Chromogenic Photonic\nCrystal Detectors for Monitoring Small Molecule Diffusion at Solid–Solid\nInterfaces Using Stimuli-Responsive Shape Memory Polymers

In situ monitoring of small molecule diffusion at solid–solid\ninterfaces is challenging, even with sophisticated equipment. Here,\nnovel chromogenic photonic crystal detectors enabled by integrating\nbioinspired structural color with stimuli-responsive shape memory\npolymer (SMP) for detecting trace amounts of small molecule interfacial\ndiffusion are reported. Colorless macroporous SMP membranes with deformed\nmacropores can recover back to the “memorized” photonic\ncrystal microstructures and the corresponding iridescent structural\ncolors when triggered by diffused small molecules. Systematic experimental\nand theoretical investigations using various microscopes, optical\nspectroscopy and modeling, spatio-resolved energy-dispersive X-ray\nspectroscopy, and theoretical diffusion calculations confirm the diffusion-induced\nshape memory and chromogenic mechanisms. Importantly, proof-of-concept\nsensing of temporospatial-resolved diffusion of bioactive ingredients\nused in drug delivery, including anti-inflammatory methyl salicylate\nin pain relieving patches and vitamin E barriers loaded in contact\nlens, and phthalates plasticizers in commercial PVC products has been\ndemonstrated. These innovative detectors are inexpensive, reusable,\nand easy to operate and deploy for both qualitative and quantitative\nanalyses, promising for opening new avenues in biomedical research,\nthreat detection, and monitoring of plastics, food, and environmental\nsafety. Moreover, reconfigurable photonic crystals with micrometer-scale\nresolution, which are of great importance in tunable and integrated\nnanooptics, can be fabricated by diffusion-enabled microcontact printing.