Cavity‐Enhanced Eye‐Readable Detection of 0.1% H 2 via Active Modulation of Charge‐Transfer Plasmons

Eye‐readable hydrogen sensors (EHSs) are essential for the safe deployment of hydrogen energy technologies, owing to their intuitive optical readout capabilities. Nevertheless, current plasmonic EHS designs are hindered by high critical discoloration concentrations (CDCs), slow response times, and inconsistent discoloration behavior. Herein, we present a Fabry‐Pérot cavity‐type EHS composed of close‐packed Au@Pd@oxide core‐shell nanoparticles that operates via a hydrogen‐induced charge‐transfer (HICT) mechanism. Hydrogen exposure reduces of the oxide shell of Au@Pd@oxide and forms Au@Pd, transforming the plasmonic resonance from a gap mode to a charge‐transfer mode, resulting in a vivid, reliable color change. This design achieves an ultra‐low CDC of 0.1% H 2 , a rapid response time of 4 s, and robust performance across diverse environmental conditions, surpassing U.S. Department of Energy safety targets. This work establishes HICT as a novel strategy for active plasmon modulation and highlights cavity‐engineered plasmonic architectures as promising platforms for next‐generation hydrogen safety sensors.