Recent progress in low-swellable polymer-based smart photonic crystal sensors

Low-swelling polymers (LSPs) generally refer to materials with a low solvent absorption ratio or volume expansion rate at swelling equilibrium. LSPs with exceptional responsiveness could be upgraded to smart sensors with structural color self-reporting by bridging photonic crystals (PCs). Based on the regulation of swelling to effective refractive index, lattice spacing, the order-disorder arrangement of nanostructures, and incident/detection angle, the structural color feedback of smart photonic crystal sensors (SPCSs) can quantitatively and visually reveal the stimulus, which greatly promotes the interdisciplinary development of nanophotonic technology in the fields of chemical engineering, materials science, engineering mechanics, biomedicine, environmental engineering, etc. Herein, to clarify the role of the photonic structures and polymer molecules in high-performance SPCSs, LSP-based SPCSs are summarized and discussed, including general swelling mechanisms, color change strategies, structural design, and typical functional applications. It aims to figure out the combination rule between PC structures and LSPs, optimize the design of PC structures, and expound the corresponding structural color sensing mechanisms, inspiring the fabrication of next-generation SPCSs. Finally, perspectives on future structural design and sensing applications are also presented. It is believed that SPCSs are multifunctional nanophotonic tools for the interdisciplinary development of numerous engineering fields in the future.