Wireless, Passive Paper-Based LC Humidity Sensor with RFID Frequency of 13.56 MHz

A low-cost paper-based LC humidity sensor is presented herein. The sensor comprises an interdigitated electrode capacitor and a planar spiral inductor in parallel. We used a high-frequency structure simulator to design and optimize the structure of the sensor, making the sensor work at a standard radio frequency identification frequency of 13.56 MHz. The sensors were fabricated with four types of paper substrates by a screen-printing process. The effect of paper surface morphology on sensor performance was studied. Among four types of sensors, the Kodak photographic paper-based sensor exhibited the best sensitivity of 2.32 kHz/%RH (relative humidity) and low wet hysteresis of 2.9 %RH. To further improve the sensitivity of the sensor, graphene oxide (GO) dispersions with different mass percentages (0.1 %, 0.2 %, and 0.5 %) were sprayed on the Kodak photographic paper substrate. Experimental results showed that the sensor with 0.5 % GO had the highest sensitivity of 3.65 kHz/%RH, a low wet hysteresis (3.3 %RH), short response/recovery times (45 s/140 s), and high linearity (R ² = 0.998). Compared to other flexible sensors, the sensor has good performance, and it has application potential to the Internet of Things, for example, in storage and express transportation.