  The homochirality of biomolecules remains one of the outstanding puzzles
concerning the beginning of life. Chiral amplification of a randomly perturbed
racemic mixture of chiral molecules is a well-accepted prerequisite for all
routes to biological homochirality. Some models have suggested that such
amplification occurred due to asymmetric discrimination of chiral biotic or
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 themselves has not been addressed. Here we report strong chiral
amplification in the colloidal synthesis of intrinsically chiral lanthanide
phosphate nanocrystals, quantitatively measured via the 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 left- and right-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.
