A Spiral-Interlocking Microneedle Enabling Long-Term in Planta Glucose Monitoring

Continuous monitoring of plant metabolic dynamics remains challenging because existing sensing interfaces often fail to maintain stable mechanical coupling with deformable plant tissues during prolonged electrochemical measurements. Here, a spiral-interlocking microneedle electrode (SI-MNE) is presented for long-term in planta glucose monitoring through mechanically persistent tissue anchoring and minimally invasive biointerfacing. Unlike conventional conical or array-type microneedles that suffer from limited interfacial stability and motion-induced signal fluctuations, the SI-MNE employs a helical geometry that enables rotation-assisted insertion and 3D interlocking within plant tissue, thereby substantially enhancing mechanical retention. Cyclic-voltammetry-derived interfacial capacitance progressively increased during rotational insertion and reached a maximum under full-locking conditions, indicating enhanced electrochemical contact formation. Mechanical testing demonstrated approximately 10-fold higher pull-out resistance compared with conventional conical microneedles, while optical coherence tomography directly visualized stable insertion of the spiral architecture within plant tissue. To establish electrochemical glucose sensing functionality, a multilayer sensing interface consisting of a conductive carbon layer, PEDOT:PSS/Pt nanoparticle catalytic layer, and chitosan/glucose oxidase enzymatic layer was conformally integrated onto the spiral surface. The SI-MNE exhibited concentration-dependent amperometric glucose responses across 1-100 mM with a sensitivity of 350.3 nA/mM and high selectivity against representative plant sap interferents. The device further demonstrated stable operational reproducibility during continuous measurements and prolonged storage conditions. Using a three-electrode SI-MNE configuration, continuous glucose monitoring in living tomato plants successfully captured reproducible diurnal glucose fluctuations under natural light-dark cycles for 7 consecutive days without observable signal degradation or severe tissue damage. The proposed SI-MNE establishes a mechanically robust and electrochemically reliable biointerface for continuous metabolic monitoring in plants and provides a broadly applicable strategy for long-term in situ plant biosensing.