Light-Induced Assembly and Reconfiguration of Chiral Nanostructures

Abstract

Matured syntheses of inorganic nanocolloids today have allowed production of diverse asymmertic nanoparticles (NP) and even more complex assemblies, introducing a variety of nanomaterials with unique properties. Chirality is one of most intriguing symmetry groups in field of material science due to the optical rotatory power of chiral substances. Strong optical activity of inorganic nanoparticles (NPs) afford photosynthetic routes to chiral superstructures using circularly polarized photons (1). Although plasmonic NPs are promising candidates for such synthetic routes due to the strong rotatory power of highly delocalized plasmonic states (2), realization of light- driven synthesis of chiral nanostructures has been more challenging for plasmonic NPs than for the semiconductor due to the short lifetime of the plasmonic states. The process also can be difficult to recognize, and therefore requires unconventional approaches for the quantification of chiral products. Here, we demonstrate that illumination of Au precursors in the presence of citric acid with right- (left-) handed circularly polarized light (CPL) induces the formation of chiral Au nanostructures. Despite seemingly irregular shape of the particles, the resulted colloids showed distinctive mirror-imaged opposite circular dichroism (CD) bands according to the handedness of illuminated CPL. Circular dichroic response and geometric chirality from these seemingly achiral structures were successfully demonstrated and quantified by implementing three-dimensional (3D) electron tomography (e-tomo) into computational model. The mechanism of the light-driven assembly of chiral nanostructures is based on the asymmetric displacement of NPs in dynamic assemblies by plasmonic fields fol-lowed by particle-to-particle attachment. The ability of Au nanostructures to retain the polarization information of incident photons can be applicable to light absorbing materials, thus to create a variety of chiral nanomaterials. Similar examination of seemingly irregular NP through suggested chirality measure can also elucidate previously puzzling optical response from unclear geometric asymmetry of nanomaterials.