Programmable Photonic Response in pH-Responsive Block Copolymer Colloids

Three-dimensionally ordered photonic colloids offer a unique opportunity to directly correlate internal nanoscale morphology with the macroscale optical response. Herein, we present pH-responsive poly(styrene-block-2-vinylpyridine) (PS-b-P2VP) colloids with stacked lamellar architecture that exhibit tunable structural color through controlled swelling dynamics. Cross-linking P2VP domains using 1,8-dibromooctane increases initial domain spacing, shifting color from violet to blue-green, while simultaneously restricting acid-induced swelling and red shift. In contrast, lower cross-linking permits significant anisotropic expansion upon protonation, enabling large color shifts from violet to red. These swelling behaviors arise from the formation of pyridinium cations and their interactions with counteranions, where anion hydration energy governs water uptake: highly hydrated species facilitate greater expansion, while less hydrophilic anions impose steric and electrostatic constraints. By systematically tuning cross-linking density, acid identity, and the molecular weight of PS-b-P2VP, we achieve continuous, full-spectrum modulation of structural color. This system enables real-time optical tracking of internal structural changes at the single-particle level, establishing a versatile platform for stimuli-responsive photonic materials with programmable color output.