Gradient Protein Thin-Film-Based Bragg's Mirror.

Conventional structural color sensors with an even thickness can hardly detect tiny structural changes caused by analytes at extremely low concentrations. Herein, we construct a gradient protein thin-film-based Bragg's mirror, which reveals a highly effective signaling translation from the 3D gradient surface to 2D interference patterns and thus realizes an extremely high sub 0.5 Å of spatial resolution by using a simple CCD camera, exhibiting 1000-fold improvement than the color recognition. Bragg's mirror consists of responsive "core-shell" structural protein nanocrystals, which is applied for sensing humidity and putrescine vapor through adsorption-swelling and crystallization-swelling mechanisms, respectively. Through an exposure-development strategy, we successfully demonstrated the real-time monitoring of seafood spoilage in high-humidity environments. Compared to professional spectrometer-based reflectance measurements, our Bragg pattern analysis shows 2.6-5× sensitivity improvement in both detection modes. This work introduces a means to open the door to types of high-performance and low-cost colorimetric sensors that are competitive with and offer greater functionality than bulky/expensive modern optical sensors.