Engineering of inorganic nanostructures with hierarchy of chiral geometries at multiple scales.

Another interesting type of system showing how chiral hierarchically organized architectures could arise from chiral molecules by non-classical pathway is optically active chiral supraparticles decorated with spikes or sheets.174-177 For example, right-handed twisting rod-like cellulose nanocrystals (CNCs) were used as a chiral template and building block for the formation of chiral CuO/CNC composite supraparticles (Figure 15A).177 These CNCs had three levels of chirality: (i) molecular chirality of glucose units, (ii) structural chirality of the right-handed nanorods, and (iii) chirality of cholesteric liquid crystal structure. Other geometrical shapes include left-handed ( B ) b and right-handed ( B ) b helices obtained by folding DNA strands.73 Complex chiral shapes can also be obtained by interconnecting NPs with multiple "springs" represented by DNA.74 Their chirality may not be initially obvious by visual comparison with familiar mirror-asymmetric shapes, such as helices, but they can be identified using chirality measures, for instance, Osipov-Pickup-Dunmur index that changes sign depending on the handedness of the structure.75 Continuing the consideration of scale, one can foresee the translation of the chirality of NPs into sub-millimeter, millimeter, and macroscale.76 Connection with biology Considering the rapid expansion of chiral nanostructures toward biology and medicine, it is instructive and insightful to draw parallels between hierarchical chirality in biological compounds and inorganic nanostructures. The optical properties of these nanostructures were compared to those of -HgS nanostructures, which also possessed crystallographic chirality but had achiral morphologies (nanocubes, nano-ellipsoids, nanorods, and nanowires), revealing that the optical activity of -HgS nanostructures with achiral morphologies originated from the chiral crystal lattice. [Extracted from the article]