Tunable Structural Color in Copolymer Microgels Through Controlled Synthesis and Thermally Induced Assembly

Structural color formation in soft colloidal systems represents a promising approach toward stimuli-responsive photonic materials. In this study, the assembly of poly(N-isopropylacrylamide) (PNIPAm)-based microgels incorporating varying percentages of anionic (methacrylic acid), neutral (acrylamide), and cationic (2-(dimethylamino)ethyl methacrylate) comonomers to create multifunctional colloidal building blocks capable of forming dynamic structural colors was systematically investigated. Through optimization of synthesis conditions, including comonomer selection, their concentration, and surfactant-mediated size control, libraries of copolymer microgels with precisely tailored particle diameter, surface charge, and stimuli-responsive swelling behavior were obtained. Structural color tuning across the visible spectrum was generated by thermal colloidal assembly. The interparticle spacing and consequently the reflected wavelengths were modulated by microgel concentration and their stimuli-responsive behavior: Finely tuned responsiveness of the microgels allowed for precise shifts in the reflected color upon environmental stimuli (temperature or pH). Such changes of the structural colors upon stimuli were clearly observable in the colloidal crystal assemblies of anionic and neutral microgels, while the amorphous assembly of the cationic microgels limited the detection of small shifts. Our findings provide key insights into the interplay between microgel responsiveness, compressibility, and the resulting structural color formation that are expected to support advanced multifunctional colloidal sensors, coatings, and optical tags.