Low-drift microneedle ion-selective electrodes for in situ monitoring of Mg2+ in plants.

Real-time in situ monitoring of magnesium ions (Mg2+) concentration in plants is essential for understanding ion transport and advancing precision agriculture. However, real-time in situ monitoring using all-solid-state ion-selective electrodes (ASS-ISEs) remains challenging because a water-layer tends to form at the interface between the sensing membrane and the metallic substrate, which leads to substantial potential drift. Here, we present an interfacial-engineering strategy for the microneedle electrode fabrication process, along with a method that enables accurate and stable in situ monitoring of Mg2+ in the stem xylem of tomato seedlings. Through comparative evaluation of candidate solid-contact materials, a graphene composite waterborne coating was identified as the optimal solid contact; it suppresses water-layer formation primarily by enhancing interfacial compactness rather than hydrophobicity alone. The optimized electrode delivered a near-Nernstian slope of 29.00 ± 0.20 mV/dec, a linear range of 1.0 × 10-5-1.0 × 10-1 mol/L, a detection limit of 1.0 × 10-6.97±0.06 mol/L, a low drift of ∼0.3 mV/h, a charge-transfer resistance of 0.74 MΩ, and a storage lifetime of ∼13 months. Under both magnesium-deficient and high-magnesium stress conditions, the in situ monitoring data agreed well with the trends measured by inductively coupled plasma optical emission spectrometry (ICP-OES), confirming the reliability of the electrode. Overall, this work offers mechanistic insight into water-layer suppression in ASS-ISEs and highlights the potential of microneedle electrodes as a versatile platform for smart agriculture and plant science.