12:00 PM - 12:15 PM
[MIS11-05] Chiral asymmetry in NaClO3 Chiral Crystallization Induced by Circularly Polarized Laser Trapping of Plasmonic Nanoparticles
Chiral crystallization, in which chirality spontaneously emerges in the course of crystallization, has been received attention from the viewpoint of emergence of chirality. The exploration of physical factors that induce a significant chiral bias in chiral crystallization should provide implications for the origin of biohomochirality. However, light-based chiral bias in chiral crystallization still remains unreported. The conceivable reason is that CD is intrinsically small. Here, we provide the first demonstration of a statistically-significant chiral bias in sodium chlorate (NaClO3) chiral crystallization induced by CPL irradiation with the help of CD enhancement by localized surface plasmon resonance (LSPR) of silver nanoparticles (AgNPs), which is caused by electronic state chiral materials coupled with the collective oscillation of free electron on the surface of metal nanoparticles.
A continuous-wave (CW) CPL green laser (wavelength is 532 nm, laser power is 940±5 mW, ellipticity is ›93%) was focused onto the air-liquid interface of the undersaturated NaClO3 solution (6 microlitter) containing AgNPs by using a 60× objective lens (NA = 0.9) equipped on an inverted polarized light microscope. We repeated crystallization and chirality identification of the NaClO3 crystal 150 times by using l-CPL and 150 times by using r-CPL, respectively. The number of the resulting enantiomorphs was counted. The crystallization dynamics was examined by in-situ microscopic observation of Raman scattering and polarized image.
Upon the laser irradiation, aggregates of the AgNPs occasionally floated on the air-liquid interface were trapped at the focal spot because of optical trapping. Crystallization was induced simultaneously with the optical trapping of AgNPs. The observation of Raman scattering showed that crystallization occurred at the moment when surface-enhanced Raman scattering (SERS)  active particle was optically trapped, showing that the excitation of LSPR plays significant role for nucleation . As the result of counting the given enantiomorphs, the crystallization using l-CPL(r-CPL) yielded l-enantiomorph 55(93) times and d-enanitomorph 95(57) times, respectively. These results indicate that the “dominant” enantiomorph can be switchable by changing the handedness of incident CPL, i.e. enantioselectivity. This chiral bias is statistically significant because the number of the “dominant” enantiomorph deviates 99% interval of the binomial distribution B(n,p) = B(150, 0.5), where n is the number of trials and p is the probability that the “dominant” enantiomorph crystallizes out. The chiral bias, which was approximately 25% in crystal enantiomeric excess, is much higher than that in typical enantiomeric excess in photosynthesis ranging from 0.5 to 2% .
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