Multifunctional photonic microobjects with asymmetric response in radial direction and their anticounterfeiting performance

There are few explorations that have integrated multiple properties into photonic microobjects in a facile and controlled manner. In this work, we present a straightforward method to integrate different functions into individual photonic microobject. Droplet-based microfluidics was used to produce uniform droplets of an aqueous dispersion of monodispersed SiO₂ nanoparticles (NPs). The droplets evolved into opal-structured photonic microballs upon complete evaporation of water. After infiltration of an aqueous solution of acrylamide (AAm) and acrylic acid (AAc) monomers into the interstices among SiO₂ NPs, opal-structured SiO₂ NPs/pAAm-co-AAc hydrogel composite photonic microballs were obtained upon UV irradiation. Afterwards, a wet etching process was introduced to etch the microballs in a controlled manner, yielding individual photonic microball composed of an SiO₂ NPs/pAAm-co-AAc composite opal core and a neat pAAm-co-AAc shell. The pendant carboxylic acid groups in the skeleton of the hydrogel matrix were further utilized to react with positively charged compounds, such as Ruthenium compound containing fluorescent polymers. The resulting photonic microobjects eventually featured with localized stimulus-responsive properties and multiple colors under different modes. The multifunctional photonic microobjects were discovered to have fivefold of anticounterfeiting properties when used as building blocks for anticounterfeiting structures and may have other potential applications.