess. These characteristics suggest a conceptual framework for chiral amplification, based on the statistical thermodynamics of critical phenomena, which we use to quantitatively account for the observations. Our results demonstrate how chiral minerals with high enantiomeric excess could have grown locally in a primordial racemic aqueous environment.