A Printable CNT-Based FM Passive Wireless Sensor Tag on a Flexible Substrate With Enhanced Sensitivity

In this paper, we report a frequency-modulated (FM) passive wireless sensor tag for ammonia (NH₃) sensing. The passive wireless sensor tag consists of a single-walled carbon nanotube (SWCNT) network based NH₃ sensor, a radio frequency antenna, a ring oscillator, and other supporting circuits. The SWCNT network-based NH₃ sensor is fabricated on a flexible plastic substrate through printable processes. The printable SWCNT-based NH₃ sensor shows an enhanced sensitivity of 0.76% per part per million primarily due to the large surface area of the SWCNT network. The sensor also exhibits a high linearity between the resistance of the sensor and logarithm of the NH₃ concentration (referred to as log [NH₃] henceforth). A simple FM circuit is designed to convert the resistance change of the sensor to the oscillating frequency shift of the circuit. By properly designing the circuit, we have obtained a linear response between the frequency shift and log [NH₃]. The linear response allows one to precisely predict the NH₃ concentration by measuring the frequency shift of the FM wireless sensor tag. Such an FM-modulated passive wireless sensor tag with linear response and enhanced sensitivity is promising for power-less stand-alone low-level NH₃ sensing and monitoring with high accuracy.