Abstract
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.
Highlights
Photonic crystals (PCs) have attracted considerable interest as a promising paradigm in the photonics research studies from both fundamental and technological viewpoints because of their outstanding features to confine and manipulate photons in a spatiotemporal way.[1,2] The PCs are the periodically modulated microarchitectures of different dielectric materials with the intervals comparable to the lengths of electromagnetic waves
These results suggested that these PNIPA hydrogel microparticles may not be suitable for the fabrication of colloidal photonic crystals (CPCs) structures by the selfassembly owing to the substantially high coefficient of variation (CV) in the diameter, that is, the broad distribution of the particle diameter
The emulsion polymerization of PNIPA hydrogel precursors with an sodium dodecyl sulfate (SDS) surfactant facilitated the preparation of monodisperse PNIPA hydrogel microparticles, whose diameters could be finely controlled by adjusting the SDS concentration during the polymerization
Summary
Photonic crystals (PCs) have attracted considerable interest as a promising paradigm in the photonics research studies from both fundamental and technological viewpoints because of their outstanding features to confine and manipulate photons in a spatiotemporal way.[1,2] The PCs are the periodically modulated microarchitectures of different dielectric materials with the intervals comparable to the lengths of electromagnetic waves. CPCs for photonic applications such as colorimetric sensors that are responsive to environmental stimuli, the general fabrication strategy is to fill the void space between the 3D ordered colloidal microparticles of CPCs with soft materials such as hydrogels, elastomers, liquid crystals, and so forth In this context, the active CPC films of microparticles stabilized with stimuli-responsive hydrogels of poly(N-isopropylacrylamide) (PNIPA) and its derivatives are one of the most frequently investigated CPC systems.[21−27] This is because the PNIPA hydrogels swell and shrink around the lower critical solution temperature of ∼33 °C in water through the reversible volume changes and absorb and expel large amounts of water in response to changes in temperature, respectively.[28−30]. These purifiable and reusable materials would be greatly advantageous in overcoming the serious issues, such as mass production, mass consumption, and waste mismanagement, related to the use of finite resources worldwide in the development of a sustainable society
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