Development and Performance Evaluation of a Hydrogel Microneedle Sensor for In Situ Monitoring of Potassium Ions in Rice Plants

The dynamic balance of potassium ions (K ⁺ ) in rice plants is critical to their growth, development, and stress resistance. To achieve in-situ, real-time monitoring of K ⁺ levels in rice plants and overcome the limitations of traditional destructive sampling methods, this study developed a biosensor based on ion-selective hydrogel microneedles. Key performance parameters of the sensor, including its calibration curve and sensitivity, were systematically evaluated via in vitro electrochemical tests. Meanwhile, the mechanical strength and microstructure of the microneedles were characterized using micro-force testing. The practical applicability of the sensor was validated through agarose gel recovery experiments and in vivo K ⁺ monitoring in rice plants under salt stress, with results cross-validated against ion chromatography as a reference method. The sensor exhibited a sensitivity close to the Nernstian response (59.0 ± 0.11 mV/decade), a linear detection range of 10 ^− 4 to 10 ^− 1 M, and a detection limit of 3.0×10 ^− 5 M. It also demonstrated a fast response time (T ₉₅  < 20s), excellent stability, and high batch-to-batch reproducibility (relative standard deviation, RSD < 0.2%). The microneedles achieved a mechanical strength of 25 mN, which is well above the threshold required for penetrating the rice plant epidermis. During in vivo testing, the sensor successfully tracked the rapid K ⁺ loss in rice leaves under salt stress, showing a strong correlation with the standard method (R ²  = 0.985). In conclusion, the developed hydrogel microneedle sensor is a stable, reliable, and effective tool for in-situ K ⁺ analysis in rice plants, providing valuable insights into plant ion physiology and the mechanisms underlying responses to environmental stress.