High sensitivity and fast response wireless humidity sensor enabled by MXene-Lys for finger proximity detection and health monitoring applications

Designing integrated, miniaturized, and wireless wearable humidity tracking devices is of great significance for realizing human-related intelligent sensing and health monitoring. In this work, a MXene-Lys (lysine-modified MXene) enabled, fast-response, high-sensitivity, small-sized wireless Bluetooth humidity sensor for real-time monitoring of human finger position, respiration, and physical activity is proposed. Theoretical and experimental results demonstrate that the synthesis of MXene-Lys improves the humidity sensing properties of the sensor by expanding the interlayer distance and enhancing hydrophilic ability. In experimental measurements, the DC (Direct Current) resistance response of the MXene-Lys humidity sensor reaches 30,161 when the relative humidity changes from 20 % to 90 %. Additionally, the equilibrium response time and recovery time of the MXene-Lys sensor were measured as 0.22 s and 0.45 s, respectively, during a rapid humidity shift between 45 % and 75 %. In device recovery tests, the sensor demonstrated a low hysteresis of only 2.77 % RH. Furthermore, the device showed excellent long-term stability, maintaining consistent performance over a 20-day period. The sensor was successfully applied for non-contact finger detection and human respiration rate monitoring, accurately distinguishing between fast and slow breathing patterns and detecting finger positions within 9 mm. Finally, the sensor, along with its battery, Bluetooth transmission module, and protective layer, was encapsulated into a compact wireless humidity sensor (approximately 35 mm × 15 mm × 12 mm). By receiving signals through a mobile device, the sensor enables long-duration and real-time (5.56 h) monitoring of human motion states.