SPR-Amplified TiO2–NanoMXene Heterojunction for Ultrasensitive Contactless Human Activity Recognition

Contactless human activity recognition (HAR) is a cornerstone of next-generation intelligent systems, but vision/inertial sensor-based progress is limited by reliability and privacy concerns. Monitoring subtle moisture signatures offers a smart alternative, yet constrained by either sensitive material or sensor architecture design and facing the trade-off among ultrasensitivity, rapid response, and scalable multichannel operation. We address this via a new moisture-sensing paradigm: a plasmon-amplified TiO2–nanoMXene heterojunction integrated onto a fiber-optic tip, leveraging light as an active catalyst in a self-reinforcing feedback loop. Surface plasmon resonance (SPR) injects hot electrons, amplifying the built-in electric field of the heterojunction to enhance H2O polarization and boost the trace moisture adsorption. This alters dielectric constant, feeding back to SPR signals. This self-reinforcing cycle translates subtle moisture signatures into a robust optical output, enabling a new class of HAR with ultrahigh sensitivity, fast response/recovery, minimal hysteresis, and robust environmental resilience. Its transformative potential is validated through contactless finger sensing, noninvasive diaper wetness monitoring, and clinical-grade respiratory analysis.