Surface Acoustic Wave Humidity Sensor Based on Flake-Like Nanodiamond-Chitosan Composite Sensitive Film

A surface acoustic wave (SAW) relative humidity (RH) sensor based on flake-like nanodiamond-chitosan (ND-CS) composite film was prepared. Flake-like NDs were uniformly dispersed into the CS and combined with the CS by hydrogen bonding. The abundant hydrophilic groups of both ND and CS are beneficial for the adsorption of water molecules. The obtained humidity sensor exhibits good linear response under wide RH range of 10%–90% with a tiny humidity hysteresis of 3.9%. The sensitivity of the sensor is −4.93 kHz/% for the adsorption process and −4.83 kHz/% for the desorption process. The short response time of 17.6 s and recovery time of 5.2 s are obtained when the RH switches between 10% and 70%. Meanwhile, the prepared ND-CS-coated SAW sensor also shows excellent repeatability, long-time stability, temperature stability, and selectivity.