# Naked‐Eye Visual Warning of ppmv‐Level Fault‐Free Humidity Upper Limit

## TL;DR

This paper introduces a low-cost, portable method for visually detecting ppmv-level humidity thresholds using a paper-like material that provides early warnings with the naked eye.

## Contribution

The paper introduces a novel 'leap change' response mode and 'enzyme-like' construction method for precise ppmv-level humidity threshold detection.

## Key findings

- The material enables high-contrast, naked-eye detection of ppmv-level humidity thresholds.
- It provides a 20% early warning signal before reaching the critical humidity value.
- The method is portable, cost-effective, and suitable for real-time monitoring in critical environments.

## Abstract

Monitoring and warning of ppmv‐level fault‐free humidity upper limit (ppmv‐FFHUL) are crucial for ensuring system reliability in critical fields such as high‐tech manufacturing, and aerospace. However, existing ppmv‐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 ppmv‐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 ppmv‐FFHUL only by naked‐eye. Moreover, it holds promise as an easily deployable ppmv‐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 ppmv‐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.

In this study, based on the urgent need in many critical fields for ppmv‐level fault‐free humidity upper limit (ppmv‐FFHUL) monitoring, an innovative threshold gated humidity sensing technology was developed. It exhibits superior portability (as thin and light as paper) and highly economic feature (without auxiliary instruments at all), and can achieve real‐time monitoring and efficient early warning of ppmv‐FFHUL.