Double-Fluorescent Response Spiropyran-Functionalized Graphitic Carbon Nitride Solvent Decoder and Nanofiber Mat-Based Visual Colorimetric Sensor for Humidity Detection within Grain Heaps

Spiropyran-functionalized graphitic carbon nitride (MCH@g-C₃N₄) in acidic conditions was prepared for the first time. MCH@g-C₃N₄ exhibited dual-wavelength fluorescence emission with two distant bands and distinct solvatochromic behavior owing to the different molecular structures of spiropyran. A three-dimensional organic solvent decoder was fabricated for solvent identification based on the emission wavelength and fluorescence quantum yield of MCH@g-C₃N₄ in different polar solvents. The ratiometric fluorescence sensing of the water content in organic solvent was realized using water to induce the structural change of spiropyran. A solid-phase MCH@g-C₃N₄/polyvinylpyrrolidone (PVP) nanofiber mat-based sensor was prepared via cospinning MCH@g-C₃N₄ with hydrophilic PVP. A fluorescent humidity sensor was fabricated using a commercial syringe equipped with a stainless-steel needle (catcher) to obtain the air inside the grain heap, which enabled the accurate location of the sampling spot. The space between the syringe and piston was used as an enclosed sensing space. Thus, a simple, accurate, and intuitive visual colorimetric method for the internal humidity of a grain heap was realized by analyzing the recorded images.