A Printed LC Resonator-Based Flexible RFID for Remote Potassium Ion Detection

This article presents a flexible printed radio-frequency identification (RFID) sensor based on a printed inductive–capacitive (LC) resonator circuit and a potassium ion-selective electrode (ISE) for remote potassium ion sensing. The potassium ion concentration of the contact solution can be monitored by measuring the change of the resonant frequency of the RFID sensor. The resonant frequency of the sensor can be directly detected by measuring the induced change in the reflection coefficient (
$S_{11}$
) of an external interrogator coil that is inductively coupled to the RFID sensor. Results obtained for the RFID sensor exhibited a second-order exponential relationship between the resonant frequency of the sensor and the K
⁺
concentration of the solution over 0.001–2 mol/L dynamic range values. Effects of varying separation distance between the sensor and the interrogator coil and the effect of temperature variations on sensor’s measurement are shown. With less than 2-s response time and the long-term stability, the wireless passive printed sensor has potential for low-cost K
⁺
monitoring applications such as K
⁺
monitoring in food packages.