In Planta Nitrate Sensor Using a Photosensitive Epoxy Bioresin

Nitrogen management through monitoring of crop nitrate status can improve agricultural productivity, profitability, and environmental performance. Current plant nitrate test methods require expensive instruments, time-intensive labor, and trained personnel. Frequent monitoring of in planta nitrate levels of the stalks in living plants can help to better understand the nitrogen cycle and the physiological responses to environmental variations. Although existing enzymatic electrochemical sensors provide high selectivity, they suffer from short shelf life, high cost, low-temperature storage requirement, and potential degradation over time. To overcome these issues, an artificial enzyme (vitamin B12 or VB12) and a two-dimensional material (graphene oxide or GO) are introduced into a conventional photoresist (SU8) to form a bioresin SU8-GO-VB12 that can be patterned with photolithography and laser-pyrolyzed into a carbon-based nanocomposite C-GO-VB12. The electrocatalytic activity of the cobalt factor in VB12, the surface enhancement properties of GO, and the porous feature of pyrolytic carbon are synergized through design to provide C-GO-VB12 with a superior ability to detect nitrate ions through redox reactions. In addition, laser writing-based selective pyrolysis allows applying thermal energy to target only SU8-GO-VB12 for selective pyrolysis of the bioresin into C-GO-VB12, thus reducing the total energy input and avoiding the thermal influence on the materials and structures in other areas of the substrate. The C-GO-VB12 nitrate sensor demonstrates a year-long shelf lifetime, high selectivity, and a wide dynamic range that enables a direct nitrate test for the extracted sap of maize stalk. For in situ monitoring of the nitrate level and dynamic changes in living maize plants, a microelectromechanical system-based needle sensor is formed with C-GO-VB12. The needle sensor allows direct insertion into the plant for in situ measurement of nitrate ions under different growth environments over time. The needle sensor represents a new method for monitoring in planta nitrate dynamics with no need for sample preparation, thus making a significant impact in plant sciences.