Boron/MXene Hollow Spheres for Humidity Sensing and Respiration Monitoring

Boron nanosheets with high surface hydrophilicity and proton conductivity are promising for ionic devices. However, their poor electrical conductivity has severely limited their applications in resistive-type electronic devices. Herein, electrically conductive three-dimensional boron/MXene hollow spheres were prepared by a freeze-drying-assisted self-assembly method. The composites were fabricated into resistive-type humidity sensors, which showed high sensitivity with a three-orders-of-magnitude variation in resistance, a wide detection range (11-97%), fast response and recovery time (4 s/3.8 s), and long-term stability. The excellent humidity sensing performance can be attributed to the abundant hydrophilic groups on both the inner and outer surfaces of open spheres as well as to a coupled hole/proton conducting process. As a proof of concept, real-time remote monitoring of human respiration was achieved by integrating our humidity sensor with a Bluetooth module, enabling differentiation of breathing frequency as rapid as 0.8 s.