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 (IDE) 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 (RFID) 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 (45/140 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.