Preparation and performance study of hydrogel microneedle sensors for in situ monitoring of potassium ions in rice plants.

To achieve real-time in situ monitoring of potassioum ions (K⁺) concentrations within plants and overcome the limitations of traditional destructive sampling methods, this study developed a microneedle (MN) biosensor featuring an ion-selective hydrogel at its tip. The sensor’s calibration curve and sensitivity were evaluated through in vitro electrochemical testing. Microforce testing and scanning electron microscopy were used to analyze the mechanical strength and microstructure of the MNs. Practical performance was validated through agarose gel recovery experiments and in vivo salt stress monitoring in rice via ion chromatography. The results indicate that the sensor exhibits near-Nernstian sensitivity toward K⁺ (59.3 ± 0.35 mV/decade), with a linear range from 0.1 mM to 100 mM and a detection limit of 3.5 × 10⁻³ mM. The sensor exhibited a rapid response (T95% = 15 ± 3 s), excellent stability, and high batch-to-batch reproducibility (RSD = 0.038%). The average breaking force of the MNs was 25.3 ± 2.1 mN, significantly exceeding the puncture threshold of the rice leaf epidermis (approximately 5–15 mN). In vivo experiments demonstrated that the sensor successfully monitored rapid K⁺ efflux dynamics in rice leaves under salt stress, and the results were highly consistent with those of ion chromatography (R² = 0.999). The hydrogel microneedle sensor developed in this study demonstrates reliable performance, providing a robust in situ analytical tool for investigating plant ion physiology and mechanisms of response to environmental stresses.