Birefringence Of Chiral Hedgehog Particles

Spiky hedgehogs can be self-assembled from twisted ribbons formed by gold-copper-cysteine complexes.¹ Furthermore, chirality of these twisted ribbons can be controlled by the cysteine amino acid and their arrangement into supraparticles is dictated by the ratio of L- vs D-form during the initial stages of self-assembly. Here we report that the seemingly spherically symmetric arrangement of twisted ribbons displays a maltese cross pattern typical of spherulites and liquid crystals under cross-polarizers.² This optical effect is observed at the single particle level³ with tunable diameters of 1-10 um. We investigate the optical effects in chiral hedgehogs using single particle methodology and find that the emergence of Maltese cross pattern is dependent on the sphericity of the supraparticle and the spacing between individual spikes. We understand the interaction of light with hierarchically self-assembled structures through finite-element based electromagnetic simulations and reconstruct the distribution of electromagnetic fields around the hedgehogs in three dimensions. These hedgehogs are omnidispersible in a variety of solvents and offset the limitations of commonly observed organic molecule based spherulitic arrangement for biomolecular sensing under harsh environments.⁴<br/>1. Jiang, W. <i>et al.</i> Emergence of complexity in hierarchically organized chiral particles. <i>Science (1979)</i> <b>368</b>, 642–648 (2020).<br/>2. Shtukenberg, A. G., Punin, Y. O., Gunn, E. & Kahr, B. Spherulites. <i>Chemical Reviews</i> <b>112</b>, 1805–1838 (2011).<br/>3. Bahng, J. H. <i>et al.</i> Mie Resonance Engineering in Meta-Shell Supraparticles for Nanoscale Nonlinear Optics. <i>ACS Nano</i> <b>14</b>, 17203–17212 (2020).<br/>4. Sivakumar, S., Wark, K. L., Gupta, J. K., Abbott, N. L. & Caruso, F. Liquid crystal emulsions as the basis of biological sensors for the optical detection of bacteria and viruses. <i>Advanced Functional Materials</i> <b>19</b>, 2260–2265 (2009).