Droplet Microfluidics-Assisted Fabrication of Toroidal Photonic Microobjects With Precisely Tunable Photonic Properties.

Toroidal photonic microobjects (TPMs) formed via evaporation-induced assembly of colloid-containing droplets have attracted increasing interest, yet existing strategies often suffer from poor control over droplet templates or require simultaneous regulation of multiple parameters, limiting structural precision and scalability. Here, we report a droplet microfluidics-assisted approach for fabricating highly uniform TPMs. Monodisperse microfluidic droplets enable precise control over the toroidal geometry, while the porous PTFE substrate guides the formation of TPMs with a distinctive bottom flat surface. SiO2 colloid-containing droplets deposited on porous PTFE evolve into toroidal microobjects with an upper curved surface and a bottom flat surface. By tuning SiO2 particle size and droplet composition, including the incorporation of polydopamine colloids, the structural colors and photonic properties of TPMs can be readily controlled. Owing to their distinct curved and flat surfaces, TPMs exhibit markedly different angle-dependent photonic responses. Ru(bpy)3Cl2 post-modification further endows TPMs with fluorescence functionality. As a proof of concept, TPMs with diverse structural colors and fluorescence are assembled into a quasi-Olympic rings pattern, demonstrating their potential for patterned photonic structures and advanced pigments.