A wireless and passive RFID tag sensor for the detection of Pb2+ in soil combining chemiresistive sensing and impedance mismatch: A new method for onsite detection of hazardous metals

In this work, a wireless and passive radio frequency identification (RFID) tag sensor, which integrated a reduced graphene oxide/ion-selective membrane (rGO/ISM) chemiresistive sensing component, was developed for the onsite detection of Pb²⁺ in soil. Additionally, a new detection method combining impedance mismatch and spectral sensing technique for Pb²⁺ was proposed. Furthermore, the sensing mechanism of the RFID tag sensor was investigated in terms of the field-effect transistor (FET) transfer curve and antenna reflection coefficient. The charge density in the sensing component was altered by Pb²⁺, reducing resistance and modifying the tag antenna impedance. Impedance mismatch between the antenna and RFID chip was thereby induced, with varying degrees of mismatch directly altering the light-emitting diode (LED) luminous intensity on the tag sensor. Rapid detection (<2 s) via luminous intensity monitoring eliminated a vector network analyser (VNA), achieving a detection limit of 1.96 μg/L. Ultimately, the sensing performance of the RFID tag sensor was verified through the detection of Pb²⁺ in real soil samples (recovery rates of 99–101%, and RMSEs of 2.08–7.32 μg/kg), indicating that the tag sensor offers a simple and effective solution for rapid and onsite detection of hazardous heavy metals in soil, with potential for practical application.