Naked‐Eye Visual Warning of ppm v ‐Level Fault‐Free Humidity Upper Limit

ABSTRACT Monitoring and warning of ppm v ‐level fault‐free humidity upper limit (ppm v ‐FFHUL) are crucial for ensuring system reliability in critical fields such as high‐tech manufacturing, and aerospace. However, existing ppm v ‐FFHUL warning technologies are scarce, and constrained by “gradual change” response modes, resulting in complicated equipment, high costs, and insufficient portability, which greatly limits their flexible application. To address this, we propose a “leap change” response mode and an “enzyme‐like” construction method for precise ppm v ‐level humidity threshold modulation. It is achieved through synergistic supramolecular interactions between two size‐matched auxiliary mediums to precisely construct an “enzyme like” microenvironment containing a matching number of multi‐level, spatially fixed sensitizing functional groups around an elaborately designed humidity‐sensitive component. The as‐prepared material combines paper‐like flexibility, facile fabrication and handling, and enables high‐contrast readout of the target ppm v ‐FFHUL only by naked‐eye. Moreover, it holds promise as an easily deployable ppm v ‐FFHUL warning labels and provide a visual pre‐alert signal 20% ahead of the critical value. It offers a portable and low‐cost solution for specialized scenarios that require early warning and rapid screening of ppm v ‐level humidity. Furthermore, the “enzyme‐like” precise construction method and the “leap change” response mode provide an innovative perspective for the design of other high‐performance trace‐level monitoring materials.