Assembly of Colloids on Different Hydrogel Surfaces and Their Visualization Applications

Ordered colloidal assembly is a common strategy for generating structural colors. Conventional evaporation‐induced self‐assembly on rigid substrates suffers from the “coffee ring” effect, which limits the fabrication of uniform films. This paper presents a simple strategy for assembling poly(styrene‐methyl methacrylate‐acrylic acid) (p(St‐MMA‐AA)) nanoparticles on synthetic (polyacrylamide, PAM), natural (gelatin), and hybrid (PAM/gelatin) hydrogel surfaces. The synthetic hydrogel drives assembly through water absorption, producing structural colors within minutes. In contrast, the natural hydrogel relies on evaporation drying, yielding high‐quality colors but at a slower rate. While structural colors on most hydrogel surfaces are unstable in water, those on crosslinked gelatin remain intact. By integrating both components, the hybrid hydrogel enables rapid assembly and maintains color stability in water for over 20 h. Leveraging these distinct behaviors, PAM hydrogels serve as canvases for rapid patterning; gelatin‐based hydrogels act as visual sensors for bacterial concentration; and hybrid hydrogels allow naked‐eye monitoring of drug release. Overall, hydrogel surfaces offer an innovative pathway for advanced photonic crystal engineering.