Wearable platform based on 3D-printed solid microneedle potentiometric pH sensor for plant monitoring

The continuous chemical monitoring of analytes in plants can provide a better understanding of plant dynamics and the identification of health conditions. pH can be an indicator of plant abiotic stress due to changes in environmental conditions or biotic stress as a result of damage done to the plant by other living organisms. With the intensification of climate change, abiotic and biotic stresses are exacerbated. Hence, the engineering of miniaturized tools to monitor chemical signaling such as pH can be highly valuable as smart sensors for precision agriculture. Herein, 3D-printed microneedle-based electrochemical sensors are presented for in-plant monitoring of pH in leaf sap. First, affordable 3D-printed microneedle arrays (MNAs) of 900 µm in height, 300 µm in width, and 30 µm in tip diameter were manufactured. Subsequently, a metallic layer was sputtered on top of the MNAs to create microneedle electrodes. A novel plug-in two-electrode setup based on two MNAs was used to develop the MNA pH sensor employing polyaniline as a pH-sensitive layer. The MNA pH sensor was analytically characterized exhibiting near-Nernstian response (i.e., −59.9 ± 1.5 mV pH-1) even after several insertions in a leaf proving its mechanical robustness. Ex vivo analysis with plant sap from different species was successfully validated with a standard glass pH electrode. Finally, MNA sensors were used to continuously monitor pH in two plant species under regular conditions for four days. Interestingly, MNA sensors were also able to distinguish shifting pH events in plants under abiotic stress conditions (i.e., drought and watering). This new design brings a leap forward in smart sensors for applications in precision agriculture.