A carbonaceous-material/PVDF-modified all-solid-state microneedle potentiometric array for multiplexed in vivo ion-selective monitoring in salt-stressed wheat

A 4-channel solid-state microneedle array was developed for synchronized multi-ion sensing. • Carbonaceous-material/PVDF improved interfacial potential stability. • PU/PEG coating enhanced membrane adhesion and mechanical robustness. • The array enabled in vivo, simultaneous tracking of multi-ion dynamics under salt stress. Understanding ion regulation during plant salt stress requires real-time in vivo monitoring of coordinated multi-ion dynamics. However, the intrinsic complexity of living plant tissues poses substantial challenges to reliable multiplexed ion sensing. To address these challenges, a four-channel all-solid-state microneedle platform was developed for simultaneous in vivo monitoring of Na + -, K + -, H + -, and Ca 2+ -selective potentiometric responses in wheat seedling stems. The platform employs a graphene/PPy/Fe(CN)₆ solid contact for ion-to-electron transduction, a carbonaceous-material/PVDF composite dispersed within the ion-selective membranes, and an outer polyurethane/polyethylene glycol protective coating. Before PU/PEG deposition, the optimized Na + -, K + -, and Ca 2+ -selective electrodes showed linear activity-based calibration responses over the nominal concentration range of 10 −5 –10 −1  M, with mean activity-based detection limits of pNaLOD = 5.6, pKLOD = 5.4, and pCaLOD = 5.5, respectively; the H + -selective electrode showed a linear response over pH 2–11, with a mean pHLOD of 11.1. Under continuous immersion in MS medium, the PU/PEG-protected electrodes maintained the predefined near-Nernstian calibration-slope thresholds for approximately 10–15 days, depending on the ion channel, whereas the corresponding unprotected electrodes reached these thresholds earlier. Spike-recovery tests of the Na + -, K + -, and Ca 2+ -selective electrodes in wheat stem extracts yielded recoveries of 86.0–128.3%, while multi-ion perturbation experiments showed limited cross-channel signal coupling under the tested conditions. The platform was further applied to two wheat cultivars for real-time monitoring of relative Na + -, K + -, H + -, and Ca 2+ -selective EMF dynamics in seedling stems under salt stress. The two cultivars exhibited distinct late-stage ion-selective ΔEMF patterns across all four channels, with DK961 showing larger Na + - and Ca 2+ -selective ΔEMF responses, a more negative K + -selective ΔEMF response, and a smaller H + -selective ΔEMF response than Landon. Together, these results support the analytical and operational feasibility of the microneedle array for multiplexed in vivo monitoring of relative ion-selective EMF dynamics in salt-stressed wheat stems. Download: Download high-res image (142KB) Download: Download full-size image