Synthesis and Optical Levitation of Ellipsoidal Silica Particles

Levitodynamics, an emerging technique in optomechanics, is the optical vacuum trapping of mesoscopic particles. Here, it was used to isolate individual ellipsoidal silica particles from their environment to measure their geometric properties with great precision. By evaluating damping rates, we observed an average aspect ratio of 1.5 while measuring the particles in 3D space, which is consistent with the TEM analysis. Levitodynamics proves an appropriate technique to study such nano-objects and could be developed for torque sensing at the nanoscale. Spherical colloidal silica particles can be produced by well-known and well-controlled chemical reactions and are used in many studies, from biology to optics. Their anisotropic counterparts are much less described, yet would present many alternative properties. Herein, we employed a sol-gel method to synthesize elliptic silica particles using a mixture of CTAB and l-tyrosine as structure-directing agents. Our results demonstrate that l-tyrosine is essential for the formation of the ellipsoidal morphology, as reactions carried out at varying CTAB concentrations only formed spherical to rod-like particles. With increased concentrations of l-tyrosine, the produced particles go from isotropic spherical particles to anisotropic prolate and oblate ellipsoidal particles with increased dimensions. Transmission electron microscopy (TEM) shows that a typical synthesis of ellipsoidal particles presents an average major axis of 137 nm and an average aspect ratio of 1.6.