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 (NH
₃
) sensing. The passive wireless sensor tag consists of a single-walled carbon nanotube (SWCNT) network based NH
₃
sensor, a radio frequency antenna, a ring oscillator, and other supporting circuits. The SWCNT network-based NH
₃
sensor is fabricated on a flexible plastic substrate through printable processes. The printable SWCNT-based NH
₃
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 NH
₃
concentration (referred to as log [NH
₃
] 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 [NH
₃
]. The linear response allows one to precisely predict the NH
₃
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 NH
₃
sensing and monitoring with high accuracy.