Modulating the perovskite lattice and fluorescence color via the "water fingerprint": A versatile platform for visual trace H2O sensing

The accurate and on-site measurement of trace water content in industrial processes is essential for product quality and production safety. However, most existing methods remain restricted to laboratory environments and are not readily applicable under complex field conditions. Herein, a universal visual sensing platform for rapid and accurate trace water content monitoring is proposed, based on H 2 O-actuated lattice regulation of the lead halide perovskite nanocrystals (HPNCs). Water specifically triggers the redox reaction between iodine and sulfur dioxide, and the resulting iodide ions subsequently undergo a halogen exchange with bromide ions in the HPNCs, thereby modulating their band gap and inducing distinct changes in emission wavelength and fluorescence color, thus enabling visual detection of trace water content. Despite the simple operation and instrument-free visual detection characteristics, its performance is comparable to the standard Karl Fischer volumetric titration method. Owing to the highly specific and rapid kinetics of H 2 O-triggered modulation of the perovskite structure, this strategy enables selective detection of water content within minutes. Integrated with a standard colorimetric card or a common smartphone, it enables semi-quantitative or quantitative on-site determination of water content in various real samples, pioneering a completely new platform for portable trace H 2 O sensing.