A Wireless Passive RFID-Integrated Sensor for the Detection of Pb ²⁺ in Soil: An Innovative Method Using Impedance Mismatch for On-Site Detection of Heavy Metals

In this study, a wireless passive radio frequency identification (RFID) integrated sensor was proposed for on-site rapid detection of Pb²⁺ in soil, eliminating the need for intricate detection circuits or high-cost equipment (e.g., vector network analyzer (VNA)). The RFID-integrated sensor comprised an RF chip, a tag antenna, and an rGO/LISM chemiresistive sensing unit, with a modular design (reusable housing/peripherals, clamp-connected replaceable sensing unit) to curtail long-term detection costs. Moreover, a detection method based on impedance mismatch was developed, using an experimental setup that captured changes in the minimum response power (MRP) of the RFID-integrated sensor. Specifically, Pb²⁺ in soil extracts selectively permeated through the LISM and underwent specific chemisorption with oxygen-containing functional groups on rGO, which altered the hole carrier concentration of the sensing unit, increased its resistance, and further modified the impedance of the tag antenna. This alteration induced impedance mismatch between the RF chip and the tag antenna, where varying degrees of impedance mismatch directly modulated the MRP. Validated via simulations and testing, the RFID-integrated sensor achieved rapid detection (<2 min) of Pb²⁺ in real soil samples, with RMSEs < 0.30 mg/kg, relative errors <15%, and recovery rates of 85.23%-107.14%, supporting it as an economical and rapid solution for Pb²⁺ detection. This method additionally offered an efficient and straightforward approach for the on-site rapid detection of heavy metals in soil, boasting practical application prospects.