Symmetry Breaking in Chiral Lanthanide-Based Nanocrystals—A Model System to Study Nucleation and Growth

Many inorganic compounds crystallize in chiral space groups, such as quartz, for example. Our group has been studying the breaking of left-right symmetry in the formation of nanocrystals of such compounds. We have shown, that using small chiral bio-molecules, which interact with the crystals’ building blocks, it is possible to achieve such a symmetry break. In particular, we have been working with the chiral TbPO₄●H₂O nanocrystals, and have shown that their handedness can be controlled by preparing the nanocrystals in the presence of certain natural chiral molecules, such as tartaric acid.¹ We used circularly polarized luminescence measurements of Eu³⁺dopant ions in the nanocrystals to follow the handedness and enantiomeric purity of the produced nanocrystals. Using single particle circularly polarized luminescence microscopy we were able to determine the handedness of individual nanocrystals and confirmed that we obtain a single enantiomer of the terbium phosphate nanocrystals when prepared with tartaric acid molecules.² This system exhibits ultra-high chirality amplification and can completely break symmetry on seeding with minute amounts of enantiomericaly-pure seed particles. Such effects are typically attributed to autocatalytic effects also termed secondary nucleation. We study the nature of these effects through comparison of symmetry breaking by chiral ligands vs. addition of chiral seed particles. We also obtain important mechanistic information by following the nanocrystals’ formation kinetics by in-situ monitoring total luminescence and circularly polarized luminescence over time.³ We believe that further studies of this model system might shed more light on the nature of nucleation in general and secondary-nucleation in particular.<br/>¹ U. Hananel, A. Ben-Moshe, H. Diamant, G. Markovich, "Spontaneous and directed symmetry breaking in the formation of chiral nanocrystals", <i>Proc. Natl. Acad. Sci. USA</i> <b>116</b>, 11159-11164 (2019).<br/>² E. Vinegrad, U. Hananel, G. Markovich, O. Cheshnovsky, "Determination of Handedness in a Single Chiral Nanocrystal via Circularly Polarized Luminescence", <i>ACS Nano</i> <b>13</b>, 601–608 (2019).<br/>³ G. Schwartz, U. Hananel, L. Avram, A. Goldbourt, G. Markovich, "A Kinetic Isotope Effect in the Formation of Lanthanide Phosphate Nanocrystals", <i>J. Am. Chem. Soc.</i> <b>144</b>, 9451-9457 (2022).