Precisely Tunable Humidity Color Indicator Based on Photonic Polymer Films

Abstract

Photonic crystals (PCs) are attractive for utilization as label-free self-display colorimetric sensors. In this study, onedimensional (1D) photonic humidity color indicators with color transitions across the whole visible spectral range are reported. For this, we synthesized photo-cross-linkable poly(2-vinylnaphthaleneco-benzophenone acrylate) (P(2VN-co-BPA)) as a high-refractiveindex polymer and poly(4-vinylpyridine-co-benzophenone acrylate) (P(4VP-co-BPA)) with its four quaternized derivatives (P4QP- 14%, P4QP-25%, P4QP-37%, and P4QP-51%) as low-refractiveindex polymers. The 1D PC films are fabricated via the sequential alternating layer deposition of high/low-refractive-index polymers onto a black poly(ethylene terephthalate) (PET) substrate. Distinct colorimetric humidity detection without an additional label is achieved by controlling the degree of quaternization of the P(4VP-co-BPA) layers and fine-thickness modulation of the P(2VN-coBPA) and P4QP layers. The films exhibit remarkable color transitions with reliable sensitivity, reversibility, and reproducibility according to changes in the relative humidity (RH), covering the entire visible spectrum (435?756 nm) for 10?90% RH.