Multiresponsive Photonic Microspheres Formed by Hierarchical Assembly of Colloidal Nanogels for Colorimetric Sensors

Soft photonic materials formed from nanoscale colloidal crystals are versatile platforms for sensing and signaling in biomedical, chemical, and mechanical application scenarios. A particularly leveraged attribute of such materials is structural coloration; when applied in reconfigurable systems, dynamic coloration can be achieved, akin to the ability of chameleons to change their color appearance through nanocrystal structures beneath their skin. To date, many hybrid photonic materials composed of hard, passive, and soft adaptable systems have been explored for their dynamic color-changing performance; however, a lack of multifunctional and fully adaptable systems with sufficient control of properties on nanoscale persists. In addition, in hybrid systems with multiple stimuli-responsive components, interactions among components tend to be overlooked. Here, we propose a facile and scalable method to prepare multiresponsive photonic materials composed of soft and adaptable components, to tune the structural coloration over the entire visible spectral range. Temperature-responsive hydrogel microspheres of diameters of several tens of microns host an array of pH-responsive colloidal nanogels; the hierarchical assembly of nanogels scaffolded within the hydrogel microspheres form soft photonic microspheres that display a dynamic color change spanning the entire visible spectral range, through a variation in pH or temperature. pH change triggers a volume change in the nanogels, as does temperature change in the scaffolding hydrogel. A model predicting the equilibrium sizes of the microspheres as a function of temperature and pH has been proposed and validated. We also discuss the mutual interactions between the colloidal nanogels and the scaffolding hydrogel. These photonic microspheres could be applied as optically interrogated sentinels in label-free multiplexed chemical, thermal, and mechanical sensing with sub-millimeter resolution.