Colloidal Photonic Crystals of Reusable Hydrogel Microparticles for Sensor and Laser Applications

[Image: see text] Although a wide variety of techniques have been developed to date for the fabrication of high-quality colloidal photonic crystals (CPCs) using monodisperse silica and polystyrene microparticles, poly(N-isopropylacrylamide) (PNIPA) hydrogel microparticles have rarely been utilized for the preparation of active CPCs despite the intriguing feature of temperature-responsive volume changes. This report describes the promising potential abilities of PNIPA hydrogel microparticles for sensor and laser applications. Monodisperse PNIPA hydrogel microparticles were synthesized by emulsion polymerization, and the microparticle diameter was finely controlled by adjusting the surfactant concentration. Such hydrogel microparticles spontaneously formed uniform CPCs with visible Bragg reflection even in fluid suspensions. The addition of small amounts of ionic substances into the centrifuged and deionized CPC suspensions enabled the on-demand color switching between Bragg reflection and white turbidity with temperature, leading to temperature- and ion-sensing applications. Moreover, our expanding experiments successfully demonstrated the optically excited laser action with a single and narrow peak from CPC suspensions with light-emitting dyes by the photonic band gap effect. After the light-emitting dyes were simply removed from the CPC suspensions by centrifugation, the purified PNIPA hydrogel microparticles were permanently reusable as the CPC laser microcavities to generate the laser action at other wavelengths using different dyes. This study contributes the circular economy concept using reusable hydrogel microparticles for the realization of a sustainable society.