Fluorescent and ratiometric humidity sensor based on cyano-substituted oligo(p-phenylene)/silica gel hybrid materials

Abstract

We have synthesized a kind of cyano-substituted oligo(p-phenylene) hybrid materials by Knoevenagel condensation reaction, which can be used as ratiometric relative humidity (RH) sensor. When the RH increased from 5 % to 90 %, the maximum emission peak of C1-3/SG gradually redshifted from 530 nm to 575 nm with the fluorescence changed from green to orange. This phenomenon can be attributed to the water vapor induced aggregation of dyes on the matrix. The non-covalent force between the dyes and the silica gel could be adjusted by altering the structures of the dyes, and the RH sensing property of the final hybrid materials could be further regulated. R2-3/SG hybrid fabricated by R2 with large steric isooctyl groups possessed indiscernible RH sensing properties, while the R1-3/SG without hydrogen bond interactions between dyes and substrate was more susceptible to water vapor. The surface properties and specific surface area of the substrate were critical for RH sensing. RH sensing properties were not presented for the substrates without hydroxyl groups or low specific surface area such as PET or PP fibers. Moreover, we have prepared a kind of C1 loaded paper strips, which exhibited potentially ability in anticounterfeiting application.