A supramolecular liquid crystal/photonic crystal composite film with broad diffraction-wavelength shifts/fast responses to pH changes

Abstract

Responsive photonic crystals (RPCs) have attracted considerable interest due to their fascinating potential in various fields, such as sensors, color displays, anticounterfeiting, paints and inks. Herein, we presented a supramolecular liquid crystal (SLC) inverse opal composite film (SLC/PC) characterized by rapid and broad wavelength shifts in response to pH changes. This composite film was prepared based on the hydrogen-bonded (H-bonded) SLCs using silica colloids as the template. The H-bonds served as a functional “switch” that could reversibly turn on (break) and off (recover) in response to changes in environmental pH or temperature. This reversible activation led to the swelling or shrinkage of the SLC layer, resulting in significant changes in lattice spacing and diffraction-wavelength shifts according to Bragg's law. Meanwhile, the structural color of the SLC/PC varied significantly, and a naked-eye monitoring of pH and temperature was realized. Moreover, the film exhibited rapid response times (less than 0.5 s), complete reversibility, and broad diffraction-wavelength shifts (up to 250 nm, nearly covering the entire visible spectrum) when exposed to pH changes. The alignment of molecules as well as the SLC structure within the film played crucial roles in achieving this high responsiveness. The film also demonstrated exceptional chemical stability and could be recycled effectively. Such a functional film, offering rapid, completely reversible, and broad wavelength shifts, along with excellent reusability and stability, holds great promise for use as a pH sensor in environmental monitoring.