Novel robust ion-specific responsive photonic hydrogel elastomers

Abstract

Embedded photonic crystal hydrogels and responsive photonic crystal microspheres that can change structural color as a result of external stimuli have shown great potential for various applications, but their practical applications have been greatly limited due to the poor mechanical properties of hydrogel materials and their sometimes unideal optical properties. Although some progress has been achieved, it is still a challenge to develop photonic crystal hydrogel materials with the desired mechanical properties and structural color. Inspired by the excellent mechanical properties of polyurethane, this article presents a novel smart composite material, which combines nonionic UV-curable polyurethane and cross-linked polystyrene-co-poly(N-isopropylacrylamide) (PS-co-PNIPAM) microspheres for the first time. This photonic hydrogel demonstrates the desired optical properties and a narrow reflection peak half-width, while the tensile strength of the cured elastomers changes between 0.1 MPa and 0.43 MPa when the swelling ratio is changed; this multifunctional material can recover from deformation to its original size quickly. As proof-of-concept, SCN⁻ was selected as a target analyte. A hydrogel sensor enables the detection of SCN⁻ ions quantitatively and selectively in water through the ion-specific deswelling behavior of hydrogels (the Hofmeister series principle) and corresponding changes in the wavelength of the diffraction peak; the limit of detection is 5 μM. Our strategy is a straightforward way to design simple smart materials whose response and mechanical properties can be tuned through the judicious choice of the hydrophilic group type and content, extending the potential application of photonic hydrogels.