Brush‐Like Polymeric Gels Enabled Photonic Crystals toward Ultrasensitive Cosolvent Chromism

Abstract Responsive photonic crystals (RPCs) exhibit dynamic chromism upon trigger by various solvents, showing potential applications in qualitative identification and quantitative analysis of multicomponent solvents. However, distinguishing similar solvents, especially traces of cosolvents, remains challenging due to the limited sensitivity of RPCs. To address this, we herein introduce brush‐like polymeric gels inside photonic crystals, forming a brush‐like polymeric photonic gel (BPPG) that can trace tiny component changes. In this BPPG system, the acrylate backbones and polyethylene glycol (PEG) side‐chains stretch incrementally due to the cosolvency of ethanol‐water mixtures, resulting in highly sensitive chromatic responses within ethanol‐rich concentrations. With water content varying slightly from 0 to 1 vol %, the reflection wavelength of BPPG can sharply redshift over 30 nm, leading to very noticeable changes in structural color. This enhanced sensitivity makes BPPG suitable for real‐time, in situ purity monitoring of absolute ethanol during storage, transportation, and other applications.