Controllable Fabrication of Toroidal Photonic Microobjects with a Janus Structure by Evaporation-Induced Assembly Coupled with a Magnetic Field Modulation

Abstract

Toroidal photonic microobjects (TPMs) have been shown to possess better performances than their spherical counterparts in terms of multiple aspects. However, existing TPMs all possess a homogeneous structure, which may limit their multiple functions and broader applications. In this study, we report the construction of TPMs with a Janus structure via the evaporation of droplets of binary colloidal particles (CPs) on a hydrophobic surface, coupled with a magnetic field-guided redistribution of CPs. The individual TPM consists of one portion that is rich in magnetic CPs and another portion that is rich in SiO2 CPs The heterostructure of the resulting TPMs was precisely controlled by regulating the concentrations of magnetic particles in the initial droplet and the intensity of the magnetic field. The obtained opal-structured TPMs were subsequently transformed into hydrogel-based inverse opal TPMs, which not only exhibited a reversible optical response to pH values and ethanol concentrations but also demonstrated efficient adsorption and cyclic regeneration capability for heavy metal ions (e.g., Cu2+), owing to the presence of abundant carboxyl groups and magnetic CPs within the hydrogel skeleton. This study offers a novel approach for designing functional photonic particles with heterostructures and showcases their applications in fields such as environmental remediation.