Soft Capsules That Change Shape and Exhibit Structural Color Due to Nanoparticle Assembly in Their Cores

Dyes and pigments are the primary source of color for most materials. However, they are susceptible to environmental (light, heat, etc.) or chemical damage and can easily fade over time causing the color to fade. Coloration that is resistant to damage and fading is desirable. For instance, structural color or iridescence is a phenomenon primarily observed in nature (butterflies, mollusks, birds, etc.) that shows brilliant colors due to the nanoscale morphology and structure of the material instead of chemical dyes. Structural color emerges due to the long-range ordered arrangement of non-absorbing nanostructured particles that diffract light at a particular wavelength corresponding to their size. Since structural color emerges from nanoscale morphology of the material, the colors are non-fading if the microstructure remains. Most natural and synthetic materials exhibit permanent structural color, while complex nanocomposites are required to generate materials that exhibit reconfigurable structural color.<br/>Here we demonstrate a simple approach to obtain structural color within a millimeter sized hydrogel capsule. We fabricate a thin spherical polymer hydrogel shell of alginate surrounding an aqueous liquid core containing a concentrated dispersion of silica nanoparticles. The hydrogel shell acts as a selective permeation barrier, allowing water and small molecules to pass between the liquid core and surrounding fluid, but excluding large polymers and nanoparticles. Placing the capsule in a concentrated polymer solution (<i>e.g.</i>, polyacrylic acid, sodium alginate) induces a positive osmotic pressure gradient on the capsule that causes water to diffuse out of the capsule, causing it to collapse. Nanoparticles are entrained in the fluid flow out of the capsule and captured on the inner surface of the hydrogel shell. Electron microscopy and dark field optical microscopy show that the nanoparticles formed a conformal layer and colloidal crystal particles that were tens of microns in size on the inner hydrogel surface. Video rate fluorescence microscopy of fluorescent polystyrene tracer particles show there were recirculating fluid flows inside the capsule during osmotic pressure-induced collapse, which deposited the colloids onto the inner surface of the hydrogel shell. The final collapse step of the capsule, where the hydrogel shell with positive curvature buckled into a complex 3D shape with negative curvature, such as a bowl or folded shape, served to pack the colloids into ordered structures responsible for structural color. We demonstrate that this phenomenon is reversible and can be applied to a range of nanoparticle sizes. Interestingly, we find that the final shape and geometry of the collapsed capsule is dependent on the type and concentration of osmolyte used.